A Multiwavelength Survey of Wolf-Rayet Nebulae in the Large Magellanic Cloud
Clara Shang Hung, Po-Sheng Ou, You-Hua Chu, Robert A. Gruendl, Chuan-Jui Li
DD RAFT VERSION J ANUARY
6, 2021Typeset using L A TEX twocolumn style in AASTeX63
A Multiwavelength Survey of Wolf-Rayet Nebulae in the Large Magellanic Cloud C LARA S HANG H UNG ( 洪 宇 函 ) ,
1, 2 P O -S HENG O U ( 歐 柏 昇 ) ,
1, 3 Y OU -H UA C HU ( 朱有 花 ) ,
1, 3, 4 R OBERT
A. G
RUENDL , AND C HUAN -J UI L I ( 李 傳 睿 ) Institute of Astronomy and Astrophysics, Academia Sinica, No.1, Sec. 4, Roosevelt Rd., Taipei 10617, Taiwan, R.O.C. Summit Public School: K2, El Cerrito, CA 94530, U.S.A. Department of Physics, National Taiwan University, No.1, Sec. 4, Roosevelt Rd., Taipei 10617, Taiwan, R.O.C. Department of Astronomy, University of Illinois, 1002 West Green Street, Urbana, IL 61801, U.S.A.
ABSTRACTSurveys of Wolf-Rayet (WR) stars in the Large Magellanic Cloud (LMC) have yielded a fairly completecatalog of 154 known stars. We have conducted a comprehensive, multiwavelength study of the interstel-lar/circumstellar environments of WR stars, using the Magellanic Cloud Emission Line Survey (MCELS) im-ages in the H α , [O III ] , and [S II ] lines; Spitzer Space Telescope µ m images; Blanco 4m Telescope H α CCD images; and Australian Telescope Compact Array (ATCA) + Parkes Telescope H I data cube of the LMC.We have also examined whether the WR stars are in OB associations, classified the H II environments of WRstars, and used this information to qualitatively assess the WR stars’ evolutionary stages. The 30 Dor giant H II region has active star formation and hosts young massive clusters, thus we have made statistical analyses for 30Dor and the rest of the LMC both separately and altogether. Due to the presence of massive young clusters, theWR population in 30 Dor is quite different from that from elsewhere in the LMC. We find small bubbles ( < ∼
12% of WR stars in the LMC, most of which are WN stars and not in OB associations.The scarcity of small WR bubbles is discussed. Spectroscopic analyses of abundances are needed to determinewhether the small WR bubbles contain interstellar medium or circumstellar medium. Implications of the statis-tics of interstellar environments and OB associations around WR stars are discussed. Multiwavelength imagesof each LMC WR star are presented.
Keywords:
ISM: bubbles– stars: Wolf–Rayet INTRODUCTIONWolf-Rayet (WR) stars are often surrounded by beautifulring-shaped nebulae in H α images. Since the discovery ofthe first three WR ring nebulae, NGC 2359, NGC 6888, andS308 (Johnson & Hogg 1965), systematic surveys of WRnebulae have been conducted in our Galaxy (e.g., Chu 1981;Heckathorn et al. 1982; Chu et al. 1983; Miller & Chu 1993;Marston et al. 1994a,b; Stock & Barlow 2010) and the LargeMagellanic Cloud (LMC; e.g., Chu & Lasker 1980; Dopita etal. 1994). It is found that ring nebulae around WR stars canhave a variety of morphologies and sizes, possibly indicat-ing different formation mechanisms and evolutionary stages.To carry out a comprehensive investigation of ring nebulaearound WR stars, it is beneficial to compile a complete in-ventory of WR stars in a galaxy and use deep nebular lineimages to search for associated nebulae.It is difficult to achieve completeness in the catalog of WRstars in our Galaxy because of the heavy extinction along theGalactic plane. Even the distances of the Galactic WR starswere uncertain before the Gaia data became available (GaiaCollaboration et al. 2018). It would thus be difficult to per- form statistical and quantitative analyses on the WR nebulaein our Galaxy. The LMC, on the other hand, is at a known50 kpc distance (Pietrzy´nski et al. 2019) with little internaland foreground extinction; thus, it is possible to achieve amore complete inventory of WR stars and carry out a thor-ough multiwavelength search for associated nebulae.An initial search of WR stars in the LMC by Westerlund& Smith (1964) found 58 objects, and subsequent searcheshave increased the number of known LMC WR stars to 80 byFehrenbach et al. (1976); 100 by Breysacher (1981); 135 byBreysacher et al. (1999); and more recently 154 by Neugentet al. (2018). The latest LMC WR star catalog is likely fairlycomplete.The search for WR nebulae in the LMC began with Chu &Lasker (1980) using photographic plates taken with the 0.61m Curtis Schmidt Telescope. Later, Dopita et al. (1994) usedthe Australian National University 2.3 m Telescope with aspectrograph in the imaging mode, as well as an imager, toconduct a deeper survey on WR nebulae. Stock & Barlow(2010) used digitized H α photographic plates taken with the1.2 m UK Schmidt Telescope to search for WR nebulae in a r X i v : . [ a s t r o - ph . GA ] J a n the LMC. The former two surveys used H α and [O III ] im-ages, while the latter used only H α images. Meanwhile, theMagellanic Cloud Emission Line Survey (MCELS) has beenconducted with the Curtis Schmidt Telescope and a CCDcamera, providing H α , [O III ], and [S II ] images for the en-tire LMC (Smith, & MCELS Team 1999); furthermore, the Spitzer Space Telescope has surveyed the LMC (Meixner etal. 2006) in infrared passbands, and the Australian TelescopeCompact Array (ATCA) has surveyed the H I in the LMCwith high resolution (Kim et al. 2003). Using these new sur-veys of neutral and ionized gas and the nearly complete cat-alog of WR stars in the LMC, we are able to, for the firsttime, probe the multiphase interstellar environment of WRstars and investigate interactions between the stars and theirambient medium.This paper reports our survey and analysis of WR nebulaein the LMC. Section 2 expands upon the expected coevolu-tion of a WR star and its environment, Section 3 describesthe data and methodology used to conduct this investigation,Section 4 presents the results, Section 5 discusses the impli-cations of our results, and Section 6 summarizes this work. COEVOLUTION OF WR STARS AND THEIRAMBIENT MEDIUMWhen WR nebulae were first discovered, neither the evolu-tionary history of WR stars nor the formation mechanisms ofthe nebulae were clearly understood. Over the years, stellarevolution models have shown that WR stars are just evolvedmassive stars, of which some are well beyond the main se-quence (MS) and some are still burning hydrogen (Langer2012). The massive stars’ winds during different evolution-ary stages have become better known. Hydrodynamic simu-lations of interactions between massive stars’ winds and theirambient medium have been made (e.g., Garcia-Segura et al.1996a,b), and their results can be used to guide our searchfor WR nebulae. In this section, we first summarize our earlyperception of WR nebulae based on data with inadequate res-olutions and our incomplete knowledge of stellar evolutionand stellar wind interaction. The misunderstandings shouldbe superseded by our current understanding based on numer-ical simulations made with improved knowledge of stellarevolution and its associated stellar wind properties.2.1.
Previous Perception
WR stars are massive stars characterized by broad emis-sion lines in their spectra, indicating the presence of fast stel-lar winds. Among massive stars, WR stars’ winds have thehighest mechanical luminosities. The powerful WR windssweep up the ambient medium to form shell structures thatappear as “arcs” or “rings” in H α images and are called “ringnebulae.”WR nebula surveys used only morphological informationin H α images to identify ring nebulae. More often than not, a WR star is projected near a curved nebular filament, and thephysical relationship between the WR star and the nebularfeature is tantalizing but not confidently established. Despitethe uncertainty, such objects were still identified as WR ringnebulae. Sometimes, a round H II region with a WR star inits central region is also identified as a ring nebula. Thus,WR nebulae became a heterogeneous class of objects.To investigate the nature of WR ring nebulae, Chu (1981)incorporated internal kinematic properties of the WR nebulaeand classified WR ring nebulae into three categories, basedon the kinematics and morphologies of the nebulae:• W-type nebulae are wind-blown bubbles whose dy-namic ages are smaller than the lifetime of a WR phaseto ensure that the WR winds are responsible for the for-mation of the bubbles. Such a ring nebula shows a finefilamentary shell around the central star, which is usu-ally projected near the center or toward the brightestpart of the nebula.• E-type nebulae consist of stellar ejecta. The E-typenebulae were introduced to have chaotic and irregu-lar expansions and clumpy morphologies. However,it was later realized that the chaotic expansion re-ported for M1-67 and RCW 58 (Chu & Treffers 1981;Chu 1982a) was an artifact caused by the large en-trance apertures of the Fabry-Perot scanning obser-vations, whereas long-slit high-dispersion spectra ac-tually show uniform expansion pattern despite thelarge density fluctuations (Solf & Carsenty 1982; Chu1988). Chemical abundance observations of the E-typeWR nebulae show enrichment of nitrogen (e.g., Este-ban et al. 1991), confirming that they consist of ejectedstellar material, which is called circumstellar medium(CSM).• R-type nebulae are characterized as “radiatively ex-cited H II regions," consisting of interstellar medium(ISM). R a denotes amorphous H II regions, while R s denotes shell nebulae whose dynamic ages are muchlarger than the lifetime of the WR phase.When deeper and higher-resolution images are available,it becomes clear that the above classification is too primitive.A more sophisticated approach is needed. The perception ofWR nebulae in this subsection should be superseded by theunderstanding outlined in the next subsection.2.2. Current Understanding
The current knowledge of WR nebulae derives from a bet-ter understanding of stellar evolution. We now know that WRstars evolve from MS O stars through either the luminousblue variable (LBV) or red supergiant (RSG) phase (Langer2012). Garcia-Segura et al. (1996a,b) showed for the firsttime a hydrodynamic model of the gaseous environment co-evolving with the central star from the MS phase to the WRphase. Similar hydrodynamic calculations have been carriedout by Dwarkadas (2007), Toalá & Arthur (2011), and vanMarle & Keppens (2012), all of which have produced quali-tatively similar results, as described below.The WR stars that evolve from the MS through the LBVphase are the most massive, e.g., (cid:38) M (cid:12) , while those thatevolve from MS via the RSG phase are less massive, e.g., ∼ M (cid:12) . In the MS phase, massive stars lose mass at rates of ∼ − M (cid:12) yr − in the form of fast stellar winds with termi-nal velocities of ∼ − (Prinja et al. 1990; Pulset al. 1996). During this stage, the MS stellar wind sweepsup the ambient ISM to form an interstellar bubble (Weaver etal. 1977). As a massive star evolves off the MS and into theLBV or RSG phase, the mass loss takes place in the form ofcopious slow winds, of which the wind velocities are ∼ − and mass-loss rates ∼ − M (cid:12) yr − (van Loonet al. 2005). The stellar material lost in this wind expandsslowly away from the star, forming a small circumstellar neb-ula (consisting of CSM) inside the cavity of the interstellarbubble formed previously when the star was in the MS stage.When the star enters a WR phase, the stellar wind velocityand mass-loss rate increase considerably, with terminal ve-locities of ∼ − and mass-loss rates of ∼ − M (cid:12) yr − (Nugis & Lamers 2000). This fast WR wind sweepsup the slowly expanding CSM into a shell, called a circum-stellar bubble. Therefore, we expect to see a nested shellstructure consisting of a small circumstellar bubble within alarger interstellar bubble around the WR star.If a WR star has a high velocity with respect to the ambi-ent ISM, either ejected from a cluster or kicked by the super-nova explosion of a binary companion, it is a runaway star.For a WR star moving in the ISM, an interstellar bow shockwill form in the direction of the star’s trajectory. However,if a WR star is surrounded by CSM, which travels togetherwith the WR star, a circumstellar bubble can still form, butthe leading edge will be compressed by the ISM and becomesharper, denser, and brighter, as seen in the circumstellar bub-ble around the LBV star S119 (Danforth & Chu 2001).We have also come to understand that whether a WR star isan isolated single star or located in a cluster environment asa member of an OB association affects the evolution of shellnebulae around the WR star. In the presence of other mas-sive stars, the stellar winds and supernova explosions fromall stars collectively form a large shell, called a superbubble(McCray & Kafatos 1987). The formation of a circumstel-lar WR bubble inside a superbubble is trickier than inside asmall interstellar bubble because of possible impacts of stel-lar winds and supernova explosions from neighboring mas-sive stars. The largest interstellar structures in a galaxy are supergiantshells (SGSs) whose sizes can approach 1000 pc. The for-mation of such large shells require multiple generations ofstar formation to provide the energy. If a WR star is in thelow-density interior of an SGS, it may not have a detectableinterstellar bubble, although its circumstellar bubble shouldstill be present.With the known characteristics of the different stages inthe evolution of WR stars, we expect to see circumstellarbubbles, interstellar bubbles, superbubbles, and SGSs aroundWR stars. However, to discern between circumstellar bub-bles and interstellar bubbles, observations of nebular abun-dances are needed, but they are not available to us. Thus,we can only use morphological information to search forsmall bubbles centered on individual WR stars, superbubblesaround WR stars and their host OB associations, and SGSson the largest scales. DATA AND METHODOLOGYWe have used multiwavelength images to examine the ion-ized and neutral gaseous environments of the LMC WR stars.We used optical emission-line images to examine the distri-bution and excitation of ionized gas, the H I
21 cm line datacube to investigate the distribution and kinematics of neutralatomic gas, 8 µ m images to identify the partially dissociatedregions, and 24 µ m images to locate emission from heateddust. 3.1. Data Used
MCELS1 is an emission-line survey of the MagellanicClouds (Smith, & MCELS Team 1999). The survey usedthe Curtis Schmidt Telescope at Cerro Tololo Inter-AmericanObservatory (CTIO) and a CCD camera to take images in theH α , [O III ] λ II ] λλ λ λ α images show the overall dis-tribution of ionized gas; the [O III ] images, compared withthe H α images, reveal the excitation of ionized gas; and the[S II ] images diagnose shocks and ionization fronts.A higher-resolution H α survey of the Magellanic Cloudshas been conducted with the MOSAIC II camera on theBlanco 4m Telescope at CTIO; this survey has been calledMCELS2 (PI: You-Hua Chu). The MOSAIC II camera,which is a mosaic of eight SITe 4096 × . (cid:48)(cid:48) × . (cid:48)(cid:48)
27 and a field-of-view of 36 (cid:48) × (cid:48) .An H α filter with 80 Å width was used to record images. Incelebration of the 30-year anniversary of the Hubble SpaceTelescope (HST), a public release of HST H α image of NGC2020 became available, and this image is used for WR 71.The H I data cube was constructed by Kim et al. (2003)using the ATCA survey data in conjunction with single-dishobservations from the Parkes Telescope. This data cube cov-ers 11.1 ◦ × . ◦ of the sky with an angular resolution of 1 . (cid:48) (cid:48)(cid:48) . The heliocentric velocity coverage, −
33 to +627 km s − , is centered at ∼
300 km s − , the bulk ra-dial velocity of the LMC. We use the zeroth moment map topresent the H I column densities and position-velocity plotsto show the kinematic structures of H I gas.The Spitzer Space Telescope
Infrared Array Camera(IRAC; Fazio et al. 2004) 8 µ m images are used to identifypolycyclic aromatic hydrocarbons (PAHs) associated withpartially dissociated regions, and the Multiband ImagingPhotometer for Spitzer (MIPS; Rieke et al. 2004) 24 µ m im-ages to reveal emission from heated dust. The Spitzer im-ages of the LMC were from the Legacy Program Surveyingthe Agents of a Galaxy’s Evolution (SAGE; Meixner et al.2006). 3.2.
Methodology
We have used our understanding of the coevolution of WRstars in their ambient medium, as detailed in Section 2.2, toguide our search for nebular shells around WR stars. Morespecifically, to identify wind-blown bubbles, we search forsmall shells less than ∼
50 pc in diameter with the WR starin a preferred location indicating that wind-ISM/CSM inter-action is responsible for forming the shell. For example, theWR star should be either near the center of a uniform shellor closer to the brighter part of a nonuniform shell; if thestructure is a bow-shock-like arc, then the WR star should benear the arc’s center of curvature. We identify superbubblesby their large shell sizes, much larger than 50 pc in diameter,or by a central cluster or OB association. For example, theshell around the R136 cluster measures 37.5 pc ×
21 pc insize yet we denote it as a superbubble because of its promi-nent central cluster. On still a larger scale, we identify SGSsthat are greater than ∼
500 pc in diameter to contextualize theextended star formation history in the WR star’s large-scaleenvironment.To search for nebular shells around WR stars, we first ex-amined the MCELS1 H α images to obtain a census of ion-ized gas around the WR stars. We noted the nebular mor-phology, specifically filaments, arcs, and shell structure in thevicinity of the star. We measured and recorded the dimen-sions of shell-like features centered on or surrounding thestar. In cases where there were no shells, we also noted anyemission nebulae around the star. We further used MCELS2H α images for a high-resolution view of the filamentarystructures in the WR nebula.To corroborate our findings on the WR nebulae from theH α images, we examined MCELS1 [O III ] and [S II ] imagesto gain insight into the nebular excitation and physical condi-tions. For early WR stars, i.e., high stellar effective temper-atures, we expect to see high [O III ] λ α ratios; thus, bubbles of stars with early spectral types stand out against theambient medium in the [O III ] images better than in the H α images. Conversely, for late WR stars, the [O III ] λ α ratios are low, and the nebulae are usually not detected in[O III ] images.Beyond examining optical images of ionized gas aroundWR stars, we also extracted the H I zeroth moment map andposition-velocity plots for north-south and east-west strips of20 (cid:48)(cid:48) width in order to show the H I column density distri-bution and the kinematics of the H I gas, respectively. Alow-intensity region in the zeroth moment map indicates adepression in the H I column density. A bow-shaped veloc-ity split in a position-velocity plot implies an expanding shellstructure.The dust emissions highlighted by Spitzer ’s IRAC 8 µ mand MIPS 24 µ m images were used to gain a more compre-hensive understanding of the physical conditions of the WRstar’s interstellar and circumstellar environments. Some verylate-type WR stars with dusty envelopes are also very brightin the 24 µ m band.To see whether a WR star is a member of an OB association(Lucke & Hodge 1970), we compared the WR star’s locationwith the boundaries of OB associations provided in the find-ing charts in P. Lucke’s Ph.D. thesis. WR stars not withinthe boundaries of an OB association but are within 100 pcto an OB association are also noted, with single parenthesesfor within 50 pc and double parentheses for 50–100 pc fromthe center of the OB association. Our results are more quan-titative and complete than the compilation by Neugent et al.(2018), who have missed a few associations, such as WR 38and 39 in LH45, WR 43 in LH 50, WR 53 in LH 62, and WR55 in LH 61. We record this information in case the WR staris a runaway star from the OB association.We also identify the H II regions around the WR stars inthe Davies et al. (1976) and Henize (1956) H II region cata-logs. We further classify their H II regions in three morpho-logical classes. Class 1 refers to a bright, amorphous H II region without filamentary or shell structures. Class 2 in-dicates that the H II region has a large shell structure, e.g.,a superbubble, and it is further divided into two subclasses,with 2a referring to a highly nonuniform shell with the ion-izing stars/cluster closer to the brightest rim of the shell and2b referring to a relatively uniform shell around the ionizingstars/cluster. Lastly, Class 3 indicates a very low-density en-vironment of the WR star. It is further divided into three sub-classes: with 3a indicating faint but detectable diffuse emis-sion in the vicinity of the star, 3b indicating the low-densityinterior of a large shell, and 3c reflecting no detectable dif-fuse emission in a large-scale diffuse field. RESULTS
Fig. Set 2. LMC WR stars
900 pc1 degree
Figure 1.
The positions of all the 154 WR stars in the LMC. The running numbers are from Neugent et al. (2018).
The 154 WR stars from Neugent et al. (2018) are markedon the MCELS1 H α image of the LMC in Figure 1. Foreach WR star or a close group of WR stars, we make a figurethat includes the MCELS2 H α image; MCELS1 H α , [O III ],and [S II ] images; Spitzer µ m images; and H I ze-roth moment map and position-velocity plots. An example isshown in Figure 2, and the rest in Fig. Set 2, LMC WR stars.The complete descriptions of individual WR stars and theirenvironments are provided in the Appendix. We summarize our findings on the stellar and interstellarenvironments of all 154 WR stars in the LMC in Table 1.Column 1 is the running number of the WR star in Neugent etal. (2018); columns 2 and 3 are the R.A. and decl. of the WRstar; columns 4 and 5 are the running numbers of the WR starfrom Breysacher et al. (1999) and Breysacher (1981), respec-tively; column 6 is the spectral type of the WR star from Neu-gent et al. (2018); and column 7 is the host OB associationfrom Lucke & Hodge (1970), where single parentheses mark -67°43'00"44'00"45'00"46'00"47'00"48'00" D e c ( ) H-alpha 4m MOSAIC +
HST -67°40'00"41'00"42'00"43'00"44'00"45'00" D e c ( ) H MCELS1 -67°40'00"41'00"42'00"43'00"44'00"45'00" D e c ( ) [OIII] MCELS1 D e c ( ) [SII] MCELS1 8 m IRAC 4
24 m MIPS 1HI mom0 V e l o c i t y ( k m / s )
250 300 350Velocity (km/s)
WR71
Figure 2.
Multiwavelength images of WR stars in the LMC. Each figure has the WR star name (or names) labeled on the top and positionmarked by a red cross or circle. The origin and passband information is marked on the upper-left corner of each panel. The H I position-velocityplots along the EW and NS directions are plotted above and to the right of the zeroth moment map respectively. The figure of WR 71 is shownhere as an example. The complete figure set (99 figures) is available in the online journal.
WR stars not in the OB association but are within 50 pc anddouble parentheses denote WR stars beyond 50 pc but within100 pc to the center. Columns 8 and 9 are the associatedH II regions from the Henize (1956) and Davies et al. (1976)catalogs; column 10 is the H II morphology classification;columns 11 and 12 are the dimensions of any bubble and su-perbubble associated with the WR star, respectively; column13 lists the H II supergiant shells from Meaburn (1980); andcolumn 14 lists the H I giant and supergiant shells from Kimet al. (1999). Note that the use of single parentheses aroundthe H II morphology and bubble/superbubble sizes indicateuncertainty.In the statistical analysis of WR stars and their associatednebular environments, the 30 Doradus (30 Dor) region needsspecial consideration because of its extreme properties. 30Dor is an archetypical giant H II region where star forma-tion is characterized as “starburst” and internal gas dynam-ics is violent. Early low-resolution radio images of 30 Dorshow three peaks in 30 Dor (Le Marne 1968): the brightestpeak, 30 Dor A (N157A), corresponds to roughly the 100 pcradius region centered on the R136 cluster, or OB associa-tion LH 100; the second brightest peak, 30 Dor B (N157B),corresponds to the H II region around LH 99 and contains asupernova remnant with a pulsar wind nebula (Wang et al.2001); and the third peak, 30 Dor C (N157C), correspondsto the superbubble around LH 90. These three componentsare encompassed in DEM L263 (Davies et al. 1976) or N157(Henize 1956), and within this region exist 43 known WRstars, almost 30% of the whole WR population in the LMC.In the following statistical analyses, we will treat 30 Dor andthe rest of the LMC separately, as well as together for theentire LMC. These results will be compared and discussed inthe next section.Of all the variously sized shells encompassing the WRstars, small bubbles are the most relevant to each individualstar as the WR wind may be directly responsible for formingthe bubble. Thus, we have compiled all small bubbles fromTable 1 into Table 2 to examine the statistics of the bubblesizes and WR spectral types. The small bubble around WR97 in 30 Dor is listed in Table 2 but separated by a horizontalline at the bottom. The H α images of the 18 small bubblesare shown in Figure 3. We have compiled all superbubblesoutside and inside 30 Dor from Table 1 into Tables 3 and 4,respectively, to probe the relationship between WR stars, OBassociations, and superbubbles. Images of example super-bubbles are shown in Figure 4.As a reference for the tendency of WR stars’ locations inOB associations, we have compiled in Table 5 numbers ofWR stars for different spectral types and for the entire LMC,the LMC excluding 30 Dor, and 30 Dor, respectively. Toobtain the statistical significance, we combine spectral typesinto subgroups: WN2-4 (early WN), WN5-6 (mid WN), WN7-L (late WN), WC4, WC5-6, WO3-4, and “other” forthe small number of remaining objects. The LMC WR starcatalog of Neugent et al. (2018) has 154 entries, but two en-tries contain double WR stars and thus the total number ofstars in Table 5 is 156. The percentage of WR stars that arein LH OB associations is given in parentheses after the num-bers of WR stars in OB associations. It is quite clear thatthe WR population in the LMC is dominated by WN stars, ∼ ∼
50% of the WR stars are in OB asso-ciations; however, WN5-6 and WC stars are more likely tobe in OB associations than WN2-4 and WN7-L. Outside 30Dor, WC stars have the highest percentage to be in OB asso-ciations, while inside 30 Dor, WN5-6 stars have the highestpercentage to be in OB associations.Table 6 presents the statistics of the WR star populationin the LMC and the fraction of each spectral type with bub-bles/superbubbles. In this table, column 1 gives the spectraltype grouping, column 2 presents the number (and percent-age) of WR stars with bubbles, column 3 presents the num-ber of WR stars with bubbles in OB associations, column 4presents the number (and percentage) of WR stars in super-bubbles, and column 5 presents the number of WR stars withsuperbubbles in OB associations for all WR stars in the LMC;columns 6-9 present the same information as columns 2-5 forall WR stars in the LMC excluding 30 Dor, respectively; col-umn 10 presents the number (and percentage) of WR stars insuperbubbles in 30 Dor and column 11 presents the numberof 30 Dor WR stars in superbubbles in OB associations.In 30 Dor, few small bubbles are identified: WR 97is surrounded by a small bubble-like structure, while WR118/119/120 are projected on the southern rim of a triangularshell structure that could be connected to the central super-bubble around the R136 cluster, and thus, the bubble natureof the triangular shell is highly uncertain. Several large shellstructures in 30 Dor can be identified as superbubbles (Chu& Kennicutt 1994), and WR stars projected within superbub-bles in 30 Dor are compiled in Table 4, along with the dimen-sions of the superbubble and host OB associations. Owing tothe scarcity of small WR bubbles in 30 Dor, we only givestatistics of WR stars in superbubbles in 30 Dor in Table 6.We define the H II morphology classes to represent the evo-lutionary stages of the ISM surrounding the WR stars. Thus,the correlation between the spectral types and the H II mor-phology classes may be used to diagnose the WR star’s pro-gression in its evolution. We have tallied the numbers of WRstars for different spectral types in H II regions of differentmorphological classes in Table 7. The results are reportedseparately for 30 Dor and the LMC excluding 30 Dor, as wellas the entire LMC. DISCUSSIONBelow we first examine the WR star population in theLMC, using their stellar and interstellar environments to as-sess their properties, then correlate their bubbles and super-bubbles with their spectral types to further probe the natureof the WR spectral types. Finally, we note the impact of WRnebulae on the supernova remnants that are formed after thesupernova explosions.5.1.
WR Star Population in the LMC
30 Dor versus the Rest of the LMC
The distribution of WR stars in the LMC is by no meansuniform. Most conspicuously, about 30% of the WR stars inthe LMC are concentrated in the 30 Dor giant H II region. Asshown in Table 5, the largest difference in the WR star popu-lation inside 30 Dor and outside 30 Dor is in the number ra-tio of WN2-4 to WN5-6 stars, roughly 1:3 in 30 Dor and 8:1in the rest of the LMC. This reversal in the number ratio ofWN2-4 to WN5-6 stars is caused by 30 Dor’s young age andmassive clusters, where the most massive bins of the initialmass function can be stochastically populated. The evolutionof massive stars has been reviewed by Langer (2012), whoseFigure 10 shows that the most massive stars evolve into late-type WN stars even during the core hydrogen-burning stage.The large number of WN5-6 stars in 30 Dor coexist withthe numerous very massive O2-3 stars (Crowther et al. 2010,2016), highly suggestive that these WN stars are very mas-sive hydrogen-burning stars, which have been shown to bevery luminous and still have hydrogen on their surface (deKoter et al. 1997; Massey & Hunter 1998).5.1.2. WR Stars and OB Associations
The starburst in 30 Dor has produced massive star clustersor OB associations, namely, LH 100 (R136 cluster) in 30 DorA (Hunter et al. 1995; Massey & Hunter 1998), LH 99 in 30Dor B (Chu 1997), and LH 90 in 30 Dor C (Lortet & Testor1984; Testor et al. 1993). It is thus not surprising that 70%of the WR stars in 30 Dor are in OB associations. Amongdifferent types of WR stars in 30 Dor, WN5-6 has a higherpercentage of being in OB associations than WN2-4, WN7-L, and WC. The R136 cluster is mostly responsible for thishigh percentage of WN5-6 stars in OB associations in 30 Dor,and these WN5-6 stars are most likely massive hydrogen-burning stars.In contrast, only 45% of WR stars outside 30 Dor are inOB associations (see Table 5). Interestingly, about 87% ofthe WC stars outside 30 Dor are in OB associations. In 30Dor, four out of seven WC stars are in OB associations, cor-responding to 57%; however, this is small number statistics. Furthermore, two WC stars in 30 Dor are within 50 pc fromOB associations and could be associated; thus, we do notthink the percentage of WC stars in OB associations differsmuch between 30 Dor and outside 30 Dor.5.1.3.
WR Stars and Surrounding Ionized ISM
Massive stars inject energy into the ambient ISM via UVradiation and fast stellar winds during their lifetime and su-pernova explosions at the end. The energy feedback is ex-pected to produce an amorphous H II region when the mas-sive stars are very young. Gradually, the fast stellar winds ofmassive stars sweep up the H II region into a shell structurecalled a bubble (for an isolated massive star) or superbubble(for OB association). Eventually, the shell structure dissi-pates into the ISM, leaving behind a low-density ISM. Thesethree stages correspond to the three morphological classes ofH II environments defined in Section 3.2, with class 1 beingthe youngest and class 3 being the most evolved stage.Table 7 shows that in 30 Dor, 15% (7/46) of WR stars aresuperposed on dense ionized gas (class 1), over 72% (33/46)of WR stars are associated with superbubbles or shell-likestructures (class 2), and 13% (6/46) of WR stars in the out-skirts of 30 Dor are superposed on diffuse nebulosity withlow surface brightness (class 3). Among the 33 WR stars inclass 2 H II regions, 29 are inside superbubbles, 3 are pro-jected on the southern rim of the superbubble Shell 5, and 1is inside a faint shell structure. Twenty-eight of the 29 WRstars in superbubbles are in the two major OB associations(LH 90 and LH 100) that are both surrounded by superbub-bles.Outside 30 Dor, 5% (6/110) WR stars are associated withH II class 1, 36% (40/110) are in superbubbles, and 58%(64/110) are in a very tenuous medium with density lowerthan 1 H-atom cm − . It is interesting to note that WN2-4 andWN7-L have the highest percentages associated with H II class 3, the most evolved state. Based on the massive starevolution summarized in Figure 10 of Langer (2012), we sug-gest that these WN2-4 stars in a very tenuous medium haveprogenitors with 20–30 M (cid:12) initial masses, and that theseWN7-L stars are the massive helium-burning ones whoseprogenitors have ≥ M (cid:12) initial masses. Note, however, thatthis conclusion can change if stellar rotation, close binaryevolution, and magnetic fields are considered in models ofmassive star evolution (Meynet et al. 2017).5.2. WR Bubbles: Observation versus Expectation
Too Few WR Bubbles are Detected
According to our current understanding of nebulae aroundWR stars, as detailed in Section 2.2, we may expect everyWR star to be surrounded by a small circumstellar bubbleand enclosed by a larger interstellar bubble. However, onlysix WR stars (WR 2, 4, 17, 52, 61, 154) are inside nested
WR2 (WN2) WR4 (WN3+O6V) WR12 (WC4)WR14 (O2If*/WN5) WR17 (WN3) WR19 (WN7h)WR21 (WN3/O3) WR25 (WC4) WR29 (WN3)
Figure 3.
4m MOSAIC H α images of small bubbles around WR stars in the LMC. The field of view of each panel is 4’ × small and large shells, which could be candidates for circum-stellar and interstellar bubbles. In fact, only 12% of WR starsare surrounded by small bubbles and only a small fraction ofthese bubbles have abundance observations to confirm thatthey are in fact circumstellar bubbles.The scarcity of small bubbles observed around WR starsneeds explanation. It is possible that a close binary compan-ion may perturb and redirect the mass outflow of the WR star’s progenitor, prohibiting the formation of a circumstel-lar bubble. However, ∼
24% of LMC WR stars with smallbubbles are binary, and among all LMC WR stars, ∼
22% arebinary, not too different from that for WR stars with smallbubbles, suggesting that binarity may not be a main factor inprohibiting the formation of circumstellar bubbles.A more plausible cause for the nondetection of circumstel-lar bubbles may be in its evolution. It is conceivable that0
WR52 (B3I+WN5) WR54 (WN7h) WR61 (WN3+O7.5)WR71 (WN3+OB) WR75 (WN3) WR76 (WN4h)WR79 (WN3) WR97 (WN3) WR154 (WN3)
Figure 3. (cont.) 4m MOSAIC H α images of small bubbles around WR stars in the LMC. The field of view of each panel is 4’ × as a circumstellar bubble expands, its density decreases andits emission measure rapidly drops below the detection limit.(Emission measure ≡ n L , where n is the electron densityand L is the emitting path length). The maximum emissionmeasure of a shell is along the path length tangent to theshell’s inner rim. For a uniform shell with mass M , radius R ,and fractional shell thickness ∆ R / R , the maximum emissionmeasure will be n L = 32 − / π − R − ( ∆ R / R ) − / ( M / m H ) , where m H is the mass of a hydrogen atom. Using observer-friendly units, the emission measure can be expressed in unitsof cm − pc as n L ≈ R − ( ∆ R / R ) − / M , where n is in unitsof cm − , L in parsecs, R in parsecs, and M in M (cid:12) .It is evident that as the shell expands, the density drops andthe maximum emission measure of the shell will decreaserapidly in proportion to R − . As shown in Figure 5, for a10 M (cid:12) shell with ∆ R / R = 0.05, the maximum emission mea-1 Figure 4.
4m MOSAIC H α images of example superbubbles around WR stars in the LMC. The field of view of each panel is 15’ × − pc at a radius of ∼ − , producing very weak shocks and compression.Without a large density jump, the interstellar bubble cannotstand out against the background, and the bubble cannot bemorphologically identified in direct images. Therefore, in-terstellar bubbles formed by MS stars are not expected to beseen around WR stars unless the WR star has gone throughan RSG phase when the ambient ionized interstellar gas re-combines and cools. The isothermal sound velocity of neu-tral H I gas at 100 K is ∼ − , and an interstellar bubbleexpanding at 15-20 km s − would generate strong shocksand compression, causing a large density jump. Such an in-terstellar bubble will become visible in H α when the WRphase starts, and the bubble and ambient ISM become pho-toionized. Whether an interstellar bubble is visible around aWR star depends a lot on the evolutionary path and physicalconditions of the ambient ISM.5.2.2. Circumstellar versus Interstellar Bubbles
The most definitive diagnostic of a circumstellar bubble isthe N/O abundance ratio (e.g., Esteban et al. 1991). Of thesmall WR bubbles identified, abundance observations havebeen made for only three–WR 2, 12, and 19–and only thebubble around WR 19 shows high N/O ratio confirming itscircumstellar bubble nature (Garnett & Chu 1994; Stock etal. 2011). The small nebula near WR 47 (BAT99-38) wasalso observed by Stock et al. (2011); however, this small neb-ula consists of bright-rimmed Bok globules with embeddedstar formation (Chu et al. 2005) and not a WR nebula, as de-scribed in more detail in the Appendix.In Section 5.2.1 we have shown that circumstellar bubblesexpand and fade below the detection limit when the bubblesize is greater than ∼
20 pc. The smallest WR bubbles arethus more likely to consist of CSM; however, WR 2 and WR19 have bubbles of similarly small sizes but only the lattershows enhanced N/O ratio. Future spectroscopic observa-tions of the small WR bubbles are needed to determine un-ambiguously whether they consist of CSM.5.2.3.
Correlation with Stellar Properties and Environments
The statistics of small WR bubbles is summarized in Table6. About 67% of the small bubbles are found around WN2-4 stars, and more strikingly, none of these WN2-4 stars arein OB associations. If all massive stars are born in clustersor OB associations, the absence of OB associations aroundthese WN2-4 stars with small bubbles suggests that the par-ent OB associations have dissolved after losing the more massive stars in supernova explosions and that these WN2-4 stars must have lower initial masses, ∼ M (cid:12) . AmongWN2-4 stars in superbubbles, only ∼
50% are in OB associ-ations. This superbubble statistic for WN2-4 stars might becaused by two possible evolutionary paths, with the ones inOB associations more massive than those not in OB associa-tions.In contrast, two WC4 stars are in small bubbles, and bothare in OB associations. This may look like small numberstatistics, but all 12 WC stars in superbubbles are in OB as-sociations as well. The implication of “WC stars in bubblesand superbubbles are all in OB associations” is intriguing butnot clear.5.3.
Preexisting Environmental Conditions of SupernovaRemnants
Massive as they are, WR stars may explode as core-collapse supernovae near the end of their evolution. We haveshown through this survey of bubbles around WR stars thatcircumstellar bubbles can be observed only at a young age,and as they expand, the density drops and the bubble fadesto oblivion. While the interstellar and circumstellar bubblesare not always detected, their shell densities are still abovethose of the diffuse ISM. Thus, when the central star even-tually explodes as a supernova, the supernova ejecta quicklyexpands in the low-density bubble interior until it hits the cir-cumstellar or interstellar bubble shell. This is called cavityexplosion. Supernova remnants formed through cavity ex-plosion include N132D and N63A (Hughes 1987; Hughes etal. 1998), and they are characterized by their large size andhigh X-ray luminosity. SUMMARY AND CONCLUSIONWe have conducted a multiwavelength survey of ISM andCSM around WR stars in the LMC. The latest LMC WR cat-alog by Neugent et al. (2018) contains 154 entries and shouldbe fairly complete. A number of survey data sets are avail-able for us to probe the ionized and neutral medium aroundthe WR stars: the MCELS images in the H α , [O III ] , and[S II ] lines; Blanco 4m Telescope MOSAIC images in theH α line; Spitzer Space Telescope µ m images; andATCA + Parkes Telescope H I
21 cm line data cube of theLMC.As a massive star evolves from the MS to WR phase, itsfast MS wind turns into a slow RSG or LBV wind and thena fast WR wind. The interactions between these winds andthe ambient medium should form a circumstellar bubble in-side an interstellar bubble (Garcia-Segura et al. 1996a,b). Weexpect the interstellar and circumstellar bubbles to be small, <
50 pc in size. Our search finds small bubbles around ∼ ∼
15% of WR stars inthe LMC but outside 30 Dor. Most of these small bubbles3
Radius (pc) E m i ss i o n M e a s u r e ( c m p c ) M / M Radius (pc) E m i ss i o n M e a s u r e ( c m p c ) R / R Figure 5.
Expected emission measures of circumstellar bubbles for a fixed shell thickness ratio ∆ R / R = 0 .
05 and different bubble masses (leftpanel), and for 10 M (cid:12) and different ∆ R / R (right panel). The dashed line marks the emission measure 10 cm − pc as the detection limit. are around WN stars and most of them are not in OB asso-ciations. Only the small bubble around WR 19 has a highN/O abundance ratio to confirm its circumstellar bubble na-ture. The scarcity of small bubbles can be caused by severalfactors. As a bubble expands, its surface brightness drops inproportion to (radius) − and fades below the detection limitwithin a short time. Furthermore, it has been observed thatinterstellar bubbles expand at 15–20 km s − , which cannotproduce strong compression for the bubble shell to stand outagainst the background; thus, interstellar bubbles can be di-agnosed kinematically but not morphologically (Nazé et al.2003).WR stars and nearby massive stars can jointly blow a su-perbubble with their fast stellar winds and supernova explo-sions. In 30 Dor, 63% of its 46 WR stars are in superbubbles,and 28 of the 29 WR stars in superbubbles are also in OBassociations. Outside 30 Dor, only 33% of the 110 WR starsare in superbubbles; about two-thirds of these WR stars insuperbubbles are also in OB associations. The contrastingstatistics is caused by the prominent starburst in 30 Dor thatproduced the massive OB association LH 90 in 30 Dor C andmore impressively the super star cluster R136 (LH 100) inthe central superbubble of 30 Dor A. While both inside andoutside 30 Dor WN stars contribute to ∼
80% of the WR pop-ulation, the number ratio of WN2-4 to WN5-6 stars is 1:3 in30 Dor and 8:1 outside 30 Dor. The dominant WN4-5 starsin 30 Dor are probably massive H-burning stars.We have also classified the H II environments of WR starsinto three categories: class 1 for bright amorphous H II re-gions; class 2 for shell H II regions, such as superbubbles;and class 3 for low-density diffuse medium. These threeclasses correspond to a progression in the dynamical disper-sal of ISM around massive stars. Inside 30 Dor, the distri-bution of WR stars in classes 1-3 is 15%, 72%, and 13%,respectively. The stars in class 3 H II environments are in the outskirts of 30 Dor. Outside 30 Dor, the distribution of WRstars in classes 1–3 is 5%, 36%, and 58%, respectively. Thecontrast between 30 Dor and elsewhere reflects the age andmass differences in the WR star population. For example,among the 65 WN2-4 stars outside 30 Dor, about 68% arenot in OB associations and 63% are in class 3 H II environ-ments. The WN2-4 stars not in OB associations and in class 3H II environments are most likely He-burning WR stars fromprogenitors with initial masses below ∼ M (cid:12) .Finally, spectroscopic analyses of abundances are neededto determine whether the small WR bubbles contain ISM orCSM. It is important to determine the physical properties andnature of small WR bubbles, as they will define how the fu-ture supernova remnants develop.ACKNOWLEDGMENTSC.S.H. acknowledges the hospitality of ASIAA for host-ing her for the duration of the research. We acknowledgegrant support of MOST 108-2112-M-001-045 and MOST108-2811-M-001-58 from the Ministry of Science and Tech-nology of Taiwan, Republic of China.4 T a b l e . L M C W R S t a r s a nd T h e i r S t e ll a r a nd I n t e r s t e ll a r E nv i r on m e n t s W R α ( J ) δ ( J ) B A T B r e y S p ec t r a l O B a ss c . a N e bu l a N e bu l a H II B ubb l e S up e r bubb l e H II S G S H I S h e ll b N u m . N u m . T yp e L HN u m . H e n i ze ND E M L M o r pho l ogy ( a r c m i n )( a r c m i n ) L M C - ( )( )( )( )( )( )( )( )( )( )( )( )( )( ) . - . W N ......... c ............ . - . W N ... a . × . . × . ...... . - . W N ......... c ............ . - . ...... W N + O V (( )) ... a . × . . × G S . - . a W N L / O f ? (( )) ...... ... × G S . - . W N a ...... G S
15 7045507 . - . a ... W N (( )) ... a ...... G S
15 8045531 . - . ... W N ec ......... ... × . ...... . - . W C ... . × . G S
18 10045611 . - . W C ... a ... ( × ) ...... . - . W C + O B a ... × ... G S
16 12045724 . - . W C / a × ... S G S . - . ...... W N / O ......... c ...... G S
18 14045727 . - . ... O If * / W N ...... a . × . ......... . - . ... W N ... a ............ . - . W N + O B ......... c ...... S G S . - . W N ... A a × . × . ...... . - . a ... W N + a b s ......... ... × ...... . - . W N ...... a . × . ......... . - . W N ...... a / ... . × . ... . - . ...... W N / O ... a × . ...... S G S . - . W N ...... c ...... ... . - . ...... W N / O (( )) ... a ...... ... . - . W N + O B A a ... . × ... S G S . - . a W C A / a . × . ...... S G S . - . W N + O B ( ) ... ( . × . ) ... G S
44 27051354 . - . W N ... ... × . ... G S T a b l e c on ti nu e d T a b l e ( c on ti nu e d ) W R α ( J ) δ ( J ) B A T B r e y S p ec t r a l O B a ss c . a N e bu l a N e bu l a H II B ubb l e S up e r bubb l e H II S G S H I S h e ll b N u m . N u m . T yp e L HN u m . H e n i ze ND E M L M o r pho l ogy ( a r c m i n )( a r c m i n ) L M C - ( )( )( )( )( )( )( )( )( )( )( )( )( )( ) . - . ... W N ( ) / ( ) a ......... G S
44 29051412 . - . W N ( ) a . × . ...... G S
46 30051417 . - . ...... O . If * / W N ( ) ... . × . ... G S
44 31051457 . - . a W N a ...... a ......... G S
47 32051638 . - . W N ... ... a / c ......... G S
51 33051810 . - . ...... W O a a ......... G S
54 34051810 . - . ...... W O a a ......... G S
54 35051819 . - . B I + W N a / ......... G S
54 36051916 . - . W C + O - a ............ . - . W N + O B ... ... . × . ... S G S . - . ...... W N / O ... a / ...... S G S . - . W N ... ...... S G S . - . W N C ............ . - . W N ... ... . × . ... G S
61 42052259 . - . ... O f p e / W N ............ . - . W C + a b s ... . × . G S
61 44052318 . - . W N ...... c ...... S G S . - . W N / W C E + O B ... ...... ... . - . W N (( )) ... ... . × . S G S . - . W C + a b s D ... × . S G S
11 48052456 . - . ...... W N / O ... E a ( . × . ) ... , S G S ,
11 49052630 . - . W C + O III / V a ... × S G S
12 50052636 . - . W N a ... × S G S
12 51052642 . - . W N ... a ...... S G S
12 52052645 . - . B I + W N a . × . × S G S
12 53052737 . - . W N + O B a / ... . × . ... . - . W N ... . × . ... S G S
12 55052752 . - . ... W N → L B V a ...... S G S
12 56052817 . - . W N (( )) a ...... S G S T a b l e c on ti nu e d T a b l e ( c on ti nu e d ) W R α ( J ) δ ( J ) B A T B r e y S p ec t r a l O B a ss c . a N e bu l a N e bu l a H II B ubb l e S up e r bubb l e H II S G S H I S h e ll b N u m . N u m . T yp e L HN u m . H e n i ze ND E M L M o r pho l ogy ( a r c m i n )( a r c m i n ) L M C - ( )( )( )( )( )( )( )( )( )( )( )( )( )( ) . - . ...... W N / O ... a ...... S G S
12 58052912 . - . W N ... c ...... S G S
12 59052918 . - . ...... W N / O ... ... c ...... S G S
12 60052931 . - . W N ... ... c ...... S G S
12 61052933 . - . a W N + O . (( )) a . × . × G S
70 62052953 . - . W N ... ... c ...... S G S
12 63053002 . - . W N (( )) c ...... S G S
12 64053012 . - . W C ......... c ...... S G S
11 65053038 . - . W C + a b s a ... × G S
70 66053118 . - . ... W N ... a ...... S G S
12 67053125 . - . ... W N ... a ...... S G S
12 68053132 . - . W N (( )) a ... . × . ... . - . W N (( )) ...... a ...... S G S
11 70053207 . - . W N ... a ...... S G S
12 71053310 . - . W N + O B (( )) C . × . ......... . - . W N + O B ... a ............ . - . W C a ... × ... G S
73 74053436 . - . ...... B I + W N a ... × ... G S
73 75053437 . - . W N ... A a . × . ... S G S
11 76053452 . - . W N ...... a . × . ... S G S
11 77053459 . - . W N + O a ... × ... G S
73 78053500 . - . ...... W N / O ... a ............ . - . W N (( )) a . × . ......... . - . ...... W N + O - III a ... × ... G S
73 81053529 . - . W N ( h ) ......... a ...... S G S
11 82053541 . - . ... W C C ... . × . ... G S
75 83053542 . - . W N C ... . × . ... G S
75 84053542 . - . O . If * / W N C ... . × . ... G S
75 85053543 . - . W C C ... . × . ... G S T a b l e c on ti nu e d T a b l e ( c on ti nu e d ) W R α ( J ) δ ( J ) B A T B r e y S p ec t r a l O B a ss c . a N e bu l a N e bu l a H II B ubb l e S up e r bubb l e H II S G S H I S h e ll b N u m . N u m . T yp e L HN u m . H e n i ze ND E M L M o r pho l ogy ( a r c m i n )( a r c m i n ) L M C - ( )( )( )( )( )( )( )( )( )( )( )( )( )( ) . - . W N + a b s ... a ...... ... . - . W N + a b s ... a ...... ... . - . W N ... a ...... ... . - . a W N + a b s ... a ...... ... . - . W N ......... c ...... S G S
11 91053554 . - . ... W N ... a ...... ... . - . W N C ... . × . ... G S
75 93053559 . - . W N C ... . × . ... G S
75 94053559 . - . W N C ... . × . ... G S
75 95053559 . - . ... W N C ... . × . ... G S
75 96053612 . - . a W N B a ............ . - . W N ( ) . × . ( . × . ) ...... . - . ... O f p e / W N → L B V c ............ . - . W C ( + O B ) ... . × . ...... . - . W C ( + O B )(( )) a ............ . - . W N ( ) A a ............ . - . W C a ............ . - . a W N / W C E ( ) A a ............ . - . W N ( ) A a ............ . - . W C ( ) a ............ . - . W N B a ............ . - . ...... W N + O V ... ( . × . ) ...... . - . B I + W N (( , )) A a ... × ...... . - . ... O If *99157 B ............ . - . ...... W N a ............ . - . W N / ec a ............ . - . W N ( ) A ......... G S
78 113053836 . - . W N ( ) A ......... G S
78 114053838 . - . ... O . If * / W N ( ) A ......... G S T a b l e c on ti nu e d T a b l e ( c on ti nu e d ) W R α ( J ) δ ( J ) B A T B r e y S p ec t r a l O B a ss c . a N e bu l a N e bu l a H II B ubb l e S up e r bubb l e H II S G S H I S h e ll b N u m . N u m . T yp e L HN u m . H e n i ze ND E M L M o r pho l ogy ( a r c m i n )( a r c m i n ) L M C - ( )( )( )( )( )( )( )( )( )( )( )( )( )( ) . - . W N A a ... . × . ... G S
78 116053840 . - . ... O . If * / W N A a ... . × . ... G S
78 117053840 . - . ... W N A a ... . × . ... G S
78 118053841 . - . ... W C A ( . × . ) ...... G S
78 119053841 . - . ... W N A ( . × . ) ...... G S
78 120053841 . - . ... W N ( h ) + O A ( . × . ) ...... G S
78 121053841 . - . ... O If * / W N A a ... . × . ... G S
78 122053842 . - . ... O If *100157 A a ... . × . ... G S
78 123053842 . - . ... W N . A a ... . × . ... G S
78 124053842 . - . ... W N A a ... . × . ... G S
78 125053842 . - . ...... O If * / W N A a ... . × . ... G S
78 126053842 . - . ... W N A a ... . × . ... G S
78 127053842 . - . ... O If * / O If * / W N A a ... . × . ... G S
78 128053842 . - . ... W N . A a ... . × . ... G S
78 129053843 . - . ... O If * / W N A a ... . × . ... G S
78 130053843 . - . ... O If * / W N A a ... . × . ... G S
78 131053844 . - . ... W C A a ... . × . ... G S
78 132053844 . - . ... W N . A a ... . × . ... G S
78 133053847 . - . W N . A a ... × ...... . - . ... W N / + W N / A a ... × ...... . - . ...... W N ( ) A ......... G S
78 136053857 . - . W N + O . If * / W N A a ... . × . ... G S
78 137053858 . - . W N ............ . - . ... W C ( ) A ............ . - . W N (( )) A ............ . - . W O ...... a ............ . - . a W N ...... S G S
19 142053956 . - . W C + a b s a ... . × . S G S
19 143054003 . - . ...... W N / O a ... × S G S T a b l e c on ti nu e d T a b l e ( c on ti nu e d ) W R α ( J ) δ ( J ) B A T B r e y S p ec t r a l O B a ss c . a N e bu l a N e bu l a H II B ubb l e S up e r bubb l e H II S G S H I S h e ll b N u m . N u m . T yp e L HN u m . H e n i ze ND E M L M o r pho l ogy ( a r c m i n )( a r c m i n ) L M C - ( )( )( )( )( )( )( )( )( )( )( )( )( )( ) . - . W N + O a ... . × . S G S
19 145054013 . - . ... W C + O a ... . × . S G S
19 146054013 . - . ...... B [ e ] + W N ? a ... . × . S G S
19 147054050 . - . W N ( ) a ...... S G S
19 148054117 . - . ...... W N / O ... a ( . × . )( . × . ) S G S
19 149054148 . - . W N + O V ...... a ............ . - . ... W N ... a ...... S G S
19 151054453 . - . W N a ... × ... G S
94 152054524 . - . W N (( )) ... c ............ . - . ... W N (( )) ... a ... × ... G S
94 154054646 . - . W N ... a . × . . × . ... G S a S i ng l e p a r e n t h e s e s a r ound O B a ss o c i a ti on i nd i ca t e t h a tt h e s t a r i s w it h i n50p c o f t h ece n t e r o f t h e O B a ss o c i a ti on , doub l e p a r e n t h e s e s i nd i ca t e t h a tt h e s t a r i s w it h i n100p c , a ndnop a r e n t h e s e s i nd i ca t e t h a tt h e s t a r i s w it h i n t h ea ss o c i a ti on . E lli p s e s i nd i ca t e no a ss o c i a ti on . b G S : g i a n t s h e ll , S G S : s up e r g i a n t s h e ll . Table 2.
WR Stars with Small BubblesWR × × × × × ×
30 39 1214 O2If*/WN5 2.2 × ×
21 40 ...17 WN3 2 × ×
30 45 ...19 WN7h 0.7 × × × ×
42 66 ...25 WC4 2.8 × ×
21 86 3129 WN3 2.8 × ×
39 108 (35)52 B3I+WN5 0.8 × ×
12 199 5854 WN7h 3.0 × ×
36 210 ...61 WN3+O7.5 1.4 × ×
21 221 ((66))71 WN3+OB 1.5 × ×
18 231 ((76))75 WN3 0.7 × × × × × × × × † WN3 1.5 × × OTE — † In 30 Dor.2
18 231 ((76))75 WN3 0.7 × × × × × × × × † WN3 1.5 × × OTE — † In 30 Dor.2 Table 3.
WR Stars in Superbubbles (Excluding 30 Dor)WR × ×
33 6 ...4 WN3+O6V 4.3 × ×
60 10 ((2))5 WNL/Of? 18 ×
12 270 ×
180 ... ((5))8 WN3pec 11 × ×
114 ... ...9 WN3 7.6 × ×
99 36 811 WC4+OB 12 × ×
120 34 917 WN3 10.2 × ×
126 45 ...18 WN3+abs 20 ×
20 300 ×
300 ... ...20 WN4 7.7 × × × ×
45 86 3127 WN9 7 × ×
69 110 3930 O3.5If*/WN5 10.2 × ×
111 105 (36)37 WN3+OB 13.8 × ×
192 137 4341 WN6h 7.7 × × × × × ×
57 175 ((52))47 WC4+abs 12 × × ×
10 150 ×
150 199 5850 WN4 10 ×
10 150 ×
150 199 5852 B3I+WN5 10 ×
10 150 ×
150 199 5853 WN3+OB 14.4 × × ×
11 210 ×
165 221 ((66))65 WC4+abs 14 ×
11 210 ×
165 221 6968 WN3 11.5 × × ×
15 315 ×
225 246 8174 B0I+WN 21 ×
15 315 ×
225 246 8177 WN3+O 21 ×
15 315 ×
225 246 8180 WN3+O8-9III 21 ×
15 315 ×
225 246 8799 WC4(+OB) 6.9 × ×
78 261 96142 WC4+abs 7.5 × × × ×
120 284 103144 WN3+O7 7.5 × × × × ×
15 255 ×
225 309 116153 WN11 17 ×
15 255 ×
225 308 ((116))154 WN3 7.7 × × Table 4.
WR Stars in Superbubbles in 30 DorWR × ×
78 N157C 9083 WN5h 7.5 × ×
78 N157C 9084 O3.5If*/WN7 7.5 × ×
78 N157C 9085 WC4 7.5 × ×
78 N157C 9092 WN7 7.5 × ×
78 N157C 9093 WN4 7.5 × ×
78 N157C 9094 WN7 7.5 × ×
78 N157C 9095 WN5 7.5 × ×
78 N157C 90108 B1I+WN3 8 × ×
105 Shell 3 ...115 WN6 2.5 × ×
21 Central Shell 100116 O2.5If*/WN6 2.5 × ×
21 Central Shell 100117 WN6h 2.5 × ×
21 Central Shell 100121 O2If*/WN5 2.5 × ×
21 Central Shell 100122 O2If* 2.5 × ×
21 Central Shell 100123 WN4.5h 2.5 × ×
21 Central Shell 100124 WN5h 2.5 × ×
21 Central Shell 100125 O2If*/WN5 2.5 × ×
21 Central Shell 100126 WN5h 2.5 × ×
21 Central Shell 100127 O2If*/O3If*/WN6 2.5 × ×
21 Central Shell 100128 WN4.5h 2.5 × ×
21 Central Shell 100129 O2If*/WN5 2.5 × ×
21 Central Shell 100130 O2If*/WN5 2.5 × ×
21 Central Shell 100131 WC5 2.5 × ×
21 Central Shell 100132 WN4.5h 2.5 × ×
21 Central Shell 100133 WN4.5 8 × ×
60 Shell 5 100134 WN5/6+WN6/7 8 × ×
60 Shell 5 100136 WN6+O3.5If*/WN7 2.5 × ×
21 Central Shell 100
Table 5.
LMC WR Stars in LH OB AssociationsThe LMC The LMC −
30 Dor 30 DorSpec. Type Table 6.
Spectral Types of WR Stars in Bubbles and SuperbubblesThe LMC The LMC - 30 Dor 30 DorSpectral
Table 7. H II Region Morphology Classes of DifferentTypes of WR StarsThe LMC LMC–30 Dor 30 DorSpectral H II Classes H II Classes H II ClassesType 1 2 3 1 2 3 1 2 3WN2-4 3 26 43 3 21 41 0 5 2WN5-6 2 23 6 0 4 4 2 19 2WN7-L 5 6 12 2 2 12 3 4 0WC4 2 14 5 1 11 3 1 3 2WC5-6 0 1 1 0 0 1 0 1 0WO3-4 0 0 3 0 0 3 0 0 0Other 1 3 0 0 2 0 1 1 0Total 13 73 70 6 40 64 7 33 6 I and H II features are abbreviated as the following: LHn – OB association from Lucke & Hodge (1970), Nn – LH α II region from Henize (1956), DEM Ln – LMC H II region from Davies et al. (1976), H II SGS LMC-n – ionized SGS identified from H α images by Meaburn (1980), H I GS n – H I giant shell from Kim et al. (1999), and H I SGS n – H I SGS from Kim et al. (1999).
WR1 is in a field without any nebulosity in the vicinity orany identifiable large shell structure.
WR2 is surrounded by a small, incomplete shell with di-mensions 0 . (cid:48) × . (cid:48) . (cid:48) × . (cid:48) II λ ∼
16 km s − (Chu et al. 1999). On a largerscale, these features are projected on the northwestern rimof H II SGS LMC-7 (Meaburn 1980). There is a known su-pernova remnant, MCELS J0449-6921 (Maggi et al. 2016),centered ∼ . (cid:48) II ] image. WR2 is more than 100 pc from LH1, thusnot likely a member of this OB association. WR3 is located in a field with a network of faint H α fil-aments without obvious shell structure. In the 8 µ m and 24 µ m images, there is a long, thin filament extending from theeastern vicinity to 1 . (cid:48) I gas shows two radial velocity compo-nents at 282 and 295 km s − , which may indicate a large-scale expanding structure, but no H I giant shells have beenidentified. The relationship between the gas kinematics andthe WR star is unclear. WR4 is surrounded by a 0 . (cid:48) . (cid:48) × (cid:48) shell extending from the luminous H II regionDEM L10 around the OB association LH2. The small shellis best seen in the 24 µ m image. WR4 is inside H I GS 9, ascorroborated by the split velocity components in the H I . TheWR star is projected at ∼
80 pc from LH2, thus not likely amember of this OB association. Details of this object havebeen reported by Gvaramadze et al. (2014).
WR5 is in a complex network of faint H α filaments lo-cated on the northern rim of H II SGS LMC-7, and somefilaments appear to form a large shell measuring 18 (cid:48) × (cid:48) with its major axis oriented along the NE-SW direction. Nei- ther the shell nature nor the physical association between thestar and the shell is certain. WR5 is also projected inside H I GS 9. This H I shell is more extended than the H α shell inthe southeast direction. The star is ∼
90 pc from LH5, thusnot likely a member of this OB association.
WR6 is superposed on diffuse H α emission in the outskirtsof the luminous H II region DEM L22 and associated with theOB association LH5. These features are all on the northernrim of H II SGS LMC-7. The WR star is projected insideH I GS 15, which is corroborated by the H I position-velocityplot. Note, however, that H II SGS LMC-7 is much moreextended than H I GS 15.
WR7 is superposed on diffuse H α emission and projectedat ∼
80 pc from LH5. The 8 µ m and 24 µ m images show a 4 (cid:48) long filamentary arc structure to the west of the star, roughlyfollowing the surface of the H I cloud. The infrared arc struc-ture and the apparent H I cavity around the WR star suggesta wind-ISM interaction. The WR star is projected in the inte-rior of H I GS 15 and on the northern rim of H II SGS LMC-7.
WR8 is projected within a faint, filamentary shell-structurewith dimensions 11 (cid:48) × . (cid:48)
6. The [O
III ] λ α ratio ofthis shell is higher than other H II regions, indicating a higherexcitation. As WR8 is an early type WN3 star with a higheffective temperature, it is likely that WR8 has photoionizedthis nebula. WR9 is projected in an incomplete, filamentary shell-likestructure with dimensions 7 . (cid:48) × . (cid:48) I GS 18 and inside H II SGS LMC-7. The H I position-velocity plots show split velocity components from the ex-pansion of H I GS 18.
WR10 is located in the northern outskirts of the H II com-plex N11. The overall morphology of this region suggests anoutflow from the prominent central superbubble around LH9,and the H I position-velocity plots show velocity splits indi-cating expansion. In the H α image, WR10 is surrounded by a4 (cid:48) × (cid:48) arc in the north, resembling a half shell, and a straightfilament to its west; however, the [O III ] image shows that thestraight filament is of lower excitation and thus extraneous.The WR star may be partially responsible for the half-shellstructure. WR10 is more than 100 pc away from the OB as-sociations LH9 and LH10, thus, it cannot be a member ofeither.
WR11 is in the prominent 12 (cid:48) × (cid:48) superbubble in the H II complex N11 and a member of the central OB associationLH9. The superbubble is inside H I GS 16. The expansionof the superbubble is clearly seen in the H I position-velocityplots.6 WR12 is in a 3 (cid:48) × (cid:48) shell in the H II region DEM L39around the OB association LH12, of which the WR star is amember. This OB association and its H II region are on thenorthwestern rim of H I SGS 2 and H II SGS LMC-6. Thebubble of WR12 was first identified by Chu & Lasker (1980)and has been observed to have an average expansion velocityof 42 km s − (Chu 1983). WR13 is located in H I GS 18 and H II SGS LMC-7. The8 µ m and 24 µ m images show an arc around the star, whichmeasure ∼ . (cid:48) WR14 is a known runaway star with a radial velocity of ≤
130 km s − relative to the ambient medium (Gvaramadzeet al. 2010). There is a 2 . (cid:48) × . (cid:48) I gas shows two velocitycomponents, although the WR star is not in any cataloguedH I shells. WR15 is surrounded by some complex filamentary fea-tures without apparent shell morphology. WR15 is locatedon the southeastern rim of the N11 complex. The WR star isnot in any catalogued OB associations in N11.
WR16 does not show any obvious H α emission in itsvicinity. The WR star is located inside H I SGS 2 and H II SGS LMC-6. Note that the coordinates of this star in Table 1have been corrected from Neugent et al. (2018).
WR17 is in a small 2 (cid:48) × (cid:48) shell (Dopita et al. 1994) insidea larger 10 . (cid:48) × . (cid:48) III ] line becausethe spectral type of the WR star is WN3, thus, its photoion-ized gas has high excitation and high [O
III ] λ α ratio. WR18 has no obvious nebulosity in its vicinity; however,on a larger scale, it is surrounded by a faint shell-like struc-ture with dimensions 20 (cid:48) × (cid:48) . The northern part of the shellis brighter and coincides with H I GS 22. The relationshipbetween the optical shell and the H I shell is not clear as theformer is twice as extended as the latter. WR18 is projectedoutside H I GS 22.
WR19 has a small, incomplete elliptical shell surround-ing the star with dimensions 0 . (cid:48) × . (cid:48) − (Chu et al.1999). The WR star is also on the northwest outskirts of theH II region DEM L5. WR20 is in the diffuse emission region DEM L68 adjacentto the bright H II region DEM L67. The [O III ] image shows adiffuse large ring with a much higher [O
III ] λ α ratiothan the bright H II region DEM L67. The ring measures 7 . (cid:48) × . (cid:48) II SGS LMC-8. The H I position-velocity plots show velocity splits, but no H I shellwas cataloged. WR21 is inside a 3 . (cid:48) × . (cid:48) II region DEM L66. The WR star is not in known OB associ-ations. WR21 is also projected on the northern rim of H I SGS 5. The H I gas shows two velocity components possiblyassociated with H I SGS 5.
WR22 has no nearby nebulosity but is located in H II SGSLMC-8. The WR star is over 100 pc away from LH18 andLH26, which is too distant for a membership.
WR23 is surrounded by some faint diffuse and filamentaryH α emission features but with no recognizable shell struc-tures. The WR star is within 50 pc to the rim of LH26 butover 100 pc to its center, thus, it is unlikely to be a member.WR23 is also projected in H II SGS LMC-8.
WR24 is located in a blister-like structure of dimensions4 . (cid:48) × (cid:48) extending from the bright H II region DEM L86 tothe west. There are fainter filaments extending to the northand south of this blister structure, but the relationship be-tween them is not clear. DEM L86 is photoionized by theOB association LH31, of which WR24 is a member. TheWR star is near the eastern interior of H I SGS 5. The H I gasshows two velocity components possibly associated with theexpansion of H I SGS 5.
WR25 is at the base of an east-west elongated shell, mea-suring 2 . (cid:48) × . (cid:48) II regionDEM L86 to the east (Dopita et al. 1994). DEM L86 is pho-toionized by the OB association LH31, of which WR25 is amember. The complex is on the eastern rim of H I SGS 5. TheH I gas shows two velocity components possibly associatedwith the expansion of SGS 5. WR26 is on the bright southern rim of the 10 . (cid:48) × . (cid:48) I GS 44.
WR27 is a member of the OB association LH39 and insidethe incomplete shell structure DEM L110 with dimensions 7 (cid:48) × . (cid:48)
6. It is likely that the OB association is responsible forshaping the shell structure. Interestingly, the 8 µ m imageshows that WR27 is near the center of an apparent cavity.DEM L110 is inside H I GS 45.
WR28 is superposed on some diffuse H α emission with-out any filamentary or shell morphology between the shellH II regions DEM L105 and DEM L106, which are centeredon the OB associations LH36 and LH38, respectively. At adistance of ∼ II regions DEM L105 and DEM L106are located inside H I GS 44.7
WR29 is inside a 2 . (cid:48) × . (cid:48) II region DEM L108. The shell is better seen inthe 8 µ m and 24 µ m images. The WR star is ∼
10 pc fromthe edge the OB association LH35 and within 50 pc from thecenter. WR29 is projected within H I GS 46.
WR30 is projected near the eastern interior of the 10 . (cid:48) × . (cid:48) II region DEM L105 and is located ∼
25 pcoutside the OB association LH36, which is at the center ofDEM L105. The WR star and DEM L105 are inside H I GS 44.
WR31 is in the diffuse H II region DEM L119 with somefilaments on the bright southern part of the H II region. TheWR star is projected on the southwest rim of H I GS 47.
WR32 is superposed on some diffuse emission to the westof DEM L132a. There are some random filaments nearbybut no organized shell structure. The WR star is projectedon H I GS 51. The H I gas shows two velocity componentsassociated with the expansion of H I GS 51.
WR33 and WR34 is in a tight cluster in the OB associ-ation LH41 and located inside the H II region DEM L132a.The cluster is in H I GS 54.
WR35 is on the edge of the H II region DEM L132b andis a member of the OB association LH41. Due to the largenumber of massive stars in the vicinity, multiple clusters inLH41, and complexity in the nebula structure, it is not possi-ble to identify specific features associated with the WR star.No small bubbles are seen around the star. The star is pro-jected in H I GS 54.
WR36 is a member of the OB association LH42 and is onthe northwest rim of a small 1 . (cid:48) × . (cid:48) µ m image shows a 2 . (cid:48) × (cid:48) cavity around the WR star, and the H I position velocityplots also show a slowly expanding shell structure around theWR star. The WR star could be responsible for excavatingthis cavity. The bright H II region DEM L143 is associatedwith the OB association LH42. WR37 is located in the OB association LH43 and insidethe large 13 . (cid:48) × . (cid:48) − (Chu 1982b). The WR star is also projectedin H I SGS 6.
WR38, WR 39 are near some nebulosities and are associ-ated with the OB association LH45. These two WR stars areinside H II SGS LMC-5 with complex filamentary structure.No specific structures can be identified to be specifically as-sociated with these WR stars. H II SGS LMC-5 is associatedwith H I SGS 7.
WR40 is located inside the OB association LH47 on thesouthwest rim of the superbubble in DEM L152. The WRstar is also in the base of a blowout-like structure extending1 . (cid:48) . (cid:48) WR41 is in the large elliptical shell DEM L165 of dimen-sions 8 . (cid:48) × . (cid:48) (cid:48) (Chu & Lasker 1980).The expansion velocity of this shell has been determined tobe less than 10 km s − (Chu 1982b). The H I position-velocityplot indicates an expansion with an expansion velocity of ∼
12 km s − . The WR star is not in any known OB asso-ciations and is located on the south rim of H I GS 61.
WR42 is in the bright diffuse H II region DEM L160,which has nonuniform surface brightness with dust lanes andemission filaments but does not have any discernable shellstructure around the star. The WR star is a member of LH49. WR43 is inside the 15 . (cid:48) × . (cid:48) II SGS LMC-9 and inside H I GS 61. The largest linesplitting of the H I position-velocity plot shows an expansionof H I GS 61 of up to 20 km s − . WR44 is projected inside the H II SGS LMC-5. Some faintH α filaments are seen within the central cavity of LMC-5 andtogether, these filaments are called DEM L154. The filamentto the south of the star is curved away from the star, thus, itis unlikely that the WR star is responsible for shaping thisfeature. The WR star is also inside H I SGS 7.
WR45 is in the H II region DEM L174, which shows stria-tions in its morphology (Chu & Lasker 1980). Although thereis an apparent cavity near the star and some curved filamentssuggesting wind-ISM interaction, the overall H II morphol-ogy does not indicate any shell structure. The internal motiondoes not indicate an expansion (Chu 1982b). The WR star isprojected outside the northwest rim of H II SGS LMC-3.
WR46 is in a small emission nebula with numerous dustlanes to the north of the star. This small nebula is interior tothe northern rim of a 7 . (cid:48) × . (cid:48) ∼ II SGS LMC-5 and in H I SGS 7.
WR47 is inside the 12 × . (cid:48) II SGS LMC-4 and H I SGS 11. Stock et al. (2011) reported a small nebula aroundthis star and that its elemental abundances are similar to thoseof LMC H II regions. However, Hubble Space Telescope images have resolved this small nebula into bright-rimmeddust globules, and
Spitzer Space Telescope has revealed em-bedded star formation (Chu et al. 2005); thus, this nebulahas no association with the WR star. These bright-rimmeddust globules can be seen in the MCELS2 H α image, andthe young stellar objects in the dust globules can be seen inthe 8 µ m and the 24 µ m images. The surface of the glob-ules might be photoionized by WR47, but this nebula is not8shaped by the WR star and there is no evidence of wind-ISMinteraction. WR48 is inside a 3 . (cid:48) × . (cid:48) II SGS LMC-4 and H II SGS LMC-5 and the interactionzone between H I SGS 7 and SGS 11. The shell’s large size,50 pc across, makes it uncertain whether it is a bubble blownby WR48.
WR49 and WR50 are members of the OB associationLH58 in the 10 (cid:48) × (cid:48) superbubble DEM L199, which is onthe west rim of H II SGS LMC-3. The WR star is also in H I SGS 12.
WR51 is projected along a curved filament of the H II re-gion DEM L198, which is inside H II SGS LMC-3. The WRstar is also in H I SGS 12.
WR52 is member of the OB association LH58 in the 10 (cid:48) × (cid:48) superbubble DEM L199, which is on the west rim of H II SGS LMC-3. A continuum-subtracted [O
III ]/H α ratio mapreveals a small 0 . (cid:48) × . (cid:48) III ]/H α map, only partial northwestern and southeastern rims of thisshell can be seen in the MCELS2 H α and MCELS1 [O III ]images. This small bubble has also been noted by Dopita etal. (1994). The WR star is also in H I SGS 12.
WR53 is in the OB association LH62 and inside the super-bubble DEM L208, measuring 14 . (cid:48) × . (cid:48) α image (Chu & Lasker 1980).DEM L208 is on the northeastern rim of H II SGS LMC-9.
WR54 is superposed on the faint, diffuse H II regionDEM L210 with a filamentary shell-like structure, measuring3 (cid:48) × . (cid:48)
4, extending from the star to the south. DEM L210 isprojected inside in H II SGS LMC-3 and H I SGS 12.
WR55 is in the faint diffuse emission region DEM L210and is a member of the OB association LH61. DEM L210 isprojected within H II SGS LMC-3 and H I SGS 12.
WR56 is in the faint diffuse emission region DEM L210and is ∼
60 pc from LH61 thus not a member of this OBassociation. DEM L210 is projected within H II SGS LMC-3and H I SGS 12.
WR57 is in the faint diffuse emission region DEM L210,which is projected within H II SGS LMC-3 and H I SGS 12.
WR58 is a member of the OB association LH64 and pro-jected on the northern interior of H II SGS LMC-3 and H I SGS 12.
WR59 is projected on the southern interior of H II SGSLMC-3 and H I SGS 12.
WR60 is located near some diffuse nebulosity. The WRstar is inside H II SGS LMC-3 and H I SGS 12.
WR61 is surrounded by an apparent bow-shock like fea-ture, measuring 1 . (cid:48) × . (cid:48)
4, which has been known to emit He II λ α and MCELS1 [O III ] images show that the“bow-shock” may not be a coherent structure. The WR staris ∼
70 pc from LH66 and more than 100 pc from LH69,although it is projected within the 14 (cid:48)(cid:48) × (cid:48)(cid:48) superbubbleDEM L221. The WR star is also projected in H II SGS LMC-9 and H I GS 70. The H I position-velocity plots show splitcomponents originating from the expansion of H I GS 70.
WR62 is in a cavity of H II SGS LMC-3 and H I SGS 12with no nearby nebulosity.
WR63 is ∼
70 pc from the OB association LH64 and isprojected on the northern interior of H II SGS LMC-3 andH I SGS 12.
WR64 is on the southern interior of H II SGS LMC-4 with-out any obvious nebulosity nearby. The WR star is also pro-jected within H I SGS 11, whose expansion is corroboratedby the two velocity components in the H I position-velocityplots. WR65 is near some filamentary structure within the su-perbubble DEM L221 and on the southwest rim of H II SGSLMC-9. This WR star is a member of the OB associationLH69, which is responsible for shaping the 14 (cid:48) × (cid:48) super-bubble DEM L221 and the surrounding H I GS 70.
WR66 is superposed on a filament on the eastern rim ofH II SGS LMC-3 and ≥
100 pc from the OB associationsLH74 and LH67 thus not likely a member of either.
WR67 is near some filaments on the eastern rim of H II SGS LMC-3 and ≥
100 pc from the OB associations LH74and LH67 thus not likely a member of either. It is interestingto note that WR67 is a very late type WN star (WN11); thebright 24 µ m emission indicates a very dense stellar wind.There is a small emission nebula at 15 (cid:48)(cid:48) northeast of the WRstar, and may consist of material ejected by the star. Spectro-scopic observations of the nebular abundance are needed todetermine its nature. WR68 is ∼
60 pc from LH76, thus likely not a member.The WR star is also projected on the western rim of the 11 . (cid:48) × . (cid:48) II SGS LMC-4.
WR69 is not superposed on any detectable nebulosity butis projected near a faint filament on the southern interior ofH II SGS LMC-4 and within H I SGS 11. The WR star is also ∼
90 pc from LH70 thus likely not a member.
WR70 is located in a dusty, complex environment of theH II region DEM L227 with a large arc northeast of the star,but no shell structure can be claimed to be formed by thisWR star. DEM L227 is located on the northern rim of H II SGS LMC-3, associated with H I SGS 12.
WR71 is ∼
75 pc from the OB association LH76, thuslikely not a member. The WR star is inside the small 1 . (cid:48) × . (cid:48) ± − (Chu 1983).9 WR72 is in a diffuse emission field of DEM L224, whichcontains a collection of random filaments without any shell-like structure. This WR star is not in any known OB associ-ation.
WR73, WR74, and WR77 are members of the OB as-sociation LH81, which is projected inside the 21 (cid:48) × (cid:48) su-perbubble DEM L246 within H I GS 73. The H I position-velocity plots show the expansion of H I GS 73.
WR75 is inside a 0 . (cid:48) × . (cid:48) α image. DEM L239 is on the northern rimof H II SGS LMC-4 and H I SGS 11.
WR76 is in a small 1 . (cid:48) × . (cid:48) II SGS LMC-4 and H I SGS 11.This shell was first observed by Chu & Lasker (1980), andCowley et al. (1984) reported the high radial velocity of thestar, ∼
470 km s − , with respect to the LMC velocity, ∼ − . WR78 is superposed on the filamentary conglomerationDEM L263. There are no obvious wind interaction featuresthat can be associated with this star.
WR79 is in a 0 . (cid:48) × . (cid:48) . (cid:48) II region embedded in a diffuse field within theconglomeration of DEM L263. This ring structure, identifiedas an oval ring nebula by Dopita et al. (1994) and studiedby Stock & Barlow (2010), is best seen in the MCELS2 H α image. Comparisons between the H α and [O III ] images in-dicate that this ring nebula has a higher [O
III ]/H α emissionline ratio than the neighboring H II region; this high excita-tion is expected from WR79’s WN3 spectral type. WR79 is ∼
90 pc from the rim of LH89 thus likely not a member.
WR80 is a member of the OB association LH87. The staris projected inside the 21 (cid:48) × (cid:48) superbubble DEM L246,which is inside H I GS 73. To the east of the WR star a fil-amentary shell structure can be seen along the northeast rimof the superbubble DEM L246.
WR81 is projected on the eastern interior of H II SGSLMC-4 and H I SGS 11. The WR star is superposed on dif-fuse emission with no apparent stellar wind interaction fea-tures.
WR82, WR83, WR84, and WR85 are members of LH90and are projected along the western rim of the 7 . (cid:48) × . (cid:48) I giant shell associated with thissuperbubble is H I GS 75. The H I position-velocity plotsshow that H I GS75 is more extended than the ionized super-bubble.
WR86, WR87, WR89, and WR91 are superposed onfaint, diffuse emission between 30 Dor and H II SGS LMC-3and are members of LH89.
WR88 is superposed on faint, diffuse emission between 30Dor and H II SGS LMC-3 and is a member of LH85.
WR90 is projected on the eastern interior of H II SGSLMC-4 and H I SGS 11. The WR star is superposed on veyfaint diffuse emission with no apparent stellar wind interac-tion features.
WR92, WR93, WR94, and WR95 are members of LH90near the center of superbubble 30 Dor C, measuring 7 . (cid:48) × . (cid:48)
2. The H I giant shell associated with this superbubble isH I GS 75, and its expansion velocity assessed from the H I velocity splits is 12–15 km s − . WR96 is a member of LH88 and is projected in a super-nova remnant in DEM L241.
WR97 has an incomplete shell of radius 0 . (cid:48)
75 that can beseen in the H α image but shows the highest contrast againstthe background in the [O III ] image. On a larger scale, thereappears to be a 6 . (cid:48) × . (cid:48) α and [O III ], and the MCELS2 H α image reveals sharp fila-ments along the shell rim. The filaments on the eastern sideof the shell structure appear to be connected with other sharpfilaments on the southeast rim of 30 Dor C, forming a largearc structure and making the apparent superbubble structurearound WR97 highly uncertain. The WR star is located onthe northeastern outskirts of 30 Dor C and is ∼
45 pc fromthe OB association LH90 thus likely not a member.
WR98 is a member of LH94 and located in an empty fieldbetween filamentary H α emission features that do not seemto be associated with the WR star or LH94. The H I position-velocity plots show multiple velocity components from theISM. WR99 is superposed on a band of H α emission withoutany wind interaction features and is a member of LH96. Ona larger scale, the WR star is surrounded by a 6 . (cid:48) × . (cid:48) WR100 is just outside the eastern rim of the superbubble30 Dor C with no associated wind interaction features. TheWR star is ∼
60 pc from LH90 thus likely not a member ofthis OB association.
WR101 is in a diffuse emission region on the western edgeof 30 Dor. No wind interaction features are seen in the vicin-ity of the star.
WR102 is superposed on some diffuse field emission inDEM L261 and south of 30 Dor B without any wind interac-tion features. The WR star is a member of LH97.
WR103 and WR104 are in a diffuse emission region onthe western edge of 30 Dor and ∼
25 and ∼
33 pc from thecenter of LH99, respectively. No wind interaction featuresare seen in the vicinity of the stars. The H I position-velocityplots show very complex motions in the ISM. WR105 is superposed on diffuse emission to the south of30 Dor B without any wind interaction features and is ∼ I position-velocity plotsshow at least four velocity components, indicating a verycomplex interstellar environment.0 WR106 is in a small cluster with diffuse field emission onthe northern edge of 30 Dor B and a member of LH99. Nowind interaction features are seen in the vicinity of the star.
WR107 is in an apparent cavity of dimensions 4 . (cid:48) × . (cid:48) I position-velocity plots show complex motions in theISM. The WR star is located in DEM L261 to the south of 30Dor B and is a member of LH97. WR108 is superposed on diffuse emission with some un-organized filaments in Shell 3 of 30 Dor (Wang & Helfand1991). It is ∼
70 pc from LH100 and ∼
75 pc from LH99 andis not likely a member of either OB association.
WR109 is superposed on diffuse emission on the north-eastern part of 30 Dor B without any wind interaction fea-tures. It is a member of LH99.
WR110 and WR111 are superposed near some faint dif-fuse emission in DEM L269 without any wind-ISM interac-tion features. It is a member of LH101.
WR112 is 20 pc northwest of the R136 cluster inside 30Dor. The WR star is outside the western rim of a 1 . (cid:48) × . (cid:48) WR113 and WR114 are projected at 12–15 pc southwestof the R136 cluster in 30 Dor. These WR stars are outside the2 . (cid:48) × . (cid:48) WR115 is on the outskirts of the R136 cluster, LH100, inthe center of 30 Dor and is interior to the western end of a2 . (cid:48) × . (cid:48) WR116, WR117, WR121, WR122, WR123, WR124,WR125, WR126, WR127, WR128, WR129, WR130,WR131, and WR132 are in the R136 cluster, or LH100, inthe center of 30 Dor. This cluster is interior to the westernend of a 2 . (cid:48) × . (cid:48) I GS 78. The H I column densities are so high at the core of 30 Dor that self-absorption occurs, causing the apparent depression in surfacebrightness. WR118, WR119, WR120 are at ∼
12 pc north of the R136cluster in 30 Dor, but are still within LH100. They are outsidethe northern edge of the 2 . (cid:48) × . (cid:48) . (cid:48) × . (cid:48) WR133, WR134 are 80 and 60 pc, respectively, north ofthe R136 cluster, LH100, in 30 Dor. Both are inside Shell 5of 30 Dor (Wang & Helfand 1991), measuring 8 (cid:48) × (cid:48) . WR135 is in 30 Dor but is ∼
30 pc from the R136 cluster.The WR star is superposed on the edge of a dark cloud. Nowind-ISM interaction features can be seen.
WR136 is not in the R136 cluster in the center of 30 Dorbut is interior to the eastern end of the same 2 . (cid:48) × . (cid:48) WR137 is superposed on bright diffuse emission inDEM L269 with no obvious wind interaction features asso-ciated with the star. The WR star is a member of LH101.
WR138 is superposed on bright emission of 30 Dor andis ∼
45 pc from the R136 cluster, LH100. No wind-ISM in-teraction features can be identified close to the star, and theenvironment is too complex to unambiguously make any as-sociations.
WR139 is projected just outside the eastern rim of Shell 5of 30 Dor (Wang & Helfand 1991). It is ∼
75 pc from LH100and thus likely not a member.
WR140 is in the faint diffuse outskirts of 30 Dor, about330 pc from the R136 cluster. There are some broad filamen-tary structures in the field, but none are curved around theWR star to indicate wind-ISM interaction.
WR141 is in the bright H II region on the southwest rim ofthe superbubble in DEM L284, which sits in the ridge of veryactive star formation on the west rim of H II SGS LMC-2.The surrounding of WR141 is quite dusty, and the apparentshell morphology in the [O
III ] image is caused by embeddeddust lanes. This environment is too complex to identify phys-ical structures unambiguously. The WR star is a member ofLH103.
WR142, WR144, WR145 are in the central cavity of the7 . (cid:48) × . (cid:48) II SGSLMC-2, corresponding to H I SGS 19. The WR star is amember of LH104 that is responsible for the superbubble inDEM L269.
WR143 is projected within the 12 (cid:48) × (cid:48) superbubble ofDEM L284, which sits in the ridge of very active star forma-tion on the west rim of H II SGS LMC-2. The H I position-velocity plot shows very complex velocity structure. The WRstar is a member of LH103. WR146 is on the northeast rim of the 7 . (cid:48) × . (cid:48) II SGS LMC-2.The WR star is a member of LH104 which is responsible forthe superbubble.
WR147 is outside the superbubble of DEM L269 in theridge of active star formation on the western base of H II SGSLMC-2. The WR star is ∼
60 pc from LH104, and is thuslikely not a member of this OB association.
WR148 is to the west of a long north-south oriented fila-ment, out of which a faint 2 . (cid:48) × . (cid:48) . (cid:48) × . (cid:48) II SGS LMC-2, DEM L310, and to the east of 30 Dor. The H I position-velocity plots show multiple velocity components, indicating1complex kinematic structures, rendering the aforementionedapparent shells somewhat uncertain. WR149 is near the small diffuse H II region DEM L294. WR150 is projected in the central cavity of H II SGS LMC-2, or DEM L310. There are some filamentary features nearthe star, but nothing can unambiguously be identified to beassociated with the star.
WR151 is at the southern edge of the H II regionDEM L309 and projected in the northern interior of a largeionized shell measuring 17 (cid:48) × (cid:48) , which is associated withH I GS 94. The WR star is a member of LH116.
WR152 is projected on the northern exterior of the H II re-gion DEM L309 and is ∼
55 pc from LH116. The WR staris near the northern boundary of the large filamentary ion-ized shell measuring 17 (cid:48) × (cid:48) , which is associated with H I GS 94.
WR153 is superposed on faint diffuse emission ofDEM L308 inside a large filamentary ionized shell measur-ing 17 (cid:48) × (cid:48) , which is associated with H I GS 94. The H I position-velocity plots show velocity splits indicating an ex-pansion velocity of ∼
20 km s − for H I GS 94. The WR staris over 60 pc from LH116 and thus not likely a member ofthis OB association.
WR154 is projected in a small 1 . (cid:48) × . (cid:48) . (cid:48) × . (cid:48) ∼
47 km s − (Chu 1983).DEM L315 is on the northeastern rim of a large 17 (cid:48) × (cid:48) ionized shell associated with H I GS 94.REFERENCES
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