A submillimeter study of the IR dust bubble S 21 and its environs
C. E. Cappa, N. U. Duronea, J. Vasquez, M. Rubio, V. Firpo, C.-H. López-Caraballo, J. Borissova
aa r X i v : . [ a s t r o - ph . GA ] N ov Manuscript for
Revista Mexicana de Astronom´ıa y Astrof´ısica (2007)
A SUBMILLIMETER STUDY OF THE IR DUSTBUBBLE S 21 AND ITS ENVIRONS
C. E. Cappa,
N. U. Duronea, J. Vasquez,
M. Rubio, V. Firpo,
C.-H. L´opez-Caraballo, and J. Borissova, Draft version: September 11, 2018
RESUMENBasados en la emisi´on molecular en las l´ıneas CO(2-1) y CO(2-1), y enla emisi´on en el continuo en el mediano y lejano infrarrojo hacia la burbujaS 21, analizamos las caracter´ısticas f´ısicas del gas y polvo asociado con S 21y la presencia de objetos estelares j´ovenes (YSOs) en su entorno. La emisi´onmolecular revela una c´ascara grumosa de 1.4 pc de radio rodeando a S 21. Sumasa molecular es de 2900 M ⊙ y la densidad ambiental original en la regi´on,2.1 × cm − , indicando que la burbuja evoluciona en un medio de alta den-sidad. La imagen a 24 µ m muestra polvo tibio dentro de la burbuja, mientrasque la emisi´on en el rango 250 a 870 µ m revelan que hay polvo fr´ıo en lavecindad, coincidente con el gas molecular. La detecci´on de emisi´on en el con-tinuo de radio indica que S 21 es una regi´on H ii compacta. Una b´usqueda deYSOs utilizando criterios fotom´etricos permiti´o identificar muchos candidatoscoincidentes con los grumos moleculares. Se analiza si el proceso de collectand collapse ha dado origen a una nueva generaci´on de estrellas.ABSTRACTBased on the molecular emission in the CO(2-1) and CO(2-1) lines, and thecontinuum emission in the MIR and FIR towards the S 21 IR dust bubble, weanalyze the physical characteristics of the gas and dust linked to the nebulaand the presence of young stellar objects (YSOs) in its environs. The lineemission reveals a clumpy molecular shell, 1.4 pc in radius, encircling S 21. Thetotal molecular mass in the shell amounts to 2900 M ⊙ and the original ambientdensity, 2.1 × cm − , indicating that the bubble is evolving in a high densityinterstellar medium. The image at 24 µ m shows warm dust inside the bubble,while the emission in the range 250 to 870 µ m reveal cold dust in its outskirts,coincident with the molecular gas. The detection of radio continuun emissionindicates that the bubble is a compact H ii region. A search for YSOs usingphotometric criteria allowed to identify many candidates projected onto the Instituto Argentino de Radioastronom´ıa, CONICET, Argentina. Facultad de Ciencias Astron´omicas y Geof´ısicas, Universidad Nacional de la Plata,Paseo del Bosque s/n, 1900, La Plata, Argentina. Departamento de Astronom´ıa, Universidad de Chile, Chile. Gemini-CONICYT, Departamento de F´ısica y Astronom´ıa, Universidad de La Serena,La Serena, Chile. Gemini Observatory, Southern Operations Center, La Serena, Chile. Universidad Cat´olica de Chile, Santiago, Chile. Instituto de F´ısica y Astronom´ıa, Universidad de Valpara´ıso, Chile. Millennium Institute of Astrophysics (MAS), Santiago, Chile. collect and collapse process has triggereda new generation of stars.
Key Words:
H II regions — ISM: molecules — ISM: individual (S 21) — Stars:protostars 1. INTRODUCTIONMassive (O and B-type) stars have an enormous impact on their surround-ings due to their ultraviolet (UV) ionizing radiation and energetic winds. Ion-ized gas in H ii regions produces strong infrared (IR), optical, and thermalradio continuum emission. In addition, ionized gas mixed with heated dustmake an H ii region bright in thermal IR emission.A common spatial shape reported for individual Galactic H ii regions at IRwavelengths is the ring morphology, or “bubble” seen in projection. A visualexamination of the images at 8.0 µ m from the Galactic Legacy Infrared SurveyExtraordinaire (GLIMPSE; Benjamin et al. 2003) allowed the identificationof about 600 full or partial IR dust bubbles (IRDBs) in the inner Galacticplane (Churchwell et al. 2006, 2007) between longitudes from –60 ◦ to +60 ◦ .Presently, more than 5000 IRDBs have been identified (Simpson et al. 2012).The main characteristics of many of these bubbles were investigated by sev-eral authors (see for example Deharveng et al. 2010; Alexander et al. 2013).These bubbles are about 1 ′ - 3 ′ in size, show filamentary appearence, andmany of them lie close to massive stars and coincide (or enclose) classical andultracompact H ii regions.At 8 µ m, most of the emission originates in strong features of polycyclicaromatic hydrocarbons (PAH) molecules, which are considered to be goodtracers of warm UV-irradiated photodissociation regions (PDR; Hollenbach &Tielens 1997). Since these complex molecules are destroyed in the ionized gas(Povich et al. 2007; Lebouteiller et al. 2007), they delineate the ionization frontand indicate the presence of substantial amounts of molecular gas surroundingthe bubbles. Therefore, these bubbles provide a good insight of the sculptinginfluences of the UV photons of massive stars on the molecular clouds wherethey are born.The geometry of the IR bubbles is also important for understanding trig-gered star formation scenarios. Classical models, like the collect and collapse mechanism (C&C; Elmegreen & Lada 1977) and the radiative driven implosion process (RDI; Lefloch & Lazareff 1994), suggest that the formation of starscan be triggered by the action of H ii regions over their parental molecularenvironment. Molecular condensations lying at the border of many galac-tic bubble-shaped H ii regions are then among the most likely sites for stellarbirths and to look for early stages of star formation (e.g. Deharveng et al. 2008;Zavagno et al. 2010; Brand et al. 2011; Samal et al. 2014). Detailed studies ofIR bubbles have shown the presence of young stellar objects (YSOs) in theirenvironments, although the triggered formation scenario not always can beproven (e.g. Dewangan & Ojha 2013; Alexander et al. 2013).ULTIWAVELENGTH STUDY OF S 21 3 Fig. 1. Composite IRAC image of S 21. The emission at 3.6 µ m is in blue, at 4.5 µ mis in green, and at 8.0 µ m is in red. The positions of IRAS 16495-4418, G341.3553-00.2885, and the catalogued B3-star are indicated with white, yellow, and greencrosses, respectively. As part of a project aimed to study and characterize galactic bubbles,we have selected S 21 from the sample of bubbles reported by Churchwell etal. (2006) to perform a study of its molecular and dust environment, andsearch for candidates to YSOs in their vecinity. S 21 is located 10 ′ east fromS 24 (Cappa et al. 2016).S 21 is an almost complete circular bubble (see Fig. 1) of ∼ . ′
75 in radiuscentered at RA,Dec.(J2000) = 16 h m s , − ◦ ′ . ′′
1. The dusty nebulaappears filamentary at 8 µ m, with a rather steep inner border and a morediffuse outer one (Fig. 1). The point source IRAS 16495-4418 (RA,Dec.(J2000)= 16 h m s , − ◦ ′ ′′ ) coincides with the bubble. Watson et al. (2010)determined the dust temperature inside the bubble using the emission at 24and 70 µ m from Spitzer-MIPS and a modified blackbody. They found atemperature gradient with the highest temperatures (85 K) close to the centerof the bubble and the lowest ones (71 K) close to the border.Figure 1 reveals a bright point source detected in the IRAC bands, namedG341.3553-00.2885 (RA,Dec.(J2000) = 16 h m s , − ◦ ′ . ′′ h m s , − ◦ ′ . ′′
28) has been identified.The distance to S 21 is matter of some debate. Adopting a mean velocityof −
44 km s − for the gas linked to S 21 (see Sect. 3.1), circular galacticrotation models predict near and far kinematical distances of 3.7 and 12-13kpc (e.g. Brand & Blitz 1993). Bearing in mind that most of the IRDBs CAPPA ET AL.are closer than 8 kpc (Churchwell et al. 2006), we will adopt for S 21 the nearkinematical distance of 3.7 kpc. A distance uncertainty of 0.5 kpc results aftertaking into account a velocity dispersion of 6 km s − for the interstellar gas.In this study, we present a molecular line and dust continuum analysistoward S 21 and its environs, with the aim of studying the distribution andphysical properties (mass, densities, temperature, kinematics, etc.) of themolecular gas and dust associated with the bubble. We based our studyon CO(2-1) and CO(2-1) data obtained with the APEX telescope , andcomplementary archival IR, optical, and radiocontinuum data.The simple morphology of S21, along with the strong evidence of starformation in its environs (see below), make this object an excellent laboratoryfor the investigation of possible scenarios of triggered star formation. Withthat aim, we also analyze the spatial distribution of the candidates to YSOs intheir vicinity and their relation to the bubble, and search for probable excitingstars. 2. DATA2.1 . Molecular line observations The characteristics of the molecular gas were investigated using CO(2-1) (at 230.538000 GHz, HPBW = 30 ′′ ) and CO(2-1) (at 220.398677 GHz,HPBW = 28 ′′ ) line observations obtained in October 2010 with the AtacamaPathfinder Experiment (APEX) 12-m telescope (G´’usten et al. 2006) at Llanode Chajnantor, Chile (Project C-086.F-0674B-2010, P.I. M. Rubio). As frontend for the observations, we used the APEX-1 receiver of the Swedish Hetero-dyne Facility Instrument (SHeFI; Vassilev et al. 2008). The back end for theobservations was the eXtended bandwidth Fast Fourier Transform Spectrome-ter2 (XFFTS2) with a 2.5 GHz bandwidth divided into 4096 channels. Undergood weather conditions, this leads to APEX-1 DSB system temperatures ofabout 150 K.The region was observed in the position switching mode using the OTFtechnique with a space between dumps in the scanning direction of 9 ′′ . Therms noise of a single spectrum in the OTF mode was 0.35 K. The off-sourceposition free of CO emission was located at RA,Dec.(J2000) = (16 h m s , − ◦ ′ . ′′ O(2-1)line at 219.560357 GHz was also observed, although the emission is very lowand it was not used in the analysis.The spectra were reduced using the Continuum and Line Analysis Single-dish Software (CLASS) of the Grenoble Image and Line Data Analysis Soft-ware (GILDAS) working group . A linear baseline fitting was applied to the APEX, the Atacama Pathfinder EXperiment, is a collaboration between Max PlanckInstitut fur Radioastronomie (MPIfR), Onsala Space Observatory (OSO), and the EuropeanSouthern Observatory (ESO). ULTIWAVELENGTH STUDY OF S 21 5 -2 0 2 4 6 8 10-80000 -70000 -60000 -50000 -40000 -30000 -20000 -10000 0 10000 −70 −60 −50 −40 −30 −20 −10 −0Velocity (km/s)1086420 T m b ( K ) Fig. 2. CO(2-1) (red line), CO(2-1) (green line), and C O(2-1) (blue line)averaged molecular line spectra obtained toward S 21.Fig. 3. Overlay of the 8 µ m emission (colorscale) and the CO emission (in contours)integrated in the velocity range from − − − . Contour levels gofrom 3 K ( ∼ rms ) to 6.9 K in steps of 0.3 K and from 6.9 in steps of 1 K. Thewhite crosses show the position of candidate YSOs identified in the VVV databaseprojected onto the molecular shell (see Sect. 7). CAPPA ET AL.data. The observed line intensities are expressed as main-beam brightnesstemperatures T mb , by dividing the antenna temperature T A by the main-beam efficiency η mb , equal to 0.72. The Astronomical Image Processing Sys-tem (AIPS) package and CLASS software were used to perform the analysis.The final molecular data were smoothed to 0.3 km s − , with a final rms noiseof 0.2 K. 2.2 . Archival dust continuum data . Herschel data The archival data comes from the Hi-GAL key program (Hi-GAL:
Herschel
Infrared GALactic plane survey, Molinari et al. 2010), OBSIDs: 1342204094and 1342204095). These data include PACS images at 70 and 160 µ m (Poglitschet al. 2010) and SPIRE images at 250, 350, and 500 µ m (Griffin et al. 2010).Thedata were re-reduced using the Herschel
Interactive Processing Environment(HIPE v12 , Ott 2010) as described in Cappa et al. (2016). The angular res-olutions of the final dust continuum images spans from 8 ′′ to 35 . ′′ µ mto 500 µ m, respectively. 2.3 . Complementary data We use archival images of ATLASGAL at 870 µ m (345 GHz) (Schulleret al. 2009). This survey has an rms noise in the range 0.05 - 0.07 Jybeam − . The calibration uncertainty in the final maps is about of 15%. The Large APEX BOlometer CAmera (LABOCA) used for these observations, is a295-pixel bolometer array developed by the Max-Planck-Institut fur Radioas-tronomie (Siringo et al. 2007). The beam size at 870 µ m is 19 . ′′ µ m from the Galactic LegacyInfrared Mid-Plane Survey Extraordinaire (GLIMPSE; Benjamin et al. 2003),and at 24 µ m from the MIPS Inner Galactic Plane Survey (MIPSGAL; Careyet al. 2005) were used.Radio continuum data from the Sydney University Molonglo Sky Sur-vey (SUMSS , Bock et al. 1999) at 843 MHz with a resolution of 43 ′′ × ′′ csc(Decl.) and an rms noise level of 1 mJy beam − and at 1.4 GHz fromthe Southern Galactic Plane Survey (beam size = 1 . ′
7, Haverkorn et al. 2006)were used. 2.4 . Search for young stellar objects
To investigate the existence of candidates to YSOs projected onto theregion we used infrared point sources from the Vista Variables in the ViaLactea ESO Public Survey (VVV, ESO programme ID 179.B-2002; Minniti etal. 2010; Saito et al. 2012), and the Spitzer (Fazio et al. 2004) and Wide-fieldInfrared Survey Explorer (WISE; Wright et al. 2010) point source catalogues. HIPE is a joint development by the Herschel Science Ground Segment Consortium,consisting of ESA, the NASA Herschel Science Center, and the HIFI, PACS and SPIREconsortia members, see http://herschel.esac.esa.int/HerschelPeople.shtml. ULTIWAVELENGTH STUDY OF S 21 7TABLE 1PARAMETERS OF THE MOLECULAR CLUMPS.
Clump R.A.(J2000) Decl.(J2000) T peak ∆v T peak ∆v T exc τ τ (h m s) ( ◦ ′ ′′ ) (K) (km s − ) (K) (km s − ) (K)1 16:53:03 -44:22:16.3 11.5 3.6 7.5 1.6 16.6 0.96 34.32 16:53:00 -44:22:57.3 8.5 2.5 7.3 2.1 13.5 1.83 79.53 16:53:01 -44:22:09.2 13.0 4.3 10.2 1.7 18.1 1.50 33.54 16:53:08 -44:22:01.3 8.8 3.8 6.5 1.8 13.1 1.28 33.35 16:53:12 -44:23:13.7 10.5 4.0 7.8 1.9 15.6 1.30 33.1 TABLE 2PROPERTIES DERIVED FOR THE MOLECULAR CLUMPS.
Clump ∆v T − mb ∆v T − mb N ( CO) R eff M (H ) n H M V IR MV IRM (H2) (km s − ) (K) (K km s − ) (10 cm − ) (pc) (M ⊙ ) (10 cm − ) (M ⊙ )1 5.0 3.75 12.0 2.4 0.65 350 ±
105 4.6 209-316 0.5-1.32 5.4 4.52 14.4 4.0 0.68 650 ±
195 7.5 377-519 0.4-1.13 3.7 5.25 16.8 4.2 0.77 880 ±
260 7.0 280-422 0.2-0.74 4.8 3.85 12.3 2.8 0.64 400 ±
120 5.5 261-393 0.3-1.45 4.6 4.39 14.0 3.2 0.75 620 ±
185 5.3 341-514 0.4-1.2
3. MOLECULAR GAS LINKED TO THE BUBBLE3.1 . Molecular gas distribution
In Fig. 2 we show the CO(2-1), CO(2-1), and C O(2-1) spectra av-eraged in a region ∼ ′ × ′ around the central position of S 21. The bulk ofthe molecular emission appears concentrated between −
55 km s − and − − , with three components peaking approximately at −
45 km s − , − − , and −
30 km s − . The spatial distribution of these components re-veals that only the molecular component peaking at −
45 km s − shows amorphological correspondence with S 21. The emission distribution in the ve-locity interval from − − − shows a bubble fully inmersed in itsparental molecular cloud. CO shells have been reported for many other IRDBs(see for example Arce et al. 2011). Figure 3 displays the integrated CO(2-1)emission distribution in the mentioned velocity range in contours overlayedonto the emission at 8 µ m. A clumpy circular structure coincident with theinfrared bubble with a minimum in the emission projected onto its center canbe discerned. The mean radius of the molecular shell is 1 . ′
3. The molecularcorrespondence between the IR emission at 8 µ m and the molecular emissionwithin the quoted velocity interval indicates that the shell is the molecularcounterpart of the IR bubble, and that they are physically associated.Using clumpfind we have identified several condensations in the molecularstructure around S 21, which will be referred as “clumps” (Blitz 1993; Williamset al. 2000). The location of the clumps, labeled from 1 to 5, are depicted in CAPPA ET AL. Fig. 4.
Panel a : CO emission in the velocity range from − − − .Contour levels go from 3 K ( ∼ rms ) to 6.9 K in steps of 0.3 K and from 6.9 K insteps of 1 K. White dotted lines indicate the limits chosen to define the clumps (seetext). The beam size is indicated in the lower left corner of this panel. Panels b tof : Channel maps of CO in intervals of 0.6 km s − (white contours) superimposedon the GLIMPSE 8.0 µ m red colorscale. Contours go from 4.5 K ( ∼
32 K rms ) to10 K in steps of 0.5 K, and from 10 K in steps of 1 K.
ULTIWAVELENGTH STUDY OF S 21 9Fig. 4a. We were left only with clumps adjacent to the bubble, which werevery likely formed in the collected layers of the molecular gas. The coordinatesand peak temperatures of the clumps are listed in Table 1.In order to unambiguously ascertain the relationship of each clump withthe nebula and to provide a better visual display, in panels b to f of Fig. 4we show the channel maps of CO(2-1) in velocity intervals of 0.6 km s − ,overlaid onto the GLIMPSE 8.0 µ m emission image. Between − − − (Fig. 4b) clumps 1 and 2 become noticeable, bordering the bubble fromits north-western side. In this velocity range, two bright molecular featurescan be also detected to the south of S 21, unconnected to this bubble. Clumps1 and 2 achieve their maximum brightness temperature in the velocity intervalfrom − − − (Fig. 4c). Clump 2 is still detected between − − − (Fig. 4d), where clump 3 becomes first noticeable. In thisvelocity interval, clump 5 appears as an extension of an elongated feature thatborders the eastern side of S 21. From − − − (Fig. 4e) clumps3, 4, and 5 attain their peak temperature. They surround the nebula fromsouth to east (clump 3) and from south to north (clumps 4 and 5). Finally,between − − − (Fig. 4f), clumps 4 and 5 are barely detectedbut clump 3 is very bright. Clump 3 dissapears at ∼ − − (not shownhere). At a distance of 3.7 kpc, the radius of the molecular shell is ∼ µ m close to the PDR.3.2 . Physical parameters of the molecular gas We have estimated some properties for the identified clumps, which arepresented in Tables 1 and 2. Assuming that all rotational levels are ther-malized with the same excitation temperature (LTE conditions) and that theemission in the CO(2-1) line is optically thick, we derived the excitationtemperature T exc (Column 8, Table 1) from the emission in the CO(2-1)line using T = T ∗ "(cid:18) e T ∗ T exc − (cid:19) − − (cid:18) e T ∗ T bg − (cid:19) − , (1)where T ∗ = hν /k , being ν the frequency of the CO(2-1) line, and T bg =2 . CO(2-1) line ( T ) (column 4, Table 1) we used the spectrum toward the positionof maximum emission of the clump.The optical depth τ (Column 9, Table 1) was obtained from the CO(2-1) line by assuming that the excitation temperature is the same for CO(2-1)and CO(2-1) emission lines using the expression τ = − ln − T peak13 T ∗ "(cid:18) e T ∗ T exc − (cid:19) − − (cid:18) e T ∗ T bg − (cid:19) − − , (2)0 CAPPA ET AL.where T ∗ = hν /k , being ν the frequency of the CO(2-1) line. We alsoestimated the optical depth of the CO(2-1) line (Column 10, Table 1) fromthe CO(2-1) line with τ = (cid:20) ν ν (cid:21) × (cid:20) ∆v ∆v (cid:21) × (cid:20) CO CO (cid:21) τ , (3)where CO/ CO is the isotopic ratio (assumed to be ∼
62; Langer & Penzias1993); ∆v and ∆v are the full width at half maximum (FWHM) of thespectra of the CO and CO lines, respectively. These values are indicatedin columns 5 and 7 of Table 1.In LTE, the CO column density (Column 5, Table 2) can be estimatedfrom the CO(2-1) line data following N ( CO) = 2 . × e T ∗ T exc − e − T ∗ T exc T exc Z τ dv (cm − ) . (4)The integral of Eq. 4 can be approximated by T exc Z τ dv ≈ τ − e ( − τ ) Z T mb d v . (5)This approximation helps to eliminate to some extend optical depth effectsand is good within 15% for τ < τ < T mean − mb (with T mean − mb equal to the average T mb within the area of the clump) and is listed in column 4, Table 2. Then,the total hydrogen mass (Column 7, Table 2) can be calculated usingM(H ) = ( m sun ) − µ m H A N (H ) d ( M ⊙ ) , (6)where m sun is the solar mass ( ∼ × g), µ is the mean molecular weight,which is assumed to be equal to 2.76 after allowing for a relative heliumabundance of 25% by mass Allen (1973), m H is the hydrogen atom mass( ∼ × − g), A is the solid angle of the CO emission (included in Ta-ble 2 as the effective radius R eff = p A/π , Column 6), and d is the adopteddistance expressed in cm. To obtain the masses, we adopted an abundance N (H ) / N ( CO) = 5 × (Dickman 1978). Uncertainties in molecularmasses are about 30%, while they are about 50% in ambient densities, andoriginate mainly in distance uncertainties.Mean volume densities of the clumps are in the range (4.5-7) × cm − (Column 8 of Table 2). We estimate the original mean volume ambient densityin the region of the bubble by assuming a uniform gas distribution before thering was formed. This density was obtained by distributing the total shellmass within a sphere with the outer radius of the shell (1.7 pc), and amountsto 2.1 × cm − . This value indicates that the bubble is evolving in a highdensity interstellar medium.ULTIWAVELENGTH STUDY OF S 21 11 Fig. 5. Composite images showing the emission in the near-, mid-, and far-infrared.All the images display the emissions at 8 µ m (in red) and at 3.6 µ m (in blue) fromIRAC-GLIMPSE. The emissions in green correspond to different IR wavelengthsindicated in the upper left corner of each image. The angular resolution of the IRimages is shown in the lower left corner. . Dust distribution Dust associated with the bubble can be analyzed from the distribution ofthe emission at different wavelengths in the infrared.Each panel of Fig. 5 shows a composite image of the emissions at 8 µ m(in red) and 3.6 µ m (in blue) from IRAC-GLIMPSE, and the emissions at24, 70, 160, 250, 500, and 870 µ m (in green) from MIPS, Herschel -PACS and-SPIRE, and ATLASGAL.The emission at 24 µ m is seen projected onto the inner part of the bubble,with its maximum near the central point source. The emission at 70 µ mcoincides fairly well with that at 8 µ m. The emission differs significantly atlarger wavelengths. At 160 µ m it encircles externally the bubble completelyand appears more extended than at 70 µ m. At 250 and 500 µ m, the emissionis brighter than at lower wavelengths and extends toward the western andnorthern parts of the bubble. The SPIRE emission distribution at 350 µ m(not shown here) is similar to that at 250 and 500 µ m. Similarly to the case ofS 24, cold dust emission detected at 870 µ m resembles that at 250 and 500 µ m,although the emission at 250 and 500 µ m, which also shows the distributionof cold dust, appears more extended. We have to bear in mind that largescale dust continuum emission might be filtered out in LABOCA bolometricobservations.Two facts can be concluded from these images. On one hand the spatialemission distribution in the mid- and far-IR seems to show a gradient indust temperature, with lower values in the outer regions of the bubble wherecontinuum emission at larger wavelengths dominates. Indeed, emission at λ > µ m is present well outside the PDR. On the other hand, the emissionat 24 µ m inside S 21 is indicative of the presence of exciting sources. Bothstatements will be analyzed in some detail in the next sections.A comparison with the molecular gas distribution around the bubble (seeFig. 6) shows that clumps 1 to 4 partially coincide with the cold dust coun-terpart identified in the Herschel -SPIRE and in the LABOCA images.The observed dust continuum emission at different wavelengths in the IRcoincides with previous findings toward other IR dust bubbles.The molecular emission from the CO(2-1) line may contribute to thethermal emission at 870 µ m. In our case, this contribution is less than about1% of the emission at 870 µ m, and consequently, within calibration uncer-tainties. The other process that contribute to the emission at 870 µ m is thefree-free emission from ionized gas. Their contribution, bearing in mind theflux density at 1.4 GHz (see Sect. 5), is less than 0.1% of the emission at 870 µ m, and again, within calibration uncertainties.4.2 . Dust temperatures and mass Dust temperatures T dust in the environs of the S 21 region can be obtainedusing the SPIRE images at 250 µ m and 350 µ m. To perform this we convolvedULTIWAVELENGTH STUDY OF S 21 13 Fig. 6. Composite image showing the emission at 8 µ m (in red) and at 870 µ m (inblue), and the same CO contours of Fig. 3. the image at 250 µ m down to the angular resolution at 350 µ m and assumedthat the emission is optically thin. The color-temperature map was constructedas the inverse function of the ratio map of Herschel 250 µ m and 350 µ m color-and-background-corrected maps, i.e., T dust = f ( T ) − , where f ( T ) is: f ( T ) = S S = B ν (250 µm, T) B ν (350 µm, T) (cid:18) (cid:19) β d (7)In this expresion S and S are the flux densities in Jy beam − , B ν ( ν, T) isthe blackbody Planck function and β d , the spectral index of the thermal dustemission. The pixel-to-pixel temperature was calculated assuming β d = 2.This is a typical value adopted for irradiated regions.The dust temperature map is shown in Fig. 7. The highest dust temper-atures (33 K) are present at the NE extreme of the 8 µ m bubble. Valuesin the range 24-33 K coincide with the eastern section of the bubble, whilelower values (21 K) were obtained for the western side. Low dust tempera-tures coincide with molecular gas and with regions with faint emission at 8 µ m. Watson et al. (2010) derived dust temperatures for the interior of S 21based on images from MIPS at 24 µ m (whose emission is detected inside thebubble) and 70 µ m (detected up to the border of the bubble). Our estimates,based on images in the far IR, sample colder dust present in the outkirts ofthe bubble.Dust masses can be estimated from the expression (Hildebrand 1983)M dust = S d κ B ( T dust ) (8)4 CAPPA ET AL. Fig. 7. Dust temperature map derived from the Herschel emission at 250 and 350 µ m superimposed onto the image at 8 µ m. Blue color scale goes from 15 to 33 K.Brighter blue regions indicate higher dust temperatures. Contour levels correspondto 15 to 30 K, in steps of 3 K. where S is the flux density at 870 µ m, d = 3.7 ± κ = 1.0 cm /gr isthe dust opacity per unit mass (Ossenkopf & Henning 1994), and B ( T dust )is the Planck function for a temperature T dust .The flux density S obtained by integrating the emission over the ob-served emitting area linked to S 21 at this wavelength (see Fig. 5f) amountsto 1.9 ± T dust = 30 K for S 21, a dust mass M dust = 1.53 ± ⊙ can be estimated. For gas-to-dust ratios in the range100-186 (Beuther et al. 2011), the gas mass amounts to 153-285 M ⊙ .5. THE IONIZED GASFigure 8 shows an overlay of the SUMSS image at 843 MHz (in whitecontours) and the emissions at 8 µ m and 24 µ m (in colorscale). The 843MHz image (synthesized beam = 43 ′′ × ′′ ) shows a radio source coincidentwith S 21 catalogued by Murphy et al. (2007) with a size of 75 . ′′ × . ′′ S . = 49.2 mJy. The elongated shape of the source is dueto the synthesized beam of the data. The source is also detected at 1.4 GHz(SGPS, Haverkorn et al. 2006) with an estimated flux density of S . = 52mJy. With these values, the derived spectral index α ≃ +0.1 ( S ν ∝ ν α ).Within uncertainties, this value is consistent with thermal emission of an H ii region optically thick at 843 MHz.Thus, these results indicate the presence of ionized gas, and consequently,the existence of at least one exciting source, compatible with warm dust insidethe bubble as shown by the 24 µ m image. The emission at 8 µ m due to PAHsreveals a PDR bordering the ionized region and the presence of molecular gasin its exterior. The radius of the ionized region inside the 8 µ m dust bubble isULTIWAVELENGTH STUDY OF S 21 15 Fig. 8. Overlay of the radio continuum emission at 843 MHz (white contours), theIRAC emission at 8 µ m (in green), and the MIPSGAL emission at 24 µ m (in blue).Contours correspond to 10, 15, 20, and 25 mJy beam − . ′′ or 0.7 pc at 3.7 kpc. The derived spectral index confirms the classificationof S 21 as H ii region by Anderson et al. (2015). The characteristics of theregion, visible in the IR and in the radio continuum, as well as the large meanoriginal ambient density of 2100 cm − (see Sect. 3.2) suggest a compact H ii region (Urquhart et al. 2013).To produce the observed flux at 1.4 GHz we requiere an UV photon fluxthat can be obtained from Matsakis et al. (1976) N Ly = 7 . × S . ν . d T − . e s − (9)where S . is in units of mJy, ν in GHz, d in kpc, and the electron temperature T e in 10 K. Considering the galactic electron temperatures for H ii regions(Quireza et al. 2006), we assumed T e = 7000 K. The required UV flux amountsto 6.5 × s − . This ionizing flux is underestimated since part of the stellarUV photons are used to heat the dust. Considering that half of the stellarphotons are absorbed by dust (Inoue 2001), the UV photon flux would be1.3 × s − . According to Martins et al. (2005), this value indicates thatthe ionization of the gas could be produced by at least a O9.5V or earlier typestar. 6. SEARCH FOR AN EXCITING STARIdentifying the exciting star of this H ii region is not an easy task. Theextinction towards the inner part of the IR bubble can help to identity thisstar since we expect a similar extinction. This extinction can be deduced from6 CAPPA ET AL.the expresion by Bohlin et al. (1978) N ( HI ) + 2 N ( H ) = 5 . × E ( B − V ) (10)Considering only the H column density toward the IR source and taking intoaccount N ( H ) = 2.9 × cm − (estimated from the CO emission), wecalculate a visual absorption of 30 mag.To search for exciting stars the color-magnitude diagrams were build usingthe VVV DR4 catalog (Fig. 8). The foreground Main Sequence stars aresituated around J − K s = 0.7 mag, and the mean reddening is estimated as E ( J − K s ) = 0.5 mag, some field red giants can be also identified. The 76extremely red stars (( J − K s ) > and TLUSTY tools and all the available detections atdifferent wavelengths (taking into account stellar atmosphere modelling for Oand B stars) suggests that the star might be a O8V of a B0I star. From thecomparison of the observed colors taken from the 2MASS catalogue (source2MASSJ 16530711-4423239) with the intrinsic magnitudes and colors takenfrom Martins & Plez (2006) for O8V stars and from Bibby et al. (2008) for B0Istars, we estimate visual absorptions and distances of 21 mag and 0.58 ± ± ii regions. To help testing if thisis the case for the S 21 bubble, we can analyze the stability of the molecularclumps by comparing virial to LTE masses. Following MacLaren et al. (1988),the virial mass can be obtained as M vir M ⊙ = k (cid:20) Rpc (cid:21) (cid:20) ∆V kms − (cid:21) (11) ULTIWAVELENGTH STUDY OF S 21 17
J-KS K S G341.5-00.28B3
Av=10
0 1 2
H-KS
G341.5-00.28B3
Av=10 -0.5 0.0 0.5 1.0 1.5 2.0 2.5 3.0
H-KS J - H Av=10
Fig. 9.
Upper panel. ( J − K s ) vs K s and ( H − K s ) vs K s VVV color magnitudediagrams within a radius of 1 . ′ E ( J − K s ) = 0.5 mag (the mean reddening of thefield stars). The candidate YSOs projected onto the molecular shell are overplottedwith red squares. The red arrow shows the reddening vector corresponding to Av=10mag. G341.3553-00.2885 and HD 329056 sources are labeled. Bottom panel.
The( H − K s ) vs ( J − H ) color-color diagram. The continuous and dashed lines representthe sequence of the zero-reddening stars of luminosity classes I (Koornneff et al. 1983)and V (Schmidt-Kaler 1982). R and ∆V are the radius of the region and the velocity width measuredfrom the CO(2-1) emission, and k depends on the density distribution inthe clump, being 190 or 126 according to ρ ∝ r − or ρ ∝ r − , respectively.Virial masses are included in column 9 of Table 2 for the two density profiles.The ratio M vir M ( H ) = γ is listed in column 10.As classical virial equilibrium analysis establishes, a ratio γ > M ( H ) and 15% for M V IR ). Minor errors are due touncertainties in the boundaries of the clumps resulting in errors in their areasand in the LTE masses.Virial masses are not free of additional errors, since the existence of mag-netic fields might overestimate by up to a factor of 2 the derived values (Ma-cLaren et al. 1988). Other source of error is the density profile of the clump,which is unknown. Uncertainties in distance in the LTE mass and those dueto the density profile in virial masses were taken into account in the value of γ . To investigate if YSOs are detected toward the molecular clumps we per-formed a search for candidates in the available point source catalogues withina region of 1 . ′ J − H ) / ( H − K ) color - color diagram we selected all stars which are at least 3 σ distant from the reddening line that marks the colors of dwarf stars. The listthereby obtained was cross-matched with GLIMPSE measurements. They areshown in Fig. 10. The objects with [ K S − [3 . > . . − [4 . > . collect-and-collapse mechanism (C&C; Elmegreen &Lada 1977) proposes that the expansion of an ionization front over its parentalmolecular cloud can trigger the star formation process. The molecular gas mayULTIWAVELENGTH STUDY OF S 21 19 -0.5 0.0 0.5 1.0 1.5 [3.6-4.5] [mag] K s - [ . ] [ m ag ] VVV-Glimpse -0.5 0.0 0.5 1.0 1.5 2.0 2.5
H-Ks -0.50.00.51.01.52.02.53.0 J - H Av=10YSO candidates
H-Ks J - K s Av=10Very Red Stars
Fig. 10.
Upper panel.
The K S − [3 . . − [4 .
5] color-color plot of stars that aredetected in GLIMPSE I. The red dashed lines represent the limits used to selectclass I and class II YSOs.
Middle panel.
The ( J − H ) vs. ( H − K S ) color-colordiagram of the sample. The continuous and dashed lines represent the sequenceof the zero-reddening stars of luminosity classes I (Koornneef et al. 1983) and V(Schmidt-Kaler 1982). Lower panel.
The extremely red stars detected in the field. ii regions, the model predicts the age of the H ii region at which thefragmentation occurs (the fragmentation time scale), t frag , the size of theH ii region at that moment, R frag , the mass of the fragments, M frag , andtheir separation along the compressed layer, r frag . The parameters requiredto derive these quantities are the UV photon flux of the exciting star, N Ly ,the ambient density of the surrounding medium into which the H ii region isevolving, n , and the isothermal sound speed in the shocked gas, a s .To estimate these parameters we take into account a large range of spectraltypes, i.e. from O3V to O9.5V stars, with UV fluxes in the range N Ly = (43-0.4) × s − (Martins et al. 2005). Using the mean H ambient density n H = 2100 cm − (see Sect. 3.2), and a s = 0.2-0.6 km s − , we obtained t frag =(1.0-1.5) × yr, R frag = 4.3-3.0 pc, M frag = 20-29 M ⊙ , and, r frag = 0.5-0.3pc. The dynamical age of the H ii region can be estimated using the equation(Dyson & Williams 1997) t dyn = 4 R S c s "(cid:18) RR S (cid:19) / − (12)where R S is the original Str¨omgren radius, equal to 0.13-0.63 pc for theadopted spectral types, and c s is the sound velocity in the ionized gas. Deriveddynamical ages span the range (1.4-33) × yr. We find that the dynami-cal age is significantly smaller than the fragmentation time scale t frag for theadopted ambient density, and then, the C&C process does not seem to be re-sponsible for the triggering of star formation in the envelope. An RDI scenariocould be investigated, however evidences of this process (such the presence ofpillars) appear to be absent. The rest of the parameters, R frag , M frag , and r frag , confirm that the H ii region is too young to start triggering.8. CONCLUSIONSWe performed a multiwavelength study of the IR dust bubble S 21 usingAPEX observations of the CO(2-1) and CO(2-1) lines and complemen-tary images in the near-, mid-, and far-IR from IRAC-Glimpse, MIPSGAL,Herschel, and ATLASGAL.The molecular emission in the CO(2-1) and CO(2-1) lines obtainedwith the APEX telescope toward the IR dust bubble S 21 revealed a molecularshell encircling the bubble and partially coincident with the PDR shown bythe IRAC emission at 8 µ m. With a mean radius of 1.4 pc, the molecularshell is larger than the 8 µ m bubble. This shell is detected in the velocityinterval from –45.8 to –42.6 km s − . The velocity of the shell confirms thatS 21 belongs to the same complex than S 24. Five clumps were identified in theULTIWAVELENGTH STUDY OF S 21 21molecular shell, with radii in the range 0.64-0.75 pc, LTE masses of 350-880M ⊙ , and volume densities of (4.5-7) × cm − . Virial masses for the clumpssuggest that at least one of them can collapse. The original ambient densityin the region was about 2100 cm − .Complementary images in the near-, mid-, and far-IR from IRAC-Glimpse,Herschel, and ATLASGAL were used to characterize the dust linked to thebubble. The emission at 24 µ m coincides with the inner part of the bubble,indicating warm dust inside. The spatial distribution of the emission in thefar-IR from 70 to 160 µ m coincides with the 8 µ m bubble and the molecularemission, while the emission at 500 and 870 µ m resembles that at 250 µ m.The spatial distribution of the Herschel-PACS and Spire, and ATLASGALemissions shows a cold dust component coincident with the molecular gas.Dust temperature determinations using the emissions at 250 and 350 µ mallowed to estimate dust temperatures in the range 21-33 K for the cold dustcomponent linked to the 8 µ m bubble.Thermal radio continuum emission at 843 MHz and 1.4 GHz was detectedfrom inside the bubble, indicating the existence of ionized gas and excitationsources, in agreement with the presence of warm dust. We conclude that acompact H ii region has developed. However, the identification of the excitingstar is a difficult task and deserves additional studies.A search for candidate YSOs was performed. We were able to identifymany candidates in the VVV database projected onto the molecular clumps,although it is not clear whether all these candidates are linked to the molecularshell. This result suggests that star formation has been active recently. TheH ii region is probably very young for the C&C process to be active.C.E.C. acknowledges the kind hospitality of M. Rubio and her family dur-ing her stay in Chile. V.F. acknowledges support from CONICYT AstronomyProgram-2015 Research Fellow GEMINI-CONICYT (32RF0002) and from theFaculty of the European Space Astronomy Centre (ESAC), and would like tothank Ivan Valtchanov, Bruno Altieri, and Luca Conversi for their supportand valuable assistance in Herschel data processing. We acknowledge theanonymous referee for very helpful comments. The ATLASGAL project is acollaboration between the Max-Planck-Gesellschaft, the European SouthernObservatory (ESO) and the Universidad de Chile. This project was partiallyfinanced by CONICET of Argentina under project PIP 0356, UNLP underproject 11/G120, and CONICyT of Chile through FONDECYT grant No.1140839. We gratefully acknowledge use of data from the ESO Public Surveyprogramme ID 179.B-2002 taken with the VISTA telescope and data productsfrom the Cambridge Astronomical Survey Unit. Support for JB is providedby the Ministry of Economy, Development, and Tourism’s Millennium Sci-ence Initiative through grant IC120009, awarded to The Millennium Instituteof Astrophysics, MAS. This work is based [in part] on observations made withthe Spitzer Space Telescope, which was operated by the Jet Propulsion Labo-ratory, California Institute of Technology under a contract with NASA. 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