Search for OB stars running away from young star clusters. II. The NGC 6357 star-forming region
aa r X i v : . [ a s t r o - ph . S R ] S e p Astronomy&Astrophysicsmanuscript no. NGC6357 c (cid:13)
ESO 2018June 23, 2018
Search for OB stars running away from young star clusters.II. The NGC 6357 star-forming region
V.V. Gvaramadze , , , A.Y. Kniazev , , P. Kroupa , and S. Oh Argelander-Institut f¨ur Astronomie, Universit¨at Bonn, Auf dem H¨ugel 71, 53121 Bonn, Germany e-mail: pavel;[email protected] Sternberg Astronomical Institute, Moscow State University, Universitetskij Pr. 13, Moscow 119992, Russia e-mail: [email protected] Isaac Newton Institute of Chile, Moscow Branch, Universitetskij Pr. 13, Moscow 119992, Russia South African Astronomical Observatory, PO Box 9, 7935 Observatory, Cape Town, South Africa e-mail: [email protected] Southern African Large Telescope Foundation, PO Box 9, 7935 Observatory, Cape Town, South AfricaReceived 22 July 2011 / Accepted 7 September 2011
ABSTRACT
Dynamical few-body encounters in the dense cores of young massive star clusters are responsible for the loss of a significant fractionof their massive stellar content. Some of the escaping (runaway) stars move through the ambient medium supersonically and can berevealed via detection of their bow shocks (visible in the infrared, optical or radio). In this paper, which is the second of a seriesof papers devoted to the search for OB stars running away from young ( < ∼ several Myr) Galactic clusters and OB associations, wepresent the results of the search for bow shocks around the star-forming region NGC 6357. Using the archival data of the MidcourseSpace Experiment ( MSX ) satellite and the
Spitzer Space Telescope , and the preliminary data release of the Wide-Field Infrared SurveyExplorer (WISE), we discovered seven bow shocks, whose geometry is consistent with the possibility that they are generated by starsexpelled from the young ( ∼ ff ectively lose massive starsat the very beginning of their dynamical evolution (long before the second mechanism for production of runaway stars, based on asupernova explosion in a massive tight binary system, begins to operate) and lends strong support to the idea that probably all fieldOB stars have been dynamically ejected from their birth clusters. A by-product of our search for bow shocks around NGC 6357 is thedetection of three circular shells typical of luminous blue variable and late WN-type Wolf-Rayet stars. Key words. stars: kinematics and dynamics – stars: massive – open clusters and associations: general – open clusters and associations:individual: Pismis 24 – open clusters and associations: individual: AH03 J1725 −
1. Introduction
Close few-body dynamical encounters in the dense cores ofyoung massive star clusters are responsible for the loss of a sig-nificant fraction of OB stars at the early stages of cluster evo-lution (Poveda et al. 1967; Aarseth & Hills 1972; Gies 1987;Leonard & Duncan 1990; Kroupa 2004; Pflamm-Altenburg &Kroupa 2006; Moeckel & Bate 2010). The high central densi-ties in young clusters (the necessary condition for the produc-tion of runaway stars) could be either primordial (e.g. Clarke &Pringle 1992; Murray & Lin 1996; Clarke & Bonnell 2008) orcaused by dynamical mass segregation (e.g. Portegies Zwart etal. 1999; G¨urkan et al. 2004; Allison et al. 2009). In both cases,the clusters start to eject stars long before the most massive clus-ter members explode in supernovae, i.e. long before the binary-supernova ejection mechanism (Blaauw 1961) begins to operate.Moreover, runaway stars could leave their parent clusters alreadyduring the cluster formation process if a core of massive proto-stars forms before bulk gas expulsion from the embedded cluster.The runaway OB stars can be revealed either directly, viameasurement of their proper motions and / or radial velocities(e.g. Mo ff at et al. 1998; Mdzinarishvili & Chargeishvili 2005;Massey et al. 2005; Evans et al. 2010; Tetzla ff , Neuh¨auser & Hohle 2011), or indirectly, through the detection of bow shocksaround them (Gvaramadze & Bomans 2008b; Gvaramadze etal. 2010a; Gvaramadze, Kroupa & Pflamm-Altenburg 2010d;Gvaramadze, Pflamm-Altenburg & Kroupa 2011a; Gvaramadzeet al. 2011b). The latter possibility is especially helpful for thoserunaways whose proper motions are still not available or aremeasured with a low significance. The geometry of detectedbow shocks can be used to infer the direction of the stellarmotion and thereby to determine possible parent clusters forthe bow-shock-producing field stars (Gvaramadze & Bomans2008b; Gvaramadze et al. 2010a,d, 2011a).The present paper is the second of a series of papers devotedto the search for OB stars running away from young ( < ∼ sev-eral Myr) Galactic clusters and OB associations. In Gvaramadze& Bomans (2008b; hereafter Paper I), we reported the detec-tion of three bow shocks produced by OB stars running awayfrom the star cluster NGC 6611. Now we report the discoveryof seven bow shocks within ∼ . ◦ − ated with NGC 6357. Sect. 3 presents the results of the search forbow shocks around NGC 6357. The bow-shock-producing starsare identified and discussed in Sect. 4. In Sect. 5 we present thediscovery of three circular shells around NGC 6357. Sect. 6 dealswith questions related to the content of the paper. We summarizein Sect. 7.
2. Star-forming region NGC 6357 and its associatedstar clusters
NGC 6357 is an extended ( ∼ ′ × ′ or ∼
20 pc ×
30 pc ata distance of 1.7 kpc; see below) H ii region of ongoing mas-sive star-formation in the Sagittarius arm. At radio wavelengthsNGC 6357 is dominated by the two components G 353.2 + + − Pismis 24 was recognized as an open star cluster by Pismis(1959). It is believed that the massive stars of this cluster are themain ionizing source responsible for the origin of the H ii regionNGC 6357 (Lortet, Testor & Niemela 1984; Bohigas et al. 2004;Cappa et al. 2011). Line observations of a molecular cloud asso-ciated with NGC 6357 showed that most of the molecular emis-sion arises from the regions behind or to the north of Pismis 24,which indicates that the cluster is immersed in a blister-like H ii region viewed face-on (Massi, Brand & Felli 1997).The cluster contains two very massive O3.5-type stars(Massey, DeGioia-Eastwood & Waterhouse 2001; Walborn etal. 2002), one of which, Pismis 24-1, was believed to be one ofthe most massive known stars in the Galaxy, with an inferredmass larger than 200 M ⊙ (Walborn et al. 2002). Subsequent Hubble Space Telescope observations resolved Pismis 24-1 intotwo visual components, while follow-up spectroscopy showedthat one of the components (Pismis 24-1SW) is an O4 III(f + ) star(of mass of ∼
100 M ⊙ ) and the second one (Pismis 24-1NE) isa very massive ( ∼
100 M ⊙ ) short-period spectroscopic binary(Ma´ız Apell´aniz et al. 2007).Because of the high visual extinction towards Pismis 24 ( ≃ −
15 mag; Wang et al. 2007), the stellar content of the clusterwas poorly studied until recently, so that only the most lumi-nous and less reddened cluster members (about two dozen alto-gether) were identified, either by means of photometry or spec-troscopy (Mo ff at & Vogt 1973; Neckel 1978, 1984; Lortet et al.1984; Massey et al. 2001). One of these stars is a binary system(HD 157504) composed of a WC7 Wolf-Rayet star (WR 93) andan O7-9 star (van der Hucht 2001).Thanks to the ability of X-ray observations to pen-etrate heavy absorption, the situation was improved with Chandra / ACIS observations of NGC 6357, which allowed thedetection of even the low-mass pre-main-sequence populationof Pismis 24 (with masses extending down to ∼ . ⊙ ), therebyincreasing the number of the cluster members by a factor of ∼ ≃ ≃ ≃ . , while the re-maining stars are spread with an approximately exponential dis-tribution over a halo of radius of ≃
11 arcmin ( ≃ . Chandra observation also detected two dozens of X-ray sources,whose luminosities and colours suggest that they could be OBstars. If spectroscopic follow-ups of these stars will prove thatthey are indeed massive, then the OB star content of Pismis 24will be doubled. Assuming that
Chandra detected all stars withmass > . ⊙ and using the Kroupa (2001) IMF (which is atwo-part power-law IMF with a slope α = . .
08 and 0 . ⊙ , and a Salpeter slope α = . ∼ M ⊙ and the expected number of OB stars in the cluster of ∼ . In the latter case, onecan expect to find numerous OB stars all around the cluster (seeSect. 3).The simultaneous presence in Pismis 24 of several veryyoung ( ∼ ≃ −
100 M ⊙ ) stars (Ma´ızApell´aniz et al. 2007) and the more evolved ( > ∼ − . ∼ ≃ ? ; Paper I).Numerous distance estimates for Pismis 24 range from ≃ . ff at & Vogt 1973; Neckel 1978, 1984; Lortetet al. 1984; Massey et al. 2001), putting the cluster in theSagittarius spiral arm. To constrain the distance to Pismis 24, weused the optical (Massey et al. 2001) and near-infrared (2MASS;Skrutskie et al. 2006) photometry of all (six) known dwarf Ostars in the cluster (see Table 2 in Massey et al. 2001) and the U BV JHK synthetic photometry of Galactic O stars by Martins& Plez (2006). Assuming that the standard total-to-selective ab- Note that the coordinates of Pismis 24 given in the SIMBADdatabase are significantly ( ∼
16 arcmin) o ff the position of these starsand rather correspond to the second star cluster, AH03 J1725 − // / webda / ) as well. The discrepancy between the ”observed” and expected number ofOB stars in Pismis 24 would be less severe if most massive stars residingin the cluster are binaries with mass ratio close to unity (cf. Clarke &Pringle 1992).2varamadze et al.: Bow shocks around young star clusters. II. NGC 6357 region sorption ratio R V = A V / E ( B − V ) = . ≃ . ff erentcalibration of stellar parameters. On the other hand, adopting theextinction law from Rieke & Rebofsky (1985), so that A K s = . E ( J − K s ) , (1)and using the 2MASS photometry, one finds a true distance mod-ulus of ≃ .
15 mag and a distance of 1.7 kpc. The discrep-ancy between the two distance estimates could be understood ifthe reddening towards Pismis 24 is anomalous, i.e. R V > . R V ≃ . R V = . A V using the rela-tionship A V = . E ( V − J ) , (2)which, according to Tapia et al. (1988), is valid across theGalaxy, irrespective of high R V values (see also Persi & Tapia2008). This distance is consistent with that to the H ii regionNGC 6334 ( ≃ . ∼ ◦ to the southwest from NGC 6357. It is believed that boththese H ii regions form a single high-mass star-forming complex(Russeil et al. 2010 and references therein). In the following weadopt a distance of 1.7 kpc for Pismis 24 (and the star-formingregion NGC 6357 as a whole) so that 1 ◦ corresponds to ≃
30 pc. − The second cluster, AH03 J1725 − ≃ . ◦
26 (or ≃ . − − Chandra observationsof NGC 6357, so that the actual extent and the stellar content ofthe cluster are still unknown. The presence of two very massivestars in the cluster, however, suggests that it should be as young( ∼ −
3. Search for bow shocks
A rich massive stellar content of Pismis 24 and the presenceof very massive (binary) stars in both clusters associated withNGC 6357 suggest that this star-forming region was very ef-ficient in producing runaway stars (e.g. Gvaramadze 2007;Gvaramadze, Gualandris & Portegies Zwart 2009a; Gvaramadze& Gualandris 2011). One can, therefore, expect to find someof the runaways via detection of their associated bow shocks –the natural attributes of supersonically moving stars (Baranov,Krasnobaev & Kulikovskii 1971; Weaver et al. 1977). The char-acteristic scale of a bow shock, l , depends on the number den-sity of the ambient medium, n , on the stellar mass-loss rate,˙ M , and on the stellar space velocity, v ∗ , as follows: l ∝ n − / , l ∝ ˙ M / and l ∝ v − ∗ (Baranov et al. 1971). The bow shocksare usually most prominent in the mid-infrared (e.g. van Buren& McCray 1988; van Buren, Noriega-Crespo & Dgani 1995; Paper I; Gvaramadze et al. 2010d, 2011b), but can also be de-tected in the optical (e.g. Kaper et al. 1997; Brown & Bomans2005; Paper I) and radio (Benaglia et al. 2010) wavebands. Itshould be noted that only a minority ( ≃
20 per cent) of run-away OB stars are associated with (detectable) bow shocks(van Buren 1993; van Buren et al. 1995; Hutho ff & Kaper2002; Gvaramadze et al. 2010d). The paucity of the bow-shock-producing stars is mostly due to the fact that the majority ofrunaway stars are moving through a low-density, hot medium,so that the emission measure of their bow shocks is below thedetection limit or the bow shocks cannot be formed at all be-cause the sound speed in the local interstellar medium is higherthan the stellar space velocity (Kaper, Comer´on & Barziv 1999;Hutho ff & Kaper 2002). Moreover, the bow shocks generated byrunaway stars receding from NGC 6357 and interacting with thedense material of the background (parent) molecular cloud (seeSect. 2) would be very compact and therefore hardly detectable,while those projected against the H ii region might be hiddenby its bright emission (see Fig. 1). From this it follows that theactual number of stars ejected from the clusters in NGC 6357could several times exceed the number of detected bow-shock-producing stars.To search for bow shocks around NGC 6357, we used thearchival data from the Mid-Infrared Galactic Plane Survey (Priceet al. 2001), the 24 and 70 Micron Survey of the Inner GalacticDisk with MIPS (MIPSGAL; Carey et al. 2009) and the prelim-inary data release of the Mid-Infrared All Sky Survey carried outwith the Wide-field Infrared Survey Explorer (WISE; Wright etal. 2010). The first survey, carried out with the Spatial InfraredImaging Telescope onboard the Midcourse Space Experiment ( MSX ) satellite, covers the entire Galactic plane within | b | < ◦ and provides images at 18 arcsec resolution in four mid-infraredspectral bands centred at 8.3 µ m (band A), 12.1 µ m (band C),14.7 µ m (band D), and 21.3 µ m (band E). The MIPSGAL sur-vey (carried out with the Spitzer Space Telescope ) mapped 278square degrees of the inner Galactic plane ( | b | < ◦ is coveredfor 5 ◦ < l < ◦ and 298 ◦ < l < ◦ and | b | < ◦ is covered for | l | < ◦ ) and provides 24 µ m images at 6 arcsec resolution. Thecurrent release of the WISE survey covers 57 per cent of the skyand provides images at four wavelengths: 3.4, 4.6, 12 and 22 µ m,with angular resolution of 6.1, 6.4, 6.5 and 12.0 arcsec, respec-tively. The advantage of the MSX and WISE surveys is that theycover the whole Galactic plane and extend to higher Galacticlatitudes than the MIPSGAL one, thereby allowing us to searchfor high-velocity runaways ejected at large angles to the Galacticplane (which could be o ff the region covered by the MIPSGALsurvey). On the other hand, the better angular resolution of theMIPSGAL survey could be vital for the detection of bow shocksgenerated by stars moving through the dense gas of the parentmolecular cloud. To search for possible optical counterparts tothe bow shocks and to identify their associated stars, we usedthe Digitized Sky Survey II (DSS-II; McLean et al. 2000).Assuming that the age of the star clusters in NGC 6357 is ∼ − ∼ −
50 km s − , one finds that stars leaving the clusters atthe very beginning of their dynamical evolution would be con-fined within ∼ − ◦ from NGC 6357. Thus, one can neglect thee ff ect of the Galactic gravitational potential on their trajectories.Correspondingly, one can expect that the bow shocks producedby the ejected stars would be directed away from their parent MIPS = Multiband Imaging Photometer for
Spitzer (Rieke et al.2004). 3varamadze et al.: Bow shocks around young star clusters. II. NGC 6357 region : : . : : . : : . Galactic longitude G a l a c t i c l a t i t ude EN Fig. 1. ◦ × ◦ µ m ( MSX band E) image of the H ii region NGC 6357 (containingthe star clusters Pismis 24 andAH03 J1725 − MSX and
Spitzer aremarked by large and smallcircles, respectively. The po-sition of the two most mas-sive stars in Pismis 24 (whichdefine the centre of the clus-ter) is indicated by a largediamond. The position of themost massive star in the clusterAH03 J1725 − − clusters, provided that there are no peculiar large-scale flows inthe interstellar medium through which the stars are moving.First, we searched for bow shocks using the MSX data.The search was carried out in a 12 ◦ wide area elongated alongthe Galactic plane and centred at the longitude of NGC 6357( l ≃ ◦ ). Along the Galactic latitude, the search was lim-ited by the MSX coverage ( | b | < ◦ ), so that potentially wewere able to detect bow shocks produced by stars leaving thecluster immediately after its formation and moving perpendicu-larly to the Galactic plane with a peculiar (tangential) velocityof ∼
100 km s − . The visual inspection of MSX µ m imagesrevealed five bow shocks (indicated in Fig. 1 by large circles).All of them have a clear arc-like structure that opens towardsNGC 6357 (Figs. 2-6), which suggests that these structures aregenerated by stars expelled from this star-forming region. Bowshock 4 has a prominent gap on its leading edge (Fig. 5; see alsobelow). All five bow shocks are most prominent at 21.3 µ m, al-though some of them are visible in other MSX wavebands aswell. Bow shock 3 has an obvious optical counterpart in theDSS-II (see Fig. 4), while bow shocks 1 and 5 are embeddedin H ii regions (the H ii region associated with bow shock 5 isknown as GAL 351.66 + MSX were covered by this sur-vey and we give their MIPS 24 µ m images in Fig. 7. We alsodiscovered two new bow shocks whose orientation is consis- a r c m i n N E
Fig. 2.
Left: MSX µ m image of bow shock 1. Right:
DSS-II(red band) image of the same field. The position of the bow-shock-producing star (star 1) is marked by a circle.tent with the possibility that their associated stars were ejectedfrom NGC 6357 (see Figs. 8 and 9 and compare them with Fig. 1,where the positions of these bow shocks are indicated by smallcircles). The angular extent of both bow shocks ( ∼
30 arcsec)is several times smaller than that of the bow shocks discoveredwith
MSX , which can be understood if the stars generating thesetwo bow shocks are either moving through the denser ambi-ent medium (e.g. the background molecular cloud) or / and haveweaker winds and higher peculiar velocities. Neither of thesebow shocks were detected in the optical range. a r c m i n NE Fig. 3.
Left: MSX µ m image of bow shock 2. Right:
DSS-II(red band) image of the same field. The position of the bow-shock-producing star (star 2) is marked by a circle. arcm i n N E
Fig. 4.
Left: MSX µ m image of bow shock 3. Right:
DSS-II(red band) image of the same field. The position of the bow-shock-producing star (star 3 or HD 319881) is marked by a cir-cle. a r c m i n NE Fig. 5.
Left: MSX µ m image of bow shock 4. Right:
DSS-II(red band) image of the same field. The position of the bow-shock-producing star (star 4) is marked by a circle. a r c m i n NE Fig. 6.
Left: MSX µ m image of bow shock 5. Right:
DSS-II(red band) image of the same field. The position of the bow-shock-producing star (star 5 or [N78] 34) is marked by a circle.
N E
Fig. 7.
From left to right, and from top to bottom: MIPS 24 µ mimages of bow shocks 1, 2, 3, and 5. The positions of the associ-ated stars are marked by circles. The orientation and the scale ofthe images are the same. " NE Fig. 8.
Left:
MIPS 24 µ m image of bow shock 6. Right:
DSS-II(red band) image of the same field. The position of the bow-shock-producing star (star 6) is marked by a circle. " NE Fig. 9.
Left:
MIPS 24 µ m image of bow shock 7. Right:
DSS-II(red band) image of the same field. The position of the bow-shock-producing star (star 7) is marked by a circle.The use of the WISE data did not result in discovery of newbow shocks around NGC 6357. All seven bow shocks detectedwith
MSX and
Spitzer are clearly visible in the WISE data aswell. In Fig. 10 we present images of bow shock 4 (not coveredby the MIPSGAL survey) at all four WISE wavelenghts, show-ing its curious fine structure (most prominent at 12 and 22 µ m). a r c m i n N E
Fig. 10.
From left to right, and from top to bottom: WISE 22,12, 4.6 and 3.4 µ m images of bow shock 4. The position of theassociated star (star 4) is marked by a circle. The orientation andthe scale of the images are the same.The possible origin of the gap in the leading edge of bow shock 4and the cirrus-like filaments around it are discussed in Sect. 6.A by-product of our search for bow shocks using the Spitzer data is the discovery of a compact arcuate nebula attached toone of the candidate OB stars revealed with
Chandra (namely,star 9 from Table 7 of Wang et al. 2007). The MIPS 24 µ m im-age of this nebula is saturated, so that we give in Fig. 11 the Spitzer
8, 4.5 and 3 . µ m images obtained with the InfraredArray Camera (IRAC; Fazio et al. 2004) within the GalacticLegacy Infrared Mid-Plane Survey Extraordinaire (GLIMPSE;Benjamin et al. 2003). The orientation and the small angularsize of the nebula (embedded in the more extended region ofinfrared emission known as IRAS 17207 − IRAS source can be in-terpreted as an H ii region created by the ionizing emission ofthe star (cf. Paper I; Gvaramadze et al. 2011b). Alternatively, thenebula could be a circumstellar (toroidal) shell, similar to thoseobserved around some evolved massive stars (see, e.g., Fig. 4 inGvaramadze, Kniazev & Fabrika 2010c for the IRAC 5.8 µ m im-age of a nebula around the candidate luminous blue variable starMN 56). The 2MASS photometry of the star, however, suggeststhat it could be a main-sequence late O-type star (see Sect. 4),so that we consider the bow-shock interpretation of the arcuatenebula as more preferable. Note that the candidate OB star asso-ciated with the nebula is separated from the centre of Pismis 24by only ≃ Spitzer is the discovery of three ring-like shells with centralpoint sources. We present these shells and discuss their originin Sect. 5.
N E a r c m i n Fig. 11.
From left to right, and from top to bottom: IRAC 8, 4.5and 3.6 µ m, and DSS-II (red band) images of the candidate bow-shock-producing star (star 8) embedded in the extended regionof infrared emission (known as IRAS 17207 −
4. Bow-shock-producing stars
Inspection of the DSS-II (red band) images suggested that thebow shocks discovered with
MSX and
Spitzer are associated withstars indicated in Figs. 2 – 9 by circles. The details of these starsare given in Table 1. Columns 2, 3, 5, and 6 give the equato-rial coordinates of the stars and their J - and K S -magnitudes (alltaken from the 2MASS catalogue; Skrutskie et al. 2006), whilecolumn 4 gives the visual magnitudes of the stars taken fromthe NOMAD catalogue (Zacharias et al. 2004). Columns 7 and8 give the K S -band interstellar extinction towards the stars andtheir absolute magnitudes in the same band derived under theassumption that the stars are located at the same distance asNGC 6357. Column 9 provides spectroscopic and photometric(enclosed in brackets) spectral types of the stars (see below).Using the SIMBAD database and the VizieR catalogue ac-cess tool , we found that two of the five bow shocks discov-ered with MSX are generated by known OB stars (with spec-troscopically derived spectral types). Namely, it turns out thatbow shocks 3 and 5 are produced by the O6 Vn (Drilling &Perry 1981) star HD 319881 and the O8 / B0 III / V (Neckel 1984)star [N78] 34, respectively. To determine the nature of the otherthree stars (hereafter stars 1, 2 and 4), we obtained their spectrawith the South African Astronomical Observatory (SAAO) 1.9-m telescope (see Sect. 4.1). The stars associated with the bowshocks discovered with
Spitzer (hereafter stars 6 and 7) still haveto be observed spectroscopically. If, however, we assume thatthese two stars are located at the same distance as NGC 6357,then one can estimate their spectral types using the 2MASSphotometry and calibration of absolute magnitudes and intrin-sic colours (see Sect. 4.2). http: // simbad.u-strasbg.fr / simbad / http: // webviz.u-strasbg.fr / viz-bin / VizieR6varamadze et al.: Bow shocks around young star clusters. II. NGC 6357 region
Table 1.
Details of seven bow-shock-producing stars (stars 1–7) and one candidate bow-shock-producing star (star 8) detectedaround NGC 6357. The last column provides spectroscopic and photometric (enclosed in brackets) spectral types of the stars (seetext for details).
Star RA (2000) Dec. (2000)
V J K s A K s M K s Spectral type1 17 27 11.23 −
34 14 34.9 11.77 8.42 7.84 0.52 − .
84 O7.5: a (O7 V)2 17 22 03.43 −
34 14 24.1 13.59 8.94 8.01 0.75 − .
89 O5.5: a (O6.5-7 V)3 or HD 319881 17 28 21.67 −
34 32 30.3 10.14 7.54 7.10 0.43 − .
48 O6 Vn b (O5 V)4 17 18 15.40 −
34 00 06.1 11.80 8.19 7.53 0.57 − .
19 O6.5: a (O6 V)5 or [N78] 34 17 22 05.62 −
35 39 55.5 13.11 8.68 7.82 0.71 − .
04 O8 / B0 III / V c (O6.5 V)6 17 22 50.02 −
34 03 22.4 13.90 9.68 8.81 0.71 − .
05 (B0 V)7 17 27 12.53 −
33 30 40.0 12.18 9.21 8.65 0.51 − .
01 (B0 V)8 17 24 05.62 −
34 07 09.5 11.74 9.03 8.46 0.51 − .
20 (O9-9.5 V)
Notes. ( a ) this work. ( b ) Drilling & Perry (1981). ( c ) Neckel (1984).
The observations of stars 1, 2 and 4 were performed with theCassegrain spectrograph using a long slit of 3 ′ × ′′ on 2009June 1 and 2. Grating with 300 lines mm − was used with the266 × ∼ − ∼ − ∼ − (FWHM ∼ . ′′ − . The see-ing during the observations varied from 1.0 to 2.1 arcsec fromobject to object, but was stable for each object. Exposure timeswere 3 × ×
600 and 3 ×
300 sec for stars 1, 2 and 4, respec-tively. The observations of star 2 were made in variable condi-tions, with drastic drops in transparency, resulting in the worsequality of the resulting spectrum. Spectra of Cu–Ar comparisonarcs were obtained to calibrate the wavelength scale. The spec-trophotometric standard stars LTT 7379 and LTT 3864 were ob-served with a slit width of 5 arcsec at the beginning and at theend of the nights for flux calibration. Data reduction was per-formed using standard procedures (see, e.g., Kniazev et al. 2008for details).Figure 12 presents the resulting normalized spectra of stars 1,2 and 4 in the λλ − i and He ii ,which is typical of O-type stars. Si iv λ iii λ iii λ ff use interstellarbands (DIBs) are present in the spectra. A blend of strong Na i λ ,
96 absorption lines is of circumstellar or interstellar ori-gin, while the strong absorptions visible at λ > i λ ii λ ± iii λ iv λ i λ iv λ λλ − i λ ii λ = (9 . ± . + (4 . ± .
23) log EW / EW . (3)Using Eq. (3) and EWs given in Table 2, we found SpT ≃ . andEW (Kerton et al. 1999):SpT = (4 . ± . + (7 . ± . , (4)SpT = (12 . ± . − (7 . ± . . (5)From Eqs. (4) and (5), we found SpT ≃ . ff er from the large errors of measured EWs (seeTable 2).The lack of an exact spectral classification does not allowus to compare the spectroscopic distances to stars 1, 2 and 4with the distance to NGC 6357. Instead, we use the assumptionthat these stars were ejected from NGC 6357 to constrain theirspectral types and luminosity classes. Using Eq. (1) and adopt-ing the intrinsic colour, for O stars, ( J − K s ) = − .
21 mag(Martins & Plez 2006), we calculated the K s -band extinction, A K s , towards these stars and their absolute magnitudes M K s (seeTable 1). Then using the calibration of absolute infrared magni-tudes for Galactic O stars by Martins & Plez (2006), we foundthat all three stars should be on the main-sequence with spectraltypes of O7 V, O6.5–7 V, and O6 V, which agree well with thosederived from spectroscopy (see Table 1, where the photometricspectral types are enclosed in brackets). Similarly, assuming that the other bow-shock-producing starsaround NGC 6357 are located at the same distance as this H ii Fig. 12.
Normalized spectra of stars associated with bow shocks 4 (top), 1 (middle) and 2 (bottom) with principal lines and mostprominent DIBs indicated. The spectra of stars 4 and 2 are shifted upwards and downwards by ≃ . Table 2.
Equivalent widths (EWs; in Å) of absorption lines used for spectral classification of bow-shock-producing stars 1, 2 and 4.
Star Si iv λ i λ i λ ii λ i λ ii λ ± ± ± ± ± ± ± ± ± ± ± ± ± ± region, we calculated A K s and M K s for these stars and estimatedtheir photometric spectral types (see Table 1). For stars 3 and 5we found spectral types of O5 V and O6.5 V, respectively. Theformer estimate agrees reasonably well with the spectroscopi-cally derived spectral type, while the latter one is inconsistentwith the spectral classification reported by Neckel (1984). Wenote, however, that the spectral types given by Neckel (1984)are systematically later than those derived from the more re-cent observations (Massey et al. 2001; Damke et al. 2006). Forexample, one of the brightest (and most massive) stars in thecluster AH03 J1725 − In Table 3 we provide two representative proper motion mea-surements for each bow-shock-producing star, found withVizieR. For most stars we used the measurements from themost recent catalogues, namely UCAC3 (Zacharias et al. 2010)and PPMXL (R¨oser, Demleitner & Schilbach 2010). When theUCAC3 proper motion is marked “with doubts” (because it re-lies on “less than two good matches”), we used the data from theNOMAD catalogue (Zacharias et al. 2004). Although most mea-surements are insignificant (i.e. the measurement uncertaintiesare comparable to the measurements themselves), it is of inter-
Table 3.
Proper-motion measurements for the seven bow-shock-producing stars (stars 1–7) and one candidate bow-shock-producingstar (star 8) around NGC 6357. Two measurements are given for each star to indicate the uncertainties in the measurements. Foreach data set, the peculiar (transverse) velocities (in Galactic coordinates) were calculated and added to the table. A likely parentcluster for each star is indicated in the last column.
Star µ α cos δ µ δ Ref. v l v b Parent clustermas yr − mas yr − km s − km s − − . ± . − . ± . − . ± . . ± . − − . ± . − . ± . − . ± . . ± . − . ± . − . ± . − . ± . . ± . − . ± . − . ± . − . ± . . ± .
33 (HD 319881) 0 . ± . − . ± . − . ± . − . ± . . ± . − . ± . . ± . − . ± . − . ± . . ± . − . ± . . ± . / AH03 J1725 − − . ± . . ± . − . ± . . ± .
65 ([N78] 34) − . ± . . ± . . ± . . ± . − − . ± . . ± . . ± . . ± . − . ± . . ± . − . ± . . ± . − − . ± . . ± . − . ± . . ± . − . ± . . ± . − . ± . . ± . − . ± . − . ± . − . ± . . ± . − . ± . . ± . − . ± . . ± . − . ± . − . ± . − . ± . . ± . References. (1) UCAC3 (Zacharias et al. 2010); (2) PPMXL (R¨oser et al. 2010); (3) NOMAD (Zacharias et al. 2004). est to check whether or not the orientation of the stellar peculiarvelocities implied by these measurements is consistent with thedistribution of the stars on the sky and with the orientation oftheir associated bow shocks.To convert the observed proper motions into the transversepeculiar velocities of the stars, we used the Galactic constants R = . Θ =
240 km s − (Reid et al. 2009) andthe solar peculiar motion ( U ⊙ , V ⊙ , W ⊙ ) = (10 . , . , .
2) km s − (McMillan & Binney 2010). The derived velocity components inGalactic coordinates are given in columns 5 and 6 of Table 3. Forthe error calculation, only the errors of the proper motion mea-surements were considered. Obviously, the orientation of pecu-liar velocities of stars 1, 2, 3, 4, 6 and 8 are consistent (withintheir margins of error) with the orientation of their bow shocksand with the possibility that these stars are moving away fromNGC 6357. For stars 5 and 7 we found that their peculiar ve-locities have ”wrong” orientations, i.e. are inconsistent with theorientation of the associated bow shocks.Because of the low significance of proper motion measure-ments, we rely only on the orientation of the bow shocks in theidentification of parent clusters for bow-shock-producing stars.We caution, however, that the close proximity of the clusters onthe sky and the absence of proper motion measurements for theclusters make the identification ambiguous. Thus, the likely par-ent clusters indicated in the last column of Table 3 should beconsidered as tentative.
5. Three circular shells
As we mentioned in Sect. 3, one of the by-products of the searchfor bow shocks around NGC 6357 with
Spitzer is the discov-ery of three circular shells. The positions of these shells on thesky are marked in Fig. 1 by crosses. The appearance of all threeshells (see Figs. 13– 15) is typical of circumstellar shells createdby luminous blue variable (LBV) stars and Wolf-Rayet stars oflate WN-types (WNL) (see Gvaramadze et al. 2010c for
Spitzer images of such shells).One of the shells (hereafter shell 1) is located at only ≃ . ◦ ≃
18 pc in projection) to the northwest of NGC 6357. " NE Fig. 13.
Left : MIPS 24 µ m image of shell 1. Right : 2MASS K s band image of the same field with the position of the geometriccentre of shell 1 indicated by a circle. " NE Fig. 14.
Left : MIPS 24 µ m image of shell 2. Right : IRAC 8 µ mimage of the same field.The MIPS 24 µ m image of this shell (see left panel of Fig. 13)shows a clear ring-like structure with a diameter of ≃
25 arcsecand enhanced brightness along the southeast rim faced towardsNGC 6357. Fig. 13 also shows the central point-like source be-ing smeared-out somewhat because of the 6 arcsec resolution ofthe MIPS 24 µ m image. We interpret this source as a massiveevolved star physically related to the shell. Many dozens of sim-ilar evolved stars were recently revealed through the detection a r c m i n N E
Fig. 15.
Left and
Middle : MIPS 24 µ m and WISE 22 µ m images of shell 3, respectively. The MIPS image is truncated because theMIPSGAL survey covers only a part of the shell. Right : DSS-II (red band) image of the same field. The central star of shell 3 ismarked by a circle.
Table 4.
Circular shells around NGC 6357 and their central stars.
Shell RA (2000) Dec. (2000) V [3 .
6] [8 .
0] Diameter (arcsec)1 17 22 36.7 −
33 49 10 – – – 252 17 26 19.11 a −
35 32 37.7 a – 13.67 a a
403 17 20 31.76 b −
33 09 48.9 b b – – 210 Notes. ( a ) GLIMPSE Source Catalog (http: // irsa.ipac.caltech.edu / applications / Gator / ). ( b ) NOMAD (Zacharias et al. 2004). of their infrared circumstellar shells with
Spitzer and follow-upspectroscopy (Gvaramadze et al. 2009b, 2010a,b,c; Wachter etal. 2010, 2011). The DSS-II and 2MASS images, however, donot show any counterpart to the putative central star of shell 1.The most plausible explanation of the absence of the 2MASScounterpart is that the central star is highly absorbed by the fore-ground dusty material. In this connection, we note that the cen-tral stars of some circumstellar shells presented in Gvaramadzeet al. (2010c) also cannot be seen in the 2MASS images, whilethey are visible in the IRAC (3.6, 4.5, 5.8 and 8.0 µ m) ones.Unfortunately, shell 1 was not covered by the GLIMPSE sur-vey, so that at the moment we cannot prove the existence of thecentral source that is apparently visible at 24 µ m. Deep infraredimaging of shell 1 is therefore highly desirable. Detection of thecentral star and its follow-up spectroscopy would allow us to un-veil the nature of the shell and potentially link the star to thestar-forming region NGC 6357.If one assumes that shell 1 is produced by a massive starejected from NGC 6357, then the linear size of the shell is ≃ . ≃ − ≃ − K s magnitude of the 2MASS survey of 14.8and the K s -band absolute magnitude of LBV and WNL stars of ≃ − − A K s > ∼ −
13 mag (or A V > ∼ ≃ . ◦ µ m (rightpanel) and IRAC 8 µ m images of this shell. The ring-like struc-ture of shell 2 is more obvious in the 8 µ m image, from whichwe measured a diameter of the shell of ≃
40 arcsec ( ≃ . µ m, but also in all four IRAC bands. We also founda weak counterpart to this star in the 2MASS K s -band image,which, however, is not listed in the 2MASS All-Sky Catalog ofPoint Sources (Skrutskie et al. 2006).The third shell (hereafter shell 3) is located at ≃ . ◦ ≃ . µ m image (middle panel of Fig. 15), whichshows an almost circular limb-brightened nebula and its centralstar (visible also in the DSS-II image; right panel of Fig. 15).Using the SIMBAD database, we identified the central star withthe emission-line star Hen 3-1383, classified in the literature asan Me (The 1961; Sanduleak & Stephenson 1973; Allen 1978)or a candidate symbiotic (Henize 1976) star. Our recent opticalspectroscopy of Hen 3-1383 with the Southern African LargeTelescope (SALT), however, showed that the spectrum of thisstar is almost identical to those of the prototype LBV P Cygni(see, e.g., Stahl et al. 1993) and the recently discovered candi-date LBV MN 112 (Gvaramadze et al. 2010b), which implies theLBV classification for Hen 3-1383 as well (Gvaramadze et al., inpreparation).The details of the three shells are summarized in Table 4. Inthis table we give approximate coordinates of the geometric cen-tre of shell 1 and the coordinates of the central stars associatedwith two other shells. For the central star of shell 2 we give itsIRAC 3.6 and 8.0 µ m magnitudes (taken from the GLIMPSE Source Catalog ), while for the central star of shell 3 we give itsvisual magnitude from the NOMAD catalogue. The last columnof the table contains the angular size of the shells.
6. Discussion
We explored the idea that young star clusters lose a significantfraction of their massive stars because of gravitational few-bodyinteractions between the cluster members. The e ffi ciency of dy-namical star ejection depends mainly on the star number den-sity in the cluster core, which is maximum either already duringcluster formation or becomes maximum later on owing to theSpitzer’s mass segregation instability (Spitzer 1969). Mass seg-regation could be a rapid ( < ∼ . ff ective well before the clusterstarts to expel its members via binary-supernova explosions (thelatter process becomes e ffi cient several Myr after the formationof the first massive binary stars).The runaway OB stars spread all around the parent clusterconstitute the population of massive field stars. It is believedthat most, if not all, field OB stars were formed in the clusteredway (Lada & Lada 2003; Smith, Longmore & Bonnell 2009)and subsequently found themselves in the field either becauseof the dynamical processes in the parent clusters or because ofrapid cluster dissolution (e.g., Kroupa & Boily 2002; de Witet al. 2005; Schilbach & R¨oser 2008; Gvaramadze & Bomans2008b; Pflamm-Altenburg & Kroupa 2010). Detection of mas-sive stars in the field around young clusters and linking them tothese clusters is therefore important not only for the problem ofthe origin of field OB stars, but also for the problem of massivestar-formation in general.The onset of OB star ejection from a young cluster verymuch depends on whether the massive stars form as tight bina-ries near the cluster centre or not (Clarke & Pringle 1992). ThusN-body experiments can be performed with di ff erent assump-tions on the OB stellar population. To see which of the theo-retical (numerical) models of cluster dynamical evolution bettercorrespond to observations, one should identify as many starsejected from a given star cluster as possible. The detected run-away stars would constitute a sample for the future high-preciseproper motion and parallax measurements with the space as-trometry mission Gaia , which are required to determine the tim-ing of ejections (e.g. Paper I) and thereby to constrain the initialconditions for young star clusters (Kroupa 2008) and to distin-guish between the primordial and the dynamical origin of masssegregation in these clusters.In the absence of reliable proper motion measurements fordistant and / or highly obscured field OB stars, the only viablepossibility to prove their runaway status and to identify their par-ent clusters is through the detection of bow shocks that are pos-sibly generated by these stars . The characteristic scale of bowshocks is determined (for the given number density of the ambi-ent interstellar medium and the peculiar velocity of the star) bythe strength of the stellar wind, which makes the massive starsprone for producing detectable bow shocks. On the other hand,the fact that dynamical star ejection is most e ffi cient in mass- Available at http: // irsa.ipac.caltech.edu / applications / Gator / . The runaway status of some field OB stars can also be confirmed viameasurement of their high peculiar radial velocities, but this channel fordetection of runaways does not allow us to determine the star’s directionof motion on the sky and therefore cannot be used to identify their birthclusters. segregated clusters (Oh et al., in preparation) implies that duringthe early dynamical evolution of these clusters the ejected starsare preferentially the most massive ones. This inference is con-sistent with the observational fact that the percentage of O starsamong the runaways is higher than that of B stars (e.g. Stone1991) and could also be derived from the existence of an ex-tended (tens of parsecs) halo of very massive (O2-type) starsaround the very young ( ∼ − ff ect of stellarwinds and ionizing emission can create bow shocks around (low-velocity; not necessarily massive) stars in the cluster’s halo. Inthis case, the bow shocks are facing towards the cluster’s centre.Numerous examples of such bow shocks were detected aroundyoung clusters and OB associations: Orion Cluster (Bally, ODell& McCaughrean 2000), Trumpler 14 (Ascenso et al. 2007), M17and RCW 49 (Povich et al. 2008), Cyg OB2 (Kobulnicky, Gilbert& Kiminki 2010). One can also imagine the situation when aslowly ( ∼
10 km s − ) moving massive star encounters a densemedium (e.g. the remainder of the parent molecular cloud) andits wind starts to interact with the ionized gas outflowing fromthe cloud surface towards the star with a velocity comparableto the sound speed ( ≃
10 km s − ). In the reference frame ofthis gas, the stellar motion is supersonic so that a bow shockis formed ahead of the (slowly) moving star. We speculate thatbow shock 1 was probably generated in this way. This possibil-ity is suggested by the small separation ( ≃
10 pc in projection)of star 1 from its likely birth cluster AH03 J1725 − −
10 km s − , provided that it was ejected from the cluster ∼ − ii region, from which one can infer that star 1 hasmet and ionized a density enhancement on its way.Finally, we discuss the origin of the gap at the leading edgeof bow shock 4 and the curious cirrus-like filaments around it. InGvaramadze et al. (2011b), we suggested that similar cirrus-likefilaments around the bow shock generated by the high-mass X-ray binary Vela X-1 (see Fig. 2 in Gvaramadze et al. 2011b) arecaused by interstellar dust grains aligned with the local interstel-lar magnetic field and heated by the radiation of Vela X-1. Thissuggestion implies that bow shock 4 propagates along the localinterstellar magnetic field, which a ff ects the compression ratio ofthe shocked gas by making it less dense along the flanks of the NE a r c m i n Fig. 16.
MIPS 24 µ m image of the bow shock generated by theBC0.7 Ia star HD 2905 and cirrus-like filaments around it (seetext for details).bow shock (e.g. Draine & McKee 1993). The density asymme-try in turn could a ff ect the heating of the dust grains accumulatedbehind the front of bow shock 4 and thereby could be responsi-ble for its brightness asymmetry. In the absence of reliable dataon parameters of bow shock 4 and its associated star we refrainfrom a more detailed discussion of the problem. We note, how-ever, that similar gaps are typical of many other bow shocks.One of them (although less impressive and not at the symmetryaxis of the bow shock) is shown in Fig. 16, which presents theMIPS 24 µ m image (Program Id.: 30088, PI: A.Noriega-Crespo)of a bow shock produced by the BC0.7 Ia (Walborn 1972) starHD 2905. Like bow shock 4, the bow shock around HD 2905 issurrounded by cirrus-like filaments, some of which are appar-ently attached to the front of the bow shock near the gap. Toconclude, if our suggestion on the origin of gaps is correct, thenthe cirrus-like filaments around bow shocks could serve as trac-ers of the local magnetic field, while the bow-shock-producingstars could serve as probes of the Galactic magnetic field.
7. Summary
Search for bow-shock-producing stars around the star-formingregion NGC 6357 with two embedded young ( ∼ − − Acknowledgements.
We are grateful to the referee for comments that allowedus to improve the presentation of the paper. VVG acknowledges financial sup-port from the Deutsche Forschungsgemeinschaft. AYK acknowledges supportfrom the National Research Foundation of South Africa. This research has madeuse of the NASA / IPAC Infrared Science Archive, which is operated by the JetPropulsion Laboratory, California Institute of Technology, under contract withthe National Aeronautics and Space Administration, the SIMBAD database andthe VizieR catalogue access tool, both operated at CDS, Strasbourg, France.
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