Massive Runaway Stars in the Large Magellanic Cloud Sive (O2-Type) Stars Is Located at 100 200 Pc in Projection Table 1

arXiv:1006.0225v2 [astro-ph.GA] 10 Jun 2010 hs rprmtosaesilntaalbeo esrdwit measured or available st not OB still field are those motions for proper even whose clusters parent dete possible thereby m the 1995), stellar mining Dgani of & detected direction Noriega-Crespo the of Buren, infer (Van geometry to tion one The allow 1988). would superson shocks McCray of bow Kulikovsk & attributes & Buren detec- Krasnobaev natural Van via Baranov, the 1971; (e.g. proved – objects be shocks moving can bow cally stars their OB of field most tion Alternativel the that clusters. of in show nature formed to runaway are approach stars this OB Röser field of Galactic & clusters use Schilbach extensive parent 2001). make Zeeuw to de (2008) orbits & their d Bruijne kinematic back-trace de available Hoogerwerf, to the use stars to runaway these is the stars on OB prove field to 1991; the way of Stone nature 1986; straightforward ejec Bolton A stars & 2003). Gies runaway Zinnecker 1961; likely (Blaauw transve cluster most or a therefore from radial are (either and peculiar field Galactic velocities measured Some OB high 1995). al. and have 1990; et stars Garmany clusters Massey 1983; 1982; known Conti Chiosi & sep- from Massey & are parsecs Conti which (Garmany, o of of associations some population hundreds field, significant by the in a arated exists associations, stars massive their OB in young and situated are clusters stars massive ent of majority the Although Introduction 1. coe 4 2018 14, October srnm Astrophysics & Astronomy hthv enpooe scniaernwysas sn ar Using stars. runaway candidate as proposed shoc w been paper bow have this detected that In clusters. of parent geometry possible their The determining stars. runaway by produced ihoeo u rgam tr,teO (f) trB 3,wh 237, BI star V((f*)) O2 the stars, programme our of one with eue natraieapoc o ovn h rbe (firs problem the solving for approach alternative an used We prahi,hwvr opiae yteln itnet th to distance movi long are the they by that show complicated however, to and is, stars approach ev field dynamical the cluster’s of parent motions their proper of beginning very the at ftebwsoksget htB 3 a jce rmteO as OB the from ejected was 237 BI that suggests shock bow the of ypouto u erhi h eeto fbwsok gener shocks bow of detection the is search our of by-product osblt htterascae tr eeeetdfo t from ejected 269 were (HD stars R81 associated variable their that blue possibility luminous candidate the to attached eevd2 pi 2010 April 27 Received in:idvda:L 2–oe lsesadascain:i associations: and clusters open – 82 LH individual: tions: 2 3 h rgno asv edsasi h ag aelncClou Magellanic Large the in stars o field massive of origin The 1 eoiypoie osbeepaaino hseim and enigma this of explanation possible a provides velocity o h rbe fteoii frnwysasadteerydy early the and words. stars Key runaway of origin the of problem the for ff trbr srnmclIsiue ocwSaeUniversit e-mail: State Moscow Institute, Astronomical Sternberg sa etnIsiueo hl,Mso rnh Universite Branch, Moscow Chile, of Institute Newton Isaac e-mail: Universität Bonn, für Astronomie, Argelander-Institut es( sets ∼ asv uaa tr nteLreMglai Cloud Magellanic Large the in stars runaway Massive [email protected](VVG) [email protected] [email protected] P) [email protected]. (PK); [email protected] 0 ms km 100 tr:knmtc n yais–sas niiul I27– 237 BI individual: stars: – dynamics and kinematics Stars: − 1 ewe h eicnrcrda eoiiso oevr ma very some of velocities radial heliocentric the between ) / cetd2 a 2010 May 26 Accepted aucitn.LMC2 no. manuscript V.V. Gvaramadze 1 , 2 , 3 .Kroupa P. , e3 oau trfrigcmlx edsusimplication discuss We complex. star-forming Doradus 30 he rsn h eut fasac o o hcsaon i mas six around shocks bow for search a of results the present e u e ¨gl7,511Bn,Germany Bonn, 53121 Hügel 73, dem Auf dvda:R3 H 38268) (HD R136 ndividual: ,the y, lto.Asrihfradwyt rv hsepaainis explanation this prove to way straightforward A olution. ugssta h edsasaernwysaseetdfo t from ejected stars runaway are stars field the that suggests (e.g. ABSTRACT ,UiesttkjP.1,Mso 192 Russia 119992, Moscow 13, Pr. Universitetskij y, par- rse) a h ted ars ata chival M,wihmksacrt rprmto esrmnsdi measurements motion proper accurate makes which LMC, e tdb orO tr ntefil fteLCada r-iestru arc-like an and LMC the of field the in stars OB four by ated aia vlto fsa clusters. star of evolution namical ple o aatcfil tr) ae ntesac o bo for search the on based stars), field Galactic for applied t o- si r 3 ocw199,Russia 119992, Moscow 13, Pr. tskij LC a ogbe neim.Tercn esrmnso la of measurements recent The enigma. an been long has (LMC) d 2) h emtyo w fteebwsok scnitn wi consistent is shocks bow these of two of geometry The 128). r- i- gaa rmoeo h erysa lseso Bassociation OB or clusters star nearby the of one from away ng ii f c stefis-vrdtcino o hcsi h M.Orien LMC. the in shocks bow of detection first-ever the is ich swudalwu oifrtedrcino tla oin th motion, stellar of direction the infer to us allow would ks oito H8 lctdat (located 82 LH sociation hcsi h M sn archival using tele- LMC the infrared in shocks modern with resolved be still scopes. can shocks bow itneo 0kcfrteLC(isn20)s ht1 that so 2000) us (Gibson We LMC 4. dynami- Sect. the early for in kpc discussed the 50 are and of clusters stars distance the star runaway around of of evolution located origin cal for are the findings stars our of of these problem Implications of 3). (Sect. our two nebula Doradus of i and 30 other by-product stars, several A by OB produced 237. shocks BI lated one bow star of with detection veloc V((f*)) associated O2 is radial search shock the peculiar bow stars, large a these qualifie iso- their discovered of been We by of 2). previously basis (Sect. produced have the ties shocks on that bow runaways stars detect as massive to very was lated, goal prime Our ta.(95 a hw htalrenme fyugvr mas- very young of number large a Massey that by shown LMC the has in (1995) population al. star et massive the of study The runaways as stars field massive Very 2. pnsto sponds rudfil Bsasi h ag aelncCod(LMC) Cloud Magellanic di Large are shoc measurements the bow motion for in proper search accurate stars where to OB tempting therefore field is around Gvaramadze It 2010b). 2008b; al. Bomans et & (Gvaramadze significance low pte pc Telescope Space Spitzer 1 n .Pflamm-Altenburg J. and , nti ae epeetterslso erhfrbow for search a of results the present we paper this In tr:idvda:H 618–oe lsesadassocia- and clusters open – 269128 HD individual: stars: ≃ e(JP-A) de 4pc. 14 sv O-ye edsasadtessei LMC systemic the and stars field (O2-type) ssive ≃ aa efudabwsokassociated shock bow a found we data, 2 ci rjcinfo h tr.A star). the from projection in pc 120 1 pte pc Telescope Space Spitzer forfindings our of s erbirthplaces heir omauethe measure to iefil stars field sive shocks w

c ffi .This s. ffi S 2018 ESO tation hthe th ereby ut while cult, cture cult. rge ′ corre- data. the so- a e ks i- d 1 V.V. Gvaramadze et al.: Massive runaway stars in the Large Magellanic Cloud sive (O2-type) stars is located at 100 200 pc in projection Table 1. Summary of candidate runaway stars in the LMC. from star clusters and OB associations.∼ This− finding was inter- 1 preted as indicating that the field can produce stars as massive as Star Spectral type v ( km s− ) those born in clusters (Massey et al. 1995). An obvious alterna- N11-026 O2.5 III(f*) 35(a) ≃ (b) tive to this interpretation is that the massive field stars were actu- Sk 67◦22 O2If* 150 ally formed in a clustered environment and subsequently ejected BI237− O2V((f*)) ≃ 120(b) from their birth sites via dynamical processes (e.g. Clarke & 30Dor016 O2III-If* ≃ 85(c) ≃ (d) Pringle 1992; Pflamm-Altenburg & Kroupa 2006); i.e., the field BI253 O2V((f*)) 55 ∼ (d) Sk 68◦137 O2III(f*) 100 massive stars are runaway stars (Walborn et al. 2002; Brandl et − ∼ al. 2007). The large separations from the possible parent clus- (a) (b) (c) ters and the young ( 2 Myr) ages of the very massive field Notes. Evans et al. 2006. Massey et al. 2005. Evans et al. 2010. (d) Walborn et al. 2002. stars imply that their (transverse)∼ velocities should be as high as 1 50 100 kms− (Walborn et al. 2002). The large offsets from ∼the parent− clusters and the high peculiar velocities are not un- under the assumption that both stars were ejected 2 Myr ago usual for Galactic massive runaway stars. For example, the O4If from R136 (Walborn et al. 2002). ∼ star BD+43 3654 ejected from the CygOB2 association is lo- ◦ From our experience in the search for bow shocks produced cated at about 80 pc from the core of the association (Comerón& by OB stars ejected from Galactic star clusters (Gvaramadze & Pasquali 2007; Gvaramadze & Bomans 2008a), while the helio- Bomans 2008a,b; Gvaramadze et al. 2010b) using the archival centric radial velocity of the star is offset by56 kms 1 from the − data from the Midcourse Space Experiment (MSX) satellite systemic velocity of Cyg OB2 (Kobulnicky, Gilbert & Kiminki (Price et al. 2001) and the Spitzer Space Telescope (Werner et 2010), which with the transverse peculiar velocity of the star of al. 2004), we know that the bow shocks are visible mostly in 40 kms 1 (Comerón & Pasquali 2007) corresponds to the to- − 21.3 µm(MSX band E) images and 24 µm images obtained with tal≃ peculiar velocity of 70 kms 1. − the Multiband Imaging Photometer for Spitzer (MIPS; Rieke et The runaway interpretation≃ of the massive field stars in the al. 2004). The resolution of Spitzer 24 µm images ( 6 ) is three LMC received strong support after the discovery that some of ′′ times better than those of the MSX, so that in the search∼ for bow them have high peculiar radial velocities, much greater than the shocks in the LMC we utilized the MIPS data alone. systemic velocity of the LMC (Massey et al. 2005; Evans et al. The typical (transverse) size of bow shocks generated by 2006, 2010). Earlier, Nota et al. (1994) and Danforth & Chu Galactic OB stars (i.e. the extent of a bow shock in the direction (2001) had found that the systemic velocity of the candidate lu- perpendicular to the vector of the stellar motion) is several par- minous blue variable S119 (HD269687) and its circumstellar secs; e.g. the size of the bow shocks associated with the above- nebula is 100 kms 1 lower than that of the LMC, so they sug- − mentioned massive runaway stars, BD+43 3654 and λ Cep, is gested that∼ S119 could be a runaway star. ◦ 5.0 and 2.3 pc, respectively. If placed at the distance of the To prove the runaway nature of the field massive stars in LMC,≃ these bow shocks will have an angularsize of 10 20 , the LMC unambiguously, it is necessary to ascertain their par- ′′ ′′ which is comparable to or several times greater than≃ the angu− lar ent clusters. The proper motion measurements cannot help solve resolution of the MIPS 24 µm images. Thus, the bow shocks in this problem in the near future. At the distance of the LMC, the LMC can be resolved with the Spitzer imaging data! the transverse velocity of 50 100 kms 1 corresponds to a − Visual inspection of MIPS 24 µm images1 of fields contain- proper motion of 0.2 ∼0.4masyr− 1, which is too low to be − ing our programme stars revealed a bow shock associated with measured with high∼ confidence− using the ground-based obser- only one of them, namely BI237. The non-detection of bow vations. The simpler solution is to infer the direction of stellar shocks around the remaining five programme stars is consis- motion via the geometry of bow shocks produced by runaway tent with the observational fact that only a small fraction (< 20 stars (Gvaramadze & Bomans 2008b) or, in the case of evolved per cent) of runaway OB stars produce (observable) bow shock∼ s massive runaway stars, through the asymmetry in the bright- (Gvaramadze & Bomans 2008b and references therein). ness distribution of associated circumstellar nebulae (Danforth & Chu 2001; Gvaramadzeet al. 2009). It is worth notingthat two Figure1 gives an overview of the region northwest of BI237 with two associations, LH88 and LH82, whose centres are sep- very massive Galactic runaway stars, BD+43 3654 and λ Cep ◦ arated in projection by 65 and 120 pc from the star. (The ap- (O6I(n)f; Walborn 1973), are both associated with spectacular ≃ bow shocks (Comerón & Pasquali 2007; Van Buren et al. 1995). proximate boundariesof the associations are indicated by dashed circles; Bica et al. 1999.) The orientation of the bow shock One can therefore expect that some of the field stars in the LMC will manifest themselves in such secondary attributes of high- generated by BI237 (see Fig.2) suggests that the more likely velocity runaway stars. parent association of the star is LH82. LH82 contains another very massive star, Sk 67◦ 211 (O2III(f*); Walborn et al. 2004). Assuming that BI237− was indeed ejected from LH82 and given 3. Search for bow shocks in the LMC the young ( 2 Myr) age of the star, one finds that its transverse ∼ 1 velocity should be 60 kms− if the star escaped from the core To search for bow shocks, we selected four isolated massive stars of LH82 soon after∼ the birth or higher if the ejection event oc- with high peculiar radial velocities (Massey et al. 2005; Evans et curred later on, so that the total peculiar velocity of the star is al. 2006, 2010) and added two isolated O2-type stars, BI253 130 kms 1. The angularsize of the bow shock of 20 corre- (O2V((f*))) and Sk 68 137 (O2III(f*)), which Walborn et ≥ − ≃ ′′ − ◦ sponds to the linear size of 4.8 pc, i.e., a figure typical of mas- al. (2002) suggests are runaways owing to their large separa- sive runaway stars (see above).≃ Using these estimates, one can tion from their plausible birthplace in the central cluster, R136 constrain the number density of the ambient interstellar medium, (HD38268), of the 30Doradus nebula. The details of these stars (listed in order of their RA) are summarized in Table1. For the 1 The images, obtained in the framework of the Spitzer Survey of the first four stars, we give their peculiar radial velocities, while the Large Magellanic Cloud (Meixner et al. 2006), were retrieved from the transverse velocities are listed for the remaining two, inferred NASA/IPAC Infrared Science Archive (http://irsa.ipac.caltech.edu).

2 V.V. Gvaramadze et al.: Massive runaway stars in the Large Magellanic Cloud

Table 2. Details of bow shock-producing stars. N LH 82 Star Spectral type Association (a) E Sk 66◦ 16 O9.7 Ib KMHK 268 − (b) Sk 68◦ 86 B1: [SL63] 495 − (b) Sk -67 211 Sk 69◦ 206 B2: 30 Doradus LH 88 − (b) Sk 69◦ 288 B0.5 30 Doradus ? − Notes. (a) Evans et al. 2006. (b) Rousseau et al. 1978.

a N b 2 arcmin BI 237 E

Fig. 1. MIPS 24 µm image of the associations LH82 and LH88 (indicated by dashed circles). The position of the O2V((f*)) star BI237 and its bow shock are marked by a black solid circle, 30" while the position of the O2III(f*) star Sk 67 211 in LH82 − ◦ is indicated by a white circle. At the distance of the LMC, 1′ corresponds to 14 pc. ≃

c d

Fig. 3. MIPS 24 µm images of bow shocks associated with (a) Sk 69 206, (b) Sk 69 288, (c) Sk 66 16, and (d) Sk 68 − ◦ − ◦ − ◦ − ◦ 30" 86. The orientation and the scale of the images are the same.

Fig. 2. Left: MIPS24 µm image of the bow shock associated with the O2V((f*)) star BI237. The position of BI237 is marked by culiar velocities of the candidate runaway stars in the LMC (see a circle. Right: 2MASS J band image of the same field. Table 1). The bow shock produced by Sk 69◦ 288 is situated (at least in projection) within the association− LH113. The geometry of the bow shock suggests that Sk 69◦ 288 is moving away from nISM. For the characteristic (transverse) size of a (parabolic) bow − the 30Doradus nebula, which is located at 21′ ( 300 pc) ˙ 2 1/2 shock of 2 √2R0, where R0 = (Mv /4πρISMv⋆) is the stand- to the west of the star (Fig.4). Although we have≃ no≃ arguments off distance∼ of the bow shock, M˙ and∞ v are the mass-loss rate ∞ against the association between Sk 69◦ 288 and LH113, one and terminal velocity of the stellar wind, ρISM = 1.4mHnISM is cannot exclude the possibility that the− actual birthplace of the the density of the interstellar medium, mH the mass of the hy- star is the 30Doradus nebula. In this connection, it is worth 1 drogen atom, and v ( 130 kms− ) the velocity of the star rel- noting that the O5V star ALS19631 (Hanson 2003) was sug- ∗ ≥ ative to the ambient medium, and using the wind parameters of gested as a member of the CygOB2 association on the basis of ˙ 7 1 1 BI 237, M = 7.8 10− M yr− and v = 3400 kms− (Mokiem its location within the confines of the association (Comerónetal. et al. 2007), one× finds n ⊙ 0.1cm∞3, i.e., a reasonable num- ISM ≤ − 2002). The astrometric data on ALS19631 and the geometry of ber. the bow shock generated by this star, however, suggest that this A by-productof our search is the detection of bow shocks as- runaway was instead ejected from the open cluster NGC6913 sociated with four other OB stars in the field of the LMC (Fig3). centred 3◦.4 west of the star (Gvaramadze & Bomans 2008a). The details of these stars are given in Table2. The spectral types ≃ The bow shocks associated with Sk 66◦ 16 and Sk 68◦ 86 of the stars were found using the VizieR catalogue access tool2. − − are located, respectively, at 3.5′ ( 50 pc) and 2.5′ ( 35 pc) The last column gives the possible birthplaces of the stars. from the clusters KMHK268∼ (Fig.5)≃ and [SL63]495≃ ≃ (Fig.6). The bow shock produced by Sk 69◦ 206 is located at 17′ Sk 66 16 is located in the N11 star-forming region, not far − ≃ ◦ ( 240 pc) to the west of R136 – the central cluster of the from− our programme star N11-026 (see Fig.5). We note the de- ≃ 30Doradus nebula (Fig.4). The orientation of the bow shock tection of a bow shock-like structure associated with one of the is consistent with the possibility that the associated star was most massive stars in N11, the ON2IIIf*: star N11-031 (Evans ejected from the 30Doradus nebula. Assuming that the ejection et al. 2006). This structure is facing towardsthe centre of the par- event occurred 2 Myr ago, one finds the transverse velocity ent association LH10 (Fig.7a). Interestingly, the radial velocity ∼ 1 of Sk 69◦ 206 of 120 kms− , which is comparable to the pe- of N11-031 is 30 kms 1 greater than the median velocity of − − stars in N11 (Evans≃ et al. 2006), which could be considered as 2 http://webviz.u-strasbg.fr/viz-bin/VizieR indicating that this star is a runaway as well.

3 V.V. Gvaramadze et al.: Massive runaway stars in the Large Magellanic Cloud

N 6 arcmin Sk -68 137

BI 253

Sk -69 288 R136 Sk -69 206

LH 113 30 Dor 016

Fig. 4. MIPS 24 µm image of the 30Doradus nebula and its surroundings. The positions of Sk 69◦ 206 and Sk 69◦ 288 and their − − bow shocks are marked by solid circles. The positions of the three programme stars, Sk 68◦ 137, BI253, and 30Dor016, are marked by crosses. The diamond point shows the position of R136. The dashed ellipsoid indicates− the approximate boundaryof the association LH113.

N N N11-026

E E LH 10

Sk -66 16 [SL63] 495

N11-031

Sk -68 86 2 arcmin KMHK 268 2 arcmin

Fig. 5. MIPS 24 µm image of the N11 star-forming region with the programme star N11-026 marked by a large cross. The Fig. 6. MIPS 24 µm image of the bow shock associated with small cross indicates the position of the ON2IIIf*: star N11- Sk 68 86 (marked by a solid circle). The position of the cluster − ◦ 031. The solid circle shows the position of Sk 66◦ 16 and its [SL63] 495 is indicated by a dashed circle. bow shock. The positions of the association LH10− and the cluster KMHK268 are indicated by a dashed ellipse and a dashed circle, respectively. Massey et al. 2005; Evans et al. 2006, 2010). The young ages ( 2 Myr) of BI237 and other O2-type field stars decidedly ar- ∼ For the sake of completeness, we note also the detection gue for their peculiar velocities being explained by supernova of an arc-like nebula (Fig.7b) attached to the candidate lumi- explosions in binary systems (Blaauw 1961); the massive com- nous blue variable R81 (HD 269128; Wolf et al. 1981; van panion (primary) stars would simply have no time to end their Genderen 2001; cf. Gvaramadze, Kniazev & Fabrika 2010a) and lives in supernovae. Moreover, the high (measured or inferred) the 24 µm counterpart to the circumstellar nebula around the O9f peculiar velocities of these stars cannot be accounted for within star Sk 69 279 (Weis et al. 1997). the framework of the binary-supernovascenario since it requires − ◦ that the stellar supernova remnant (a 5 10 M black hole) − ⊙ 1 receive an unrealistically high (> 200 300 kms− ) kick ve- ∼ − 4. Discussion and conclusion locity at birth (Gvaramadze & Bomans 2008a; cf. Gvaramadze 2009). The only viable alternative is that the massive stars were The discovery of a bow shock produced by BI237 lends strong ejected in the field via dynamical three- or four-body encounters support to the idea that this and other isolated massive stars in (Poveda et al. 1967; Leonard & Duncan 1990; Kroupa 1998; the field of the LMC are runaway stars (Walborn et al. 2002; Pflamm-Altenburg & Kroupa 2006; Gvaramadze, Gualandris &

4 V.V. Gvaramadze et al.: Massive runaway stars in the Large Magellanic Cloud

a b References Baranov, V.B., Krasnobaev, K.V., & Kulikovskii, A.G. 1971, Soviet Phys. Doklady, 15, 791 Bica, E.L.D., Schmitt, H.R., Dutra, C.M., & Oliveira, H.L. 1999, AJ, 117, 238 Blaauw, A. 1961, Bull. Astron. Inst. Netherlands, 15, 265 Brandl, B.R., Portegies Zwart, S.F., Moffat, A.F.J., & Chernoff, D.F. 2007, in Massive Stars in Interactive Binaries, ed. N. St.-Louis & A.F.J. Moffat (San Francisco: ASP), 629 Brandl, B.R., Sams, B.J., Bertoldi, F., et al. 1996, ApJ, 466, 254 N Campbell, B., Hunter, D.A., Holtzman, J.A., et al. 1992, AJ, 104, 1721 Clarke, C.J., & Pringle, J.E. 1992, MNRAS, 255, 423 30" Comerón, F., & Pasquali, A. 2007, A&A, 467, 23 E Comerón, F., Pasquali, A., Rodighiero, G., et al. 2002, A&A, 389, 874 Danforth, C.W., & Chu, Y.-H. 2001, ApJ, 552, L155 de Grijs, R., Johnson, R.A., Gilmore, G.F., Frayn, C.M. 2002, MNRAS, 331, Fig. 7. MIPS 24 µm images of (a) a bow shock-like structure as- 228 sociated with N11-031 and (b) an arc-like nebula attached to the Evans, C.J., Lennon, D.J., Smartt, S.J., & Trundle, C. 2006, A&A, 456, 623 Evans, C.J., Walborn, N.R., Crowther, P.A., et al. 2010, ApJL, 715, L74 candidate luminous blue variable R81 (HD269128). The posi- Garmany, C.D. 1990, in Properties of Hot Luminous Stars (San Francisco: ASP), tions of both stars are indicated by circles. The orientation and 16 the scale of the images are the same. Garmany, C.D., Conti, P.S., & Chiosi, C. 1982, ApJ, 263, 777 Gibson, B.K. 2000, MmSAI, 71, 693 Gies, D.R., & Bolton, C.T. 1986, ApJS, 61, 419 Gvaramadze, V.V. 2009, MNRAS, 395, L85 Gvaramadze, V.V., & Bomans, D.J. 2008a, A&A, 485, L29 Portegues Zwart 2008, 2009). Naturally, less massive (late B- Gvaramadze, V.V., & Bomans, D.J. 2008b, A&A, 490, 1071 type) stars are also ejected from their birth clusters by dynami- Gvaramadze, V.V., Gualandris, A., & Portegies Zwart, S. 2008, MNRAS, 385, 929 cal interactions (e.g. Kroupa 1998), but they would be difficult Gvaramadze, V.V., Gualandris, A., & Portegies Zwart, S. 2009, MNRAS, 400, to observe in the LMC. 524 The large separation of some of the O2-type field stars from Gvaramadze, V.V., Fabrika, S., Hamann, W.-R., et al. 2009, MNRAS, 400, 524 Gvaramadze, V.V., Kniazev, A.Y., & Fabrika, S. 2010a, MNRAS, in press (astro- their plausible birthplaces implies that these stars were ejected ph/0909.0458) soon after birth (which also argues against the binary-supernova Gvaramadze, V.V., Kniazev, A.Y., Hamann, W.-R., et al. 2010b, MNRAS, 403, scenario). This implication has an important consequence for un- 760 derstanding the early dynamical evolution of star clusters since it Hanson, M.M. 2003, ApJ, 597, 957 suggests that mass segregation in young clusters (the necessary Hoogerwerf, R., de Bruijne, J.H.J., & de Zeeuw, P.T. 2001, A&A, 365, 49 Hunter, D.A., Shays, E.J., Holtzman, J.A., Light, R.M., O’Neil E.J., & Lynds, condition for effective production of runaway OB stars) should R., 1995, ApJ, 448, 179 be primordial rather than caused by the Spitzer instability. For Kobulnicky, H.A., Gilbert, I.J., & Kiminki, D.C. 2010, ApJ, 710, 549 example, R136 was found to already be mass-segregated at its Kroupa, P. 1998, MNRAS, 298, 231 age of about 2 Myr or younger (Campbell et al. 1992; Hunter et Kroupa, P. 2008, in Arseth S.J., Tout C.A., Mardling R.A., eds, The Cambridge N-Body Lectures, Lecture Notes in Physics, Vol. 760. Springer, Berlin, p. al. 1995; Brandl et al. 1996; de Grijs et al. 2002). But the Spitzer 181 instability could be a very fast (< 0.5 Myr) process if the birth Leonard, P.J.T., & Duncan, M.J. 1990, AJ, 99, 608 cluster is very dense (e.g. Kroupa∼ 2008). High-precision proper Massey, P., & Conti, P.S. 1983, ApJ, 273, 576 motion measurements for the massive field stars are therefore Massey, P., Lang, C.C., Degioia-Eastwood, K., & Garmany, C.D. 1995, ApJ, required to determine the timing of their ejections, thereby dis- 438, 188 Massey, P., Puls, J., Pauldrach, A.W.A., et al. 2005, ApJ, 627, 477 tinguishing between the primordial and the dynamical origins Meixner, M., Gordon, K.D., Indebetouw, R., et al. 2006, AJ, 132, 2268 of mass segregation in young clusters (Gvaramadze & Bomans Mokiem, M.R., de Koter, A., Evans, C.J., et al. 2007, A&A, 465, 1003 2008b). Future proper motion measurements with the space as- Nota, A., Drissen, L., Clampin, M., et al., 1994, in Circumstellar Media in the trometry mission Gaia will allow us to solve this problem. At Late Stages of Stellar Evolution, ed. R.E.S. Clegg, I.R. Stevens, & W.P.S. Miekle (Cambridge: Cambridge Univ. Press), 89 the same time, N-body experiments are required to quantify the Pflamm-Altenburg, J., & Kroupa, P. 2006, MNRAS, 373, 295 expected differences between the two types of mass segregation Poveda, A., Ruiz, J., & Allen, C. 1967, Bol. Obs. Tonantzintla Tacubaya, 4, 86 in terms of the ejection of massive stars. Price, S.D., Egan, M.P., Carey, S.J., Mizuno, D.R., & Kuchar, T.A. 2001, AJ, To conclude, the search for bow shocks in star-forming re- 121, 2819 Rieke, G.H., Young, E.T., Engelbracht, C.W., et al. 2004, ApJS, 154, 25 gions and subsequent identification of their associated stars serve Rousseau, J., Martin, N., Prévot, L., et al. 1978, A&AS, 31, 243 as a useful tool for detecting runaway OB stars (e.g. Gvaramadze Schilbach, E., & Röser, S. 2008, A&A, 489, 105 & Bomans 2008b; Gvaramadze et al. 2010b), hence for con- Stone, R.C. 1991, AJ, 102, 333 straining the dynamical evolution of their parent clusters. Further Van Buren, D. & McCray, R. 1988, ApJ, 329, L93 Van Buren, D., Noriega-Crespo, A., & Dgani, R. 1995, AJ, 110, 2914 search for bow shocks aroundyoung massive clusters and OB as- van Genderen, A.M. 2001, A&A, 366, 508 sociations in the LMC (when necessary, accompanied by follow- Walborn, N.R. 1973, AJ, 78, 1067 up spectroscopy of their associated stars) is therefore warranted. Walborn, N.R., Howarth, I.D., Lennon, D.J., et al. 2002, AJ, 123, 2754 Walborn, N.R., Morrell, N.I., Howarth, I.D., et al. 2004, ApJ, 608, 1028 Weis, K., Chu, Y.-H., Duschl, W.J., & Bomans, D.J. 1997, A&A, 325, 1157 Acknowledgements. We are grateful to S.Röser, H.Zinnecker, and the anony- Werner, M.W., Roellig, T.L., Low, F.J., et al. 2004, ApJS, 154, 1 mous referee for carefully reading the manuscript and for useful comments, al- Wolf, B., Stahl, O., de Groot, M.J.H., & Sterken, C. 1981, A&A, 99, 351 lowing us to improve the presentation of the paper. VVG acknowledges finan- Zinnecker, H. 2003, in A Massive Star Odyssey: From Main Sequence to cial support from the Deutsche Forschungsgemeinschaft. This research has made Supernova, ed. K. van der Hucht, A. Herrero, & C. Esteban, IAU use of the NASA/IPAC Infrared Science Archive, which is operated by the Jet Symposium, 212, 80 Propulsion Laboratory, California Institute of Technology, under contract with the National Aeronautics and Space Administration, the SIMBAD database, and the VizieR catalogue access tool, both operated at CDS, Strasbourg, France.