arXiv:0711.4719v1 [astro-ph] 29 Nov 2007 zdgs,ado tr ndi in of and gas), ized for- of distributions the di spatial on since light scenarios, cast analyze mation can To regions lacking. forming still is cloud stars molecular into entire converts un- an Detailed how formation. of star derstanding of stages in sug- involved first is the which gravity scales Galactic-scale that Galactic gests on and law Schmidt the Kennicut of universality the small with (tur- on scales environment trigger) initial local compression, the bulence, of influence nent omto rcs nlwmtlcnetenvi- the content about metal hints low in derive process to formation allow clusters stars of and population of young process Their formation. the star study to regions are promising (MCs), very Clouds and Magellanic , the particular Group in the Local between models. distinguish to proposed help can stages ary h rcs fsa omto ndi in formation star of process The Introduction 1. iomnsi a rmbigudrto.In understood. di being is it from particular, far is vironments 282 75, Vol. S.A.It. Mem. edo Send c ff At2004 SAIt rn opnns(oeua lus ion- clouds, (molecular components erent ff ago ainhsbe ciefraln eidi h ein rmaf a from region, the II in and period I long YSO a objects for active Spitzer been of has presence mation the Colour-Magnitude and the tracks of quence comparison in The candidates sequence Clouds. pre-main Magellanic of discovery the present We siuoNzoaed srfiia–Osraoi Astronomi Osservatorio – Astrofisica e-mail: Italy Padova, I-35122 di 5, Osservatorio Nazionale Istituto Abstract. rn eussto requests print edsusteyugpplto fsasadcutr nteM the in clusters and stars of population young the discuss We ffi utt eocl h promi- the reconcile to cult .Vallenari A. : ff rn evolution- erent ff rn en- erent .Vallenari A. [email protected] hc h rsneo eashsasignificant a on has cooling, metals e the of presence and the heating balance which gas the on the depends between stars of since formation important the particularly is This ronments. oa est nacmnsta ih produce might that enhancements causes density it local prevent scale local to on while globally, is collapse e turbulence net compressible The between highly gravitation. balance the and for- by turbulence cluster regulated and are literature. star mation theory, in turbulence proposed the star In been field and have cluster formation of mechanisms Several formation cluster and star Field 2. section in given 6. LMC are the remarks Final in presented. 11 dis- is N pre- in of are discovery candidates Way the sequence 5 main Milky and 4 the section in and cussed, be- MCs interaction the gravitational tween the stars of e the field formation 3 section of in mechanism clusters, and formation the cuss ff c.I hsppr nScin2w rtdis- first we 2 Section in paper, this In ect. scenario 1 h euaN1 nteLarge the in 11 N the iga ihpemi se- pre-main with diagram w10 ew ugs httesa for- star the that suggest od aoa Vicolo Padova, di co 5 r osvrlMyr several to yrs glai Clouds. agellanic Memorie ff cso h star the on ects della ff c of ect Vallenari: Young population in the MCs 283 a collapse, under suitable conditions. If the sur- & Vallenari 2007, Tosi et al 2007, Noel et al rounding flow is not strong enough to continue 2007). These results point out that the most re- to drive the cloud, the turbulence will quickly cent interactions between SMC and LMC trig- dissipate giving rise to an active . gered cluster and field star formation in the The properties of the fragmenting clouds and SMC, in agreement with the expectations from the mass of the individual proto-stellar cores, Bekki & Chiba (2005) while at older ages, giving birth to isolated stars rather then clus- the tidal interaction between the Magellanic ters of objects, depend in a complex and still Clouds and the was not able to unclear way on the values of the politropic in- give rise to significant star formation events dex which, in turn, is related to the metal con- (see Vallenari 2007 for a wider discussion). tent of the gas. When turbulence dominates, However, as expected, the star formation pro- the star formation is inefficient and slow and cess cannot be explained uniquely as the result stars build up in small groups. Turbulent con- of gravitational trigger. Comparing field stars trol of the star formation predicts that star clus- with the cluster age distribution, Chiosi et al ters form in regions where the support of the (2006) find that there is not a complete coin- turbulence is insufficient or where only large cidence between young cluster and field star scale driving is working (McLow & Klessen formation, suggesting that different modes of 2004). Star formation on scales of galaxies as formation might be at work. a whole is expected to be controlled by the bal- ance between turbulence and self-gravity just like star formation on scale of individual gas 4. The stellar population of N 11 in clouds, but might be modulated by additional the LMC: a case of triggered star effects, such as cooling, differential rotation, formation? gravitational interaction (Sasao 1973, Li et al Most stars form in groups and clusters. This 2004). An alternative mechanism is suggested implies that recently formed stellar objects by Bekki et al (2004): during interac- must interact with their environment. In this tions and mergers clusters can form as a re- respect, one of the most intriguing aspects of sult of relatively high velocity cloud-cloud col- the star formation is that of the triggering. lisions. Young massive stars are expected to inject en- ergy into the nearby interstellar medium, heat- 3. Young Cluster and field SF in the ing and compressing the surrounding gas. This SMC: star formation by can have destructive or constructive effects, de- gravitational trigger pending on the balance between heating and gravity but it is still unclear what regulates Star formation by gravitational trigger is well it. To distinguish between sequential and trig- known to take place in mergers/ interactions. gered star formation is indeed very difficult. The Magellanic Clouds represents one of the Star formation is often found to be sequential, best studied examples. i.e. stellar population can independently form Formation episodes involving both cluster in adjacent clouds with little age difference, but and field star formation happened at 5, 20, and no causal relationship is present. A hierarchi- finally at 100-150 Myr, in coincidence with cal system of three or more generationsof stars SMC perigalactic passage (Chiosi et al 2006). in the same region is often interpreted as more As far as the old population is concerned, the stringent sign of triggers (i.e. causal relation- star formation was not continuous but pro- ship between episodes of star formation)(Oey ceeded in a number of bursts taking place at et al 2005, Deharveng et al 2005). The stellar 3 Gyr and 6 Gyr ago. These bursts are tem- population in N 11 is often presented as one porally coincident with past peri-galactic pas- of the best example of triggered star forma- sages of the SMC about the Milky Way. Only tion. N 11 is the second largest nebula of the a very low efficiency of the star formation at LMC after the 30 Dor Nebula. It is located in epochs older than 6-8 Gyr is found (Chiosi the north-western corner of the LMC. It has a 284 Vallenari: Young population in the MCs

HD32228 HD32228

Fig. 1. HD 32228 is the central cluster of the association LH 9. Its age is derived by Walborn et al (1999) on the basis of WC star found there. The PMS sequence is clearly visible and well separated by the MS stars (left panel). Right panel: CMD of HD 3228 after that the field population contamination is subtracted. Pre- isochrones by Siess et al (2000) are plotted for AV = 0.2 (dashed lines), AV = 0.8 (solid lines) and ages of 0.5, 1.0,1.5,10, 15 Myr. Photometric errors on the color are shown for increasing values of the magnitude (solid error bars). peculiar morphology with a central hole with (2006)find a spatial correlationof OB stars and no emissions and several filaments around. The Herbig Ae/Be star candidates with the radio central cavity has been evacuated by the OB continuum, Hα, CO and X-ray. Herbig Ae/Be stars in the association LH 9, located near the stars are found in all the associations, but in- center of N 11.The whole complexhas a diam- side LH 9. This suggests thatin LH 9 such very eter of 45’, corresponding to a linear extent of young objects would have already disappeared, 705 pc, assuming a distance for the LMC of 54 losing their circumstellar envelopes/disks. This kpc. In addition to LH 9, this complex includes leads to the conclusion that LH 9 is slightly the OB associations LH 10 in N11B, LH 13 in older than the other associations and has pos- N11C, LH 14 in N11E. Bica & Dutra (2000) sibly triggered star formation episodes in the find at least 18 objects, including associations surrounding regions. and clusters, in the age range 0-30 Myr. Walborn & Parker (1992) suggest a two stage star-burst hypothesis: an initial star burst 5. Pre-MS candidate stars in in LH 9 triggered a secondary burst in LH 10, a few Myr later. This hypothesis was re- PMS stars are found in the LMC only in few cently confirmed by the results by Hatano et young associations/clusters: in the region of al (2006), Mokiem et al (2007) who detect SN 1987 (Panagia et al 2000), in the young 127 Herbig Ae/Be star candidates from near- double cluster NGC 1850 (Gilmozzi et al infrared photometry in this region, mainly in 1994), in 30 Dor region (Brandner et al 2001, the periphery of LH 9. Herbig Ae/Be star are Romaniello et al 2006), in LH 95 (Gouliermis intermediate mass pre-main sequence stars (7- et al 2002) and in LH 52(Gouliermis et al 3M⊙) andhaveage range1-3Myr.Hatanoet al 2007). Vallenari: Young population in the MCs 285

HST ACS/WFC archive photometry in the seems to be in agreement with a turbulent sce- region of N 11 shows the presence of pre- nario of star formation. main sequence candidates (Vallenari & Chiosi 2007). The studied fields are partially covering the associations LH 9, LH 10, and LH 13. The 6. Conclusions most conspicuous concentrations of pre-main sequence stars are seen in N11B associated In this paper we discuss the young popula- with LH 10, and in LH 9. These concentrations tion of stars and clusters in the Magellanic are corresponding to the location of OB stars. Clouds. The last gravitational interaction be- A small group of PMS candidates are found in tween LMC and SMC has triggered a star for- N11C (associated with LH 13). Spectroscopic mation episode in both the cluster and field determination of the age of LH 13 are in the stars. We find that the SMC was relatively qui- range 3-5 Myr, while LH 10 is 3±1 Myr old escent at ages older than 6-8 Gyr. This result (Heydari-Malayeri et al 2000). The presence suggests that at older ages, the tidal interaction of OB stars in LH 9 set its age at 7.0 ±1 Myr between the Magellanic Clouds and the Milky (Mokiem et al 2007). Fig.1 shows the CMD Way was not able to trigger significant star for- of the cluster HD 3228 located at the periph- mation events. Field star and cluster formation ery of the association LH 9, where the PMS are not completely correlated, as expected on star candidates are clearly visible as a sequence the basis of theoretical models and different well separated from the field population. The star formation modes are expected to act in the age of this cluster is set to about 3-4 Myr by field and in cluster populations. Young stars are Walborn et al (1999) based on the presence well known to trigger star formation on local of a WC star. Pre-main sequence tracks by scale by injecting energy in the surrounding in- Siess et al (2000) suggest an age ranging from terstellar medium. N 11 region in the LMC is 1 to 10 Mys, depending on the adopted ex- one of the most promising candidate of trig- tinction (AV=0.2 –0.8). The comparison with gered star formation. In this paper we discuss the isochrones shows that pre-main sequence the star formation process in this region. We stars have masses from 1.3 M⊙ to 2.0 M⊙. report on the discovery of a PMS candidates Far infrared Spitzer Space Telescope IRAC and associated with N 11 from HST ACS/WFC MIPS observations of the region are made in photometry. The PMS are consistent with an the framework of the SAGE project (Meixner age going from 1 to 10 Myr, although a more et al 2006). These observations are comple- precise determination is not possible due to mentary to HST photometry: while HST ob- the uncertainties on the interstellar extinction. servations can give information about faint, ex- The comparison with the isochrones shows that posed pre-main sequence candidates, near-IR pre-main sequence stars having masses from data allow to detect embedded young stellar 1.3 M⊙ to 2.0 M⊙ are present. Spitzer IRAC objects(YSO). Comparing the magnitudes and and MIPS observations of the region, made in colors of the objects with photometric models the framework of the SAGE project reveals the by Robitaille et al (2006), Vallenari & Chiosi presence of young stellar object (YSO) candi- (2007) select young stellar object candidates. dates. YSO type I and II having ages < 1Myr They find out that YSO type I (showing large are found at the same location than the candi- infalling envelopes) and II (characterized by date PMS stars. The data seems to suggest that the presence of an optically thick disk) having the star formation in the region is a long last- ages from 0.1 to 1 Myr are found at the same ing process where stars from 0.1 to 7 Myr are location than the candidate PMS stars. While widely distributed, as expected in a turbulent it cannot be excluded that LH 9 triggered the scenario of star formation. star formation in the surroundings,however the data seems to suggest that the star formation in Acknowledgements. This work was done in collab- the region is a long lasting process where stars oration with E. Chiosi, E. Held, A. Moretti, G. from 0.1 to 7 Myr are widely distributed. This Bertelli, L. Rizzi 286 Vallenari: Young population in the MCs

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