A Close Look At The Heart Of RCW 108 - ESO

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c ESO 2007Astronomy & Astrophysics manuscript no. iras16362July 19, 2007A close look at the heart of RCW 1081?2F. Comerón and N. Schneider1ESO, Karl-Schwarzschild-Str. 2, D-85748 Garching bei München, Germanye-mail:[email protected]/CEA Saclay, F-91191 Gif-sur-Yvette, Francee-mail:[email protected]; acceptedABSTRACTContext. The IRAS 16362-4845 star-forming site in the RCW 108 complex contains an embedded compact cluster that includessome massive O-type stars. Star formation in the complex, and in particular in IRAS 16362-4845, has been proposed to be externallytriggered by the action of NGC 6193.Aims. We present a photometric study of the IRAS 16362-4845 cluster sensitive enough to probe the massive brown dwarf regime. Inparticular, we try to verify an apparent scarcity of solar-type and low-mass stars reported in a previous paper (Comer ón et al. 2005,A&A, 433, 955).0Methods. Using NACO at the VLT we have carried out adaptive optics-assisted imaging in the J H KS L bands, as well as throughnarrow-band lters centered on the Brγ and the H2 S (1) v 1 0 lines. We estimate individual line-of-sight extinctions and, forstars detected in the three J H KS lters, we estimate the contribution to the KS ux caused by light reprocessed in the circumstellarenvironment. We also resolve close binary and multiple systems. We use the K luminosity function as a diagnostic tool for the characteristics of the underlying mass function.Results. IRAS 16362-4845 does contain young low-mass stars. Nevertheless, they are far less than those expected from the extrapo-lation of the bright end of the K luminosity function towards fainter magnitudes. We estimate a total stellar mass of 370 M . Nearly0all the cluster members display L excesses, whereas KS excesses are in general either absent or moderate ( 1 mag). We also detect0an extremely red object with (KS L ) 9, likely to be a Class I source.Conclusions. The fact that solar-type and low-mass stars are present in numbers much smaller than those expected from the numberof more massive members hints at an initial mass function de cient in low mass stars as compared to that of other young clusterssuch as the Trapezium. The origin of this difference is unclear, and we speculate that it might be due to external triggering havingstarted star formation in the cluster, perhaps producing a top-heavy initial mass function. We also note that there are no detectablesystematic differences between the spatial distributions of bright and faint cluster members. Such absence of mass segregation in thespatial distribution of stars may also support external triggering having played an important role in the history of the RCW 108 region.Key words. ISM: HII regions - ISM: individual object: RCW 108 - Stars: luminosity function, mass function - Open clusters andassociations: IRAS 16362-48451. Introductioninterpreted as evidence for star formation triggered by radiation-The RCW 108 HII region in the Ara OB1 association has longbeen regarded as one of the best case studies illustrating the eroding action of newly formed clusters containing massive stars onthe molecular gas in their environments (Shaver & Goss 1970,Straw et al. 1987, Comerón et al. 2005). In visible-light imagesdriven implosion of dense cores (e.g. Bertoldi 1989, Miao etal. 2006). This mechanism may well be at work in RCW 108,as noted by the indications of triggered star formation in the region recently discussed by Urquhart et al. (2004) and Comerónet al. (2005, hereafter CSR05).of the region (see e.g. Petersen 2001) RCW 108 appears as aThe most conspicuous star forming site in RCW 108 is thebright rim nebula (SFO 79 in the catalog of Sugitani et al. 1991)compact HII region IRAS 16362-4845, rst noted by Shaveron a size scale of several 10 arcmin, de ning a sharp boundary& Goss (1970). It is deeply embedded in the dense molecularbetween an extended molecular cloud in the west and a regioncloud and associated with a cluster of IR-sources discovered bypredominantely lled with ionized gas in the east. The rim rep-Straw et al. (1987). Its stellar contents has been more recentlyresents the ionization front, produced when the ultraviolet radia-studied by Urquhart et al. (2004), based on the 2MASS catalogtion from O-type stars of the neighboring cluster NGC 6193 hit(Skrutskie et al. 2006); by CSR05 by means of dedicated deeper,and progressively destroy the molecular cloud.higher resolution J H KS imaging; and by Wolk et al. (2007) us-Infrared sources indicating active star forming sites are com-ing X-ray emission as a tracer of young stellar populations. Themon among externally ionized molecular clouds (Sugitani etstudy of CSR05 indicates that the IRAS 16362-4845 embeddedal. 1989, 1991, 1995; Sugitani & Ogura 1994), which has beenaggregate is a Trapezium-like cluster containing at least one lateO-type star, in consistency with the visible spectrum of the heav-Send offprint requests to: F. Comerón?Based on observations obtainedily obscured HII region. The mass of the cluster was estimatedattheEuropeanSouthernby CSR05 to be 210 M , with a rather large uncertainty.Observatory using the Very Large Telescope (VLT) (programme 077.C-Furthermore, that study noted as an intriguing feature of the0660(A)), on Cerro Paranal, Chile.color-magnitude diagram an apparent lack of stars fainter than

2F. Comerón and N. Schneider: A close look at the heart of RCW 108MK' 1.0and with amounts of foreground reddening in theTable 1. Log of observationsrange covered by the brightest stars in the cluster, which mightbe indicative of a peculiar mass function.FilterDITN DITNEXPJ13 s51629/30 June 2006in clusters dominated by massive stars, some practical limita-H20 s41627/28 July 2006tions of the observations presented in that work advised a furtherKS20 s41622/23 July 20060.175 s1802829/30 June 20062.176511622/23 July 20062.126511617/18 July 2006While the result hinted at by CSR05 may be potentially relevant to understand the build-up of the initial mass function (IMF)Date29/30 June 2006analysis based on material of higher quality. First and foremost,Lthough the observations were deep enough to penetrate well into024/25 July 200629/30 June 2006the area of the color-magnitude diagram where the lack of thestars was noted, the completeness of the census in that range22/23 July 2006was difficult to assess. This was due to the presence of brightnebulosity pervading the cluster with large brightness variations28/29 July 2006over small angular scales. Secondly, although the observed stellar images have a full-width at half maximum (FWHM) belowone arcsecond, the combination of crowdedness and nebulositystill hampered the detection of faint members relatively closeJ, H, and KS bands and the narrow-band. The resulting eldto brighter stars. For these reasons, higher quality observationssize, 54 , matches well the angular size of the IRAS 16362-0000were needed to place our tentative conclusion of a de cit of faint4845 cluster ( 30 ), whereas the location of CSR-012A nearmembers in IRAS 16362-4845 on a rm standing.the northern border of the cluster allows us to sample a portiontheof the dark cloud located to the north of IRAS 16362-4845. ForIRAS 16362-4845 cluster carried out using adaptive optics near-the L band observations the maximum eld size of the eld at-infrared imaging at the Very Large Telescope (VLT), which pro-tainable with NACO is 27 , at a scale of 27 milliarcseconds pervide a far deeper and sharper view of the cluster than previouslypixel. Our observations through that lter thus sample only theavailable. This new material allows us to reassess the stellar con-densest part of the cluster centered on ts of the cluster, and address questions related to possible pe-000The observations were obtained on six different nights be-culiarities of its IMF, the frequency of infrared excesses amongtween 29/30 June and and 28/29 July 2006. The images wereits members, the abundance of massive binary stars, or the spa-performed through two series of exposures in each of the J, KS ,0µm, and 2.17 µm lters, and one series of exposures intial distribution of high- and low-mass members, also providingL , 2.12some further insights on the structure of the associated nebula.the H lter. We used the common technique of stacking a number N DIT of frames, each with a detector integration time DIT ,centered on a number N E X P of closely spaced telescope point-2. Observationsings on a random dither pattern, for which we used an amplitude00of 10 . The image resulting from each exposure was constructed2.1. Data acquisition and reductionby combining the sky-subtracted individual stacks after correct-Our observations were carried out in service mode using NACO,ing for the telescope offset between pointings. This procedurethe adaptive optics near-infrared camera and spectrograph atwas also used for the L band observations, rather than the al-the VLT (Rousset et al. 2002, Lenzen et al. 2003), in imag-ternative chop-and-nod technique. Table 1 gives the log of theing mode. Broad-band images were obtained through the Jobservations, including the individual exposure parameters. Skysubtraction from our on-target images was performed by stack-in KS -band images of the cluster, star #12 from CSR05 (here-the median- ltered on-target frames uncorrected for telescope(1.66µm), KSµm), and L0µm) lters,as well as through two narrow-band lters centered on 2.12 µmand 2.17 µm respectively sampling the nebular emission in theH2 S (1) v 1 0 and Brγ lines. The brightest star in the eld(1.26µm), H0(2.18(3.80ing intercalated images obtained around a separate sky position0located 3 away from IRAS 16362-4845, as the bright nebulositypervading the cluster prevented us from using for that purposeand was used for wavefrontoffsets, as is normally done on uncrowded, nebulosity-free elds.sensing, using the near-infrared wavefront sensor in NAOS, theThe detector readout mode was selected for each lter as the bestadaptive optics module of NACO. Different available dichroicscompromise between the necessary sensitivity and the dynamicwere chosen, depending on the band of the observations: the Krange available. For the lters in which two images were ob-dichroic for the J and H observations (90% transmitted light intained on separate observations, one combined frame was pro-those bands), the J H K dichroic for the Lduced and used for further analysis. Figure 1 shows a composite1after CSR-012A ) has KS 8.360observations (also90% transmission in that band), and the N20C80 dichroic forthe KS , 2.12These choices provide the best compromises between the signalfrom the wavefront reference star needed for a good adaptive optics correction and the light transmitted to CONICA, the cameraof NACO. We selected the wide- eld camera optics yielding apixel scale of 54 milliarcseconds per pixel for the images in the1of the nal frames obtained through the J H KS lters.µm, and 2.17 µm observations (80% transmission).We use the nomenclature CSR-nnn to refer to the stars listed inTable 2 of CSR05. Stars detected in the present work and not listed inCSR05 are named CS-nnn. Members of multiple systems are referredto by adding A, B,. to their numbers. In this way, CSR-012A is the2.2. PhotometryOur observations in the J H KS bands were calibrated using theinfrared photometry of non-saturated sources in common with0those listed in CSR05. The L photometry was calibrated usingas a reference the stars in the eld observed in that same band byStraw et al. (1987).The intrinsic difficulty of performing stellar photometry inobservations using adaptive optics, due to the noticeable vari-brightest member of the system collectively identi ed as Star #12 ination of the point-spread function (PSF) across the eld, isCSR05, which the current observations show to be composed of vecompounded in our case with the additional complication ofmembers.a variable nebular background pervading the area of the clus-

F. Comerón and N. Schneider: A close look at the heart of RCW 1083Fig. 1. A color composite of the frames of IRAS 16362-4845 obtained through the J (blue), H (green) and KS (red) lters. The frames are centeredhms 0000000on the brightest source at near-infrared wavelengths, CSR-012A (α(2000) 16 40 00 2, δ(2000) 48 51 40 ). The eld covered is 52 57 ,with North at the top and East to the left. A comparison with Figure 12 of CSR05 gives an appreciation of the increase in quality between theobservations discussed in that paper and those presented here. The small red ring just outside the upper right edge of the nebula is actually anartifact due to out-of-focus ghosts of the brightest star in the cluster. The dashed line corresponds toα(2000) hms16 40 01 78. As explained inSection 3.2 contamination by background sources is higher to the East of this line, and we exclude that region from our analysis.ter, which makes it difficult to accurately estimate the contribu-The radius of the aperture for photometry was chosen as vetion to the measurement of the extended wings of the PSF. Thetimes the FWHM of the PSF of a bright, non-saturated star lo-strong and complicated variation of the PSF, particularly in the Jcated 15 away from the wavefront reference star, which pro-band where the adaptive optics correction is poorer and degradesvided an average PSF of the eld. We veri ed that the relativelyfastest with the distance to the wavefront reference star, led uslarge number by which the FWHM of the PSF of that star wasnot to consider PSF tting as a suitable method. Instead, we ob-multiplied ensured a negligible aperture correction elsewehere intained better results by performing aperture photometry at thethe eld. The counts within this aperture were computed by di-position of each detected object. Source detection was carriedviding the aperture into concentric rings each with a width of oneout automa