A&A 399, 147–167 (2003) Astronomy DOI: 10.1051/0004-6361:20021717 & c ESO 2003 Astrophysics Star formation in the Vela molecular clouds, V. Young stellar objects and star clusters towards the C-cloud F. Massi1, D. Lorenzetti2, and T. Giannini2 1 Osservatorio Astrofisico di Arcetri, INAF, Largo E. Fermi 5, 50125 Firenze, Italy 2 Osservatorio Astronomico di Roma, INAF, Via Frascati 33, 00040 Monte Porzio Catone, Roma, Italy e-mail: [email protected], [email protected] Received 4 July 2002 / Accepted 14 November 2002 Abstract. We present the latest results from a sensitive (K ∼ 18 mag) near-infrared (JHK) imaging survey of IRAS selected young stellar objects associated with the Vela molecular ridge. These enlarge the sample of 12 fields, previously studied, adding 10 sites of recent star formation. The spectral energy distributions derived from near-infrared and 1.3-mm photometry allowed to identify at least 5 Class I sources. Their bolometric luminosities indicate that they are protostellar objects of intermediate mass (∼2–10 M). Herbig Ae/Be stars and compact UCHII regions could account for the far infrared emission towards some of the remaining fields. The most luminous IRAS sources have also been found associated with young embedded star clusters. The physical properties of the clusters have been determined and used to improve on the statistical relationships already suggested by our previous work. They have sizes of ∼>0.1 pc and volume densities of 103–104 stars pc−3. Where identified, the Class I sources tend to lie near the centre of the clusters and it is confirmed that the most massive ones are associated with the richest 2 clusters. The less luminous Class I sources (∼10 L) are found either isolated or within small groups of young stellar objects. It is proposed to use the relationship between the bolometric luminosity of the IRAS sources and the total number of cluster members as a test of the initial mass function at the highest masses. Key words. stars: formation – stars: pre-main sequence – infrared: stars – ISM: individual objects: Vela molecular ridge 1. Introduction transition. These authors further subdivided the emission area into 4 main regions (named A, B, C and D) based on the The far-infrared (FIR) maps of the sky provided by the IRAS location of the intensity peaks. The issue of distance is dis- satellite have long been a cornerstone in driving a number cussed in Paper I, where a value of 700 ± 200 pc is derived for of studies on star formation processes. Along the same lines, clouds A, C and D. Details on the VMR and its star formation Liseau et al. (1992) and Lorenzetti et al. (1993), hereafter history can be found in the quoted literature. Paper I and Paper II respectively, used the IRAS Point Source Catalogue (PSC) for retrieving all listed sources with colours Complementing the selected PSC entries with near-infrared typical of young stellar objects on the sky area of the Vela (NIR) single-channel photometry and mm observations, a Molecular Ridge (VMR) as designated by Murphy & May Spectral Energy Distribution (SED) is determined for each ob- (1991). The VMR is a giant molecular cloud complex lo- ject and a catalogue of Class I sources (Lada & Wilking 1984) cated in the galactic plane (b ≈±3◦) outside the solar circle associated with the VMR is eventually provided in Papers I (l ≈ 260◦−275◦). Murphy & May (1991) delineated its struc- and II. More precisely, given the bolometric luminosities im- 2 4 ture by low resolution (∼30) mm-observations in the CO(1−0) plied by the FIR fluxes (10 –10 L), these objects are actually the analogues of Class I sources in the regime of intermedi- ate mass stars (2–10 M). An analysis of the large scale star Send offprint requests to: F. Massi, formation activity hosted by the VMR is attempted in Papers I e-mail: [email protected] Based on observations collected at the European Southern and II, as well. Due to the intrinsic low resolution of IRAS Observatory, Chile. data and NIR single channel photometry, the issue of the envi- ff The complete version of Table 5 is only available in electronic ronmental e ects on the derived SEDs could not be addressed form at the CDS via anonymous ftp to cdsarc.u-strasbg.fr in the two works. Hence, Massi et al. (1999) and Massi et al. (130.79.128.5)orvia (2000), hereafter Paper III and Paper IV, studied NIR images http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/399/147 (JHK) of a subsample of 12 sources (all those listed in the final Article published by EDP Sciences and available at http://www.aanda.org or http://dx.doi.org/10.1051/0004-6361:20021717 148 F. Massi et al.: Star formation in the Vela molecular clouds catalogue of Papers I-II and believed to belong to cloud D) in sources meeting the following conditions: order to find out the counterparts of the FIR emission. These images also unveiled the presence of a number of young em- 1. having passed through all steps described in Papers I and II bedded star clusters among the most luminous IRAS sources. and being eventually identified as Class I protostellar can- These are discussed in Paper IV. The preliminary results of didates; larger field and much deeper NIR imaging towards a few of 2. belonging to the VMR-C cloud (as designated by Murphy the IRAS sources studied in Papers III–IV (Testi et al. 2001) & May 1991). confirm the findings of Paper IV on the embedded young clus- ters. Another feature of the sites hosting the IRAS sources is To these, we added 3 more IRAS sources belonging to the D and C clouds either representing Class I sources which had sketched by Lorenzetti et al. (2002) who searched for proto- been previously rejected (IRS 70) or being remarkable for their stellar jets towards the 12 sources of Papers III–IV (and 3 more ones, also associated with the VMR) using NIR imaging in nar- association with HII regions and young star clusters (IRS 16 and IRS 34). Thus, in both Papers III/IV and the present work row bands centred at the 2.12 µm transition of H2 and at the 1.7 µm transition of [FeII]. The case of a well developed jet we examine a flux-limited sample of 22 IRAS sources with red colours [1 Jy < F(12 µm) < F(25 µm) < F(60 µm); in the fol- discovered towards IRS 17 (also known as IRAS 08448–4343) lowing, F(i µm) is abbreviated to F ] and associated to molec- is examined in detail with the aid of NIR spectroscopy. i ular gas whose velocity is in the typical range of clouds C and A few important recent works on observations at mm wave- D(0∼< v ∼< 14 km s−1). lengths are worth to be mentioned. Wouterloot & Brand (1999) LSR observed 12CO(1 − 0), 13CO(1 − 0), C18O(1 − 0) and CS(2 − 1) The 10 IRAS sources are listed in Table 1, along with their towards nine sources in the Vela region, presenting maps of equatorial coordinates and the gas velocities obtained from / molecular clumps and unveiling the presence of a few molec- pointed observations in CO and or CS transitions (see Papers I ular outflows. Four of the sources are listed in the catalogue and II). The last column (# IRS) refers to the internal classifi- of Papers I–II and are among those studied in Papers III– cation adopted in Papers I and II. In Fig. 1 the positions of all IV. But the most important advancement in the mm range IRAS sources (including those discussed in Paper III) are over- 13 − is represented by the results of the large scale observations laid on the map of CO(1 0) integrated intensity taken from with NANTEN: Yamaguchi et al. (1999) present new large- Yamaguchi et al. (1999; their Fig. 1b). scale higher-resolution (∼3) maps of integrated emission in the 12CO(1−0), 13CO(1−0) and C18O(1−0) transitions. These ob- 2.2. Near-IR imaging: Data reduction, photometry servations allow to better delineate the morphology and struc- and astrometry ture of the VMR (see Fig. 1 for the 13CO map) and suggest that cloud C is the less evolved of the complex. The authors also The NIR images were obtained in February 1993 (IRS 16, analyse the star formation history in the region, comparing their IRS 22, IRS 26, IRS 70) and in February 1994 (IRS 31, IRS 32, mm data with the locations of IRAS sources and optical tracers IRS 33, IRS 34, IRS 73, IRS 74) with IRAC2 (Moorwood et al. of early evolutionary phases. Moriguchi et al. (2001) discuss 1992) on the ESO/MPI 2.2-m telescope at La Silla (Chile) the more general distribution of 12CO(1 − 0) towards the Vela through standard J (1.25 µm), H (1.65 µm) and K (2.20 µm) Supernova Remnant (SNR) also from NANTEN observations. filters. During the 1993 run, a plate scale of 0.49 arcsec/pixel As for the VMR, they address the possibility of interaction with was used (resulting in a field of view of about 2 × 2arcmin2), the SNR, an issue also considered in Paper I where it is shown whereas, during the 1994 run, a plate scale of 0.27 arcsec/pixel that the current generation of Class I sources cannot have been was selected in order to better sample the PSF (resulting in a induced by compression from the SNR.