ISOCAM observations of the $\mathsf{\rho}$ Ophiuchi cloud: Luminosity and mass functions of the pre-main sequence embedded cluster

Abstract

Abstract. We present the results of the first extensive mid-infrared (IR) imaging survey of the ρ Ophiuchi embedded cluster, performed with the ISOCAM camera on board the ISO satellite. The main ρ Ophiuchi molecular cloud L1688, as well as the two secondary clouds L1689N and L1689S, have been completely surveyed for point sources at 6.7 µm and 14.3 µm. A total of 425 sources are detected in ∼0.7 deg 2 , including 16 Class I, 123 Class II, and 77 Class III young stellar objects (YSOs). Essentially all of the mid-IR sources coincide with near-IR sources, but a large proportion of them are recognized for the first time as YSOs. Our dual-wavelength survey allows us to identify essentially all the YSOs with IR excess in the embedded cluster down to Fν ∼ 10-15 mJy. It more than doubles the known population of Class II YSOs and represents the most complete census to date of newly formed stars in the ρ Ophiuchi central region. There are, however, reasons to believe that several tens of Class III YSOs remain to be identified below L ∼ 0.2 L . The mid-IR luminosities of most (∼65%) Class II objects are consistent with emission from purely passive circumstellar disks. The stellar luminosity function of the complete sample of Class II YSOs is derived with good accuracy down to L ∼ 0.03 L . It is basically flat (in logarithmic units) below L ∼ 2 L , exhibits a possible local maximum at L ∼ 1.5 L , and sharply falls off at higher luminosities. A modeling of the luminosity function, using available pre-main sequence tracks and plausible star formation histories, allows us to derive the mass distribution of the Class II YSOs which arguably reflects the initial mass function (IMF) of the embedded cluster. After correction for the presence of unresolved binary systems, we estimate that the IMF in ρ Ophiuchi is well described by a two-component power law with a low-mass index of −0.35 ± 0.25, a high-mass index of −1.7 (to be compared with the Salpeter value of −1.35), and a break occurring at M flat = 0.55 ± 0.25 M . This IMF is flat with no evidence for a low-mass cutoff down to at least ∼0.06 M .

Multiple protostellar systems

“…We have built our samples from the object lists from Motte & André (2001) in Taurus and Bontemps et al (2001) in Ophiuchus. The list of our targets is presented in Tables 1 and 2, together with their near-infrared photometry and nearto mid-infrared spectral index.…”

“…The classification of each source is based on the infrared spectral indices (see text); "I" stands for Class I sources and "FS" for flat spectrum sources. The last three columns indicate whether the source i) has at least one visual companion within 10 , ii) is surrounded by an extended nebulosity in our images (see also Park & Kenyon 2002), and iii) has an extended (>1000 AU) envelope as seen in millimeter continuum mapping (from Motte & André 2001 between Class I, flat-spectrum and T Tauri (Class II) sources, where we adopted α IR = 0.55 and α IR = −0.05 thresholds between these categories (see Bontemps et al 2001). Note that because of the large line-of-sight extinctions towards some of our targets, the use of the observed infrared spectral index is not completely secure.…”

“…With all three J, H and K filters, we obtained a sequence of 10 × 1.18 s exposures using a random jittering pattern in a 20 × 20 box. Bontemps et al (2001), while the near-infrared magnitudes are from Barsony et al (1997), Greene et al (1994) and the 2MASS database (2MASS uses a Ks filter rather than a K filter; the difference in magnitude for an object can reach a few tenths of a magnitude). The source marked with a † symbol has an infrared spectral index typical of a normal Class II source but Ressler & Barsony (2001) have shown that this system contains a Class I source and it is classified as such here.…”

“…It is believed that most stars form in much richer, cluster-like structures with several hundred objects within a 1 pc-radius (e.g., Carpenter 2000; Adams & Myers 2001). While the Orion Trapezium cluster is a well known example of this "clustered" mode of star formation, the Ophiuchus molecular cloud also belongs to this category, as it hosts several hundred objects with an average spatial density of order 10 3 stars/pc 3 (Bontemps et al 2001;Allen et al 2002). Taurus and Ophiuchus are therefore excellent candidates for investigating whether the star formation process is sensitive to environmental conditions.…”

“…Firstly, we focus on homogeneous and nearly complete samples of embedded YSOs in two star-forming regions only, Taurus and Ophiuchus, in order to be able to contrast at a significant statistical level the multiplicity rate resulting from the isolated and clustered star formation modes. Second, the 0.6 angular resolution of the present survey allows us to explore closer systems down to a separation of 110 AU at the distance of these two clouds (140 pc, Bertout et al 1999;Bontemps et al 2001). This paper is organized as follows: in Sect.…”