A WFI survey in the Chamaeleon II dark cloud

Astronomy & Astrophysics, Jul 2007

Aims.We present the results of an optical multi-band survey for low-mass pre-main sequence (PMS) stars and young brown dwarfs (BDs) in the Chamaeleon II (Cha II) dark cloud. This survey constitutes the complementary optical data to the c2d Spitzer Legacy survey in Cha II.Methods.Using the wide-field imager (WFI) at the ESO 2.2 m telescope, we surveyed a sky area of about 1.75 square degrees in Cha II. The region was observed in the Rc, Ic, and z broad-bands, in Hα and in two medium-band filters centred at 856 and 914 nm. We selected PMS star and young BD candidates using colour-magnitude diagrams (CMDs) and theoretical isochrones reproduced ad-hoc for the WFI at the ESO2.2m telescope system. We reinforced the selection criteria by using the previously known PMS stars in Cha II to define the PMS locus on the CMDs and by investigating the infrared (IR) colours of the candidates. By exploiting the WFI intermediate-band photometry we also estimated the effective temperature and the level of Hα emission of the candidates.Results.Our survey, which is one of the largest and deepest optical surveys conducted so far in Cha II, recovered the majority of the PMS stars and 10 member candidates of the cloud from previous IR surveys. In addition, the survey revealed 10 new potential members. From our photometric characterisation, we estimate that some 50% of the 20 candidates will result in true Cha II members. Based on our temperature estimates, we conclude that several of these objects are expected to be sub-stellar and give a first estimate of the fraction of sub-stellar objects.

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A WFI survey in the Chamaeleon II dark cloud

A&A A WFI survey in the Chamaeleon II dark cloud L. Spezzi 0 1 2 J. M. Alcalá 1 A. Frasca 2 E. Covino 1 D. Gandolfi 0 2 0 Dipartimento di Fisica e Astronomia, Universitá di Catania , via S. Sofia 78, 95123 Catania , Italy 1 INAF - Osservatorio Astronomico di Capodimonte , Salita Moiariello 16, 80131 Napoli , Italy 2 INAF - Osservatorio Astrofisico di Catania , via S. Sofia 78, 95123 Catania , Italy Aims. We present the results of an optical multi-band survey for low-mass pre-main sequence (PMS) stars and young brown dwarfs (BDs) in the Chamaeleon II (Cha II) dark cloud. This survey constitutes the complementary optical data to the c2d Spitzer Legacy survey in Cha II. Methods. Using the wide-field imager (WFI) at the ESO 2.2 m telescope, we surveyed a sky area of about 1.75 square degrees in Cha II. The region was observed in the Rc, Ic, and z broad-bands, in Hα and in two medium-band filters centred at 856 and 914 nm. We selected PMS star and young BD candidates using colour-magnitude diagrams (CMDs) and theoretical isochrones reproduced ad-hoc for the WFI at the ESO2.2m telescope system. We reinforced the selection criteria by using the previously known PMS stars in Cha II to define the PMS locus on the CMDs and by investigating the infrared (IR) colours of the candidates. By exploiting the WFI intermediate-band photometry we also estimated the effective temperature and the level of Hα emission of the candidates. Results. Our survey, which is one of the largest and deepest optical surveys conducted so far in Cha II, recovered the majority of the PMS stars and 10 member candidates of the cloud from previous IR surveys. In addition, the survey revealed 10 new potential members. From our photometric characterisation, we estimate that some 50% of the 20 candidates will result in true Cha II members. Based on our temperature estimates, we conclude that several of these objects are expected to be sub-stellar and give a first estimate of the fraction of sub-stellar objects. stars; low-mass; brown dwarfs - stars; formation - stars; pre-main sequence - ISM; clouds - ISM; individual objects; Chamaeleon II 1. Introduction Recent investigations in star-forming regions (SFRs) point out that the fraction of young brown dwarfs (BDs) relative to lowmass and more massive pre-main sequence (PMS) stars may vary significantly among different SFRs (Kroupa 2002). In particular, the fraction is different in T and in OB associations (Briceño et al. 2002; Hillenbrand & Carpenter 2000; Luhman et al. 2000; Muench et al. 2003; Preibisch et al. 2003). Different environments may give rise to different initial conditions for star and planet formation and, hence, to differences in the observed spectrum of masses (Kroupa 2001, 2002). This may have strong consequences on the initial mass function (IMF), in particular in the sub-stellar domain. It has also been proposed that BDs may form as members of small groups of objects, which may be ejected by dynamical interactions before they can grow to stellar masses (Reipurth & Clarck 2001); as a result, many low-mass and sub-stellar objects might have escaped detection in surveys that concentrate sharply in the cores of SFRs. Thus, one possible reason for the differences in the fraction of young sub-stellar objects in T and OB associations can in principle be ascribed to Based on observations carried out at the European Southern Observatory, La Silla, Chile under proposals numbers 67.C-0225 and 68.C-0311. Appendices and Fig. 14 are only available in electronic form at http://www.aanda.org Table 7 is available in electronic form at http://www.aanda.org and at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/470/281 the photometric and spatial incompleteness of the imaging surveys. Spatially complete deep-imaging surveys are thus crucial in order to single out low-mass star and BD candidates to be investigated by follow-up spectroscopy. Only then, problems like mass segregation in SFRs and the low-mass end of the IMF can be addressed. In this paper, optical wide-field imaging observations in the Chamaeleon II dark cloud (hereafter Cha II), complemented with JHK photometry from 2MASS (Cutri et al. 2003) , are used to search for low-mass PMS stars and young BDs. Our survey covers almost 2 square degrees in the Cha II cloud. Because of its proximity to the Sun (d ≈ 178 pc, Whittet et al. 1997), young age (0.1–10 Myr, Hughes & Hartigan 1992), and relatively high galactic latitude (b ≈ −15 deg, Schwartz 1991), which decreases the effects of contamination by background stars, the Cha II dark cloud is particularly well-suited to studies of low-mass PMS stars and young BDs. It is indeed characterised by the presence of objects with Hα emission (Hartigan 1993; Hughes & Hartigan 1992), as well as of embedded Class-I and Class-II IR sources (Larson et al. 1998; Prusti et al. 1992; Whittet et al. 1991) and X-rays sources (Alca (...truncated)


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L. Spezzi, J. M. Alcalá, A. Frasca, E. Covino, D. Gandolfi. A WFI survey in the Chamaeleon II dark cloud, Astronomy & Astrophysics, 2007, pp. 281-294, Volume 470, Issue 1, DOI: 10.1051/0004-6361:20066500