On the stability and star formation related to Bok globules: An infrared approach

1. On the stability and star formation related to Bok globules: An infrared approach Germán A. Racca and Ramiro de la Reza Observatório Nacional – ON/MCT Southern Bok globules are less studied than those of the northern hemisphere. We began a systematic survey of these clouds on the southern hemisphere in order to study two main points: (a) their internal structure and stability and (b) a related low mass star formation. Our first step was a near-infrared survey of Globule 2 (Racca, Gómez, & Kenyon 2002). This cloud is the densest and most massive condensation embedded in the southern Coalsack. We found two near-infrared candidates young stellar objects whose visual magnitudes are expected to be 17 and 19. Star formation signatures in Bok globules are difficult to be obtained and appears to be related to denser condensations with characteristic infrared properties. Only large telescopes are necessary to obtain valuable spectro-photometry in order to confirm the pre-main sequence nature of low mass stars related to Bok globules. Having this in mind, we expect first to confirm these initial studies and continue this research on southern hemisphere Bok globules. The problem The clumpy structure of the Coalsack is similar to other well-known clouds, such as Taurus, Chamaeleon I and Lupus, that are actively forming stars. Despite this similarity, previous searches of the Coalsack have failed to detect any signs of recent star formation. Why? DSS V-band Image 13CO intensity map of the Coalsack and C18O intensity map of the densest region at the western edge of the complex (Kato et al. 1999). 2MASS K-band Image Globule 2 The Globule 2 is the densest [n(H2)  4  1013 cm-3] and more massive (10 M) of the C18O cores. It is an obvious, roughly circular patch (6’ radius) of extinction on shallow optical and near-IR surveys. The Coalsack is a conspicuous, nerby (d  180 pc) dark cloud located close to the Galactic plane in the southern Milky Way (l  303°, b  0°). The estimated total mass from 12CO data is  3550 M. Star counts indicate an average extinction of AV 5 mag. Previous work Recently, Racca, Gómez & Kenyon (2002) have surveyed the Globule 2 in JHK. From the large number of background stars ( 2500 sources), they derived an accurate near-IR extinction law for the cloud. They used the J-H/H-K color-color diagram to identify two potencial young stars with K < 14 in that region. GMOS long slit GEMINI application Color-color diagram for near-IR sources with K < 14 in the survey region. The large red stars show the position of two near-IR candidate young stellar objects (Racca, Gómez & Kenyon 2002). In our project recently submitted to the Gemini South, we intend to obtain a single long slit spectrum of the two target stars of V magnitudes 17 and 19 respectively. The exposure time is calculated in order to obtain the best spectrum possible for the brighest target and detect the lithium line in absorption and H in emission. This same spectrum will probably permit the detection of the H line in emission only of the second and fainter target. What we expect? Considering the mass of Globule 2 of 4.5 M (Racca, Gómez & Kenyon 2002) we expect, if this cloud has formed stars recently, to observe low mass (< 1 M) classical T Tauri stars (strong H line in emission and resonance lithium line in absorption). No weak T Tauri stars are expected because these stars do not present sufficient IR excesses to be distinguished as is the case of our two candidates.