Probing the embedded YSOs of the R CrA region through VLT-ISAAC spectroscopy

Abstract

Near IR spectra at low (R ∼ 800) and medium (R ∼ 9000) resolution, obtained with ISAAC at VLT, have been used to pose constraints on the evolutionary state and accretion properties of a sample of five embedded YSOs located in the R CrA core. This sample includes three Class I sources (HH100 IR, IRS2 and IRS5), and two sources with NIR excesses (IRS6 and IRS3). IRS5 and IRS6 have been discovered to be binaries with a separation between the two components of 78 and 97 AU, respectively. Absorption lines, typical of late-type photospheres, have been detected in the medium resolution spectra of all the observed targets, including HH 100 IR and IRS2 which have high values of infrared continuum veiling (r K = 6 and 3, respectively). These two sources also present low resolution spectra rich in emission lines (HI, CO and plenty of other permitted lines from neutral atoms) likely originating in the disk-star-wind connected regions. Among the features observed in HH100 IR and IRS2, Na I at 2.205 μm and CO at 2.3 μm, which are more commonly used for stellar classification, are detected in emission instead of absorption. Several strong photospheric lines which lie around 2.12 and 2.23 μm and whose ratio is sensitive to both effective temperature and gravity are proposed as independent diagnostic tools for this type of sources. We derived spectral types, veiling and stellar luminosity for the five observed sources, which in turn have been used to infer their mass (ranging between 0.3-1.2 M ○. ) and age (between 10 5 and 10 6 yr) adopting pre-main sequence evolutionary tracks. We find that in HH100 IR and IRS2 most of the bolometric luminosity is due to accretion (L acc /L bol ∼ 0.8 and 0.6 respectively), while the other three investigated sources, including the Class I object IRS5a, present low accretion activity (L acc /L bol < 0.2). Mass accretion rates of the order of 2 x 10 -6 and 3 x 10 -7 M ○. yr -1 are derived for HH100 IR and IRS2, respectively, i.e. higher by an order of magnitude than the average values derived for T Tauri stars. We observe a general correlation between the accretion luminosity, the IR veiling and the emission line activity of the sources. In particular, we find that the correlation between L acc and L Brγ , previously reported for optical T Tauri stars, can be extended to the embedded sources, up to at least one order of magnitude larger line luminosity. A correlation between the accretion activity and the spectral energy distribution slope is recognizable with the notable exception of IRS5a. Our analysis therefore shows how the definition of the evolutionary stage of deeply embedded YSOs by means of IR colors needs to be refined.