Spectroscopic properties of young stellar objects in the Lupus molecular clouds

Mon. Not. R. Astron. Soc. 418, 1194–1207 (2011) doi:10.1111/j.1365-2966.2011.19570.x Spectroscopic properties of young stellar objects in the Lupus molecular clouds Annelies Mortier,1,2 Isa Oliveira2 and Ewine F. van Dishoeck2,3 1 Centro de Astrofı́sica & Faculdade de Ciências, Universidade do Porto, Rua das Estrelas, 4150-762 Porto, Portugal Observatory, Leiden University, PO Box 9513, 2300 RA Leiden, the Netherlands 3 Max-Planck-Institut für Extraterrestriche Physik, PO Box 1312, D-85741 Garching, Germany 2 Leiden Accepted 2011 August 2. Received 2011 July 11; in original form 2011 April 11 ABSTRACT Key words: Hertzsprung–Russell and colour–magnitude diagrams – stars: pre-main-sequence – ISM: individual objects: Lupus. 1 I N T RO D U C T I O N The Lupus molecular clouds are the generic denomination of a loosely connected concentration of dark clouds and low-mass premain-sequence (PMS) stars located in the Scorpius–Centaurus OB association at 16h 20m < α < 15h 30m and −43◦ < δ < −33◦ . Due to its large size, close distance (d = 150–200 pc) and substantial mass of molecular gas, the Lupus clouds have been the subject of many studies at all wavelengths over the years (e.g. Hughes et al. 1994; Merı́n et al. 2008; see Comerón 2008 for a review; Comerón, Spezzi & López Martı́ 2009; Tothill et al. 2009). Lupus is one of the five clouds selected by the Spitzer Legacy Program ‘From Molecular Cores to Planet-Forming Disks’, also referred to as c2d (cores to discs, Evans et al. 2003). Using 400 hours of observations and all three instruments of Spitzer, the c2d programme studies the process of star and planet formation from the earliest stages of molecular cores to the epoch of planet-forming discs. The five observed clouds cover a range of cloud types broad  E-mail: enough to study all modes of low-mass star formation, and large enough to allow statistical conclusions. A rich population of lowmass young stellar objects (YSOs) still surrounded by their circumstellar material have been discovered by Spitzer in these clouds (Harvey et al. 2007a,b; Merı́n et al. 2008; Evans et al. 2009). Discs around YSOs, called protoplanetary discs, consist mainly of dust and gas. Dust particles of sub-μm size dominate the disc opacity, making it easily observable at infrared (IR) and (sub-)millimetre wavelengths by re-emitting some of the received stellar radiation. Dust emission is also temperature-dependent with colder dust emitting at longer wavelengths than warm dust. Protoplanetary discs evolve in time, ending up in different scenarios like a planetary system or a debris disc, that may or may not also harbour planets. The stellar radiation spectrum depends on the effective temperature of the star. Because of the properties of dust emission, the IR excess in a spectral energy distribution (SED) of this star+disc system provides information about the geometry and properties of the dusty disc. To separate star and disc emission, the stellar characteristics need to be known. The inner regions of the circumstellar  C 2011 The Authors C 2011 RAS Monthly Notices of the Royal Astronomical Society  The results of an optical spectroscopic survey of a sample of young stellar objects and premain-sequence stars in the Lupus clouds are presented. 92 objects were observed with the Very Large Telescope/Fibre Large Array Multi Element Spectrograph. All of those objects show infrared excess as discovered by the Spitzer Legacy Program ‘From Molecular Cores to Planet-Forming Disks’ (c2d). After reduction, 54 spectra with good signal-to-noise ratio are spectrally classified. Effective temperatures and luminosities are derived for these objects, and used to construct Hertzsprung–Russell diagrams for the population. The sample consists of mostly M-type stars, with 10 per cent K-type stars. Individual ages and masses are inferred for the objects according to theoretical evolutionary models. The mean population age is found to be between 3.6 and 4.4 Myr, depending on the model, while the mean mass is found to be ∼0.3 M for either model. Together with literature data, the distribution of spectral types is found to be similar to that in Chamaeleon I and IC 348. The Hα line in emission, found in 49 per cent of the sample, is used to distinguish between classical and weak-line T Tauri stars. 56 per cent of the objects show Hα in emission and are accreting T Tauri stars. Mass accretion rates between 10−8 and 10−11 M yr−1 are determined from the full width at 10 per cent of the Hα peak intensity. These mass accretion rates are, within a large scatter, consistent with the Ṁac ∝ M 2 relation found in the literature. YSOs in the Lupus molecular clouds 2 SAMPLE SELECTION Merı́n et al. (2008) used the Spitzer c2d Point Source Catalog to identify the YSO population in the Lupus clouds. Objects are classified as YSOs if they show an IR excess in the SED. To obtain an optimal separation between young stars, background galaxies and Galactic post-asymptotic giant branch (post-AGB) stars, Merı́n et al. (2008) used the selection criteria developed by the c2d team on its official point source catalogue (Evans et al. 2007). The method relies on an empirical probability function that depends on the relative position of any given source in several colour–colour and colour–magnitude diagrams where diffuse boundaries have been determined. The list of YSOs from Evans et al. (2007) was then adopted by Merı́n et al. (2008). Visual inspection was performed to subtract suspected galaxies or binaries, leaving the list with 94 YSOs. The final list of Merı́n et al. (2008) was merged with 65 PMS stars and PMS candidates. Here, the term PMS star is used for other objects added to the list whose youth had already been confirmed using other observational techniques, mainly optical spectroscopy. If an object has not been spectroscopically confirmed as young but it was selected by its optical and near-IR photometry as such, then it is labelled as a PMS candidate. This final list of 159 young objects is used for the observations presented here. 3 O B S E RVAT I O N S A N D DATA R E D U C T I O N The data presented here were taken in the second half of the nights of 2008 February 20–25 with the Very Large Telescope (VLT)  C 2011 The Authors, MNRAS 418, 1194–1207 C 2011 RAS Monthly Notices of the Royal Astronomical Society  and the instrument Fibre Large Array Multi Element Spectrograph (FLAMES)/GIRAFFE (ID: 080C.0473-A, PI: Oliveira). The instrument was used in the MEDUSA mode, with wavelength coverage of 6437–7183 Å and spectral resolution of 0.79 Å. This wavelength range was chosen for containing temperature-sensitive features, useful for spectral classification. Additionally, it covers the Hα line, an accretion diagnostic. MEDUSA has 135 fibres available, each with an aperture of 1.2 arcsec. In total, 250 stars in 19 fields were observed (on average: 14 stars per field). Of those, 158 are field stars, leaving the Lupus (...truncated)