X-shooter spectroscopy of young stellar objects: I - Mass accretion rates of low-mass T Tauri stars in \sigma Orionis

We present high-quality, medium resolution X-shooter/VLT spectra in the range 300-2500 nm for a sample of 12 very low-mass stars in the \sigma Orionis cluster. The sample includes stars with masses ranging from 0.08 to 0.3 M$_\odot$. The aim of this first paper is to investigate the reliability of the many accretion tracers currently used to measure the mass accretion rate in low-mass, young stars. We use our spectra to measure the accretion luminosity from the continuum excess emission in the UV and visual; the derived mass accretion rates range from 10$^{-9}$ M$_{\odot}$ yr$^{-1}$ down to 5$\times10^{-11}$ M$_{\odot}$ yr$^{-1}$, allowing us to investigate the behavior of the accretion-driven emission lines in very-low mass accretion rate regimes. We compute the luminosity of ten accretion-driven emission lines, from the UV to the near-IR, obtained simultaneously. Most of the secondary tracers correlate well with the accretion luminosity derived from the continuum excess emission. We confirm the validity of the correlations between accretion luminosities and line luminosities given in the literature, with the possible exception of H\alpha. When looking at individual objects, we find that the Hydrogen recombination lines, from the UV to the near-IR, give good and consistent measurements of accretion luminosities, often in better agreement than the uncertainties introduced by the adopted correlations. The average accretion luminosity derived from several Hydrogen lines, measured simultaneously, have a much reduced error. This suggests that some of the spread in the literature correlations may be due to the use of non-simultaneous observations of lines and continuum. Three stars in our sample deviate from this behavior, and we discuss them individually.Comment: 15 pages, 14 figure

Search for young stars among ROSAT All-Sky Survey X-ray sources in and around the R CrA dark cloud

We present the ROSAT All-Sky Survey data in a 126 deg^2 area in and around the CrA star forming region. With low-resolution spectroscopy of unidentified ROSAT sources we could find 19 new pre-main sequence stars, two of which are classical T Tauri stars, the others being weak-lined. The spectral types of these new T Tauri stars range from F7 to M6. The two new classical T Tauri stars are located towards two small cloud-lets outside of the main CrA cloud. They appear to be ~10 Myrs old, by comparing their location in the H-R diagram with isochrones for an assumed distance of 130 pc, the distance of the main CrA dark cloud. The new off-cloud weak-line T Tauri stars may have formed in similar cloud-lets, which have dispersed recently. High-resolution spectra of our new T Tauri stars show that they have significantly more lithium absorption than zero-age main-sequence stars of the same spectral type, so that they are indeed young. From those spectra we also obtained rotational and radial velocities. For some stars we found the proper motion in published catalogs. The direction and velocity of the 3D space motion - south relative to the galatic plane - of the CrA T Tauri stars is consistent with the dark cloud being formed originally by a high-velocity cloud impact onto the galactic plane, which triggered the star formation in CrA. We also present VRIJHK photometry for most of the new T Tauri stars to derive their luminosities, ages, and masses.Comment: A&A Suppl. in pres

The Spitzer c2d survey of large, nearby, interstellar clouds. X. The Chamaeleon II pre-main-sequence population as observed with IRAC and MIPS

We discuss the results from the combined IRAC and MIPS c2d Spitzer Legacy survey observations and complementary optical and NIR data of the Chamaeleon II (Cha II) dark cloud. We perform a census of the young population in an area of similar to 1.75 deg^(2) and study the spatial distribution and properties of the cloud members and candidate pre-main-sequence (PMS) objects and their circumstellar matter. Our census is complete down to the substellar regime (M approximate to 0.03 M☉). From the analysis of the volume density of the PMS objects and candidates we find two groups of objects with volume densities higher than 25 M☉ pc^(-3) and 5-10 members each. A multiplicity fraction of about 13% +/- 3% is observed for objects with separations 0.8" < θ < 6.0" (142-1065 AU). No evidence for variability between the two epochs of the c2d IRAC data set, Δt ~ 6 hr, is detected. We estimate a star formation efficiency of 1%-4%, consistent with the estimates for Taurus and Lupus, but lower than for Cha I. This might mean that different star formation activities in the Chamaeleon clouds reflect a different history of star formation. We also find that Cha II is turning some 6-7 M☉ into stars every Myr, which is low in comparison with the star formation rate in other c2d clouds. The disk fraction of 70%-80% that we estimate in Cha II is much higher than in other star-forming regions and indicates that the population in this cloud is dominated by objects with active accretion. Finally, the Cha II outflows are discussed; a new Herbig-Haro outflow, HH 939, driven by the classical T Tauri star Sz 50, has been discovered

VizieR Online Data Catalog: HeI 5876 & 10830Å EWs of solar-type stars (Andretta+, 2017)

A total of 134 FEROS spectra (R=48000) of our targets (including telluric standards) were acquired on the night of UT 2011 December 6-7; spectral coverage from 3500 to 9200Å. The Fiber Extended-range Optical Spectrograph (FEROS) was mounted at the 2.2m Max-Planck Gesellschaft/European Southern Observatory (MPG/ESO) telescope at La Silla (Chile). The HeIλ10830 spectroscopic observations were carried out on the same night as the FEROS D3 observations, using the CRyogenic high-resolution InfraRed Echelle Spectrograph (CRIRES), mounted at Unit Telescope 1 (Antu) of the VLT array at Cerro Paranal. The details of the observations is given in table 1. (3 data files)

The young population of the Chamaeleon II dark cloud

We discuss the results of the optical spectroscopic follow-up of pre-main-sequence (PMS) objects and candidates selected in the Chamaeleon II dark cloud based on data from the Spitzer Legacy survey "From Molecular Cores to Planet Forming Disks" (c2d) and from previous surveys. Our sample includes both objects with infrared excess selected according to c2d criteria and referred to as young stellar objects and other cloud members and candidates selected from complementary optical and near-infrared data. We characterize the sample of objects by deriving their physical parameters. The vast majority of objects have masses M ≤ 1 M_☉ and ages <6 Myr. Several of the PMS objects and candidates lie very close to or below the hydrogen-burning limit. A first estimate of the slope of the initial mass function in Cha II is consistent with that of other T associations. The star formation efficiency in the cloud (1%-4%) is consistent with our own estimates for Taurus and Lupus, but significantly lower than for Cha I. This might mean that different star formation activities in the Chamaeleon clouds may reflect a different history of star formation. We also find that the Cha II cloud is turning some 8 M_☉ into stars every megayear, which is less than the star formation rate in the other c2d clouds. However, the star formation rate is not steady and evidence is found that the star formation in Cha II might have occurred very rapidly. The Hα emission of the Cha II PMS objects, as well as possible correlations between their stellar and disk properties, is also investigated

The star formation in the L1615/L1616 cometary cloud

The present work aims at performing a comprehensive census and characterisation of the pre-main sequence (PMS) population in the cometary cloud L1615/L1616, in order to assess the significance of the triggered star formation scenario and investigate the impact of massive stars on its star formation history and mass spectrum. Our study is based on UBVRcIc and JHKs photometry, as well as optical multi-object spectroscopy. We performed a physical parametrisation of the young stellar population in L1615/L1616. We identified 25 new T Tauri stars mainly projected on the dense head of the cometary cloud, almost doubling the current number of known members. We studied the spatial distribution of the cloud members as a function of the age and H$\alpha$ emission. The star formation efficiency in the cloud is about 7-8 %, as expected for molecular clouds in the vicinity of OB associations. The slope of the initial mass function (IMF), in the mass range 0.1<M<5.5 $M_{\odot}$, is consistent with that of other T and OB associations, providing further support of an universal IMF down to the hydrogen burning limit, regardless of environmental conditions. The cometary appearance, as well as the high star formation efficiency, can be explained in terms of triggered star formation induced by the strong UV radiation from OB stars or supernovae shockwaves. The age spread as well as both the spatial and age distribution of the PMS objects provide strong evidence of sequential, multiple events and possibly still ongoing star formation activity in the cloud.Comment: 59 pages, 14 figures, accepted for publication in Ap

X-shooter spectroscopy of young stellar objects in Lupus. Accretion properties of class II and transitional objects

The mass accretion rate, Ṁacc, is a key quantity for the understanding of the physical processes governing the evolution of accretion discs around young low-mass (M⋆ ≲ 2.0 M☉) stars and substellar objects (YSOs). We present here the results of a study of the stellar and accretion properties of the (almost) complete sample of class II and transitional YSOs in the Lupus I, II, III and IV clouds, based on spectroscopic data acquired with the VLT/X-shooter spectrograph. Our study combines the dataset from our previous work with new observations of 55 additional objects. We have investigated 92 YSO candidates in total, 11 of which have been definitely identified with giant stars unrelated to Lupus. The stellar and accretion properties of the 81 bona fide YSOs, which represent more than 90% of the whole class II and transition disc YSO population in the aforementioned Lupus clouds, have been homogeneously and self-consistently derived, allowing for an unbiased study of accretion and its relationship with stellar parameters. The accretion luminosity, Lacc, increases with the stellar luminosity, L⋆, with an overall slope of 1.6, similar but with a smaller scatter than in previous studies. There is a significant lack of strong accretors below L⋆ ≈ 0.1 L☉, where Lacc is always lower than 0.01 L⋆. We argue that the Lacc - L⋆ slope is not due to observational biases, but is a true property of the Lupus YSOs. The log Ṁacc - log M⋆ correlation shows a statistically significant evidence of a break, with a steeper relation for M⋆ ≲ 0.2 M☉ and a flatter slope for higher masses. The bimodality of the Ṁacc - M⋆ relation is confirmed with four different evolutionary models used to derive the stellar mass. The bimodal behaviour of the observed relationship supports the importance of modelling self-gravity in the early evolution of the more massive discs, but other processes, such as photo-evaporation and planet formation during the YSO's lifetime, may also lead to disc dispersal on different timescales depending on the stellar mass. The sample studied here more than doubles the number of YSOs with homogeneously and simultaneously determined Lacc and luminosity, Lline, of many permitted emission lines. Hence, we also refined the empirical relationships between Lacc and Lline on a more solid statistical basis. Based on observations collected at the European Southern Observatory at Paranal, under programs 084.C-0269(A), 085.C-0238(A), 086.C-0173(A), 087.C-0244(A), 089.C-0143(A), 095.C-0134(A), 097.C-0349(A), and archive data of programmes 085.C-0764(A) and 093.C-0506(A)