The mass accretion rate,Ṁ acc , is a key quantity for the understanding of the physical processes governing the evolution of accretion discs around young low-mass (M 2.0M ) 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, L acc , 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 ≈0.1L , where L acc is always lower than 0.01 L . We argue that the L acc -L slope is not due to observational biases, but is a true property of the Lupus YSOs. The logṀ acclogM 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 theṀ acc -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 L acc and luminosity, L line , of many permitted emission lines. Hence, we also refined the empirical relationships between L acc and L line on a more solid statistical basis.

We present VLT/X-shooter observations of a sample of 36 accreting low-mass stellar and substellar objects (YSOs) in the Lupus star-forming region, spanning a range in mass from ∼0.03 to ∼1.2 M , but mostly with 0.1 M < M < 0.5 M . Our aim is twofold: firstly, to analyse the relationship between excess-continuum and line emission accretion diagnostics, and, secondly, to investigate the accretion properties in terms of the physical properties of the central object. The accretion luminosity (L acc ), and in turn the accretion rate (Ṁ acc ), was derived by modelling the excess emission from the UV to the near-infrared as the continuum emission of a slab of hydrogen. We computed the flux and luminosity (L line ) of many emission lines of H , He , and Ca ii, observed simultaneously in the range from ∼330 nm to 2500 nm. The luminosity of all the lines is well correlated with L acc . We provide empirical relationships between L acc and the luminosity of 39 emission lines, which have a lower dispersion than relationships previously reported in the literature. Our measurements extend the Paβ and Brγ relationships to L acc values about two orders of magnitude lower than those reported in previous studies. We confirm that different methodologies of measuring L acc andṀ acc yield significantly different results: Hα line profile modelling may underestimateṀ acc by 0.6 to 0.8 dex with respect toṀ acc derived from continuum-excess measures. These differences may explain the probably spurious bi-modal relationships betweenṀ acc and other YSOs properties reported in the literature. We derivedṀ acc in the range 2 × 10 −12 -4 × 10 −8 M yr −1 and conclude thatṀ acc ∝ M 1.8(±0.2) , with a dispersion lower by a factor of about 2 than in previous studies. A number of properties indicate that the physical conditions of the accreting gas are similar over more than 5 orders of magnitude inṀ acc , confirming previous suggestions that the geometry of the accretion flow controls the rate at which the disc material accretes onto the central star.

Context. A key science goal of the Gaia-ESO survey (GES) at the VLT is to use the kinematics of low-mass stars in young clusters and star forming regions to probe their dynamical histories and how they populate the field as they become unbound. The clustering of low-mass stars around the massive Wolf-Rayet binary system γ 2 Velorum was one of the first GES targets. Aims. We empirically determine the radial velocity precision of GES data, construct a kinematically unbiased sample of cluster members and characterise their dynamical state. Methods. Targets were selected from colour-magnitude diagrams and intermediate resolution spectroscopy was used to derive radial velocities and assess membership from the strength of the Li i 6708 Å line. The radial velocity distribution was analysed using a maximum likelihood technique that accounts for unresolved binaries. Results. The GES radial velocity precision is about 0.25 km s −1 and sufficient to resolve velocity structure in the low-mass population around γ 2 Vel. The structure is well fitted by two kinematic components with roughly equal numbers of stars; the first has an intrinsic dispersion of 0.34 ± 0.16 km s −1 , consistent with virial equilibrium. The second has a broader dispersion of 1.60 ± 0.37 km s −1 and is offset from the first by 2 km s −1 . The first population is older by 1-2 Myr based on a greater level of Li depletion seen among its M-type stars and is probably more centrally concentrated around γ 2 Vel. Conclusions. We consider several formation scenarios, concluding that the two kinematic components are a bound remnant of the original, denser cluster that formed γ 2 Vel, and a dispersed population from the wider Vela OB2 association, of which γ 2 Vel is the most massive member. The apparent youth of γ 2 Vel compared to the older (≥10 Myr) low-mass population surrounding it suggests a scenario in which the massive binary formed in a clustered environment after the formation of the bulk of the low-mass stars.

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 $1.75 deg 2 and study the spatial distribution and properties of the cloud members and candidate preY main-sequence ( PMS) objects and their circumstellar matter. Our census is complete down to the substellar regime (M % 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 À3 and 5Y10 members each. A multiplicity fraction of about 13% AE 3% is observed for objects with separations 0:8 00 < < 6:0 00 (142Y1065 AU ). No evidence for variability between the two epochs of the c2d IRAC data set, Át $ 6 hr, is detected. We estimate a star formation efficiency of 1%Y4%, 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 6Y7 M into stars every Myr, which is low in comparison with the star formation rate in other c2d clouds. The disk fraction of 70%Y80% 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.

We discuss the results of the optical spectroscopic follow-up of preYmain-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%Y4%) 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 H emission of the Cha II PMS objects, as well as possible correlations between their stellar and disk properties, is also investigated.