S. Randich

Accretion in the

Aims. The aim of this paper is to provide a measurement of the mass accretion rate in a large, complete sample of objects in the core of the star forming region p Oph. Methods. The sample includes most of the objects (104 out of 111) with evidence of a circumstellar disk from mid-infrared photometry; it covers a stellar mass range from about 0.03 to 3 M ⊙ and it is complete to a limiting mass of ∼0.05 M ⊙ . We used J and K-band spectra to derive the mass accretion rate of each object from the intensity of the hydrogen recombination lines, Pap or Bry. For comparison, we also obtained similar spectra of 35 diskless objects. Results. The results show that emission in these lines is only seen in stars with disks, and can be used as an indicator of accretion. However, the converse does not hold, as about 50% of our disk objects do not have detectable line emission. The measured accretion rates show a strong correlation with the mass of the central object (Mace oc M 1.8±0.2 *) and a large spread, of two orders of magnitude at least, for any interval of M*. A comparison with existing data for Taurus shows that the objects in the two regions have similar behaviour, at least for objects more massive than ∼0.1 M ⊙ . The implications of these results are briefly discussed.

\mathsf{\rho}

The early Universe had a chemical composition consisting of hydrogen, helium and traces of lithium; almost all other elements were subsequently created in stars and supernovae. The mass fraction of elements more massive than helium, Z, is known as ‘metallicity’. A number of very metal-poor stars has been found, some of which have a low iron abundance but are rich in carbon, nitrogen and oxygen. For theoretical reasons and because of an observed absence of stars with Z < 1.5 × 10−5, it has been suggested that low-mass stars cannot form from the primitive interstellar medium until it has been enriched above a critical value of Z, estimated to lie in the range 1.5 × 10−8 to 1.5 × 10−6 (ref. 8), although competing theories claiming the contrary do exist. (We use ‘low-mass’ here to mean a stellar mass of less than 0.8 solar masses, the stars that survive to the present day.) Here we report the chemical composition of a star in the Galactic halo with a very low Z (≤ 6.9 × 10−7, which is 4.5 × 10−5 times that of the Sun) and a chemical pattern typical of classical extremely metal-poor stars—that is, without enrichment of carbon, nitrogen and oxygen. This shows that low-mass stars can be formed at very low metallicity, that is, below the critical value of Z. Lithium is not detected, suggesting a low-metallicity extension of the previously observed trend in lithium depletion. Such lithium depletion implies that the stellar material must have experienced temperatures above two million kelvin in its history, given that this is necessary to destroy lithium.

-Ophiuchi pre-main sequence stars

We have performed a new and homogeneous analysis of all the Li data available in the literature for main sequence stars (spectral-types from late F to K) in open clusters. In the present paper we focus on a detailed investigation of MS Li depletion and its time scales for stars in the 6350-5500 K effective temperature range. For the first time, we were able to constrain the age at which non-standard mixing processes, driving MS Li depletion, appear. We have also shown that MS Li depletion is not a continuous process and cannot be simply described by a t^(-alpha) law. We confirm that depletion becomes ineffective beyond an age of 1-2 Gyr for the majority of the stars, leading to a Li plateau at old ages. We compared the empirical scenario of Li as a function of age with the predictions of three non-standard models. We found that models including only gravity waves as main mixing process are not able to fit the Li vs. age pattern and thus this kind of mixing can be excluded as the predominant mechanism responsible for Li depletion. On the other hand, models including slow mixing induced by rotation and angular momentum loss, and in particular those including also diffusive processes not related to rotation, can explain to some extent the empirical evidence. However, none of the currently proposed models can fit the plateau at old ages.

An extremely primitive star in the Galactic halo

We present X-Shooter/VLT observations of a sample of 36 accreting low-mass stellar and sub-stellar objects (YSOs) in the Lupus star forming region, spanning a range in mass from ~0.03 to ~1.2Msun, but mostly with 0.1Msun < Mstar < 0.5Msun. Our aim is twofold: firstly, 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 (Lacc), and from it the accretion rate (Macc), is derived by modelling the excess emission, from the UV to the near-IR, as the continuum emission of a slab of hydrogen. The flux and luminosity (Ll) of a large number of emission lines of H, He, CaII, etc., observed simultaneously in the range from ~330nm to 2500nm, were computed. The luminosity of all the lines is well correlated with Lacc. We provide empirical relationships between Lacc and the luminosity of 39 emission lines, which have a lower dispersion as compared to previous relationships in the literature. Our measurements extend the Pab and Brg relationships to Lacc values about two orders of magnitude lower than those reported in previous studies. We confirm that different methodologies to measure Lacc and Macc yield significantly different results: Ha line profile modelling may underestimate Macc by 0.6 to 0.8dex with respect to Macc derived from continuum-excess measures. Such differences may explain the likely spurious bi-modal relationships between Macc and other YSOs properties reported in the literature. We derive Macc in the range 2e-12 -- 4e-8 Msun/yr and conclude that Macc is proportional to Mstar^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 in Macc.

Time scales of Li evolution: a homogeneous analysis of open clusters from ZAMS to late-MS

The birth of stars involves not only accretion but also, counter-intuitively, the expulsion of matter in the form of highly supersonic outflows. Although this phenomenon has been seen in young stars, a fundamental question is whether it also occurs among newborn brown dwarfs: these are the so-called ‘failed stars’, with masses between stars and planets, that never manage to reach temperatures high enough for normal hydrogen fusion to occur. Recently, evidence for accretion in young brown dwarfs has mounted, and their spectra show lines that are suggestive of outflows. Here we report spectro-astrometric data that spatially resolve an outflow from a brown dwarf. The outflows characteristics appear similar to, but on a smaller scale than, outflows from normal young stars. This result suggests that the outflow mechanism is universal, and perhaps relevant even to the formation of planets.

X-shooter spectroscopy of young stellar objects - IV. Accretion in low-mass stars and substellar objects in Lupus

We present lithium abundances for50 X-ray selected candidate members of the 30{50 Myr old open clusters IC 2602 and IC 2391. These data enlarge and extend to cooler temperatures previous Li surveys of these clusters by Stauer et al. (1989) and Randich et al. (1997). We also give for the rst time an estimate of the metallicity of the two clusters which turns out to be close to solar. Radial velocity measurements together with H chromospheric emission and the presence/absence of other spectroscopic features are used to ascertain the membership status for the sample stars not yet conrmed as cluster members; rotational velocities have also been determined for all sample stars. Stars more massive than1 M in both clusters show no sign of signicant Li depletion, while lower mass stars are all lithium depleted, with the amount of Li depletion increasing to cooler temperatures. We conrm that the late{G and early{K stars in IC 2602 present a star-to-star scatter in Li abundances similar to, but not as large as the one in the Pleiades. A scatter is also seen among late{K and M dwarfs. Unlike in the Pleiades and Alpha Per clusters, the scatter among early{K stars in IC 2602 shows only marginal correlation with rotation. Our data suggest that the drop-o of lithium towards lower masses may start at an earlier color in IC 2391 than in IC 2602, but larger cluster samples are needed to conrm this result. In addition, whereas G and early K stars in the two clusters are, on average, more Li rich than their counterparts in the Pleiades, a fraction of the coolest stars, in particular in IC 2391, are as depleted as the lowest-Li Pleiades stars of the same mass. If they continue depleting Li on their way to the main sequence, they are expected to be more Li depleted than the Pleiades at the age of the latter cluster.

A resolved outflow of matter from a brown dwarf.

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 Orionis cluster. The sample includes eight stars with evidence of disks from Spitzer and four without, with masses ranging from 0.08 to 0.3 M . 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 and the accuracy of the correlations between these secondary tracers (mainly accretion line luminosities) found in the literature. 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 yr 1 down to 5 10 11 M 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, all obtained simultaneously. In general, most of the secondary tracers correlate well with the accretion luminosity derived from the continuum excess emission. We recompute the relationships between the accretion luminosities and the line luminosities, we confirm the validity of the correlations given in the literature, with the possible exception of H . Metallic lines, such as the CaII IR triplet or the Na I line at 589.3 nm, show a larger dispersion. 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 Lacc often in better agreement than the uncertainties introduced by the adopted correlations. The average Lacc 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.

Membership, lithium, and metallicity in the young open clusters IC 2602 and IC 2391: Enlarging the sample

Context: The discovery and chemical analysis of extremely metal-poor stars permit a better understanding of the star formation of the first generation of stars and of the Universe emerging from the Big Bang. aims: We report the study of a primordial star situated in the centre of the constellation Leo (SDSS J102915+172027). method: The star, selected from the low resolution-spectrum of the Sloan Digital Sky Survey, was observed at intermediate (with X-Shooter at VLT) and at high spectral resolution (with UVES at VLT). The stellar parameters were derived from the photometry. The standard spectroscopic analysis based on 1D ATLAS models was completed by applying 3D and non-LTE corrections. results: An iron abundance of [Fe/H]=--4.89 makes SDSS J102915+172927 one of the lowest [Fe/H] stars known. However, the absence of measurable C and N enhancements indicates that it has the lowest metallicity, Z<= 7.40x10^{-7} (metal-mass fraction), ever detected. No oxygen measurement was possible. conclusions: The discovery of SDSS J102915+172927 highlights that low-mass star formation occurred at metallicities lower than previously assumed. Even lower metallicity stars may yet be discovered, with a chemical composition closer to the composition of the primordial gas and of the first supernovae.

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

Context. The discovery of true solar analogs is fundamental to a better understanding of the Sun and of the solar system. Despite a number of efforts, this search has brought only limited results for field stars. The open cluster M 67 offers a unique opportunity to search for solar analogs, because its chemical composition and age are very similar to those of the Sun. Aims. We analyse FLAMES spectra of a large number of M 67 main sequence stars to identify solar analogs in this cluster. Methods. We first determined cluster members that are not likely binaries, by combining proper motions and radial velocity measurements. We concentrate our analysis on determining stellar effective temperature, using analyses of line-depth ratios and Hα wing and making a direct comparison to the solar spectrum obtained with the same instrument. We also computed the lithium abundance for all the stars. Results. Ten stars have the temperature derived both by line-depth ratios and by the Hα wings within 100 K from the Sun. From these stars we derive, assuming a cluster reddening E(B −V) = 0.041, the solar color (B −V)� = 0.649 ± 0.016 and a cluster distance modulus of 9.63. Five stars are most similar (within 60 K) to the Sun and candidates to be true solar twins. These stars also have a low Li content, comparable to the photospheric abundance of the Sun, likely indicating a similar mixing evolution. Conclusions. We find several candidates for the best solar analogs ever. These stars are amenable to further spectroscopic investigations and planet searches. The solar colors are determined with fairly high accuracy with an independent method, as is the cluster distance modulus.

A primordial star in the heart of the Lion

This work was partially supported by the Gaia Research for European Astronomy Training (GREAT-ITN) Marie Curie network, funded through the European Union Seventh Framework Programme [FP7/2007-2013] under grant agreement 264895 and supported by the European Union FP7 programme through ERC grant number 320360 and by the Leverhulme Trust through grant RPG-2012-541. We acknowledge the support from INAF and Ministero dell’ Istruzione, dell’ Universita e della Ricerca (MIUR) in the form of the grant “Premiale VLT 2012”. RJJ acknowledges financial support from the UK Science and Technology Facilities Council.