The X-shooter Spectral Library (XSL): I. DR1. Near-ultraviolet through optical spectra from the first year of the survey

We present the first release of XSL, the X-Shooter Spectral Library. This release contains 237 stars spanning the wavelengths 3000--10200 \AA\ observed at a resolving power $R \equiv \lambda / \Delta\lambda \sim 10000$. The spectra were obtained at ESO's 8-m Very Large Telescope (VLT). The sample contains O -- M, long-period variable (LPV), C and S stars. The spectra are flux-calibrated and telluric-corrected. We describe a new technique for the telluric correction. The wavelength coverage, spectral resolution and spectral type of this library make it well suited to stellar population synthesis of galaxies and clusters, kinematical investigation of stellar systems and studying the physics of cool stars.Comment: 41 pages, 38 figures, 5 tables. Accepted for publication in A&A. Webpage: http://xsl.u-strasbg.fr

Flux calibration of medium-resolution spectra from 300 nm to 2500 nm: Model reference spectra and telluric correction

While the near-infrared wavelength regime is becoming more and more important for astrophysics there is a marked lack of spectrophotometric standard star data that would allow the flux calibration of such data. Furthermore, flux calibrating medium- to high-resolution \'echelle spectroscopy data is challenging even in the optical wavelength range, because the available flux standard data are often too coarsely sampled. We will provide standard star reference data that allow users to derive response curves from 300nm to 2500nm for spectroscopic data of medium to high resolution, including those taken with \'echelle spectrographs. In addition we describe a method to correct for moderate telluric absorption without the need of observing telluric standard stars. As reference data for the flux standard stars we use theoretical spectra derived from stellar model atmospheres. We verify that they provide an appropriate description of the observed standard star spectra by checking for residuals in line cores and line overlap regions in the ratios of observed (X-shooter) spectra to model spectra. The finally selected model spectra are then corrected for remaining mismatches and photometrically calibrated using independent observations. The correction of telluric absorption is performed with the help of telluric model spectra.We provide new, finely sampled reference spectra without telluric absorption for six southern flux standard stars that allow the users to flux calibrate their data from 300 nm to 2500 nm, and a method to correct for telluric absorption using atmospheric models.Comment: Reference spectra available at CDS. Published in A&A 568, A9, 201

Carbon stars in the X-shooter Spectral Library

We provide a new collection of spectra of 35 carbon stars obtained with the ESO/VLT X-shooter instrument as part of the X-shooter Spectral Library project. The spectra extend from 0.3$\mu$m to 2.4$\mu$m with a resolving power above $\sim$ 8000. The sample contains stars with a broad range of (J-K) color and pulsation properties located in the Milky Way and the Magellanic Clouds. We show that the distribution of spectral properties of carbon stars at a given (J-K) color becomes bimodal (in our sample) when (J-K) is larger than about 1.5. We describe the two families of spectra that emerge, characterized by the presence or absence of the absorption feature at 1.53$\mu$m, generally associated with HCN and C$_2$H$_2$. This feature appears essentially only in large-amplitude variables, though not in all observations. Associated spectral signatures that we interpret as the result of veiling by circumstellar matter, indicate that the 1.53$\mu$m feature might point to episodes of dust production in carbon-rich Miras.Comment: 29 pages, 21 figures, 9 tables, Accepted for publication in A&

THESEUS1 and RALF34 monitor cell wall integrity

The cell wall is a rigid network being the first barrier between a plant cell and its environment, but at the same time has to be a dynamic network whose cell growth and shape is given by deposition and remodeling of the cell wall. Maintaining cell wall integrity (CWI) is essential for correct plant development and stress response. Members of the family of Catharanthus roseus receptor-like kinase 1-like (CrRLK1L) proteins have been shown to play a role in cell wall homeostasis, mechanoperception, CWI maintenance and growth control. One of the 17 members in Arabidopsis, THESEUS1 (THE1), was identified in a suppressor screen of a cellulose deficient mutant, revealing that the reduction in growth is part of a THE1-mediated compensatory response to cell wall perturbation (Hématy et al., 2007). Interestingly, several CrRLK1L members have been shown to be receptors for Rapid Alkalinisation Factor (RALF) peptides. RALFs are on average 50 amino acids highly basic, cysteine-rich peptides, most of which are predicted to be cleaved from a highly acidic prodomain by a subtilisin protease. Recently, THE1 has been identified as a receptor for RALF34 (Gonneau et al., 2018). However, this peptide might not be the only THE1 ligand, since ralf34 loss-of-function mutants do not phenocopy all aspects of the1 mutants. RALF24 and RALF31 clustered together with RALF34, based on expression values across different tissues. We generated CRISPR/Cas9 mutants on RALF24, RALF31 and RALF34 with the aim to study which THE1 responses depend on these peptides. Our data suggest that RALF34 could not be the ligand for CWI response of THE1. The presence of RALF34 is acting negatively through THE1, inhibiting its response to cell wall damage.Universidad de Málaga. Campus de Excelencia Internacional Andalucía Tech

Modelling simple stellar populations in the near-ultraviolet to near-infrared with the X-shooter Spectral Library (XSL)

We present simple stellar population models based on the empirical X-shooter Spectral Library (XSL) from NUV to NIR wavelengths. The unmatched characteristics of relatively high resolution and extended wavelength coverage ($350-2480$ nm, $R\sim10\,000$) of the XSL population models bring us closer to bridging optical and NIR studies of intermediate and old stellar populations. It is now common to find good agreement between observed and predicted NUV and optical properties of stellar clusters due to our good understanding of the main-sequence and early giant phases of stars. However, NIR spectra of intermediate-age and old stellar populations are sensitive to cool K and M giants. The asymptotic giant branch, especially the thermally pulsing asymptotic giant branch, shapes the NIR spectra of $0.5-2$ Gyr old stellar populations; the tip of the red giant branch defines the NIR spectra of populations with ages larger than that. We construct sequences of the average spectra of static giants, variable-rich giants, and C-rich giants to include in the models separately. The models span the metallicity range $-2.2<[Fe/H]<+0.2$ and ages above 50 Myr, a broader range in the NIR than in other models based on empirical spectral libraries. Our models can reproduce the integrated optical colours of the Coma cluster galaxies at the same level as other semi-empirical models found in the literature. In the NIR, there are notable differences between the colours of the models and Coma cluster galaxies. The XSL models expand the range of predicted values of NIR indices compared to other models based on empirical libraries. Our models make it possible to perform in-depth studies of colours and spectral features consistently throughout the optical and the NIR range to clarify the role of evolved cool stars in stellar populations.Comment: 30 pages, 26 figures, accepted to Astronomy & Astrophysics, models will be available on http://xsl.astro.unistra.fr/ upon publishin

The Gaia-ESO Survey: Membership probabilities for stars in 32 open clusters from 3D kinematics

The Gaia-ESO Survey (GES) observed many open clusters as part of its programme to spectroscopically characterise the various Milky Way populations. GES spectroscopy and Gaia astrometry from its second data release are used here to assign membership probabilities to targets towards 32 open clusters with ages from 1-3800 Myr, based on maximum likelihood modelling of the 3D kinematics of the cluster and field populations. From a parent catalogue of 14398 individual targets, 5033 stars with uniformly determined 3D velocities, $T_{\rm eff}$, $\log g$ and chemistry are assigned cluster membership with probability $>0.9$, and with an average probability of 0.991. The robustness of the membership probabilities is demonstrated using independent membership criteria (lithium and parallax) in two of the youngest clusters. The addition of radial velocities improves membership discrimination over proper motion selection alone, especially in more distant clusters. The kinematically-selected nature of the membership lists, independent of photometry and chemistry, makes the catalogue a valuable resource for testing stellar evolutionary models and investigating the time evolution of various parameters

The Gaia-ESO Survey: Empirical estimates of stellar ages from lithium equivalent widths (EAGLES)

We present an empirical model of age-dependent photospheric lithium depletion, calibrated using a large, homogeneously-analysed sample of 6200 stars in 52 open clusters, with ages from 2--6000 Myr and $-0.3<{\rm [Fe/H}]<0.2$, observed in the Gaia-ESO spectroscopic survey. The model is used to obtain age estimates and posterior age probability distributions from measurements of the Li I 6708A equivalent width for individual (pre) main sequence stars with $3000 < T_{\rm eff}/{\rm K} <6500$, a domain where age determination from the HR diagram is either insensitive or highly model-dependent. In the best cases, precisions of 0.1 dex in log age are achievable; even higher precision can be obtained for coeval groups and associations where the individual age probabilities of their members can be combined. The method is validated on a sample of exoplanet-hosting young stars, finding agreement with claimed young ages for some, but not others. We obtain better than 10 per cent precision in age, and excellent agreement with published ages, for seven well-studied young moving groups. The derived ages for young clusters ($<1$ Gyr) in our sample are also in good agreement with their training ages, and consistent with several published, model-insensitive lithium depletion boundary ages. For older clusters there remain systematic age errors that could be as large as a factor of two. There is no evidence to link these errors to any strong systematic metallicity dependence of (pre) main sequence lithium depletion, at least in the range $-0.29 < {\rm [Fe/H]} < 0.18$. Our methods and model are provided as software -- "Empirical AGes from Lithium Equivalent widthS" (EAGLES).Comment: Accepted for publication in Monthly Notices of the Royal Astronomical Societ

The X-shooter Spectral Library (XSL): Data Release 3

We present the third data release (DR3) of the X-shooter Spectral Library (XSL). This moderate-to-high resolution, near-ultraviolet-to-near-infrared ($350-2480$ nm, R $\sim$ 10 000) spectral library is composed of 830 stellar spectra of 683 stars. DR3 improves upon the previous data release by providing the combined de-reddened spectra of the three X-shooter segments over the full $350-2480$ nm wavelength range. It also includes additional 20 M-dwarf spectra from the ESO archive. We provide detailed comparisons between this library and Gaia EDR3, MILES, NGSL, CaT library, and (E-)IRTF. The normalised rms deviation is better than $D=0.05$ or 5$\%$ for the majority of spectra in common between MILES (144 spectra of 180), NGSL (112$/$116), and (E-)IRTF (55$/$77) libraries. Comparing synthetic colours of those spectra reveals only negligible offsets and small rms scatter, such as the median offset(rms) 0.001$\pm$0.040 mag in the (box1-box2) colour of the UVB arm,-0.004$\pm$0.028 mag in (box3-box4) of the VIS arm, and -0.001$\pm$0.045 mag in (box2-box3) colour between the UVB and VIS arms, when comparing stars in common with MILES. We also find an excellent agreement between the Gaia published (BP-RP) colours and those measured from the XSL DR3 spectra, with a zero median offset and an rms scatter of 0.037 mag for 449 non-variable stars. The unmatched characteristics of this library, which combine a relatively high resolution, a large number of stars, and an extended wavelength coverage, will help us to bridge the gap between the optical and the near-IR studies of intermediate and old stellar populations, and to probe low-mass stellar systems.Comment: 26 pages, 25 figures, accepted to Astronomy & Astrophysics. The data are available at the CDS via anonymous ftp to cdsarc.u-strasbg.fr(130.79.128.5) or via http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/ or on the XSL web-page http://xsl.astro.unistra.f

The Gaia -ESO Survey: Galactic evolution of lithium at high metallicity

Context. Reconstructing the Galactic evolution of lithium (Li) is the main tool used to constrain the source(s) of Li enrichment in the Galaxy. Recent results have suggested a decline in Li at supersolar metallicities, which may indicate reduced production. Aims. We exploit the unique characteristics of the Gaia-ESO Survey open star cluster sample to further investigate this issue and to better constrain the evolution of Li at high metallicity. Methods. We trace the upper envelope of Li abundance versus metallicity evolution using 18 clusters and considering members that should not have suffered any Li depletion. Results. At variance with previous claims, we do not find any evidence of a Li decrease at high metallicity. The most metal-rich clusters in the sample ([Fe/H] = ∼0.3) actually show the highest Li abundances, with A(Li) > 3.4. Our results clearly show that previous findings, which were based on field stars, were affected by selection effects. The metal-rich population in the solar neighbourhood is composed of relatively old and cool stars that have already undergone some Li depletion; hence, their measured Li does not represent the initial interstellar medium abundance, but a lower limit to it