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The Very Large Telescope’s Ultraviolet and Visual Echelle Spectrograph (UVES) recently marked 20 years of operations, but the job is not done for this workhorse instrument, write Instrument Scientist Luca Sbordone and Fellow Camila Navarrete.

A primitive star in the Galactic halo For theoretical reasons and because of an apparent absence of stars with low metallicities (abundance of elements heavier than helium), it has been suggested that low-mass stars cannot form until the interstellar medium has been enriched above a critical value of metallicity, Z , estimated as lying between 1.5 × 10 −8 and 1.5 × 10 −6 . Caffau et al . now describe a star with a primordial-type composition, suggesting that, in fact, long-lived low-mass stars can form when the concentration of complex nuclei is low. The star is in the Galactic halo, has very low metallicity ( Z ≤ 6.9 × 10 −7 ) and no enrichment of carbon, nitrogen or oxygen. Its chemical composition should provide clues as to how the first stars formed. The early Universe had a chemical composition consisting of hydrogen, helium and traces of lithium 1 ; 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 2 , 3 , some of which have a low iron abundance but are rich in carbon, nitrogen and oxygen 4 , 5 , 6 . For theoretical reasons 7 , 8 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 9 . (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 10 ) and a chemical pattern typical of classical extremely metal-poor stars 2 , 3 —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 11 . 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.

We present the status of an ongoing project to map the detailed chemical abundances of stars across the main body of the Sagittarius dwarf Spheroidal galaxy (Sgr dSph). The Sgr dSph is the closest known dwarf galaxy, and is being tidally destroyed by its interaction with the Milky Way (MW), leaving behind a massive stellar stream. Sgr dSph is a chemically outstanding object, with peculiar abundance ratios, clear center-outskirts abundance gradients, and spanning more than 3 orders of magnitude in metallicity. We present here detailed abundances from UVES@VLT spectra for more than 50 giants across 8 fields along the major and minor axes of Sgr dSph, and 5 more outside the galaxy main body, but possibly associated to its stellar stream.

Simple spherical, non-rotating stellar models are inadequate when describing real stars in the limit of very fast rotation: Both the observable spectrum and the geometrical shape of the star deviate strongly from simple models. We attempt to approach the problem of modeling geometrically distorted, rapidly rotating stars from a new angle: By constructing distorted geometrical models and integrating standard stellar models with varying temperature, gravity, and abundances, over the entire surface, we attempt a semi-empirical approach to modeling. Here we present our methodology, and present simple examples of applications.

We present the abundance analysis of three very metal poor stars, CS 22166-0030 ([Fe/H]=−2.96), CS 22186-0005 ([Fe/H]=−2.70), and CS 30344-0033 ([Fe/H]=−2.90). Our study is based on high resolution spectra which were obtained from SARG (on TNG), HARPS (on 3.6m), and UVES (on VLT) spectrographs and one-dimensional ATLAS9 model atmospheres. We derived the abundances for 2, 9, and 16 atomic species in the spectrum of CS 22166-0030, CS 22186-0005, and CS 30344-0033, respectively. The Na and Mg abundances of CS 22166-0030 are highly under-abundant with respect to the solar values. The abundance patterns of CS 22186-0005 and CS 30344-0033 are consistent with the other halo stars within abundance uncertainties.

We successfully ported the suite of codes developed by R. L. Kurucz for stellar atmosphere modelling, abundance determination and synthetic spectra calculation, to run under GNU-Linux. The ported codes include ATLAS 9 and ATLAS 12 for 1-D plane-parallel atmosphere model calculation, DFSYNTHE, which calculates the Opacity Distribution Functions (ODF) to be used with ATLAS 9, WIDTH to derive chemical abundances from measured line Equivalent Widths (EW) and SYNTHE to calculate synthetic spectra. The codes input and output files remain fully compatible with the VMS versions, while the computation speed has been greatly increased due to the high efficiency of modern PC CPUs. As an example, ATLAS 9 model calculations and the computation of large (e.g. 10 nm) synthetic spectra can be executed in a matter of minutes on any mainstream laptop computer. Arbitrary chemical compositions can be used in calculations (by using ATLAS 12 through opacity sampling or by calculating ad-hoc ODFs for ATLAS 9). The large set of scripting languages existing under Linux (shell, perl, python. . .) and the availability of low-cost multiprocessor Linux architectures (such as Beowulf) makes the port highly effective to build model farms to produce large quantities of atmosphere models or synthetic spectra (e.g. for the production of integrated light synthetic spectra). The port is hosted on a dedicated website including a download section for source codes, precompiled binaries, needed data (opacities, line lists and so on), sample launch scripts and documentation.

Globular clusters (GCs) associated with the Sagittarius dwarf spheroidal galaxy (Sgr dSph) have evolved under the gravitational potential of both Sgr dSph and the Milky Way. The effects of these potentials are most pronounced in the extra-tidal regions as compared to the central regions of the GCs.We aim to study the extra-tidal regions of the GCs that are possibly associated with Sgr dSph, namely Arp 2, Terzan 8, NGC 5634, NGC 6284, Terzan 7, NGC 2419, NGC 4147, M 54 and Pal 12, using data from the {\\it Gaia} early data release 3. We selected the extra-tidal candidates based on their angular distances from the cluster centre in the RA-Dec plane, proper motions of the clusters and the individual extra-tidal star candidates, and their positions on the colour-magnitude diagrams of the clusters. We found extra-tidal candidates for the nine studied GCs. For eight of them, the surface density of candidate extra-tidal stars in the vicinity of the clusters is in significant excess with respect to more distant surrounding fields. No extended extra-tidal features beyond 5 tidal radii were detected for any of the clusters. We publish a list of the most probable extra-tidal candidates that we determined using Gaia astrometric and photometric data. Our analysis shows that the clusters that are associated with Sgr dSph are more likely affected by the gravitational potential of the Sgr, as the distribution of extra-tidal stars is elongated in the same direction as the local stream. NGC 4147 is the only exception. We found some high-probability candidate extra-tidal stars in several of the analysed clusters. We failed to detect any coherent large-scale tidal tail around them.