In the last decade, the photospheric solar metallicity as determined from spectroscopy experienced a remarkable downward revision. Part of this effect can be attributed to an improvement of atomic ...data and the inclusion of NLTE computations, but also the use of hydrodynamical model atmospheres seemed to play a role. This “decrease” with time of the metallicity of the solar photosphere increased the disagreement with the results from helioseismology. With a CO
5
BOLD 3D model of the solar atmosphere, the CIFIST team at the Paris Observatory re-determined the photospheric solar abundances of several elements, among them C, N, and O. The spectroscopic abundances are obtained by fitting the equivalent width and/or the profile of observed spectral lines with synthetic spectra computed from the 3D model atmosphere. We conclude that the effects of granular fluctuations depend on the characteristics of the individual lines, but are found to be relevant only in a few particular cases. 3D effects are not responsible for the systematic lowering of the solar abundances in recent years. The solar metallicity resulting from this analysis is
Z
=0.0153,
Z
/
X
=0.0209.
Context.
The measurement of
α
-element abundances provides a powerful tool for placing constraints on the chemical evolution and star formation history of galaxies. The majority of studies on the
α
...-element sulfur (S) are focused on local stars, making S behavior in other environments an astronomical topic that is yet to be explored in detail.
Aims.
The investigation of S in the Galactic bulge was recently considered for the first time. This work aims to improve our knowledge on S behavior in this component of the Milky Way.
Methods.
We present the S abundances of 74 dwarf and sub-giant stars in the Galactic bulge, along with 21 and 30 F and G thick- and thin-disk stars, respectively. We performed a local thermodynamic equilibrium analysis and applied corrections for non-LTE on high resolution and high signal-to-noise UVES spectra. S abundances were derived from multiplets 1, 6, and 8 in the metallicity range of − 2 < Fe/H < 0.6, by spectrosynthesis or line equivalent widths.
Results.
We confirm that the behavior of S resembles that of an
α
-element within the Galactic bulge. In the S/Fe versus Fe/H diagram, S presents a plateau at low metallicity, followed by a decreasing of S/Fe with the increasing of Fe/H, before reaching S/Fe ~ 0 at a super-solar metallicity. We found that the Galactic bulge is S-rich with respect to both the thick- and thin-disks at − 1 < Fe/H < 0.3, supporting a scenario of more rapid formation and chemical evolution in the Galactic bulge than in the disk.
Abstract
We have developed a model atom for Cu with which we perform statistical equilibrium computations that allow us to compute the line formation of Cu i lines in stellar atmospheres without ...assuming local thermodynamic equilibrium (LTE). We validate this model atom by reproducing the observed line profiles of the Sun, Procyon and 11 metal-poor stars. Our sample of stars includes both dwarfs and giants. Over a wide range of stellar parameters, we obtain excellent agreement among different Cu i lines. The 11 metal-poor stars have iron abundances in the range − 4.2 ≤ Fe/H ≤ -1.4, the weighted mean of the Cu/Fe ratios is −0.22 dex, with a scatter of −0.15 dex. This is very different from the results from LTE analysis (the difference between NLTE and LTE abundances reaches 1 dex) and in spite of the small size of our sample, it prompts for a revision of the Galactic evolution of Cu.
ABSTRACT
The very massive first stars (m > 100 $\rm M_{\odot }$) were fundamental to the early phases of reionization, metal enrichment, and supermassive black hole formation. Among them, those with ...$140\le \rm m/\rm M_{\odot }\le 260$ are predicted to evolve as Pair Instability Supernovae (PISN) leaving a unique chemical signature in their chemical yields. Still, despite long searches, the stellar descendants of PISN remain elusive. Here we propose a new methodology, the PISN-explorer, to identify candidates for stars with a dominant PISN enrichment. The PISN-explorer is based on a combination of physically driven models, and the FERRE code; and applied to data from large spectroscopic surveys (APOGEE, GALAH, GES, MINCE, and the JINA data base). We looked into more than 1.4 million objects and built a catalogue with 166 candidates of PISN descendants. One of which, 2M13593064+3241036, was observed with UVES at VLT and full chemical signature was derived, including the killing elements, Cu and Zn. We find that our proposed methodology is efficient in selecting PISN candidates from both the Milky Way and dwarf satellite galaxies such as Sextans or Draco. Further high-resolution observations are highly required to confirm our best selected candidates, therefore allowing us to probe the existence and properties of the very massive First Stars.
The CoRoT and Kepler space-borne missions have provided us with a wealth of high-quality observational data that allows for seismic inferences of stellar interiors. This requires the computation of ...precise and accurate theoretical frequencies, but imperfect modeling of the uppermost stellar layers introduces systematic errors. To overcome this problem, an empirical correction has been introduced by Kjeldsen et al. and is now commonly used for seismic inferences. Our aim is to constrain the surface-effect corrections across the Hertzsprung-Russell (HR) diagram using a set of 3D hydrodynamical simulations. We used a grid of these simulations computed with the COsup 5 BOLD code to model the outer layers of solar-like stars. Upper layers of the corresponding 1D standard models were then replaced by the layers obtained from the horizontally averaged 3D models. Surface-effect corrections vary significantly across the HR diagram. Therefore, empirical relations like those by Kjeldsen et al. must not be calibrated on the Sun but should instead be constrained using realistic physical modeling as provided by 3D hydrodynamical simulations.
Aims. A probable carbon enhanced metal-poor (CEMP) star, Pisces II 10694, was discovered recently in the ultra-faint (UFD) galaxy Pisces II. This galaxy is supposed to be very old, suspected to ...include dark matter, and likely formed the bulk of its stars before the reionisation of the Universe. Methods. New abundances have been obtained from observations of Pisces II 10694 at the Kueyen ESO VLT telescope, using the high-efficiency spectrograph: X-shooter. Results. We found that Pisces II 10694 is a CEMP-no star with Fe/H = −2.60 dex. Careful measurements of the CH and C2 bands confirm the enhancement of the C abundance (C/Fe = +1.23). This cool giant has very probably undergone extra mixing and thus its original C abundance could be even higher. Nitrogen, O, Na, and Mg are also strongly enhanced, but from Ca to Ni the ratios X/Fe are similar to those observed in classical very metal-poor stars. With its low Ba abundance (Ba/Fe = −1.10 dex) Pisces II 10694 is a CEMP-no star. No variation in the radial velocity could be detected between 2015 and 2017. The pattern of the elements has a shape similar to the pattern found in galactic CEMP-no stars like CS 22949-037 (Fe/H = −4.0) or SDSS J1349+1407 (Fe/H = −3.6). Conclusions. The existence of a CEMP-no star in the UFD galaxy Pisc II suggests that this small galaxy likely hosted zero-metallicity stars. This is consistent with theoretical predictions of cosmological models supporting the idea that UFD galaxies are the living fossils of the first star-forming systems.
The photospheric solar oxygen project Steffen, M; Prakapavicius, D; Caffau, E ...
Astronomy and astrophysics (Berlin),
11/2015, Letnik:
583
Journal Article
Recenzirano
Odprti dostop
The solar photospheric oxygen abundance is still widely debated. Adopting the solar chemical composition based on the "low" oxygen abundance, as determined with the use of three-dimensional (3D) ...hydrodynamical model atmospheres, results in a well-known mismatch between theoretical solar models and helioseismic measurements that is so far unresolved. We carry out an independent re-determination of the solar oxygen abundance by investigating the center-to-limb variation of the O I IR triplet lines at 777 nm in different sets of spectra. The high-resolution and high signal-to-noise solar center-to-limb spectra are analyzed with the help of detailed synthetic line profiles based on 3D hydrodynamical CO5BOLD model atmospheres and 3D non-LTE line formation calculations with NLTE3D. The analysis of various observations of the triplet lines with different methods yields oxygen abundance values that fall in the range 8.74 < A(O) < 8.78, and our best estimate of the 3D non-LTE solar oxygen abundance is A(O) = 8.76 + or - 0.02.
Context. The current and planned high-resolution, high-multiplexity stellar spectroscopic surveys, as well as the swelling amount of underutilized data present in public archives, have led to an ...increasing number of efforts to automate the crucial but slow process of retrieving stellar parameters and chemical abundances from spectra. Aims. We present MyGIsFOS1, a code designed to derive atmospheric parameters and detailed stellar abundances from medium- to high-resolution spectra of cool (FGK) stars. We describe the general structure and workings of the code, present analyses of a number of well-studied stars representative of the parameter space MyGIsFOS is designed to cover, and give examples of the exploitation of MyGIsFOS very fast analysis to assess uncertainties through Monte Carlo tests. Methods. MyGIsFOS aims to reproduce a “traditional” manual analysis by fitting spectral features for different elements against a precomputed grid of synthetic spectra. The lines of Fe i and Fe ii can be employed to determine temperature, gravity, microturbulence, and metallicity by iteratively minimizing the dependence of Fe i abundance from line lower energy and equivalent width, and imposing Fe i-Fe ii ionization equilibrium. Once parameters are retrieved, detailed chemical abundances are measured from lines of other elements. Results. MyGIsFOS replicates closely the results obtained in similar analyses on a set of well-known stars. It is also quite fast, performing a full parameter determination and detailed abundance analysis in about two minutes per star on a mainstream desktop computer. Currently, its preferred field of application are high-resolution and/or large spectral coverage data (e.g., UVES, X-shooter, HARPS, Sophie).
Aims. The abundance patterns of the neutron-capture elements in metal-poor stars provide a unique record of the nucleosynthesis products of the earlier massive primitive objects. Methods. We measured ...new abundances of so-called light neutron-capture of first peak elements using local thermodynamic equilibrium (LTE) 1D analysis; this analysis resulted in a sample of 11 very metal-poor stars, from Fe/H = –2.5 to Fe/H = –3.4, and one carbon-rich star, CS 22949-037 with Fe/H = –4.0. The abundances were compared to those observed in two classical metal-poor stars: the typical r-rich star CS 31082-001 (Eu/Fe > +1.0) and the r-poor star HD 122563 (Eu/Fe < 0.0), which are known to present a strong enrichment of the first peak neutron-capture elements relative to the second peak. Results. Within the first peak, the abundances are well correlated in analogy to the well-known correlation inside the abundances of the second-peak elements. In contrast, there is no correlation between any first peak element with any second peak element. We show that the scatter of the ratio of the first peak abundance over second peak abundance increases when the mean abundance of the second peak elements decreases from r-rich to r-poor stars. We found two new r-poor stars that are very similar to HD 122563. A third r-poor star, CS 22897-008, is even more extreme; this star shows the most extreme example of first peak elements enrichment to date. On the contrary, another r-poor star (BD–18 5550) has a pattern of first peak elements that is similar to the typical r-rich stars CS 31082-001, however this star has some Mo enrichment. Conclusions. The distribution of the neutron-capture elements in our very metal-poor stars can be understood as the combination of at least two mechanisms: one that enriches the forming stars cloud homogeneously through the main r-process and leads to an element pattern similar to the r-rich stars, such as CS 31082-001; and another that forms mainly lighter, first peak elements.
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