Mn abundances in the stars of the Galactic disc with metallicities -1.0 < [Fe/H] < 0.3

In this work we present and discuss the observations of the Mn abundances for 247 FGK dwarfs, located in the Galactic disc with metallicity -1 < [Fe/H]< +0.3. The observed stars belong to the substructures of the Galaxy thick and thin discs, and to the Hercules stream. The observations were conducted using the 1.93 m telescope at Observatoire de Haute-Provence (OHP, France) equipped with the echelle type spectrographs ELODIE and SOPHIE. The abundances were derived under the LTE approximation, with an average error for the [Mn/Fe] ratio of 0.10 dex. For most of the stars in the sample Mn abundances are not available in the literature. We obtain an evolution of [Mn/Fe] ratio with the metallicity [Fe/H] consistent with previous data compilations. In particular, within the metallicity range covered by our stellar sample the [Mn/Fe] ratio is increasing with the increase of metallicity. This due to the contribution to the Galactic chemical evolution of Mn and Fe from thermonuclear supernovae. We confirm the baseline scenario where most of the Mn in the Galactic disc and in the Sun is made by thermonuclear supernovae. In particular, the effective contribution from core-collapse supernovae to the Mn in the Solar system is about 10-20%. However, present uncertainties affecting the production of Mn and Fe in thermonuclear supernovae are limiting the constraining power of the observed [Mn/Fe] trend in the Galactic discs on, e.g., the frequency of different thermonuclear supernovae populations. The different production of these two elements in different types of thermonuclear supernovae needs to be disentangled by the dependence of their relative production on the metallicity of the supernova progenitor

Neutron Star Mergers Are the Dominant Source of the r-process in the Early Evolution of Dwarf Galaxies

There are many candidate sites of the r-process: core-collapse supernovae (including rare magnetorotational core-collapse supernovae), neutron star mergers, and neutron star/black hole mergers. The chemical enrichment of galaxies---specifically dwarf galaxies---helps distinguish between these sources based on the continual build-up of r-process elements. This technique can distinguish between the r-process candidate sites by the clearest observational difference---how quickly these events occur after the stars are created. The existence of several nearby dwarf galaxies allows us to measure robust chemical abundances for galaxies with different star formation histories. Dwarf galaxies are especially useful because simple chemical evolution models can be used to determine the sources of r-process material. We have measured the r-process element barium with Keck/DEIMOS medium-resolution spectroscopy. We present the largest sample of barium abundances (almost 250 stars) in dwarf galaxies ever assembled. We measure [Ba/Fe] as a function of [Fe/H] in this sample and compare with existing [alpha/Fe] measurements. We have found that a large contribution of barium needs to occur at more delayed timescales than core-collapse supernovae in order to explain our observed abundances, namely the significantly more positive trend of the r-process component of [Ba/Fe] vs. [Fe/H] seen for [Fe/H] <~ -1.6 when compared to the [Mg/Fe] vs. [Fe/H] trend. We conclude that neutron star mergers are the most likely source of r-process enrichment in dwarf galaxies at early times.Comment: Accepted to ApJ on 2018 October 2

Sulphur in the Sculptor dwarf spheroidal galaxy - Including NLTE corrections

In Galactic halo stars, sulphur has been shown to behave like other $\alpha$-elements, but until now, no comprehensive studies have been done on this element in stars of other galaxies. Here, we use high-resolution ESO VLT/FLAMES/GIRAFFE spectra to determine sulphur abundances for 85 stars in the Sculptor dwarf spheroidal galaxy, covering the metallicity range $-2.5\leq \text{[Fe/H]} \leq-0.8$. The abundances are derived from the S~I triplet at 9213, 9228, and 9238~\AA. These lines have been shown to be sensitive to departure from local thermodynamic equilibrium, i.e. NLTE effects. Therefore, we present new NLTE corrections for a grid of stellar parameters covering those of the target stars. The NLTE-corrected sulphur abundances in Sculptor show the same behaviour as other $\alpha$-elements in that galaxy (such as Mg, Si, and Ca). At lower metallicities ($\text{[Fe/H]}\lesssim-2$) the abundances are consistent with a plateau at $\text{[S/Fe]}\approx+0.16$, similar to what is observed in the Galactic halo, $\text{[S/Fe]}\approx+0.2$. With increasing [Fe/H], the [S/Fe] ratio declines, reaching negative values at $\text{[Fe/H]}\gtrsim-1.5$. The sample also shows an increase in [S/Mg] with [Fe/H], most probably because of enrichment from Type Ia supernovae.Comment: 9 pages, 11 figures, 2 tables + 3 online tables, accepted in A&

An enigma of Przybylski's star: is there promethium on its surface?

We carried out a new attempt to check for the presence promethium lines in the spectrum of HD101065 (Przybylski's star). The neutron capture element promethium does not have stable isotopes and the maximum half-life time is about 18 years. Thus its presence in this peculiar star would indicate an ongoing process of irradiation of its surface layers with free neutrons. Unfortunately, almost all promethium lines are heavily blended with lines of other neutron capture elements and other species. We selected and analysed three lines of promethium (Pm I and Pm II) and came to the conclusion that at present it is impossible to definitely claim the presence of this element in Przybylski's star atmosphere.Comment: Accepted for publication in Astronomische Nachrichte

Pressure-induced magnetic transitions with change of the orbital configuration in dimerised systems

We suggest a possible scenario for magnetic transition under pressure in dimerised systems where electrons are localised on molecular orbitals. The mechanism of transition is not related with competition between kinetic energy and on-site Coulomb repulsion as in Mott-Hubbard systems, or between crystal-field splitting and intra-atomic exchange as in classical atomic spin-state transitions. Instead, it is driven by the change of bonding-antibonding splitting on part of the molecular orbitals. In the magnetic systems with few half-filled molecular orbitals external pressure may result in increase of the bonding-antibonding splitting and localise all electrons on low-lying molecular orbitals suppressing net magnetic moment of the system. We give examples of the systems, where this or inverse transition may occur and by means of ab initio band structure calculations predict that it can be observed in α−MoCl(4) at pressure P ~ 11 GPa

Neutron Star Mergers are the Dominant Source of the r-process in the Early Evolution of Dwarf Galaxies

There are many candidate sites of the r-process: core-collapse supernovae (CCSNe; including rare magnetorotational core-collapse supernovae), neutron star mergers (NSMs), and neutron star/black hole mergers. The chemical enrichment of galaxies—specifically dwarf galaxies—helps distinguish between these sources based on the continual build-up of r-process elements. This technique can distinguish between the r-process candidate sites by the clearest observational difference—how quickly these events occur after the stars are created. The existence of several nearby dwarf galaxies allows us to measure robust chemical abundances for galaxies with different star formation histories. Dwarf galaxies are especially useful because simple chemical evolution models can be used to determine the sources of r-process material. We have measured the r-process element barium with Keck/DEIMOS medium-resolution spectroscopy. We present the largest sample of barium abundances (almost 250 stars) in dwarf galaxies ever assembled. We measure [Ba/Fe] as a function of [Fe/H] in this sample and compare with existing [α/Fe] measurements. We have found that a large contribution of barium needs to occur at more delayed timescales than CCSNe in order to explain our observed abundances, namely the significantly more positive trend of the r-process component of [Ba/Fe] versus [Fe/H] seen for [Fe/H] ≾ -1.6 when compared to the [Mg/Fe] versus [Fe/H] trend. We conclude that NSMs are the most likely source of r-process enrichment in dwarf galaxies at early times

Nitrogen Enrichment in Atmospheres of A- and F- Type Supergiants

Using new accurate fundamental parameters of 30 Galactic A and F supergiants, namely their effective temperatures Teff and surface gravities log g, we implemented a non-LTE analysis of the nitrogen abundance in their atmospheres. It is shown that the non-LTE corrections to the N abundances increase with Teff. The nitrogen overabundance as a general feature of this type of stars is confirmed. A majority of the stars has a nitrogen excess [N/Fe] between 0.2 and 0.9 dex with the maximum position of the star's distribution on [N/Fe] between 0.4 and 0.7 dex. The N excesses are discussed in light of predictions for B-type main sequence (MS) stars with rotationally induced mixing and for their next evolutionary phase, i.e. A- and F-type supergiants that have experienced the first dredge-up. Rotationally induced mixing in the MS progenitors of the supergiants may be a significant cause of the nitrogen excesses. When comparing our results with predictions of the theory developed for stars with the mixing, we find that the bulk of the supergiants (28 of 30) show the N enrichment that can be expected (i) either after the MS phase for stars with the initial rotational velocities v0 = 200-400 km s-1, (ii) or after the first dredge-up for stars with v0 = 50-400 km s-1. The latter possibility is preferred on account of the longer lifetime for stars on red-blue loops following the first dredge-up. Two supergiants without a discernible N enrichment, namely HR 825 and HR 7876, may be post-MS objects with the relatively low initial rotational velocity of about 100 km s-1. The suggested range for v0 is approximately consistent with inferences from the observed projected rotational velocities of B-type MS stars, progenitors of A and F supergiants.Comment: 14 pages, 13 figure

Neutron Star Mergers are the Dominant Source of the r -process in the Early Evolution of Dwarf Galaxies

International audienceThere are many candidate sites of the r-process: core-collapse supernovae (CCSNe; including rare magnetorotational core-collapse supernovae), neutron star mergers (NSMs), and neutron star/black hole mergers. The chemical enrichment of galaxies—specifically dwarf galaxies—helps distinguish between these sources based on the continual build-up of r-process elements. This technique can distinguish between the r-process candidate sites by the clearest observational difference—how quickly these events occur after the stars are created. The existence of several nearby dwarf galaxies allows us to measure robust chemical abundances for galaxies with different star formation histories. Dwarf galaxies are especially useful because simple chemical evolution models can be used to determine the sources of r-process material. We have measured the r-process element barium with Keck/DEIMOS medium-resolution spectroscopy. We present the largest sample of barium abundances (almost 250 stars) in dwarf galaxies ever assembled. We measure [Ba/Fe] as a function of [Fe/H] in this sample and compare with existing [α/Fe] measurements. We have found that a large contribution of barium needs to occur at more delayed timescales than CCSNe in order to explain our observed abundances, namely the significantly more positive trend of the r-process component of [Ba/Fe] versus [Fe/H] seen for [{Fe}/{{H}}]≲ -1.6 when compared to the [Mg/Fe] versus [Fe/H] trend. We conclude that NSMs are the most likely source of r-process enrichment in dwarf galaxies at early times