NLTE analysis of Sr lines in spectra of late-type stars with new R-matrix atomic data

Nordström

et al. 2016

A&A

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Context. An increasing fraction of carbon-enhanced metal-poor (CEMP) stars is found as their iron abundance, [Fe/H], decreases below [Fe/H] = −2.0. The CEMP-s stars have the highest absolute carbon abundances, [C/H], and are thought to owe their enrichment in carbon and the slow neutron-capture (s-process) elements to mass transfer from a former asymptotic giant branch (AGB) binary companion. The most Fe-poor CEMP stars are normally single, exhibit somewhat lower [C/H] than CEMP-s stars, but show no s-process element enhancement (CEMP-no stars). Abundance determinations of CNO offer clues to their formation sites. Aims. Our aim is to use the medium-resolution spectrograph X-Shooter/VLT to determine stellar parameters and abundances for C, N, Sr, and Ba in several classes of CEMP stars in order to further classify and constrain the astrophysical formation sites of these stars. Methods. Atmospheric parameters for our programme stars were estimated from a combination of V − K photometry, model isochrone fits, and estimates from a modified version of the SDSS/SEGUE spectroscopic pipeline. We then used X-Shooter spectra in conjunction with the 1D local thermodynamic equilibrium spectrum synthesis code MOOG, 1D ATLAS9 atmosphere models to derive stellar abundances, and, where possible, isotopic 12 C/ 13 C ratios. Results. Abundances (or limits) of C, N, Sr, and Ba are derived for a sample of 27 faint metal-poor stars for which the X-Shooter spectra have sufficient signal-to-noise ratios (S/N). These moderate resolution, low S/N (∼10−40) spectra prove sufficient to perform limited chemical tagging and enable assignment of these stars into the CEMP subclasses (CEMP-s and CEMP-no). According to the derived abundances, 17 of our sample stars are CEMP-s and 3 are CEMP-no, while the remaining 7 are carbon-normal. For four CEMP stars, the subclassification remains uncertain, and two of them may be pulsating AGB stars. Conclusions. The derived stellar abundances trace the formation processes and sites of our sample stars. The [C/N] abundance ratio is useful for identifying stars with chemical compositions unaffected by internal mixing, and the [Sr/Ba] abundance ratio allows us to distinguish between CEMP-s stars with AGB progenitors and the CEMP-no stars. Suggested formation sites for the latter include faint supernovae with mixing and fallback and/or primordial, rapidly-rotating, massive stars (spinstars). X-Shooter spectra have thus proved to be valuable tools in the continued search for their origin.

Section: Line Listsmentioning

confidence: 99%

Abundances of carbon-enhanced metal-poor stars as constraints on their formation

Nordström

et al. 2016

A&A

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“…The lines to synthesize were selected from the line list in Hansen et al (2011) to find the lines that are most likely detectable in RR Lyrae stars. The synthesis was conducted using a line list containing atomic data from , Hansen et al (2013), Bergemann et al (2012), Gallagher et al (2012), Ivans et al (2006), and Lawler et al (2001).…”

Section: Stellar Parameters and Abundancesmentioning

confidence: 99%

“…Previously, most of the high-to-hyper-velocity stars were thought to be young (relatively massive) hot stars originating from the Galactic center (e.g., Hills 1988;Brown 2015, and references therein). However, the recent SEGUE study by Palladino et al (2014) used a combination of radial velocities and proper motions to show that lowmass F and G stars could be traveling with high enough speed to allow them to escape the Galaxy, and that these stars did not originate in the Galactic center (see also Kollmeier et al 2010;Nidever et al 2012).…”

Section: Introductionmentioning

confidence: 99%

Chemical abundances in a high-velocity RR Lyrae star near the bulge

Hansen¹,

Rich

Koch

et al. 2016

A&A

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Low-mass variable high-velocity stars are interesting study cases for many aspects of Galactic structure and evolution. Until recently, the only known high-or hyper-velocity stars were young stars thought to originate from the Galactic center. Wide-area surveys such as APOGEE and BRAVA have found several low-mass stars in the bulge with Galactic rest-frame velocities higher than 350 km s −1 . In this study we present the first abundance analysis of a low-mass RR Lyrae star that is located close to the Galactic bulge, with a space motion of ∼-400 km s −1 . Using medium-resolution spectra, we derived abundances (including upper limits) of 11 elements. These allowed us to chemically tag the star and discuss its origin, although our derived abundances and metallicity, at [Fe/H] = −0.9 dex, do not point toward one unambiguous answer. Based on the chemical tagging, we cannot exclude that it originated in the bulge. However, its retrograde orbit and the derived abundances combined suggest that the star was accelerated from the outskirts of the inner (or even outer) halo during many-body interactions. Other possible origins include the bulge itself, or the star might have been stripped from a stellar cluster or the Sagittarius dwarf galaxy when it merged with the Milky Way.

“…In this study, they find a difference of up to ∼ 0.7 dex between the Mo I abundances and the Mo II upper limits. Since the studies in the UV often use adjusted log gf values, we would like to stress that possible NLTE effects might still affect both the Mo and Fe abundances, and that these effects are often seen to increase with increasing line strength (Lind et al 2012, for Fe, and for Sr see Andrievsky et al 2011;Bergemann et al 2012or Hansen et al 2013.…”

Section: Abundances Of Mo and Rumentioning

confidence: 99%

Stellar abundances and presolar grains trace the nucleosynthetic origin of molybdenum and ruthenium

Hansen¹,

Andersen

Christlieb³

2014

A&A

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This work presents a large consistent study of molybdenum (Mo) and ruthenium (Ru) abundances in the Milky Way. These two elements are important nucleosynthetic diagnostics. In our sample of 71 Galactic metal-poor field stars, we detect Ru and/or Mo in 52 of these (59 including upper limits). The sample consists of high-resolution, high signal-to-noise spectra covering both dwarfs and giants from [Fe/H] = −0.63 down to −3.16. Thus we provide information on the behaviour of Mo I and Ru I at higher and lower metallicity than is currently known. In this sample we find a wide spread in the Mo and Ru abundances, which is typical of heavy elements. We confirm earlier findings of Mo enhanced stars around [Fe/H] = −1.5 and add ∼15 stars both dwarfs and giants with normal (<0.3 dex) Mo and Ru abundances, as well as more than 15 Mo and Ru enhanced (>0.5 dex) stars to the currently known stellar sample. This indicates that several formation processes, in addition to high entropy winds, can be responsible for the formation of elements like Mo and Ru. We trace the formation processes by comparing Mo and Ru to elements (Sr, Zr, Pd, Ag, Ba, and Eu) with known formation processes. Based on how tight the two elements correlate with each other, we are able to distinguish if they share a common formation process and how important this contribution is to the element abundance. We find clear indications of contributions from several different formation processes, namely the p-process, and the slow (s-), and rapid (r-) neutron-capture processes. From these correlations we find that Mo is a highly convolved element that receives contributions from both the s-process and the p-process and less from the main and weak r-processes, whereas Ru is mainly formed by the weak r-process as is silver. We also compare our absolute elemental stellar abundances to relative isotopic abundances of presolar grains extracted from meteorites. Their isotopic abundances can be directly linked to the formation process (e.g. r-only isotopes) providing a unique comparison between observationally derived abundances and the nuclear formation process. The comparison to abundances in presolar grains shows that the r-/s-process ratios from the presolar grains match the total elemental chemical composition derived from metal-poor halo stars with [Fe/H] around −1.5 to −1.1 dex. This indicates that both grains and stars around and above [Fe/H] = −1.5 are equally (well) mixed and therefore do not support a heterogeneous presolar nebula. An inhomogeneous interstellar medium (ISM) should only be expected at lower metallicities. Our data, combined with the abundance ratios of presolar grains, could indicate that the AGB yields are less efficiently mixed into stars than into presolar grains. Finally, we detect traces of s-process material at [Fe/H] = −1.5, indicating that this process is at work at this and probably at even lower metallicity.