Stellar substructures in the solar neighbourhood. II. Abundances of neutron-capture elements in the kinematic Group 3 of the Geneva-Copenhagen survey

The evolution of chemical elements in a galaxy is linked to its star formation history. Variations in star formation history are imprinted in the relative abundances of chemical elements produced in different supernova events and asymptotic giant branch stars. We determine detailed elemental abundances of s- and r-process elements in stars belonging to Group3 of the Geneva-Copenhagen survey and compare their chemical composition with Galactic disc stars. The aim is to look for possible chemical signatures that might give information about the formation history of this kinematic group of stars, which is suggested to correspond to remnants of disrupted satellites. High-resolution spectra were obtained with the FIES spectrograph at the Nordic Optical Telescope, La Palma, and were analysed with a differential model atmosphere method. Comparison stars were observed and analysed with the same method. Abundances of chemical elements produced mainly by the s-process are similar to those in the Galactic thin-disc dwarfs of the same metallicity, while abundances of chemical elements produced predominantly by the r-process are overabundant. The similar elemental abundances are observed in Galactic thick-disc stars. The chemical composition together with the kinematic properties and ages of stars in Group3 of the Geneva-Copenhagen survey support a gas-rich satellite merger scenario as the most likely explanation for the origin. The similar chemical composition of stars in Group3 and the thick-disc stars might suggest that their formation histories are linked.Comment: 8 pages, 8 figures, accepted for publication in Astronomy & Astrophysics, 2013. arXiv admin note: text overlap with arXiv:1203.619

Detection and Characterization of Oscillating Red Giants: First Results from the TESS Satellite

Since the onset of the "space revolution" of high-precision high-cadence photometry, asteroseismology has been demonstrated as a powerful tool for informing Galactic archeology investigations. The launch of the NASA Transiting Exoplanet Survey Satellite (TESS) mission has enabled seismic-based inferences to go full sky—providing a clear advantage for large ensemble studies of the different Milky Way components. Here we demonstrate its potential for investigating the Galaxy by carrying out the first asteroseismic ensemble study of red giant stars observed by TESS. We use a sample of 25 stars for which we measure their global asteroseimic observables and estimate their fundamental stellar properties, such as radius, mass, and age. Significant improvements are seen in the uncertainties of our estimates when combining seismic observables from TESS with astrometric measurements from the Gaia mission compared to when the seismology and astrometry are applied separately. Specifically, when combined we show that stellar radii can be determined to a precision of a few percent, masses to 5%-10%, and ages to the 20% level. This is comparable to the precision typically obtained using end-of-mission Kepler data

Chemical composition of kinematically identified galactic stellar group

A number of stellar streams, moving and kinematic groups were identified in the Milky Way galaxy. Some of them are suspected to originate from accreted satellites. Can we also find such traces of ancient merger events in the solar neighbourhood? Helmi et al. (2006) identified three new coherent groups of stars in the Geneva-Copenhagen survey (Nordström et al. 2004) and suggested that those might correspond to remains of disrupted satellites. With the detailed chemical composition analysis of the newly identified kinematic group we aim to contribute to the Galactic substructure studies. The main aim of the study is to perform a high-resolution spectroscopic elemental abundance analysis in stars belonging to one of the newly identified kinematic groups and to compare the results with other stars in the Galactic disc. We performed the detailed chemical analysis of 21 stars attributed to Group 3 of the Geneva-Copenhagen survey and six comparison Galactic thin disc stars. The main atmospheric parameters and abundances of 22 chemical elements were determined. All programme stars are overabundant in oxygen and α-elements compared with the Galactic thin-disc. This abundance pattern has similar characteristics to those of the Galactic thick disc. The abundances of chemical elements produced predominantly by the r-process are overabundant in comparison with Galactic thin-disc dwarfs of the same metallicity. The abundances of iron-group elements and chemical elements produced mainly by the s-process are similar to those in the Galactic thin-disc dwarfs of the same metallicity. The chemical composition of stars in Group 3 is similar to the thick-disc stars, which might suggest that their formation histories are linked. The chemical composition together with the kinematic properties and ages of stars in the investigated Group 3 of the Geneva-Copenhagen survey support a gas-rich satellite merger scenario as the most suitable origin for Group 3

Galactic thick disk chemical evolution

Šio darbo tikslas buvo ištirti Galaktikos storojo disko žvaigždžių cheminę evoliuciją. Devynių F-G-K spektrinės klasės žvaigždžių spektrai buvo gauti SOFIN spektrografu su Šiaurės šalių teleskopu. Iš spektrų nustačiau pagrindinius žvaigždžių atmosferų fizikinius parametrus: efektinę temperatūra Teff., gravitacinį pagreitį žvaigždės paviršiuje lg g, mikroturbulencijos greitį vt. ir deguonies, geležies, α, r- ir s-procesų elementų gausas devyniose storojo Galaktikos disko žvaigždėse. Rezultatus palyginau su kitų mokslininkų grupių darbais: tai Tautvaišienės ir kt. darbas (2001 m.); Bensby ir kt. darbai (2004, 2005, 2007 m.); Reddy ir kt. darbas (2006 m.). Geležies gausai [Fe/H] esant intervale nuo ≈ –0,9 iki ≈ –0,3 dešimtųjų žvaigždės, pasižyminčios storojo Galaktikos disko kinematika, yra turtingesnės deguoniu, magniu ir alfa-proceso elementais, nei plonojo disko žvaigždžių populiacija. Kitų cheminių elementų gausos storajame ir plonajame Galaktikos diskų žvaigždėse yra panašios.The aim of this research is to study the Galactic thick disk chemical evolution. Using high-resolution spectra I have derived the chemical composition of a sample of Galactic thick disk stars. High resolution spectra of 9 F-G-K spectral type stars in the Galactic thick disk have been obtained by the SOFIN spectrograph on the Nordic Optical Telescope. From these stellar spectra I determined the basics stellar atmosphere parameters: effective temperatures Teff [K], surface gravities lg g, microturbulent velocities vt [km/s], and abundances of oxygen, iron, α, r- and s-process elements in the Galactic thick disk. The results are similar to those of other investigations: Tautvaišienė et al., 2001; Bensby et al., 2004, 2005, 2007; Reddy et al., 2006. For a given [Fe/H] the stars with kinematics typical of the thick disk are more enriched in oxygen and α- process elements (Mg, Si, Ca, Ti) than the stars with kinematics typical of the thin disk. The abundance of other elements (Al, Na, Cr, Ni, V, Sc, Mn, Co, Cu, Y, Zr, Ba, La and Nd) is similar for the two disks. The abundance ratios of oxygen, α, r- and s-process elements to iron provide further evidence that the thick disk star population have a different chemical history compared to the thin disk. The stars formation rate was probably higher in the Galactic thick disk than in the Galactic thin disk.Švietimo akademijaVytauto Didžiojo universiteta

On the metallicity dependence of the [Y/Mg]–age relation for solar-type stars

Several recent studies of solar twins in the solar neighbourhood have shown a tight correlation between various elemental abundances and age, in particular [Y/Mg]. If this relation is real and valid for other types of stars as well as elsewhere in the Galaxy, it would provide a very powerful tool to derive ages of stars without the need to resort to determining their masses (evolutionary stage) very precisely. The method would also likely work if the stellar parameters have relatively large errors. The studies presented in the recent literature span a narrow range of [Fe/H]. By studying a larger sample of solar neighbourhood dwarfs with a much larger range of [Fe/H], we find that the relation between [Y/Mg] and age depends on the [Fe/H] of the stars. Hence, it appears that the [Y/Mg]–age relation is unique to solar analogues