Kinematics and chemical properties of the Galactic stellar populations

Abstract

Aims. We analyzed chemical and kinematical properties of about 850 FGK solar neighborhood long-lived dwarfs observed with the HARPS high-resolution spectrograph. The stars in the sample have log g ≥ 4 dex, 5000 ≤ T eff ≤ 6500 K, and −1.39 ≤ [Fe/H] ≤ 0.55 dex. The aim of this study is to characterize and explore the kinematics and chemical properties of stellar populations of the Galaxy in order to understand their origins and evolution. Methods. We applied a purely chemical analysis approach based on the [α/Fe] vs. [Fe/H] plot to separate Galactic stellar populations into the thin disk, thick disk, and high-α metal-rich (hαmr). Then, we explored the population's stellar orbital eccentricity distributions, their correlation with metallicity, and rotational velocity gradients with metallicity in the Galactic disks to provide constraints on the various formation models. Results. We identified a gap in the [α/Fe]-[Fe/H] plane for the α-enhanced stars, and by performing a bootstrapped Monte Carlo test we obtained a probability higher than 99.99% that this gap is not due to small-number statistics. Our analysis shows a negative gradient of the rotational velocity of the thin disk stars with [Fe/H] (-17 km s −1 dex −1 ), and a steep positive gradient for both the thick disk and hαmr stars with the same magnitude of about +42 km s −1 dex −1 . For the thin disk stars we observed no correlation between orbital eccentricities and metallicity, but observed a steep negative gradient for the thick disk and hαmr stars with practically the same magnitude (≈-0.18 dex −1 ). The correlations observed for the nearby stars (on average 45 pc) using high-precision data, in general agree well with the results obtained for the SDSS sample of stars located farther from the Galactic plane. Conclusions. Our results suggest that radial migration played an important role in the formation and evolution of the thin disk. For the thick disk stars it is not possible to reach a firm conclusion about their origin. Based on the eccentricity distribution of the thick disk stars only their accretion origin can be ruled out, and the heating and migration scenario could explain the positive steep gradient of V φ with [Fe/H]. When we analyzed the hαmr stellar population we found that they share properties of both the thin and thick disk population. A comparison of the properties of the hαmr stars with those of the subsample of stars from the N-body/SPH simulation using radial migration suggest that they may have originated from the inner Galaxy. Further detailed investigations would help to clarify their exact nature and origin.

TheGaia-ESO Survey: characterisation of the [α/Fe] sequences in the Milky Way discs

“…This result is in agreement with previous studies, (e.g. Adibekyan et al 2013;Bensby et al 2014;Nidever et al 2014, most recently); see also Bensby et al (2007), Kordopatis et al (2013bKordopatis et al ( , 2015 for further evidence of metal-rich, thick-disc stars, defined kinematically. We find that the dispersion in [Mg/Fe] at fixed iron abundance for thick-disc stars is marginally smaller than the values derived for thin-disc stars.…”

“…This overlap in metallicity was already suggested from the α-abundance trends derived from very high-resolution studies of stars in the immediate solar neighbourhood (d < 100 pc; e.g. Reddy et al 2006;Adibekyan et al 2013;Bensby et al 2014;Nissen 2015), and by Nidever et al (2014) for stars in a wide range of Galactic radii (0 R 15 kpc). We can further assess the presence of thin-disc stars below [Fe/H] = −0.5 dex by making a rough separation of the stars within a given [Fe/H] bin into either high-α ([Mg/Fe] > 0.3 dex) or low-α ([Mg/Fe] < 0.2 dex, see also Sect.…”

“…The top, middle, and bottom rows show the Gaia-ESO iDR2 results for the inner Galaxy, solar suburb and outer Galaxy, respectively. The over-plotted grey points in the panel 7.5 < R < 8.5 kpc and 0 < |Z| < 1 kpc are the α-abundance measurements of Adibekyan et al (2013), obtained for a very local sample (d 150 pc). No significant differences in the derived trends are found as a function of S/N, which is indicative of the robustness of our fitting method.…”

The evolution of the Milky Way: new insights from open clusters

“…A similar behaviour is observable for Galactic field stars where the older thin and thick disc field dwarfs from Bensby et al (2014) yield more positive [α/Fe] ratios than the younger field dwarfs with thin disc kinematics. The similarity in [α/Fe] between the thin and thick disc clusters with ages of 4 to 9 Gyr suggest that age cannot be the key variable, but different proportions of Type II to Type Ia supernovae products in clusters formed at dif- Adibekyan et al 2013;Bensby et al 2014). Although the origin of such objects is still in debate, Adibekyan et al (2011Adibekyan et al ( , 2013 have suggested a possible origin of these α-enhanced metal-rich stars in the inner Galaxy (bulge) followed by a subsequent migration to the solar vicinity.…”

Constraining the Milky Way assembly history with Galactic Archaeology

“…This model needs to be improved to account also for the gap in the [α/Fe]-[Fe/H] relation seen for a long time in high-resolution spectroscopic data in the solar vicinity (e.g., Adibekyan et al 2013;Bensby et al 2013;Fuhrmann 2004) and more recently found in a range of Galactic radii using APOGEE data (Anders et al 2014;Nidever et al 2014). This separation into low-and high-[α/Fe] sequences (sometime referred to as the thin and thick discs) 8 is most likely the result of a gap in the SFH at high redshift, as first suggested in the Two-Infall Model by Chiappini et al (1997).…”