The Age-Metallicity-Velocity relation in the nearby disk

The age-metallicity and the age-velocity relation in the nearby Galactic disk is investigated using a sample of sub-giants. The sample was carefully selected from the RAdial Velocity Experiment (RAVE) and the Geneva-Copenhagen Survey (GCS). Further observations were needed to improve the stellar parameters in order to clean the sample and get accurate ages. We obtained a total of 1253 low resolution spectra from the Double Beam Spectrograph (DBS) on the ANU 2.3-m telescope in Siding Spring Observatory (Australia). The resolving power was 400, with a spectral range from 3200 to 6200 Å. The age-metallicity and the age-velocity relation in the nearby Galactic disk is investigated using a sample of sub-giants. The sample was carefully selected from the RAdial Velocity Experiment (RAVE) and the Geneva-Copenhagen Survey (GCS). Further observations were needed to improve the stellar parameters in order to clean the sample and get accurate ages. We obtained a total of 1253 low resolution spectra from the Double Beam Spectrograph (DBS) on the ANU 2.3-m telescope in Siding Spring Observatory (Australia). The resolving power was 400, with a spectral range from 3200 to 6200 Å. We derive the stellar parameters via flux calibrated spectra using an empirical stellar library (MILES). This technique provides a widely applicable new method of deriving stellar parameters from low resolution spectrophotometry. A comparison between Teff , [m/H] and log g derived in this work and those in a high-resolution catalogue (PASTEL) present a rms ∼ 145 K, 0.16 and 0.23 dex respectively. We also estimate [Mg/Fe] and [Ca/Fe] from spectrophotometry using the abundances ratios obtained in de Castro Milone et al. (2011) and Venn et al. (2004) as reference. We determine the age of stars via isochrones fitting. Adopting a Gaussian probability density for Teff , log g and [m/H] centered at the measured values we determine the probability density distribution for the age. The procedure makes no assumption on the initial mass function, the metallicity distribution, or the star formation rate. We demonstrate that sub-giants are suitable for dating the Galaxy and we find that around 80% of the stars present a total relative error lower than 1.5 Gyr. We derive an age-metallicity relation with an intrinsic cosmic dispersion in metal abundances of 0.14 dex, a factor of two smaller than that found by Edvardsson et al. (1993), Nordstr¨om et al. (2004), Casagrande et al. (2011). The mean metallicity shows a slow, steady increase with time. We also find a relation between the α-elements and the age for the thin disk stars. Stars around 5.0 Gyr old present [α/Fe] slightly higher than solar (∼ +0.02) while the oldest thin disk stars show values around +0.1 dex. For the thick disk stars we find values from +0.1 dex up to +0.3 dex in [α/Fe]. These results suggest a less need for radial migration in the Galactic disk. The metallicity-velocity relation shows that U, V and W velocities are independent of metallicity for a range between +0.2 and -0.6 dex. We also find that the mean rotational velocity is independent of the metallicity for the thin disk. Furthermore, the velocity dispersion remains roughly constant for a metallicity range from +0.2 to -0.5 dex. For metal-poor stars ([m/H] < -0.7 dex) the velocity dispersion clearly increase for the three velocity components. The age-velocity dispersion relation shows that the heating for the thin disk take place for the first ∼ 3.0 Gyr, and then saturates when σw ~ 20 km s−1. This result is consistent with inefficient heating caused by scattering from tightly wound transient spiral structure and with the very efficient heating mechanism caused by the GMCs during the first 3.0 Gyr. We find an abrupt increase of the velocity dispersion for stars older than 10.0 Gyr. The shape of the velocity ellipsoid is also investigated. For the thin disk we find σu : σv : σw ~ 1.0:0.7:0.5 and for the thick disk, σu : σv : σw ~ 1.0:1.0:0.7. Mechanisms of formation of the thick disk such as accretion events remain compatible with our findings.