The kinematical evolution of the Galactic disk

Abstract

We present preliminary results from follow-up observations of a carefully selected sample of subgiant stars from RAVE and Geneva-Copenhagen surveys. We find evidence of rapid saturation of the heating mechanism in the disk, with an abrupt increase of the velocity dispersion for stars older than 9 Gyr. 1. SAMPLE DEFINITION AND FOLLOW-UP OBSERVATIONS Using high resolution spectroscopy data (7) showed the value of subgiants for dating the Galactic disk. We have selected ∼2000 subgiants candidates from the RAVE survey using RAVEs stellar parameters (6, 8). We have also selected 404 subgiants (no binaries) from the Geneva-Copenhagen survey (GCS) (3 )i n the Mv−Teff plane. We performed follow-up observations with the Double Beam Spectrograph (DBS) instrument mounted at the ANU 2.3 m telescope in Siding Spring Observatory. We use a long slit of 5 �� width and the 300B grating, giving a spectral resolution of R = /∼400. The recorded spectra cover the wavelength range from 3200 A to 6600 A. Observations consist of a sequence of target exposure, standard stars, calibration lamps, flats and bias. During the observational campaign we used a wide slit of 5 �� , which assures that 100% of the star light is collected (depending on the seeing), while avoid tracking problems. We also placed the slit parallel to the direction of atmosphere dispersion, i.e, aligning the slit at the parallactic angle to avoid recording incorrect intensities as a function of wavelength that could spoil our flux calibration. Standard stars with the same instrumental setup were obtained each night covering a large range of airmass over the night. This allows us to control the atmospheric extinction in the observed spectra. The final spectra present a typical S/N∼130 at 5500A and a S/N∼80 for the Ca II H & K region around 3950 A. A total of 1253 subgiant candidates spectra were obtained. 2. STELLAR PARAMETERS FROM SPECTROPHOTOMETRY We derived the stellar parameters using a chi-square minimization technique using empirical model atmospheres from the MILES library (4). The library consist of ∼1000 stars spanning a large range in atmospheric parameters. The spectra cover the range 3525-7500A at 2.3A (FWHM) spectral resolution. This technique allows us to derive accurate parameters from low resolution spectrophotometry via flux calibrated spectra. For reddening corrections we use the standard value of R(V) = A(V)/ E(B − V) = 3.1. We redden the model and we use the E(B − V) as a free parameter together with Teff , (M/H) and log g in the 2 fitting. Reddening effects are not large for our stars, as the selected RAVE stars are at |b |≥ 25 degrees and the GCS stars are in a very local volume with a radius lower than 100 pc.