Kinematics of Field Subdwarf O Stars

Astronomy and Astrophysics – Astronomy

Scientific paper

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Scientific paper

Radial velocities of a sample of 87 subdwarf O (sdO) stars drawn from the Palomar Green (PG) and Kitt Peak Downes (KPD) UV-excess surveys are analyzed to derive the solar motion with respect to the population and its velocity ellipsoid. The velocities were measured from high resolution (0.1 Angstroms) echelle spectra of the He II lambda 4686 absorption line, a feature prominent in the spectra of these hot, subluminous stars, and which is uncontaminated by blending with Balmer and metal line absorption also present in the spectra. The typical single measurement error is 5-20\ km\ s(-1) . The observational data are supplemented with high precision velocities drawn from the literature. Standard solar motion and space velocity dispersion analyses demonstrate that the population belongs kinematically to the old part of the thin disk. Implications for the evolutionary history of the population are discussed. A series of Monte Carlo simulations is used to investigate the standard assumption that the total space motions of the sample vanish in the mean in the velocity centroid of the population, and to examine the effects of sampling error on the derived kinematic parameters. It is shown that the errors in the derived parameters are completely dominated by the sampling error. The simulations provide a quantitative measure of the precision of the measured radial velocities required to achieve a specified precision in the derived parameters for a sample of a given size.

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