COOL METALLIC-LINE WHITE-DWARFS, RADIAL-VELOCITIES, AND INTERSTELLAR ACCRETION

From observations of the spectra of 15 cool metallic-line white dwarfs we derive their radial velocities from the positions of the H and K lines of Ca II. A pressure shift correction is included, and the resulting mean gravitational redshift is 35 km/s. By projecting their spatial distribution and space velocity components onto and perpendicular to the galactic plane, and including 13 additional stars not observed by us, we compare the present and past spatial distribution of the white dwarfs with the distribution of local interstellar matter. If the metallic-line white dwarfs are accreting their metals from the interstellar medium, one would expect a spatial correlation between the stars and the local gas. However, we conclude that only very few of the stars could possibly just have accreted or presently be accreting metals from local clouds. We also consider several scenarios for interstellar accretion that may have taken place within the last million years, assuming that the local medium has not substantially changed. Our results show that Ca accretion from local interstellar matter in the past cannot have proceeded at a rate high enough to explain the observed photospheric abundances of Ca, at least in the cool metallic-line degenerates. The main factors in limiting the accretion rates are the lack of dense clouds in the local medium and low effective stellar temperatures that reduce the accretion rate much below the fluid rate. Alternatives to accretion from the local interstellar medium must be seriously considered.