Abstract
{Abridged}. An analysis of the calcium and hydrogen abundances, Galactic positions and kinematics of 146 DZ stars from the Sloan Digital Sky Survey demonstrates that interaction with the interstellar medium cannot account for their externally polluted atmospheres. The calcium-to-hydrogen ratios for the 37 DZA stars are dominated by super-solar values, as are the lower limits for the remaining 109 DZ stars. All together their metal-contaminated convective envelopes contain 10^{20+-2} g of calcium, commensurate with the masses of calcium inferred for large asteroids. It is probable that these stars are contaminated by circumstellar matter; the rocky remains of terrestrial planetary systems. In this picture, two predictions emerge: 1) at least 3.5% of all main sequence A- and F-type stars build terrestrial planets; and 2) the DZA stars are externally polluted by both metals and hydrogen, and hence constrain the frequency and mass of water-rich, extrasolar planetesimals.
Highlights
The curious presence of heavy elements in the atmospheres of cool white dwarfs has been known since the discovery of the first de-C 2010 The Authors
As discussed by Dupuis et al (1993a), white dwarfs accreting solar abundance matter within the interstellar medium (ISM) at 3 × 1011 g s−1 for 106 yr can accumulate the heavy element mass fractions observed in the convective envelopes of DZ stars, such as plotted in Fig. 2
The metal abundances of 146 DZ stars shows no connection with speed relative to the local standard of rest (LSR), nor with height above the Galactic disc
Summary
Aannestad et al (1993) provided an extensive spatial, kinematical and calcium abundance analysis against which they tested likely scenarios of interstellar accretion based on the best available data at that time, concluding interstellar accretion could not explain the abundances in any of their 15 DZ white dwarfs Their landmark study was limited to targets within 50 pc for the most part, and to stars that were proper-motion selected, and biased towards higher space velocities (Sion et al 1988). It is argued here that the hundreds of newly identified, cool, helium-rich white dwarfs from the SDSS provide an excellent, spatially and kinematically unbiased sample upon which to test the ISM accretion hypothesis These stars form a large statistical sample that favours an alternative to the accretion of Galactic gas and dust; as a whole, the SDSS DZ stars do not show the spatial, kinematical or elemental abundance correlations expected from accretion episodes within the plane of the Galaxy. The SDSS DZ and DC (helium-rich, featureless) stars appear to be similar populations of polluted and non-polluted stars broadly sharing all relevant characteristics save photospheric calcium, and perhaps hydrogen
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