Karen G. Schaefer

A Redetermination of the Mass of Procyon

The parallax and astrometric orbit of Procyon have been redetermined from PDS measurements of over 250 photographic plates spanning 83 years, with roughly 600 exposures used in the solution. These data are combined with two modern measurements of the primary–white dwarf separation, one utilizing a ground-based coronagraph, the other, the Planetary Camera (PC) of the Hubble Space Telescope. Together with the redetermined astrometric orbit and parallax, these yield new estimates of the component masses. The derived masses are 1.497 ± 0.037 M⊙ for the primary and 0.602 ± 0.015M⊙ for the white dwarf secondary. These mass values are heavily weighted by the PC separation measurement, which, while being somewhat discordant with the ground-based measures, we argue is more precise and more accurate and thus deserving of its greater weight. This stated, the long-standing discrepancy between previous determinations of the observed mass of Procyon A (1.75 M⊙) and the value supported by stellar evolution models (1.50 M⊙) appears to be reconciled.

Hubble Space Telescope Observations of an Accreted Silicon Spot on the White Dwarf in V471 Tauri

We have used the Hubble Space Telescopes (HST) Goddard High-Resolution Spectrograph to detect a photospheric metallic absorption line, Si III λ1206, in the spectrum of the magnetic white dwarf component of the Hyades precataclysmic binary V471 Tauri. The Si III feature is modulated on the soft X-ray/EUV/optical 9.25 minute rotational period of the white dwarf and is strongest at the time of soft X-ray/EUV minimum and optical maximum. A model in which the soft X-ray/EUV magnetic pole is dark because of metallic and helium absorption, and bright in the optical due to flux redistribution, is strongly supported. We derive a Si abundance of 0.10+ 0.03−0.07 times solar in the accretion cap. Assuming equilibrium between mass accretion onto—and diffusion of Si out of—the photosphere, we find the white dwarf to be accreting from its dK companions wind at only 3.8×10−18 M☉ yr-1, some 5 orders of magnitude lower than the Bondi-Hoyle fluid rate. This strongly suggests operation of a magnetic-centrifugal propeller mechanism that rejects most of the material that attempts to accrete. We tentatively detect Zeeman splitting of the Si III line, implying a polar field strength of ~350 kG. V471 Tau is destined, in the distant future, to become a DQ Her-type cataclysmic binary.

HST Snapshot Images of Planetary Nebulae

During Cycle 5 of the HST General Observer program we have been carrying out a “snapshot” survey of central stars of planetary nebulae. The snapshots are short exposures in the V (F555W) and / (F814W) filters, taken with the Wide Field Planetary Camera (WFPC2) during brief scheduling opportunities.