The Einstein Redshift in White Dwarfs

Abstract

Low-dispersion radial velocities of 53 white dwarfs have been measured on Palomar spectrograms. Table 1 contains the type, velocity, space-motion components, photometrically deduced temperature and radius, for each star. Table 4 contains 39 additional radial velocities of very low weight. A few members of wide binary systems and 6 white dwarfs in the Hyades provide direct measures of the Einstein gravitational redshift, with a mean value of +51 km/sec. Omitting the very-high-velocity star LP9-231, there are 37 DA stars, with a mean K-term (expansion velocity) of +65.6 km/sec. If the Hyades stars are omitted, the mean K term is +62.5 km/sec. A number of white dwarfs are members of the high-velocity population. Systematic wavelength shifts of He i lines in DB stars make their velocities more negative than those of DA stars; similar negative shifts may exist for metallic lines. The temperature scale is obtained from colors and, combined with luminosities, gives radii. The broad distribution of radii and redshifts is shown in Figure 2, and median values are derived. The median radial velocity for 37 DA stars is +58 km/sec, and the median radius 0.0107 R_⊙; the redshift and radius give a mass of 0.98 M_⊙. However, this value is almost certainly too high, if we expect accordance with the theoretical mass-radius relation. The theoretical M-R relation of a zero-temperature degenerate star predicts a redshift, for given mass, for various compositions. Two corrections could bring the theoretically expected redshifts into agreement with the observations. Either a systematic change in luminosity, ΔM_v of +0.25 mag, or a reciprocal temperature change of Δθ = —0.03, reduces the median radius to 0.0093 R_⊙. The mass derived from the redshift is then 0.86 M_⊙. These values are in accordance with the Hamada-Salpeter mass-radius relation, if the composition in the interior is pure helium. A carbon or magnesium interior also gives a radius not too different from the colorimetric radius. An iron core gives a mass of 0.73 M_⊙, but a radius of 0.008 R_⊙, sufficiently smaller to require substantial changes in the temperature scale. The mass now derived from the radial velocities is higher than that previously found from radii only and closer to the Chandrasekhar limit.

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