The Einstein Redshift in White Dwarfs

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

The Einstein Redshift in White Dwarfs

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

A Chandra Search for Coronal X Rays from the Cool White Dwarf GD 356

We report observations with the Chandra X-ray Observatory of the single, cool, magnetic white dwarf GD 356. For consistent comparison with other X-ray observations of single white dwarfs, we also re-analyzed archival ROSAT data for GD 356 (GJ 1205), G 99-47 (GR 290 = V1201 Ori), GD 90, G 195-19 (EG250 = GJ 339.1), and WD 2316+123 and archival Chandra data for LHS 1038 (GJ 1004) and GD 358 (V777 Her). Our Chandra observation detected no X rays from GD 356, setting the most restrictive upper limit to the X-ray luminosity from any cool white dwarf -- L_{X} < 6.0 x 10^{25} ergs/s, at 99.7% confidence, for a 1-keV thermal-bremsstrahlung spectrum. The corresponding limit to the electron density is n_{0} < 4.4 x 10^{11} cm^{-3}. Our re-analysis of the archival data confirmed the non-detections reported by the original investigators. We discuss the implications of our and prior observations on models for coronal emission from white dwarfs. For magnetic white dwarfs, we emphasize the more stringent constraints imposed by cyclotron radiation. In addition, we describe (in an appendix) a statistical methodology for detecting a source and for constraining the strength of a source, which applies even when the number of source or background events is small.Comment: 27 pages, 4 figures, submitted to the Astrophysical Journa

Perspectives on Astrophysics Based on Atomic, Molecular, and Optical (AMO) Techniques

About two generations ago, a large part of AMO science was dominated by experimental high energy collision studies and perturbative theoretical methods. Since then, AMO science has undergone a transition and is now dominated by quantum, ultracold, and ultrafast studies. But in the process, the field has passed over the complexity that lies between these two extremes. Most of the Universe resides in this intermediate region. We put forward that the next frontier for AMO science is to explore the AMO complexity that describes most of the Cosmos.Comment: White paper submission to the Decadal Assessment and Outlook Report on Atomic, Molecular, and Optical (AMO) Science (AMO 2020

LSST Science Book, Version 2.0

A survey that can cover the sky in optical bands over wide fields to faint magnitudes with a fast cadence will enable many of the exciting science opportunities of the next decade. The Large Synoptic Survey Telescope (LSST) will have an effective aperture of 6.7 meters and an imaging camera with field of view of 9.6 deg^2, and will be devoted to a ten-year imaging survey over 20,000 deg^2 south of +15 deg. Each pointing will be imaged 2000 times with fifteen second exposures in six broad bands from 0.35 to 1.1 microns, to a total point-source depth of r~27.5. The LSST Science Book describes the basic parameters of the LSST hardware, software, and observing plans. The book discusses educational and outreach opportunities, then goes on to describe a broad range of science that LSST will revolutionize: mapping the inner and outer Solar System, stellar populations in the Milky Way and nearby galaxies, the structure of the Milky Way disk and halo and other objects in the Local Volume, transient and variable objects both at low and high redshift, and the properties of normal and active galaxies at low and high redshift. It then turns to far-field cosmological topics, exploring properties of supernovae to z~1, strong and weak lensing, the large-scale distribution of galaxies and baryon oscillations, and how these different probes may be combined to constrain cosmological models and the physics of dark energy.Comment: 596 pages. Also available at full resolution at http://www.lsst.org/lsst/sciboo

Perspectives on Astrophysics Based on Atomic, Molecular, and Optical (AMO) Techniques

About two generations ago, a large part of AMO science was dominated by experimental high energy collision studies and perturbative theoretical methods. Since then, AMO science has undergone a transition and is now dominated by quantum, ultracold, and ultrafast studies. But in the process, the field has passed over the complexity that lies between these two extremes. Most of the Universe resides in this intermediate region. We put forward that the next frontier for AMO science is to explore the AMO complexity that describes most of the Cosmos