G. L. Hammond

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

Abundances in the hot DZ star CBS 127 - How efficient is hydrogen screening

A revised and updated grid of cool helium-rich model atmospheres which includes the effect of varying hydrogen abundance is used to analyze the temperature and abundance of the DZ white dwarf CBS 127. Comparisons of the star to several other DZ white dwarfs with similar T(e)s show that CBS 137 is the hottest DZ star and lies near the boundary temperature above which it has been proposed that hydrogen accretion may be prevented. Based on the results, a specifically defined single lower boundary temperature for screening is rejected, and it is demonstrated that the propeller must operate over a very broad range of T(eff), down to T(eff) as low as 5800 K, with highly variable efficiency. 39 refs.

UV observations of the cool DBQA5 white dwarf LDS 678A - Limits on the atmospheric composition, pressure shift, and gravitational redshift derived from C I 2479

A high-resolution ultraviolet spectrum of the helium-rich degenerate LDS 678A, obtained with the International Ultraviolet Explorer (IUE) satellite, is presented. LDS 678A is the coolest metallic line degenerate (DQ or DZ) yet observed with the IUE echelle. These observations provide a detailed line profile of the strong C I 2479 absorption line and equivalent width W2479 = 2.35 + or - 0.06 A from which theoretical profile fits yield a carbon abundance of log C/He = (-6.7 + or- 0.2). The presence of carbon in a He-rich atmosphere lends credence to the notion that LDS 678A is a transitional case between the DB white dwarfs with nearly pure helium atmospheres and the helium-rich DQ white dwarfs which exhibit carbon bands. Corrected for an inferred pressure shift Vp = + 38 + or - 4 km/s for the C I 2479 line, a gravitational redshift of Vrs = + 26 + or - 13 km/s is deduced from which a most probable mass of 0.55 solar mass is derived. 27 refs.

Abundances in Cool DZA and DAZ White Dwarfs: New Results Using Laboratory Damping Constants

We have determined H/He ratios, effective temperatures, and metal abundances for two cool white dwarfs, G77-50 and G74-7, that exhibit CaII lines and hydrogen Balmer lines. Using model atmosphere techniques and our recently published MMT spectra, we find helium to be the dominant atmospheric element in both stars, and the[Ca/H] ratios to be extremely metal deficient relative to the solar value. The observed Balmer decrements for these stars are compatible with the derived H/He ratios when a line broadening treatment properly combining resonant and non-resonant neutral interactions is applied. The derived abundances are consistent with predictions of accretion from the local interstellar medium.

Hubble Space Telescope observations of cool white dwarf stars: Detection of new species of heavy elements

Observations of cool white dwarf stars with the Hubble Space Telescope (HST) has uncovered a number of spectral features from previouslly unobserved species. In this paper we present the data on four cool white dwarfs. We present identifications, equivalent width measurements, and brief summaries of the significance of our findings. The four stars observed are GD 40 (DBZ3, G 74-7 (DAZ), L 745-46A (DZ), and LDS 749B (DBA). Many additional species of heavey elements were detected in GD 40 and G 74-7. In L 745-46A, while the detections are limited to Fe 1, Fe II, and Mg II, the quality of the Mg II h and K line profiles should permit a test of the line broadening theories, which are so crucial to abundance determinations. The clear detection of Mg II h and k in LDS 749 B should, once an abundance determination is made, provide a clear test of the hypothesis that the DBA stars are the result of accretion from the interstellar medium. This star contains no other clear features other than a tantalizing hint of C II 1335 with a P Cygni profile, and some expected He 1 lines.

Are There Any Hydrogen-Rich, Cool White Dwarfs?

We have calibrated the Balmer decrement as a He/H abundance ratio diagnostic for cool degenerates. Using a detailed treatment of neutral collision broadening, we find that the sensitivity of the decrement to composition is far greater than to surface gravity, and thus the problem of the ambiguity between those parameters is avoided. This calibration confirms the suggestion of several earlier authors that steep Balmer decrements in cool DA’s indicate significant helium dilution of the hydrogen in the outer layers. The He/H ratios obtained with this method have been combined with our He/H ratio results for the DZ class of stars from the MMT Atlas, and with other published model atmosphere results, to yield an overall view of the He/H ratio in white dwarfs cooler than 20,000 K.