Element abundances in cool white dwarfs

Abstract

Abstract. We present a small data base of homogeneously derived photospheric element abundances of DZ white dwarfs and related objects. Our previous investigations are supplemented with the analysis of ultraviolet spectra for nine white dwarfs. Of particular interest is the detection of Lα absorption in van Maanen 2 and a determination of the effective temperature of this star. The new value is about 1000 K lower than previous results due to the strong ultraviolet absorption by metals which has to be considered consistently. The metal abundances of our sample stars are compatible with the predictions from the two-phase accretion model of Dupuis et al. (1992Dupuis et al. ( , 1993aDupuis et al. ( , 1993b. Small deviations can be observed for the abundance ratios in some objects. This could indicate non-solar metal-to-metal ratios in the accreted material. Hydrogen can be detected in virtually all of our objects. However, its average accretion rate must be at least two orders of magnitude lower than the metal accretion rate.

Collisional effects in the far red wing of Lyman-$\sf \alpha$

“…A small fraction of the DA white dwarfs are known to have effective temperatures T eff ≤ 4000 K. Such objects were discussed by Wolff et al (2002) and Kowalski & Saumon (2006).…”

Model Atmosphere Analysis of the Weakly Magnetic DZ White Dwarf G165‐7

“…Previous studies (e.g., Dupuis et al 1993;Wolff et al 2002) have shown, however, that DZ stars have less hydrogen in their atmospheres than what is expected if the accreted material has a solar composition. Consequently, the hydrogen accretion rate must be reduced relative to that of metals in order to account for the relatively low hydrogen abundances (with respect to heavier elements) observed in DZ stars.…”

“…We note that it is unlikely that nonideal effects in the equation of state (Coulomb corrections, finite volume effect, line quenching) are important, since the pressure in our model never exceeds 10 10 dynes cm À2 . Based on a modified version of the equation of state of Mihalas et al (1990), Wolff et al (2002) found only a weak influence in van Maanen 2, another DZ star in which the atmospheric pressure is certainly higher than that in G165-7, since it is cooler and possesses a smaller metal content. We note finally that the sample of DZ stars displayed in Figures 10 and 11 of Harris et al (2003) show several objects with similar asymmetric profiles, some of them certainly too hot ($8000 K ) to be explained by molecular absorption.…”

“…The chemical abundances cannot be determined individually, however, since most of these elements are not observed spectroscopically. We thus initially assume that the relative abundances are consistent with solar ratios, a reasonable assumption according to Wehrse & Liebert (1980) (see also Wolff et al [2002] for other DZ stars) and our own preliminary analysis based on a chemical abundance analysis of the observable elements.…”

“…Atmospheric abundance analyses of DZ stars are usually based on a restricted number of absorption lines detected in the optical portion of the spectrum (see, e.g., Liebert et al 1987), although a more important set of lines can be used when UV spectroscopy is available (Zeidler-K. T. et al 1986;Koester & Wolff 2000;Wolff et al 2002). G165-7 (also known as WD 1328+307, LHS 2745; V ¼ 16:03) is a particularly unusual DZ white dwarf, which has practically no flux in the UV because of the severe blanketing by a large number of metallic lines, but it is also characterized by a very rich optical spectrum showing hundreds of absorption lines from various heavy elements such as Ca, Cr, Fe, Mg, and Na.…”