Heavy-element abundance patterns in hot DA white dwarfs

Abstract

We present a series of systematic abundance measurements for 25 hot DA white dwarfs in the temperature range ∼20 000-110 000 K, based on far-ultraviolet spectroscopy with the Space Telescope Imaging Spectrograph (STIS)/Goddard High Resolution Spectrograph (GHRS) onboard Hubble Space Telescope, IUE and FUSE. Using our latest heavy-element blanketed non-local thermodynamic equilibrium (non-LTE) stellar atmosphere calculations we have addressed the heavy-element abundance patterns, making completely objective measurements of abundance values and upper limits using a χ 2 fitting technique to determine the uncertainties in the abundance measurements, which can be related to the formal upper limits in those stars where particular elements are not detected.We find that the presence or absence of heavy elements in the hot DA white dwarfs largely reflects what would be expected if radiative levitation is the supporting mechanism, although the measured abundances do not match the predicted values very well, as reported by other authors in the past. Almost all stars hotter than ∼50 000 K contain heavy elements. For most of these the spread in element abundances is quite narrow and similar to the abundances measured in G191-B2B. However, there is an unexplained dichotomy at lower temperatures with some stars having apparently pure H envelopes and others having detectable quantities of heavy elements. The heavy elements present in these cooler stars are often stratified, lying in the outermost layers of the envelope. A few strong temperature/evolutionary effects are seen in the abundance measurements. There is a decreasing Si abundance with temperature, the N abundance pattern splits into two groups at lower temperature and there is a sharp decline in Fe and Ni abundance to zero, below ∼50 000 K. When detected, the Fe and Ni abundances maintain an approximately constant ratio, close to the cosmic value of ∼20. For the hottest white dwarfs observed by STIS, the strongest determinant of abundance appears to be gravity.

Structure and Evolution of White Dwarfs and their Interaction with the Local Interstellar Medium

“…Furthermore, we now have direct observational evidence (Barstow et al 1999;Dreizler & Wolff 1999) that photospheric heavy elements are not necessarily homogeneously distributed (by depth) and that more complex stratified structures must be considered. While almost all stars hotter than ~50,000K contain heavy elements, as expected, there is an unexplained dichotomy at lower temperatures, with some stars having apparently pure H atmospheres and others detectable quantities of heavy elements (Barstow et al 2003b, e.g. figure 4).…”

“…A survey of 25 hot DA white dwarfs, based on IUE and HST data (Barstow et al 2003b), shows that, while the presence or absence of heavy elements largely reflects what would be expected if radiative levitation were the supporting mechanism, the measured abundances do not match predicted values very well. These and earlier results are forcing us to confront complexities in the real physical structure of the stars.…”

“…figure 4). In many of these objects the photospheric opacity does not reveal itself in EUV photometric or spectroscopic observations, implying that the observed material resides in a thin layer in the uppermost region of the photosphere (see Holberg et al 1999, Barstow et al 2003b). The effect of this stratification can be observed directly with high resolution UV spectroscopy.…”

O VI in the local interstellar medium

“…From an observational point of view, there are no detections of interstellar N V or C IV along the lines-of-sight to any local white dwarf observed with IUE (Holberg Barstow & Sion 1998). Furthermore, careful radial velocity analyses of a smaller but higher signal-to-noise sample of DA white dwarfs studied with HST/STIS showed detections of both photospheric and circumstellar N V and C IV (Barstow et al 2003a, Bannister et al 2003, but no interstellar features. Savage & Lehner (2006) take the reasonable position that if no stellar or circumstellar heavy element lines are detected in a spectrum then any detected O VI is interstellar in origin.…”

Hot DA white dwarf model atmosphere calculations: including improved Ni PI cross-sections

“…For this work, we adopted T eff = 52 500K and log g = 7.53 as measured by Barstow et al (2003) for G191-B2B for Models 1 to 4. These values were used for consistency with the work described by Preval et al (2013).…”