Per A. Aannestad

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

HYDROMAGNETIC SHOCK WAVES AND THEIR INFRARED EMISSION IN H I REGIONS.

Shock wave structure in H I taking into account magnetic fields and radiative losses to evaluate IR intensities due to various cooling transitions

An atlas of optical spectra of DZ white dwarfs and related objects

An atlas of optical spectra and equivalent width measurements for DZ stars and several related objects is described. These data should improve abundance measurements for Ca/He, Mg/He, and Fe/He in these stars and provide tests for calculations of accretion, diffusion, and radiative transfer in white-dwarf atmospheres. Also reported is the possible detection of He I (3888-A) in three DZ white dwarfs, 0246 + 735, 1705 + 030, and 2215 + 388. 25 refs.

Effects of silicate-graphite dust in H II regions

Interstellar dust that is present in H II regions will most likely absorb H-ionizing photons more efficiently than He-ionizing photons. Assuming a silicate-graphite dust composition, the effects of such selective absorption on the ionization structure of H, He, O, and N are investigated with emphasis on a blister geometry. Scattering by the dust grains is included in the generalized on-the-spot approximation. For a relatively soft radiation field of effective temperature about 35,000 K, the radiation hardening may cause the He(+) zone to be nearly coextensive with the H(+) zone, and the O(++) volume is strongly increased relative to the N(++) volume. The smaller grains (radii about 0.005 micron) contribute most to the selective absorption. We show that because of a cancellation of effects a simple analytical formula for the fraction of ionizing photons absorbed by the dust given by Petrosian, Silk, and Field is quite accurate in the case of silicate-graphite dust, even for blister regions. 25 refs.

Particle Shapes and the 2175 Å Feature

The Galactic interstellar extinction curve exhibits a pronounced feature at 2175 ?. The most striking characteristic of this feature is the relatively fixed nature of its central wavelength, while the width of the feature may vary. Small graphite particles are often proposed as the carrier of this UV bump. However, graphite grains of a single size and shape cannot reproduce the observed width and peak characteristics simultaneously. Here, we investigate to what extent these characteristics may be reproduced by invoking a shape distribution of prolate and oblate graphite particles. The graphite constants calculated by Draine and Lee do not yield the narrowest 2175 ? features that have been observed. We have recalculated the dielectric function of astronomical graphite such that a spherical grain of radius 50 ? produces a feature with a peak at x0 = 4.6 ?m-1 and a width ? = 0.8 ?m-1. Using grains in the Rayleigh approximation, we have calculated extinction curves for combinations of prolate, spherical, and oblate grain shapes and successfully reproduce the stationary nature of the peak. In addition, such shape variations do give a standard deviation in the width of the feature of ~1/2 the observed standard deviation. However, the absolute values of the calculated widths fall short by ~10% compared with observed widths. The observed lack of correlation between peak position and width is not reproduced by the shape variations considered here.

Hydrogen and calcium in DB white dwarfs - a case for interstellar accretion

Detections of calcium in two DBA stars and in two DB stars as well as possible detections of calcium and/or hydrogen in three additional DB degenerates, from moderate-resolution spectra obtained at the Multiple Mirror Telescope are reported. Calcium and hydrogen abundances and abundance limits which are close to the cosmic abundance have been derived. This finding would appear to indicate that accretion of interstellar hydrogen and grains can effectively occur in some single helium-rich degenerates up to effective temperatures of at least 20,000 K. If a propeller mechanism is operative in DB stars, the results would suggest that the mechanism may break down or be less efficient in a significant fraction of these objects. 23 references.

Temperature fluctuations in interstellar dust grains

Temperature fluctuations in interstellar dust grains caused by absorption of ultraviolet photons are discussed. Temperature probability distributions are presented for various assumptions about the grain specific heat and far infrared emissivity. Grains smaller than or about 0.01 μ may suffer large temperature fluctuations and would spend most of their time at very low temperatures. The ‘equilibrium temperature’ is not a good measure for most average temperatures of such grains. It is shown that, although small grains spend only a small fraction of their time at the higher temperatures, the emitted far infrared spectrum is significantly shifted towards shorter wavelengths than predicted for grains assumed to be at the equilibrium temperature.

Hot H

The rotational excitation of H2 molecules by interstellar shock waves is calculated. Level populations integrated through a shock in a cloud with a density of 10 per cu cm have nearly exponential distributions, with effective rotational temperatures T sub r = 15 V to the 1.5 power K, where V is the shock velocity in km/sec. Ortho-para conversion is effective for high shock velocities, but not for low ones, so that the degree of alternation of intensities of rotational lines in the Lyman absorption band should be correlated with V, and therefore with T sub 2. The results are compared with observations of H2 rotational levels made by the Copernicus satellite.

sub 2

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.

and interstellar shocks

A silicate-graphite model of interstellar extinction has been compared with recent observational information about the ultraviolet part of the extinction curve. The model has large (>0.01 μm) and small (0.005 μm) grains of each composition, and the number densities of the four components are assumed to vary independently and randomly within certain limits. A total of 50 extinction curves have been generated and subjected to analysis similar to that for the ultraviolet observations