New features in the spectra of white dwarfs.

Abstract

view Abstract Citations (4) References Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS New features in the spectra of white dwarfs. Greenstein, Jesse L. Abstract Some aspects of the spectra of white dwarfs have been studied in a spectrophotometric investigation using the 38 and 18 A/mm dispersions of the ~oo-inch Hale reflector coude'. A general characteristic of those white dwarfs with strong hydrogen lines is the steep Balmer decrement. H~/H7 is commonly less than 0 1 the confluence of the strong wings results in a depression of the continuum with respect to which the lines are measured. In contrast, in a normal A dwarf, the ratio of these equivalent widths is usually near 0.5. This steep decrement is characteristic both of normal white dwarfs, like 40 Eri B, where W(H~) is 31 A, and of the weak-lined white dwarfs like HZ 43 which has W(H~) 9 A. The group of "sharp-lined" white dwarfs like L 870-2 has a steep Balmer decrement, but line-widths at half intensity only o.6 that of 40 Eri B, and small equivalent widths comparable to those in B~ to Ao dwarfs. In addition a group of white dwarfs exists with very broad, shallow weak lines e.g., HZ 43 and LDS 678A. The most striking observation in the helium- rich white dwarfs is the extraordinary width and variety of line profiles, varying from line to line. Superposed on broad stellar lines are ` `shell' lines, XX3888, 3965, arising from strongly metastable levels; cores of negative velocity seem to vary in velocity and strength. Since the velocity of escape from a white dwarf is near 5000 km/ sec, the ejection of matter involves enormous and as yet unknown forces. The star HZ 3 proves to resemble Luyten's helium-rich white dwarfs, but has in addition weak, very broad hydrogen lines. It may be intermediate in characteristics, like + 2804211, between extreme subdwarfs and the true white dwarf. Van Maanen 2 is much cooler, near 60000, has a deduced radius near 0.01 R 0, surface gravity io8 dynes and Einstein shift expected between 20 and 6o km/sec. The older observed radial velocity was anomalously large, +238 km/sec. The new spectra show no lines longward of A3968. The H and K lines are strong, broad and show pronounced cores of shell characteristics. The core velocity is +36km/sec; underexposed plates give a velocity of +114 km/sec, presumably referring more to the whole width of the line, with some effect of the displaced core. Low- dispersion plates on successive nights gave +21 km/sec on an over-exposed plate, and +216 km/ sec on a thin plate. The true velocity of the star is thus not known; the star presumably has some type of variable extended envelope. In the ultraviolet, extremely broad Fe I lines dominate the spectrum, appearing in clumps 20 to 50 A wide, in spite of a possible resolution of about o.8 A on the plate. An attempt was made to synthesize the spectrum by using King's f-values, an excitation temperature of 50000, and blurring the predicted line-absorption coefficient by a damping-type profile. A homogeneous atmosphere at a single temperature and pressure was used. The width of individual lines at half intensity proved to be 8 A, which corresponds to 6o,ooo times the radiation damping width. This is only a first determination, because the effect of integration through a model atmosphere will almost certainly greatly affect this estimated value of the collisional damping. The absence of the X4045 multiplet of Fe I, and of other strong lines from excited levels, suggests that the high pressures seriously shift and alter the atomic energy states. Mt. Wilson and Palomar Observatories, Pasadena, Calsf. Publication: The Astronomical Journal Pub Date: October 1954 DOI: 10.1086/107029 Bibcode: 1954AJ.....59..322G full text sources ADS |