Astronomy and Astrophysics – Astronomy
Scientific paper
Aug 1994
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1994apjs...94..163s&link_type=abstract
The Astrophysical Journal Supplement Series, vol. 94, no. 1, p. 163-182
Astronomy and Astrophysics
Astronomy
8
B Stars, Photosphere, Resonance Lines, Stellar Winds, Ultraviolet Astronomy, Ultraviolet Spectra, Iue, Shell Stars, Stellar Motions
Scientific paper
Equivalent widths of 16 lines of C I, C II, C III, C IV, Si II, Si III, Si IV, Al II, Al III, Fe II, and Fe III, plus centriod and edge velocities of the Si IV and C IV lines, were measured in International Ultraviolet Explorer spectra of 39 Ble-B8e and 18 B1-B8 standard non-emission-line stars. These suggest the following: (1) Certain line ratios of Si II/III, C II/III, Al II/III, and Fe II/III are very sensitive to spectral type and represent excellent UV criteria for spectral classification. (2) UV line strengths and line ratios show that there are no significant differences between the photospheric line spectra of Be and normal, non-emission-line stars of corresponding type. (3) The Si IV and C IV wind lines in the Be stars are correlated with both spectral type and luminosity class in the sense that the hottest stars have the strongest lines with the largest centroid and edge velocities, and the giants and subgiants have stronger lines than the main-sequence stars. (4) The Si IV wind lines persist to spectral type B8 in both the Be stars and the standard stars but are stronger in the Be stars than in the standards for the earlier types. (5) The C IV wind lines persist to spectral type B8 in the Be stars, but only to B3 in the standard stars, and are stronger in the Be stars than in the standards at all spectral types. (6) The equivalent widths of the Si IV and C IV wind lines are only very weakly correlated with v sin i, if at all, but a threshold in v sin i near 150 kn/sec exists, below which no large equivalent widths of Si IV or C IV may be seen. Assuming that the Be stars are all rapid rotators, such a correlation is essentially a correlation with i and suggests that the winds from Be stars arise preferentially from the equatorial regions. (7) Shell stars have weaker C IV absorption and smaller centroid and edge velocities than other Be stars, suggesting that they have weaker winds. Since there is considerable evidence that these are stars with cool, low-velocity disks which are being viewed edge-on or nearly edge-on, the winds may be inhibited and modified by the denser material in the equatorial regions. (8) Mg II emission is detected in about half of the program Be stars with long-wavelength IUE spectra, and seems not to be correlated with spectral type, v sin i, or strength of the Si IV wind lines. Since the Mg II emission presumably originates in the cool, low-velocity envelope, and since Mg II emission also correlates with hydrogen Balmer emission in the Be stars, this sugests that there is no strong physical relationship between the stellar winds and the cool disk. (9) The Be shell stars have stronger resonance lines of Si II, C I, C II, Al II, Fe II, presumably formed in the cool shells, than the other Be stars and the normal, non-emission-line stars of the same spectral types, consistent with the strong lines arising from metastable levels in the optical spectra of these stars.
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