D. van Blerkom

(C-12)O emission from the envelopes of cool stars in the solar neighborhood

Results are presented on observations of the CO J = 1-0 line emission from all M giants, S stars, and C stars listed in the Two-Micron Sky Survey having strong FIR emission and lying north of delta = -10 deg. The data from this survey and other data for C and S stars show that the line profiles of these stars look like flattened parabolas and have roughly the same shape for different stars. In contrast, the shapes of the spectral lines from giant M stars are diverse, ranging from triangular to spiked and asymmetric, suggesting that the envelopes of M stars have complex kinematics and structure. The outflow velocities inferred from the line profiles of the stars surveyed span a range of more than an order of magnitude, with the velocities of C stars correlating with IR color. 54 refs.

The geometry of VY Canis Majoris derived from SiO maser lines

A radiation reveals transfer calculation reveals that the SiO maser lines observed in the spectrum of VY CMa are formed in a rotating equatorial disk seen nearly edge-on. Other geometries are considered and eliminated. It is suggested that the SiO lines of NML Cyg also show evidence that they are formed in a disk. (AIP)

Mass loss from P Cygni. II. An equivalent two-level-atom approach

The formation of the Balmer-line spectrum of P Cygni is investigated by using an equivalent two-level-atom formalism. This approach eliminates the artificial distinction between a continuum-emitting core and line-emitting envelope, and also does not employ a supersonic approximation in the line-transfer problem. The results support the conclusion that a velocity distribution similar to the linear law upsilon (r) proportionalr exists in the envelope, and can account for the observed Balmer lines. A more complicated velocity law derived from infrared continuum measurements does not give a good representation of the Balmer lines.

Neutral helium emission in Wolf-Rayet envelopes

A non-LTE analysis of neutral helium emission is performed for Wolf-Rayet stars. The statistical equilibrium equations are derived and simplified by using the escape probability method. Using a thirty-six-bound-level model helium atom, the relative intensities of allowed transitions are calculated for various helium abundances at T/sube/=10,000 and 20,000 K and are compared with the observations of MR 119. (AIP)

The normalized on-the-spot approximation for line transfer problems

Abstract A simple approximation for line transfer problems is presented which is useful when photons scatter only a few times before being destroyed. Comparison is made to solutions of high accuracy.

Interpretation of the line profiles of the 8 micron band of silicon monoxide from VY canis majoris

Observations of the vibration-rotation transitions of silicon monoxide from the expanding circumstellar envelope of VY Canis Majoris have previously been interpreted in terms of line formation in a thin shell of gas surrounding an opaque core of dust which emits a continuous spectrum. We show that the use of this model is the cause of the apparent displacement of the stellar velocity derived from the 8 ..mu..m lines by 14 km s/sup -1/ from the value based on the v = 0, J = 2..-->..1 SiO line and on the midpoint of the OH maser emission. No such displacement occurs if a model of line formation is employed in which emission from the entire envelope contributes to the emergent profile. This eliminates the need to postulate asymmetries in the envelope and agrees with infrared continuum studies.

A simple estimate of the effect of a thick circumstellar dust shell on photospheric line profiles

A star embedded in an optically thick dust shell may have a line spectrum that closely resembles the spectrum of some unobscured star--e.g., VY CMa (shrouded) and ..cap alpha.. Her (unshrouded). This implies that the surface temperatures of the two stars are nearly equal, but since one is subject to a back-warming effect, the distributions of temperature with depth in the respective atmospheres are dissimilar. We find that effective temperatures and absorption line strengths are reduced in shrouded stars below the values that obtain in their unshrouded counterparts. For models of VY CMa that yield grain temperatures at the inner boundary of the dust shell consistent with observation, the reduction in strength of a visible line is small (< or approx. =15%). Thus the spectral distribution of ..cap alpha.. Her may be used as an approximation to that of VY CMa before it is scattered by the dust, without the necessity of first correcting for back warming.