UC San Diego

Twenty-four late-type variable stars exhibiting intrinsic variable polarization have been monitored over the range 3.5 μ to 11 μ for several cycles. No conclusive evidence for gross changes in the amount of c1rcumstellar grains have been found. Thus, circumstellar infrared emission is attributed to the total abundance of grams surrounding the star which does not change by a large amount with time while intrinsic polarization is attributed to more localized scattering and absorption effects or possibly to aligned grains. Spectrophotometry with Δ λ/λ ≈ 0.015 over the 7.5-13.5 μ wavelength range of several stars with different chemical composition indicate excess emission characteristic of 3 types of grains: (1) "blackbody" grains, (2) silicate grains and (3) silicon carbide grains.

It is found that the Mira carbon stars and V Hya have an energy distribution that qualitatively different than the semi-regular carbon stars and the oxygen-rich stars. In addition to the spectral feature due to silicon carbide grains, which also appears in the semi-regular carbon variables, a major fraction, from 0.3 to 0.8, of the observed energy from the Mira carbon stars appears as a cool component characterized by an approximately 950 ºK blackbody. Thus, the ratio silicon carbide to "blackbody" excess appears to vary greatly, among the carbon stars. A plausible mechanism to explain the excess over all wavelengths seen in the carbon Miras is absorption of starlight and thermal re-radiation by graphite grains in the circumstellar envelope.

Similar Infrared observations of the variable star R CrB over the period 1970-1974 have been made and discussed in terms of a single or double star model for this object. On the single star model it is found that dust made on or about JD 2,441,033 was subsequently ejected from the star with an apparent recessionalvelocity of 27 km/s. There was no visual event directly associated with this infrared activity but if the apparent recession is extrapolated linearly back in time, the material would have left the stellar surface on JD ~ 2,440,400, and about this time there was a visual event. Spectrophotometry with Δ λ/λ ≈ 0.02 over the 3-4 μ and Δ λ/λ ≈ 0.015 over the 8-12.5 μ wavelength ranges show an excess emission which is completely smooth and featureless to the accuracy obtained, in distinct contrast to the spectra of all the M and C stars observed in this study which do show features.

Models to explain the intrinsic polarization of starlight in terms of scattering and absorption by circumstellar dust grains are discussed. It is found that the rapid variability of polarization and the optical depth of the shell inferred from infrared measurements put severe constraints on the asynmetric envelope models and suggest small clouds, perhaps close to the star, must be responsible for the polarization. Alternatively, it is found that asymetric grains in the extended circumstellar envelope may be aligned by the Davis-Greenstein para-magnetic relaxation mechanism. The time scales for alignment would be 1-3 days and if the field is dipolar (1) a stellar surface field of approximately 16-100 gauss is required, and (2) the expected infrared polarization may not necessarily large.

The mass loss from late-type stars has been re-investigated and it is found that the observed persistence of dust grains in the circumstellar envelope requires continuous condensation and ejection of grains leading to a minimum gas loss rate ⪆ 10 -6 M/yr necessary to restrain the gram outflow. The grains must be momentum coupled to the gas as suggested by Gilman for otherwise they would exit too rapidly. In addition, it is found that the momentum coupling of gas to dust and the minimum gas loss rate consistent with the observed dust loss rate limits the supersonic terminal velocity of dust grains relative to the gas to approximately 5-10 km/s. For the oxygen-rich Mira variables the calculated mass loss rates (using a simplified model for the circumstellar envelope) are ~ 3-7 x 10 -6 M/yr and the dust loss rate is ~ 1/300 of this in silicate grains. For the carbon Miras the gas and graphite dust loss rates are approximately twice this and, in addition, the calculated rates agree with the lower limit to mass loss given by the minimum radiation pressure exerted on the grains (consistent with the observed infrared excess and conservation of energy) and the conservation of momentum. Thus, the idea of Gehrz and Woolf and Gilman that mass loss in late-type stars may be driven by radiation pressure on the circumstellar dust grains is supported.