Irene R. Little-Marenin

Additional late-type stars with technetium

The results of a survey of 279 late-type giants and supergiants for the spectral lines of the radioactive element technetium (Tc I) at 4297, 4262, and 4238 A are presented. The following conclusions are reached: (1) the presence of Tc correlates very strongly with the existence of light variability; (2) evolutionary MS stars show Tc and spectroscopic MS stars do not show Tc; (3) single S stars show Tc; (4) SC stars show Tc; (5) about 75 percent of the C stars show Tc; and (6) Ba II stars do not show Tc. The findings are compatible with predictions from stellar evolution theory. 78 references.

The formation and annealing of circumstellar dust, as gauged by IRAS low-resolution spectra and the microwave maser chronology

By comparing IRAS mean colors and LRS spectral differences in a large sample of Mira variables with the chronological sequence for development of microwave maser emission among such stars, it appears possible to describe a dust grain formation and annealing sequence. The early time spectra are dominated by aluminum oxides, which are then overrun by the emergence of silicates. Rather than the growth of silicate mantles on aluminate cores, this sequence is interpreted as a simple result of the higher electron affinity of aluminum for oxygen, resulting in an initial abundance of AlO bonds in the underoxidized grains. Once all the Al becomes fully oxidized, the relative abundances of Al and Si dictate that the AlO signature will be swamped by the growth of the SiO stretching fundamental. The correlations among the proposed dust chronology, the increasingly thick circumstellar envelopes, and the light curve asymmetries of Mira variables are instructive for understanding the evolutionary changes occurring in such stars. 10 refs.

The Carbon-Rich Dust Sequence: Infrared Spectral Classification of Carbon Stars

We have developed a classification system for the infrared spectral emission from carbon stars, using a sample of 96 bright carbon-rich variables associated with the asymptotic giant branch. In addition to the stellar contribution, most spectra include the 11.2 μm emission feature from SiC and either a smooth, cool continuum from amorphous carbon or a secondary emission feature at 9.0 μm. We have identified a carbon-rich dust sequence along which the amorphous carbon component grows while the 9.0 μm feature declines in strength. Along this spectral sequence, the proportion of Mira variables increases, as does the period of variability, the mass-loss rate, and the thickness of the circumstellar shell. Thus the carbon-rich dust sequence appears to be an evolutionary sequence. One class of spectra shows a particularly strong 9.0 μm feature, enhanced C/O ratio, and several other unusual properties that suggest a different sequence, perhaps related to J stars.

8-14 micron spectroscopy of carbon stars associated with silicate dust

8-14 μm spectra of two known and two proposed silicate carbon stars are presented. The data confirm the positional association of BM Gem and V778 Cyg with the IRAS Low Resolution Spectrometer silicate spectra. Moderate silicate emission has been discovered in the carbon star HD 189605. HD 100764, on the other hand, has no significant silicate emission

A search for technetium (Tc II) in barium stars

The authors searched without success for the lines of Tc II at 2647.02, 2610.00 and 2543.24 A in IUE spectra of the barium stars HR 5058, Omicron Vir, and Zeta Cap. The lack of Tc II implies that the observed s-process enhancements were produced more than half a million years ago and supports the suggestion that the spectral peculiarities of barium stars are probably related to the binary nature of the stars.

Water masers associated with two carbon stars: EU Andromedae and V778 Cygni

We observed the 22.2 GHz H2O maser line from the two C stars EU And and V778 Cyg. The intensity of the line has varied by more than a factor of 5 over several month and the line has shown additional weaker components. We interpret the system as being binary with a C and an M star component with a thick shell.

Water maser and optical variability for V778 CYG and EU and 1987–1991

The water maser spectra of V778 Cyg show periodic variations with the same period as the optical carbon star (P=302d). However, a phaselag of 0.23 is present. The central velocity of the features may show long term variability. The 22 GHz spectra of EU And and the optical light curve show semi-periodic variations with periods ranging between 140 to 175 days.

Emission features in IRAS LRS spectra of MS,'S and SC stars

We observe a progression of emission features in the 8–11 μm region in MS, S and SC stars that parallels their increasing C:O ratio and s-process enhancements.

An analysis of IRAS low resolution spectra of S stars

The Infrared Astronomical Satellite (IRAS) carried a low resolution spectrometer (LRS) with a wavelength response covering the 8 to 22 micrometer region. About 5000 LRS spectra exist of point sources brighter than 10 Jy at 12 and 25 micrometers. Spectra of 67 known S stars were identified by the IRAS catalogue search. We have analyzed the 32 highest quality S star spectra and find that 19 show measurable emission features in the 9 to 13 micrometer region. Most of the emission features were relatively weak, the strongest being about 30% of the strength of the underlying continuum and the weakest about 10%. Four stars, Y Lyn, RS Cnc, W And, and RAFGL-2425, show emission features resembling the silicate features of M stars. One star, UY Cen, shows a nearly pure SiC feature. Most (14) S Star emission features were centered near 10.5 micrometers, intermediate between the silicate emission features of M stars and the silicon carbide (SIC) features of C stars (see the paper by Little-~renin and Wilton at this conference). The identification of the intermediate wavelength feature is at present unknown, but two possible interpretations may be suggested: i) A previously unknown molecular species may occur in circumstellar shells only when stars are progressing from M to S to C by stellar evolution on the asymptotic giant branch, and while the photospheric ratio of C/0 changes respectively from i. We believe that the M to S to C change occurs on a time sca~e of greater than thousands of years, and that the circumstellar shell, with a lifetime of 10 to 100 years, should represent the chemical abundances in the photosphere. It has been suggested at this conference (see following remarks) that the unknown molecular species may be a form of polycyclic hydrocarbon. Our intermediate wavelength feature is similar to that observed by Gehrz, et al. (Ap. J., 281, 303, 1984) in Nova Aql 1982 as it evolved during its dust shell development phase. 2) The intermediate wavelength feature shown by our 14 S stars may be a blend of SiC and silicate emission. We are able to model the intermediate wavelength features of our 14 S stars with mixtures ranging