Stephen J. Little

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.

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.

High-resolution H and K mapping of W51

The H-II region W51 has been mapped in the near-infrared H and K wavebands with a resolution of about 6 arcsec over an area of about 14 sq arcsec. The W51 IRS 1 source has been resolved into six components, and an arc-like extension of small H-II regions is found that seems to parallel the edge of the molecular cloud. Measurements of the fluxes and extinctions of these sources are presented. A close correspondence is noted between the present K maps and the radio maps of Scott (1978). 22 refs.

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