77 research outputs found
Advances in Spectral Classification
In this article we give an overview of the developments in the field of
spectral classification and its continued importance in the fields of stellar
and galactic evolution. The extension of MK system to cool stars as well as
refined classification of the hot stars using the new data obtained from modern
ground based facilities and space missions has been described. A brief summary
of automated methods of spectral classifications developed for the quick and
objective classification of the ongoing and future surveys is presented. A new
spectral class encoding system developed for exploitation of databases is
described. Recent large scale spectral classification catalogues are listed.Comment: To appear in BASI, 33 pages, 9 figure
Abundance Analyses of Field RV Stars, V: DS Aquarii, UY Arae, TW Camelopardis, BT Librae, U Moncerotis, TT Ophiuchi, and RV Tauri
Abundance analyses are presented and discussed for eight RV Tauri variables.
The RV B star UY Ara shows the abundance anomalies seen in other RV B stars,
namely elements that condense into grains at high temperature are underabundant
but elements of low condensation temperature are much less underabundant. This
pattern is ascribed to a separation of dust from gas with accretion of gas but
not dust by the atmosphere. Abundances for two RV C stars with earlier results
for other RV C stars show that these intrinsically metal-poor stars do not show
effects of dust-gas separation. Analyses of five RV A stars show that these
cooler stars are very largely unaffected by dust-gas separation. It is proposed
that the deeper convective envelope of cooler stars dilutes anomalies resulting
from dust-gas separation - the dusty wind off the RV Tauri variable or a dusty
circumbinary disk - are reviewed and observational tests are suggested.Comment: 42 pages, 6 figures, 8 tables, this may be listed as ApJ in pres
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Is the Post-AGB Star SAO 40039 Mildly Hydrogen-Deficient?
We have conducted an LTE abundance analysis for SAO 40039, a warm post-AGB star whose spectrum is known to show surprisingly strong He I lines for its effective temperature and has been suspected of being H-deficient and He-rich. High-resolution optical spectra are analyzed using a family of model atmospheres with different He/H ratios. Atmospheric parameters are estimated from the ionization equilibrium set by neutral and singly ionized species of Fe and Mg, the excitation of Fe I and Fe II lines, and the wings of the Paschen lines. On the assumption that the He I lines are of photospheric and not chromospheric origin, a He/H ratio of approximately unity is found by imposing the condition that the adopted He/H ratio of the model atmosphere must equal the ratio derived from the observed He I triplet lines at 5876, 4471, and 4713 angstrom, and singlet lines at 4922 and 5015 angstrom. Using the model with the best-fitting atmospheric parameters for this He/H ratio, SAO 40039 is confirmed to exhibit mild dust-gas depletion, I. e., the star has an atmosphere deficient in elements of high condensation temperature. The star appears to be moderately metal-deficient with [Fe/H] = -0.4 dex. But the star's intrinsic metallicity as estimated from Na, S, and Zn, elements of a low condensation temperature, is [Fe/H](o) similar or equal to -0.2 ([Fe/H](o) refers to the star's intrinsic metallicity). The star is enriched in N and perhaps O as well, changes reflecting the star's AGB past and the event that led to He enrichment.National Science FoundationRobert A. Welch Foundation of Houston, Texas F-634McDonald Observator
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