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Astron. Astrophys. 326, 1069-1075 (1997)

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1. Introduction

RV Tau variables are one of several classes of low mass variable stars thought to populate the region of the HR Diagram to the left of the AGB-branch, and above the horizontal branch. These stars appear to be at the very end of their evolution along the asymptotic giant branch (AGB), or post-AGB stars. The standard picture of this stage of evolution is of an electron degenerate C-O core, with overlying layers of He and H. Mixing between the two outer layers should result in enhancement of the surface layers with s-process elements, the by-products of neutron capture reactions occurring during He-burning. So investigation into the s-process abundances should reveal valuable insights into the nature of the stars.

Although there has been very little work done on the detailed elemental abundance analyses of RV Tau variables, and stars like them, what has been done (Aliev, 1967; Baird, 1979; Yoshioka, 1979; Luck and Bond, 1984 &1989; Giridhar et al., 1994), reveals evidence for systematic s-process under-abundances. This has serious implications for our accepted view of AGB evolution, and the nature of RV Tau variables.

Luck and Bond (1989) noted that the s-process abundances were reminiscent of those found in extreme halo red giants. Yet RV Tau variables have much higher metallicities than extreme halo stars. Luck and Bond suggest that either:

  1. the stars reflect their original s-process deficiencies relative to Fe, but severe mass-loss has reduced the hydrogen abundance in the atmosphere, thereby raising the Fe/H ratios to the observed levels.
  2. the stars have normal s-process abundances, but because of their low 2nd ionization potentials, these elements are selectively over-ionized by Lyman-continuum photons. The key to this possibility is the observed systematic deficiency in Sc in these stars; the only light metal that shares a sufficiently low 2nd ionization potential.

These two alternatives could be tested by observing RV Tau variables in globular clusters, and comparing them with their red giant companions. If a hydrogen deficiency is the cause of the s-process deficiencies, RV Tau variables should show higher [Fe/H] ratios than the normal red giants, but the same [s- process/Fe] ratios. If over-ionization is the cause, RV Tau variables should show the same [Fe/H] ratios as the red giants, but lower [s-process/Fe] ratios.

Giridhar et al. (1994) pointed out that there were so few detailed abundance studies of RV Tau variables in the literature, that any patterns in their abundances are impossible to discern. Since there are just six stars studied in the literature, this survey of 3 stars adds considerably to the total number of RV Tau variables analysed. It is not unreasonable, therefore to reanalyse the complete data set in this work, to look for patterns that may be present.

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© European Southern Observatory (ESO) 1997

Online publication: April 8, 1998