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Astron. Astrophys. 351, 597-606 (1999)

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1. Introduction

Cepheids are stars, whose investigation is of paramount importance for the critical testing of our knowledge about the detailed evolutionary characteristics of the intermediate mass stars. Numerous studies of the chemical composition of Cepheids and non-variable yellow supergiants showed that the observational results agree qualitatively with the theoretical predictions (see, e.g. Luck & Lambert 1981, Luck & Lambert 1985, Luck 1994, Andrievsky et al. 1996, Kovtyukh et al. 1996). However, a more detailed investigation indicates that there are still some contradictions between the theory and observations that are not completely understood and have not been explained to date. One can mention the papers by Luck & Lambert (1985), Luck et al. (1998), where the problem was posed and some attempts to solve it were undertaken. The problem itself arises from the following observational results:

  1. The considerable oxygen deficiency observed in the intermediate mass ([FORMULA]) supergiants is not explained by the standard dredge-up scenario.

  2. The observed carbon deficiency is often greater than the theoretically predicted one for these stars.

    These contradictions are likely to be connected with more general problems of the stellar spectroscopic analysis.

  3. Spectroscopic gravities (based on [FORMULA]/[FORMULA] balance) appear to be systematically lower than those determined from the stellar physical parameters.

  4. For the great majority of the elements that presented in the spectrum by two ionization stages, the ionization balance is not reached when the gravity is determined using [FORMULA] and [FORMULA] lines.

Luck & Lambert (1985) were the first to clearly point out that for intermediate mass supergiants so-called "physical" (or "theoretical") gravities, whose determination is based on the masses and radii, are not consistent with the "spectroscopic" gravities found by keeping the [FORMULA]/[FORMULA] ionization balance. These authors have emphasized that the difference between the spectroscopic and theoretical gravities is well correlated with the determined carbon and oxygen abundances. Remarkable oxygen deficiency appears in the case, when spectroscopically determined gravities [FORMULA]sp are less than theoretically estimated by a factor of 2 or more (see Fig. 11 from that paper). Oxygen abundance can be changed to approximately solar value only if [FORMULA]ph instead of [FORMULA]sp is used in the analysis, but this immediately leads to a significant imbalance in the ionization equilibrium for metals. Such a disequilibrium cannot be removed by any reasonable [FORMULA] changes. The problems of the spectroscopic analysis are clearly seen, first of all, in the resulting abundances of carbon and oxygen, which are the elements of great importance from the evolutionary point of view. In their concluding remarks, Luck & Lambert (1985) speculate that the oxygen underabundance found for intermediate mass stars can be explained by either a specific chemical composition of their progenitors, or systematic errors attributed to traditional spectroscopic abundance analysis. We show below that the mentioned contradictions can be removed using a more realistic approach in the chemical composition analysis. For this aim we present the results of a detailed study of the elemental abundances in the atmosphere of the famous classical Cepheid [FORMULA] Cep observed at seven different phases. This star was selected because 1) as a bright object, it enables one to obtain spectra of high quality, 2) as a star belonging to the class of pulsating variables, it gives a possibility to estimate its luminosity, mass, radius and effective temperature with rather high accuracy.

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© European Southern Observatory (ESO) 1999

Online publication: November 3, 1999