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

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9. Conclusion

Above we showed that standard spectroscopic analysis of intermediate mass supergiant stars, based on determination of the atmospheric parameters with the help of numerous [FORMULA] lines, produces some discrepancy with the theoretical expectation for surface gravity and abundances of some chemical elements. This discrepancy is likely due to a sensitivity of the lines of neutral iron to NLTE effects, which are ignored in the LTE analysis.

The use of [FORMULA] lines, which are seen as being be free of significant influence from NLTE effects, allows one to avoid this problem giving physically more acceptable results.

One can then ask whether all previously obtained results on chemical abundances, at least for yellow supergiants, are wrong. Fortunately, the situation seems to be not so dramatic. The differences discussed between the two considered approaches become noticeable when the internal errors of the analysis are small (say, about 0.1 dex, which is rarely reached in the quantitative analysis).

This is relevant to spectroscopic studies based on high quality observations (high resolution and high signal-to-noise spectra). As one can see from Tables 2 and 3 (averaged data), despite the significant differences in the microturbulence parameter and surface gravity values, both applied methods give similar results for some elements. Let us consider iron, in particular. In fact, both methods produce nearly the same mean iron abundance, which is actually close to the value given by the weakest [FORMULA] lines. Nevertheless, adopting the standard approach we underestimate the microturbulent velocity and artificially decrease the spectroscopic gravity. In the case of non-standard approach we reach practically the same iron abundance by analysing the [FORMULA] lines and only the weakest (formally having W=0 mÅ) lines of neutral iron (both the line samples are regarded to be free of the NLTE influence). Being physically more grounded, the second method gives the following advantages:

  • it removes so-called problem of the spectroscopic gravities, fitting them to the gravities based on the estimated stellar masses and radii;

  • it brings the abundances of some elements (among them carbon, nitrogen, oxygen) in accordance with the theoretical prediction for the stars of intermediate masses;

  • it allows us to keep an ionizational balance for some elements seen in two ionization stages together with [FORMULA]/[FORMULA] balance. In the next papers we plan to report similar results obtained for some Cepheids having reliably determined physical characteristics.

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© European Southern Observatory (ESO) 1999

Online publication: November 3, 1999
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