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Astron. Astrophys. 345, 915-924 (1999)


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Probing the Na I  D and K I [FORMULA]7699 resonance lines sensitivity to background opacity in late-type stars *

A. Tripicchio 1, M.T. Gomez 1, G. Severino 1, E. Covino 1, R.J. GarcíaLópez 2 and L. Terranegra 1

1 Osservatorio Astronomico di Capodimonte, Via Moiariello 16, I-80131 Naples, Italy
2 Instituto de Astrofísica de Canarias, E-38200 La Laguna, Tenerife, Spain

Received 16 November 1998 / Accepted 19 March 1999

Abstract

We have measured the equivalent width [FORMULA] of the K I resonance line at 7699 Å for a large sample of low activity late-type stars observed with high spectral resolution and we have verified that the relation [FORMULA] vs. [FORMULA] is monotonically decreasing, for both dwarf and giant stars. This behaviour is different from that of the NaI  D lines for stars of the same type, which showed that the relation [FORMULA] vs. [FORMULA] has a maximum for [FORMULA] K, which is better defined for giants than for dwarfs (Tripicchio et al. 1997).

The fit of the observed KI equivalent widths by means of a NLTE spectral line synthesis using conventional background opacity shows that, for dwarf stars, the adopted models overestimate the observed [FORMULA] for temperatures [FORMULA] K. This result is similar to that discussed for the NaI  D lines in our previous paper. On the other hand, for giant stars with [FORMULA] [FORMULA] K these models in general underestimate [FORMULA].

The discrepancies between observed and computed [FORMULA] and [FORMULA] for cool stars are much stronger than the variations due to uncertainties in either atmospheric model or line parameters, like effective temperature and surface gravity, or Van der Waals broadening. For M dwarf stars, the most convincing explanation for the disagreement is the lack of atomic and molecular line opacity in the adopted models. In fact, a NLTE spectral synthesis including an additional background opacity reproduces with a good level of accuracy the equivalent widths, as well as the general shape of the profiles for both the NaI  D and KI lines, in a subsample of early-M dwarfs.

Key words: stars: late-type – stars: atmospheres – line: formation – radiative transfer – line: profiles

* Based on observations collected at the European Southern Observatory (ESO), La Silla, Chile, and at the McDonald Observatory, Mt. Locke, Texas, USA

Send offprint requests to: A. Tripicchio

Correspondence to: tripicchio@na.astro.it

© European Southern Observatory (ESO) 1999

Online publication: April 28, 1999

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