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Issue A&A
Volume 421, Number 1, July I 2004
Page(s) 295 - 303
Section Stellar atmospheres
DOI http://dx.doi.org/10.1051/0004-6361:20034193



A&A 421, 295-303 (2004)
DOI: 10.1051/0004-6361:20034193

An atmospheric model for UZ Librae from mean H $\alpha$-line profiles

M. Zboril1, K. G. Strassmeier1 and E. H. Avrett2

1  Astrophysikalisches Institut Potsdam (AIP), An der Sternwarte 16, 14482, Germany
    e-mail: kstrassmeier@aip.de
2  SAO, Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, MA 02138, USA
    e-mail: avrett@cfa.harvard.edu

(Received 14 August 2003 / Accepted 25 March 2004 )

Abstract
We present the results from fitting a semi-empirical atmospheric model including a chromosphere and a transition region to the mean (seasonal) Balmer H $\alpha$ line profiles of the RS CVn-type K0-giant UZ Librae. As a first step, a static 1D spherical model was applied to the mean component of the H $\alpha$-emission core and its profile. The main result of the fitting is that the transition region begins at a log mass depth of  -1.8 g cm -2 at a temperature of 7400 K (approximately 3000 K warmer than the photosphere) and then has a steep increase to the peak temperature of  $\approx$10 6 K. A stellar model in plane-parallel mode with "partial-frequency redistribution" option in the line transfer gave roughly the best fit. Subsequently, two-Gaussian fitting of the phase-dependent H $\alpha$-line profiles yields a complex velocity field. The radial velocities, from both the absorption reversal and the main emission component, display rotational and/or orbital modulation. The largest differences between consecutive line profiles occur mostly in the red line wings, suggesting the existence of both an inward-pointed velocity field and sporadic radiation events possibly related to flares. The total H $\alpha$ emission, as derived from the equivalent width, possibly also displays rotational modulation. Indirect evidence is presented for the existence of circumstellar matter in form of an H $\alpha$ absorption shell. Finally, a preliminary elemental abundance analysis suggests sub-solar metallicity of  $\rm [M/H]\approx -0.5$ dex based on ATLAS models, or  -0.7 dex based on PHOENIX models.


Key words: stars: late-type -- stars: activity -- stars: atmospheres -- stars: chromospheres -- stars: individual: UZ Lib

Offprint request: M. Zboril, mzboril@aip.de

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