An atmospheric model for UZ Librae from mean Hα-line profiles

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

Corresponding author: M. Zboril, mzboril@aip.de

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 Hα 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 Hα-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 ≈10⁶ 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 Hα-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 Hα 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 Hα absorption shell. Finally, a preliminary elemental abundance analysis suggests sub-solar metallicity of  dex based on ATLAS models, or -0.7 dex based on PHOENIX models.