Hysteresis of atmospheric parameters of 12 RR Lyrae stars based on multichannel simultaneous Strömgren photometry^*

Argelander Institut für Astronomie, Universität Bonn, Auf dem Hügel 71, 53121 Bonn, Germany e-mail: deboer@astro.uni-bonn.de

Received: 8 January 2010
Accepted: 17 May 2010

Abstract

Aims. RR Lyrae stars have been observed to improve the insight into processes at work in their atmospheres.

Methods. Simultaneous Strömgren-photometry allows to obtain a rapid sequence of measurements in which photometric indices are unaffected by non-optimum observing conditions. The indices y, b–y, and c₁ are used with an established calibration to derive T_eff and to determine the gravity, log g_BJ from the Balmer jump, throughout the pulsation cycle. By employing the equations for stellar structure, additional parameters can be derived.

Results. Strömgren photometry and its calibration in terms of T_eff and log g  can be used to determine the run of R and the atmosphere pulsation velocity. We find that the Balmer-line strengths are correlated with T_eff and that the strength of the Ca ii K line correlates well with the radius of the star and thus the pulsation-dependent density of the atmosphere. The density in the stellar atmosphere fluctuates as indicated by the changes in the gravity log g_BJ, derived from c₁, between 2.3 and 4.5 dex. Also the Strömgren metal index, m₁, fluctuates. We find a disagreement between log g(T,L,M), the gravity calculated from T_eff, L, and the mass M, and the gravity log g_BJ. This can be used to reassess the mass and the absolute magnitude of an individual star. The curves derived for the pulsational velocity V_pul differ from curves obtained from spectra needed to apply the Baade-Wesselink method; we think these differences are due to phase dependent differences in the optical depth levels sampled in continuum photometry and in spectroscopy. We find an atmospheric oscillation in these fundamental mode RR Lyrae stars of periodicity P/7.

Conclusions. Carefully conducted Strömgren-photometry allows to derive a large number of parameters for RR Lyrae stars. It provides a means of deriving masses and absolute magnitudes. When comparing photometry results with spectroscopic analyses it appears that optical depth effects affect all interpretations.