The helium weak silicon star HR 7224^*

II. Doppler Imaging analysis

¹ Thüringer Landessternwarte Tautenburg, 07778 Tautenburg, Germany e-mail: lehm@tls-tautenburg.de
² Tavrian National University, Dep. Astronomy, Simferopol, Ukraine e-mail: andr309@yandex.ru; vad@simfi.net
³ Departamento de Física UFSC, CP 476, 88040-900 Florianópolis, SC, Brazil
⁴ Institut für Astronomie, Unversität Wien, Türkenschanzstr. 17, 1180 Wien, Austria
⁵ Astrophysical Research Center for the Structure and Evolution of the Cosmos, Sejong University, Seoul 143-747, Korea
⁶ Astronomical Observatory, Odessa National University, Shevchenko Park, Odessa 65014, Ukraine e-mail: davidm@sejong.ac.kr

Received: 23 April 2007
Accepted: 7 June 2007

Abstract

Aims.We investigate the abundance distributions of silicon and iron on the surface of HR 7224 and check for the presence of magnetic fields. We try further to find an explanation for the observed Balmer line variations, alternative to that by surface temperature gradients given in a previous article.

Methods.Based on time series of more than 570 high-resolution spectra we investigate the abundance distribution of silicon and iron using the Doppler Imaging technique applying two independent program codes and using single and multiple components atmosphere models. We further calculate the combined effect of abundance changes of Si, Fe, and He on the H_β line profile and compare it with the observations. The mean longitudinal magnetic field strength of HR 7224 is measured spectropolarimetrically.

Results.The Doppler Imaging analysis of HR 7224 shows the presence of large silicon and iron spots on the star's surface, large gradients of the abundances of these elements, and hints to enhanced line strengths near the visible pole of the star. The explanation of the observed Balmer line variations by inhomogeneous abundance distributions alone requires unrealistic large gradients of the helium abundance. Magnetic field measurements gave no definite results, a field strength of the order of +400 G is likely.

Conclusions.We can exclude the hypothesis that the hydrogen line profile variations of HR 7224 are caused by inhomogeneous abundance distributions of Fe, Si, and He alone or by pressure gradients like in the case of Ap stars. Still the observed variations can be described only by the combined effect of abundance and temperature gradients. In this case, the derived Doppler imaging maps will reflect also the temperature gradients and the surface will be cooler in regions of enhanced line strength like at the visible pole. At present state, the physical origin of possible surface temperature gradients remains unknown.