The helium weak silicon star HR 7224

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.

The magnetic field and spectral variability of the He-weak star HR 2949

“…This phenomenon has been suggested as the reason for Balmer line LPV in the He-weak Si star HR 7224 (B9p, B d < 0.4 kG Lehmann et al 2007). In this case chemical spots were specifically excluded as the possible source of the LPV.…”

Modelling the light variability of the Ap starε Ursae Majoris

“…First, Krtička et al (2007) succeeded in reproducing the light variability of a hot (T eff = 22 000 K) Bp star HD 37776 based on the DI maps of He and Si derived by Khokhlova et al (2000). Later, Krtička et al (2009) successfully fitted the light curves of a cooler (T eff = 14 500 K) Bp star HR 7224 using Fe and Si maps presented by Lehmann et al (2007). The main result of both studies is a very good, parameter-free theoretical explanation of both the shapes and amplitudes of the observed light curves.…”

“…Slow rotation (and, consequently, weak meridional currents), globally structured magnetic fields, and weak convection allow a build up of the prominent abundance anomalies and inhomogeneities as a result of particle diffusion driven by the radiation field (first proposed by Michaud 1970). These inhomogeneities are frequently seen as abundance spots on stellar surfaces (see, for example, Khokhlova et al 2000;Kochukhov et al 2004;Lehmann et al 2007), and as clouds of chemical elements concentrated at different heights in stellar atmospheres (Ryabchikova et al 2002(Ryabchikova et al , 2008Kochukhov et al 2006;Shulyak et al 2009, and references therein).…”

“…With the appearance of detailed abundance Doppler images (DI) of the stellar surfaces (e.g., Lüftinger et al 2003;Kochukhov et al 2004;Lehmann et al 2007) and advanced computers it became possible to numerically explore the role of uneven distributions of chemical elements as a source of the light variability. First, Krtička et al (2007) succeeded in reproducing the light variability of a hot (T eff = 22 000 K) Bp star HD 37776 based on the DI maps of He and Si derived by Khokhlova et al (2000).…”

The mechanism of the light variability of chemically peculiar stars

“…Silicon is overabundant on the visible disc around the phase φ = 0. Right: The variation of iron abundances on the visible disc of HR 7224 for different rotational phases after Lehmann et al (2007). Iron is overabundant on the visible disc around the phase φ = 0.…”