The nature of the light variability of the silicon star HR 7224

Abstract

Context. Although photometric variations of chemically peculiar (CP) stars are frequently used to determine their rotational periods, the detailed mechanism of their light variability remains poorly understood. Aims. We simulate the light variability of the star HR 7224 using the observed surface distribution of silicon and iron. Methods. We used the TLUSTY model atmospheres calculated for the appropriate silicon and iron abundances to obtain the emergent flux and to predict the rotationally modulated light curve of the star. We also obtained additional photometric measurements and employed our own regression procedure to derive a more precise estimate of the light elements. Results. We show that the light variation of the star can be explained as a result of i) the uneven surface distribution of the elements, ii) the flux redistribution from the ultraviolet to the visible part of the spectrum, and iii) rotation of the star. We show that the silicon bound-free transitions and iron bound-bound transitions provide the main contribution to the flux redistribution, although an additional source of opacity is needed. We confirm that numerous iron lines significantly contribute to the well-known depression at 5200 Å and discuss the connection between iron abundance and the value of peculiarity index a. Conclusions. The uneven surface distribution of silicon and iron is able to explain most of the rotationally modulated light variation in the star HR 7224.