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

J. Krticka, Z. Mikulasek, G. W. Henry, J. Zverko, J. Ziznovsky, J. Skalicky, P. Zverina

Department of Theoretical Physics and Astrophysics, Masaryk University, Brno, Czech Republic
Center of Excellence in Information Systems, Tennessee State University, Nashville, Tennessee, USA
Astronomical Institute, Slovak Academy of Sciences, Tatranska Lomnica, Slovak Republic

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. We simulate the light
variability of the star HR 7224 using the observed surface distribution of
silicon and iron. 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. 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 A and discuss the
connection between iron abundance and the value of peculiarity index a. The
uneven surface distribution of silicon and iron is able to explain most of the
rotationally modulated light variation in the star HR 7224.

Reference: A&A, in press
Status: Manuscript has been accepted