The Inclination Dependence of the Mean Spectral Line Shapes of RS CVn Stars: Further Evidence for Polar Spots
The mean spectral line shape of the Ca I 6439 Å line profile is examined in four RS CVn-type stars having different stellar inclinations. It is found that the low-inclination stars (HR 1099 and EI Eri) have a calcium line with a more pronounced flat-bottomed shape compared to the two high-inclination stars (AR Lac and V471 Tau) whose line shapes are close to those of rotationally broadened profiles. If the flat-bottomed line shape seen in many RS CVn stars arises purely from the atmosphere of the star and is not due to the surface temperature distribution, then one should expect to see flat-bottomed profiles in high-inclination stars as well, and this is not the case. Four models are considered as an explanation for the line shapes seen in the low-inclination RS CVn stars: (1) gravity darkening, (2) a bright band at the equator of the star, (3) surface differential rotation, and (4) a cool polar spot that is viewed with different projected areas. It is shown that gravity darkening cannot produce a flat-bottomed profile even when taking into consideration the temperature sensitivity of the Ca I 6439 Å line. A bright equatorial band is capable of producing a flat-bottomed profile, but the change in line shape with stellar inclination is not as dramatic as is observed with actual stars. Likewise, differential rotation with the poles of the star rotating faster than the equatorial regions can also produce a box-shaped line profile, but an unreasonable amount of differential rotation is required (the poles must rotate with an angular velocity at least twice that of the equator). The polar spot model is the only one of the four considered that is capable of producing a flat-bottomed profile whose shape changes significantly with stellar inclination. This model is also consistent with the presence of cool spots on RS CVn stars that have been established via photometry and the presence of persistent TiO molecular features in the spectra of these stars and provides further support to the presence of polar spots on RS CVn stars.