THE INCLINATION DEPENDENCE OF THE MEAN SPECTRAL-LINE SHAPES OF RS CVNSTARS - FURTHER EVIDENCE FOR POLAR SPOTS

The mean spectral line shape of the Ca I 6439 Angstrom line profile is examined in four RS CVn-type stars having different stellar inclinati ons. It is found that the low-inclination stars (HR 1099 and EI Eri) h ave a calcium line with a more pronounced flat-bottomed shape compared to the two high-inclination stars (AR Lac and V471 Tau) whose line sh apes are close to those of rotationally broadened profiles. If the fla t-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 dist ribution, 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-incli nation RS CVn stars: (1) gravity darkening, (2) a bright band at the e quator 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 whe n taking into consideration the temperature sensitivity of the Ca I 64 39 Angstrom line. A bright equatorial band is capable of producing a h at-bottomed profile, but the change in line shape with stellar inclina tion is not as dramatic as is observed with actual stars. Likewise, di fferential rotation with the poles of the star rotating faster than th e equatorial regions can also produce a box-shaped line profile, but a n unreasonable amount of differential rotation is required (the poles must rotate with an angular velocity at least twice that of the equato r). The polar spot model is the only one of the four considered that i s capable of producing a flat-bottomed profile whose shape changes sig nificantly with stellar inclination. This model is also consistent wit h the presence of cool spots on RS CVn stars that have been establishe d via photometry and the presence of persistent TiO molecular features in the spectra of these stars and provides further support to the pre sence of polar spots on RS CVn stars.