A spectroscopic study of the cataclysmic binary star SU Ursae Majoris

Abstract

We report spectroscopic observations of SU UMa, the prototype of a class of short-period cataclysmic bin- aries. From Ha radial velocity observations obtained at McGraw-Hill Observatory, we find the orbital period to be 0.076351 ± 4.3 x 10“5 days (=110 minutes). We also present an analysis of high-quality spectrograms from the Hale 5 m telescope and double spectrograph. We find the velocity amplitude of the emission line source to be 58 ± 4 km s“1, leading to a mass function -1.5 x 10“3 M0. This implies a moderately low inclination when interpreted using conventional ideas about cataclysmic variables. From multicomponent line profile fits, we find that asymmetries in the He i A4471 emission line probably arise from blending with Mg n A4481, which has never before been detected in emission in a dwarf nova at minimum light. In the Ha and Hß lines, the line cores are found to lead the wings slightly in orbital phase, contrary to the predictions of simple “hot spot” models of the line core. Also, we find that the velocity widths of the Balmer series increase toward higher members of the series, which suggests that pressure broadening may contribute nonnegligibly to the emission line widths. Some support for this idea comes from the absence of double peaks in the line profiles, despite more than adequate spectral resolution to see them at the expected separation.