Analysis of the Si IV Ultraviolet Spectra of U Sagittae

Abstract

Archival IUE spectra of U Sge have been used to study hot (~10⁵ K) circumstellar gas in this system. The observed spectra contain several UV resonance lines, of which the Si IV lines (λλ1394 and 1405) are the strongest. These lines are observed in absorption, so we must remove the photospheric absorption line contribution to the spectra in order to see the emission produced by the circumstellar gas. We have developed a robust method for creating such difference spectra from IUE data. In the observed spectra we see a variation in the line profiles and line centers with an orbital phase suggesting the presence of circumstellar gas. The residual emission seen in the difference spectra is strongest between phases φ=0.3 and φ=0.7, with a strength of up to 0.2 of the continuum flux. This is confirmed by the presence of an emission region visible in the Doppler tomogram of the λ1394 line. This is consistent with the location of an Hα ``absorption zone'' seen in tomograms of U Sge and U CrB, and with theoretical predictions of a high temperature accretion region.