IUE and Optical Observations of AM Herculis in Its Low State

Abstract

Ten spectra of AM Herculis were obtained with the SWP instrument of the lUE over 2.5 orbital periods during a faint state in 1992 June. Significant periodic variability was seen in the UV flux and spectra. On the same day, UBVRI photometry was obtained at the 1.25 m optical telescope of the Crimean Astrophysical Observatory. A periodic brightening was seen in the UV. The UV spectra and optical photometry at the same orbital phase were fitted to synthetic spectra derived from the stellar atmosphere models of Hubeny (1988) to determine the temperature and log (g). When AM Her was faint, a single cool (T = 20,000 K) model fit the data, while at phases when AM Her was bright, a second component with T = 35000±5,000 K covering ≤8% of the white dwarf surface significantly improved the fit. The value of log (g) determined from the fits is 8.0 + 0.2. A significant excess is seen in the R-band data, probably due to emission from the secondary. The next day we obtained 16 spectra with the Russian 6 m telescope. We discuss the possibility that the white dwarf cools with a timescale similar to that of dwarf novae, but moderate and variable emission-line intensities suggest highly variable accretion at a moderate level. The observed Zeeman features lead to a magnetic field strength of ∼13 MG.