Detection of photospheric Zeeman features and cyclotron emission lines in V834 CEN in a low state

Abstract

Simultaneous spectroscopic and polarimetric observations of the AM Herculis system V834 Cen during a low state of accretion are reported. The polarization data show a reversal in the sign of the circular polarization when lambda is greater than about 5000 A, which lasts for about 0.1 in phase and which is not present during a high state. The reversal can be interpreted as the result of the dominance of photospheric radiation at these phases. The photometric observations show, for the first time, nearly sinusoidal UBV light curves with minima shifted by 0.5 in phase from previous high-state observations. This behavior is explained by assuming that the radiation in the blue bands is dominated by blackbody emission from the heated photosphere near the accretion region. The spectroscopic observations show broad, resolvable cyclotron harmonics and strong photospheric Zeeman features. Theoretical models suggest the presence of a dipole of polar field strength of about 31 MG offset by about -0.1 Rwd from the center of the white dwarf along the dipole axis. The cyclotron emission region is located about 10 deg from the pole and has B of about 23 MG.