Phase-resolved Hubble Space Telescope/STIS Spectroscopy of the Exposed White Dwarf in the High-Field Polar AR Ursae Majoris

Abstract

Phase-resolved Hubble Space Telescope (HST)/Space Telescope Imaging Spectrograph (STIS) ultraviolet spectroscopy of the high-field polar AR UMa confirms that the white dwarf photospheric Lyα Zeeman features are formed in a magnetic field of ~200 MG. In addition to the Lyα π and σ⁺ components, we detect the forbidden hydrogen 1s₀-->2s₀ transition, which becomes ``enabled'' in the presence of both strong magnetic and electric fields. Overall, the combined ultraviolet and optical low-state spectrum is similar to that of the single white dwarf PG 1031+234, in that the optical continuum has a steeper slope than the ultraviolet continuum and that the depth of the Lyα Zeeman lines reaches only 30%-50% of the continuum level. Our attempt in fitting the low-state data with single-temperature magnetic white dwarf models remains rather unsatisfactory, indicating either a shortcoming in the present models or a new physical process acting in AR UMa. As a result, our estimate of the white dwarf temperature remains somewhat uncertain, T_wd=20,000+/-5000 K. We detect a broad emission bump centered at ~1445 Å and present throughout the entire binary orbit, and a second bump near ~1650 Å, which appears only near the inferior conjunction of the secondary star. These are suggestive of low harmonic cyclotron emission produced by low-level (M~10^-13 M_solar yr^-1) accretion onto both magnetic poles. However, there is no evidence in the power spectrum of light variations for accretion in gas blobs. The derived field strengths are B~240 MG and B>~160 MG for the northern and the southern poles, respectively, broadly consistent with the field derived from the Zeeman lines. The observed Lyα emission line shows a strong phase dependence with maximum flux and redshift near orbital phase φ~0.3, strongly indicating an origin on the trailing hemisphere of the secondary star. An additional Lyα absorption feature with similar phasing as the Lyα emission, but a ~700 km s^-1 blueshift could tentatively be ascribed to absorption of white dwarf emission in a moderately fast wind. Finally, the high signal-to-noise STIS data provide important information on the intergalactic absorption toward AR UMa. We derive a column density of neutral hydrogen of N_H=(1.1+/-1.0)×10¹⁸ cm^-2, the lowest of any known polar, making AR UMa an excellent candidate for further EUV observations. Based on observations made with the NASA/ESA Hubble Space Telescope, obtained at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy Inc., under NASA contract NAS 5-26555.