Phase-resolved HST/STIS spectroscopy of the exposed white dwarf in the high-field polar AR UMa

Abstract

Phase-resolved HST/STIS ultraviolet spectroscopy of the high-field polar AR UMa confirms that the WD photospheric Ly alpha Zeeman features are formed in a magnetic field of ~200 MG. In addition to the Ly alpha pi and sigma+ components, we detect the forbidden hydrogen 1s0->2s0 transition, which becomes ``enabled'' in the presence of both strong magnetic and electric fields. Our attempt in fitting the overall optical+UV low state spectrum with single temperature magnetic WD 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 WD temperature remains somewhat uncertain, Twd=20000+-5000K. We detect a broad emission bump centered at ~1445A and present throughout the entire binary orbit, and a second bump near ~1650A, which appears only near the inferior conjunction of the secondary star. These are suggestive of low harmonic cyclotron emission produced by low-level (M-dot~1e-13 Msun/yr) accretion onto both magnetic poles. However, there is no evidence in the power spectrum of light variations for accretion in gas blobs. The observed Ly alpha emission line shows a strong phase dependence with maximum flux and redshift near orbital phase phi~0.3, strongly indicating an origin on the trailing hemisphere of the secondary star. An additional Ly alpha absorption feature with similar phasing as the Ly alpha emission, but a \~700km/s blueshift could tentatively be ascribed to absorption of WD emission in a moderately fast wind. We derive a column density of neutral hydrogen of NH=(1.1+-1.0)1e18 cm**-2, the lowest of any known polar.

arXiv comment26 pages, 10 figures, AAS TeX 5.0, accepted for publication in the Astrophysical Journal

References in corpus (2)

• Phase-resolved far-ultraviolet HST spectroscopy of the peculiar magnetic white dwarf RE J0317-853
• The Cyclotron Fundamental Exposed in the High-Field Magnetic Variable V884 Her

Cited by in corpus (21)

• Magnetic White Dwarfs
• Cataclysmic Variable Primary Effective Temperatures: Constraints on Binary Angular Momentum Loss
• Measuring White Dwarf Accretion Rates via their Effective Temperatures
• Magnetic fields in isolated and interacting white dwarfs
• Analysis of hydrogen-rich magnetic white dwarfs detected in the Sloan Digital Sky Survey
• Far-ultraviolet Spectroscopy of Magnetic Cataclysmic Variables
• New Low Accretion-Rate Magnetic Binary Systems and their Significance for the Evolution of Cataclysmic Variables
• A catalogue of cataclysmic variables from 20 years of the Sloan Digital Sky Survey with new classifications, periods, trends and oddities
• FUSE and HST/STIS far-ultraviolet observations of AM Herculis in an extended low state
• The nature of the close magnetic white dwarf + probable brown dwarf binary SDSS J121209.31+013627.7
• The accreting white dwarfs in BW Scl, BC UMa and SW UMa
• Dwarf novae in the Hamburg Quasar Survey: Rarer than expected
• GALEX and Optical Light Curves of EF Eridanus During a Low State: the Puzzling Source of UV Light
• DAHe white dwarfs from the DESI survey
• A 150MG magnetic white dwarf in the cataclysmic variable RX J1554.2+2721
• GALEX, Optical and IR Light Curves of MQ Dra: UV Excesses at Low Accretion Rates
• Analyzing the Low State of EF Eridani with Hubble Space Telescope Ultraviolet Spectra
• Peeking Between the Pulses: The Far-UV Spectrum of the Previously Unseen White Dwarf in AR Scorpii
• A 110 MG cyclotron harmonic in the optical spectrum of RX J1554.2+2721
• Time-Variable Aluminum Absorption in the Polar AR Ursae Majoris, and an Updated Estimate for the Mass of the White Dwarf
• A half-ring of ionized circumstellar material trapped in the magnetosphere of a white dwarf merger remnant