Kinematics of the ultracompact helium accretor AM Canum Venaticorum

• Published in: Monthly notices of the Royal Astronomical Society Vol. 371; no. 3; pp. 1231 - 1242
• Main Authors: Roelofs, G. H. A., Groot, P. J., Nelemans, G., Marsh, T. R., Steeghs, D.
• Format: Journal Article
• Language: English
• Published: Oxford, UK Blackwell Publishing Ltd 21.09.2006; Blackwell Science; Oxford University Press
• Subjects: accretion; accretion discs; accretion, accretion discs; Astronomical research; Astronomy; binaries: close; cataclysmic variables; Double stars; Earth, ocean, space; Exact sciences and technology; Helium; Kinematics; novae; novae, cataclysmic variables; Spectrum analysis; stars: individual: AM CVn; binaries: close; stars: individual: AM CVn; accretion, accretion discs; novae, cataclysmic variables; Spectroscopy; White dwarf stars; Eccentricity; Gravitational waves; Helium stars; Detectability; Accretion disks; Phase velocity; Kinematics; Mass ratio; Tomography; Models; Resonance; Emission line
• ISSN: 0035-8711; 1365-2966
• DOI: 10.1111/j.1365-2966.2006.10718.x

We report on the results from a five-night campaign of high-speed spectroscopy of the 17-min binary AM Canum Venaticorum (AM CVn), obtained with the 4.2-m William Herschel Telescope on La Palma. We detect a kinematic feature that appears to be entirely analogous to the ‘central spike’ known from the long-period, emission-line AM CVn stars GP Com, V396 Hya and SDSS J124058.03−015919.2, which has been attributed to the accreting white dwarf. Assuming that the feature indeed represents the projected velocity amplitude and phase of the accreting white dwarf, we derive a mass ratio q= 0.18 ± 0.01 for AM CVn. This is significantly higher than the value found in previous, less direct measurements. We discuss the implications for AM CVn's evolutionary history and show that a helium star progenitor scenario is strongly favoured. We further discuss the implications for the interpretation of AM CVn's superhump behaviour, and for the detectability of its gravitational-wave signal with the Laser Interferometer Space Antenna (LISA). In addition, we demonstrate a method for measuring the circularity or eccentricity of AM CVn's accretion disc, using stroboscopic Doppler tomography. We test the predictions of an eccentric, precessing disc that are based on AM CVn's observed superhump behaviour. We limit the effective eccentricity in the outermost part of the disc, where the resonances that drive the eccentricity are thought to occur, to e= 0.04 ± 0.01, which is smaller than previous models indicated.