Abstract
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.
Highlights
AM Canum Venaticorum was found in a survey of faint, blue objects by Humason & Zwicky (1947)
As in the aforementioned systems, the phase of the low-velocity component is consistent with the expected phase of the accreting white dwarf, and its relatively low velocity amplitude is difficult to reconcile with any emission site in the accretion disc
Marsh 1999; Morales-Rueda et al 2003). Based on these three properties, we interpret the feature as the perfect analogue of the central spike in GP Com, V396 Hya, and SDSS J124058.03−015919.2
Summary
AM Canum Venaticorum was found in a survey of faint, blue objects by Humason & Zwicky (1947). The star was shown to be a possible ultra-compact white dwarf binary by Smak (1967), who discovered photometric variations on an 18minute period; quickly thereafter, Paczynski (1967) noted that it would be a prime example of a binary whose evolution is expected to be governed by gravitational-wave radiation, and which could serve as an excellent test for the existence of gravitational radiation. The interpretation as an interacting binary analogous to the Cataclysmic Variables was first proposed by Warner & Robinson (1972) upon their discovery of rapid flickering in the light-curve. The discussion as to the true orbital period of the system was put to. Our current data-set fully samples the proposed 13.37-h accretion disc precession period (Patterson, Halpern & Shambrook 1993; Skillman et al 1999) for the first time, allowing for a characterisation of the spectroscopic appearance of the system as a function of orbital period, precession period, and superhump (beat) period
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