The Thermal State of the Accreting White Dwarf in AM Canum Venaticorum Binaries

We calculate the heating and cooling of the accreting white dwarf (WD) in theultracompact AM Canum Venaticorum (AM CVn) binaries and show that the WD cancontribute significantly to their optical and ultraviolet emission. We estimatethe WD's effective temperature, Teff, using the optical continuum for a numberof observed binaries, and show that it agrees well with our theoreticalcalculations. Driven by gravitational radiation losses, the time averagedaccretion rate, , decreases monotonically with increasing Porb, coveringsix orders of magnitude. If the short period (Porb<10 min) systems accrete at arate consistent with gravitational radiation via direct impact, we predicttheir unpulsed optical/UV light to be that of the Teff>50,000$ K accreting WD.At longer Porb we calculate the Teff and absolute visual magnitude, M_V, thatthe accreting WD will have during low accretion states, and find that the WDnaturally crosses the pulsational instability strip. Discovery and study ofpulsations could allow for the measurement of the accumulated helium mass onthe accreting WD, as well as its rotation rate. Accretion heats the WD core,but for Porb>40 minutes, the WD's Teff is set by its cooling as plummets. For the two long period AM CVn binaries with measured parallaxes, GPCom and CE 315, we show that the optical broadband colors and intensity arethat expected from a pure helium atmosphere WD. This confirms that the WDbrightness sets the minimum light in wide AM CVn binaries, allowing formeaningful constraints on their population density from deep optical searches,both in the field and in Globular Clusters.Comment: 8 pages, 2 figures (emulateapj), accepted to ApJ; minor clarifications, added an object to tabl