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Register a new userNew Ultraviolet Observations of AM CVn
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Richard A. Wade
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We have obtained observations of the ultraviolet spectrum of AM CVn, an ultra-short-period helium cataclysmic variable, using the Space Telescope Imaging Spectrograph (STIS) aboard the Hubble Space Telescope (HST). We obtained data in time-tag mode during two consecutive orbits of HST, covering 1600-3150 and 1140-1710 Angstrom, respectively. The mean spectrum is approximately flat in f-nu. The absorption profiles of the strong lines of N V, Si IV, C IV, He II, and N IV are blue-shifted and in some cases asymmetric, evidencing a wind that is partly occulted by the accretion disk. There is weak red-shifted emission from N V and He II. The profiles of these lines vary mildly with time. The light curve shows a decline of ~20% over the span of the observations. There is also flickering and a 27 s (or 54 s) dwarf nova oscillation, revealed in a power-spectrum analysis. The amplitude of this oscillation is larger at shorter wavelengths. We assemble and illustrate the spectral energy distribution (s.e.d.) of AM CVn from the ultraviolet to the near-infrared. Modeling the accretion phenomenon in this binary system can in principle lead to a robust estimate of the mass accretion rate on to the central white dwarf, which is of great interest in characterizing the evolutionary history of the binary system. Inferences about the mass accretion rate depend strongly on the local radiative properties of the disk, as we illustrate. Uncertainty in the distance of AM CVn and other parameters of the binary system presently limit the ability to confidently infer the mass accretion rate.
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We present the results of XMM-Newton observations of four AM CVn systems -- AM CVn, CR Boo, HP Lib and GP Com. Their light curves show very different characteristics. The X-ray light curves show no coherent pulsations, suggesting the accreting white dwarfs have relatively low magnetic field strengths. Their spectra were best modelled using a multi-temperature emission model and a strong UV component. We find that CR Boo and HP Lib have X-ray spectra with abundances consistent with relatively low temperature CNO processed material, while AM CVn and GP Com show an enhancement of nitrogen. A large fraction of the accretion luminosity is emitted in the UV. We determine accretion luminosities of ~1.6x10^{33} ergs/s and 1.7x10^{31} ergs/s for AM CVn and GP Com respectively. Comparing the implied mass transfer rates with that derived using model fits to optical and UV spectra, we find evidence that in the case of AM CVn, we do not detect a significant proportion of the accretion energy. This missing component could be lost in the form of a wind.
AM CVn systems are ultra-compact, helium-rich, accreting binaries with degenerate or semi-degenerate donors. We report the discovery of five new eclipsing AM CVn systems with orbital periods of 61.5, 55.5, 53.3, 37.4, and 35.4 minutes. These systems were discovered by searching for deep eclipses in the Zwicky Transient Facility (ZTF) lightcurves of white dwarfs selected using Gaia parallaxes. We obtained phase-resolved spectroscopy to confirm that all systems are AM CVn binaries, and we obtained high-speed photometry to confirm the eclipse and characterize the systems. The spectra of two long-period systems (61.5 and 53.3 minutes) show many emission and absorption lines, indicating the presence of N, O, Na, Mg, Si, and Ca, and also the K and Zn, elements which have never been detected in AM CVn systems before. By modelling the high-speed photometry, we measured the mass and radius of the donor star, potentially constraining the evolutionary channel that formed these AM CVn systems. We determined that the average mass of the accreting white dwarf is $approx0.8$$mathrm{M_{odot}}$, and that the white dwarfs in long-period systems are hotter than predicted by recently updated theoretical models. The donors have a high entropy and are a factor of $approx$ 2 more massive compared to zero-entropy donors at the same orbital period. The large donor radius is most consistent with He-star progenitors, although the observed spectral features seem to contradict this. The discovery of 5 new eclipsing AM~CVn systems is consistent with the known observed AM CVn space density and estimated ZTF recovery efficiency. Based on this estimate, we expect to find another 1--4 eclipsing AM CVn systems as ZTF continues to obtain data. This will further increase our understanding of the population, but will require high precision data to better characterize these 5 systems and any new discoveries.
We consider initial stage of the evolution of AM CVn type stars with white dwarf donors, which is accompanied by thermonuclear explosions in the layer of accreted He. It is shown that the accretion never results in detonation of He and accretors in AM CVn stars finish their evolution as massive WDs. We found, for the first time, that in the outbursts the synthesis of n-rich isotopes, initiated by the ${mathrm{^{22}{Ne}(alpha,n)^{25}Mg}}$ reaction becomes possible.
We present the results of XMM-Newton observations of two AM CVn systems - V396 Hya and SDSS J1240-01. Both systems are detected in X-rays and in the UV: neither shows coherent variability in their light curves. We compare the rms variability of the X-ray and UV power spectra of these sources with other AM CVn systems. Apart from ES Cet, AM CVn sources are not strongly variable in X-rays, while in the UV the degree of variability is related to the systems apparent brightness. The X-ray spectra of V396 Hya and SDSS J1240-01 show highly non-solar abundances, requiring enhanced nitrogen to obtain good fits. We compare the UV and X-ray luminosities for 7 AM CVn systems using recent distances. We find that the X-ray luminosity is not strongly dependent upon orbital period. However, the UV luminosity is highly correlated with orbital period with the UV luminosity decreasing with increasing orbital period. We expect that this is due to the accretion disk making an increasingly strong contribution to the UV emission at shorter periods. The implied luminosities are in remarkably good agreement with predictions.
AM CVn binaries are hydrogen deficient compact binaries with an orbital period in the 5-65 min range and are predicted to be strong sources of persistent gravitational wave radiation. Using Gaia Data Release 2, we present the parallaxes and proper motions of 41 out of the 56 known systems. Compared to the parallax determined using the HST Fine Guidance Sensor we find that the archetype star, AM CVn, is significantly closer than previously thought. This resolves the high luminosity and mass accretion rate which models had difficulty in explaining. Using Pan-STARRS1 data we determine the absolute magnitude of the AM CVn stars. There is some evidence that donor stars have a higher mass and radius than expected for white dwarfs or that the donors are not white dwarfs. Using the distances to the known AM CVn stars we find strong evidence that a large population of AM CVn stars have still to be discovered. As this value sets the background to the gravitational wave signal of LISA, this is of wide interest. We determine the mass transfer rate for 15 AM CVn stars and find that the majority have a rate significantly greater than expected from standard models. This is further evidence that the donor star has a greater size than expected.
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