The population of AM CVn stars from the Sloan Digital Sky Survey

The AM Canum Venaticorum stars are rare interacting white dwarf binaries, whose formation and evolution are still poorly known. The Sloan Digital Sky Survey provides, for the first time, a sample of 6 AM CVn stars (out of a total population of 18) that is sufficiently homogeneous that we can start to study the population in some detail. We use the Sloan sample to `calibrate' theoretical population synthesis models for the space density of AM CVn stars. We consider optimistic and pessimistic models for different theoretical formation channels, which yield predictions for the local space density that are more than two orders of magnitude apart. When calibrated with the observations, all models give a local space density of 1-3x10^{-6} pc^{-3}, which is lower than expected. We discuss the implications for the formation of AM CVn stars, and conclude that at least one of the dominant formation channels (the double-degenerate channel) has to be suppressed relative to the optimistic models. In the framework of the current models this suggests that the mass transfer between white dwarfs usually cannot be stabilized. We furthermore discuss evolutionary effects that have so far not been considered in population synthesis models, but which could be of influence for the observed population. We finish by remarking that, with our lower space density, the expected number of Galactic AM CVn stars resolvable by gravitational-wave detectors like LISA should be lowered from current estimates, to about 1,000 for a mission duration of one year.Comment: Accepted to MNRA

Simulation of the White Dwarf -- White Dwarf galactic background in the LISA data

LISA (Laser Interferometer Space Antenna) is a proposed space mission, which will use coherent laser beams exchanged between three remote spacecraft to detect and study low-frequency cosmic gravitational radiation. In the low-part of its frequency band, the LISA strain sensitivity will be dominated by the incoherent superposition of hundreds of millions of gravitational wave signals radiated by inspiraling white-dwarf binaries present in our own galaxy. In order to estimate the magnitude of the LISA response to this background, we have simulated a synthesized population that recently appeared in the literature. We find the amplitude of the galactic white-dwarf binary background in the LISA data to be modulated in time, reaching a minimum equal to about twice that of the LISA noise for a period of about two months around the time when the Sun-LISA direction is roughly oriented towards the Autumn equinox. Since the galactic white-dwarfs background will be observed by LISA not as a stationary but rather as a cyclostationary random process with a period of one year, we summarize the theory of cyclostationary random processes and present the corresponding generalized spectral method needed to characterize such process. We find that, by measuring the generalized spectral components of the white-dwarf background, LISA will be able to infer properties of the distribution of the white-dwarfs binary systems present in our Galaxy.Comment: 14 pages and 6 figures. Submitted to Classical and Quantum Gravity (Proceedings of GWDAW9

Time-resolved X-Shooter spectra and RXTE light curves of the ultra-compact X-ray binary candidate 4U 0614+091

In this paper we present X-Shooter time resolved spectroscopy and RXTE PCA light curves of the ultra-compact X-ray binary candidate 4U 0614+091. The X-Shooter data are compared to the GMOS data analyzed previously by Nelemans et al. (2004). We confirm the presence of C III and O II emission features at ~ 4650 {\AA} and ~ 5000 {\AA}. The emission lines do not show evident Doppler shifts that could be attributed to the motion of the donor star/hot spot around the center of mass of the binary. We note a weak periodic signal in the red-wing/blue-wing flux ratio of the emission feature at ~ 4650 {\AA}. The signal occurs at P = 30.23 +/- 0.03 min in the X-Shooter and at P = 30.468 +/- 0.006 min in the GMOS spectra when the source was in the low/hard state. Due to aliasing effects the period in the GMOS and X-Shooter data could well be the same. We deem it likely that the orbital period is thus close to 30 min, however, as several photometric periods have been reported for this source in the literature already, further confirmation of the 30 min period is warranted. We compare the surface area of the donor star and the disc of 4U 0614+091 with the surface area of the donor star and the disc in typical hydrogen-rich low-mass X-ray binaries and the class of AM Canum Venaticorum stars and argue that the optical emission in 4U 0614+091 is likely dominated by the disc emission. Additionally, we search for periodic signals in all the publicly available RXTE PCA light curves of 4U 0614+091 which could be associated with the orbital period of this source. A modulation at the orbital period with an amplitude of ~ 10% such as those that have been found in other ultra-compact X-ray binaries (4U 0513-40, 4U 1820-30) is not present in 4U 0614+091.Comment: Accepted for publication in MNRAS, 11 pages, 7 figure

Kinematics of the ultracompact helium accretor AM canum venaticorum

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

Photometric Variability in the Faint Sky Variability Survey

The Faint Sky Variability Survey (FSVS) is aimed at finding photometric and/or astrometric variable objects between 16th and 24th mag on time-scales between tens of minutes and years with photometric precisions ranging from 3 millimag to 0.2 mag. An area of 23 deg$^2$, located at mid and high Galactic latitudes, was covered using the Wide Field Camera (WFC) on the 2.5-m Isaac Newton Telescope (INT) on La Palma. Here we present some preliminary results on the variability of sources in the FSVS.Comment: 4 pages, 3 figures, to appear in 14th European Workshop on White Dwarfs, ASP Conference Series, eds. D. Koester, S. Moehle

On the orbital periods of the AM CVn stars HP Librae and V803 Centauri

We analyse high-time-resolution spectroscopy of the AM CVn stars HP Librae and V803 Centauri, taken with the New Technology Telescope (NTT) and the Very Large Telescope (VLT) of the European Southern Observatory, Chile. We present evidence that the literature value for V803 Cen's orbital period is incorrect, based on an observed `S-wave' in the binary's spectrogram. We measure a spectroscopic period P=1596.4+/-1.2s of the S-wave feature, which is significantly shorter than the 1611-second periods found in previous photometric studies. We conclude that the latter period likely represents a `superhump'. If one assumes that our S-wave period is the orbital period, V803 Cen's mass ratio can be expected to be much less extreme than previously thought, at q~0.07 rather than q~0.016. This relaxes the constraints on the masses of the components considerably: the donor star does then not need to be fully degenerate, and the mass of the accreting white dwarf no longer has to be very close to the Chandrasekhar limit. For HP Lib, we similarly measure a spectroscopic period P=1102.8+/-0.2s. This supports the identification of HP Lib's photometric periods found in the literature, and the constraints upon the masses derived from them.Comment: Accepted for publication in MNRA

Phase-resolved spectroscopy of the helium dwarf nova 'SN 2003aw' in quiescence

High time resolution spectroscopic observations of the ultracompact helium dwarf nova 'SN 2003aw' in its quiescent state at V similar to 20.5 reveal its orbital period at 2027.8 +/- 0.5 s or 33.80 min. Together with the photometric 'superhump' period of 2041.5 +/- 0.5 s, this implies a mass ratio q approximate to 0.036. We compare both the average and time-resolved spectra of 'SN 2003aw' and Sloan Digital Sky Survey (SDSS) J124058.03-015919.2. Both show a DB white dwarf spectrum plus an optically thin, helium-dominated accretion disc. 'SN 2003aw' distinguishes itself from the SDSS source by its strong calcium H & K emission lines, suggesting higher abundances of heavy metals than the SDSS source. The silicon and iron emission lines observed in the SDSS source are about twice as strong in 'SN 2003aw'. The peculiar 'double bright spot' accretion disc feature seen in the SDSS source is also present in time-resolved spectra of 'SN 2003aw', albeit much weaker

UVES and X-Shooter spectroscopy of the emission line AM CVn systems GP Com and V396 Hya

We present time-resolved spectroscopy of the AM CVn-type binaries GP Com and V396 Hya obtained with VLT/X-Shooter and VLT/UVES. We fully resolve the narrow central components of the dominant helium lines and determine radial velocity semi-amplitudes of $K_{\rm spike} = 11.7\pm0.3$ km s$^{-1}$ for GP Com and $K_{\rm spike} = 5.8\pm0.3$ km s$^{-1}$ for V396 Hya. The mean velocities of the narrow central components show variations from line to line. Compared to calculated line profiles that include Stark broadening we are able to explain the displacements, and the appearance of forbidden helium lines, by additional Stark broadening of emission in a helium plasma with an electron density $n_e\simeq 5\times 10^{15}$ cm$^{-3}$. More than $30$ nitrogen and more than $10$ neon lines emission lines were detected in both systems. Additionally, $20$ nitrogen absorption lines are only seen in GP Com. The radial velocity variations of these lines show the same phase and velocity amplitude as the central helium emission components. The small semi-amplitude of the central helium emission component, the consistency of phase and amplitude with the absorption components in GP Com as well as the measured Stark effect shows that the central helium emission component, the so-called central-spike, is consistent with an origin on the accreting white dwarf. We use the dynamics of the bright spot and the central spike to constrain the binary parameters for both systems and find a donor mass of $9.6$ - $42.8$ M$_{\rm Jupiter}$ for GP Com and $6.1$ - $30.5$ M$_{\rm Jupiter}$ for V396 Hya. We find an upper limit for the rotational velocity of the accretor of $v_{\rm rot}<46$ km s$^{-1}$ for GP Com and $v_{\rm rot}<59$ km s$^{-1}$ for V396 Hya which excludes a fast rotating accretor in both systems.Comment: Accepted for publication in MNRAS, 15 pages, 14 figures, 5 table

The formation of black hole low-mass X-ray binaries: through case B or case C mass transfer?

The formation of low-mass X-ray binaries containing a rather massive (M >~ 7 \msun) black hole is problematic because in most recent stellar evolutionary calculations the immediate progenitors of these black holes (Wolf-Rayet stars) lose so much mass via their stellar wind that their final masses are well below the observed black hole masses. We discuss the recently proposed solution that these binaries are formed through case C mass transfer (i.e. mass transfer after core helium burning is completed), avoiding a long Wolf-Rayet phase and thus significant mass loss. We show that only some of the currently available models for the evolution of massive stars allow this formation channel. We also investigate the effect of the downward revised Wolf-Rayet mass-loss rate as is suggested by observations, and conclude that in that case Wolf-Rayet stars end their lives with significantly higher masses than previously found and may be able to form a black holes.Comment: Accepted for publication by A&

XMM-Newton observations of AM CVn binaries : V396 Hya and SDSS J1240–01

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