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

The AM Canum Venaticorum (AM CVn) 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 six AM CVn stars (out of a total population of 18) that are 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  ρ₀= 1–3 × 10⁻⁶ 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 the Laser Interferometer Space Antenna ( LISA ) should be lowered from current estimates, to about 1000 for a mission duration of 1 yr.     

An XMM–Newton observation of the neutron star X-ray transient 2S 1803−245 in quiescence

We observed the neutron star X-ray transient 2S 1803−245 in quiescence with the X-ray satellite XMM–Newton , but did not detect it. An analysis of the X-ray bursts observed during the 1998 outburst of 2S 1803−245 gives an upper limit to the distance of ≤7.3 kpc, leading to an upper limit on the quiescent 0.5–10 keV X-ray luminosity of  ≤2.8 × 10³² erg s⁻¹  (3σ). Since the expected orbital period of 2S 1803−245 is several hours, this limit is not much higher than those observed for the quiescent black hole transients with similar orbital periods.     

Shock breakout in Type Ibc supernovae and application to GRB 060218/SN 2006aj

Recently, a soft blackbody component was observed in the early X-ray afterglow of GRB 060218, which was interpreted as shock breakout from the thick wind of the progenitor Wolf–Rayet (WR) star of the underlying Type Ic supernova 2006aj. In this paper, we present a simple model for computing the characteristic quantities (including energy, temperature and time duration) for the transient event from the shock breakout in Type Ibc supernovae produced by the core-collapse of WR stars surrounded by dense winds. In contrast to the case of a star without a strong wind, the shock breakout occurs in the wind region rather than inside the star, caused by the large optical depth in the wind. We find that, for the case of a WR star with a dense wind, the total energy of the radiation generated by the supernova shock breakout is larger than that in the case of the same star without a wind by a factor of >10. The temperature can be either hotter or colder, depending on the wind parameters. The time duration is larger caused by the increase in the effective radius of the star due to the presence of a thick wind. Then, we apply the model to GRB 060218/SN 2006aj. We show that, to explain both the temperature and the total energy of the blackbody component observed in GRB 060218 by the shock breakout, the progenitor WR star has to have an unrealistically large core radius (the radius at optical depth of 20), larger than 100 R_⊙. In spite of this disappointing result, our model is expected to have important applications to the observations on Type Ibc supernovae in which the detection of shock breakout will provide important clues to the progenitors of Type Ibc supernovae.