The dwarf nova SS Cygni: what is wrong?

Since the Fine Guiding Sensor (FGS) on the Hubble Space Telescope (HST) was used to measure the distance to SS Cyg to be $166\pm12$ pc, it became apparent that at this distance the disc instability model fails to explain the absolute magnitude during outburst. It remained, however, an open question whether the model or the distance have to be revised. Recent observations led to a revision of the system parameters of SS Cyg and seem to be consistent with a distance of d\gta 140 pc. We re-discuss the problem taking into account the new binary and stellar parameters measured for SS Cyg. We confront not only the observations with the predictions of the disc instability model but also compare SS Cyg with other dwarf novae and nova-like systems. We assume the disc during outburst to be in a quasi stationary state and use the black-body approximation to estimate the accretion rate during outburst as a function of distance. Using published analysis of the long term light curve we determine the mean mass transfer rate of SS Cyg as a function of distance and compare the result with mass transfer rates derived for other dwarf novae and nova-like systems. At a distance of d\gta 140 pc, both the accretion rate during outburst as well as the mean mass transfer rate of SS Cyg contradict the disc instability model. More important, at such distances we find the mean mass transfer rate of SS Cyg to be higher or comparable to those derived for nova-like systems. Our findings show that a distance to SS Cyg \gta 140 pc contradicts the main concepts developed for accretion discs in cataclysmic variables during the last 30 years. Either our current picture of disc accretion in these systems must be revised or the distance to SS Cyg is $\sim 100$ pcComment: 6 pages, 3 figures, accepted for publication in Astronomy and Astrophysic

Reversing the verdict: Cataclysmic variables could be the dominant progenitors of AM CVn binaries after all

Context. AM CVn binaries are potential progenitors of thermonuclear supernovae and strong sources of persistent gravitational wave radiation. For a long time, it has been believed that these systems cannot descend from cataclysmic variables (CVs), at least not in large numbers, because the initial conditions need to be fine-tuned and, even worse, the resulting surface hydrogen abundance would be high enough to be detected which contradicts a defining feature of AM CVn binaries. Aims. Here we show that both claimed weaknesses of the CV formation channel for AM CVn binaries are model-dependent and rely on poorly constrained assumptions for magnetic braking. Methods. We performed binary evolution simulations with the MESA code for different combinations of post-common-envelope white dwarf and companion masses as well as orbital periods assuming the CARB model for strong magnetic braking. Results. We found that AM CVn binaries with extremely-low surface hydrogen abundances are one natural outcome of CV evolution if the donor star has developed a non-negligible helium core prior to the onset of mass transfer. In this case, after hydrogen envelope exhaustion during CV evolution, the donor becomes degenerate and its surface hydrogen abundance substantially drops and becomes undetectable. Our simulations also show that the CV formation channel is able to explain the observed AM CVn binaries with very low mass and bloated donor stars (Gaia14aae and ZTF J1637+49). Conclusions. CVs with evolved donors are likely the progenitors of at least a fraction of AM CVn binaries.Comment: Accepted for publication in A&

Formation and Evolution of Accreting Compact Objects

Accreting compact objects are crucial to understand several important astrophysical phenomena such as Type Ia supernovae, gravitational waves, or X-ray and $\gamma$-ray bursts. In addition, they are natural laboratories to infer fundamental properties of stars, to investigate high-energy phenomena and accretion processes, to test theories of stellar and binary evolution, to explore interactions between high-density plasma and very strong magnetic fields, to examine the interplay between binary evolution and dynamical interactions (in the case they belong to dense star clusters), and they can even be used as a probe for the assembling process of galaxies over cosmic time-scales. Despite the fundamental importance of accreting compact objects for astrophysics and recent progress with the comprehension of these fascinating objects, we still do not fully understand how they form and evolve. In this chapter, we will review the current theoretical status of our knowledge on these objects, and will discuss standing problems and potential solutions to them.Comment: Invited chapter for the Handbook of X-ray and Gamma-ray Astrophysics (Editors: Cosimo Bambi, Andrea Santangelo; Publisher: Springer Singapore

Cold giant planets evaporated by hot white dwarfs

Atmospheric escape from close-in Neptunes and hot Jupiters around Sun-like stars driven by extreme ultraviolet (EUV) irradiation plays an important role in the evolution of exoplanets and in shaping their ensemble properties. Intermediate and low mass stars are brightest at EUV wavelengths at the very end of their lives, after they have expelled their envelopes and evolved into hot white dwarfs. Yet the effect of the intense EUV irradiation of giant planets orbiting young white dwarfs has not been assessed. We show that the giant planets in the solar system will experience significant hydrodynamic escape caused by the EUV irradiation from the white dwarf left behind by the Sun. A fraction of the evaporated volatiles will be accreted by the solar white dwarf, resulting in detectable photospheric absorption lines. As a large number of the currently known extrasolar giant planets will survive the metamorphosis of their host stars into white dwarfs, observational signatures of accretion from evaporating planetary atmospheres are expected to be common. In fact, one-third of the known hot single white dwarfs show photospheric absorption lines of volatile elements, which we argue are indicative of ongoing accretion from evaporating planets. The fraction of volatile contaminated hot white dwarfs strongly decreases as they cool. We show that accretion from evaporating planetary atmospheres naturally explains this temperature dependence if more than 50% of hot white dwarfs still host giant planets

The cataclysmic variable orbital period gap: More evident than ever

Recently, large and homogeneous samples of cataclysmic variables (CVs) identified by the Sloan Digital Sky Survey (SDSS) were published. In these samples, the famous orbital period gap, which is a dearth of systems in the orbital period range ~2-3 hr and the defining feature of most evolutionary models for CVs, has been claimed not to be clearly present. If true, this finding would completely change our picture of CV evolution. In this Letter we focus on potential differences with respect to the orbital period gap between CVs in which the magnetic field of the white dwarf is strong enough to connect with that of the donor star, so-called polars, and non-polar CVs as the white dwarf magnetic field in polars has been predicted to reduce the strength of angular momentum loss through magnetic braking. We separated the SDSS I-IV sample of CVs into polars and non-polar systems and performed statistical tests to evaluate whether the period distributions are bimodal as predicted by the standard model for CV evolution or not. We confirm the existence of a period gap in the SDSS I-IV sample of non-polar CVs with >98 per cent confidence. The boundaries of the orbital period gap are 147 and 191 minutes, with the lower boundary being different to previously published values (129 min). The orbital period distribution of polars from SDSS I-IV is clearly different and does not show a similar period gap. The SDSS samples as well as previous samples of CVs are consistent with the standard theory of CV evolution. Magnetic braking does indeed seem get disrupted around the fully convective boundary, which causes a detached phase during CV evolution. In polars, the white dwarf magnetic field reduces the strength of magnetic braking and consequently the orbital period distribution of polars does not display an equally profound and extended period gap as non-polars.Comment: Accepted for publication in Astronomy & Astrophysics Letter