AM CVn stars

I review our observational and theoretical knowledge of AM CVn stars, focusing on recent developments. These include newly discovered systems, the possibility that two recently discovered extremely short period objects are AM CVn stars and an update on X-ray, UV an optical studies. Theoretical advances include the study of the details of both the donor and accretor, and the physics of the helium accretion discs. I review our (limited) knowledge of the formation of AM CVn stars and the apparent success of the now more than 25 year old suggestion that in these objects the mass transfer is driven by gravitational wave radiation losses. The exciting prospect of directly detecting these gravitational waves and the possibilities this brings conclude this contribution.Comment: Error in table 1 corrected: orbital period GP Com is 2794 sec. Review to appear in "The astrophysics of cataclysmic variables and related objects", ASP Conf. Ser.,eds. J.M. Hameury and J.P. Lasot

Constraining parameters of white-dwarf binaries using gravitational-wave and electromagnetic observations

The space-based gravitational wave (GW) detector, \emph{evolved Laser Interferometer Space Antenna} (eLISA) is expected to observe millions of compact Galactic binaries that populate our Milky Way. GW measurements obtained from the eLISA detector are in many cases complimentary to possible electro-magnetic (EM) data. In our previous papers, we have shown that the EM data can significantly enhance our knowledge of the astrophysically relevant GW parameters of the Galactic binaries, such as the amplitude and inclination. This is possible due to the presence of some strong correlations between GW parameters that are measurable by both EM and GW observations, for example the inclination and sky position. In this paper, we quantify the constraints in the physical parameters of the white-dwarf binaries, i.e. the individual masses, chirp mass and the distance to the source that can be obtained by combining the full set of EM measurements such as the inclination, radial velocities, distances and/or individual masses with the GW measurements. We find the following $2-\sigma$ fractional uncertainties in the parameters of interest. The EM observations of distance constrains the the chirp mass to $\sim 15-25$, whereas EM data of a single-lined spectroscopic binary constrains the secondary mass and the distance with factors of 2 to $\sim 40$. The single-line spectroscopic data complemented with distance constrains the secondary mass to $\sim 25-30$. Finally EM data on double-lined spectroscopic binary constrains the distance to $\sim 30$. All of these constraints depend on the inclination and the signal strength of the binary systems. We also find that the EM information on distance and/or the radial velocity are the most useful in improving the estimate of the secondary mass,inclination and/or distance.Comment: 12 pages, 15 figures, accepted for publication by Ap

Constraining the formation of black-holes in short-period Black-Hole Low-Mass X-ray Binaries

The formation of stellar mass black holes is still very uncertain. Two main uncertainties are the amount of mass ejected in the supernova event (if any) and the magnitude of the natal kick the black hole receives at birth (if any). Repetto et al. (2012), studying the position of Galactic X-ray binaries containing black holes, found evidence for black holes receiving high natal kicks at birth. In this Paper we extend that study, taking into account the previous binary evolution of the sources as well. The seven short-period black-hole X-ray binaries that we use, are compact binaries consisting of a low-mass star orbiting a black hole in a period less than $1$ day. We trace their binary evolution backwards in time, from the current observed state of mass-transfer, to the moment the black hole was formed, and we add the extra information on the kinematics of the binaries. We find that several systems could be explained by no natal kick, just mass ejection, while for two systems (and possibly more) a high kick is required. So unless the latter have an alternative formation, such as within a globular cluster, we conclude that at least some black holes get high kicks. This challenges the standard picture that black hole kicks would be scaled down from neutron star kicks. Furthermore, we find that five systems could have formed with a non-zero natal kick but zero mass ejected (i.e. no supernova) at formation, as predicted by neutrino-driven natal kicks.Comment: 15 pages, 14 figures, accepted for publication in MNRA

Measuring tides and binary parameters from gravitational wave data and eclipsing timings of detached white dwarf binaries

The discovery of the most compact detached white dwarf (WD) binary SDSS J065133.33+284423.3 has been discussed in terms of probing the tidal effects in white dwarfs. This system is also a verification source for the space-based gravitational wave (GW) detector, evolved Laser Interferometer Space Antenna (eLISA) which will observe short-period compact Galactic binaries with $P_{orb}\lesssim 5$ hrs. We address the prospects of doing tidal studies using eLISA binaries by showing the fractional uncertainties in the orbital decay rate and the rate of that decay, $\dot{f}, \ddot{f}$ expected from both the GW and EM data for some of the high-$f$ binaries. We find that $\dot{f}$ and $\ddot{f}$ can be measured using GW data only for the most massive WD binaries observed at high-frequencies. Form timing the eclipses for $\sim 10$ years, we find that $\dot{f}$ can be known to $\sim 0.1$ for J0651. We find that from GW data alone, measuring the effects of tides in binaries is (almost) impossible. We also investigate the improvement in the knowledge of the binary parameters by combining GW amplitude and inclination with EM data with and without $\dot{f}$. In our previous work we found that EM data on distance constrained 2-$\sigma$ uncertainty in chirp mass to $15-25$ whereas adding $\dot{f}$ reduces it to $0.11$. EM data on $\dot{f}$ also constrains 2-$\sigma$ uncertainty in distance to $35$. EM data on primary mass constrains the secondary mass $m_2$ to factors of 2 to $\sim40$ whereas adding $\dot{f}$ reduces this to $25$. And finally using single-line spectroscopic constrains 2-$\sigma$ uncertainties in both the $m_2, d$ to factors of 2 to $\sim 40$. Adding EM data on $\dot{f}$ reduces these 2-$\sigma$ uncertainties to $\leq 25$ and $6$ respectively. Thus we find that EM measurements of $\dot{f}$ and radial velocity will be valuable in constraining binary parameters.Comment: 10 pages, 3 figures, Accepted for publication in Ap

Observational clues to the progenitors of Type-Ia supernovae

Type-Ia supernovae (SNe Ia) are important distance indicators, element factories, cosmic-ray accelerators, kinetic-energy sources in galaxy evolution, and endpoints of stellar binary evolution. It has long been clear that a SN Ia must be the runaway thermonuclear explosion of a degenerate carbon-oxygen stellar core, most likely a white dwarf (WD). However, the specific progenitor systems of SNe Ia, and the processes that lead to their ignition, have not been identified. Two broad classes of progenitor binary systems have long been considered: single-degenerate (SD), in which a WD gains mass from a non-degenerate star; and double-degenerate (DD), involving the merger of two WDs. New theoretical work has enriched these possibilities with some interesting updates and variants. We review the significant recent observational progress in addressing the progenitor problem. We consider clues that have emerged from the observed properties of the various proposed progenitor populations, from studies of their sites, pre- and post-explosion, from analysis of the explosions themselves, and from the measurement of event rates. The recent nearby and well-studied event, SN 2011fe, has been particularly revealing. The observational results are not yet conclusive, and sometimes prone to competing theoretical interpretations. Nevertheless, it appears that DD progenitors, long considered the underdog option, could be behind some, if not all, SNe Ia. We point to some directions that may lead to future progress.Comment: to appear in Annual Reviews of Astronomy and Astrophysics, 2014. For near-final published version see http://www.annualreviews.org/doi/abs/10.1146/annurev-astro-082812-14103

The influence of the distribution of cosmic star formation at different metallicities on the properties of merging double compact objects

Binaries that merge within the local Universe originate from progenitor systems that formed at different times and in various environments. The efficiency of formation of double compact objects is highly sensitive to metallicity of the star formation. Therefore, to confront the theoretical predictions with observational limits resulting from gravitational waves observations one has to account for the formation and evolution of progenitor stars in the chemically evolving Universe. In particular, this requires knowledge of the distribution of cosmic star formation rate at different metallicities and times, probed by redshift (SFR(Z,z)). We investigate the effect of the assumed SFR(Z,z) on the properties of merging double compact objects, in particular on their merger rate densities. Using a set of binary evolution models from Chruslinska et al. (2018) we demonstrate that the reported tension between the merger rates of different types of double compact objects and current observational limits in some cases can be resolved if a SFR(Z,z) closer to that expected based on observations of local star-forming galaxies is used, without the need for changing the assumptions about the evolution of progenitor stars of different masses. This highlights the importance of finding tighter constraints on SFR(Z,z) and understanding the associated uncertainties.Comment: 6 pages, 4 figures, resubmitted to MNRAS after minor revisio