K. Schenker

AE Aquarii: how cataclysmic variables descend from supersoft binaries

AE Aquarii (AE Aqr) is a propeller system. It has the shortest spin period among cataclysmic variables (CVs), and this is increasing on a 107 yr time-scale. Its ultraviolet spectrum shows very strong carbon depletion versus nitrogen, and its secondary mass indicates a star far from the zero-age main sequence. We show that these properties strongly suggest that AE Aqr has descended from a supersoft X-ray binary. We calculate the evolution of systems descending through this channel, and show that many of them end as AM CVn systems. The short spin-down time-scale of AE Aqr requires a high birth rate for such systems, implying that a substantial fraction of cataclysmic variables must have formed in this way. A simple estimate suggests that this fraction could be of the order of one-third of current CVs. We emphasize the importance of measurements of the C/N abundance ratio in CVs, particularly via the C iv 1550/N v 1238 ratio, in determining how large the observed fraction is.

The ultraviolet line spectrum of the soft X‐ray transient XTE J1118+480: a CNO‐processed core exposed

We compare ultraviolet (UV) spectra of the recent soft X-ray transients XTE J1118+480 and XTE J1859+226. The emission line strengths in XTE J1118+480 strongly suggest that the accreting material has been CNO processed. We show that this system must have come into contact with a secondary star of about 1.5 M, and an orbital period 15 h, very close to the bifurcation value at which the nuclear and angular momentum loss time-scales are similar. Subsequent evolution to the current period of 4.1 h was driven by angular momentum loss. In passing through a period of 7.75 h the secondary star would have shown essentially normal surface abundances. XTE J1118+480 could thus represent a slightly later evolutionary stage of A0620-00. We briefly discuss the broad Lyα absorption wings in XTE J1118+480.

A New Evolutionary Channel for Type Ia Supernovae

ABSTRACT We show that long–period dwarf novae offer a promising route for making Type Iasupernovae. For typical dwarf nova duty cycles d ∼ 0.1 − 0.01, mass is accreted bythe white dwarf mainly during dwarf nova outbursts at rates allowing steady nuclearburning of most of the accreted matter. Mass gains up to ∼ 0.4M ⊙ are possible in thisway. Although these are too small to allow a 0.7M ⊙ WD to reach the Chandrasekharmass, they are sufficient if the WD grew to >∼1M ⊙ in a previous episode of thermal–timescale mass transfer, i.e. for those long–period dwarf novae which descend fromsupersoft binaries. A further advantage of this picture is that the supernova alwaysoccurs in a binary of small secondary/primary mass ratio, with the secondary havingverylittle remaining hydrogen.Both features greatlyreduce the possibility of hydrogencontamination of the supernova ejecta.Key words: accretion, accretion discs – binaries: general – X–rays: binaries – stars:dwarf novae – supernovae: general – galaxies: stellar content – cosmology: distancescale

The minimum orbital period in thermal time-scale mass transfer

We show that the usual picture of supersoft X-ray binary evolution as driven by conservative thermal time-scale mass transfer cannot explain the short orbital periods of RX J0537.7–7034 (3.5 h) and 1E 0035.4–7230 (4.1 h). Non-conservative evolution may produce such periods, but requires very significant mass loss, and is highly constrained.

Nova-induced Mass Transfer Variations

We investigate variations of the mass transfer rate in cataclysmic variables (CVs) that are induced by nova outbursts. The ejection of nova shells leads to a spread of transfer rates in systems with similar orbital period. The effect is maximal if the specific angular momentum in the shell is the same as the specific orbital angular momentum of the white dwarf. We show analytically that in this case the nova-induced widening of the mass transfer rate distribution can be significant if the system, in the absence of nova outbursts, is close to mass transfer instability (i.e., within a factor of ~1.5 of the critical mass ratio). Hence, the effect is negligible below the period gap and for systems with high-mass white dwarfs. At orbital periods between about 3 and 6 hr the width of the mass transfer rate distribution exceeds an order of magnitude if the mass accreted on the white dwarf prior to the runaway is larger than a few 10-4 M☉. At a given orbital period in this range, systems with the highest transfer rate should on average have the largest ratio of donor to white dwarf mass. We show results of population synthesis models that confirm and augment the analytic results.

The short period supersoft source in M31

We show that the recently discovered short period supersoft source in M31 is probably a progenitor of a magnetic cataclysmic variable (CV). The white dwarf spins asynchronously because of the current high accretion rate. However its fieldstrength is typical of an AM Herculis system, which is what it will ultimately become. We discuss the relevance of this system to CV evolution, and its relation to some particular CVs with special characteristics.

The population of black widow pulsars

We consider the population of black widow pulsars (BWPs). The large majority of these are members of globular clusters. For minimum companion masses 0.1 M⊙ are systems relaxing to equilibrium after a relatively rece nt capture event. We point out that all binary millisecond pulsars (MSPs) with orbital periods P < 10 hr are BWPs (our line of sight allows us to see the eclipses in 10 out of 16 cases). This implies that recycled MSPs emit either in a wide fan beam or a pencil beam close to the spin plane. Simple evolutionary ideas favour a fan beam.

AE Aquarii: how CVs descend from supersoft binaries

AE Aquarii (AE Aqr) is a propeller system. It has the shortest spin period among cataclysmic variables (CVs), and this is increasing on a 107 yr time-scale. Its ultraviolet spectrum shows very strong carbon depletion versus nitrogen, and its secondary mass indicates a star far from the zero-age main sequence. We show that these properties strongly suggest that AE Aqr has descended from a supersoft X-ray binary. We calculate the evolution of systems descending through this channel, and show that many of them end as AM CVn systems. The short spin-down time-scale of AE Aqr requires a high birth rate for such systems, implying that a substantial fraction of cataclysmic variables must have formed in this way. A simple estimate suggests that this fraction could be of the order of one-third of current CVs. We emphasize the importance of measurements of the C/N abundance ratio in CVs, particularly via the C iv 1550/N v 1238 ratio, in determining how large the observed fraction is.

Thermal-timescale mass transfer and magnetic CVs

We investigate the spin evolution of the unusual magnetic CV AE Aqr. As a prototype for a potentially large population of CVs subject to a thermally unstable phase of mass transfer, understanding its future is crucial. We present a new definition of the magnetospheric radius in terms of the white dwarfs spin period, and use this along with numerical simulations to follow the spin evolution of AE Aqr. We also present preliminary SPH results suggesting the existence of a stable propeller state. These results highlight the complexity of mCVs and may provide am improved understanding of the evolution of all types of CVs.

AE aquarii: How cataclysmic variables descend from supersoft binaries

AE Aquarii (AE Aqr) is a propeller system. It has the shortest spin period among cataclysmic variables (CVs), and this is increasing on a 107 yr time-scale. Its ultraviolet spectrum shows very strong carbon depletion versus nitrogen, and its secondary mass indicates a star far from the zero-age main sequence. We show that these properties strongly suggest that AE Aqr has descended from a supersoft X-ray binary. We calculate the evolution of systems descending through this channel, and show that many of them end as AM CVn systems. The short spin-down time-scale of AE Aqr requires a high birth rate for such systems, implying that a substantial fraction of cataclysmic variables must have formed in this way. A simple estimate suggests that this fraction could be of the order of one-third of current CVs. We emphasize the importance of measurements of the C/N abundance ratio in CVs, particularly via the C iv 1550/N v 1238 ratio, in determining how large the observed fraction is.