Curriculum Vitae

Selected Publications

August 02 - MNRAS 337, 1105-1112

K. Schenker, A.R. King, U. Kolb, G.A. Wynn, Z. Zhang
AE Aquarii: how CVs descend from supersoft binaries

AE Aquarii is a propeller system. It has the shortest spin period among cataclysmic variables, and this is increasing on a 10^7 yr timescale. Its UV spectrum shows very strong carbon depletion vs 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 spindown timescale of AE Aqr requires a high birthrate 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 order one-third of current CVs. We emphasize the importance of measurements of the C/N abundance ratio in CVs, particularly via the CIV1550/NV1238 ratio, in determining how large the observed fraction is.

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April 02 - ASP Conf. Ser. 261, 242-251

K. Schenker, A.R. King
A New Evolutionary Picture for CVs and LMXBs II. The Impact of Thermal-Timescale Mass Transfer

Depending on the outcome of pre-CV formation, mass transfer may set in under thermally unstable conditions in a significant number of systems. Full computations have shown that such an early phase of thermal-timescale mass transfer usually leads to ordinary looking CVs, but these do also show some unusual properties (e.g. chemical anomalies in later stages).

Rather than investigating the common envelope evolution leading to pre-CVs, we study the properties of multiple evolutionary tracks starting with a phase of thermal-timescale mass transfer. Apart from fitting unusual CVs (like AE Aqr), global properties of the CV population as a whole give indications that this is indeed the channel where many CVs come from.

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April 02 - ASP Conf. Ser. 261, 233-241

A.R. King, K. Schenker
A New Evolutionary Picture for CVs and LMXBs

We consider an alternative to the standard picture of CV and LMXB evolution, namely the idea that most CVs (and by extension LMXBs) may not yet have had time to evolve to their theoretical minimum orbital periods. We call this the Binary Age Postulate (BAP). The observed short-period cutoff in the CV histogram emerges naturally as the shortest period yet reached in the age of the Galaxy, while the post-minimum-period space density problem is removed. The idea has similar desirable consequences for LMXBs. In both cases systems with nuclear-evolved secondary stars form a prominent part of the short-period distributions. Properties such as the existence and nature of ultrashort-period systems, and the spread in mass transfer rates at a given orbital period, are naturally reproduced.

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January 02 - MNRAS 329, L43-L46

A.R. King, J.P. Osborne, K. Schenker
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 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.

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August 01 - ApJ 563, 958-970

U. Kolb, S. Rappaport, K. Schenker, S. Howell
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 Msol. 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.

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March 01 - MNRAS 321, 327-332

A.R. King, K. Schenker, U. Kolb, M.B. Davies
The Minimum Orbital Period in Thermal-Timescale Mass Transfer

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

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June 00 - ASP Conf. Ser. 229, 321-332

K. Schenker
Thermal-Timescale Mass Transfer and CV Evolution

A detailed study has begun to investigate the role of thermal-timescale mass transfer in compact binaries. The properties of systems with initially thermally unstable mass ratios are discussed both analytically and using full computations of binary evolution, in which the mass transfer rate is allowed to adjust itself accordingly. It is shown that in such a model supersoft X-ray sources in the period range ~6..20 hr appear as natural progenitors of CVs bearing the signature of unusual chemical composition. As a particular well constrained example, the case of AE Aqr is discussed.

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September 99 - PhD Thesis

K. Schenker
Radial Pulsations in Envelopes of Classical Novae

The aim of this work is to investigate the pulsation hypothesis as the physical origin of the oscillatory phase in the declining part of the light curve of some classical novae. During their outburst, novae evolve through a region in the Hertzsprung-Russell diagram where their envelopes are expected to be strongly unstable towards pulsations. Static models of nova envelopes, consisting of an isothermal CO-core plus an envelope with typical nova-like C- and O-enriched chemical composition, can be constructed on small portions of the corresponding evolutionary paths. These regions belong either to low white dwarf masses and/or to the final stage of hydrogen shell burning at very high effective temperatures. Such envelopes show strong instabilities in linear, radial, non-adiabatic stability analyses.

To verify and extend these results into the non-linear long-term behaviour, selected linear models have been studied in detail with the help of full radiation hydrodynamical simulations. The growth of radial instabilities can be followed starting from a static initial configuration similar to those the linear analysis was performed on, until a regular sequence of shock waves is passing through most of the envelope below the photosphere. Furthermore a significant expansion of the background model can be observed during the late non-linear phase. In combination of both the linear and non-linear results and with new analytical concepts the interpretation of strongly non-adiabatic radial pulsations is given a new direction.

A direct observational confirmation of the claimed instability is suggested in the X-ray band. Other consequences during earlier phases of the nova outburst are subject to debatable assumptions. The rapid growth of the instabilities is providing a challenge for linear studies alone, inasmuch as exact observable quantities can only be derived using more complicated methods.

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June 98 - MNRAS 297, 633-647

K. Schenker, U. Kolb, H. Ritter
Properties of discontinuous and nova-amplified mass transfer in CVs

We investigate the effects of discontinuous mass loss in recurrent outburst events on the long-term evolution of cataclysmic variables (CVs). Similarly we consider the effects of frictional angular momentum loss (FAML), i.e. interaction of the expanding nova envelope with the secondary. The Bondi-Hoyle accretion model is used to parameterize FAML in terms of the expansion velocity v_exp of the nova envelope at the location of the secondary; we find that small v_exp causes strong FAML.

Numerical calculations of CV evolution over a wide range of parameters demonstrate the equivalence of a discontinuous sequence of nova cycles and the corresponding mean evolution (replacing envelope ejection by a continuous wind), even close to mass transfer instability. A formal stability analysis of discontinuous mass transfer confirms this, independent of details of the FAML model.

FAML is a consequential angular momentum loss which amplifies the mass transfer rate driven by systemic angular momentum losses such as magnetic braking. We show that for a given v_exp and white dwarf mass the amplification increases with secondary mass and is significant only close to the largest secondary mass consistent with mass transfer stability. The amplification factor is independent of the envelope mass ejected during the outburst, whereas the mass transfer amplitude induced by individual nova outbursts is proportional to it.

In sequences calculated with nova model parameters taken from Prialnik & Kovetz (1995) FAML amplification is negligible, but the outburst amplitude in systems below the period gap with a white dwarf mass ~0.6 M_sun is larger than a factor of 10. The mass transfer rate in such systems is smaller than 10^-11 M_sun/yr for ~0.5 Myr (~10 % of the nova cycle) after the outburst. This offers an explanation for intrinsically unusually faint CVs below the period gap.

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March 95 - Diploma Thesis (german)

K. Schenker
Novaausbrüche und die Langzeitentwicklung von kataklysmischen Doppelsternen

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