Observational constraints on the evolution of cataclysmic variable stars

Abstract

I provide observational constraints on the size and period distribution of the Galactic cataclysmic variable (CV) population, and derive the implications that these constraints have for the theory of CV evolution. The results are based on quantitative modelling applied to three observational CV samples, two of which are newly constructed here. Large differences between the size and other properties of the known sample of CVs and the predictions of the theory of binary star evolution have long been recognized. However, because all existing observational CV samples suffer from strong selection effects, observational biases, must be taken into account before it is possible to tell whether there are real inconsistencies. In order to address this, I implement a Monte Carlo method to model selection effects in observed CV samples, and make a new measurement of the space density of CVs. I illustrate the effects of the biases that are introduced by several of the most common observational strategies for identifying CVs. Also, by simulating the selection criteria of the Palomar-Green (PG) Survey, I show that selection effects cannot reconcile the relative sizes of the long- and short-period CV populations predicted by standard CV evolution theory with the observed sample. k The selection criteria used to define most CV samples (including the PG sample) discriminate heavily against the discovery of intrinsically faint, short-period systems. The situation can be improved by selecting systems for the presence of emission lines; I have therefore constructed a homogeneous new sample of 17 CVs, selected on the basis of Ha emission from the AAO/UKST SuperCOSMOS Ha Survey (SHS). I present observations of the CVs discovered in this search, and use the sample to constrain the properties of the intrinsic CV population. I show that even very generous allowance for selection effects is not sufficient to reconcile the ratio of short- to long-period CVs predicted by standard CV evolution theory with the observed sample, confirming the result based on the PG survey. The most likely implication is that short-period systems evolve faster than predicted by the disrupted magnetic braking model. This would require that an angular momentum loss mechanism besides gravitational radiation acts on CVs with orbital periods below the period gap. To bring the model into agreement with observations, the rate of angular momentum loss below the period gap must be increased by a factor of at least 3, unless the model also overestimates the angular momentum loss rate of long-period CVs. In order to constrain the size of the Galactic CV population, I construct a small, but purely X-ray flux-limited sample of CVs, using the ROSAT North Ecliptic Pole (NEP) survey. The sample includes only 4 systems, 2 of which are new discoveries. Orbital periods are measured for both these systems from time-resolved spectroscopy, and the distances of all the CVs in this sample are estimated. The space density of the CV population represented by the sample is l.lij)!? x 10~5pc~3. I also place upper limits on the space density of any population of CVs too faint to be included in the NEP survey—if the overall space density of CVs is as high as 2 x 10~4 pc~3 (as has been predicted theoretically), the vast majority of CVs must have X-ray luminosities below ~ 2 x 1029ergs~1.

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