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Astron. Astrophys. 320, 136-146 (1997)

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Modelling Population II cataclysmic variables

R. Stehle 1, 2, U. Kolb 1, 3 and H. Ritter 1

1 Max-Planck-Institut für Astrophysik, Karl-Schwarzschild-Str. 1, D-85740 Garching, Germany
2 Astronomical Institute 'Anton Pannekoek', Kruislaan 403, 1098 SJ Amsterdam, The Netherlands
3 University of Leicester, Astronomy Group, Leicester LE1 7RH, UK

Received 29 January 1996 / Accepted 12 September 1996

Abstract

We study the long-term evolution of cataclysmic variables (CVs) as a function of the companion star metallicity. It is shown that CVs with secondaries of low metallicity (Z= [FORMULA]) which belong to population II (PopII CVs) pass through a detached phase with a smaller period width, a shorter minimum period, and display a slightly higher mass transfer rate, resulting in shorter evolutionary timescales compared to CVs with a solar chemical composition of the secondary. A population synthesis model for PopII CVs with a star formation period lasting for [FORMULA] yr after the onset of Galaxy formation shows that most PopII CVs have already evolved beyond the minimum period. As these systems are faint, they are difficult to observe. Extracting a magnitude-limited sample from the computed intrinsic models and taking into account the sample's incompleteness towards fainter apparent magnitudes we derive a period distribution both for PopII CVs and the more common population I CVs. From a differential comparison of the models we calculate the probability that a CV at a given vector distance [FORMULA] from the Sun is member of the population II. Based on our results, we find that a reasonable observable sample of PopII CVs can only be found for [FORMULA] pc.

Key words: novae, cataclysmic variables – stars: evolution – stars: Population II – binaries: close

Send offprint requests to: R. Stehle (rms@mpa-garching.mpg.de)

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© European Southern Observatory (ESO) 1997

Online publication: July 3, 1998
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