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Abstract
It is shown that the secondary components of cataclysmic binaries with orbital periods of less than ∿10 hours are indistinguishable from ordinary low-mass main-sequence stars and that, therefore, they are essentially unevolved. On the other hand, it is shown that, depending on the mass ratio of the progenitor system, the secondary of a cataclysmic binary could be significantly evolved. The fact that nevertheless most of the observed secondaries are essentially unevolved can be accounted for by assuming that the probability distribution for the initial mass ratio is not strongly peaked towards unity mass ratio.
Recent developments and ideas concerning the secular evolution of cataclysmic binaries are briefly summarized. In particular the following topics are addressed: basic observational properties, stability against mass transfer, mechanisms for driving the mass transfer, the reaction of the secondary to mass loss, the period gap model with its implications, the minimum orbital period, the secular evolution of the white dwarf primary, the mass spectrum of the white dwarfs in cataclysmic binaries, precataclysmic binaries, and finally the evolution of systems containing an evolved secondary.
Cataclysmic variables (CV's) is the common name of a subgroup of eruptive variables consisting of the classical novae, the dwarf novae, the recurrent novae and of the nova-like objects. Since Kraft's pioneering investigation about twenty years aga (Kraft, R. P.: 1973, Adv. Astron. Astrophys. 2, 43) we know that probably all of the CV's are close binaries. However among the roughly 500 CV's known at present, only for about 50 objects has the bi nary nature been established by observations. Hereafter these objects will be referred to as cataclysmic binaries (CB's). From the histogram of their orbital periods, shown in Fig. 1, it is seen that CB's have extremely short orbital periods, typically only a few hours. Moreover the histogram shows a remarkable gap of orbital periods in the range between about 2 and 3 hours. This gap has been found to be statistically highly significant. Apparently CB's are divided into two subgroups, i. e. into the ultrashort- period CB's (hereafter USPCB's) with orbital periods P = 3h
Radial velocities for DQ Herculis have been measured from peaks of
double emission lines on 27 spectrograms obtained in 1978. These
velocities are analyzed within a three-body approximation for the
rotation of the outer parts of the disk. The results are: K1 = 145 km/s,
mu = M2/(M1 + M2) = 0.43, r(disk) = 0.30; a crude analysis of the
eclipse gives r(disk) = 0.29 and i = 77 deg. The mass of the primary,
white dwarf companion is M1 = 0.5 = + or - 0.2 solar mass. The secondary
component, with M2 = 0.35 + or - 0.1 solar mass, is slightly oversized
and probably underluminous as compared to main sequence stars.
Near-infrared CCD spectra of six cataclysmic binaries have been obtained
in an attempt to detect their red dwarf components. Similar observations
of M dwarfs yield a classification scheme, based on TiO bands, which can
be used to describe the red dwarfs in cataclysmic binaries. Observations
of AM Her while in its low state revealed a red dwarf of type M4(+) V
and a distance of 80 pc. The red dwarf in DQ Her was detected in an
observation centered on mid-eclipse. The red star in DQ Her has a
spectral type of M3(+) V, a mass = 0.32 solar mass, and a radius = 0.45
solar radius. The red star in DQ Her does not lie on the main sequence;
although its temperature is correct for its mass, the radius is 25% too
large. A spectroscopic distance estimate of 400 pc to the DQ Her binary
system was obtained; in four additional systems, the red dwarf defied
detection. Lower limits to their distances are given.
Spectrophotometry of U Geminorum from 3200 to 10,900 A shows an upturn
in flux for wavelengths longward of 6000 A. The energy distribution can
be interpreted as the sum of fluxes from an M4.5 or M5 star and a source
giving a flat continuum. The presence of an M star is confirmed by a
higher-resolution spectrum in the red and near-infrared and by the work
of Stauffer, Spinrad, and Thorstensen (1978). The M companion is less
dense than a main-sequence star of the same color index. The absolute
photometry allows a distance modulus of 4.4 + or 0.4 mag to be derived
for U Geminorum. The absolute visual magnitude of the quiescent system
at orbital phase 0.08 is 10.3 + or - 0.8.
Photoelastic UBV photometry and spectroscopy of the southern dwarf nova
BV Centauri is analyzed to determine the orbital period and basic
parameters of the system. The light curves obtained indicate a period of
about 0.61 days, with a hump shape typical of cataclysmic variables.
Spectroscopic data suggest that the system consists of a hot primary
exhibiting Balmer series and He I 4471-A emission, and a G5-G8 IV-V
secondary. Masses of 1.4 solar masses are obtained from radial velocity
data for each of the components, along with orbital elements and the
absolute dimensions of the system. The light curves and B-V color
variations are interpreted in terms of the inhomogeneous atmosphere of
the red star with a slightly enhanced effective temperature around the
inner Lagrangian point. The U-B color curve is explained by the
dominance of free-free transitions in the hot spot, with the disk
characterized by a strong U-B excess due to the Balmer continuum in
emission.
H-gamma, He II 4686 A, and H-beta emission lines were observed in DQ Her
through an eclipse (phases 0.80-0.15). A total of 14 spectra with
exposure times of 300 seconds were used to investigate phenomena in the
eclispe of the emitting region by the red companion. The classical
rotational disturbance of the He II 4686 A line is observed; it changes
its velocity by over 600 km/s in 25 minutes during the eclipse. The
eclipse duration is very long (0.11 of a cycle) and indicates a mass
ratio near unity.
U Geminorum is one of the brighter dwarf novae and an eclipsing binary
system. It is pointed out that, in connection with these
characteristics, U Gem has proven crucial to an understanding of the
structure of cataclysmic variable stars. The considered investigation is
concerned with the determination of the radial velocity semiamplitude of
U Gem B, the secondary star in the U Gem system. The determination is
based on observations of the near-infrared Na I absorption doublet.
Spectroscopic conjunction occurs 0.026 + or - 0.007 periods before
mid-eclipse of the bright spot. The result that U Gem B seems to be
larger than a zero age main-sequence star of the same mass casts some
doubt on those studies of cataclysmic variables, where the component
masses have been determined under the assumption that the secondary star
obeys some particular mass-radius relation.
1982 a, preprint MPA 18, to appear in the proceedings of the workshop on “High Energy Astrophysics
- H Ritter