Cataclysmic Variable Secondaries Research Articles

An3He-DRIVEN INSTABILITY NEAR THE FULLY CONVECTIVE BOUNDARY

We report on the discovery of an instability in low mass stars just above the threshold ($\sim 0.35 \textrm{M}_{\odot}$) where they are expected to be fully convective on the main sequence. Non-equilibrium He3 burning creates a convective core, which is separated from a deep convective envelope by a small radiative zone. The steady increase in central He3 causes the core to grow until it touches the surface convection zone, which triggers fully convective episodes in what we call the "convective kissing instability". These episodes lower the central abundance and cause the star to return to a state in which is has a separate convective core and envelope. These periodic events eventually cease when the He3 abundance throughout the star is sufficiently high that the star is fully convective, and remains so for the rest of its main sequence lifetime. The episodes correspond to few percent changes in radius and luminosity, over Myr to Gyr timescales. We discuss the physics of the instability, as well as prospects for detecting its signatures in open clusters and wide binaries. Secondary stars in cataclysmic variables (CVs) will pass through this mass range, and this instability could be related to the observed paucity of such systems for periods between two and three hours. We demonstrate that the instability can be generated for CV secondaries with mass-loss rates of interest for such systems, and discuss potential implications.

A SEMI-EMPIRICAL MASS-LOSS RATE IN SHORT-PERIOD CATACLYSMIC VARIABLES

The mass-loss rate of donor stars in cataclysmic variables (CVs) is of paramount importance in the evolution of short-period CVs. Observed donors are oversized in comparison with those of isolated single stars of the same mass, which is thought to be a consequence of the mass loss. Using the empirical mass-radius relation of CVs and the homologous approximation for changes in effective temperature T_2, orbital period P, and luminosity of the donor with the stellar radius, we find the semi-empirical mass-loss rate M2_dot of CVs as a function of P. The derived M2_dot is at ~10^(-9.5)-10^(-10) Msun/yr and depends weakly on P when P > 90 min, while it declines very rapidly towards the minimum period when P < 90 min, emulating the P-T_2 relation. Due to strong deviation from thermal equilibrium caused by the mass loss, the semi-empirical M2_dot is significantly different from, and has a less-pronounced turnaround behavior with P than suggested by previous numerical models. The semi-empirical P-M2_dot relation is consistent with the angular momentum loss due to gravitational wave emission, and strongly suggests that CV secondaries with 0.075 Msun < M_2 < 0.2 Msun are less than 2 Gyrs old. When applied to selected eclipsing CVs, our semi-empirical mass-loss rates are in good agreement with the accretion rates derived from the effective temperatures T_1 of white dwarfs, suggesting that M2_dot can be used to reliably infer T_2 from T_1. Based on the semi-empirical M2_dot, SDSS 1501 and 1433 systems that were previously identified as post-bounce CVs have yet to reach the minimal period.

Barnes-Evans relations for dwarfs with an application to the determination of distances to cataclysmic variables

Context. Barnes-Evans type relations provide an empirical relationship between the surface brightness of stars and their color. They are widely used for measuring the distances to stars of known radii, as the Roche-lobe filling secondaries in cataclysmic variables (CVs).

Detecting Chromospheric Activity on the Secondary Star of Cataclysmic Variables

AbstractChromospheric activity on the secondary stars of cataclysmic variables (CVs) is a key ingredient for angular momentum loss from the system via magnetic braking. This effect is thought to drive the evolution of the system and is invoked to explain a number of observed properties of CV light curves, such as long-term modulations and high/low states. However, obtaining observational support for magnetic activity has proven difficult. We present a new method of studying chromospheric activity on the secondary stars of CVs, using near-IR spectral features. We discuss in particular the magnetic CV AM Herculis, in which satellites to the H-alpha emission line are interpreted as arising from magnetically confined gas streams (prominences). This phenomenon provides a new technique for mapping magnetic structures on CV secondaries, and advances our understanding of the nature of magnetic structures and activity on CV secondaries.

Superhumps in Cataclysmic Binaries. XXV. qcrit, \U0001d6dc(q), and Mass‐Radius

ABSTRACTWe report on successes and failures in searching for positive superhumps in cataclysmic variables, and show the superhumping fraction as a function of orbital period. Basically, all short‐period system do, all long‐period systems do not, and a 50% success rate is found at Porb = 3.1 ± 0.2 hr. We can use this to measure the critical mass ratio for the creation of superhumps. With a mass‐radius relation appropriate for cataclysmic variables, and an assumed mean white‐dwarf mass of 0.75 M⊙, we find a mass ratio qcrit = 0.35 ± 0.02. We also report superhump studies of several stars of independently known mass ratio: OU Vir, XZ Eri, UU Aqr, and KV UMa (=XTE J1118+480). The latter two are of special interest, because they represent the most extreme mass ratios for which accurate superhump measurements have been made. We use these to improve the ε(q) calibration, by which we can infer the elusive q from the easy‐to‐measure ε (the fractional period excess of Psuperhump over Porb). This relation allows mass and radius estimates for the secondary star in any cataclysmic variable (CV) showing superhumps. The consequent mass‐radius law shows an apparent discontinuity in radius near 0.2 M⊙, as predicted by the disrupted magnetic braking model for the 2.1–2.7 hr period gap. This is effectively the “empirical main sequence” for CV secondaries.

Stellar Models of Evolved Secondaries in Cataclysmic Variables

In this paper we study the impact of chemically evolved secondaries on cataclysmic variable (CV) evolution. We find that when evolved secondaries are included, a spread in the secondary mass-orbital period plane comparable to that seen in the data is produced for either the saturated prescription for magnetic braking or the unsaturated model commonly used for CVs. We argue that in order to explain this spread, a considerable fraction of all CVs should have evolved stars as secondaries. The evolved stars become fully convective at lower orbital periods. Therefore, even if there were an abrupt decrease in magnetic braking for fully convective stars (contrary to open cluster data), it would not be expected to produce a sharp break in the period distribution for CVs. We also explore recent proposed revisions to the angular momentum loss rate for single stars and find that only modest increases over the saturated prescription are consistent with the overall observed spin-down pattern. We compare predictions of our models with diagnostics of the mass accretion rate in white dwarfs and find results intermediate between the saturated and the older braking prescription. Taken together, these suggest that the angular momentum loss rate may be higher in CV secondaries than in single stars of the same rotation period but still significantly lower than in the traditional model. Alternative explanations for the CV period gap are discussed.

Understanding CV Secondaries and the Energetics of Dwarf Novae Outbursts Via Parallaxes

We combine HST and ground-based parallaxes for cataclysmic variables (CVs) to examine their outburst energetics, as well as the nature of their secondary stars. Harrison et al. (2004) have recently published parallaxes for WZ Sge, YZ Cnc, and RU Peg, and have reanalyzed the HST parallaxes for U Gem, SS Aur, and SS Cyg. Combined with existing HST parallaxes for RW Tri (MeArthur et al. 1999), TV Col (MeArthur et al. 2001), EX Hya and V1223 Sgr (Beuermann et al. 2004a,b), ten high-quality parallaxes are available to constrain the various types of outbursts of CVs. In addition, Thorstensen (2003) has published ground-based parallaxes for fourteen CV systems.

A search for brown dwarf like secondaries in cataclysmic variables

We present VTL/ISAAC infrared spectroscopy of a sample of short-orbital-period cataclysmic variables that are candidates for harbouring substellar companions. We have detected the K i and Na i absorption lines of the companion star in VY Aqr. The overall spectral distribution in this system is best fitted with an M9.5 type dwarf spectrum, implying a distance of 100 ± 10 pc. VY Aqr seems to fall far from the theoretical distribution of secondary star temperatures around the orbital period minimum. Fitting of the IR spectral energy distribution (SED) was performed by comparing the observed spectrum with late-type templates. The application of such a spectral fitting procedure suggests that the continuum shape in the 1.1–2.5 μm spectral region in short-orbital-period cataclysmic variables may be a useful indicator of the companion's spectral type. SED fitting for RZ Leo and CU Vel suggests M5-type dwarf companions, and distances of 340 ± 110 and 150 ± 50 pc, respectively. These systems may be placed in the upper evolution branch for short-period cataclysmic variables.

A search for brown dwarf like secondaries in cataclysmic variables - II

We present VTL/ISAAC infrared spectroscopy of a sample of short-orbital-period cataclysmic variables that are candidates for harbouring substellar companions. We have detected the K I and Na I absorption lines of the companion star in VY Aqr. The overall spectral distribution in this system is best fitted with an M9.5 type dwarf spectrum, implying a distance of 100 ′ 10 pc. VY Aqr seems to fall far from the theoretical distribution of secondary star temperatures around the orbital period minimum. Fitting of the IR spectral energy distribution (SED) was performed by comparing the observed spectrum with late-type templates. The application of such a spectral fitting procedure suggests that the continuum shape in the 1.1-2.5 μm spectral region in short-orbital-period cataclysmic variables may be a useful indicator of the companion's spectral type. SED fitting for RZ Leo and CU Vel suggests M5-type dwarf companions, and distances of 340 ′ 110 and 150 ′ 50 pc, respectively. These systems may be placed in the upper evolution branch for short-period cataclysmic variables.

On the late spectral types of cataclysmic variable secondaries

We investigate why the spectral type of most cataclysmic variable (CV) secondaries is significantly later than that of a ZAMS star with the same mean density. Using improved stellar input physics, tested against observations of low-mass stars at the bottom of the main sequence, we calculate the secular evolution of CVs with low-mass donors. We consider sequences with different mass transfer rates and with a different degree of nuclear evolution of the donor prior to mass transfer. Systems near the upper edge of the gap ($P \sim 3 - 6$ h) can be reproduced by models with a wide range of mass transfer rates from $1.5 \times 10^{-9} \msolyr$ to $10^{-8} \msolyr$. Evolutionary sequences with a small transfer rate and donors that are substantially evolved off the ZAMS (central hydrogen content $0.05-0.5$) reproduce CVs with late spectral types above $P \simgr$ 6 h. Systems with the most discrepant (late) spectral type should have the smallest donor mass at any given $P$. Consistency with the period gap suggests that the mass transfer rate increases with decreasing donor mass for evolved sequences above the period gap. In this case, a single-parameter family of sequences with varying $\xc$ and increasing mass transfer rate reproduces the full range of observed spectral types. This would imply that CVs with such evolved secondaries dominate the CV population.

On the late spectral types of cataclysmic variable secondaries

On the late spectral types of cataclysmic variable secondaries

The Lower Main Sequence and the Orbital Period Distribution of Cataclysmic Variable Stars

The color-magnitude diagram of the lower main sequence, as measured from a volume-limited sample of nearby stars, shows an abrupt downward jump between MV ~ 12 and 13. This jump indicates that the observed mass-radius relationship steepens between 0.3 and 0.2 M☉, but theoretical models show no such effect. It is difficult to isolate the source of this disagreement: the observational mass-radius relationship relies upon transformations that may not be sufficiently accurate, while the theoretical relationship relies upon stellar models that may not be sufficiently complete, particularly in their treatment of the complex physics governing the interior equation of state. If the features in the observationally derived mass-radius relationship are real, their existence provides a natural explanation for the well-known gap in the orbital period distribution of cataclysmic variables. This explanation relies only upon the observed mass-radius relationship of low-mass stars and does not require ad hoc changes in magnetic braking or in the structure of cataclysmic variable secondaries. If correct, it will allow broader application of cataclysmic variable observations to problems of basic stellar physics.

On the general properties of the secondaries of cataclysmic variable stars

The mass-radius, mass-orbital period, spectral type-orbital period and absolute magnitude-orbital period relationships for the secondary star components of cataclysmic variable (CV) binary stars are discussed. By comparing these with those expected for systems containing Roche lobe-filling main sequence stars, using empirical data for low-mass main sequence stars, it is shown thatas a group the CV secondaries do not differ significantly from main sequence stars.

Tidal spin-up and magnetic braking in cataclysmic variables

Based on proposed models for the tidal spin-up and magnetic braking of stars with a convective outer envelope we suggest that the rotation of secondaries in cataclysmic variables is not necessarily synchronized with the orbital revolution. This may provide an explanation for the observed large range in the mass transfer rate (at the same orbital period) of cataclysmic variables above the period gap.

Hydrodynamical modeling of mass transfer from cataclysmic variable secondaries

Resultats de simulations detaillees sur ordinateur de la perte de masse de la secondaire d'une etoile variable cataclysmique

He I line emission and the helium abundance in cataclysmic variables

Time-resolved spectroscopy of a sample of eclipsing cataclysmic variables indicates that the He I emission lines are found with the Balmer lines in the outer regions of the accretion disk. Additional observations of lower inclination noneclipsing systems demonstrate that the He I emission is frequently characterized by relatively high singlet to triplet intensity ratios and an inverted line decrement, an unusual situation which can be explained in terms of emission from an optically thick (in the lines) region in local thermodynamic equilibrium. Calculations of steady state accretion disk structure show that the observed characteristics of the He I and H I lines can be reproduced in normal disk models, but only in the helium abundance is assumed to be very high, i.e., He/H> or approx. =100 by number. Enhanced helium abundances are thus implied for some cataclysmic variable secondaries, suggesting that they are either highly evolved cores or that they have an outer layer of processed material which was accreted from the white dwarf during earlier evolution of the system.

Hydrodynamic Modeling of Mass Loss from Cataclysmic Variable Secondaries

AbstractResults of detailed computer simulations of mass loss from a cataclysmic variable secondary are presented. The calculations involve solution of the nonlinear hydrodynamical equations of stellar structure under varying degrees of approximation in an attempt to determine stability of the mass loss process. For comparison with previous theoretical studies dynamical sequences were computed assuming spherically symmetric mass loss from the secondary. The assumption of spherical symmetry is very poor since nearly all of the transferred material is lost within a small region about the inner Lagrangian point. Dynamical sequences treating only the region near the Lagrangian point were constructed in a manner consistent with the assumption of Roche geometry. Finally the effects of mass flow nonorthogonal to the Roche equipotential surfaces were treated in a very simple way. The last generalization produces stable mass loss in a model which was unstable for the less realistic approximation schemes. The finding of stable mass transfer implies that instability of the secondary star is not the mechanism leading to cataclysmic variable outbursts. This conclusion is consistent with published observations of dwarf novae made just prior to outburst.

Hydrodynamic modeling of mass loss from cataclysmic variable secondaries

Results of detailed computer simulations of mass loss from a cataclysmic variable secondary are presented. The calculations involve solution of the nonlinear hydrodynamical equations of stellar structure under varying degrees of approximation in an attempt to determine stability of the mass loss process. For comparison with previous theoretical studies dynamical sequences were computed assuming spherically symmetric mass loss from the secondary. The assumption of spherical symmetry is very poor since nearly all of the transferred material is lost within a small region about the inner Lagrangian point. Dynamical sequences treating only the region near the Lagrangian point were constructed in a manner consistent with the assumption of Roche geometry. Finally the effects of mass flow nonorthogonal to the Roche equipotential surfaces were treated in a very simple way. The last generalization produces stable mass loss in a model which was unstable for the less realistic approximation schemes. The finding of stable mass transfer implies that instability of the secondary star is not the mechanism leading to cataclysmic variable outbursts. This conclusion is consistent with published observations of dwarf novae made just prior to outburst.