Secondary Stars In Cataclysmic Variables Research Articles

SDSS J001153.08–064739.2, A CATACLYSMIC VARIABLE WITH AN EVOLVED DONOR IN THE PERIOD GAP

Secondary stars in cataclysmic variables (CVs) follow a well defined period-density relation. Thus, canonical donor stars in CVs are generally low-mass stars of spectral type M. However, several CVs have been observed containing secondary stars which are too hot for their inferred masses. This particular configuration can be explained if the donor stars in these systems underwent significant nuclear evolution before they reached contact. In this paper we present SDSSJ001153.08-064739.2 as an additional example belonging to this peculiar type of CV and discuss in detail its evolutionary history. We perform spectroscopic and photometric observations and make use of available Catalina Real-Time Transient Survey photometry to measure the orbital period of SDSSJ001153.08-064739.2 as 2.4 hours and estimate the white dwarf (Mwd>0.65Msun) and donor star (0.21Msun<Mdon<0.45Msun) masses, the mass ratio (q = 0.32 +- 0.08), the orbital inclination (47 degrees < i < 70 degrees), derive an accurate orbital ephemeris (T0 = 2453383.578 + E x 0.10028081), and report the detection of an outburst. We show that SDSSJ001153.08-064739.2 is one of the most extreme cases in which the donor star is clearly too hot for its mass. SDSSJ001153.08-064739.2 is therefore not only a peculiar CV containing an evolved donor star but also an accreting CV within the period gap. Intriguingly, approximately half of the total currently-observed sample of these peculiar CVs are located in the period gap with nearly the same orbital period.

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.

Roche tomography of the secondary stars in CVs

AbstractThe secondary stars in cataclysmic variables (CVs) are key to our understanding of the origin, evolution and behaviour of this class of interacting binary. In seeking a fuller understanding of these objects, the challenge for observers is to obtain images of the secondary star. This goal can be achieved through Roche tomography, an indirect imaging technique that can be used to map the Roche‐lobe‐filling secondary. The review begins with a description of the basic principles that underpin Roche tomography, including methods for determining the system parameters. Finally, we conclude with a look at the main scientific highlights to date, including the first unambiguous detection of starspots on AE Aqr B, and consider the future prospects of this technique. (© 2004 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

On the evidence for brown dwarf secondary stars in cataclysmic variables

We present the K-band spectrum of the cataclysmic variable LL And, obtained using NIRSPEC on Keck-II. The spectrum shows no evidence for the absorption features observed by Howell and Ciardi, which these authors used to claim a detection of a brown dwarf secondary star in LL And. In light of our new data, we review the evidence for brown dwarf secondary stars in this and other cataclysmic variables.

Roche tomography of cataclysmic variables - I. Artefacts and techniques

Roche tomography is a technique used for imaging the Roche-lobe filling secondary stars in cataclysmic variables (CVs). In order to interpret Roche tomograms correctly, one must determine whether features in the reconstruction are real, or due to statistical or systematic errors. We explore the effects of systematic errors using reconstructions of simulated datasets and show that systematic errors result in characteristic distortions of the final reconstructions that can be identified and corrected. In addition, we present a new method of estimating statistical errors on tomographic reconstructions using a Monte-Carlo bootstrapping algorithm and show this method to be much more reliable than Monte-Carlo methods which `jiggle' the data points in accordance with the size of their error bars.

Solar-type cycles of the secondary stars in cataclysmic variables

We found cyclical variations in the quiescent magnitudes and outburst intervals of 21 dwarf novae, 1 nova and 1 nova-like star produced by the solar-type cycles of the late-type secondary components. More than 70% of individual systems in this work are rst studied by Fourier periodogram analysis. It seems that the mean cycle amplitudes of the quiescent magnitudes are considerably larger for SU UMa stars. Cyclical modulations of quiescent magnitudes found for the stars, which have orbital periods below the period gap, support the hypothesis that even almost fully convective secondaries may possess activity cycles. The estimated probability density functions show peaks at 9.7, 7.9 and 8.6 yrs for cataclysmic variables (CVs), single main-sequence stars (MS) and all stars (CVs+MS), respectively. Using a larger set of data, we conrm that there is no correlation of the cycle period with the rotational regime of the secondary star. It is also conrmed that cycle periods of CVs are in the range of those of single main-sequence stars. We note that the observational cycle parameters (i.e. cycle period, Pcyc, and cycle amplitude of quiescent magnitudes, m) do not seem in relation with any other parameter such as masses of primary and secondary stars, mean outburst interval, mean outburst duration, mean decline and rise rate of outbursts, absolute magnitudes in quiescent and outburst states etc.

The secondary stars in cataclysmic variables and low-mass X-ray binaries

We critically re-examine the available data on the spectral types, masses and radii of the secondary stars in cataclysmic variables (CVs) and low-mass X-ray binaries (LMXBs), using the new catalogue of Ritter & Kolb as a starting point. We find there are 55 reliable spectral type determinations and only 14 reliable mass determinations of CV secondary stars (10 and 5, respectively, in the case of LMXBs). We derive new spectral type–period, mass–radius, mass–period and radius–period relations, and compare them with theoretical predictions. We find that CV secondary stars with orbital periods shorter than 7–8 h are, as a group, indistinguishable from main-sequence stars in detached binaries. We find that it is not valid, however, to estimate the mass from the spectral type of the secondary star in CVs or LMXBs. We find that LMXB secondary stars show some evidence for evolution, with secondary stars which are slightly too large for their mass. We show how the masses and radii of the secondary stars in CVs can be used to test the validity of the disrupted magnetic braking model of CV evolution, but we find that the currently available data are not sufficiently accurate or numerous to allow such an analysis. As well as considering secondary star masses, we also discuss the masses of the white dwarfs in CVs, and find mean values of M1 ¼ 0:69 6 0:13 M ⊙ below the period gap, and M1 ¼ 0:806 0:22 M⊙ above the period gap.

The secondary stars in cataclysmic variables and low-mass X-ray binaries

We critically re-examine the available data on the spectral types, masses and radii of the secondary stars in cataclysmic variables (CVs) and low-mass X-ray binaries (LMXBs), using the new catalogue of Ritter &38; Kolb as a starting point. We find there are 55 reliable spectral type determinations and only 14 reliable mass determinations of CV secondary stars (10 and 5, respectively, in the case of LMXBs). We derive new spectral type–period, mass–radius, mass–period and radius–period relations, and compare them with theoretical predictions. We find that CV secondary stars with orbital periods shorter than 7–8 h are, as a group, indistinguishable from main-sequence stars in detached binaries. We find that it is not valid, however, to estimate the mass from the spectral type of the secondary star in CVs or LMXBs. We find that LMXB secondary stars show some evidence for evolution, with secondary stars which are slightly too large for their mass. We show how the masses and radii of the secondary stars in CVs can be used to test the validity of the disrupted magnetic braking model of CV evolution, but we find that the currently available data are not sufficiently accurate or numerous to allow such an analysis. As well as considering secondary star masses, we also discuss the masses of the white dwarfs in CVs, and find mean values of −M = 0.69 ± 0.13 M⊙ below the period gap, and −M = 0.80 ± 0.22 M⊙ above the period gap.

Uncovering Flickering in Time-Resolved Spectroscopy for the Nova-like Variable UX Ursa Majoris

AbstractThe flickering phenomenon in cataclysmic variables (CVs) is manifest photometrically as variations in system brightness with amplitudes up to about 1 mag and on timescales from seconds to tens of minutes. Although a pronounced characteristic of CVs, flickering has been studied, in general, only tangentially and in the context of overall CV photometric properties. It is usually uncovered in CV photometry, yet my Doppler tomographic analysis of data collected on the KPNO 2.1-meter telescope in 1982 by Schlegel, Honeycutt, and Kaitchuck (1983) for the nova-like variable UX Ursa Majoris suggests that flickering also exists in this system’s spectroscopy.Using the integrated continuum flux from the UX UMa observations and Bruch’s (1992) methodology for classifying high- and low-flicker data, I constructed two data subsets corresponding to high- and low-flicker states. I generated Doppler tomograms of these high- and low-flicker states and interpret differences between the tomograms as spectroscopic signatures of flickering in the system.In UX UMa’s Hβ Doppler tomograms, a small, faint, incomplete ring of emission is visible for both flickering states. Dominating ring emission is strong emission in the (+Vx,+Vy) quadrant. The usual location for Swave emission, in the (−Vx,+Vy) quadrant, is here the region of faintest emission. Differenced Doppler tomograms, produced by subtracting from the tomograms the symmetric ring-emission component, show clear differences between the high- and low-flicker states. Further, these early results suggest Doppler tomography may be a useful technique for studying the secondary stars in CVs imaged in this manner. Enhanced chromospheric activity of the red dwarf companion is clearly visible in the high-flickering state tomogram at (0, Krd) and may, therefore, afford investigators the opportunity to estimate the chromospheric flux and radial velocity of the secondary star.

Simulations of Angular Momentum Evolution in Wind-Fed CV Precursors

It is thought that the secondary stars in cataclysmic variables (CVs) may undergo a period of mass loss in the form of a wind during the evolution of the system (Mullan et al. 1992). This wind is thought to magnetically brake the secondary star with a time-scale ~ 108 yr (e.g. van Paradijs 1986). When the secondary’s spin has been brought close to synchronism with the orbit it is possible for tidal torques to lock the secondary in synchronous rotation.We have modelled this process by adapting a method that has proved successful in explaining the properties of the intermediate polars (Wynn &amp; King 1995). We model the secondary magnetic field as a pure dipole which drags on the wind material as it crosses field lines. This has the effect of trying to force material to move along field lines. This does not model the magnetic interaction completely since it does not allow for the back reaction of the material on the field structure. However, this will only become significant in regions where the field is weak and hence where material is not exerting a large torque on the star.