V803 Cen Research Articles

THE HOT COMPONENTS OF AM CVn HELIUM CATACLYSMICS

We present the results of a multi-component synthetic spectral analysis of the archival far ultraviolet spectra of the hot components of several AM CVn double degenerate interacting binaries with known distances from trigonometric parallaxes. Our analysis was carried out using the code BINSYN (Linnell & Hubeny 1996) which takes into account the donor companion star, the shock front which forms at the disk edge and the FUV and NUV energy distribution. We fixed the distance of each system at its parallax-derived value and adopted appropriate values of orbital inclination and white dwarf mass. We find that the accretion-heated "DO/DB" WDs are contributing significantly to the FUV flux in four of the systems (ES Ceti, CR Boo, V803 Cen, HP Lib, GP Com). In two of the systems, GP Com and ES Ceti, the WD dominates the FUV/NUV flux. We present model-derived accretion rates which agree with the low end of the range of accretion rates derived earlier from black body fits over the entire spectral energy distribution. We find that the WD in ES Ceti is very likely not a direct impact accretor but has a small disk. The WD in ES Ceti has $T_{eff} \sim 40,000 \pm 10,000$K. This is far cooler than the previous estimate of Espaillat et al.(2005). We find that the WD in GP Com has $T_{eff} = 14,800 \pm500$K, which is hotter than the previously estimated temperature of 11,000K. We present a comparison between our empirical results and current theoretical predictions for these systems.

On the orbital periods of the AM CVn stars HP Librae and V803 Centauri

We analyse high time resolution spectroscopy of the AM CVn stars HP Librae and V803 Centauri, taken with the New Technology Telescope (NTT) and the Very Large Telescope (VLT) of the European Southern Observatory, Chile. We present evidence that the literature value for V803 Cen's orbital period is incorrect, based on an observed 'S-wave' in the binary's spectrogram. We measure a spectroscopic period P-V803 (Cen) = 1596.4 +/- 1.2 s of the S-wave feature, which is significantly shorter than the 1611 -s periods found in previous photometric studies. We conclude that the latter period likely represents a 'superhump'. If one assumes that our S-wave period is the orbital period, V803 Cen's mass ratio can be expected to be much less extreme than previously thought, at q similar to 0.07 rather than q similar to 0.016. This relaxes the constraints on the masses of the components considerably: the donor star then does not need to be fully degenerate, and the mass of the accreting white dwarf no longer has to be very close to the Chandrasekhar limit. For HP Lib, we similarly measure a spectroscopic period P-HP (Lib) = 1102. +/- 8 0.2 s. This supports the identification of HP Lib's photometric periods found in the literature, and the constraints upon the masses derived from them.

Arbitrarily Degenerate Helium White Dwarfs as Donors in AM Canum Venaticorum Binaries

We apply the Deloye & Bildsten isentropic models for donors in ultracompact low-mass X-ray binaries to the AM CVn population of ultracompact, interacting binaries. The mass-radius relations of these systems' donors in the mass range of interest (M2 < 0.1 M⊙) are not single-valued, but parameterized by the donor's specific entropy. This produces a range in the relationships between system observables, such as orbital period Porb and mass transfer rate . For a reasonable range in donor specific entropy, can range over several orders of magnitude at fixed Porb. We determine the unique relation between and M2 in the AM CVn systems with known donor-to-accretor mass ratios q = M2/M1. We use structural arguments, as well as each system's photometric behavior, to place limits on and M2 in each. Most systems allow a factor of about 3 variation in , although V803 Cen, if the current estimates of its q are accurate, is an exception and must have M2 ≈ 0.02 M⊙ and ≈ 10-10 M⊙ yr-1. Our donor models also constrain each donor's core temperature (Tc) range and correlate Tc with M2. We examine how variations in donor specific entropy across the white dwarf family of AM CVn systems affects this population's current Galactic distribution. Allowing for donors that are not fully degenerate produces a shift in systems toward longer Porb and higher , increasing the parameter space in which these systems can be found. This shift increases the fraction of systems whose Porb is long enough that their gravity wave (GW) signal is obscured by the background of detached double white dwarf binaries that dominate the GW spectrum below a frequency of ≈2 mHz.

V803 Centauri: Helium Dwarf Nova Mimicking a WZ Sge-Type Superoutburst

Abstract We observed the long-term behavior of the helium dwarf nova V803 Cen, and clarified the existence of at least two distinct states (a state with 77-d supercycles and a standstill-like state) that interchangeably appeared with a time-scale of 1–2 yr. We also conducted a time-resolved CCD photometry campaign during a bright outburst in 2003 June. The overall appearance of the outburst closely resembles that of the late stage of the 2001 outburst of WZ Sge, consisting of the initial peak stage (superoutburst plateau), the dip, and the oscillating (rebrightening) states. During the initial peak stage, we detected a large-amplitude superhump-type variation with a period of $0.018686(4) \,\mathrm{d} = 1614.5(4) \,\mathrm{s}$, and during the oscillation stage we detected variations with a period of $0.018728(2) \,\mathrm{d} = 1618.1(2) \,\mathrm{s}$. We consider that the former period better represents the superhump period of this system, and the latter periodicity may be better interpreted as arising from late superhumps. The overall picture of the V803 Cen outburst resembles that of a WZ Sge-type outburst, but apparently with a higher mass-transfer rate than that in hydrogen-rich WZ Sge-type stars. We suggest that this behavior may be either the result of difficulty in maintaining the hot state in a helium disk, or the effect of an extremely low tidal torque resulting from the extreme mass ratio.

Dwarf nova oscillations and quasi-periodic oscillations in cataclysmic variables - III. A new kind of dwarf nova oscillation, and further examples of the similarities to X-ray binaries

We present measurements of the periods of Dwarf Nova Oscillations (DNOs) and Quasi-Periodic Oscillations (QPOs) in Cataclysmic Variable stars (CVs), many culled from published literature, but also others newly observed (in VZ Pyx, CR Boo, OY Car, Z Cha, AQ Eri, TU Men, HX Peg, CN Ori, V893 Sco, WX Hyi and EC2117-54). These provide data for 26 systems. We show that in general P_QPO ~ 15 P_DNO and that the correlation for CVs extends by three orders of magnitude lower in frequency the similar relationship found for X-Ray binaries. In addition, we have found that there is a second type of DNO, previously overlooked, which have periods ~ 4 times those of the regular DNOs (As well as those mined from publications, we have observed them in VW Hyi, OY Car, AQ Eri, V803 Cen, CR Boo, VZ Pyx, HX Peg and EC2117-54). Often both types of DNO coexist. Unlike the standard DNOs, the periods of the new type, which we refer to as longer period DNOs (lpDNOs), are relatively insensitive to accretion luminosity and can even appear in quiescence of dwarf novae. We interpret them as magnetically channelled accretion onto the differentially rotating main body of the white dwarf primary, rather than onto a rapidly slipping equatorial belt as in the case of the standard DNOs. This is supported by published measurements of v sin(i) for some of the primaries. Some similarities of the DNOs, lpDNOs and QPOs in CVs to the three types of QPO in X-Ray binaries (burst pulsation, high and low frequency QPOs) are noted.

Superhumps in the helium dwarf nova KL Draconis

We report on the discovery of a 25.5-min superhump period for the suspected helium dwarf nova system KL Draconis in a high state. The presence of superhumps combined with the previously observed helium spectrum and large-amplitude photometric variations confirm that KL Dra is an AM CVn system similar to CR Bootis, V803 Cen and CP Eridani. We also find a low-state photometric period at 25.0 min that we suggest may be the orbital period. With this assumption, we estimate q=0.075, M1=0.76 M⊙ and M2=0.057 M⊙.

Large-Amplitude 2.65-d Oscillation in the VY Scl-Type Star V425 Cassiopeiae

From long-term photometry of a VY Scl-type star, V425 Cas, between 1998 and 2000, we discovered a short-term, large-amplitude (up to 1.5 mag) variation. The variation was well represented by a single period of 2.65 d. The large amplitude and the profile of the folded light curve suggest that the dwarf nova-type disk instability is responsible for this variation. The shortness of the period is unprecedented in hydrogen-rich cataclysmic variables. Given the recent emerging evidence that the irradiation from white dwarfs in VY Scl-type systems affect their light behavior, we propose a possibility that this unique variation in V425 Cas can be explained by the combination of the dwarf nova-type disk instability and irradiation. Similar short-period “outbursts" have been known in X-ray transients (V518 Per), and helium cataclysmic variables (CR Boo and V803 Cen). We discuss the possibility that these phenomena have a common origin to the unique variation in V425 Cas.

NLTE accretion disc models for the AM Canum Venaticorum systems

We present models of the optical spectra of two types of accretion disc atmospheres: a pure helium model and a H-He model, in a low mass ratio, helium transferring, interacting binary white dwarf system. The computations are restricted to stationary discs in LTE and NLTE cases. We also investigate the influence on the spectra of some of the disc parameters such as the mass accretion rate, the angle of inclination, the H/He mass ratio, and the inner and outer radius of the disc. Departures from LTE are also investigated in order to assess the necessity of computing more complicated and more time consuming models. We found that dierences in predicted equivalent widths of helium line computed in LTE and NLTE models range between 10 and 40 percent. Finally we compare our disc models with 4 of the AM CVn systems observed with the Nordic Optical Telescope: AM CVn, HP Lib, V803 Cen and CR Boo to determine their parameters as mass transfer rates and inclinations of their discs.

An X‐Ray Study of the Helium‐Disk Dwarf Nova V803 Centauri

ABSTRACTRossi X‐Ray Timing Explorer observations of the helium‐rich dwarf nova V803 Cen were conducted throughout an entire outburst cycle of 23 hr. The X‐ray count rate is very low and does not change during the outburst cycle. The X‐ray spectrum is consistent with a 10 keV bremsstrahlung source with a column density of 6 × 1020 cm−2. We compare the X‐ray and optical properties of this system with those of dwarf novae with hydrogen disks.

V803 Centauri: A Helium‐rich Dwarf Nova

ABSTRACTWe report 1992–1999 photometry of the helium‐rich cataclysmic variable V803 Centauri. In its high brightness state at V = 13, the star shows a strong periodic signal with P = 1618 s; this resembles the superhumps associated with many dwarf novae. However, it is unusual because the superhump appears to endure through all brightness states, including the very faint state at V = 17. The star also becomes occasionally stuck in a “cycling state,” in which the brightness varies in the range 13.4–14.5, with a period of 22 ± 1 hr. This appears to be the recurrence pattern of “normal” dwarf nova outbursts. Thus the underlying physics is probably that of a dwarf nova, but with an accretion disk dominated by helium. Reckoned as a dwarf nova, V803 Cen presents an interesting test for accretion disk theory, because it appears to display two timescales for eruption recurrence: 0.94 day at V = 14.5 and ∼5 days at V = 17.2. This is roughly consistent with the general idea that recurrence time scales inversely with accretion rate.

Non Circular Disks in am CVn Systems?

The AM CVn stars are mass transferring, interacting binary, white dwarf systems with orbital periods of 15…45 minutes. Hydrogen is completely lost from these systems, and we observe small helium disks which may show thermal and tidal instabilities if the mass transfer rate is large enough (Osaki 1995). A tidal instability brings the disk into a superoutburst state, and in the light curve we may observe superhumps. Based on the observed periods and mass transfer rates we can divide the AM CVn stars into three groups:A: In permanent superoutburst: AM CVn and EC 15330-1403B: Regular superoutbursts: CR Boo, V803 Cen and CP EriC: Not yet observed superoutburst: GP ComGroup A consists of systems with disks which are too hot to decline into a low state. These disks are in a constant superoutburst state, analogous to nova-like CVs which show no outbursts, but still exhibit permanent superhumps (Skillman &amp; Patterson 1993). Group B shows normal outbursts which can trigger superoutbursts, analogous to the VY Scl dwarf novae. The group C object may also be a superhumper, but with very infrequent outbursts analogous to the SU UMa stars (Warner 1995). In the following we will discuss evidence for superoutbursts in these systems, and the likelihood for these systems to develop elliptical disks.

The AM CVn systems: A bibliographic search for clues to their cataclysmic nature

The nature of AM CVn, GP Com, CR Boo, and V803 Cen is investigated in detail on the basis of a comprehensive review of all that to-date is known about these systems. They are classified as cataclysmic variables, within these forming the subclass of AM CVn systems; their distinguishing characteristic is an extreme H-deficiency. Besides, they are very blue and display periodicities on the order of minutes. Although in several aspects they behave like cataclysmics, in others they differ from them. The most widely accepted interpretation for their nature involves two interacting He white dwarfs in a very close orbit. The validity of such a model is evaluated in the present work.

IUE-spectra of the four interacting binary white dwarf stars: AM CVn, PG 1346+082, V803 Cen, AND GP com. A comparative study

This project is part of a world wide cooperation aimed at investigating properties of white dwarf stars to determine their constitution and evolution. The project is based on two recent developments: -The first is the discovery, of pulsational instabilities in some white dwarf stars, giving rise to non-radial pulsations which can be observed as flux modulations with low amplitude. -The second is to study the mass transfer between interacting binary white dwarf stars, which transports processed material from the interior of the donating star to the accretor. We present a study of four interacting binary white dwarf systems based on IUE spectra.

A Limit on the Space Density of Short-Period Binary White Dwarfs

We infer that detached binary white dwarfs with orbital periods of a few hours exist because we observe both their progenitors and their descendents. The binary LB 3459 has an orbital period of 6.3 hr and contains a pair of hot subdwarfs that will eventually cool to become white dwarfs (Kilkenny, Hill, and Penfold 1981). L870-2 is a pair of white dwarfs and, given enough time, its 1.55 d orbital period will decay to shorter periods (Saffer, Liebert, and Olszewski 1988). GP Com, AM CVn, V803 Cen, and PG1346+082 are interacting binary white dwarfs with orbital periods between 1051 s for AM CVn and 46.5 min for GP Com (Nather, Robinson, and Stover 1981; Solheim et al. 1984; Wood et al. 1987; O’Donoghue and Kilkenny 1988). These ultrashort period systems must be descendents of detached pairs of white dwarfs. We also expect short-period binary white dwarfs to exist for theoretical reasons. All calculations of the evolution of binary stars show that main-sequence binaries can evolve to binary white dwarfs (e.g., Iben and Tutukov 1984). Among Population I stars, 1/2 to 2/3 of all main-sequence stars are binaries and about 20% of these binaries should become double white dwarfs with short orbital periods (Abt 1983, Iben and Tutukov 1986). Thus, about 1/10 of all white dwarfs could be close binaries (Paczynski 1985). Nevertheless, no detached binary white dwarfs with extremely short periods have yet been found.

The Optical Spectra of V803 CEN

AbstractWe obtained spectrophotometry of V803 Cen in both high state (B ≈ 13.5) and low state (B ≈ 17) from 3800 Å to 7100 Å and 5 Å resolution. Essentially all the observed absorption lines are due to He I. No emission lines in either high or low state were detected, as well as no significant departures from a Planck law distribution.

High-speed spectroscopy and photometry of the interacting binary white dwarf V803 Cen (AE-1)

High-speed spectroscopy and photometry of the interacting binary white dwarf V803 Cen (AE-1)

Photometry and spectroscopy of the interacting binary white dwarf V803 Cen (AE-1)

Photometry and spectroscopy of the interacting binary white dwarf V803 Cen (AE-1)