The Masses and Evolutionary State of the Stars in the Dwarf Nova SS Cygni

Bitner, Martin A.; Robinson, E. L.; Behr, Bradford B.

doi:10.1086/517496

Cited by 46 publications

(81 citation statements)

References 51 publications

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“…The value given above is also essentially higher (by a factor of ∼2.5) than the one derived by Schreiber & Gänsicke (2002). The higher mass accretion rate is required because Bitner et al (2007) found a higher value for the inclination and a lower value for the mass of the white dwarf.…”

Section: Accretion Rate During Outburstmentioning

confidence: 58%

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The dwarf nova SS Cygni: what is wrong?

Schreiber¹,

Lasota

2007

A&A

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Context. Since the Fine Guiding Sensor (FGS) on the Hubble Space Telescope (HST) was used to measure the distance to SS Cyg to be 166 ± 12 pc, it became apparent that at this distance the disc instability model fails to explain the absolute magnitude during outburst. It remained, however, an open question whether the model or the distance have to be revised. Recent observations led to a revision of the system parameters of SS Cyg and seem to be consistent with a distance of d > ∼ 140 pc. Aims. We re-discuss the problem taking into account the new binary and stellar parameters measured for SS Cyg. We confront not only the observations with the predictions of the disc instability model but also compare SS Cyg with other dwarf novae and nova-like systems. Methods. We assume the disc during outburst to be in a quasi stationary state and use the black-body approximation to estimate the accretion rate during outburst as a function of distance. Using published analysis of the long term light curve we determine the mean mass transfer rate of SS Cyg as a function of distance and compare the result with mass transfer rates derived for other dwarf novae and nova-like systems. Results. At a distance of d > ∼ 140 pc, both the accretion rate during outburst as well as the mean mass transfer rate of SS Cyg contradict the disc instability model. More important, at such distances we find the mean mass transfer rate of SS Cyg to be higher or comparable to those derived for nova-like systems. Conclusions. Our findings show that a distance to SS Cyg > ∼ 140 pc contradicts the main concepts developed for accretion discs in cataclysmic variables during the last 30 years. Either our current picture of disc accretion in these systems must be revised or the distance to SS Cyg is ∼100 pc.

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Section: Accretion Rate During Outburstmentioning

confidence: 58%

“…As a consequence of the revised inclination and mass of the white dwarf (see Table 1), agreement with the DIM now requires distances as short as d < ∼ 100 pc. Bitner et al (2007) (right column). Also given is the derived outer radius of the disc (∼0.9 R 1 ) in units of 10 10 cm.…”

Section: Accretion Rate During Outburstmentioning

confidence: 99%

The dwarf nova SS Cygni: what is wrong?

Schreiber¹,

Lasota

2007

A&A

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“…However, we have to point out that a recent estimation gave a lesser WD mass in SS Cyg, M WD = 0.81 ± 0.18 M (Bitner et al 2007) instead of 1 M . We may expect that some BL model around a slower rotating WD with M WD = 0.8 M can have similar effective temperatures (and a similar spectrum) as the considered BL model with M WD = M and ω WD = 0.8ω K .…”

Section: Comparison With Observationsmentioning

confidence: 73%

Modeling the EUV spectra of optically thick boundary layers of dwarf novae in outburst

Suleimanov

Hertfelder

Werner

et al. 2014

A&A

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Context. Disk accretion onto weakly magnetized white dwarfs (WDs) in cataclysmic variables (CVs) leads to the formation of a boundary layer (BL) between the accretion disk and the WD, where the accreted matter loses its excess kinetic energy and angular momentum. It is assumed that angular momentum is effectively transported in the BL, but the transport mechanism is still unknown. Aims. Here we compute detailed model spectra of recently published optically thick one-dimensional radial BL models and qualitatively compare them with observed soft X-ray/extreme ultraviolet (EUV) spectra of dwarf novae in outburst. Methods. Every considered BL model with given effective temperature and surface density radial distribution is divided into a number of rings, and for each ring, a structure model along the vertical direction is computed using the stellar-atmosphere method. The ring spectra are then combined into a BL spectrum taking Doppler broadening and limb darkening into account. Results. Two sets of model BL spectra are computed, the first of them consists of BL models with fixed WD mass (1 M ) and various relative WD angular velocities (0.2, 0.4, 0.6 and 0.8 break-up velocities), while the other deals with a fixed relative angular velocity (0.8 break-up velocity) and various WD masses (0.8, 1, and 1.2 M ). The model spectra show broad absorption features because of blending of numerous absorption lines, and emission-like features at spectral regions with only a few strong absorption lines. The model spectra are very similar to observed soft X-ray/EUV spectra of SS Cyg and U Gem in outburst. The observed SS Cyg spectrum could be fitted by BL model spectra with WD masses 0.8−1 M and relative angular velocities 0.6−0.8 break up velocities. These BL models also reproduce the observed ratio of BL luminosity and disk luminosity. The difference between the observed and the BL model spectra is similar to a hot optically thin plasma spectrum and could be associated with the spectrum of outflowing plasma with a mass loss rate compatible with the BL mass accretion rate. Conclusions. The suggested method of computing BL spectra seems very promising and can be applied to other BL models for comparison with EUV spectra of dwarf novae in outburst.

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“…For example, there is very little evidence for even the presence or absence of starspots on CV secondaries. Webb et al (2002) spectroscopically inferred a spot coverage of 20% for the donor in SS Cyg, but the likely nuclearevolved nature of this star (Bitner et al 2007) makes it an unreliable calibration point for our purposes. Similarly, Roche tomography has been used to detect starspots in the donor stars of AE Aqr (Watson et al 2006), BV Cen (Watson et al 2007b), and V426 Oph (Watson et al 2007a), but one of these is a strongly magnetic system (AE Aqr) and at least one of the others contains an evolved donor (BV Cen).…”

Section: Larger-than-expected Radii In Non-interacting Low-mass Starsmentioning

confidence: 99%

The Evolution of Cataclysmic Variables as Revealed by Their Donor Stars

Knigge

Baraffe

Patterson

2011

ApJS

585

879

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We present an attempt to reconstruct the complete evolutionary path followed by cataclysmic variables (CVs), based on the observed mass-radius relationship of their donor stars. Along the way, we update the semi-empirical CV donor sequence presented previously by one of us, present a comprehensive review of the connection between CV evolution and the secondary stars in these systems, and reexamine most of the commonly used magnetic braking (MB) recipes, finding that even conceptually similar ones can differ greatly in both magnitude and functional form. The great advantage of using donor radii to infer mass-transfer and angular-momentum-loss (AML) rates is that they sample the longest accessible timescales and are most likely to represent the true secular (evolutionary average) rates. We show explicitly that if CVs exhibit long-term mass-transfer-rate fluctuations, as is often assumed, the expected variability timescales are so long that other tracers of the mass-transfer rate-including white dwarf (WD) temperatures-become unreliable. We carefully explore how much of the radius difference between CV donors and models of isolated main-sequence stars may be due to mechanisms other than mass loss. The tidal and rotational deformation of Roche-lobe-filling stars produces 4.5% radius inflation below the period gap and 7.9% above. A comparison of stellar models to mass-radius data for non-interacting stars suggests a real offset of 1.5% for fully convective stars (i.e., donors below the gap) and 4.9% for partially radiative ones (donors above the gap). We also show that donor bloating due to irradiation is probably smaller than, and at most comparable to, these effects. After calibrating our models to account for these issues, we fit self-consistent evolution sequences to our compilation of donor masses and radii. In the standard model of CV evolution, AMLs below the period gap are assumed to be driven solely by gravitational radiation (GR), while AMLs above the gap are usually described by an MB law first suggested by Rappaport et al. We adopt simple scaled versions of these AML recipes and find that these are able to match the data quite well. The optimal scaling factors turn out to be f GR = 2.47 ± 0.22 below the gap and f MB = 0.66 ± 0.05 above (the errors here are purely statistical, and the standard model corresponds to f GR = f MB = 1). This revised model describes the mass-radius data significantly better than the standard model. Some of the most important implications and applications of our results are as follows. (1) The revised evolution sequence yields correct locations for the minimum period and the upper edge of the period gap; the standard sequence does not. (2) The observed spectral types of CV donors are compatible with both standard and revised models. (3) A direct comparison of predicted and observed WD temperatures suggests an even higher value for f GR , but this comparison is sensitive to the assumed mean WD mass and the possible existence of mass-transfer-rate fluctuations. (4) The pred...

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The Masses and Evolutionary State of the Stars in the Dwarf Nova SS Cygni

Cited by 46 publications

References 51 publications

The dwarf nova SS Cygni: what is wrong?

The dwarf nova SS Cygni: what is wrong?

Modeling the EUV spectra of optically thick boundary layers of dwarf novae in outburst

The Evolution of Cataclysmic Variables as Revealed by Their Donor Stars

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