Mass Transfer In Cataclysmic Variables Research Articles

X-ray, UV, and optical observations of the accretion disk and boundary layer in the symbiotic star RT Crucis

Compared to mass transfer in cataclysmic variables, the nature of accretion in symbiotic binaries in which red giants transfer material to white dwarfs (WDs) has been difficult to uncover. The accretion flows in a symbiotic binary are most clearly observable, however, when there is no quasi-steady shell burning on the WD to hide them. RT Cru is the prototype of such non-burning symbiotics, with its hard (δ-type) X-ray emission providing a view of its innermost accretion structures. In the past 20 yr, RT Cru has experienced two similar optical brightening events, separated by ~4000 days and with amplitudes of ΔV ~ 1.5 mag. After Swift became operative, the Burst Alert Telescope (BAT) detector revealed a hard X-ray brightening event almost in coincidence with the second optical peak. Spectral and timing analyses of multi-wavelength observations that we describe here, from NuSTAR, Suzaku, Swift/X-Ray Telescope (XRT) + BAT + UltraViolet Optical Telescope (UVOT) (photometry) and optical photometry and spectroscopy, indicate that accretion proceeds through a disk that reaches down to the WD surface. The scenario in which a massive, magnetic WD accretes from a magnetically truncated accretion disk is not supported. For example, none of our data show the minute-time-scale periodic modulations (with tight upper limits from X-ray data) expected from a spinning, magnetic WD. Moreover, the similarity of the UV and X-ray fluxes, as well as the approximate constancy of the hardness ratio within the BAT band, indicate that the boundary layer of the accretion disk remained optically thin to its own radiation throughout the brightening event, during which the rate of accretion onto the WD increased to 6.7 × 10−9M⊙ yr−1 (d/2 kpc)2. For the first time from a WD symbiotic, the NuSTAR spectrum showed a Compton reflection hump at E > 10 keV, due to hard X-rays from the boundary layer reflecting off of the surface of the WD; the reflection amplitude was 0.77 ± 0.21. The best fit spectral model, including reflection, gave a maximum post-shock temperature of kT = 53 ± 4 keV, which implies a WD mass of 1.25 ± 0.02 M⊙. Although the long-term optical variability in RT Cru is reminiscent of dwarf-novae-type outbursts, the hard X-ray behavior does not correspond to that observed in well-known dwarf nova. An alternative explanation for the brightening events could be that they are due to an enhancement of the accretion rate as the WD travels through the red giant wind in a wide orbit, with a period of about ~4000 days. In either case, the constancy of the hard X-ray spectrum while the accretion rate rose suggests that the accretion-rate threshold between a mostly optically thin and thick boundary layer, in this object, may be higher than previously thought.

Evolution and Outbursts of Cataclysmic Variables

Mass transfer and accretion are very important to understand the evolution and observational properties of cataclysmic variables (CVs). Due to the lack of an accretion disk, eclipsing profiles of polars are the best source to study the character of mass transfer in CVs. By analyzing long-term photometric variations in the eclipsing polar HU Aqr, the property of mass transfer and accretion are investigated. The correlation between the brightness state change and the variation of the ingress profile suggests that both the accretion hot spot and the accretion stream are produced instantaneously. The observations clearly show that it is the variation of mass transfer causing the brightness state changes that is a direct evidence of variable mass transfer in a CV. It is shown that it is the local dark-spot activity near the L1 point to cause the change of the mass transfer rather than the activity cycles of the cool secondary star. Our results suggest that the evolution of CVs is more complex than that predicted by the standard model and we should consider the effect of variable mass accretion in nova and dwarf nova outbursts.

ANGULAR MOMENTUM LOSS MECHANISMS IN CATACLYSMIC VARIABLES BELOW THE PERIOD GAP

Mass transfer in cataclysmic variables (CVs) is usually considered to be caused by angular momentum loss (AML) driven by magnetic braking and gravitational radiation (GR) above the period gap, and solely by GR below the period gap. The best-fit revised model of CV evolution recently by \citet{kbp11}, however, indicates that AML rate below the period gap is $2.47(\pm 0.22)$ times the GR rate, suggesting the existence of some other AML mechanisms. We consider several kinds of consequential AML mechanisms often invoked in the literature: isotropic wind from the accreting white dwarfs, outflows from the Langrangian points, and the formation of a circumbinary disk. We found that neither isotropic wind from the white dwarf nor outflow from the $L_1$ point can explain the extra AML rate, while ouflow from the $L_2$ point or a circumbinary disk can effectively extract the angular momentum provided that $\sim (15-45)%$ of the transferred mass is lost from the binary. A more promising mechanism is a circumbinary disk exerting gravitational torque on the binary. In this case the mass loss fraction can be as low as $\lesssim 10^{-3}$.

Magnetized fragmented mass transfer in cataclysmic variables: AE Aquarii, a trial case

This paper examines the effect of secondary-star magnetic activity in AE Aquarii on the mass-transfer process. The results can be extrapolated to cataclysmic variables in general. Based upon the evolution of the system, the surface polar field of the secondary star has been constrained to values of the order of B 2,o ∼ 1000 G. This may indicate the presence of a global magnetic field on the star, making magnetic activity on the stellar surface at least plausible. We have shown that magnetic fields of the order of B ≥ 300 G will effectively curtail the mass-transfer process through the L1 point, resulting in a fragmentation of the mass-transfer flow as it is forced through the magnetic obstruction, while magnetic fields with values B < 250 G will be advected along with the fluid through the L1 region. This scenario will result in the field being pinched off, resulting in an inhomogeneous or 'blobby' magnetized mass flow towards the compact object. We show that magnetic flux conservation will result in these magnetic fields reaching values of B blob ∼ 3000 G close to the white dwarf, giving rise to the observed radio outbursts due to the violent encounter with the fast-rotating white-dwarf magnetosphere. Within this framework, optical flares can also be explained in terms of the collisional interaction between a spectrum of denser unmagnetized blobs in the fragmented flow after ejection from the system by the magnetospheric propeller. This may explain the apparent anticorrelation between the non-thermal radio and thermal optical flares seen in AE Aquarii. Supplementing the ideas above, it has been shown that extended prominences in the L1 region may also be instrumental in producing fragmented and magnetized mass transport. Compressed fragments of fluid will be released from these structures when the fluid pressure dominates the magnetic pressure inside. Reconnection at the foot points may initiate episodes of particle acceleration and non-thermal emission when these structures interact with the propeller, followed by enhanced mass transfer and optical brightening of the source when the mass transfer encounters the propeller, leading to the same anticorrelation between radio and optical outbursts.

Properties of discontinuous and nova-amplified mass transfer in cataclysmic variables

We investigate the effects of discontinuous mass loss in recurrent outburst events on the long-term evolution of cataclysmic variables (CVs). Similarly we consider the effects of frictional angular momentum loss (FAML), i.e. interaction of the expanding nova envelope with the secondary. The Bondi-Hoyle accretion model is used to parametrize FAML in terms of the expansion velocity vexp of the nova envelope at the location of the secondary; we find that small vexp causes strong FAML. Numerical calculations of CV evolution over a wide range of parameters demonstrate the equivalence of a discontinuous sequence of nova cycles and the corresponding mean evolution (replacing envelope ejection by a continuous wind), even close to the mass-transfer instability. A formal stability analysis of discontinuous mass transfer confirms this, independent of details of the FAML model. FAML is a consequential angular momentum loss that amplifies the mass-transfer rate driven by systemic angular momentum losses such as magnetic braking. We show that for a given vexp and white dwarf mass the amplification increases with secondary mass and is significant only close to the largest secondary mass consistent with mass-transfer stability. The amplification factor is independent of the envelope mass ejected during the outburst, whereas the mass-transfer amplitude induced by individual nova outbursts is proportional to it. In sequences calculated with nova model parameters taken from Prialnik & Kovetz, FAML amplification is negligible, but the outburst amplitude in systems below the period gap with a white dwarf mass ≃ 0.6 M⊙ is larger than a factor of 10. The mass-transfer rate in such systems is smaller than 10−11 M⊙ yr−1 for ≃ 0.5 Myr (≃ 10 per cent of the nova cycle) after the outburst. This offers an explanation for intrinsically unusually faint CVs below the period gap.

DI Ursae Majoris: Discovery of an Extreme ER UMa-Type Dwarf Nova with the Shortest Orbital Period

The long-term, time-resolved CCD photometry of a dwarf nova, DI UMa, has revealed this object to be a peculiar SU UMa-type dwarf nova having both the shortest ever observed superhump period (0.0555 d) and a very short (25 d) supercycle length. These features make DI UMa the fourth established member of a recently discovered small group of dwarf novae, called ER UMa stars or RZ LMi stars, which are now believed to be SU UMa-type dwarf novae with exceptionally high mass-transfer rates. DI UMa in all respects resembles a peculiar dwarf nova RZ LMi, but is more extreme in its short superhump period. The high-incident discovery of objects with such an extreme combination of parameters, which can not be expected from the standard evolution of cataclysmic variables (CVs), should provide a unique tool in probing the long-debated problem concerning the origin of mass-transfer in CVs and a secular alteration between CV subclasses.

Feedback Mass Transfer in Cataclysmic Variables: An Explanation of the Behaviour of VY Scl Stars

Abstract We investigate irradiation-driven mass transfer in cataclysmic variables in the presence of shielding of the radiation by an accretion disk. We have shown that the shielding can cause a temporal drop of 3–5 magnitudes in the brightness of systems with orbital periods shorter than about 4 hr. This provides a possible explanation for the brightness variations in the VY Scl stars.

The role of the dwarf nova period distribution in understanding the evolution of cataclysmic variables

view Abstract Citations (70) References (58) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS The Role of the Dwarf Nova Period Distribution in Understanding the Evolution of Cataclysmic Variables Shafter, Allen W. Abstract The dwarf nova period distribution is used to constrain the variation of mass transfer with orbital period. For orbital periods above the gap, agreement between the observed dwarf nova period distribution with those constructed from various magnetic braking models is generally poor. The most promising braking law is that of Mestel and Spruit (1987). Under certain conditions this braking law is able to produce a relatively flat M-dot(P) relation, but the braking law is not entirely satisfactory because it offers no explanation for the complete dominance of stable over unstable accretors immediately above the period gap. Speculative ideas are presented that may eventually provide a complete and satisfactory explanation for the lack of dwarf novae with periods between 3 and about 4 hr. It is suggested that the dwarf nova period distribution, and not only the overall period distribution, should be considered when applying observational constraints to theories of mass transfer in cataclysmic variables. Publication: The Astrophysical Journal Pub Date: July 1992 DOI: 10.1086/171579 Bibcode: 1992ApJ...394..268S Keywords: Accretion Disks; Cataclysmic Variables; Dwarf Novae; Stellar Evolution; Angular Momentum; Computational Astrophysics; Stellar Mass Accretion; Stellar Models; Stellar Orbits; Astrophysics; ACCRETION; ACCRETION DISKS; STARS: BINARIES: CLOSE; STARS: NOVAE; CATACLYSMIC VARIABLES full text sources ADS | data products SIMBAD (14)

Mass transfer in cataclysmic variables - Clues from the dwarf nova period distribution

Evidence is presented in support of the hypothesis that the mean mass-transfer rate at a given orbital period is not continuous across the 2-3 hr gap in the orbital period distribution for cataclysmic variables. It is pointed out that although dwarf novae comprise nearly half (48 percent) of all disk systems with orbital periods less than 10 hr, only three systems out of the 22 with periods between 3 and 4 hr appear to be dwarf novae. The overall orbital period distribution for dwarf novae in conjunction with the predictions from current theories of dwarf nova eruptions are used to argue that mass-transfer rates must be generally higher for systems with orbital periods greater than 3 hr relative to systems with periods less than 2 hr. It is further argued that the mean mass-transfer rate at a given orbital period cannot increase more steeply than P exp 1.7 unless the white dwarf mass is positively correlated with orbital period.

Magnetic braking in cataclysmic and low-mass X-ray binaries

The chromospheric-coronal emission of lower Main-Sequence single and binary stars can be correlated with an activity parameter of type R = g(B- V)P -1, where P is the rotation or orbital period and g(B-V) a function of the color resembling the convective turnover time. Observations indicate that the active region area coverage filling factor grows as R 2, and the whole stellar surface becomes filled with closed loop structures at R ≂ 3. A braking formula is proposed (Equation 4) to include all periods (0.d1 ≲ R ≲ 30d) and spectral types F-M. On the basis of this equation, the mass transfer rates in compact binaries (driven by the gradual loss of orbital angular momentum) are discussed. It is concluded that the magnetic braking has good chances of being that mechanism which drives the mass transfer in cataclysmic variables and galactic bulge X-ray sources.

Magnetic Braking in Cataclysmic and Low-Mass X-Ray Binaries

AbstractThe chromospheric-coronal emission of lower Main-Sequence single and binary stars can be correlated with an activity parameter of type R = g(B-V)P-1 where P is the rotation or orbital period and g(B-V) a function of the color resembling the convective turnover time. Observations indicate that the active region area coverage filling factor grows as R2, and the whole stellar surface becomes filled with closed loop structures at R ≂ 3. A braking formula is proposed (Equation 4) to include all periods (0.d1 ≲ R ≳ 30d) and spectral types F-M. On the basis of this equation, the mass transfer rates in compact binaries (driven by the gradual loss of orbital angular momentum) are discussed. It is concluded that the magnetic braking has good chances of being that mechanism which drives the mass transfer in cataclysmic variables and galactic bulge X-ray sources.