Thermal-tidal Instability Model Research Articles

Two-dimensional simulations of disks in close binaries

Context. Previous simulations of cataclysmic variables studied either the quiescence, or the outburst state in multiple dimensions or they simulated complete outburst cycles in one dimension using simplified models for the gravitational torques. Aims. We self-consistently simulate complete outburst cycles of normal and superoutbursts in cataclysmic variable systems in two dimensions. We study the effect of different α viscosity parameters, mass transfer rates, and binary mass ratios on the disk luminosities, outburst occurrence rates, and superhumps. Methods. We simulate non-isothermal, viscous accretion disks in cataclysmic variable systems using a modified version of the FARGO code with an updated equation of state and a cooling function designed to reproduce s-curve behavior. Results. Our simulations can model complete outburst cycles using the thermal tidal instability model. We find higher superhump amplitudes and stronger gravitational torques than previous studies, resulting in better agreement with observations.

Spectroscopic observations of V455 Andromedae superoutburst in 2007: The most exotic spectral features in dwarf nova outbursts

Abstract We present our spectroscopic observations of V455 Andromedae during the 2007 superoutburst. Our observations cover this superoutburst from around the optical peak of the outburst to the post-superoutburst stage. During the early superhump phase, the emission lines of the Balmer series, He i, He ii, Bowen blend, and C iv/N iv blend were detected. The He ii 4686 line exhibited a double-peaked emission profile, where Balmer emission lines were single-peaked, which is unexpected from its high inclination. In the ordinary superhump phase, the Balmer series transitioned to double-peaked emission profiles, and high-ionization lines were significantly weakened. These transitions of the line profiles should be related to the structural transformation of the accretion disk, as expected between the early and ordinary superhump transition in the thermal–tidal instability model. The Doppler map of Hα during the early superhump phase exhibits a compact blob centered at the primary white dwarf. In analogy to SW Sex-type cataclysmic variables, this feature could emerge from the disk wind and/or the mass accretion column on to the magnetized white dwarf. The Doppler map of He ii 4686 Å is dominated by the ring-like structure and imposed two flaring regions with the velocity of ∼300 km s−1, which is too slow for a Keplerian accretion disk. The phase of the flaring regions was coincident with the inner spiral arm structure identified during the early superhump phase. Our disk wind model with the enhanced emission from the wind component launched from the inner arm structure successfully reproduced the observed properties of He ii 4686 Å. Therefore, V455 And is the first case in dwarf nova outbursts where the presence of the disk wind is inferred from an optical spectrum.

A Comprehensive K2 and Ground-based Study of CRTS J035905.9+175034, an Eclipsing SU UMa System with a Large Mass Ratio

Abstract CRTS J035905.9+175034 is the first eclipsing SU UMa system for which a superoutburst has been observed by Kepler in the short-cadence mode. The light curve contains one superoutburst, eight normal outbursts (including a precursor to the superoutburst), and several minioutbursts that are present before—but not after—the superoutburst. The superoutburst began with a precursor normal outburst, and shortly after the peak of the precursor, the system developed large-amplitude superhumps that achieved their maximum amplitude after just three superhump cycles. The period excess of the initial superhump period relative to the orbital period implies a mass ratio of 0.281 ± 0.015, placing it marginally above most theoretical predictions of the highest-possible mass ratio for superhump formation. In addition, our analysis of the variations in eclipse width and depth, as well as the hot spot amplitudes, generally provides substantiation of the thermal-tidal instability model. The K2 data, in conjunction with our ground-based time-resolved spectroscopy and photometry from 2014 to 2016, allows us to determine many of the fundamental parameters of this system.

Detection of the supercycle in V4140 Sagittarii: First eclipsing ER Ursae Majoris-like object

Abstract We observed the deeply eclipsing SU UMa-type dwarf nova V4140 Sgr and established the very short supercycle of 69.7(3) d. There were several short outbursts between superoutbursts. These values, together with the short orbital period (0.06143 d), were similar to, but not as extreme as, those of ER UMa-type dwarf novae. The object is thus the first, long sought, eclipsing ER UMa-like object. This ER UMa-like nature can naturally explain the high (apparent) quiescent viscosity and unusual temperature profile in quiescence, which were claimed observational features against the thermal–tidal instability model. The apparently unusual outburst behavior can be reasonably explained by a combination of this ER UMa-like nature and the high orbital inclination, and there is no need to introduce mass transfer bursts from its donor star.

RZ Leonis Minoris bridging between ER Ursae Majoris-type dwarf nova and nova-like system

AbstractWe observed RZ LMi, which is renowned for its extremely short (∼19 d) supercycle and is a member of a small, unusual class of cataclysmic variables called ER UMa-type dwarf novae, in 2013 and 2016. In 2016, the supercycles of this object substantially lengthened in comparison to the previous measurements to 35, 32, and 60 d for three consecutive superoutbursts. We consider that the object virtually experienced a transition to the nova-like state (permanent superhumper). This observed behavior reproduced the prediction of the thermal-tidal instability model extremely well. We detected a precursor in the 2016 superoutburst and detected growing (stage A) superhumps with a mean period of 0.0602(1) d in 2016 and in 2013. Combined with the period of superhumps immediately after the superoutburst, the mass ratio is not as small as in WZ Sge-type dwarf novae, having orbital periods similar to RZ LMi. By using least absolute shrinkage and selection operator (Lasso) two-dimensional power spectra, we detected possible negative superhumps with a period of 0.05710(1) d. We estimated an orbital period of 0.05792 d, which suggests a mass ratio of 0.105(5). This relatively large mass ratio is even above that of ordinary SU UMa-type dwarf novae, and it is also possible that the exceptionally high mass-transfer rate in RZ LMi may be a result of a stripped secondary with an evolved core in a system evolving toward an AM CVn-type object.

PM J03338+3320: Long-period superhumps in growing phase following a separate precursor outburst

Abstract We observed the first-ever recorded outburst of PM J03338+3320, the cataclysmic variable selected by proper-motion survey. The outburst was composed of a precursor and the main superoutburst. The precursor outburst occurred at least 5 d before the maximum of the main superoutburst. Despite this separation, long-period superhumps were continuously seen between the precursor and main superoutburst. The period of these superhumps is longer than its orbital period by 6.0(1)% and can be interpreted to reflect the dynamical precession rate at the 3 : 1 resonance for a mass ratio of 0.172(4). These superhumps smoothly evolved into those in the main superoutburst. These observations provide the clearest evidence that the 3 : 1 resonance is triggered by the precursor outburst, even if it is well separated, and the resonance eventually causes the main superoutburst as predicted by the thermaltidal instability model. The presence of superhumps well before the superoutburst cannot be explained by alternative models (the enhanced mass-transfer model and the pure thermal instability one) and the present observations clearly support the thermaltidal instability model.

CC Sculptoris: Eclipsing SU UMa-type intermediate polar

Abstract We observed the 2014 superoutburst of the SU UMa-type intermediate polar CC Scl. We detected superhumps with a mean period of 0.05998(2) d during the superoutburst plateau and during three nights after the fading. During the post-superoutburst stage after three nights, a stable superhump period of 0.059523(6) d was detected. We found that this object is an eclipsing system with an orbital period of 0.058567233(8) d. By assuming that the disk radius in the post-superoutburst phase is similar to those in other SU UMa-type dwarf novae, we obtained a mass ratio of q = 0.072(3) from the dynamical precession rate of the accretion disk. The eclipse profile during outbursts can be modeled by an inclination of 80 $_{.}^{\circ}$6 ± 0 $_{.}^{\circ}$5. The 2014 superoutburst was preceded by a precursor outburst and the overall appearance of the outburst was similar to superoutbursts in ordinary SU UMa-type dwarf novae. We showed that the standard thermal-tidal instability model can explain the outburst behavior in this system and suggest that inner truncation of the disk by magnetism of the white dwarf does not strongly affect the behavior in the outer part of the disk.

Study of negative and positive superhumps in ER Ursae Majoris

AbstractWe carried out photometric observations of the SU UMa-type dwarf nova ER UMa during 2011 and 2012, which showed the existence of persistent negative superhumps even during the superoutburst. We performed a two-dimensional period analysis of its light curves by using a method called “least absolute shrinkage and selection operator” (Lasso) and the “phase dispersion minimization” (PDM) analysis, and found that the period of negative superhumps systematically changed between a superoutburst and the next superoutburst. The trend of the period change can be interpreted as a reflection of the change of the disk radius. This change is in agreement with the one predicted by the thermal tidal instability model. The normal outburst during a supercycle showed a general trend that the rising rate to its maximum becomes slower as the next superoutburst is approaching. The change can be interpreted as the consequence of the increased gas-stream flow into the inner region of the disk as a result of the tilted disk. Some of superoutbursts were found to be triggered by a precursor normal outburst when the positive superhump appeared to develop. The positive and negative superhumps coexisted during the superoutburst. Positive superhumps were prominent only for four or five days after the supermaximum, while the signal of negative superhumps became stronger after the middle phase of the superoutburst plateau. A simple combination of the positive and negative superhumps was found to be insufficient for reproduction of the complex profile variation. We were able to detect the developing phase of positive superhumps (stage A superhumps) for the first time in ER UMa-type dwarf novae. Using the period of stage A superhumps, we obtained a mass ratio of 0.100(15), which indicates that ER UMa is on the ordinary evolutional track of cataclysmic variable stars.

Study of Superoutbursts and Superhumps in SU UMa Stars by the Kepler Light Curves of V344Lyrae and V1504 Cygni

Abstract We have studied the short-cadence Kepler public light curves of two SU UMa stars, V344 Lyr and V1504 Cyg, extending over a period of more than two years by using power spectral analysis. We determined the orbital period of V344 Lyr to be Porb = 0.087903(1)d. We also reanalyzed the frequency variation of the negative superhump in a complete supercycle of V1504 Cyg with additional data of the O – C diagram, confirming that its characteristic variation is in accordance with the thermal-tidal instability model. We present a new two-dimensional period analysis based on a new method of a least absolute shrinkage and selection operator (Lasso). The new method gives very sharp peaks in the power spectra, and it is very useful for studying the frequency variation in cataclysmic variable stars. We have analyzed simultaneous frequency variations of the positive and negative superhumps. If they are appropriately converted, it is found that they vary in unison, indicating that they represent a disk-radius variation. We have also studied the frequency (or period) variations of positive superhumps during superoutbursts. These variations can be understood in a qualitative way by combining the disk-radius variation and the variation of pressure effects during a superoutburst. A sudden excitation of oscillation with a frequency range near to the negative superhump (which we call “impulsive negative superhump”) was observed in the descending branch of several outbursts of V344 Lyr. These events seem to have occurred just prior to the next superoutburst, and to act as a “lead” of the impending superoutburst.

Analysis of Three SU UMa-Type Dwarf Novae in the Kepler Field

Abstract We studied Kepler light curves of three SU UMa-type dwarf novae: a background dwarf nova of KIC 4378554, V585 Lyr, and V516 Lyr. Both the background dwarf nova and V516 Lyr showed a combination of a precursor and a main superoutburst, during which superhumps always developed in the fading branch of the precursor. This finding supports that the thermal-tidal instability theory explains the origin of superoutburst. A superoutburst of V585 Lyr recorded by Kepler did not show a precursor outburst, and the superhumps developed only after the maximum light: namely, the first-ever example in the Kepler data. Such a superoutburst is understood based on the thermaltidal instability model to be a “case B” superoutburst, discussed by Osaki and Meyer (2003, A&A, 401, 325). From the observation of V585 Lyr, Kepler first clearly revealed the positive period derivative commonly seen in the “stage B” superhumps of dwarf novae with a short orbital period. In all three objects, there was no strong signature of a transition to the dominating stream impact-type component of superhumps. This finding suggests that there is no strong indication of an enhanced mass-transfer following the superoutburst. In V585 Lyr, there were “mini-rebrightenings” with an amplitude of 0.2–0.4 mag and its period of 0.4–0.6d during the period between the superoutburst and the rebrightening. We have determined that the orbital period of V516 Lyr is 0.083999(8)d. In V516 Lyr, some of outbursts were double outbursts with varying degrees. The preceding outburst in the double was of the inside-out nature, while the following one was of the outside-in nature. One of the superoutbursts in V516 Lyr was preceded by a double precursor. The preceding precursor failed to trigger a superoutburst, and the following precursor triggered a superoutburst by developing positive superhumps. We have also developed new methods of reconstructing the light curve of superhumps, and of measuring the times of maxima from poorly sampled Kepler LC data.

The Cause of the Superoutburst in SU UMa Stars is Finally Revealed by Kepler Light Curve of V1504 Cygni

Abstract We have studied the short-cadence Kepler light curve of an SU UMa star, V1504 Cyg, which covers a period of $ \sim$ 630 d. All superoutbursts in V1504 Cyg have turned out to be of precursor-main types, and the superhump first appears near the maximum of the precursor. The superhumps grow smoothly from the precursor to the main superoutburst, showing that the superoutburst was initiated by a tidal instability (as evidenced by the growing superhump) as envisioned in the thermal-tidal instability (TTI) model proposed by Osaki (1989, PASJ, 41, 1005). We performed a power spectral analysis of the light curve of V1504 Cyg. One of the outstanding features is the appearance of a negative superhump extending over around 300 d, well over a supercycle. We found that the appearance of the negative superhump tends to decrease the frequency of occurrence of normal outbursts. Two types of supercycles are recognized in V1504 Cyg, which are similar to those of the Type L and Type S supercycles in the light curve of VW Hyi, a prototype SU UMa star, introduced by Smak (1985, Acta Astron., 35, 357). It is found that the Type L supercycle is the one accompanied by the negative superhump, and Type S is that without the negative superhump. If we adopt a tilted disk as an origin of the negative superhump, two types of the supercycles are understood to be due to a difference in the outburst interval, which is in turn caused by a difference in mass supply from the secondary to different parts of the disk. The frequency of the negative superhump varies systematically during a supercycle in V1504 Cyg. This variation can be used as an indicator of the disk-radius variation, and we have found that the observed disk-radius variation in V1504 Cyg fits very well with a prediction of the TTI model.

The orbital and superhump periods of the dwarf nova HS 0417+7445 in Camelopardalis

We present the 2005–2010 outburst history of the SU UMa-type dwarf HS 0417+7445, along with a detailed analysis of extensive time-series photometry obtained in March 2008 during the second recorded superoutburst of the system. The mean outburst interval is 197 ± 59 d, with a median of 193 d. The March 2008 superoutburst was preceded by a precursor outburst, had an amplitude of 4.2 magnitudes, and the whole event lasted about 16 days. No superhumps were detected during the decline from the precursor outburst, and our data suggests instead that orbital humps were present during that phase. Early superhumps detected during the rise to the superoutburst maximum exhibited an unusually large fractional period excess of ϵ = 0.137 ( P sh = 0.0856(88) d). Following the maximum, a linear decline in brightness followed, lasting at least 6 days. During this decline, a stable superhump period of P sh = 0.07824(2) d was measured. Superimposed on the superhumps were orbital humps, which allowed us to accurately measure the orbital period of HS 0417+7445, P orb = 0.07531(8) d, which was previously only poorly estimated. The fractional superhump period excess during the main phase of the outburst was ϵ = 0.037, which is typical for SU UMa dwarf novae with similar orbital period. Our observations are consistent with the predictions of the thermal-tidal instability model for the onset of superoutbursts, but a larger number of superoutbursts with extensive time-series photometry during the early phases of the outburst would be needed to reach a definite conclusion on the cause of superoutbursts.

Comprehensive simulations of superhumps

(Abridged) We use 3D SPH calculations with higher resolution, as well as with more realistic viscosity and sound-speed prescriptions than previous work to examine the eccentric instability which underlies the superhump phenomenon in semi-detached binaries. We illustrate the importance of the two-armed spiral mode in the generation of superhumps. Differential motions in the fluid disc cause converging flows which lead to strong spiral shocks once each superhump cycle. The dissipation associated with these shocks powers the superhump. We compare 2D and 3D results, and conclude that 3D simulations are necessary to faithfully simulate the disc dynamics. We ran our simulations for unprecedented durations, so that an eccentric equilibrium is established except at high mass ratios where the growth rate of the instability is very low. Our improved simulations give a closer match to the observed relationship between superhump period excess and binary mass ratio than previous numerical work. The observed black hole X-ray transient superhumpers appear to have systematically lower disc precession rates than the cataclysmic variables. This could be due to higher disc temperatures and thicknesses. The modulation in total viscous dissipation on the superhump period is overwhelmingly from the region of the disc within the 3:1 resonance radius. As the eccentric instability develops, the viscous torques are enhanced, and the disc consequently adjusts to a new equilibrium state, as suggested in the thermal-tidal instability model. We quantify this enhancement in the viscosity, which is ~10 per cent for q=0.08. We characterise the eccentricity distributions in our accretion discs, and show that the entire body of the disc partakes in the eccentricity.

WX Ceti: a closer look at its behaviour in quiescence and outburst

Context. WX Cet is a dwarf nova with rare outbursts of large amplitude. Aims. We compile the available data of WX Cet, compare the results with other SU UMa stars, and discuss our findings in the context of current theories of superhumps and superoutbursts to progress with our understanding of SU UMa stars. Methods. We analyse all recorded outbursts of WX Cet, based on the AAVSO archive and other published sources, and present new CCD photometry during two recent superoutbursts, including the determination of the corresponding periodicities. We perform numerical disc instability model calculations and compare its predictions with the observations. Results. WX Cet is a SU UMa type dwarf nova with a superoutburst cycle of 880 days on average, and short eruptions every 200 days. It seems that the outburst cycle length increased by nearly a factor of 2 during the past 70 years. According to our numerical simulations, this can be explained in the context of the disc instability model by assuming enhanced mass transfer during outburst and a decreasing mean mass transfer rate during the last decades. Using the data available, we refine the orbital period of WX Cet to 0.0582610 ± 0.0000002 days and interpret the orbital hump found in quiescence as emission from the hot spot. During two recent superoutbursts in July 2001 and December 2004 we observed superhumps, with a rather large positive period derivative of u Ps/Ps = 1.6 × 10 −4 , present only during the first 9 days of a superoutburst. Afterwards and during decline from the “plateau” phase, a constant superhump period of about 0.05922 days was observed. Late superhumps are present for at least 12 days after the decline from the “plateau”, with a period of 0.05927 days. We find this phenomenology difficult to interpret in the context of the standard explanation for superhumps, i.e. the thermal tidal instability model. Conclusions. We interpret the long-term light curve of WX Cet as the result of a significantly decreasing mean mass transfer rate. Highlighting the complexity of the observed superhump light curves, we emphasise the importance of WX Cet for a proper understanding of the SU UMa star outburst physics and the evolution of ultra-short period cataclysmic variables.

TV Corvi revisited: Precursor and superhump period derivative linked to the disk instability model

We report optical photometric observations of four superoutbursts of the short-period dwarf nova TV Crv. This object experiences two types of superoutbursts; one with a precursor and the other without. The superhump period and period excess of TV Crv are accurately determined to be 0.065028 +/- 0.000008 d and 0.0342 +/- 0.0021, respectively. This large excess implies a relatively large mass ratio of the binary components (M_2/M_1), though it has a short orbital period. The two types of superoutbursts can be explained by the thermal-tidal instability model for systems having large mass ratios. Our observations reveal that superhump period derivatives are variable in distinct superoutbursts. The variation is apparently related to the presence or absence of a precursor. We propose that the superhump period derivative depends on the maximum disk radius during outbursts. We investigate the relationship of the type of superoutbursts and the superhump period derivative for known sources. In the case of superoutbursts without a precursor, superhump period derivatives tend to be larger than those in precursor-main type superoutbursts, which is consistent with our scenario.

The disk instability model for dwarf nova outbursts

The development and the present state of the disk instability model for outbursts of dwarf nova are reviewed. Two intrinsic instabilities are known in dwarf nova accretion disks, i.e., the thermal instability and the tidal instability. The thermal-tidal instability model (abbreviated the TTI model) that combines these two intrinsic instabilities was first proposed in 1989 by Osaki to explain the superoutburst phenomenon of SU UMa stars. In this paper a complete account of the TTI model is presented. We first explain the basic concept of the model and how it works for the superoutburst phenomenon. We then discuss recent refinements of the model, in particular on the start and the end of superoutbursts, by which we are now able to explain wide varieties in superoutburst light curves of different stars and of different superoutbursts within one and the same star. We also discuss some exceptional cases that apparently seem to contradict the theory, such as the 1985 superoutburst of U Gem itself. It is argued that the TTI model can after all explain most of the observed phenomena related to superoutbursts and superhumps in dwarf novae. (Communicated by Yoshihide KOZAI, M. J. A.)

A Disk Instability Interpretation for the 2001 Outburst of WZ SGE

AbstractThe 2001 outburst of WZ SGE is discussed based on the thermal-tidal instability model (TTI model). It is demonstrated that the overall development of the outburst can be understood by the disk instability model (TTI model) and that no enhanced mass transfer is needed to explain observations.

Is evidence for enhanced mass transfer during dwarf-nova outbursts well substantiated?

Outburst mechanisms of SU UMa-type dwarf novae are discussed. Two competing models were proposed; a pure disk instability model called the thermal-tidal instability model (TTI model) and the enhanced mass transfer model (EMT model). Observational evidence for enhanced mass transfer from the secondary star during outbursts is critically examined. It is demonstrated that most evidence for enhanced mass transfer is not well substantiated. Patterson et al. (2002) have recently claimed to have found evidence for enhanced mass transfer during the 2001 outburst of WZ Sge. We show that their evidence is probably due to a misinterpretation of their observed light curves. Our theoretical analysis also shows that irradiation during outburst should not affect the mass transfer rate. A refinement of the TTI model is proposed that can explain why superhumps appear a few days after the superoutburst maximum in some SU UMa stars. We present our own interpretation of the overall development of the 2001 outburst of WZ Sge based on the TTI model that does not require the assumption of an unproved enhanced mass transfer.

Disk Instability Model for the AM Canum Venaticorum Stars

We examine AM CVn stars, ultra-short-period variables thought to have helium disks, from the stand-point of the disk instability model. By calculating the vertical structure of the helium accretion disks, we have confirmed that the helium accretion disks are thermally unstable, since their thermal-equilibrium curves exhibit the characteristic S-shaped form. We apply these results to AM CVn stars. We have found that the observed large-amplitude photometric variations in AM CVn stars can be explained by the thermal-tidal instability model of helium accretion disks.

Thermal-Tidal Instability Model of Dwarf Novae Below the Period Gap: A Unification Theory

AbstractA unification theory is proposed in which almost all of outburst behavior in dwarf novae may be understood in a unified way within the basic framework of the disk instability model. In particular, a rich variety of outburst behavior of cataclysmic variables below the period gap may be understood by the thermal-tidal instability model in which the coupling of the two intrinsic instabilities in the accretion disk plays a unique role.