Disk Instability Model Research Articles

A New IW And-type Star: Karachurin 12 with Tilted Disks and Diverse Cycles

The IW And-type phenomenon in cataclysmic variables presents a significant challenge to the accretion disk instability model. Using photometric data from the All-Sky Automated Survey for Supernovae, the Zwicky Transient Facility, and the Transiting Exoplanet Survey Satellite, we identify Karachurin 12 as a new noneclipsing IW And-type object with a cycle period of 35.69(3) days. We also report for the first time that Karachurin 12 is a negative superhump (NSH) system featuring precessing tilted disks, with precession, orbital, and NSH periods of 4.9588(2) days, 0.3168895(13) days, and 0.2979861(8) days, respectively. Our analysis, using dips as the index and NSHs as the probe, reveals diverse cycle patterns in Karachurin 12, with NSH amplitude varying throughout the cycle. These findings offer new insights for studying tilted disks and the IW And-type phenomenon. The mass-transfer burst model has difficulty explaining the observed variations in NSH amplitude, especially given the uncertainty surrounding the origin of the mass-transfer burst. Meanwhile, the tilted thermally unstable disk model indicates a possible connection to the IW And-type phenomenon, but it also struggles to account for the detailed variations in Karachurin 12. Therefore, a wider range of factors must be considered to fully understand the complex changes in Karachurin 12.

The outcome of collisions between gaseous clumps formed by disk instability

The disk instability model is a promising pathway for giant planet formation in various conditions. At the moment, population synthesis models are used to investigate the outcomes of this theory, where a key ingredient of the disk population evolution are collisions of self-gravitating clumps formed by the disk instabilities. In this study, we explored the wide range of dynamics between the colliding clumps by performing state-of-the-art smoothed particle hydrodynamics simulations with a hydrogen-helium mixture equation of state and investigated the parameter space of collisions between clumps of different ages, masses (1--10 Jupiter mass), various impact conditions (head-on to oblique collisions) and a range of relative velocities. We find that the perfect merger assumption used in population synthesis models is rarely satisfied and that the outcomes of most of the collisions lead to erosion, disruption or a hit-and-run. We also show that in some cases collisions can initiate the dynamical collapse of the clump. We conclude that population synthesis models should abandon the simplifying assumption of perfect merging. Relaxing this assumption will significantly affect the inferred population of planets resulting from the disk instability model.

Instability of Circumnuclear Gas Supply as an Origin of the “Changing-look” Phenomenon of Supermassive Black Holes

The origin of the “changing-look” (CL) phenomenon in supermassive black holes (SMBHs) remains an open issue. This study aims to shed light on this phenomenon by focusing on a sample that encompasses all known repeating CL active galactic nuclei (AGNs). Through the identification of a characteristic timescale for the CL phenomenon, it was observed that larger SMBHs possess shorter characteristic timescales, while smaller SMBHs exhibit longer timescales. These findings reveal a significant contrast to the traditional AGN variability that has been adequately explained by the AGN’s disk instability model. This stark discrepancy highlights a distinct origin of the CL phenomenon, distinguishing it from traditional AGN variability. By properly predicting the characteristic timescale and its dependence on SMBH mass, we propose that the CL phenomenon is likely a result of a variation in accretion rate caused by a sudden change in the supply of circumnuclear gas during the transition between active and passive SMBH fueling stages.

Study of type II migration under the framework of the disk instability model for giant planet formation

Context. Hydrodynamic simulations of the migration of planets formed by gravitational instability suggest that after an initial phase of fast migration, planets can open gaps and continue to migrate on a type II migration timescale. The simulation time length is typically on the order of 104 yr. Aims. We study the effects of the subsequent type II migration during the disk lifetime on the final orbital radii of planets. Methods. We used a numerical disk model that follows the disk formation and evolution. The disk acquires mass through the mass influx from the collapse of its parent molecular cloud core. The model reflects the influence of the properties of the parent core on the disk. Considering clumps forming at different times in a disk and also in different disks with different parent core properties, we used the type II migration rate to follow the clump migration from the formation location. We studied the dependence of the clump migration on the properties of the parent core. Results. The mass influx drag enhances the migration process. The duration and viscosity of gravitational instability, viscosity in the dead zone, and the collapse time of the parent core play important roles in planet migration. As the angular momentum and mass of the parent core increase, migration is enhanced. The final radius is sensitive to the initial radius. Clumps forming at large radii might migrate outward with the disk expansion. Conclusions. Even though type II migration is slow, clumps can migrate over significant distances. A considerable proportion of clumps migrate to the central protostar via type II migration. Our calculations support the idea that the observed pile-up of planets at <0.3 AU is explained by a scenario where planets might form at large radii, then migrate to orbits of <0.3 AU, and halt by a stopping mechanism at this location.

Discovery of a new IW And-type dwarf nova with both tilted disc and tidal instability

ABSTRACT IW And-type dwarf novae are anomalous Z Cam stars featured with outbursts happening during standstill states, which are not expected in the standard disc instability model. The physical mechanisms for these variations remain unclear. In this study, we report the discovery of a new candidate IW And-type dwarf nova J0652+2436, identified with its frequent outbursts from the slowly rising standstill states. Luckily, the Transiting Exoplanet Survey Satellite observations during a long standstill state and the earlier K2 observations give a chance to find the orbital and negative superhump period in the light curve of J0652+2436, allowing the measurement of its mass ratio of 0.366. This mass ratio is marginally possible for the tidal instability to set in according to previous smoothed particle hydrodynamics simulations. Thus, we propose that the outbursts in J0652+2436 are likely to be caused by the growing accretion disc during standstills, in favour of the previous hypothesis of the mechanisms lying in all IW And stars. We conclude that J0652+2436 might be the first IW And star with both a precessing tilted disc and tidal instability, which will be an important laboratory for studying the accretion disc dynamics and help understand the IW And phenomenon.

Cataclysmic variables and the disc instability model in the Gaia DR3 colour–magnitude diagram

Context. Cataclysmic variables (CVs) are semi-detached binaries composed of a white dwarf orbiting a lower-mass K or M star. Aims. We investigate whether CVs are responsible for a new intriguing feature (the “hook”) that appears in the Gaia DR3 colour–magnitude Hertzsprung-Russell diagram (HRD) when selecting sources with low extinction. We also aim to understand the location of CVs in the HRD based on the predictions of the disc instability model (DIM). The DIM forms the foundation of our basic understanding of stable (novae-like) and outbursting CVs (dwarf novae). Methods. We calculated the expected behaviour of CVs in the Gaia HRD while taking into account the variable light contributed by the accretion disc, the companion, the white dwarf, and the bright spot where the Roche lobe overflow stream from the companion intersects the disc. Results. We find that the hook feature is most likely composed of CVs. The hook corresponds to the limited region where stable (novae-like) CVs must be located in the HRD according to the DIM. Unstable systems giving rise to dwarf novae outbursts trace counterclockwise loops in the HRD. The overall behaviour is consistent with the location of the various CV subtypes in the HRD. Conclusions. These results can be used as a basis on which to pinpoint interesting outliers in the HRD, either due to their location or their tracks. These outliers could signal new subtypes, such as cold, stable CVs with truncated discs, or may challenge the disc instability model.

Mini‐outburst of black hole X‐ray binary GX 339‐4 in 2004

AbstractThe mini‐outburst of black hole X‐ray binaries (BHXBs) is a re‐bright event of light curve, which is often detected in BHXBs with the low orbital period, such as XTE J1650‐500 (), GRO J0422+32 () and Swift J1753.5‐0127 (). Because of its low X‐ray luminosity, the physics properties of mini‐outburst is still unclear. GX 339‐4 is a famous BHXBs with the orbital period and present many normal outburst and failed outburst over the past decades. However, the mini‐outburst is not still detected in GX 339‐4. In this work, using the Rossi X‐ray Timing Explorer (RXTE) observations, we investigated the energy spectrum properties of the GX 339‐4 outburst in 2004 and confirmed that the outburst in 2004 is a mini‐outburst. It is very interest that the mini‐outburst has been through a whole state transition from Low/Hard state to High/Soft state. We also give a comparison between the normal outburst in 2002 and the mini‐outburst in 2004. And we find that the two outburst present a similar Hardness‐Intensity diagram and spectral properties, which suggest that the normal outburst and mini‐outburst have a similar physics. We think that the mini‐outburst of GX 339‐4 in 2004 can be well explained by the disk instability model.

Nine New Cataclysmic Variable Stars with Negative Superhumps

Negative superhumps (NSHs) are signals a few percent shorter than the orbital period of a binary star and are considered to originate from the reverse precession of the tilted disk. Based on TESS photometry, we find nine new cataclysmic variable stars with NSHs. Three (ASAS J1420, TZ Per, and V392 Hya) of these stars similar to AH Her still have NSHs during dwarf nova outbursts, and the NSH amplitude varies with the outburst. The variation in the radius of the accretion disk partially explains this phenomenon. However, it does not explain the rebound of the NSH amplitude after the peak of the outburst and the fact that the NSH amplitude of the quiescence is sometimes not the largest, and it is necessary to include the disk instability model (DIM) and add other ingredients. Therefore, we suggest that the variation of NSH amplitude with outburst can be an important basis for studying the origin of NSHs and improving the DIM. The six (ASASSN-V J1137, ASASSN-V J0611, 2MASS J0715, LAMOST J0925, ASASSN-17qj, and ZTF 18acakuxo) remaining stars have been poorly studied, and for the first time we determine their orbital periods, NSHs, and superorbital signal (SOR) periods. The NSH periods and amplitudes of ASASSN-V J1137 and ASASSN-17qj vary with the SOR, and based on the comparison of the observations with the theory, we suggest that a single change in tilted disk angle does not explain the observations of the SOR and that other ingredients need to be considered as well.

Typical X-Ray Outburst Light Curves of Aql X-1

We show that the typical X-ray outburst light curve of Aql X-1 can be reproduced by accretion onto a neutron star in the frame of the disk instability model without invoking partial accretion or the propeller effect. The knee and the subsequent sharp decay in the X-ray light curve can be generated naturally by taking into account the weak dependence of the disk aspect ratio, h/r, on the disk mass-flow rate, Ṁin , in the X-ray irradiation flux calculation. This Ṁin dependence of h/r only slightly modifies the irradiation temperature profile along the hot disk in comparison to that obtained with a constant h/r. Nevertheless, this small difference has a significant cumulative effect on the hot disk radius, leading to a much faster decrease in the size of the hot disk and thereby to a sharper decay in the X-ray outburst light curve. The same model also produces the long-term evolution of Aql X-1, consistent with its observed outburst recurrence time and typical light curve. Our results imply that the source accretes matter from the disk in the quiescent state as well. We also estimate that the dipole moment of the source μ ≲ 4 × 1026 G cm3 (B ≲ 4 × 108 G at the surface).

Investigating the Lower Mass Gap with Low-mass X-Ray Binary Population Synthesis

Mass measurements from low-mass black hole X-ray binaries (LMXBs) and radio pulsars have been used to identify a gap between the most massive neutron stars (NSs) and the least massive black holes (BHs). BH mass measurements in LMXBs are typically only possible for transient systems: outburst periods enable detection via all-sky X-ray monitors, while quiescent periods enable radial velocity measurements of the low-mass donor. We quantitatively study selection biases due to the requirement of transient behavior for BH mass measurements. Using rapid population synthesis simulations (COSMIC), detailed binary stellar-evolution models (MESA), and the disk instability model of transient behavior, we demonstrate that transient LMXB selection effects introduce observational biases, and can suppress mass-gap BHs in the observed sample. However, we find a population of transient LMXBs with mass-gap BHs form through accretion-induced collapse of an NS during the LMXB phase, which is inconsistent with observations. These results are robust against variations of binary evolution prescriptions. The significance of this accretion-induced collapse population depends upon the maximum NS birth mass . To reflect the observed dearth of low-mass BHs, COSMIC and MESA models favor . In the absence of further observational biases against LMXBs with mass-gap BHs, our results indicate the need for additional physics connected to the modeling of LMXB formation and evolution.

On the survivability of a population of gas giant planets on wide orbits

ABSTRACT The existence of giant planets on wide orbits ($\stackrel{\gt }{_\sim }100\rm ~au$) challenge planet formation theories; the core accretion scenario has difficulty in forming them, whereas the disc instability model forms an overabundance of them that is not seen observations. We perform N-body simulations investigating the effect of close stellar encounters (≤1200 au) on systems hosting wide-orbit giant planets and the extent at which such interactions may disrupt the initial wide-orbit planet population. We find that the effect of an interaction on the orbit of a planet is stronger for high-mass, low-velocity perturbers, as expected. We find that due to just a single encounter there is a $\sim 17~{{\ \rm per\ cent}}$ chance that the wide-orbit giant planet is liberated in the field, a $\sim 10~{{\ \rm per\ cent}}$ chance it is scattered significantly outwards, and a $\sim 6~{{\ \rm per\ cent}}$ chance it is significantly scattered inwards. Moreover, there is a $\sim 21~{{\ \rm per\ cent}}$ chance that its eccentricity is excited to e > 0.1, making it more prone to disruption in subsequent encounters. The results strongly suggest that the effect of even a single stellar encounter is significant in disrupting the primordial wide-orbit giant planet population; in reality the effect will be even more prominent, as in a young star-forming region more such interactions are expected to occur. We conclude that the low occurrence rate of wide-orbit planets revealed by observational surveys does not exclude the possibility that such planetary systems are initially abundant, and therefore the disc–instability model may be a plausible scenario for their formation.

A New Sample of Transient Ultraluminous X-Ray Sources Serendipitously Discovered by Swift/XRT

Ultraluminous X-ray sources (ULXs) are our best laboratories for studying extreme super-Eddington accretion. Most studies of these objects are of relatively persistent sources; however, there is growing evidence to suggest a large fraction of these sources are transient. Here we present a sample of five newly reported transient ULXs in the galaxies NGC 4945, NGC 7793, and M81 serendipitously discovered in Swift/XRT observations. Swift monitoring of these sources have provided well-sampled lightcurves, allowing for us to model the lightcurves with the disk-instability model of Hameury & Lasota, which implies durations of 60–400 days and that the mass-accretion rate through the disk is close to or greater than the Eddington rate. Of the three source regions with prior Hubble Space Telescope imaging, color–magnitude diagrams of the potential stellar counterparts show varying ages of the possible stellar counterparts. Our estimation of the rates of these sources in these three galaxies is 0.4–1.3 yr−1. We find that, while persistent ULXs dominate the high end of galaxy luminosity functions, the number of systems that produce ULX luminosities are likely dominated by transient sources.

The mass of TOI-519 b: A close-in giant planet transiting a metal-rich mid-M dwarf

Abstract We report on the determination of the mass of TOI-519 b, a transiting substellar object around a mid-M dwarf. We carried out radial velocity measurements using Subaru/InfraRed Doppler (IRD), revealing that TOI-519 b is a planet with a mass of $0.463^{+0.082}_{-0.088}\, M_{\rm Jup}$. We also found that the host star is metal rich ([Fe/H] = 0.27 ± 0.09 dex) and has the lowest effective temperature (Teff = 3322 ± 49 K) among all stars hosting known close-in giant planets based on the IRD spectra and mid-resolution infrared spectra obtained with NASA Infrared Telescope Facility/SpeX. The core mass of TOI-519 b inferred from a thermal evolution model ranges from 0 to ∼30 M⊕, which can be explained by both core accretion and disk instability models as the formation origins of this planet. However, TOI-519 is in line with the emerging trend that M dwarfs with close-in giant planets tend to have high metallicity, which may indicate that they formed in the core accretion model. The system is also consistent with the potential trend that close-in giant planets around M dwarfs tend to be less massive than those around FGK dwarfs.

A dynamical mass for GJ 463 b: A massive super-Jupiter companion beyond the snow line of a nearby M dwarf

We determined the full orbital architecture and true mass of the recently Doppler-detected long-period giant planet GJ 463 b using the HIPPARCOS-Gaia proper motion anomaly in combination with the available radial velocities, constraints from the knowledge of the spectroscopic orbital parameters, and supplementary information from a sensitivity analysis of Gaia Data Release 3 astrometry. We determined an orbital inclination ib = 152−3+2 deg (for a prograde orbit) and a mass ratio q = 0.0070 ± 0.0007, corresponding to a true mass of the companion Mb = 3.6 ± 0.4 MJup. True mass determinations for a super-Jupiter companion at intermediate orbital separations beyond the snow line around low-mass stars (M⋆ ≤ 0.5 M⊙) are a rare occurrence. Its existence is possibly explained in the context of disk-instability models of planet formation.

Relating peak optical luminosity and orbital period of stellar-mass black holes in X-ray binaries

ABSTRACT We compare the peak optical luminosity with the orbital period for a sample of 22 stellar-mass black hole candidates with good measurements of both quantities. We find that the peak absolute magnitude for the outbursts follows a linear relation with MV, peak = 3.48(± 0.85) − 3.89(± 0.91) logPorb, which corresponds to a $L_V \propto P_{orb}^{1.56\pm 0.36}$ power-law relation. Excluding V4641 Sgr which is a strong outlier and not likely to have outbursts produced by the standard disc instability model, in addition to BW Cir and V821 Ara – both of which have highly uncertain distances; the new correlation for the 19 sources is found to be MV, peak = 3.01 (± 0.93) − 3.21 (± 1.04) logPorb, which corresponds to $L_V \propto P_{orb}^{1.28 \pm 0.42}$. This is an analogous relationship to the ‘Warner relation’ between orbital period and peak luminosity found for cataclysmic variables. We discuss the implications of these results for finding black hole X-ray binaries in other galaxies and in our own Galaxy with the Large Synoptic Survey Telescope and other future large time domain surveys.

A Misfired Outburst in the Neutron Star X-Ray Binary Centaurus X-4

We report on a long-term optical monitoring of the neutron star X-ray binary Centaurus X-4 performed during the last 13.5 yr. This source has been in quiescence since its outburst in 1979. Our monitoring reveals the overall evolution of the accretion disk; we detect short-duration flares, likely originating also in the disk, superimposed with a small-amplitude (<0.1 mag) ellipsoidal modulation from the companion star due to geometrical effects. A long-term (∼2300 days) downward trend, followed by a shorter (∼1000 days) upward one, is observed in the disk light curve. Such a rise in the optical has been observed for other X-ray binaries preceding outbursts, as predicted by the disk instability model. For Cen X-4, the rise of the optical flux proceeded for ∼3 yr, and culminated in a flux increase at all wavelengths (optical–UV–X-rays) at the end of 2020. This increase faded after ∼2 weeks, without giving rise to a full outburst. We suggest that the propagation of an inside-out heating front was ignited due to a partial ionization of hydrogen in the inner disk. The propagation might have stalled soon after the ignition due to the increasing surface density in the disk that the front encountered while propagating outward. The stall was likely eased by the low-level irradiation of the outer regions of the large accretion disk, as shown by the slope of the optical/X-ray correlation, suggesting that irradiation does not play a strong role in the optical, compared to other sources of emission.

A Disk Instability Model for the Quasi-periodic Eruptions of GSN 069

Abstract GSN 069 is a recently discovered quasi-periodic eruption (QPE) source recurring about every 9 hr. The mechanism for the QPEs of GSN 069 is still unclear. In this work, a disk instability model is constructed to explain GSN 069 based on Pan et al. (PLC21), where the authors proposed a toy model for the repeating changing-look active galactic nuclei. We improve the work of PLC21 by including a nonzero viscous torque condition on the inner boundary of the disk and adopting a general form for the viscous stress torque in the Kerr metric. It is found that the 0.4–2 keV light curves, the light curves at different energy bands, and the phase-resolved X-ray spectrum of GSN 069 can all be qualitatively reproduced by our model. Furthermore, the profiles of light curves in QPEs can be significantly changed by the parameter μ in the viscous torque equation, which implies that our model may also be applied to other QPEs.

Plateaus, dips, and rebrightenings during the outbursts of WZ Sge: no magnetic propeller, but a veiling curtain

ABSTRACT WZ Sge is the prototype of highly evolved, low-accretion rate dwarf novae (DNe). During the decline from eruptions, its light curve displays a ‘dip’ followed by ≃10 ‘echo outbursts’. The standard disc instability model does not account for this behaviour, which is also seen in other low-accretion rate DNe. One recent interpretation for these rapid brightness changes is that they represent transitions into and out of a magnetic propeller regime. Here, we test this scenario with time-resolved, ultraviolet spectroscopy taken with the Hubble Space Telescope just before, during and after the dip in WZ Sge’s 2001 eruption. We find no distinctive or unique signatures that could be attributed to a propeller in either the time-averaged UV spectrum or the variability spectrum. Thus the data do not support the magnetic propeller scenario. Instead of resolving the mystery of WZ Sge’s outburst light curve, our study has actually added another: the origin of the narrow absorption features seen in all outburst phases. We show explicitly that these features are likely formed in a high-density ‘veiling curtain’ with a characteristic temperature $\rm T \simeq 17,000~\mathrm{K}$. However, the nature and origin of this veil are unclear. Given that WZ Sge-type DNe are the most intrinsically common class of accreting white dwarfs, resolving these questions should be a high priority.

Challenging the Disk Instability Model. I. The Case of YZ LMi

Abstract Observations of YZ LMi show enhanced emission along the stream trajectory beyond impact at the disk rim during outbursts as well as when the quiescent disk is large. We investigated whether these features can be explained in terms of either gas stream overflow or penetration within the frameworks of the disk instability model (DIM) and the mass-transfer instability model (MTIM) of outbursting disks. Gas stream overflow is not possible because the vertical scale height of the stream is significantly lower than that of the outer disk and because there is no combination of parameters which enables stream overflow on a larger disk while preventing it on a smaller disk. Stream penetration requires the gas stream to be denser than the outer disk regions. This requirement cannot be met by a low-viscosity DIM disk because its density is significantly larger than that of the gas stream over the whole range of mass-transfer rates where the thermal-viscous instability occurs. On the other hand, the high-viscosity MTIM disk has much lower densities which decrease with increasing radius, easily allowing for gas stream penetration during outbursts (when mass-transfer rate and stream density increase) as well as in large quiescent disks. The observed features are not consistent with DIM, but can be plausibly explained by MTIM. These results suggest that the outbursts of YZ LMi are the response of a high-viscosity disk to bursts of enhanced mass-transfer rate. In this case, the outburst decline timescale of (2–3) days implies a viscosity parameter in the range α = 3–4.

The Fast Evolving, Tremendous Blue Superoutburst in ASASSN-21au Reveals a Dichotomy in the Outbursts of Long-period AM CVns

Abstract ASASSN-21au is an ultracompact accreting white dwarf binary (AM CVn type) with a period of ∼58 minutes. Using multiwavelength observations of the system, we discovered a dichotomy in the behavior of outbursts in AM CVns. The binary showed an initial increase in brightness that lasted for at least 82 days, followed by an additional increase that lasted two weeks. Afterward, ASASSN-21au went into superoutburst with a total duration of 19 days, showing an amplitude with respect to quiescence of ∼7.5 mag in g, with a precursor and an echo outburst. A correlation between X-rays, UV, and optical was identified for the first time in an AM CVn during this stage. The color evolution of ASASSN-21au indicates that during the superoutburst the dominant component was the accretion disk. The short duration, large amplitude, and color evolution of the superoutburst agree with expectations from the disk instability model. These characteristics are opposite to those observed in SDSS J080710+485259 and SDSS J113732+405458, which have periods of ∼53 minutes and ∼60 minutes, respectively. The initially slow increase in brightness in the light curve of ASASSN-21au and the behavior after the superoutburst favor a scenario in which changes in the mass-transfer rate led to disk instabilities, while the outburst mechanism of SDSS J080710+485259 and SDSS J113732+405458 has been attributed to enhanced mass transfer alone. Further observations are needed to understand the origin of this dichotomy.