Disk Instability Research Articles

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.

Galaxy Zoo: Clump Scout: Surveying the Local Universe for Giant Star-forming Clumps

Massive, star-forming clumps are a common feature of high-redshift star-forming galaxies. How they formed, and why they are so rare at low redshift, remains unclear. In this paper we identify the largest sample yet of clumpy galaxies (7050) at low redshift using data from the citizen science project Galaxy Zoo: Clump Scout, in which volunteers classified 58,550 Sloan Digital Sky Survey (SDSS) galaxies spanning redshift 0.02 < z < 0.15. We apply a robust completeness correction by comparing with simulated clumps identified by the same method. Requiring that the ratio of clump to galaxy flux in the SDSS u band be greater than 8% (similar to clump definitions used by other works), we estimate the fraction of local star-forming galaxies hosting at least one clump (f clumpy) to be . We also compute the same fraction with a less stringent relative flux cut of 3% (), as the higher number count and lower statistical noise of this fraction permit finer comparison with future low-redshift clumpy galaxy studies. Our results reveal a sharp decline in f clumpy over 0 < z < 0.5. The minor merger rate remains roughly constant over the same span, so we suggest that minor mergers are unlikely to be the primary driver of clump formation. Instead, the rate of galaxy turbulence is a better tracer for f clumpy over 0 < z < 1.5 for galaxies of all masses, which supports the idea that clump formation is primarily driven by violent disk instability for all galaxy populations during this period.

Successful nonsurgical management of lumbar radiculopathy associated with disc herniation and instability in low back pain syndrome

Successful nonsurgical management of lumbar radiculopathy associated with disc herniation and instability in low back pain syndrome

Constraints on the origins of hypervelocity stars: velocity distribution, mergers, and star formation history

ABSTRACT In recent years surveys have identified several dozen B stars in the Milky Way halo moving faster than the local escape speed. The origin of most of these hypervelocity stars (HVSs) is still poorly constrained. Here we show that the velocity distribution, and in particular the deficiency in &amp;gt;700 km s−1 HVSs is inconsistent with binary disruptions by the massive black hole (MBH) in the Galactic Centre. This conclusion holds in the full and empty loss-cone regime, and for secular instabilities in eccentric discs. Accounting for multiple close encounters between binaries and the MBH does not qualitatively change the results. Moreover, there is no observed counterpart population in the Galactic Centre that is consistent with the HVSs. The star formation history could be tuned explain the HVS velocity distribution, but this tuning would produce a mismatch with the observed HVS flight times. Frequent stellar collisions of the binary components due to interactions with the MBH do not significantly impact the velocity distribution in the Galactic halo. Such collisions, however, can leave observable remnants in the Galactic Centre, and potentially explain the origins of G2-like dust clouds.

Machine learning algorithm for minute-long burst searches

Minute-long Gravitational Wave (GW) transients are events lasting from few to hundreds of seconds. In opposition to compact binary mergers, their GW signals cover a wide range of poorly understood astrophysical phenomena such as accretion disk instabilities and magnetar flares. The lack of accurate and rapidly generated gravitational-wave emission models prevents the use of matched filtering methods. Such events are thus probed through the template-free excess-power method, consisting in searching for a local excess of power in the time-frequency space correlated between detectors. The problem can be viewed as a search for high-value clustered pixels within an image, which has been generally tackled by deep learning algorithms such as Convolutional Neural Networks (CNNs). In this work, we use a CNN as a anomaly detection tool for the long-duration searches. We show that it can reach a pixel-wise detection despite trained with minimal assumptions, while being able to retrieve both astrophysical signals and noise transients originating from instrumental coupling within the detectors. We also note that our neural network can extrapolate and connect partially disjoint signal tracks in the time-frequency plane.

Images of embedded Jovian planet formation at a wide separation around AB Aurigae

Direct images of protoplanets embedded in disks around infant stars provide the key to understanding the formation of gas giant planets like Jupiter. Using the Subaru Telescope and Hubble Space Telescope, we find evidence for a jovian protoplanet around AB Aurigae orbiting at a wide projected separation (93 au), likely responsible for multiple planet-induced features in the disk. Its emission is reproducible as reprocessed radiation from an embedded protoplanet. We also identify two structures located at 430-580 au that are candidate sites of planet formation. These data reveal planet formation in the embedded phase and a protoplanet discovery at wide, > 50 au separations characteristic of most imaged exoplanets. With at least one clump-like protoplanet and multiple spiral arms, the AB Aur system may also provide the evidence for a long-considered alternative to the canonical model for Jupiter's formation: disk (gravitational) instability.

The Super-Alfvénic Rotational Instability in Accretion Disks about Black Holes

The theory of instability of accretion disks about black holes, neutron stars, or protoplanets is revisited by means of the recent method of the Spectral Web. The cylindrical accretion disk differential equation is shown to be governed by the forward and backward Doppler-shifted continuous Alfvén spectra , where ω A is the static Alfvén frequency. It is crucial to take nonaxisymmetry (m ≠ 0) and super-Alfvénic rotation of the Doppler frames (∣mΩ∣ ≫ ∣ω A∣) into account. The continua and then overlap, ejecting a plethora of super-Alfvénic rotational instabilities (SARIs). In-depth analysis for small inhomogeneity shows that the two Alfvén singularities reduce the extent of the modes to sizes much smaller than the width of the accretion disk. Generalization for large inhomogeneity leads to the completely unprecedented result that, for mode numbers ∣k∣ ≫ ∣m∣, any complex ω in a wide neighborhood of the real axis is an approximate “eigenvalue.” The difference with genuine eigenmodes is that the amount of complementary energy to excite the modes is tiny, ∣W com∣ ≤ c, with c the machine accuracy of the computation. This yields a multitude of two-dimensional continua of quasi-discrete modes: quasi-continuum SARIs. We conjecture that the onset of 3D turbulence in magnetized accretion disks is governed not by the excitation of discrete axisymmetric magnetorotational instabilities but by the excitation of modes from these two-dimensional continua of quasi-discrete nonaxisymmetric SARIs.

Extreme Variation in Star Formation Efficiency across a Compact, Starburst Disk Galaxy

Abstract We report on the internal distribution of star formation efficiency in IRAS 08339+6517 (hereafter IRAS08), using ∼200 pc resolution CO(2 − 1) observations from NOEMA. The molecular gas depletion time changes by 2 orders-of-magnitude from disk-like values in the outer parts to less than 108 yr inside the half-light radius. This translates to a star formation efficiency per freefall time that also changes by 2 orders-of-magnitude, reaching 50%–100%, different than local spiral galaxies and the typical assumption of constant, low star formation efficiencies. Our target is a compact, massive disk galaxy that has a star formation rate 10× above the z = 0 main sequence; Toomre Q ≈ 0.5−0.7 and high gas velocity dispersion (σ mol ≈ 25 km s−1). We find that IRAS08 is similar to other rotating, starburst galaxies from the literature in the resolved Σ SFR ∝ Σ mol N relation. By combining resolved literature studies we find that the distance from the main sequence is a strong indicator of the Kennicutt-Schmidt power-law slope, with slopes of N ≈ 1.6 for starbursts from 100 to 104 M ⊙ pc−2. Our target is consistent with a scenario in which violent disk instabilities drive rapid inflows of gas. It has low values of Toomre-Q, and also at all radii, the inflow timescale of the gas is less than the depletion time, which is consistent with the flat metallicity gradients in IRAS08. We consider these results in light of popular star formation theories; in general observations of IRAS08 find the most tension with theories in which star formation efficiency is a constant. Our results argue for the need of high-spatial-resolution CO observations for a larger number of similar targets.

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.

Clump survival and migration in VDI galaxies: an analytical model versus simulations and observations

ABSTRACT We address the nature of the giant clumps in high-z galaxies that undergo violent disc instability, distinguishing between long-lived and short-lived clumps. We study the evolution of long-lived clumps during migration through the disc via an analytical model tested by simulations and confront theory with CANDELS-HST observations. The clump ‘bathtub’ model, which considers gas and stellar gain and loss, involves four parameters: the accretion efficiency α, the star formation rate (SFR) efficiency ϵd, and the outflow mass-loading factors for gas and stars, η and ηs. The corresponding time-scales are comparable to the migration time, two-three orbital times. The accretion-rate dependence on clump mass, gas, and stars, allows an analytical solution involving exponential growing and decaying modes. For the fiducial parameter values there is a main evolution phase where the SFR and gas mass are constant and the stellar mass is rising linearly with time. This makes the inverse specific SFR an observable proxy for clump age. When η or ϵd are high, or α is low, the decaying mode induces a decline of SFR and gas mass till the migration ends. Later, the masses and SFR approach an hypothetical exponential growth with a constant specific SFR. The model matches simulations with different, moderate feedbacks, both in isolated and cosmological settings. The observed clumps agree with our predictions, indicating that the massive clumps are long-lived and migrating. A challenge is to model feedback that is non-disruptive in massive clumps but suppresses SFR to match the galactic stellar-to-halo mass ratio.

Gravitational instability of gas-dust circumnuclear disks in nearby galaxies

Using the two-dimensional multi-component hydrodynamical simulations, we study the effect of dust on the development of instability in the gas-dust mini-disks observed in the central regions of galaxies. The dust is coupled with the gas gravitationally and by means of a friction force depending on the velocity difference between the two components. The presence of dust leads to the instability of a mini-disk and the development of a spiral structure observed in the gas-dust mini-disks. The spiral structure is multi-armed, which is caused by a dominance of the high order Fourier harmonics in the unstable modes. The instability of gas-dust circumnuclear disks is an important mechanism for explaining the activity of galactic nuclei associated with the accretion of matter onto a central black hole. We find that the admixture of dust with a dust-to-gas ratio of 10-20 % significantly destabilizes the gas-dust disk.

The Rate, Amplitude, and Duration of Outbursts from Class 0 Protostars in Orion

Abstract At least half of a protostar’s mass is accreted in the Class 0 phase, when the central protostar is deeply embedded in a dense, infalling envelope. We present the first systematic search for outbursts from Class 0 protostars in the Orion clouds. Using photometry from Spitzer/IRAC spanning 2004 to 2017, we detect three outbursts from Class 0 protostars with ≥2 mag changes at 3.6 or 4.5 μm. This is comparable to the magnitude change of a known protostellar FU Ori outburst. Two are newly detected bursts from the protostars HOPS 12 and 124. The number of detections implies that Class 0 protostars burst every 438 yr, with a 95% confidence interval of 161 to 1884 yr. Combining Spitzer and WISE/NEOWISE data spanning 2004–2019, we show that the bursts persist for more than nine years with significant variability during each burst. Finally, we use 19–100 μm photometry from SOFIA, Spitzer, and Herschel to measure the amplitudes of the bursts. Based on the burst interval, a duration of 15 yr, and the range of observed amplitudes, 3%–100% of the mass accretion during the Class 0 phase occurs during bursts. In total, we show that bursts from Class 0 protostars are as frequent, or even more frequent, than those from more evolved protostars. This is consistent with bursts being driven by instabilities in disks triggered by rapid mass infall. Furthermore, we find that bursts may be a significant, if not dominant, mode of mass accretion during the Class 0 phase.

Роль черного излучения в модификации критериев неустойчивости Джинса для экзопланетного пылевого плазменного диска при учете магнитной вязкости и лучевого теплообмена

Within the framework of the problem of modeling the evolution of an exoplanetary disk, the influence of blackbody radiation on the Jeans magneto-gravitational instability for a self-gravitating plasma disk is discussed, taking into account the influence on the critical length of the disturbing wave of rotation, magnetic field, dissipative processes of viscosity and thermal conductivity, as well as radiation pressure and radiation heat loss. Using the analysis of the normal mode, the general dispersion relation is derived. On its basis, modified criteria for the Jeans instability were obtained for a number of special cases associated with different relative orientations of the magnetic field, the axis of rotation, and the propagation vector of the disturbance wave. The stability of the environment is discussed using the Hurwitz criterion. Both in the case of longitudinal and transverse directions of propagation of the disturbance wave, the Jeans instability criterion is modified due to the presence of radiation heat losses and Stokes correction. The results obtained are aimed at solving the problem associated with the instability of exoplanetary accretion disks and with the formation of exoplanets.

Джинсовская неустойчивость астрофизической самогравитирующей среды при наличии высокого радиационного давления и диффузионного переноса излучения

Within the framework of the problem of modeling the evolution of a protostellar disk, the influence of radiation on the Jeans gravitational instability for a self-gravitating optically thick (for intrinsic infrared radiation) gas-dust medium is discussed, taking into account the influence of radiation pressure and diffusion transfer of radiation on the critical wavelength of the perturbing wave. Two approximations of radiative diffusion are considered: 1. the case of ideal radiative equilibrium, when the temperatures of matter and radiation are the same; 2. the case of the time dependence of the radiation field, when there is an energy decoupling between radiation and matter. Using the analysis of the normal regime, dispersion relations are derived that allow one to obtain modifications of the Jeans gravitational instability criterion under the influence of radiation pressure and radiation diffusion. In particular, it is shown that, in contrast to local radiation equilibrium, when the acoustic velocity of the gas coincides with the isothermal speed of sound, in the case of a difference in the temperatures of radiation and gas, the perturbing wave propagates with the adiabatic speed of sound in the gas. The results obtained are aimed at solving the problem of gravitational instability of individual massive protostellar disks or self-gravitating radiation media characterized by large optical depths for their own infrared radiation transformed by dust.

Finite-resolution Deconvolution of Multiwavelength Imaging of 20,000 Galaxies in the COSMOS Field: The Evolution of Clumpy Galaxies over Cosmic Time

Abstract Compact star-forming clumps observed in distant galaxies are often suggested to play a crucial role in galaxy assembly. In this paper, we use a novel approach of applying finite-resolution deconvolution on ground-based images of the COSMOS field to resolve 20,185 star-forming galaxies (SFGs) at 0.5 &lt; z &lt; 2 to an angular resolution of 0.″3 and study their clump fractions. A comparison between the deconvolved images and HST images across four different filters shows good agreement and validates image deconvolution. We model spectral energy distributions using the deconvolved 14-band images to provide resolved surface brightness and stellar-mass density maps for these galaxies. We find that the fraction of clumpy galaxies decreases with increasing stellar masses and with increasing redshift: from ∼30% at z ∼ 0.7 to ∼50% at z ∼ 1.7. Using abundance matching, we also trace the progenitors for galaxies at z ∼ 0.7 and measure the fractional mass contribution of clumps toward their total mass budget. Clumps are observed to have a higher fractional mass contribution toward galaxies at higher redshift: increasing from ∼1% at z ∼ 0.7 to ∼5% at z ∼ 1.7. Finally, the majority of clumpy SFGs have higher specific star formation rates (sSFR) compared to the average SFGs at fixed stellar mass. We discuss the implication of this result for in situ clump formation due to disk instability.

Intrinsic properties of the bars formed by the bar instability in flat stellar discs

ABSTRACT The properties of the bars formed by the bar instability are examined for flat stellar discs. The initial mass models chosen are Kuzmin–Toomre discs, for which two types of exact equilibrium distribution function (DF) are employed in order to realize different distributions of Toomre’s Q values along the radius. First, the most linearly unstable, global two-armed modes (MLUGTAMs) of these disc models are determined by numerically solving the linearized collisionless Boltzmann equation. Next, we carry out N-body simulations whose models are constructed from the DFs adopted above. The latter simulations unravel that the MLUGTAMs corresponding to those obtained from the former modal calculations are excited in the early phases of evolution, finally being deformed into bars in the non-linear regime by the bar instability. We show that for simulated bars, the length increases and the axial ratio, in essence, decreases as the amplitude increases. These correlations are almost similar to those of the observed bars. In addition, we find that these bar properties are tightly correlated with the initial typical Q value, irrespective of the DF. In conclusion, a disc with a smaller typical Q value produces a bar which is smaller in amplitude, shorter in length, and rounder in shape. This finding might suggest that the Hubble sequence for barred galaxies is the sequence of decreasing Q from SBa to SBc or SBd. The implied correlations between the initial typical Q value and each of the bar properties are discussed on the basis of the characteristics of the MLUGTAMs.

Flux-limited Diffusion Approximation Models of Giant Planet Formation by Disk Instability. II. Quadrupled Spatial Resolution

Abstract While collisional accumulation is nearly universally accepted as the formation mechanism of rock and ice worlds, the situation regarding gas giant planet formation is more nuanced. Gas accretion by solid cores formed by collisional accumulation is the generally favored mechanism, but observations increasingly suggest that gas disk gravitational instability might explain the formation of at least the massive or wide-orbit gas giant exoplanets. This paper continues a series aimed at refining three-dimensional (3D) hydrodynamical models of disk instabilities, where the handling of the gas thermodynamics is a crucial factor. Boss (2017, 2021) used the β cooling approximation to calculate 3D models of disks with initial masses of 0.091 M ⊙ extending from 4 to 20 au around 1 M ⊙ protostars. Here we employ 3D flux-limited diffusion (FLD) approximation models of the same disks, in order to provide a superior treatment of disk gas thermodynamics. The new models have quadrupled spatial resolution compared to previous 3D FLD models, in both the radial and azimuthal spherical coordinates, resulting in the highest spatial resolution 3D FLD models to date. The new models continue to support the hypothesis that such disks can form self-gravitating, dense clumps capable of contracting to form gas giant protoplanets, and suggest that the FLD models yield similar numbers of clumps as β cooling models with β ∼ 1 to ∼10, including the critical value of β = 3 for fragmentation proposed by Gammie.