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Irradiation from the central star controls the temperature structure in protoplanetary discs. Yet simulations of gravitational instability typically use models of stellar irradiation with varying complexity, or ignore it altogether, assuming heat generated by spiral shocks is balanced by cooling, leading to a self-regulated state. In this paper, we perform simulations of irradiated, gravitationally unstable protoplanetary discs using 3D hydrodynamics coupled with live Monte-Carlo radiative transfer. We find that the resulting temperature profile is approximately constant in time, since the thermal effects of the star dominate. Hence, the disc cannot regulate gravitational instabilities by adjusting the temperatures in the disc. In a |$0.1M_\odot$| disc, the disc instead adjusts by angular momentum transport induced by the spiral arms, leading to steadily decreasing surface density, and hence quenching of the instability. Thus, strong spiral arms caused by self-gravity would not persist for longer than ten thousand years in the absence of fresh infall, although weak spiral structures remain present over longer time-scales. Using synthetic images at 1.3 mm, we find that spirals formed in irradiated discs are challenging to detect. In higher mass discs, we find that fragmentation is likely because the dominant stellar irradiation overwhelms the stabilizing influence of |$P\mathrm{d}V$| work and shock heating in the spiral arms. [ABSTRACT FROM AUTHOR]

Settling‐driven gravitational instabilities (SDGIs) can form at the base of buoyant particle‐laden suspensions, modulating particle sedimentation in various settings such as meteorological and volcanic clouds, fluvial plumes, magma chambers, submarine hydrothermal plumes, or industrial emissions. These instabilities result in the formation of rapidly descending currents called 'fingers' within which fine particles settle faster collectively than individually. This study investigates SDGI triggering conditions underneath volcanic ash clouds through analogue experiments considering sedimentation from aqueous particle suspensions. We confirm that the conditions for which SDGIs develop are controlled by two dimensionless numbers: Bf (ratio of the characteristic finger velocity to the individual particle settling velocity); and Bi (ratio of timescale for individual particle settling to that for collective settling controlled by inertial drag). SDGIs are triggered for values of Bf and Bi > 1 for which particles are fully coupled with the flow within fingers. Using these parameters, we produce a regime diagram for the 2010 eruption of Eyjafjallajökull (Iceland) that describes particle settling as a function of particle concentration and size. More studies are needed to produce a general regime diagram accounting for the evolution of SDGIs properties with eruption and atmospheric parameters. Nonetheless, our study confirms that fingers affect sedimentation from volcanic clouds with high ash volume fractions above 10−6 vol.%. Our validation of criteria predicting the onset of fingers due to SDGIs constitutes a step forward toward the incorporation of these collective settling processes in volcanic ash transport and dispersion models. Plain Language Summary: Fine particles can fall more rapidly than individually when settling collectively within vertical currents that are usually termed 'fingers'. Fingers have been observed to form in different natural environments, such as the base of volcanic ash clouds, where they can result from the destabilization of the cloud base and affect the deposition of volcanic ash to the ground. We conducted laboratory experiments simulating particle settling in water to understand the conditions that trigger instabilities and the formation of fingers. We confirm that high particle concentrations of fine particles promote the occurrence of instabilities. In general, instabilities occur when particles remain coupled with the fluid, which is the case when the ratio of the finger velocity to the individual particle velocity is greater than one, as parameterized by two dimensionless numbers. Using these parameters, we created a regime diagram describing particle settling during the 2010 Eyjafjallajökull eruption. More research is needed to create a comprehensive diagram considering other factors like eruption parameters. Key Points: Settling‐driven gravitational instabilities can produce fingers under buoyant particle suspensions, impacting particle sedimentationConditions for the onset of instabilities in laboratory experiments are predicted by ratios of the terminal settling and finger velocitiesRegime diagrams predicting the formation of instabilities below volcanic ash clouds can be obtained [ABSTRACT FROM AUTHOR]

Astrophysical accretion discs that carry a significant mass compared with their central object are subject to the effect of self‐gravity. In the context of circumstellar discs, this can, for instance, cause fragmentation of the disc gas, and—under suitable conditions—lead to the direct formation of gas‐giant planets. If one wants to study these phenomena, the disc's gravitational potential needs to be obtained by solving the Poisson equation. This requires to specify suitable boundary conditions. In the case of a spherical‐polar computational mesh, a standard multipole expansion for obtaining boundary values is not practicable. We hence compare two alternative methods for overcoming this limitation. The first method is based on a known Green's function expansion (termed "CCGF") of the potential, while the second (termed "James' method") uses a surface screening mass approach with a suitable discrete Green's function. We demonstrate second‐order convergence for both methods and test the weak scaling behavior when using thousands of computational cores. Overall, James' method is found superior owing to its favorable algorithmic complexity of ∼On3$$ \sim \mathcal{O}\left({n}^3\right) $$ compared with the ∼On4$$ \sim \mathcal{O}\left({n}^4\right) $$ scaling of the CCGF method. [ABSTRACT FROM AUTHOR]

We measure the ionized gas velocity dispersions of star-forming galaxies in the MAGPI survey (⁠|$z\sim 0.3$|⁠) and compare them with galaxies in the SAMI (⁠|$z\sim 0.05$|⁠) and KROSS (⁠|$z\sim 1$|⁠) surveys to investigate how the ionized gas velocity dispersion evolves. For the first time, we use a consistent method that forward models galaxy kinematics from |$z=0$| to |$z=1$|⁠. This method accounts for spatial substructure in emission line flux and beam smearing. We investigate the correlation between gas velocity dispersion and galaxy properties to understand the mechanisms that drive gas turbulence. We find that in both MAGPI and SAMI galaxies, the gas velocity dispersion more strongly correlates with the star-formation rate surface density (⁠|$\Sigma _{\rm SFR}$|⁠) than with a variety of other physical properties, and the average gas velocity dispersion is similar, at the same |$\Sigma _{\rm SFR}$|⁠ , for SAMI, MAGPI, and KROSS galaxies. The results indicate that mechanisms related to |$\Sigma _{\rm SFR}$| could be the dominant driver of gas turbulence from |$z\sim 1$| to |$z\sim 0$|⁠ , for example, stellar feedback and/or gravitational instability. The gas velocity dispersion of MAGPI galaxies is also correlated with the non-rotational motion of the gas, illustrating that in addition to star-formation feedback, gas transportation and accretion may also contribute to the gas velocity dispersion for galaxies at |$z\sim 0.3$|⁠. KROSS galaxies only have a moderate correlation between gas velocity dispersion and |$\Sigma _{\rm SFR}$| and a higher scatter of gas velocity dispersion with respect to |$\Sigma _{\rm SFR}$|⁠ , in agreement with the suggestion that other mechanisms, such as gas transportation and accretion, are relatively more important at higher redshift galaxies. [ABSTRACT FROM AUTHOR]

The hydrodynamic instability of a magnetized, self-gravitating rotating anisotropic plasma is analyzed in the collisionless approximation and considering the heat flux vector based on the modified Chu–Goldberger–Low equations. A dispersion relation is obtained, on the basis of which simplified cases of propagation of low-amplitude disturbance waves and the derivation of modified criteria for hydrodynamic instability are discussed. Using the obtained dispersion relation, three simple cases are treated when the disturbance wave propagates across, along, and obliquely to the magnetic field vector. It is shown that the anisotropy of pressure and heat flow not only changes the classical criterion of the Jeans instability, but also leads to the appearance of new wave modes and causes the appearance of new unstable domains. It has been found that the presence of uniform rotation of the plasma reduces the critical wave number and has a stabilizing effect on the criterion of gravitational instability when the disturbance wave propagates transversely, without having an effect in the case of longitudinal propagation. These results are important for developing evolutionary magnetohydrodynamic models of astrophysical collisionless plasma. [ABSTRACT FROM AUTHOR]

Many circumbinary gas giant planets have been recently discovered. The formation mechanism of circumbinary planets on wide orbits is unclear. We investigate the formation of Delorme 1 (AB)b, a |$13 \pm 5 \ \mathrm{ M_J}$| planet, orbiting its host binary at 84 au. The planet is accreting while having an estimated age of 40 Myr, which is unexpected, as this process should have ceased due to the dissipation of the protoplanetary disc. Using the smoothed particle hydrodynamics code seren , we model three formation scenarios for this planet. In Scenario I, the planet forms in situ on a wide orbit in a massive disc (by gravitational instability), in Scenario II closer to the binary in a massive disc (by gravitational instability), and in Scenario III much closer to the binary in a less massive disc (by core accretion). Planets in Scenario I stay at the observed separation and have mass accretion rates consistent with observed value, but their final mass is too high. In Scenario II, the planet reaches the observed separation through outward migration or scattering by the binary, and has mass accretion rate comparable to the observed; however, the planet mass is above the observed value. In Scenario III, the planet's final mass and mass accretion rate are comparable to the observed ones, but the planet's separation is smaller. We conclude that all models may explain some features of the observations but not all of them, raising questions about how gas is accreted on to the planet from its circumplanetary disc, and the presumed age of the system. [ABSTRACT FROM AUTHOR]

Physical processes behind flow‐topography interactions and turbulent transitions are essential for parameterization in numerical models. We examine how the Kuroshio cascades energy into turbulence upon passing over a seamount, employing a combination of shipboard measurements, tow‐yo microstructure profiling, and high‐resolution mooring. The seamount, spanning 5 km horizontally with two summits, interacts with the Kuroshio, whose flow speed ranges from 1 to 2 m s−1, modulated by tides. The forward energy cascade process is commenced by forming a train of 2–3 nonlinear lee waves behind the summit with a wavelength of 0.5–1 km and an amplitude of 50–100 m. A train of Kelvin‐Helmholtz (KH) billows develops immediately below the lee waves and extends downstream, leading to enhanced turbulence. The turbulent kinetic energy dissipation rate is O (10−7–10−4) W kg−1, varying in phase with the upstream flow speed modulated by tides. KH billows occur primarily at the lee wave's trailing edge, where the combined strong downstream shear and low‐stratification recirculation trigger the shear instability, Ri < 1/4. The recirculation also creates an overturn susceptible to gravitational instability. This scenario resembles the rotor, commonly found in atmospheric mountain waves but rarely observed in the ocean. A linear stability analysis further suggests that critical levels, where the KH instability extracts energy from the mean flow, are located predominantly at the strong shear layer of the lee wave's upwelling portion, coinciding with the upper boundary of the rotor. These novel observations may provide insights into flow‐topography interactions and improve physics‐based turbulence parameterization. Plain Language Summary: A thorough grasp of the physics driving flow‐topography interactions and turbulent transition is crucial for precise parameterization in numerical models. This study explores energy transformation into turbulence as the Kuroshio encounters a seamount based on direct field observations and theoretical examination. We found that the Kuroshio affected by the seamount kick starts a series of processes wherein the Kuroshio transfers its energy into turbulence through the formation of lee waves and shear instability. As the flow reverses beneath the trailing edge of lee waves, a recirculation rotor is formed, enhancing the vertical shear and weakening the stratification. The strong vertical shear exceeds the limitations imposed by water column stratification, promoting the development of shear instability and ultimately leading to water roll‐ups and turbulence generation, which could dissipate the Kuroshio energy and mix the Kuroshio water. Our findings could help improve physics‐based turbulence parameterization skills for flow‐topography interaction processes in numerical models. Key Points: Prominent nonlinear lee waves are formed as the Kuroshio interacts with the seamount east of TaiwanThe enhanced shear related to the lee wave and the weakened stratification related to the rotor generate a shear unstable regionThe shear instability grows by extracting the mean flow energy at the upwelling portion of the lee wave, where ε O(10−7−10−4) W kg−1 [ABSTRACT FROM AUTHOR]

The prevalence of light element enhancement in the most metal-poor stars is potentially an indication that the Milky Way has a metallicity floor for star formation around |$\sim 10^{-3.5}$| Z |$_{\odot }$|⁠. We propose that this metallicity floor has its origins in metal-enriched star formation in the minihaloes present during the Galaxy's initial formation. To arrive at this conclusion, we analyse a cosmological radiation hydrodynamics simulation that follows the concurrent evolution of multiple Population III star-forming minihaloes. The main driver for the central gas within minihaloes is the steady increase in hydrostatic pressure as the haloes grow. We incorporate this insight into a hybrid one-zone model that switches between pressure-confined and modified free-fall modes to evolve the gas density with time according to the ratio of the free-fall and sound-crossing time-scales. This model is able to accurately reproduce the density and chemo-thermal evolution of the gas in each of the simulated minihaloes up to the point of runaway collapse. We then use this model to investigate how the gas responds to the absence of H |$_{2}$|⁠. Without metals, the central gas becomes increasingly stable against collapse as it grows to the atomic cooling limit. When metals are present in the halo at a level of |$\sim 10^{-3.7}$|  Z |$_{\odot }$|⁠ , however, the gas is able to achieve gravitational instability while still in the minihalo regime. Thus, we conclude that the Galaxy's metallicity floor is set by the balance within minihaloes of gas-phase metal cooling and the radiation background associated with its early formation environment. [ABSTRACT FROM AUTHOR]

We use smoothed particle hydrodynamics simulations of isolated Milky Way-mass disc galaxies that include cold, interstellar gas to test subgrid prescriptions for star formation (SF). Our fiducial model combines a Schmidt law with a gravitational instability criterion, but we also test density thresholds and temperature ceilings. While SF histories are insensitive to the prescription for SF, the Kennicutt–Schmidt (KS) relations between SF rate and gas surface density can discriminate between models. We show that our fiducial model, with an SF efficiency per free-fall time of 1 per cent, agrees with spatially resolved and azimuthally averaged observed KS relations for neutral, atomic, and molecular gas. Density thresholds do not perform as well. While temperature ceilings selecting cold, molecular gas can match the data for galaxies with solar metallicity, they are unsuitable for very low-metallicity gas and hence for cosmological simulations. We argue that SF criteria should be applied at the resolution limit rather than at a fixed physical scale, which means that we should aim for numerical convergence of observables rather than of the properties of gas labelled as star-forming. Our fiducial model yields good convergence when the mass resolution is varied by nearly 4 orders of magnitude, with the exception of the spatially resolved molecular KS relation at low surface densities. For the gravitational instability criterion, we quantify the impact on the KS relations of gravitational softening, the SF efficiency, and the strength of supernova feedback, as well as of observable parameters such as the inclusion of ionized gas, the averaging scale, and the metallicity. [ABSTRACT FROM AUTHOR]

The gravitational instability of hot material deposited during eruptive activity can lead to the formation of glowing avalanches, commonly known as deposit-derived pyroclastic density currents (PDCs). These currents can travel hundreds of metres to several kilometres from the source at exceptionally high temperatures, posing a catastrophic hazard to areas surrounding steep-slope volcanoes. The occurrence of deposit-derived PDCs is often associated with crater rim failure, which can be triggered by various factors such as magma thrust from dike injection, magma fingering, bulging or less commonly, powerful explosions. Here, the in-depth study of data from the multi-parametric monitoring network operating on Stromboli (Italy), including video surveillance, seismicity and ground deformation data, complemented by remote topographic sensing data, has facilitated the understanding of the events leading to the crater rim collapse on 9 October and 4 December 2022. The failures resulted in the remobilisation of 6.4 ± 1.0 × 103 m3 and 88.9 ± 26.7 × 103 m3 of material for the 9 October and the 4 December 2022, respectively, which propagated as PDCs along the NW side of the volcano and reached the sea in a few tens of seconds. These events were characterised by a preparatory phase marked by an increase in magmatic pressure in the preceding weeks, which correlated with an increase in the displacement rate of the volcano's summit. There was also an escalation in explosive degassing, evidenced by spattering accompanied by seismic tremors in the hours before the collapse. These events have been interpreted as an initial increase in magma vesicularity, followed by the release of gas once percolation threshold was reached. The degassing process induced densification of the magma, resulting in increased thrust on the conduit walls due to increased magmastatic pressure. This phase coincided with crater rim collapse, often followed or accompanied by the onset of lava overflow phases. A mechanism similar to the one proposed may shed light on similar phenomena observed at other volcanoes. The analysis performed in this study highlights the need for a multi-parametric and multi-platform approach to fully understand such complex phenomena. By integrating different data sources, including seismic, deformation and remote sensing data, it is possible to identify the phenomena associated with the different phases leading to crater rim collapse and the subsequent development of deposit-derived PDCs. [ABSTRACT FROM AUTHOR]

In our previous study (Tsukamoto et al. 2023b, PASJ, 75, 835), we investigated the formation and early evolution of protoplanetary disks with 3D non-ideal magnetohydrodynamics simulations considering dust growth, and found that the modified equations of the conventional steady accretion disk model that consider magnetic braking, dust growth, and ambipolar diffusion reproduce the disk structure (such as density and vertical magnetic field) obtained from simulations very well. In this paper, as a sequel to our previous study, we analytically investigate the structure and evolution of protoplanetary disks corresponding to Class 0/I young stellar objects using the modified steady accretion disk model combining an analytical model of envelope accretion. We estimate that the disk radius is several astronomical units at the disk formation epoch and increases to several hundred astronomical units at the end of the accretion phase. The disk mass is estimated to be |$0.01 \lesssim M_{\rm disk} \lesssim 0.1 \, M_\odot$| for a disk with a radius of several tens of astronomical units and a mass accretion rate of |$\dot{M}_{\rm disk} \sim 10^{-6} \, M_\odot \,\, {\rm yr^{-1}}$|⁠. These estimates seems to be consistent with recent observations. We also found that, with typical disk ionization rates (ζ ≳ 10−19 s−1) and a moderate mass accretion rate (⁠|$\dot{M}_{\rm disk}\gtrsim 10^{-8} \, M_\odot \,\, {\rm yr^{-1}}$|⁠), magnetorotational instability is suppressed in the disk because of low plasma β and efficient ambipolar diffusion. We argue that the radial profile of specific angular momentum (or rotational velocity) at the disk outer edge should be continuously connected to that of the envelope if the disk evolves by magnetic braking, and should be discontinuous if the disk evolves by an internal angular momentum transport process such as gravitational instability or magnetorotational instability. Future detailed observations of the specific angular momentum profile around the disk outer edge are important for understanding the angular momentum transport mechanism of protoplanetary disks. [ABSTRACT FROM AUTHOR]

Local gravitational instability (LGI) is considered crucial for regulating star formation and gas turbulence in galaxy discs, especially at high redshift. Instability criteria usually assume infinitesimally thin discs or rely on approximations to include the stabilising effect of the gas disc thickness. We test a new 3D instability criterion for rotating gas discs that are vertically stratified in an external potential. This criterion reads Q3D < 1, where Q3D is the 3D analogue of the Toomre parameter Q. The advantage of Q3D is that it allows us to study LGI in and above the galaxy midplane in a rigorous and self-consistent way. We apply the criterion to a sample of 44 star-forming galaxies at 0 ≲ z ≲ 5 hosting rotating discs of cold gas. The sample is representative of galaxies on the main sequence at z ≈ 0 and includes massive star-forming and starburst galaxies at 1 ≲ z ≲ 5. For each galaxy, we first apply the Toomre criterion for infinitesimally thin discs, finding ten unstable systems. We then obtain maps of Q3D from a 3D model of the gas disc derived in the combined potential of dark matter, stars and the gas itself. According to the 3D criterion, two galaxies with Q < 1 show no evidence of instability and the unstable regions that are 20% smaller than those where Q < 1. No unstable disc is found at 0 ≲ z ≲ 1, while ≈60% of the systems at 2 ≲ z ≲ 5 are locally unstable. In these latter, a relatively small fraction of the total gas (≈30%) is potentially affected by the instability. Our results disfavour LGI as the main regulator of star formation and turbulence in moderately star-forming galaxies in the present-day Universe. LGI likely becomes important at high redshift, but the input by other mechanisms seems required in a significant portion of the disc. We also estimate the expected mass of clumps in the unstable regions, offering testable predictions for observations. [ABSTRACT FROM AUTHOR]

Aims. We study a new mechanism of dust accumulation and planetesimal formation in a gravitationally unstable disk with suppressed magnetorotational instability and we compare it with the classical dead zone in a layered disk model. Methods. We used numerical hydrodynamics simulations in the thin-disk limit (FEOSAD code) to model the formation and long-term evolution of gravitationally unstable disks, including dust dynamics and growth. Results. We found that in gravitationally unstable disks with a radially varying strength of gravitational instability (GI), an inner region (of several astronomical units) of low mass and angular momentum transport is formed. This region is characterized by a low effective value for the αGI parameter, often used to describe the efficiency of mass transport by GI in young protoplanetary disks. The inner region is also similar in terms of characteristics to the dead zone in the layered disk model. As the disk forms and evolves, the GI-induced dead zone accumulates a massive dust ring, which is susceptible to the development of the streaming instability. The model and observationally inferred dust masses and radii may differ significantly in gravitationally unstable disks with massive inner dust rings. Conclusions. The early occurrence of the GI-induced dust ring, followed by the development of the streaming instability suggest that this mechanism may be behind the formation of the first generation of planetesimals in the inner terrestrial zone of the disk. The proposed mechanism, however, crucially depends on the susceptibility of the disk to gravitational instability and requires the magnetorotational instability to be suppressed. [ABSTRACT FROM AUTHOR]

This study examines the gravitational instability of a finitely conducting magnetized viscoelastic fluid, under the influence of radiation pressure within the framework of a generalized hydrodynamic fluid model. The general dispersion relation, valid for strongly and weakly coupled fluids under kinetic and hydrodynamic limits respectively, is derived using normal mode analysis for a transverse wave propagation mode. The obtained Jeans instability criteria, in terms of critical Jeans wavenumbers under both limits, are modified by the presence of radiative effects, viscoelastic compressional speed, and Alfvén wave velocity. The findings highlight that viscoelastic compressional speed and radiative pressure slow the Jeans instability's growth rate, exerting a stabilizing effect on the initiation of gravitational collapse. The present investigation provides valuable insights into the role of radiative mechanisms in the gravitational collapse within the strongly coupled region of molecular cloud clumps. [ABSTRACT FROM AUTHOR]

We learn the consequences of finite ion Larmor radius (FLR) corrections, finite electron inertia, Hall current and radiative heat-loss function on the Jean's-gravitational instability of an infinite homogeneous, viscous plasma integrating the impacts of finite electrical resistivity, thermal conductivity and permeability. A general dispersion relation is derived by means of the normal mode analysis method with the assistance of linearized perturbation equations of the problem. We find that the original Jeans criterion of gravitational instability is modified into the radiative instability criterion by the inclusion of thermal conductivity and radiative heat-loss function. The finite electrical resistivity eliminates the impact of the magnetic field and the viscosity of the medium eliminates the impact of FLR from radiative instability. The Hall parameter has no impact on the transverse mode of propagation. Numerical calculations demonstrate the stabilizing impact of temperature-dependent heat-loss function, FLR corrections and the destabilizing impact of density-dependent heat-loss function, finite electron inertia on the Jean's-gravitational instability. [ABSTRACT FROM AUTHOR]

The dominant structural fabric in the Eastern Goldfields is the steep north- to north-northwest-trending S2 foliation that is axial planar to upright F2 folds, developed during intense horizontal east–west shortening (D2–4 event). These structures consistently overprint D1 structures, which comprise layer-parallel S1 foliation, extensional shears, thrusts and recumbent F1 folds. The D1 event represents a separate and distinct episode of deformation with a markedly different stress regime (dominant vertical σ1), compared with the stress regime during the D2–4 events (horizontal east–west oriented σ1). There is little evidence for pre-2670 Ma ductile deformation in the Eastern Goldfields, as: (1) the ca 2730–2670 Ma greenstone sequences are deposited conformably on the older ca 2800 Ma greenstone sequences, with the first significant angular unconformities observed at the base of the post greenstone late basins; (2) a lack of schist or gneiss clasts in the late basins suggests that the surrounding uplifted sequence was largely undeformed; (3) the layer-parallel S1 foliation is observed throughout the 2720–2670 Ma greenstone sequence; and (4) the dominant low north- and south-plunges of F2 folds suggest that the sequence was predominantly flat-lying prior to strong east–west shortening (D2–4 events). The ca 2670–2655 Ma D1 event represents a period of sagduction that occurred immediately after cessation of rifting as a result of gravitational instability from deposition of dense, cold, greenstone sequences onto thinned, light, hot felsic crust. Late basins represent depocentres on the sinking greenstones during sagduction. Marked contrasts in the structural style of gold deposits, metallogeny and fluid sources, typically attributed to progressive deformation during orogenesis, could instead reflect temporally distinct mineralising events during changing tectonic regimes. In the East Yilgarn, an early plumbing system dominated by north-northwest-trending basin-controlling structures was likely established during rifting and focused fluids during multiple hydrothermal circulation events. ca 2720–2670 Ma greenstone sequences in the Eastern Goldfields were deposited in intracratonic rift basins. Rifting triggered an episode of sagduction that formed the early dome-and-basin geometry of the Eastern Goldfields and the onset of orogenesis at ca 2650 Ma (D2–4 events) modified the existing dome-and-basin geometry. Late basins represent depocentres on sinking greenstone sequences during sagduction. Gold mineralisation occurred throughout the changing tectonic regimes. [ABSTRACT FROM AUTHOR]

The evolution of many astrophysical systems depends strongly on the balance between heating and cooling, in particular star formation in giant molecular clouds and the evolution of young protostellar systems. Protostellar discs are susceptible to the gravitational instability, which can play a key role in their evolution and in planet formation. The strength of the instability depends on the rate at which the system loses thermal energy. To study the evolution of these systems, we require radiative cooling approximations because full radiative transfer is generally too expensive to be coupled to hydrodynamical models. Here, we present two new approximate methods for computing radiative cooling that make use of the polytropic cooling approximation. This approach invokes the assumption that each parcel of gas is located within a spherical pseudo-cloud, which can then be used to approximate the optical depth. The first method combines the methods introduced by Stamatellos et al. and Lombardi et al. to overcome the limitations of each method at low and high optical depths, respectively. The second method, the 'modified Lombardi' method, is specifically tailored for self-gravitating discs. This modifies the scale height estimate from the method of Lombardi et al. using the analytical scale height for a self-gravitating disc. We show that the modified Lombardi method provides an excellent approximation for the column density in a fragmenting disc, a regime in which the existing methods fail to recover the clumps and spiral structures. We therefore recommend this improved radiative cooling method for more realistic simulations of self-gravitating discs. [ABSTRACT FROM AUTHOR]

A Bayesian linear regression to determine the bias in the Nafe–Drake relationship between compressional velocity and density provides an improved model for the density structure of Kīlauea volcano, Hawaiʻi. In previous work, we combined the results of seismic tomography with the Nafe–Drake relationship between compressional velocity and density to explain the large values of gravity disturbances overlying the summits and rift zones of the island's volcanoes. These results were used to determine mechanisms for gravitational instability of the island flanks. Here, we use laboratory measurements of the relationship of velocity and density for a wide range of Hawaiʻi island rocks as a prior in a Bayesian regression, with seismic tomography, to refine the 3-D density structure for Kīlauea volcano. This refined structure shows dense bodies (3220 kg m−3) between 5 and 8 km below sea level that underly regions of magma storage, found from geodetic and geophysical studies, beneath the summit and East Rift Zone of Kīlauea volcano. Above these bodies, density isosurfaces surround and cradle sources of pressure change determined from geodetic models, both at the summit and along the East Rift Zone. Continued subsidence of the summit following the 2018 eruption is aligned with a bowl-shaped density structure, formed primarily by density isosurfaces between 2800 and 2900 kg m−3 at 4–6 km depth. These surfaces underly the ∼3 km depth at which dyke injection initiates, are largely aseismic, and from their density values are inferred to contain high concentrations of olivine. Taken together, these density structures are consistent with an olivine-rich mush with variable porosity that increases in density with depth and provides a mechanism to form olivine cumulates both at the summit and along the rift zones. This structural framework for Kīlauea volcano is consistent with melt and mush transport occurring over a large range of depths to accommodate the growth and spreading of the volcano. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

The Sr‐Nd‐Hf‐Pb isotope geochemistry of the Late Miocene Tunceli Volcanics suggests that they are the products of mixed asthenospheric and lithospheric mantle melts. The combined elemental and mineral chemistry data additionally indicate that a pyroxenite component of lithospheric origin is involved in their genesis. Calculations favor melting depths of ∼2 GPa for the Tunceli lavas, that is, deeper than the current lithosphere‐asthenosphere boundary beneath Eastern Anatolia. Geochemical data suggest that during regional Neo‐Tethyan subduction, dense (i.e., pyroxenite‐bearing) domains formed by progressive melt intrusion into the lower lithosphere resulted in gravitational instabilities. This unstable density configuration eventually led to the foundering of the eastern Anatolian lithosphere in the Late Miocene, resulting in progressive melting of fusible pyroxenite‐bearing domains at asthenospheric depths. We demonstrate that these pyroxenitic melts mixed with ambient asthenospheric melts and generated the Tunceli lavas. Plain Language Summary: The Eastern Anatolian High Plateau (EAHP) that formed after the collision of Arabian and Eurasian continents hosts a huge volcanic system. The nature of the source region from which these volcanics originated and the geological dynamics that triggered this widespread volcanic activity are still under debate. We suggest that volcanism occurred when the lithospheric mantle beneath the EAHP separated physically from the overlying crust and sank into the deep asthenosphere. In this study, we explore the geochemical evidence for this model by focusing on the Late Miocene Tunceli Volcanics, one of the early stage members of post‐collisional volcanics in the EAHP. Our data suggest that the Tunceli Volcanics are the products of mixed asthenospheric and lithospheric mantle melts. The lithosphere contains a dense pyroxenite component that forms when silica‐rich melt invades the lower lithosphere during the subduction process. Calculations have shown that these dense materials melted at higher depths than the base of the lithosphere beneath the region. Thus, we propose that the dense domains in the lower lithosphere resulted in gravitational instabilities and eventually led to the foundering of the eastern Anatolian lithosphere in the Late Miocene. Key Points: During Neo‐Tethyan subduction, pyroxenite‐bearing domains formed by melt intrusion into the lower lithosphere beneath Eastern AnatoliaThe pyroxenite‐bearing domains resulted in gravitational instabilities and led to the foundering of the lithosphere in the Late MioceneThe regional post‐collisional volcanics represent the melts derived both from asthenospheric and delaminated lithospheric mantle domains [ABSTRACT FROM AUTHOR]

Recent Atacama Large Millimeter/submillimeter Array (ALMA) observations have revealed an increasing number of compact protostellar disks with radii of less than a few tens of astronomical units and that young Class 0/I objects have an intrinsic size diversity. To deepen our understanding of the origin of such tiny disks, we have performed highest-resolution configuration observations with ALMA at a beam size of ∼0.″03 (4 au) on the very low-luminosity Class 0 protostar embedded in the Taurus dense core MC 27/L1521F. The 1.3 mm continuum measurement successfully resolved a tiny, faint (∼1 mJy) disk with a major axis length of ∼10 au, one of the smallest examples in the ALMA protostellar studies. In addition, we detected spike-like components in the northeastern direction at the disk edge. Gravitational instability or other fragmentation mechanisms cannot explain the structures, given the central stellar mass of ∼0.2 M ⊙ and the disk mass of ≳10−4 M ⊙. Instead, we propose that these small spike structures were formed by a recent dynamic magnetic flux transport event due to interchange instability that would be favorable to occur if the parental core has a strong magnetic field. The presence of complex arc-like structures on a larger (∼2000 au) scale in the same direction as the spike structures suggests that the event was not single. Such episodic, dynamical events may play an important role in maintaining the compact nature of the protostellar disk in the complex gas envelope during the main accretion phase. [ABSTRACT FROM AUTHOR]

Gravitational instability in porous media is observed in numerous natural systems in which reaction at the penetrating chemical front is a common occurrence. Most often, this reaction kinetics is difficult to capture and explain adequately using the classical linear reaction rate profiles.The nonlinearity of the reaction rate addresses the general reaction framework that may be encountered in natural settings and has substantial ramifications for the overall gravitational instability dynamics at long times. However, the way in which a nonlinear reaction affects the dissolution in combination with porous dispersion has remained largely elusive. This study used a two-dimensional (2D) numerical framework built in OpenFOAM to capture the coupled effect of nonlinear reaction kinetics and porous dispersion on the ensuing gravitational instability across a miscible fluid pair. This study quantitatively explored the instability onset times, total dissolution of heavier fluid, and convective dynamics as a function of the nonlinearity in the chemical kinetics in dispersive porous media. We hereby report intriguing and nonintuitive effects on induced gravitational instability in the presence of nonlinear chemical reactions. The results show a decrease in the nonlinear onset time and increase in overall dissolution with stronger reaction rate. The onset time and overall dissolution varied with the nonlinearity of the reaction kinetics. This study focused on the enhancement of effective dissolution employing the inherently occurring chemical reactions in natural systems, and is a model tool to understand various geological reaction-transport and sequestration phenomena. [ABSTRACT FROM AUTHOR]

In this paper, we investigate the mass accretion properties in the innermost regions of a viscously evolved protoplanetary disk and try to find some clues to the outburst events. In our newly developed one-dimensional time-dependent disk model based on the diffusion equation for surface density, we take into account the following physical effects: the gravitational collapse of the parent molecular cloud core, the irradiation from the central star to the disk, the effect of the photoevaporation mechanism, the viscosity due to the magnetorotational instability (MRI) and the gravitational instability (GI), and the thermal ionization mechanism in the inner regions. We find that the mass accretion rate M · d i s k in the innermost regions is statistically high enough to generate outbursts, although there are regions where the accretion rate is low. Additionally, we find that there is a weak correlation between the high mass accretion rate M · d i s k and the molecular cloud core's properties (angular velocity ω and mass M cd ), as well as a strong correlation with the minimum viscosity parameter α min . In general, there are two regions of outburst, the inner Region I and outer Region II. The outburst of Region I is caused by the MRI mechanism and thermal instability, while neither the MRI, the GI, nor the thermal instability causes the outburst of Region II. Our analysis suggests that the outer Region II is dominated by, or largely related to, the Rosseland mean opacity κ R and the α min parameter. [ABSTRACT FROM AUTHOR]

Gravitational instabilities occurring at the interface between a suspension of granular particles and a clear fluid are studied experimentally using a sealed Hele–Shaw cell. Special attention is paid to the effects of particle Reynolds number (Re) and the initial particle packing on the growth of the interfacial perturbation. Glass beads are immersed in the glycerin–water mixture and are placed at the bottom of the cell initially. The mass content of the glycerin in fluid mixtures is varied to obtain the desired fluid viscosities and thus the Re value. The cell is then inverted to place the suspension above the fluid to trigger the gravitational instabilities. When the particle packing is dense, the distinct interface separating the fluid transforms progressively into an asymmetrical cusp-shaped structure. In contrast, when the packing is loose, the interface between the fluid and suspension takes on a symmetrical sinusoidal form initially and evolves into a mushroom-like pattern reminiscent of the canonical Rayleigh–Taylor instability. The growth rate of this symmetrical perturbation observed for loose packing surpasses that of its asymmetrical counterpart at the equivalent Re. Both types of perturbations exhibit larger growth rates with increasing Re. Linear stability analysis adapted to the loose packing configuration predicts a growth rate that is comparable to the observed rate during the initial growth of the symmetrical perturbation and suggests a similar dependence on Re. [ABSTRACT FROM AUTHOR]

Azizi, H., Daneshvar, N., Asahara, Y., Minami, M. & Anma, R. (2023). The generation of Eocene mafic dike swarms during the exhumation of a Core complex, Biarjmand area, NE Iran. Journal of Petrology 64 , 1–18 have attributed the E–W-oriented mafic dike swarm in the Biarjmand metamorphic core complex to an Eocene extensional event which is much younger than a previously suggested Late Jurassic–Early Cretaceous age. They proposed that the emplacement of these dikes occurred in a rapid extensional regime coeval with the exhumation of the core complex after gravitational instability in the Central Iran/Eurasia collision zone. I appreciate the opportunity this article provides to shed light on specific aspects of the Late Mesozoic–Early Cenozoic continental extension within the Eurasian sector of the Neotethys subduction system. However, I here bring to attention certain discrepancies within Azizi et al. 's publication. Specifically, the assignment of an Eocene age to the emplacement of the E–W-oriented dike swarm, even though purportedly supported by U–Pb zircon dating, appears to be at odds with field observations and previously published geochronological data. Furthermore, the article contains internal contradictions in its presentation of the core complex model for the study area. It is important to note that Malekpour-Alamdari, A., Axen, G. J., Heizler, M. T. & Hassanzadeh, J. (2017). Large-magnitude continental extension in the northeastern Iranian plateau: insight from K-feldspar 40Ar/39Ar thermochronology from the Shotor Kuh-Biarjmand metamorphic core complex. Geosphere 13 , 1207–1233 previously documented the geochronological-based metamorphic core complex model of the area. Regrettably, despite its direct relevance, these earlier works have not been acknowledged in Azizi et al. 's paper. In this comment, I outline the problems with the structural and regional geology, the zircon U–Pb age of the dike samples, the age of the dike swarm, and the geodynamic interpretations in Azizi et al. 's work. [ABSTRACT FROM AUTHOR]

This paper revisits the distribution of thermodynamic variables within initial protoplanets formed via gravitational instability (GI) across a broad mass spectrum ranging from 0.3MJ to 10MJ (where 1MJ denotes 1 Jupiter mass, equal to 1.8986×1030 g), using the Homotopy Analysis Method (HAM), a novel approach in this context. Concerning heat transfer within the protoplanets, consideration is given to the convective mode. Our findings reveal a noteworthy alignment between the results obtained via the HAM, utilizing only the first four terms (third approximation), and numerical outcomes. The HAM is found to demonstrate rapid convergence towards the exact solution, showcasing its effectiveness in obtaining such solutions to nonlinear problems. Comparative graphical illustrations of approximate series solutions obtained via HAM and the Adomian decomposition method highlight the superior accuracy and analytical depth of HAM within this framework. This establishes HAM as a powerful and insightful tool for studying astrophysical phenomena. The method offers significant advantages in accuracy and analytical depth over traditional methods, demonstrating its potential for broader applications in astrophysical research and providing robust solutions where conventional approaches may fall short.

Large-scale cosmological galaxy formation simulations typically prevent gas in the interstellar medium (ISM) from cooling below |$\approx 10^4\, \mathrm{K}$|⁠. This has been motivated by the inability to resolve the Jeans mass in molecular gas (⁠|$\ll 10^5\, \mathrm{M}_{\odot }$|⁠) which would result in undesired artificial clumping. We show that the classical Jeans criteria derived for Newtonian gravity are not applicable in the simulated ISM if the spacing of resolution elements representing the dense ISM is below the gravitational force softening length and gravity is therefore softened and not Newtonian. We re-derive the Jeans criteria for softened gravity in Lagrangian codes and use them to analyse gravitational instabilities at and below the hydrodynamical resolution limit for simulations with adaptive and constant gravitational softening lengths. In addition, we define criteria for which a numerical runaway collapse of dense gas clumps can occur caused by oversmoothing of the hydrodynamical properties relative to the gravitational force resolution. This effect is illustrated using simulations of isolated disc galaxies with the smoothed particle hydrodynamics code swift. We also demonstrate how to avoid the formation of artificial clumps in gas and stars by adjusting the gravitational and hydrodynamical force resolutions. [ABSTRACT FROM AUTHOR]

The profile of tephra concentration along a volcanic plume (i.e., the tephra segregation profile) is an important source parameter for the simulation of tephra transport and deposition and thus for the tephra sedimentation load. The most commonly-used approach is to treat an eruption as a single event (i.e., with a time-averaged mass eruption rate; MER). In this case, it is common to use pre-determined profiles that feature most of the tephra segregate at the top of the plume. However, case studies based on observations have revealed that large concentration maxima also appear at the lower part of the plume. To investigate this discrepancy, the impact of plume height on the temporal variations in the MER is examined. To this end, we use the tephra transport and dispersion model Tephra4D with MER estimates obtained from geophysical monitoring and maximum plume height observations to calculate the spatial distribution of the tephra deposit load for 39 eruptive events that consisted of explosions and quasi-steady particle emission from the Sakurajima volcano, Japan. A comparison of the model results with observations from a disdrometer network revealed that for both kinds of activity, maxima in tephra segregation can occur at heights below the reported plume height. The tephra segregation profiles of Vulcanian eruptions at Sakurajima volcano are consistent with most of the modeling studies giving profiles that feature most of the tephra segregating at the top of the plume if the temporal variation of the MER is taken into consideration to properly represent the total series of eruptive events in a sequence. This highlights that even though the activity at Sakurajima volcano is commonly characterized simply as Vulcanian eruptions, in addition to the primary plume developed due to the initial instantaneous release caused by the explosion, the subsequent continuous plume that can accompany the eruption plays an important role in particle emission. Calculations could not reproduce the simultaneous deposition of particles with a wide range of settling velocities in observations, suggesting the importance of volcanic ash fingers caused by gravitational instability in tephra transport simulations. [ABSTRACT FROM AUTHOR]

Arp 263 is a single irregularly shaped galaxy with intense star formation. Its short tidal features of low brightness visible in both optics and the H  i line, as well as the strong non-circular motion of atomic gas in the outskirts, give pieces of evidence of a recent merging with some intruder, non-observed directly. We carried out spectral observations of this galaxy at the 6 m telescope of the Special Astrophysical Observatory. The spectra for three positions of the spectral slit were obtained: two of them cross the central region of the galaxy and the third one runs along the nearly straight narrow chain of clumps of emission gas with a length of 4–5 kpc. We derived the profiles of velocity, fluxes, and oxygen abundances (O/H) along the slits. We confirm that within the radius of several kpc the galaxy rotates regularly. The maximal value of the rotational velocity allows us to conclude that the dark mass of the galaxy prevails over the mass of its visible components. The observed velocity profile along the peripheral star-forming chain gives pieces of evidence that it is located outside of the plane of the disc, and the gas that formed the chain is probably falling back on to the disc after being ejected from the galaxy. We discuss that the observed chain of clumps is the result of gravitational instability of the gaseous filament. [ABSTRACT FROM AUTHOR]

Gravitational instabilities can develop at volcanoes of any size and in any geological setting and can lead to various types of volcano deformation, ranging from small‐scale landslides on the flanks of the edifice to large, deep‐seated sector collapses. As volcanoes grow, they impose an increasing load on the underlying basement, which can result in styles of gravitational deformation where the edifice sags or spreads outward under its own weight. In this study, we utilize our previously developed global catalog of volcanoes on Venus to analyze a subset of edifices that appears to have undergone gravitational deformation. We identify 162 volcanoes that display morphological evidence for gravitational deformation and classify them into four main categories based on associated deformational structures: landsliding, sector collapse, spreading, and sagging. Landsliding of volcano flanks or full sector collapse are the most common and geographically widespread deformational styles on Venus, and account for ∼64% of our data set. Edifices exhibiting structures linked to volcano spreading and sagging are relatively rare; nonetheless, we note for the first time on Venus the presence on a shield volcano of flank terraces, structures linked to sagging. We find that deformed volcanoes are distributed globally, are found at a range of elevations, are spatially proximal to a variety of tectonic structures, and are associated with various crustal thickness values, which together suggest that there are numerous drivers of volcano deformation on Venus. Plain Language Summary: The effect of gravity works to destabilize volcanoes of all shapes and sizes and in all geological settings on Earth. In fact, gravity can lead to failure of small or even large parts of a volcano. As volcanoes grow and become heavier, they either weigh down the ground below them or, if the ground is very rigid, flatten outwards. Here, we used a catalog of volcanoes on Venus that we previously published to identify and describe all the examples of gravity‐deformed volcanoes on the planet. We found more than 160 such volcanoes and divided them into four major classes based on different types of collapse features. Deformed volcanoes are located across the entire surface of Venus, with a slightly higher concentration found in the Beta–Atla–Themis region. Furthermore, these collapsed edifices are situated in areas with different tectonic structures and are located at varying elevations across the planet, suggesting that there are multiple reasons why a volcano on Venus might collapse. Key Points: We identify 162 volcanoes displaying structures associated with gravitational deformation on VenusDeformed volcanoes are geographically widespread but are somewhat concentrated in the Beta–Atla–Themis regionWe note examples of flank terraces on Tepev Montes, a structural indicator of volcano sagging that has not yet been documented on Venus [ABSTRACT FROM AUTHOR]

Starburst (SB) galaxies are a rare population of galaxies with star formation rates (SFRs) greatly exceeding those of the majority of star-forming galaxies with similar stellar mass. It is unclear whether these bursts are the result of either especially large gas reservoirs or enhanced efficiencies in converting gas into stars. Tidal torques resulting from gas-rich galaxy mergers are known to enhance the SFR by funnelling gas towards the centre. However, recent theoretical works show that mergers do not always trigger an SB and not all SB galaxies are interacting systems, raising the question of what drives an SB. We analyse a large sample of SB galaxies and a mass- and redshift-matched sample of control galaxies, drawn from the FIREbox cosmological volume at z = 0–1. We find that SB galaxies have both larger molecular gas fractions and shorter molecular depletion times than control galaxies, but similar total gas masses. Control galaxies evolve towards the SB regime by gas compaction in their central regions, over time-scales of ∼70 Myr, accompanied by an increase in the fraction of ultradense and molecular gas. The driving mechanism behind the SB varies depending on the mass of the galaxy. Massive (⁠|$M_\star \gtrsim 10^{10}~\rm {M}_\odot$|⁠) galaxies undergoing intense, long-lasting SBs are mostly driven by galaxy interactions. Conversely, SBs in non-interacting galaxies are often triggered by a global gravitational instability, which can result in a 'breathing' mode in low-mass galaxies. [ABSTRACT FROM AUTHOR]

Context. The model of disc fragmentation due to gravitational instabilities offers an alternate formation mechanism for gas giant planets, especially those on wide orbits. Aims. Our goal is to determine the 3D structure of disc-instability protoplanets and to examine how this relates to the thermal physics of the fragmentation process. Methods. We modelled the fragmentation of gravitationally unstable discs using the SPH code PHANTOM, and followed the evolution of the protoplanets formed through the first and second-hydrostatic core phases (up to densities 10−3 g cm−3). Results. We find that the 3D structure of disc-instability protoplanets is affected by the disc environment and the formation history of each protoplanet (e.g. interactions with spiral arms, mergers). The large majority of the protoplanets that form in the simulations are oblate spheroids rather than spherical, and they accrete faster from their poles. Conclusions. The 3D structure of disc-instability protoplanets is expected to affect their observed properties and should be taken into account when interpreting observations of protoplanets embedded in their parent discs. [ABSTRACT FROM AUTHOR]

Migmatites and granitic-gneisses exhumed in Archean to Phanerozoic segments are former partially molten crustal roots, display typical domes structures ranging in size from kilometres to decakilometres, and are often interpreted as resulting from the development of diapiric or convective gravitational instabilities. In previous work (part I), we determined various regimes of gravity-driven segregation, by considering a thick continental crust heated from below and containing melt related heterogeneities. These heterogeneities, represented by inclusions of distinct densities and viscosities with respect to the ambient partially molten material, can be entrained into convection cells (in the 'suspension' and 'layering' regimes) and/or accumulate as clusters (in the 'layering' and 'diapirism' regimes). Here we further investigate the specific conditions that allow for the formation and preservation of domes resulting from diapirism at the top of convective cells. We show that both the cessation of basal heating and the freezing of the buoyant inclusions density favour their stacking and preservation at ca. 15 km depth, within about 10 Myr. The buoyant inclusions form domes, 5–20 km in size, that also record several convective cycles at velocities ranging from 0.5–4 cm yr−1. 3-D models demonstrate their radial geometrical nature. The influence of the size and concentration of the inclusions is also assessed, complementing the characteristics of crustal heterogeneity in driving its differentiation and the formation of migmatite domes. [ABSTRACT FROM AUTHOR]

In this paper, the effect of dissipative energy arising from bulk-viscosity on the collapse of a self-gravitating viscoelastic medium permeated with a nonuniform magnetic field and rotation is analyzed using the standard Jeans mechanism. A local solution of the system of nondimensional linearized perturbation equations, having variable coefficients, is obtained using the normal modes analysis method. The Jeans instability criteria are derived from the characteristic equation (valid under the kinetic and hydrodynamic limits) for parallel and perpendicular wave propagation, modified due to bulk viscosity and Alfvén wave velocity. From the calculated critical values of Jeans wavenumber, it is found that the bulk-viscosity and magnetic field have stabilizing influence on the onset of gravitational instability for each mode of wave propagation. It is observed that the nonuniform rotation does not affect the instability criterion, however, the rotation strongly suppresses the growth rate of the Jeans instability in both the hydrodynamic and kinetic limits. Also, a comparison of the impact of various rotational and magnetic field orientations on the growth rate in viscoelastic fluid is also presented. From the analysis, it is also observed that the presence of dissipative energy reduces the growth rate, in both modes of wave propagation. [ABSTRACT FROM AUTHOR]

We present maps of geohazard features identified across north-western Sicily and Pantelleria in the framework of the Magic project (MArine Geohazard along Italian Coasts), which involved Italian marine geological researchers in 2007-2013. These seafloor features were recognized using high-resolution bathymetry data and rely on the morphological expression of the seafloor and shallow sub-surface processes. The north-western Sicily is a complex continental margin, affected by morphodynamic, depositional, and tectonic processes. The Egadi offshore is controlled by fault escarpments and alternating retreating and progradational processes. Ustica and Pantelleria submerged edifices show the effect of volcanic activity. The Ustica seafloor is interested in volcanic, tectonic, and gravitational instability processes, while the Pantelleria offshore underwent erosive-depositional processes and the effect of bottom currents. Two levels of interpretation are represented: the physiographic domain at a scale of 1:250.000 and the morphological units and morphobathymetric elements at a 1:100.000 scale. [ABSTRACT FROM AUTHOR]

The generation mechanism of compressible fluid turbulence at kiloparsec scales in the interstellar medium is a long-lasting puzzle. In this work, we explore the nature of large-scale turbulence in the external spiral galaxy NGC 6946. We use the visibility moment estimator to measure the H  i column density and line-of-sight turbulent velocity power spectra combining the new observations of A array configuration of Karl G. Jansky Very Large Array (VLA) with the VLA B, C, D array observations from The H  i Nearby Galaxy Survey. The estimated power spectra are obeying a power law with a slope of −0.96 ± 0.05 in column density and −1.81 ± 0.07 in line-of-sight velocity in length-scales ranging from 6 kpc to 170 pc. This points towards a forward energy cascade in the plane of the disc with a driving scale at least as large as 6 kpc. The values of the power-law indices indicate a combination of solenoidal and compressive force responsible for driving the measured turbulence. The presence of strong regular magnetic fields from the magnetic spiral arms in the galaxy is possibly contributing to the solenoidal part, while self-gravity or gravitational instability can mostly be the input for the compressive part of the forcing in the driving mechanism. [ABSTRACT FROM AUTHOR]

In contrast to several classical studies, in which gravitational instability criteria for astrophysical disks are derived within traditional hydrodynamics or kinetics, it is proposed to consider the set of loose gas–dust clusters of an accretion protoplanetary disk as a special type of continuous medium, i.e., a fractal medium whose phase velocity space contains points and areas not filled by its components. Within the Tsallis formalism of nonadditive statistics (thermodynamics), intended to describe the behavior of anomalous systems, i.e., systems with a strong gravitational interaction of its individual parts and the fractal nature of the phase space, linearized equations are obtained for oscillations of a solid-state rotating disk on the basis of modified Navier–Stokes hydrodynamic equations (the so-called q-hydrodynamics equations) and in view of dissipative effects, and a derivation is given of the dispersion equation in the WKB approximation. An analysis is conducted of axisymmetric oscillations of a differentially rotating astrophysical gas–dust space object to obtain modified Jeans and Toomre gravitational instability criteria for disks with a fractal structure. [ABSTRACT FROM AUTHOR]

Theories of planet formation predict that low-mass stars should rarely host exoplanets with masses exceeding that of Neptune. We used radial velocity observations to detect a Neptune-mass exoplanet orbiting LHS 3154, a star that is nine times less massive than the Sun. The exoplanetÕs orbital period is 3.7 days, and its minimum mass is 13.2 Earth masses. We used simulations to show that the high planet-to-star mass ratio (>3.5 × 10−3) is not an expected outcome of either the core accretion or gravitational instability theories of planet formation. In the core-accretion simulations, we show that close-in Neptune-mass planets are only formed if the dust mass of the protoplanetary disk is an order of magnitude greater than typically observed around very low-mass stars. [ABSTRACT FROM AUTHOR]

The problem of self-gravitational instability of an astrophysical rotating plasma in a strong magnetic field with an anisotropic pressure tensor is studied on the basis of the Chew–Goldberger–Low (CGL) quasi-hydrodynamic equations modified by generalized polytropic laws. Using the general form of a dispersion relation obtained by the normal-mode perturbation method, a discussion is provided of the propagation of small-amplitude perturbation waves in an infinite homogeneous plasma medium for transverse, longitudinal, and oblique directions with respect to the magnetic field vector. It is shown that different polytropic indices and anisotropic pressures not only change the classical Jeans instability condition but also cause the appearance of new unstable regions. Modified Jeans instability criteria are obtained for isotropic MHD equations and anisotropic CGL equations owing to the influence of the polytropic indices on gravitational and firehose instabilities for astrophysical plasma. It is shown that in the case of a longitudinal mode of perturbation wave propagation, the Jeans instability criterion does not depend on uniform rotation. In the case of the transverse propagation regime, the presence of rotation reduces the critical wave number and exerts a stabilizing effect on the growth rate of the unstable regime. [ABSTRACT FROM AUTHOR]

We present a dynamics study toward the G326.611+0.811 (G326) hub–filament system (HFS) cloud using new APEX observations of both 13CO and C18O (J = 2–1). The G326 HFS cloud constitutes a central hub and at least four hub-composing filaments that are divided into a major branch of filaments (F1 and F2) and a side branch (F3–F5). The cloud holds ongoing high-mass star formation as characterized by three massive dense clumps (i.e., 370–1100 M ⊙ and 0.14–0.16 g cm−2 for C1–C3) with high clump-averaged mass infalling rates (>10−3 M ⊙ yr−1) within the major filament branch, and the associated point sources bright at 70 μ m, typical of young protostars. Along the five filaments, velocity gradients are found in both 13CO and C18O (J = 2–1) emission, suggesting that filament-aligned gravitational collapse toward the central hub (i.e., C2) is responsible for the high-mass star formation therein. Moreover, a periodic velocity oscillation along the major filament branch is revealed in both 13CO and C18O (J = 2–1) emission with a characteristic wavelength of ∼3.5 pc and an amplitude of ∼0.31–0.38 km s−1. We suggest that this pattern of velocity oscillation in G326 could arise from clump-forming gas motion induced by gravitational instabilities. The prevalent velocity gradients, fragmentation of the major branch of filaments, and the ongoing collapse of the three massive dense clumps are indicative that G326 is an HFS undergoing global collapse. [ABSTRACT FROM AUTHOR]

Highly siderophile elements (HSEs; namely Ru, Rh, Pd, Re, Os, Ir, Pt, and Au) in Earth’s mantle require the addition of metals after the formation of Earth’s core. Early, large collisions have the potential to deliver metals, but the details of their mixing with Earth’s mantle remain unresolved. As a large projectile disrupts and penetrates Earth’s mantle, a fraction of its metallic core may directly merge with Earth’s core. Ensuing gravitational instabilities remove the remaining projectile’s core stranded in Earth’s mantle, leaving the latter deprived of HSEs. Here, we propose a framework that can efficiently retain the metallic components during large impacts. The mechanism is based on the ubiquitous presence of a partially molten region in the mantle beneath an impact-generated magma ocean, and it involves rapid three-phase flow with solid silicate, molten silicate, and liquid metal as well as long-term mixing by mantle convection. In addition, large low-shear-velocity provinces in the lower mantle may originate from compositional heterogeneities resulting from the proposed three-phase flow during high-energy collisions. [ABSTRACT FROM AUTHOR]

We study the initial development, structure, and evolution of protoplanetary clumps formed in three-dimensional resistive magnetohydrodynamic (MHD) simulations of self-gravitating discs. The magnetic field grows by means of the recently identified gravitational instability dynamo. Clumps are identified and their evolution is tracked finely both backward and forward in time. Their properties and evolutionary path is compared with clumps in companion simulations without magnetic fields. We find that magnetic and rotational energy are important in the clumps' outer regions, while in the cores, despite appreciable magnetic field amplification, thermal pressure is most important in counteracting gravity. Turbulent kinetic energy is of a smaller scale than magnetic energy in the clumps. Compared with non-magnetized clumps, rotation is less prominent, which results in lower angular momentum in much better agreement with observations. In order to understand the very low sub-Jovian masses of clumps forming in MHD simulations, we revisit the perturbation theory of magnetized sheets finding support for a previously proposed magnetic destabilization in low-shear regions. This can help explaining why fragmentation ensues on a scale more than an order of magnitude smaller than that of the Toomre mass. The smaller fragmentation scale and the high magnetic pressure in clumps' envelopes explain why clumps in magnetized discs are typically in the super-Earth to Neptune mass regime rather than super-Jupiter as in conventional disc instability. Our findings put forward a viable alternative to core accretion to explain widespread formation of intermediate-mass planets. [ABSTRACT FROM AUTHOR]

The present analytical study extends the problems of pressure anisotropy-driven instabilities and gravitational instability in space plasmas to mixed quantum polytropic gas in the interior of dense stars accounting for the effects of viscosity, finite Larmor radius (FLR) and self-gravitational effects. The generalized polytrope pressure laws are considered as adiabatic equations in which the pressure components depend upon the plasma density, magnitude of the magnetic field, and the polytrope indices. The modified properties of waves and instabilities in gravitating quantum plasmas have been analysed using the quantum magnetohydrodynamic (QMHD) fluid description in the magnetohydrodynamic (MHD) and Chew–Goldberger–Low (CGL) limits. In the parallel propagation, the Jeans instability modified by quantum diffraction parameters and firehose mode modified by FLR parameter is obtained separately. The Jeans instability condition depends upon the quantum diffraction term and polytrope index β, and it remains unaffected due to viscosity and ion Larmor frequency. The growth rate of the Jeans instability decreases due to viscosity and quantum diffraction parameters, while the growth rate of the firehose instability increases due to FLR corrections. In the transverse mode, a similar nature is observed in the growth rates; however, the instability region decreases significantly due to polytrope indices and different dispersion properties of MHD and CGL viscous quantum plasmas. The analytical results have been applied in dense degenerate stars to measure the characteristic parameters and understand the MHD wave propagation, pressure anisotropy-driven, and gravitationally driven instabilities. [ABSTRACT FROM AUTHOR]

The first step toward planet formation is grain growth from (sub)micrometer to millimeter/centimeter sizes. Grain growth has been reported not only in Class II protoplanetary disks but also in Class 0/I protostellar envelopes. However, early-stage grain growth occurring in Class 0/I stages has rarely been observed on the protostellar disk scale. Here we present the results from the Atacama Large Millimeter/submillimeter Array Band 3 (λ = 3.1 mm) and 7 (λ = 0.87 mm) archival data of the Class I protostellar disk WL 17 in the ρ Ophiuchus molecular cloud. Disk substructures are found in both bands, but they are different: while a central hole and a symmetric ring appear in Band 3, an off-center hole and an asymmetric ring are shown in Band 7. Furthermore, we obtain an asymmetric spectral index map with a low mean value of α = 2.28 ± 0.02, suggestive of grain growth and dust segregation on the protostellar disk scale. Our radiative transfer modeling verifies these two features by demonstrating that 10 cm sized large grains are symmetrically distributed, whereas 10 μ m sized small grains are asymmetrically distributed. In addition, the analysis shows that the disk is expected to be massive and gravitationally unstable. We thus suggest a single Jupiter-mass protoplanet formed by gravitational instability as the origin of the ring-like structure, grain growth, and dust segregation identified in WL 17. [ABSTRACT FROM AUTHOR]

Recent discoveries of gas giant exoplanets around M-dwarfs from transiting and radial velocity surveys are difficult to explain with core-accretion models. We present here a homogeneous suite of 162 models of gravitationally unstable gaseous disks. These models represent an existence proof for gas giants more massive than 0.1 Jupiter masses to form by the gas disk gravitational instability (GDGI) mechanism around M-dwarfs for comparison with observed exoplanet demographics and protoplanetary disk mass estimates for M-dwarf stars. We use the Enzo 2.6 adaptive mesh refinement (AMR) 3D hydrodynamics code to follow the formation and initial orbital evolution of gas giant protoplanets in gravitationally unstable gaseous disks in orbit around M-dwarfs with stellar masses ranging from 0.1 M ⊙ to 0.5 M ⊙. The gas disk masses are varied over a range from disks that are too low in mass to form gas giants rapidly to those where numerous gas giants are formed, therefore revealing the critical disk mass necessary for gas giants to form by the GDGI mechanism around M-dwarfs. The disk masses vary from 0.01 M ⊙ to 0.05 M ⊙ while the disk to star mass ratios explored the range from 0.04 to 0.3. The models have varied initial outer disk temperatures (10–60 K) and varied levels of AMR grid spatial resolution, producing a sample of expected gas giant protoplanets for each star mass. Broadly speaking, disk masses of at least 0.02 M ⊙ are needed for the GDGI mechanism to form gas giant protoplanets around M-dwarfs. [ABSTRACT FROM AUTHOR]

The traces left by earthquakes in the unlithified sediments, recorded as soft-sediment deformation structures (SSDS), are well reconstructed as palaeo-seismic signals, while the origin of SSDS, seismic vs. Aseismic, is challenging. The present study discusses the origin of SSDS and its implications on palaeoceanography and sediment architecture. In the Middle Jurassic succession of Spiti Himalayan region in India, the topmost part of the Ferruginous Oolitic Formation (FOF) consists of four layers of SSDS and is underlain by the lower member of the Spiti Formation (SF). The sedimentary facies analysis documents the palaeogeographic shift from the middle shelf (carbonate-shale repository: FOF) to the outer shelf (black shale: lower member of SF). The SSDS layers, exhibiting load casts, ball and pillow structures, indicate gravitational instability, while syn-sedimentary faults and insitu breccia are the results of brittle deformation. The dominance of storms in depositional sites often argues for a possible triggering agent for SSDS. Therefore, it was necessary to distinguish between seismic vs. aseismic triggering agents. The lateral continuity, vertical repetition, confinement of SSDS at the top part of FOF and sharp change of facies assemblage indicate seismicity-induced syn-sediment deformation, i.e. seismite. The transition from middle shelf to outer shelf at the onset of seismite indicates that seismic impact possibly caused the rapid subsidence, resulting in the palaeogeographic shift. The rapid transgression is recorded as carbonate-shale repository to anoxic black shale. This study highlights the importance of sedimentological analysis to distinguish the seismite and its implications on palaeogeographic evolution and sedimentary architecture. [ABSTRACT FROM AUTHOR]

Molecular cloud (MC) formation is caused by the gravitational collapse mechanism and is significantly affected by radiative heating and cooling processes. This paper analyzes the gravitational instability in strongly coupled clumpy molecular clouds (MCs), under the effects of uniform rotation, magnetic field, and heat-loss functions. The generalized hydrodynamic equations coupled with the modified energy equation (which incorporates the heating and cooling effects due to cloud-cloud collisions) are used to describe the mathematical model. Following Jeans stability analysis, it is found that the value of the critical Jeans wavenumber decreases due to the strong coupling between the plasma particles (coupling parameter) and clump stirring processes (heating rate), so both have a stabilizing influence on the onset of gravitational collapse in clumpy MCs. The influence of various parameters on the growth rate of the instability is discussed numerically, and it is found that the cooling rate parameter that describes cloud-cloud collisions shows a destabilizing effect. The region of instability is observed to be smaller in the strongly coupled clumps (kinetic limit) than in the weakly coupled (hydrodynamic limit) clumps. The results are helpful in understanding the role of heating and cooling mechanisms in the MC formation. [ABSTRACT FROM AUTHOR]

We use the eagle cosmological simulations to study the evolution of the vertical velocity dispersion of cold gas, σ z , in central disc galaxies and its connection to stellar feedback, gravitational instabilities, cosmological gas accretion, and galaxy mergers. To isolate the impact of feedback, we analyse runs that turn off stellar and (or) active galactic nuclei feedback in addition to a run that includes both. The evolution of σ z and its dependence on stellar mass and star formation rate in eagle are in good agreement with observations. Galaxies hosted by haloes of similar virial mass, |$\rm M_{200}$|⁠ , have similar σ z values even in runs where feedback is absent. The prevalence of local instabilities in discs is uncorrelated with σ z at low redshift and becomes only weakly correlated at high redshifts and in galaxies hosted by massive haloes. σ z correlates most strongly with the specific gas accretion rate onto the disc as well as with the degree of misalignment between the inflowing gas and the disc's rotation axis. These correlations are significant across all redshifts and halo masses, with misaligned accretion being the primary driver of high gas turbulence at redshifts z ≲ 1 and for halo masses |$\rm M_{200} \lesssim 10^{11.5} {\rm M}_{\odot }$|⁠. Galaxy mergers increase σ z , but because they are rare in our sample, they play only a minor role in its evolution. Our results suggest that the turbulence of cold gas in eagle discs results from a complex interplay of different physical processes whose relative importance depends on halo mass and redshift. [ABSTRACT FROM AUTHOR]

We study the Jeans gravitational instability for a mixture of baryonic and dark matter particles, in the post-Newtonian approximation. We adopt a kinetic model consisting of a coupled system of post-Newtonian collisionless Boltzmann equations, for each species, coupled to the post-Newtonian Poisson equations. We derive the stability criterion, accounting for both post-Newtonian corrections and the presence of dark matter. It is shown that both effects give rise to smaller Jeans masses, in comparison with the standard Jeans criterion, meaning that a smaller mass is needed to begin the gravitational collapse. Taking advantage of that, we confront the model with the observational stability of Bok globules, and show that the model correctly reproduces the data. [ABSTRACT FROM AUTHOR]

Recent simulations suggest that submesoscale motions with scales smaller than 30 km and frequencies greater than 1 day−1 drive upward vertical heat transport. These simulations have prompted us to revisit the mechanisms that explain high‐frequency (HF) vertical heat fluxes (VHFs) within the surface mixed layer (ML). Here, an idealized numerical simulation of a re‐entrant channel flow with an unbalanced submesoscale thermal front is used to analyze the impact of surface cooling on HF VHFs. Two types of simulations are analyzed: forced and unforced. The VHFs cospectrum analysis shows that surface diurnal cooling increases VHFs, reaching frequencies larger than 1 day−1. However, the fastest‐growing length scale of ML instabilities limits the extension of positive VHFs toward fine scales. Symmetric and gravitational instabilities are the main conduits producing ageostrophic HF and small‐scale structures, which in turn enhance upward VHFs across the diurnal frequency. A comparison between forced‐idealized simulations with the K‐profile parameterization scheme and a realistic regional simulation in the frequency‐wavenumber space, reveals that the two simulation types reproduce similar VHFs near the diurnal frequency. However, the realistic simulation displays higher VHFs than the forced‐idealized simulation. This study emphasizes that surface diurnal cooling significantly impacts HF VHFs. However, this impact is not sufficient to reach the HF VHFs estimated in realistic submesoscale‐permitting and tidal‐resolving simulations. Plain Language Summary: Recent research has highlighted the importance of balanced motions in the ocean with horizontal dimensions less than 30 km, as these motions play a crucial role in carrying heat upward with frequencies greater than 1 day−1 in the surface mixed layer (ML). These motions are influenced by atmospheric forcing, which impacts the heat flux. This study investigates how high‐frequency vertical heat fluxes (VHFs) respond to atmospheric cooling by using numerical simulations to generate structures with horizontal dimensions less than 30 km in two scenarios: one with active surface cooling and one without. The simulations reveal that surface diurnal cooling amplifies VHFs with frequencies higher than 1 day−1. This amplification occurs through the enhancement of the generation of structures driven by convective motions in the ML. Through comparison with a realistic numerical simulation that generates horizontal motions less than 30 km and tidal motions, the study finds that diurnal cooling partially explains the vertical fluxes with frequencies greater than 1 day−1 estimated in the realistic simulations. This research sheds light on the complex mechanisms involved in vertical heat transport in the ocean and highlights the roles of balanced motions with horizontal dimensions of less than 30 km and atmospheric forcing. Key Points: Surface diurnal cooling amplifies vertical heat fluxes with frequencies higher than 1 day−1Such amplification occurs through ageostrophic motions driven by symmetric and gravitational instabilitiesDiurnal cooling partially explains the high‐frequency vertical fluxes estimated in submesoscale‐permitting and tide‐resolving simulations [ABSTRACT FROM AUTHOR]