Unstable Disks Research Articles

Parameter study for the burst mode of accretion in massive star formation

ABSTRACT It is now a widely held view that, in their formation and early evolution, stars build up mass in bursts. The burst mode of star formation scenario proposes that the stars grow in mass via episodic accretion of fragments migrating from their gravitationally unstable circumstellar discs, and it naturally explains the existence of observed pre-main-sequence bursts from high-mass protostars. We present a parameter study of hydrodynamical models of massive young stellar objects (MYSOs) that explores the initial masses of the collapsing clouds (Mc = 60–$200\, \rm M_{\odot }$) and ratio of rotational-to-gravitational energies (β = 0.005–0.33). An increase in Mc and/or β produces protostellar accretion discs that are more prone to develop gravitational instability and to experience bursts. We find that all MYSOs have bursts even if their pre-stellar core is such that β ≤ 0.01. Within our assumptions, the lack of stable discs is therefore a major difference between low- and high-mass star formation mechanisms. All our disc masses and disc-to-star mass ratios Md/M⋆ > 1 scale as a power law with the stellar mass. Our results confirm that massive protostars accrete about $40\, -\, 60{{\ \rm per\ cent}}$ of their mass in the burst mode. The distribution of time periods between two consecutive bursts is bimodal: there is a short duration ($\sim 1\, -\, 10~\rm yr$) peak corresponding to the short, faintest bursts and a long-duration peak (at $\sim 10^{3}\, -\, 10^{4} \rm yr$) corresponding to the long, FU-Orionis-type bursts appearing in later disc evolution, i.e. around $30\, \rm kyr$ after disc formation. We discuss this bimodality in the context of the structure of massive protostellar jets as potential signatures of accretion burst history.

Disc fragmentation and intermittent accretion on to supermassive stars

ABSTRACT Supermassive stars (SMSs) with ∼104–105 M⊙ are candidate objects for the origin of supermassive black holes observed at redshift z > 6. They are supposed to form in primordial-gas clouds that provide the central stars with gas at a high accretion rate, but their growth may be terminated in the middle due to the stellar ionizing radiation if the accretion is intermittent and its quiescent periods are longer than the Kelvin–Helmholtz (KH) time-scales at the stellar surfaces. In this paper, we examine the role of the ionizing radiation feedback based on the accretion history in two possible SMS-forming clouds extracted from cosmological simulations, following their evolution with vertically integrated two-dimensional hydrodynamic simulations with detailed thermal and chemical models. The consistent treatment of the gas thermal evolution is crucial for obtaining the realistic accretion history, as we demonstrate by performing an additional run with a barotropic equation of state, in which the fluctuation of the accretion rate is artificially suppressed. We find that although the accretion becomes intermittent due to the formation of spiral arms and clumps in gravitationally unstable discs, the quiescent periods are always shorter than the KH time-scales, implying that SMSs can form without affected by the ionizing radiation.

Rotation Curves in z ∼ 1–2 Star-forming Disks: Evidence for Cored Dark Matter Distributions

Abstract We report high-quality, Hα or CO rotation curves (RCs) to several R e for 41 large, massive, star-forming disk galaxies (SFGs) across the peak of cosmic galaxy evolution (z ∼ 0.67–2.45), taken with the ESO-VLT, the LBT and IRAM-NOEMA. Most RC41 SFGs have reflection-symmetric RCs plausibly described by equilibrium dynamics. We fit the major axis position–velocity cuts using beam-convolved forward modeling generated in three dimensions, with models that include a bulge and turbulent disk component embedded in a dark matter (DM) halo. We include priors for stellar and molecular gas masses, optical light effective radii and inclinations, and DM masses from abundance-matching scaling relations. Two-thirds or more of the z ≥ 1.2 SFGs are baryon dominated within a few R e of typically 5.5 kpc and have DM fractions less than maximal disks (median 〈 f DM ( R e ) 〉 = 0.12 ). At lower redshift (z < 1.2), that fraction is less than one-third. DM fractions correlate inversely with the baryonic angular momentum parameter, baryonic surface density, and bulge mass. Inferred low DM fractions cannot apply to the entire disk and halo but more plausibly reflect a flattened, or cored, inner DM density distribution. The typical central “DM deficit” in these cores relative to Navarro–Frenk–White (NFW) distributions is ∼30% of the bulge mass. The observations are consistent with rapid radial transport of baryons in the first-generation massive gas-rich halos forming globally gravitationally unstable disks and leading to efficient build-up of massive bulges and central black holes. A combination of heating due to dynamical friction and AGN feedback may drive DM out of the initial cusps.

Accretion bursts in low-metallicity protostellar disks

Aims. The early evolution of protostellar disks with metallicities in the Z = 1.0 − 0.01 Z⊙ range was studied with a particular emphasis on the strength of gravitational instability and the nature of protostellar accretion in low-metallicity systems. Methods. Numerical hydrodynamics simulations in the thin-disk limit were employed that feature separate gas and dust temperatures, and disk mass-loading from the infalling parent cloud cores. Models with cloud cores of similar initial mass and rotation pattern but distinct metallicity were considered to distinguish the effect of metallicity from that of the initial conditions. Results. The early stages of disk evolution in low-metallicity models are characterized by vigorous gravitational instability and fragmentation. Disk instability is sustained by continual mass-loading from the collapsing core. The time period that is covered by this unstable stage is much shorter in the Z = 0.01 Z⊙ models than in their higher metallicity counterparts thanks to the higher rates of mass infall caused by higher gas temperatures (which decouple from lower dust temperatures) in the inner parts of collapsing cores. Protostellar accretion rates are highly variable in the low-metallicity models reflecting the highly dynamic nature of the corresponding protostellar disks. The low-metallicity systems feature short but energetic episodes of mass accretion caused by infall of inward-migrating gaseous clumps that form via gravitational fragmentation of protostellar disks. These bursts seem to be more numerous and last longer in the Z = 0.1 Z⊙ models than in the Z = 0.01 Z⊙ case. Conclusions. Variable protostellar accretion with episodic bursts is not a particular feature of solar metallicity disks. It is also inherent to gravitationally unstable disks with metallicities up to 100 times lower than solar.

Evolution of CAI-sized Particles during FU Orionis Outbursts. I. Particle Trajectories in Protoplanetary Disks with Beta Cooling

Abstract Solar-type young stellar objects undergo periodic, energetic outbursts that appear to be the result of enhanced mass accretion driven by the gravitational instability of their disks. Such FU Orionis outbursts may have profound consequences for the earliest solids in a protoplanetary disk, namely the refractory inclusions containing abundant calcium and aluminum (CAIs). We present models of the orbital evolution of centimeter-radius particles representing large CAIs in marginally gravitationally unstable disks. The hydrodynamical evolution of the disks is calculated with a fully three-dimensional code, including compressional heating and cooling in the beta cooling approximation. The particles are initially distributed uniformly throughout the disk, which extends from 1 to 10 au around a solar-mass protostar, but within ∼100 yr the particles are concentrated by gas drag into regions surrounding the spiral arms and rings formed by the gas disk. The particles settle down toward the disk midplane, only to be lofted repeatedly upward by shock fronts. Large-scale radial transport both outward and inward occurs, with significant numbers of particles reaching the outer disk (∼10 au) and surviving for considerably longer times than would be the case in a quiescent disk with gas pressure monotonically decreasing with distance from the protostar. Individual particles experience wide ranges of disk temperatures during their journeys, ranging from 60 K in the outer disk to nearly 2000 K in spiral features. Future work will consider the implications for CAI rims of the thermochemical processing experienced during FU Orionis outbursts.

New maser species tracing spiral-arm accretion flows in a high-mass young stellar object

Numerical simulations have predicted that substructures such as spiral arms can be produced through a gravitationally unstable disk around high-mass young stellar objects (HMYSOs). Recent high-resolution observations from the Atacama Large Millimeter/submillimeter Array have investigated these substructures at a spatial resolution of ∼100 au. An accretion burst, which is a manifestation of an increase in the accretion rate caused by a gravitational instability in the disk, can result in luminosity outbursting phenomena - as has been seen in several HMYSOs. However, no clear relationship between the accretion bursts and disk substructures has been established. Here we report the detections of three new molecular maser species, HDO, HNCO and 13CH3OH, from the direction of the HMYSO G358.93-0.03 during a 6.7 GHz methanol maser flaring event. High-quality imaging of the three new maser species exhibits consistent observational evidence that these masers closely trace the spiral-arm substructures around this HMYSO. The rapid decay of the spectral lines emitted from these molecules suggests that these are transient phenomena (for only ∼1 month), probably associated with rapid changes in radiation field due to an accretion burst. Therefore, these new maser species provide evidence linking the spiral-arm substructure with an accretion burst, both expected from massive disk instabilities.

Are Lower-leg And Thigh Muscle Resistance Training Equally Effective To Adl-related Functional Fitness For Community-dwelling Elderly Females?

PURPOSE: The purpose was to compare the magnitude of lower-leg training program and thigh muscle training program to ADL-related functional fitness changes for community-dwelling elderly Japanese women. METHODS: After giving written informed consent, the subjects, unable to stand on one leg for more than 25 seconds with their eyes open, were divided into a lower-leg training group (LLG; 10 females, 72.9±4.2 yrs, BMI 22.1±1.8) and a thigh muscle training group (TMG; 10 females, 70.6±2.5 yrs, BMI 22.1±1.2). The program was 60min. two times per week for 16 weeks. Each training program consisted of three parts. At first, participants learned about management skills for their physical stiffness. Secondly, they learned each resistance program. LLG participated in the program using unstable disk and elastic band. TMG learned program was to strengthen their thigh muscles with elastic band. Finally, both groups learned a three-minute arm and leg combined exercise program with music. Participants were asked to follow their learned management skill program and resistance program every day and check it on the card. ADL-related functional fitness(sitting & standing time, zig-zag walking time, self-care working time), dynamic balance ability which measured by one-leg standing time with their eyes open and knee extension strength was evaluated. Each measurement items were assessed before and after the intervention period. Student’s t-test and two-way repeated measures ANOVA were used to test the effectiveness. RESULTS: The class participation rates were 82± 4% and 81± 8% and home participation rates were 76± 10% and 72± 15% respectively. ADL-related functional fitness of TMG improved significantly compared to LLG; Sitting & standing time (TMG: 18.6±7.4 to 13.4±6.1 sec., LLG: 16.3±7.4 to 13.8± 5.7sec., F=18.00, P=0.033), self-care working time ( TMG: 16.3±5.3 to 11.3±3.2sec., LLG: 16.1±3.2 to 14.9±4.3sec., F=17.00,P=0.026), zig-zag walking time( TMG: 15.6±7.2 to 14.2±5.5sec., LLG: 14.2±3.2 to 12.6±3.0ec., F=0.043,P=0.838). Knee extension strength improved significantly in TMG (P=0.029). One-leg standing time with their eyes open improved in LLG(P=0.038) CONCLUSIONS: Thigh-muscle training was found more effective to improve ADL-related functional fitness than lower-leg muscles for community-dwelling females.

Long-Term Evolution of Convectively Unstable Disk

AbstractWe continue studying convection as a possible factor of episodic accretion in protoplanetary disks. Within the model of a viscous disk, the accretion history is analyzed at different rates and regions of matter inflow from the envelope onto the disk. It is shown that the burst-like regime occurs in a wide range of parameters. The long-term evolution of the disk is modeled, including the decreasing-with-time matter inflow from the envelope. It is demonstrated that the disk becomes convectively unstable and maintains burst-like accretion onto the star for several million years. The general conclusion of the study is that convection can serve as one of the mechanisms of episodic accretion in protostellar disks, but this conclusion needs to be verified using more consistent hydrodynamic models.

The origin of tail-like structures around protoplanetary disks

Aims. We study the origin of tail-like structures recently detected around the disk of SU Aurigae and several FU Orionis-type stars. Methods. Dynamic protostellar disks featuring ejections of gaseous clumps and quiescent protoplanetary disks experiencing a close encounter with an intruder star were modeled using the numerical hydrodynamics code FEOSAD. Both the gas and dust dynamics were taken into account, including dust growth and mutual friction between the gas and dust components. Only plane-of-the-disk encounters were considered. Results. Ejected clumps produce a unique type of tail that is characterized by a bow-shock shape. Such tails originate from the supersonic motion of ejected clumps through the dense envelope that often surrounds young gravitationally unstable protostellar disks. The ejected clumps either sit at the head of the tail-like structure or disperse if their mass is insufficient to withstand the head wind of the envelope. On the other hand, close encounters with quiescent protoplanetary disks produce three types of the tail-like structure; we define these as pre-collisional, post-collisional, and spiral tails. These tails can in principle be distinguished from one another by particular features of the gas and dust flow in and around them. We find that the brown-dwarf-mass intruders do not capture circumintruder disks during the encounter, while the subsolar-mass intruders can acquire appreciable circumintruder disks with elevated dust-to-gas ratios, which can ease their observational detection. However, this is true only for prograde collisions; the retrograde intruders fail to collect appreciable amounts of gas or dust from the disk of the target. The mass of gas in the tail varies in the range 0.85–11.8 MJup, while the total mass of dust lies in the 1.75–30.1 M⊕ range, with the spiral tails featuring the highest masses. The predicted mass of dust in the model tail-like structures is therefore higher than what was inferred for similar structures in SU Aur, FU Ori, and Z CMa, making their observational detection feasible. Conclusions. Tail-like structures around protostellar and protoplanetary disks can be used to infer interesting phenomena such as clump ejection or close encounters. In particular, the bow-shock morphology of the tails could point to clump ejections as a possible formation mechanism. Further numerical and observational studies are needed to better understand the detectability and properties of the tails.

Global simulations of galactic discs: violent feedback from clustered supernovae during bursts of star formation

ABSTRACT A suite of idealized, global, gravitationally unstable, star-forming galactic disc simulations with 2 pc spatial resolution, performed with the adaptive mesh refinement code ramses, is used in this paper to predict the emergent effects of supernova feedback. The simulations include a simplified prescription for the formation of single stellar populations of mass $\sim 100 \, {\rm M}_{\odot }$, radiative cooling, photoelectric heating, an external gravitational potential for a dark matter halo and an old stellar disc, self-gravity, and a novel implementation of supernova feedback. The results of these simulations show that gravitationally unstable discs can generate violent supersonic winds with mass-loading factors η ≳ 10, followed by a galactic fountain phase. These violent winds are generated by highly clustered supernovae exploding in dense environments created by gravitational instability, and they are not produced in simulation without self-gravity. The violent winds significantly perturb the vertical structure of the disc, which is later re-established during the galactic fountain phase. Gas resettles into a quasi-steady, highly turbulent disc with volume-weighted velocity dispersion $\sigma \gt 50 \, {\rm km\, s}^{-1}$. The new configuration drives weaker galactic winds with a mass-loading factor η ≤ 0.1. The whole cycle takes place in ≤10 dynamical times. Such high time variability needs to be taken into account when interpreting observations of galactic winds from starburst and post-starburst galaxies.

Revealing Hidden Substructures in the MBH–σ Diagram, and Refining the Bend in the L–σ Relation

Abstract Using 145 early- and late-type galaxies (ETGs and LTGs) with directly measured supermassive black hole masses, M BH, we build upon our previous discoveries that: (i) LTGs, most of which have been alleged to contain a pseudobulge, follow the relation M BH ∝ M * , sph 2.16 ± 0.32 ; and (ii) the ETG relation M BH ∝ M * , sph 1.27 ± 0.07 is an artifact of ETGs with/without disks following parallel M BH ∝ M * , sph 1.9 ± 0.2 relations that are offset by an order of magnitude in the M BH direction. Here, we searched for substructure in the diagram of M BH versus central velocity dispersion σ, using our recently published multi-component galaxy decompositions, by investigating divisions based on the presence of a depleted stellar core (major dry merger), a disk (minor wet/dry merger, gas accretion), or a bar (evolved unstable disk). The Sérsic and core-Sérsic galaxies define two distinct relations: M BH ∝ σ 5.75 ± 0.34 and M BH ∝ σ 8.64 ± 1.10 , with Δ rms ∣ BH = 0.55 and 0.46 dex, respectively. We also report on the consistency with the slopes and bends in the galaxy luminosity (L)–σ relation due to Sérsic and core-Sérsic ETGs, and LTGs that all have Sérsic light profiles. Two distinct relations (superficially) reappear in the M BH–σ diagram upon separating galaxies with/without a disk (primarily for the ETG sample), while we find no significant offset between barred and non-barred galaxies, nor between galaxies with/without active galactic nuclei. We also address selection biases purported to affect the scaling relations for dynamically measured M BH samples. Our new M BH–σ relations, dependent on morphological type, more precisely estimate M BH in other galaxies, and hold implications for galaxy/black hole coevolution theories, simulations, feedback, the pursuit of a black-hole fundamental plane, and calibration of virial f-factors for reverberation mapping.

Gravitational wave emission from unstable accretion discs in tidal disruption events

Abstract Gravitational waves can be emitted by accretion discs if they undergo instabilities that generate a time varying mass quadrupole. In this work we investigate the gravitational signal generated by a thick accretion disc of 1 M⊙ around a static supermassive black hole of 106 M⊙, assumed to be formed after the tidal disruption of a solar type star. This torus has been shown to be unstable to a global non-axisymmetric hydrodynamic instability, the Papaloizou–Pringle instability, in the case where it is not already accreting and has a weak magnetic field. We start by deriving analytical estimates of the maximum amplitude of the gravitational wave signal, with the aim to establish its detectability by the Laser Interferometer Space Antenna (LISA). Then, we compare these estimates with those obtained through a numerical simulation of the torus, made with a 3D smoothed particle hydrodynamics code. Our numerical analysis shows that the measured strain is two orders of magnitude lower than the maximum value obtained analytically. However, accretion discs affected by the Papaloizou–Pringle instability may still be interesting sources for LISA, if we consider discs generated after deeply penetrating tidal disruptions of main-sequence stars of higher mass.

The First Bird’s-eye View of a Gravitationally Unstable Accretion Disk in High-mass Star Formation

Abstract We report on the first bird’s-eye view of the innermost accretion disk around the high-mass protostellar object G353.273+0.641, taken by Atacama Large Millimeter/submillimeter Array long baselines. The disk traced by dust continuum emission has a radius of 250 au, surrounded by the infalling rotating envelope traced by thermal CH3OH lines. This disk radius is consistent with the centrifugal radius estimated from the specific angular momentum in the envelope. The lower-limit envelope mass is ∼5–7 M ☉ and accretion rate onto the stellar surface is 3 × 10−3 M ☉ yr−1 or higher. The expected stellar age is well younger than 104 yr, indicating that the host object is one of the youngest high-mass objects at present. The disk mass is 2–7 M ☉, depending on the dust opacity index. The estimated Toomre’s Q parameter is typically 1–2 and can reach 0.4 at the minimum. These Q values clearly satisfy the classical criteria for gravitational instability, and are consistent with recent numerical studies. Observed asymmetric and clumpy structures could trace a spiral arm and/or disk fragmentation. We found that 70% of the angular momentum in the accretion flow could be removed via the gravitational torque in the disk. Our study has indicated that the dynamical nature of a self-gravitating disk could dominate the early phase of high-mass star formation. This is remarkably consistent with the early evolutionary scenario of a low-mass protostar.

Giant planets and brown dwarfs on wide orbits: a code comparison project

Gas clumps formed within massive gravitationally unstable circumstellar discs are potential seeds of gas giant planets, brown dwarfs and companion stars. Simulations show that competition between three processes -- migration, gas accretion and tidal disruption -- establishes what grows from a given seed. Here we investigate the robustness of numerical modelling of clump migration and accretion with the codes PHANTOM, GADGET, SPHINX, SEREN, GIZMO-MFM, SPHNG and FARGO. The test problem comprises a clump embedded in a massive disc at an initial separation of 120 AU. There is a general qualitative agreement between the codes, but the quantitative agreement in the planet migration rate ranges from $\sim 10$% to $\sim 50$%, depending on the numerical setup. We find that the artificial viscosity treatment and the sink particle prescription may account for much of the differences between the codes. In order to understand the wider implications of our work, we also attempt to reproduce the planet evolution tracks from our hydrodynamical simulations with prescriptions from three previous population synthesis studies. We find that the disagreement amongst the population synthesis models is far greater than that between our hydrodynamical simulations. The results of our code comparison project are therefore encouraging in that uncertainties in the given problem are probably dominated by the physics not yet included in the codes rather than by how hydrodynamics is modelled in them.

Adding a Suite of Chemical Abundances to the MACER Code for the Evolution of Massive Elliptical Galaxies

Abstract We add a suite of chemical abundances to the MACER (Massive AGN Controlled Ellipticals Resolved) 2D code, by solving 12 additional continuity equations for H, He, C, N, O, Ne, Mg, Si, S, Ca, Fe, and Ni with sources from AGB stars and Type Ia and II supernovae with metal yields based on standard stellar physics. New stars, formed in Toomre unstable circumnuclear disks (of a size ≲150 pc), are assumed to have a top-heavy initial mass function with a power index of 1.65. The metal dilution effects due to cosmic accretion are also included. With a high resolution of a few parsecs in central regions, resolved black hole accretion, and active galactic nucleus (AGN) feedback, we can track the metal enrichment, transportation, and dilution throughout the modeled massive elliptical galaxy of velocity dispersion ∼280 km s−1. We retrieve the chemical composition of the broad absorption line (BAL) winds launched by the central AGN, synthesize the X-ray features of the hot ISM, and find that (1) the simulated metallicity in the BAL winds could be up to ∼8 Z ⊙, while that of the hot ISM in the host galaxy is ∼2.3 Z ⊙, matching well with SDSS observations of BLR gas; (2) the X-ray emitting hot gas is metal-enriched with a typical value ∼2.5 Z ⊙; (3) the circumunuclear cold gas disk, where the metals are condensed, further enriched, and recycled, plays a critical role in the metal enrichment; (4) the black hole accretion rate linearly correlates with the star formation rate in the circumnuclear disk, i.e., , but lagged in time by roughly 106 yr.

Effects of the Isothermal Region in Protoplanetary Disks and the Protostar Irradiation on the Disk Instability Model for Giant Planet Formation

Abstract We construct an analytical model of gravitationally unstable protoplanetary disks consisting of three regions: the inner region where the internal dissipation dominates the heating, the intermediate region where the central protostar irradiation dominates, and the outer region where background irradiation dominates. We use this analytical model and an evolutionary numerical model of protoplanetary disks to calculate the cooling time and find out the location of the isothermal region. We investigate the effects of the isothermal region on the disk instability model for giant planet formation. We find that the fragmentation region found in previous studies is contained in the isothermal region of a disk. In this case, the cooling time criterion is not applicable for fragmentation. Therefore, the constraint on the disk instability model caused by the cooling time criterion should be relieved. The viability of the disk instability model is improved. When the isothermal region is considered, the inner boundary of the fragmentation region is extended inward to ∼20 au. We also show that if the contribution of the protostar irradiation to the disk surface temperature can be included in the cooling rate, the fragmentation region defined by the cooling time criterion can be extended inward to ∼26 au. We find that a disk tends to be isothermal in the region where the cooling time criterion is satisfied. We also find that at the later stage of disk instability, the inner boundary of the fragmentation region is determined by the inner boundary of the gravitationally unstable region.

Pebble accretion in Class 0/I YSOs as a possible pathway for early planet formation

Recent theoretical works suggest that the pebble accretion process is important for planet formation in protoplanetary disks, because it accelerates the growth of planetary cores. While several observations reveal axisymmetric sharp gaps in very young disks, which may be indicative of the existence of planets. We investigate the possibility of planet formation via pebble accretion in much earlier phases, the gravitationally unstable disks of class 0/I young stellar objects. We find that under the conditions of the class 0/I disks, the pebble accretion timescales can be shorter compared to the typical protoplanetary disks due to larger gas and dust accretion rate, but also find that the accretion timescale is not always a decreasing function of the gas accretion rate. By using estimated accretion timescales, we give a required initial mass to form cores of gas giants within the lifetime of class 0/I phases under several parameters, such as radial distances from the host star, gas accretion rates, and dust-to-gas mass ratio. In the most optimistic case, for example the dust-to-gas mass ratio is $f=3f_{\rm solar}$, $\sim10^{-4}M_{\oplus}$ objects at 10 au can grow to $10M_{\oplus}$ cores during the typical lifetime of the class 0/I phases, 0.5 Myr.

Gravitational stability and fragmentation condition for discs around accreting supermassive stars

Supermassive stars (SMSs) with mass $\sim10^{5}~\rm{M}_{\odot}$ are promising candidates for the origin of supermassive black holes observed at redshift $\gtrsim6$. They are supposed to form as a result of rapid accretion of primordial gas, although it can be obstructed by the time variation caused by circum-stellar disc fragmentation due to gravitational instability. To assess the occurrence of fragmentation, we study the structure of marginally gravitationally unstable accretion discs, by using a steady one-dimensional thin disc model with detailed treatment of chemical and thermal processes. Motivated by two SMS formation scenarios, i.e., those with strong ultraviolet radiation background or with large velocity difference between the baryon and the dark matter, we consider two types of flows, i.e., atomic and molecular flows, respectively, for a wide range of the central stellar mass $10-10^5~\rm{M}_{\odot}$ and the accretion rate $10^{-3}-1~\rm{M}_{\odot}~\rm{yr}^{-1}$. In the case of a mostly atomic gas flowing to the disc outer boundary, the fragmentation condition is expressed as the accretion rate being higher than the critical value of $10^{-1}~\rm{M}_{\odot}~\rm{yr}^{-1}$ regardless of the central stellar mass. On the other hand, in the case of molecular flows, there is a critical disc radius outside of which the disc becomes unstable. Those conditions appears to be marginally satisfied according to numerical simulations, suggesting that disc fragmentation can be common during SMS formation.

The Fate of Formamide in a Fragmenting Protoplanetary Disk

Recent high-sensitivity observations carried out with the Atacama Large Millimeter Array have revealed the presence of complex organic molecules (COMs) such as methyl cyanide (CH3CN) and methanol (CH3OH) in relatively evolved protoplanetary discs. The behavior and abundance of COMs in earlier phases of disk evolution remain unclear. Here, we combine a smoothed particle hydrodynamics simulation of a fragmenting, gravitationally unstable disk with a gas-grain chemical code. We use this to investigate the evolution of formamide (NH2CHO), a prebiotic species, in both the disk and in the fragments that form within it. Our results show that formamide remains frozen onto grains in the majority of the disks where the temperatures are <100 K, with a predicted solid-phase abundance that matches those observed in comets. Formamide is present in the gas phase in three fragments as a result of the high temperatures (≥200 K), but remains in the solid phase in one colder (≤150 K) fragment. The timescale over which this occurs is comparable to the dust sedimentation timescales, suggesting that any rocky core that is formed would inherit their formamide content directly from the protosolar nebula.

The wide binary fraction of solar-type stars: emergence of metallicity dependence at a &amp;lt; 200 au

ABSTRACT We combine a catalogue of wide binaries constructed from Gaia DR2 with [Fe/H] abundances from wide-field spectroscopic surveys to quantify how the binary fraction varies with metallicity over separations 50 ≲ s/au ≲ 50 000. At a given distance, the completeness of the catalogue is independent of metallicity, making it straightforward to constrain intrinsic variation with [Fe/H]. The wide binary fraction is basically constant with [Fe/H] at large separations (s ≳ 250 au) but becomes quite rapidly anticorrelated with [Fe/H] at smaller separations: for 50 &amp;lt; s/au &amp;lt; 100, the binary fraction at $\rm [Fe/H] = -1$ exceeds that at $\rm [Fe/H] = 0.5$ by a factor of 3, an anticorrelation almost as strong as that found for close binaries with a &amp;lt; 10 au. Interpreted in terms of models where disc fragmentation is more efficient at low [Fe/H], our results suggest that 100 &amp;lt; a/au &amp;lt; 200 is the separation below which a significant fraction of binaries formed via fragmentation of individual gravitationally unstable discs rather than through turbulent core fragmentation. We provide a public catalogue of 8407 binaries within 200 pc with spectroscopically determined [Fe/H] for at least one component.