Time Scale For Fragmentation Research Articles

The critical cooling time for the fragmentation of accretion disks in the presence of bump density perturbation

In this paper, we try to study the fragmentation conditions of self-gravitational accretion disk in Dead zone. This region is located between 1 and 5 AU from the star. There is a high bump density at the outer boundary of Dead Zone. A bump surface density leads to the production of instability and vortices. Vortices can produce the spiral waves and affect the fragmentation conditions in the system. So, in this paper, we assume a bump density perturbation and obtain the Jeans mass, time scale of fragmentation and critical cooling time for fragmentation. Results show that the critical cooling time has larger values around the density peak. This value is dependent on amplitude and width of bump density (βcrit∼14 for F=1.3). Therefore, systems with long cooling time can be fragment. Also, the results show that in some areas near the peak, the cooling time can be less than the critical cooling time, which depends on the width and peak of the bump(βc<βcrit). Also the results show that in some disks, the critical cooling time is less than the cooling time of system in thermal equilibrium state. It is found that in the presence of bump density perturbation, the critical value of β is dependent on the radius and it does not have a same value in all radii. Other result obtained from this study is that the Jeans mass decreases near the peak of density and therefore the probability of gravitational instability increases in this area. The Minimum Jeans mass is obtained 0.005Msun∼5MJup for disk with low accretion rate.

ALMA Fragmented Source Catalog in Orion (FraSCO). I. Outflow Interaction within an Embedded Cluster in OMC-2/FIR 3, FIR 4, and FIR 5

We present a high-angular resolution (∼1″) and wide-field () image of the 1.3 mm continuum, CO(J = 2–1) and SiO(J = 5–4) line emissions toward an embedded protocluster, FIR 3, FIR 4, and FIR 5, in the Orion Molecular Cloud 2 obtained from the Atacama Large Millimeter/submillimeter Array. We identify 51 continuum sources, 36 of which are newly identified in this study. Their dust masses, projected sizes, and H2 gas number densities are estimated to be 3.8 × 10−5–1.1 × 10−2 M ⊙, 290–2000 au, and 6.4 × 106–3.3 × 108 cm−3, respectively. The results of a Jeans analysis show that ∼80% of the protostellar sources and ∼15% of the prestellar sources are gravitationally bound. We identify 12 molecular outflows traced in the CO(J = 2–1) emission, six of which are newly detected. We spatially resolve shocked gas structures traced by the SiO(J = 5–4) emission in this region for the first time. We identify shocked gas originating from outflows and other shocked regions. These results provide direct evidence of an interaction between dust condensation, FIR 4, and an energetic outflow driven by HOPS-370 located within FIR 3. A comparison of the outflow dynamical timescales, fragmentation timescales, and protostellar ages shows that the previously proposed triggered star formation scenario in FIR 4 is not strongly supported. We also discuss the spatial distribution of filaments identified in our continuum image by comparing it with a previously identified hub-fiber system in the N2H+ line.

Effect of O2/CO2 atmospheres on coal fragmentation

Recently, a single particle pyrolysis-combustion fragmentation model has been developed (Senneca et al., 2013, 2017) [1,2] to predict the propensity of coal particles to fragment under a wide range of heating conditions as a consequence of mechanical failure of the particle. Stress inside the particle arises from thermal shock, associated to particles’ heat up, as well as from overpressure generated by volatiles release upon devolatilization.The model is now used to calculate the propensity of coal particles to undergo fragmentation in the early stages of oxy-combustion, with gaseous atmospheres of 5–30% O2 in CO2 in entrained flow and fluidized beds reactors. Accordingly particles size of 0.1–10 mm are assumed, temperatures of 1123 and 2073 K, heating rates of 100 and 10,000 K/s.Results show that under entrained flow reactor conditions the particles break in the first 20–30 ms, producing a bimodal particle-size distribution. Under fluidized bed conditions, the particles undergo explosive fragmentation after 1–2 s, before pyrolysis is complete, generating broad particle size distribution. In both cases fragmentation occurs over short timescales compared to char combustion and gasification.Operative conditions where fragmentation occurs before or in parallel with char combustion or gasification are inferred by comparing on an Arrhenius plot the timescale of fragmentation and heterogeneous reactions for a larger array of operating conditions. The figure reveals that for high reaction temperatures, more reactive coals, larger particles size, gasification reactions can have an important role and maybe enhance porosity and percolative fragmentation.

Molecular environs and triggered star formation around the large Galactic infrared bubble N 24

Abstract A multi-wavelength analysis of the large Galactic infrared bubble N 24 is presented in this paper in order to investigate the molecular and star-formation environment around expanding H ii regions. Using archival data from Herschel and ATLASGAL, the distribution and physical properties of the dust over the entire bubble are studied. Using the Clumpfind2d algorithm, 23 dense clumps are identified, with sizes and masses in the range 0.65–1.73 pc and 600–16 300 M⊙, respectively. To analyse the molecular environment in N 24, observations of NH3 (1,1) and (2,2) were carried out using the Nanshan 26-m radio telescope. Analysis of the kinetic temperature and gravitational stability of these clumps suggests gravitational collapse in several of them. The mass–size distributions of the clumps and the presence of massive young protostars indicate that the shell of N 24 is a region of ongoing massive-star formation. The compatibility of the dynamical and fragmentation timescales and the overabundance of young stellar objects and clumps on the rim suggest that the ‘collect-and-collapse’ mechanism is in play at the boundary of the bubble, but the existence of the infrared dark cloud at the edge of bubble indicates that a ‘radiation-driven implosion’ mechanism may also have played a role there.

Ultrafast charge redistribution in small iodine containing molecules

We present studies on intra-molecular charge redistribution in iodine containing molecules upon iodine-4d photoionization. For this, we employed an XUV-pump-XUV-probe scheme based on time-delayed femtosecond pulses delivered by the free-electron laser at DESY in Hamburg (FLASH). The experimental results show delay dependent and molecule-specific iodine charge state distributions that arise upon multiple iodine-4d photoionization. Using the example of CH3I and CH2I2, we compare the delay-dependent yields of I3+. We model the involved processes using advanced ab initio electronic structure calculations which include electron correlations combined with a classical model of the nuclear motion. The qualitative agreement of our model with the experimental results allows us to relate the observed, strongly molecule-specific efficiencies of the intra-molecular charge rearrangement not only to molecule-specific fragmentation timescales but also to molecule-specific electronic structure and molecular environment.

Cosmic-ray propagation with DRAGON2: II. Nuclear interactions with the interstellar gas

Understanding the isotopic composition of cosmic rays (CRs) observed near Earth represents a milestone towards the identification of their origin. Local fluxes contain all the known stable and long-lived isotopes, reflecting the complex history of primaries and secondaries as they traverse the interstellar medium. For that reason, a numerical code which aims at describing the CR transport in the Galaxy must unavoidably rely on accurate modelling of the production of secondary particles. In this work we provide a detailed description of the nuclear cross sections and decay network as implemented in the forthcoming release of the galactic propagation code DRAGON2. We present the secondary production models implemented in the code and we apply the different prescriptions to compute quantities of interest to interpret local CR fluxes (e.g., nuclear fragmentation timescales, secondary and tertiary source terms). In particular, we develop a nuclear secondary production model aimed at accurately computing the light secondary fluxes (namely: Li, Be, B) above 1 GeV/n. This result is achieved by fitting existing empirical or semi-empirical formalisms to a large sample of measurements in the energy range 100 MeV/n to 100 GeV/n and by considering the contribution of the most relevant decaying isotopes up to iron. Concerning secondary antiparticles (positrons and antiprotons), we describe a collection of models taken from the literature, and provide a detailed quantitative comparison.

Convergence of the Critical Cooling Rate for Protoplanetary Disk Fragmentation Achieved: The Key Role of Numerical Dissipation of Angular Momentum

Abstract We carry out simulations of gravitationally unstable disks using smoothed particle hydrodynamics (SPH) and the novel Lagrangian meshless finite mass (MFM) scheme in the GIZMO code. Our aim is to understand the cause of the nonconvergence of the cooling boundary for fragmentation reported in the literature. We run SPH simulations with two different artificial viscosity implementations and compare them with MFM, which does not employ any artificial viscosity. With MFM we demonstrate convergence of the critical cooling timescale for fragmentation at . Nonconvergence persists in SPH codes. We show how the nonconvergence problem is caused by artificial fragmentation triggered by excessive dissipation of angular momentum in domains with large velocity derivatives. With increased resolution, such domains become more prominent. Vorticity lags behind density, due to numerical viscous dissipation in these regions, promoting collapse with longer cooling times. Such effect is shown to be dominant over the competing tendency of artificial viscosity to diminish with increasing resolution. When the initial conditions are first relaxed for several orbits, the flow is more regular, with lower shear and vorticity in nonaxisymmetric regions, aiding convergence. Yet MFM is the only method that converges exactly. Our findings are of general interest, as numerical dissipation via artificial viscosity or advection errors can also occur in grid-based codes. Indeed, for the FARGO code values of significantly higher than our converged estimate have been reported in the literature. Finally, we discuss implications for giant planet formation via disk instability.

Infrared dust bubble CS51 and its interaction with the surrounding interstellar medium

A multiwavelength investigation of the southern infrared dust bubble CS51 is presented in this paper. We probe the associated ionized, cold dust, molecular and stellar components. Radio continuum emission mapped at 610 and 1300 MHz, using the Giant Metrewave Radio Telescope, India, reveal the presence of three compact emission components (A, B, and C) apart from large-scale diffuse emission within the bubble interior. Radio spectral index map show the coexistence of thermal and non-thermal emission components. Modified blackbody fits to the thermal dust emission using Herschel PACS and SPIRE data is performed to generate dust temperature and column density maps. We identify five dust clumps associated with CS51 with masses and radius in the range 810 - 4600 M{\sun} and 1.0 - 1.9 pc, respectively. We further construct the column density probability distribution functions of the surrounding cold dust which display the impact of ionization feedback from high-mass stars. The estimated dynamical and fragmentation timescales indicate the possibility of collect and collapse mechanism in play at the bubble border. Molecular line emission from the MALT90 survey is used to understand the nature of two clumps which show signatures of expansion of CS51.

CO OBSERVATIONS AND INVESTIGATION OF TRIGGERED STAR FORMATION TOWARD THE N10 INFRARED BUBBLE AND SURROUNDINGS

ABSTRACT We studied the environment of the dust bubble N10 in molecular emission. Infrared bubbles, first detected by the GLIMPSE survey at 8.0 μm, are ideal regions to investigate the effect of the expansion of the H ii region on its surroundings and the eventual triggering of star formation at its borders. In this work, we present a multi-wavelength study of N10. This bubble is especially interesting because infrared studies of the young stellar content suggest a scenario of ongoing star formation, possibly triggered on the edge of the H ii region. We carried out observations of 12CO(1-0) and 13CO(1-0) emission at PMO 13.7 m toward N10. We also analyzed the IR and sub-millimeter emission on this region and compare those different tracers to obtain a detailed view of the interaction between the expanding H ii region and the molecular gas. We also estimated the parameters of the denser cold dust condensation and the ionized gas inside the shell. Bright CO emission was detected and two molecular clumps were identified from which we have derived physical parameters. We also estimate the parameters for the densest cold dust condensation and for the ionized gas inside the shell. The comparison between the dynamical age of this region and the fragmentation timescale favors the “Radiation-Driven Implosion” mechanism of star formation. N10 is a case of particular interest with gas structures in a narrow frontier between the H ii region and surrounding molecular material, and with a range of ages of YSOs situated in the region, indicating triggered star formation.

Characterising the physical and chemical properties of a young Class 0 protostellar core embedded in the Orion B9 filament

The present study aims to characterise the physical and chemical properties of the protostellar core Orion B9-SMM3. The APEX telescope was used to perform a follow-up molecular line survey of SMM3. The following species were identified from the frequency range 218.2-222.2 GHz: $^{13}$CO, C$^{18}$O, SO, para-H$_2$CO, and E$_1$-type CH$_3$OH. The on-the-fly mapping observations at 215.1-219.1 GHz revealed that SMM3 is associated with a dense gas core as traced by DCO$^+$ and p-H$_2$CO. Altogether three different p-H$_2$CO transitions were detected with clearly broadened linewidths (8.2-11 km s$^{-1}$ in FWHM). The derived p-H$_2$CO rotational temperature, $64\pm15$ K, indicates the presence of warm gas. We also detected a narrow p-H$_2$CO line (FWHM=0.42 km s$^{-1}$) at the systemic velocity. The p-H$_2$CO abundance for the broad component appears to be enhanced by two orders of magnitude with respect to the narrow line value ($\sim3\times10^{-9}$ versus $\sim2\times10^{-11}$). The detected methanol line shows a linewidth similar to those of the broad p-H$_2$CO lines, which indicates their coexistence. The CO isotopologue data suggest that the CO depletion factor decreases from $\sim27\pm2$ towards the core centre to a value of $\sim8\pm1$ towards the core edge. In the latter position, the N$_2$D$^+$/N$_2$H$^+$ ratio is revised down to $0.14\pm0.06$. The origin of the subfragments inside the SMM3 core we found previously can be understood in terms of the Jeans instability if non-thermal motions are taken into account. The estimated fragmentation timescale, and the derived chemical abundances suggest that SMM3 is a few times $10^5$ yr old, in good agreement with its Class 0 classification inferred from the spectral energy distribution analysis. The broad p-H$_2$CO and CH$_3$OH lines, and the associated warm gas provide the first clear evidence of a molecular outflow driven by SMM3.

Low-metallicity star formation: relative impact of metals and magnetic fields

Low-metallicity star formation poses a central problem of cosmology, as it determines the characteristic mass scale and distribution for the first and second generations of stars forming in our Universe. Here, we present a comprehensive investigation assessing the relative impact of metals and magnetic fields, which may both be present during low-metallicity star formation. We show that the presence of magnetic fields generated via the small-scale dynamo stabilises the protostellar disc and provides some degree of support against fragmentation. In the absence of magnetic fields, the fragmentation timescale in our model decreases by a factor of ~10 at the transition from Z=0 to Z>0, with subsequently only a weak dependence on metallicity. Similarly, the accretion timescale of the cluster is set by the large-scale dynamics rather than the local thermodynamics. In the presence of magnetic fields, the primordial disc can become completely stable, therefore forming only one central fragment. At Z>0, the number of fragments is somewhat reduced in the presence of magnetic fields, though the shape of the mass spectrum is not strongly affected in the limits of the statistical uncertainties. The fragmentation timescale, however, increases by roughly a factor of 3 in the presence of magnetic fields. Indeed, our results indicate comparable fragmentation timescales in primordial runs without magnetic fields and Z>0 runs with magnetic fields.

The density and temperature dependence of the cooling timescale for fragmentation of self-gravitating disks

The purpose of this paper is to explore the influences of cooling timescale on fragmentation of self-gravitating protoplanetary disks. We assume the cooling timescale, expressed in terms of the dynamical timescale Ω tcool, has a power-law dependence on temperature and density, Ω tcool ∝ Σ−aT−b, where a and b are constants. We use this cooling timescale in a simple prescription for the cooling rate, du/dt = −u/tcool, where u is the internal energy. We perform our simulations using the smoothed particle hydrodynamics method. The simulations demonstrate that the disk is very sensitive to the cooling timescale, which depends on density and temperature. Under such a cooling timescale, the disk becomes gravitationally unstable and clumps form in the disk. This property even occurs for cooling timescales which are much longer than the critical cooling timescale, Ω tcool ≳ 7. We show that by adding the dependence of a cooling timescale on temperature and density, the number of clumps increases and the clumps can also form at smaller radii. The simulations imply that the sensitivity of a cooling timescale to density is more than to temperature, because even for a small dependence of the cooling timescale on density, clumps can still form in the disk. However, when the cooling timescale has a large dependence on temperature, clumps form in the disk. We also consider the effects of artificial viscosity parameters on fragmentation conditions. This consideration is performed in two cases, where Ω tcool is a constant and Ω tcool is a function of density and temperature. The simulations consider both cases, and results show the artificial viscosity parameters have rather similar effects. For example, using too small of values for linear and quadratic terms in artificial viscosity can suppress the gravitational instability and consequently the efficiency of the clump formation process decreases. This property is consistent with recent simulations of self-gravitating disks. We perform simulations with and without the Balsara form of artificial viscosity. We find that in the cooling and self-gravitating disks without the Balsara switch, the clumps can form more easily than those with the Balsara switch. Moreover, in both cases where the Balsara switch is present or absent, the simulations show that the cooling timescale strongly depends on density and temperature.

Outgassing: Influence on speed of magma fragmentation

AbstractPredicting explosive eruptions remains an outstanding challenge. Knowledge of the controlling parameters and their relative importance is crucial to deepen our understanding of conduit flow dynamics and accurately model the processes involved. This experimental study sheds light on one important parameter—outgassing—and evaluates its influence on magma fragmentation behavior. We perform fragmentation experiments based on the shock tube theory at room temperature on natural pyroclastic material with a connected porosity ranging from 15% to 78%. For each sample series, we determine the initial pressure (P) required to initiate magma fragmentation (fragmentation threshold, Pth). Furthermore, we measure the permeability of each sample for P &lt; Pth and the fragmentation speed for P &gt; Pth. A significant loss of initial pressure, caused by outgassing in samples with permeability ≥1e–12 m2, is observed within the fragmentation time scale (a few milliseconds). The samples are classified into: (a) dome/conduit wall rocks and (b) pumice/scoria. Substantial outgassing during fragmentation leads to higher fragmentation thresholds. Experimental fragmentation speeds are significantly higher than the modeled fragmentation speeds for high‐permeability dome/conduit wall rocks, but lower for high‐permeability pumices. Experimental fragmentation speeds for low‐permeability dome/conduit wall rocks and low‐permeability pumice/scoria are as expected. We also find that low‐porosity, low‐permeability, altered dome/conduit wall rocks fragment at significantly higher speeds than expected. Because fragmentation threshold and fragmentation speed are among the determining parameters for the initiation, sustainment and cessation of an eruption, outgassing should be considered in the modeling of magma fragmentation dynamics.

On the convergence of the critical cooling time-scale for the fragmentation of self-gravitating discs

We carry out simulations of gravitationally unstable discs using a Smoothed Particle Hydrodynamics (SPH) code and a grid-based hydrodynamics code, FARGO, to understand the previous non-convergent results reported by Meru & Bate (2011a). We obtain evidence that convergence with increasing resolution occurs with both SPH and FARGO and in both cases we find that the critical cooling timescale is larger than previously thought. We show that SPH has a first-order convergence rate while FARGO converges with a second-order rate. We show that the convergence of the critical cooling timescale for fragmentation depends largely on the numerical viscosity employed in both SPH and FARGO. With SPH, particle velocity dispersion may also play a role. We show that reducing the dissipation from the numerical viscosity leads to larger values of the critical cooling time at a given resolution. For SPH, we find that the effect of the dissipation due to the numerical viscosity is somewhat larger than had previously been appreciated. In particular, we show that using a quadratic term in the SPH artificial viscosity (beta_{SPH}) that is too low appears to lead to excess dissipation in gravitationally unstable discs, which may affect any results that sensitively depend on the thermodynamics, such as disc fragmentation. We show that the two codes converge to values of the critical cooling timescale, beta_{crit} > 20 (for a ratio of specific heats of gamma=5/3), and perhaps even as large as beta_{crit} \approx 30. These are approximately 3-5 times larger than has been found by most previous studies. This is equivalent to a maximum gravitational stress that a disc can withstand without fragmenting of alpha_{GI,crit} \approx 0.013-0.02, which is much smaller than the values typically used in the literature. It is therefore easier for self-gravitating discs to fragment than has been concluded from most past studies.

Formation of Dense Gas and Stars near the End of the Galactic Bar

Abstract We carried out H$ ^{13}$ CO$ ^{+}$ ($ J$$ =$ 1–0) observations of a molecular cloud containing a massive star-forming region, G23.44$-$ 0.18, using the Nobeyama 45-m radio telescope. We identified three clumps, named Clumps A, B, and C, on the periphery of the H II region in the cloud. The most massive clump, Clump A, has a radius of 0.74 $ \pm$ 0.20 pc and a mass of 1100$ ^{+800}_{-\hskip.35mm 640}\ M\odot$ . Both Clumps B and C, however, have much smaller size and mass than Clump A. These three clumps seem not to be virialized. We also found four sub-clumps in Clump A. These sub-clumps may be affected by the strong outflow penetrating Clump A. The star-formation efficiency (SFE) of the entire cloud is $ \sim$ 0.4%, which is typical for galactic star-forming clouds. The SFE of Clump A is 25$ ^{+14}_{-\hskip.35mm 11}$ %. This value is higher than other embedded cluster-forming clumps, which implies that stars, including massive ones, are formed efficiently and actively in Clump A. The dense gas fraction estimated from H$ ^{13}$ CO$ ^{+}$ ($ J$$ =$ 1–0) and $ ^{13}$ CO ($ J$$ =$ 1–0) of the cloud are lower than those of other star-forming regions. The results mean that this star-forming region is young, and suggest that the formation of dense gas and stars in the cloud has just begun. Comparing the dynamical age of the H II region with the fragmentation timescale for the collect and collapse process, molecular gas is accumulating through expansion of the H II region in Clumps B and C. Since there are massive young stellar objects (YSOs) in Clump A, we suggest that this clump has already become denser than the other regions in the cloud due to converging flow, or some external factors, such as an old bubble and cloud–cloud interaction.

Influence of in-mediumNNcross sections, symmetry potential, and impact parameter on isospin observables

We explore the influence of in-medium nucleon-nucleon cross section, symmetry potential and impact parameter on isospin sensitive observables in intermediate-energy heavy-ion collisions with the ImQMD05 code, a modified version of Quantum Molecular Dynamics model. At incident velocities above the Fermi velocity, we find that the density dependence of symmetry potential plays a more important role on the double neutron to proton ratio $DR(n/p)$ and the isospin transport ratio $R_i$ than the in-medium nucleon-nucleon cross sections, provided that the latter are constrained to a fixed total NN collision rate. We also explore both $DR(n/p)$ and $R_i$ as a function of the impact parameter. Since the copious production of intermediate mass fragments is a distinguishing feature of intermediate-energy heavy-ion collisions, we examine the isospin transport ratios constructed from different groups of fragments. We find that the values of the isospin transport ratios for projectile rapidity fragments with $Z\ge20$ are greater than those constructed from the entire projectile rapidity source. We believe experimental investigations of this phenomenon can be performed. These may provide significant tests of fragmentation time scales predicted by ImQMD calculations.

Convergence of smoothed particle hydrodynamics simulations of self-gravitating accretion discs: sensitivity to the implementation of radiative cooling

Recent simulations of self-gravitating accretion discs, carried out using a three-dimensional Smoothed Particle Hydrodynamics (SPH) code by Meru and Bate, have been interpreted as implying that three-dimensional global discs fragment much more easily than would be expected from a two-dimensional local model. Subsequently, global and local two-dimensional models have been shown to display similar fragmentation properties, leaving it unclear whether the three-dimensional results reflect a physical effect or a numerical problem associated with the treatment of cooling or artificial viscosity in SPH. Here, we study how fragmentation of self-gravitating disc flows in SPH depends upon the implementation of cooling. We run disc simulations that compare a simple cooling scheme, in which each particle loses energy based upon its internal energy per unit mass, with a method in which the cooling is derived from a smoothed internal energy density field. For the simple per particle cooling scheme, we find a significant increase in the minimum cooling time scale for fragmentation with increasing resolution, matching previous results. Switching to smoothed cooling, however, results in lower critical cooling time scales, and tentative evidence for convergence at the highest spatial resolution tested. We conclude that precision studies of fragmentation using SPH require careful consideration of how cooling (and, probably, artificial viscosity) is implemented, and that the apparent non-convergence of the fragmentation boundary seen in prior simulations is likely a numerical effect. In real discs, where cooling is physically smoothed by radiative transfer effects, the fragmentation boundary is probably displaced from the two-dimensional value by a factor that is only of the order of unity.

Non-convergence of the critical cooling time-scale for fragmentation of self-gravitating discs

Abstract We carry out a resolution study on the fragmentation boundary of self-gravitating discs. We perform three-dimensional Smoothed Particle Hydrodynamics simulations of discs to determine whether the critical value of the cooling time-scale in units of the orbital time-scale, βcrit, converges with increasing resolution. Using particle numbers ranging from 31 250 to 16 million (the highest resolution simulations to date) we do not find convergence. Instead, fragmentation occurs for longer cooling time-scales as the resolution is increased. These results suggest that at the very least, the critical value of the cooling time-scale is longer than previously thought. However, the absence of convergence also raises the question of whether or not a critical value exists. In light of these results, we caution against using cooling time-scale or gravitational stress arguments to deduce whether gravitational instability may or may not have been the formation mechanism for observed planetary systems.

Non-convergence of the critical cooling timescale for fragmentation of self-gravitating discs

AbstractWe carry out a resolution study on the fragmentation boundary of self-gravitating discs. We perform three-dimensional Smoothed Particle Hydrodynamics (SPH) simulations of discs to determine whether the critical value of the cooling timescale in units of the orbital timescale, βcrit, converges with increasing resolution. Using particle numbers ranging from 31,250 to 16 million (the highest resolution simulations to date) we do not find convergence. Instead, fragmentation occurs for longer cooling timescales as the resolution is increased. These results certainly suggest that βcrit is larger than previously thought. However, the absence of convergence also questions whether or not a critical value exists. In light of these results, we caution against using cooling timescale or gravitational stress arguments to deduce whether gravitational instability may or may not have been the formation mechanism for observed planetary systems.

SuperWASP observations of the 2007 outburst of Comet 17P/Holmes

We present wide-field imaging of the 2007 outburst of Comet 17P/Holmes obtained serendipitously by SuperWASP-North on 17 nights over a 42-night period beginning on the night (2007 October 22-23) immediately prior to the outburst. Photometry of 17P's unresolved coma in SuperWASP data taken on the first night of the outburst is consistent with exponential brightening, suggesting that the rapid increase in the scattering cross-section of the coma could be largely due to the progressive fragmentation of ejected material produced on a very short timescale at the time of the initial outburst, with fragmentation timescales decreasing from t(frag)~2x10^3 s to t(frag)~1x10^3 s over our observing period. Analysis of the expansion of 17P's coma reveals a velocity gradient suggesting that the outer coma was dominated by material ejected in an instantaneous, explosive manner. We find an expansion velocity at the edge of the dust coma of v(exp) = 0.55+/-0.02 km/s and a likely outburst date of t_0=2007 October 23.3+/-0.3, consistent with our finding that the comet remained below SuperWASP's detection limit of m(V)~15 mag until at least 2007 October 23.3. Modelling of 17P's gas coma indicates that its outer edge, which was observed to extend past the outer dust coma, is best explained with a single pulse of gas production, consistent with our conclusions concerning the production of the outer dust coma.