Onset Of Gravitational Collapse Research Articles

Chemical Variation among Protostellar Cores: Dependence on Prestellar Core Conditions

Abstract Hot corino chemistry and warm carbon chain chemistry (WCCC) are driven by gas–grain interactions in star-forming cores: radical–radical recombination reactions to form complex organic molecules (COMs) in the ice mantle, sublimation of CH4 and COMs, and their subsequent gas-phase reactions. These chemical features are expected to depend on the composition of the ice mantle, which is set in the prestellar phase. We calculated the gas–grain chemical reaction network considering a layered ice mantle structure in star-forming cores to investigate how the hot corino chemistry and WCCC depend on the physical condition of the static phase before the onset of gravitational collapse. We found that WCCC becomes more active if the temperature is lower, or the visual extinction is lower in the static phase, or the static phase is longer. The dependence of hot corino chemistry on the static-phase condition is more complex. While CH3OH is less abundant in the models with a warmer static phase, some COMs are formed efficiently in those warm models because there are various formation paths of COMs. If the visual extinction is lower, photolysis makes COMs less abundant in the static phase. Once the collapse starts and visual extinction increases, however, COMs can be formed efficiently. The duration of the static phase does not largely affect COM abundances. The chemical diversity between prototypical hot corinos and hybrid sources, in which both COMs and carbon chains are reasonably abundant, can be explained by the variation of prestellar conditions. Deficiency of gaseous COMs in prototypical WCCC sources is, however, hard to reproduce within our models.

Large-scale Mixing in a Violent Oxygen–Neon Shell Merger Prior to a Core-collapse Supernova

Abstract We present a 7 minute long 4π-3D simulation of a shell merger event in a nonrotating 18.88 M ⊙ supernova progenitor before the onset of gravitational collapse. The key motivation is to capture the large-scale mixing and asymmetries in the wake of the shell merger before collapse using a self-consistent approach. The 4π geometry is crucial, as it allows us to follow the growth and evolution of convective modes on the largest possible scales. We find significant differences between the kinematic, thermodynamic, and chemical evolution of the 3D and 1D models. The 3D model shows vigorous convection leading to more efficient mixing of nuclear species. In the 3D case, the entire oxygen shell attains convective Mach numbers of ≈0.1, whereas in the 1D model, the convective velocities are much lower, and there is negligible overshooting across convective boundaries. In the 3D case, the convective eddies entrain nuclear species from the neon (and carbon) layers into the deeper part of the oxygen-burning shell, where they burn and power a violent convection phase with outflows. This is a prototypical model of a convective–reactive system. Due to the strong convection and resulting efficient mixing, the interface between the neon layer and the silicon-enriched oxygen layer disappears during the evolution, and silicon is mixed far out into the merged oxygen/neon shell. Neon entrained inward by convective downdrafts burns, resulting in lower neon mass in the 3D model compared to the 1D model at the time of collapse. In addition, the 3D model develops remarkable large-scale, large-amplitude asymmetries, which may have important implications for the impending gravitational collapse and subsequent explosion.

Hydrodynamics with gas–grain chemistry and radiative transfer: comparing dynamical and static models

Context. We study the evolution of chemical-abundance gradients using dynamical and static models of starless cores. Aims. We aim to quantify if the chemical abundance gradients given by a dynamical model of core collapse, which includes time-dependent changes in density and temperature, differ greatly from abundances derived from static models where the density and temperature structures of the core are kept fixed as the chemistry evolves. Methods. We developed a new one-dimensional spherically symmetric hydrodynamics code that couples the hydrodynamics equations with a comprehensive time-dependent gas–grain chemical model, including deuterium and spin-state chemistry, and radiative transfer calculations to derive self-consistent time-dependent chemical-abundance gradients. We apply the code to model the collapse of a starless core up to the point when the infall flow becomes supersonic. Results. The abundances predicted by the dynamical and static models are almost identical at early times during the quiescent phase of core evolution. After the onset of core collapse, the results from the two models begin to diverge: at late times the static model generally underestimates abundances in the high-density regions near the core center, and overestimates them in the outer parts of the core. Deuterated species are clearly overproduced by the static model near the center of the model core. On the other hand, simulated lines of NH3 and N2H+ are brighter in the dynamical model because they originate in the central part of the core where the dynamical model predicts higher abundances than the static model. The reason for these differences is that the static model ignores the history of the density and temperature profiles which has a large impact on the abundances, and therefore on the molecular lines. Our results also indicate that the use of a very limited chemical network in hydrodynamical simulations may lead to an overestimate of the collapse timescale, and in some cases may prevent the collapse altogether. Limiting the set of molecular coolants has a similar effect. In our model, most of the line cooling near the center of the core is due to HCN, CO, and NO. Conclusions. Our results show that the use of a static physical model is not a reliable method of simulating chemical abundances in starless cores after the onset of gravitational collapse. The abundance differences between the dynamical and static models translate to large differences in line emission profiles, showing that the difference between the models is at the observable level. The adoption of complex chemistry and a comprehensive set of cooling molecules is necessary to model the collapse adequately.

CHEMISTRY IN A FORMING PROTOPLANETARY DISK: MAIN ACCRETION PHASE

ABSTRACT We investigate the chemistry in a radiation-hydrodynamics model of a star-forming core that evolves from a cold (∼10 K) prestellar core to the main accretion phase in ∼105 years. A rotationally supported gravitationally unstable disk is formed around a protostar. We extract the temporal variation of physical parameters in ∼1.5 × 103 SPH particles that end up in the disk, and perform post-processing calculations of the gas-grain chemistry adopting a three-phase model. Inside the disk, the SPH particles migrate both inward and outward. Since a significant fraction of volatiles such as CO can be trapped in the water-dominant ice in the three-phase model, the ice mantle composition depends not only on the current position in the disk, but also on whether the dust grain has ever experienced higher temperatures than the water sublimation temperature. Stable molecules such as H2O, CH4, NH3, and CH3OH are already abundant at the onset of gravitational collapse and are simply sublimated as the fluid parcels migrate inside the water snow line. On the other hand, various molecules such as carbon chains and complex organic molecules (COMs) are formed in the disk. The COMs abundance sensitively depends on the outcomes of photodissociation and diffusion rates of photofragments in bulk ice mantle. As for S-bearing species, H2S ice is abundant in the collapse phase. In the warm regions in the disk, H2S is sublimated to be destroyed, while SO, H2CS, OCS, and SO2 become abundant.

Dynamics, CO depletion, and deuterium fractionation of the dense condensations within the fragmented prestellar core Orion B9–SMM 6

We present APEX observations of C17O(2-1), N2H+(3-2), and N2D+(3-2) towards the subfragments inside the prestellar core SMM 6 in Orion B9. We combined these spectral line data with our previous SABOCA 350-{\mu}m dust continuum map of the source. The subfragments are characterised by subsonic internal non-thermal motions ({\sigma}NT~0.5cs), and most of them appear to be gravitationally bound. The dispersion of the N2H+ velocity centroids among the condensations is very low (0.02 km/s). The CO depletion factors we derive, fD=0.8+/-0.4 - 3.6+/-1.5, do not suggest any significant CO freeze-out but this may be due to the canonical CO abundance we adopt. The fractional abundances of N2H+ and N2D+ with respect to H2 are found to be ~0.9-2.3x10^-9 and ~4.9-9.9x10^-10, respectively. The deuterium fractionation of N2H+, or the N2D+/N2H+ column density ratio, lies in the range 0.30+/-0.07 - 0.43+/-0.09. The detected substructure inside SMM 6 is likely the result of cylindrical Jeans-type gravitational fragmentation. We estimate the timescale for this fragmentation to be ~1.8x10^5 yr. The condensations are unlikely to be able to interact with one another and coalesce before local gravitational collapse ensues. Moreover, significant mass growth of the condensations via competitive-like accretion from the parent core seems unfeasible. The high level of molecular deuteration in the condensations suggests that gas-phase CO should be strongly depleted. It also points towards an advanced stage of chemical evolution. The subfragments of SMM 6 might therefore be near the onset of gravitational collapse, but whether they can form protostellar or substellar objects (brown dwarfs) depends on the local star formation efficiency and remains to be clarified.

The formation of active protoclusters in the Aquila rift: a millimeter continuum view

Abridged -- We present an analysis of the Aquila Rift complex which addresses the questions of the star formation rate (SFR), star formation efficiency (SFE) and typical lifetime of the Class 0 protostellar phase in two nearby cluster-forming clumps: the Serpens South and W40 protoclusters. We carried out a 1.2 mm dust continuum mapping of the Aquila Rift complex with the MAMBO bolometer array on the IRAM 30m telescope. We perform a systematic source extraction in our millimeter continuum map. Based on complementary data from the Herschel Gould Belt survey and Spitzer maps, we characterize the SEDs of the 77 mm continuum sources detected with MAMBO and estimate their evolutionary stages. Taking advantage of the comprehensive dataset available for the Serpens South region, spanning wavelengths from 2 microns to 1.2 mm, we estimate the numbers of young stellar objects (YSOs) at different evolutionary stages and find a ratio of Class 0 to Class I protostars N(0)/N(I)=0.19-0.27. This low ratio supports a scenario of relatively fast accretion at the beginning of the protostellar phase, and leads to a Class 0 lifetime of ~4-9x10^{4} yr. We also show that both the Serpens South and W40 protoclusters are characterized by large fractions of protostars and high SFRs ~20-50 Msun.Myr^{-1}pc^{-2}, in agreement with the idea that these two nearby clumps are active sites of clustered star formation currently undergoing bursts of star formation, and have the potential ability to form bound star clusters. While the formation of these two protoclusters is likely to have been initiated in a very different manner, the resulting protostellar populations are observed to be very similar. This suggests that after the onset of gravitational collapse, the detailed manner in which the collapse has been initiated does not affect much the ability of a clump to form stars.

The thermal–mechanical evolution of crustal orogenic belts at convergent plate boundaries: A reappraisal of the orogenic cycle

Convergent plate boundaries are characterized by the development of crustal orogenic wedges and orogenic plateaus but also by gravitational collapse of previously thickened crust leading to the opening of intermontane and eventually oceanic back-arc basins. Foreland and extensional sedimentary basins in the plate boundary region are filled by the erosional products of the orogenic crust. Metamorphic rocks forming orogenic crust attest to burial and exhumation under contrasted geothermal gradients. These features portray the crustal orogenic cycle and are first-order indicators of the thermal and mechanical evolution of the crust within the plate boundary region. This evolution is controlled by complex interactions among (i) the dynamic balance among forces that arise from plate-tectonic, gravitational potential energy, and buoyancy, (ii) the thermal balance between deformation-induced and radioactive heat production and heat advection related to subduction, orogenic deformation, and magma transfer, and (iii) the mass transfer balance between uplift and erosion. To account for these geological characteristics, a generic model, that integrates results from physical modeling, is proposed for the thermal–mechanical evolution of crustal orogenic belts and for its implication in controlling the transition between the different phases of the orogenic cycle. In this model, the transition from low to high geothermal gradient is associated with increased heat production in the thickened crust owing to radioactive decay and deformation. Partial melting and rheologic weakening of the thermally mature thickened crust triggers gravity-driven lateral flow of the lower crust and controls the transition from wedge to orogenic plateau. Destruction of the orogenic crust is achieved in part by erosion but mostly by gravitational collapse. The style of extension is controlled by the rheology of the crust at the onset of gravitational collapse and its evolution as the crust thins and cools. Gravitational collapse is permitted by a modification of lithosphere dynamics in the convergence zone and might eventually lead to opening of a new oceanic basin if collapse is followed by thinning of the lithospheric mantle.

A GENERAL RELATIVISTIC RAY-TRACING METHOD FOR ESTIMATING THE ENERGY AND MOMENTUM DEPOSITION BY NEUTRINO PAIR ANNIHILATION IN COLLAPSARS

Bearing in mind the application to the collapsar models of gamma-ray bursts (GRBs), we develop a numerical scheme and code for estimating the deposition of energy and momentum due to the neutrino pair annihilation ($\nu + {\bar \nu} \rightarrow e^{-} + e^{+}$) in the vicinity of accretion tori around a Kerr black hole. Our code is designed to solve the general relativistic neutrino transfer by a ray-tracing method. To solve the collisional Boltzmann equation in curved spacetime, we numerically integrate the so-called rendering equation along the null geodesics. For the neutrino opacity, the charged-current $\beta$-processes are taken into account, which are dominant in the vicinity of the accretion tori. The numerical accuracy of the developed code is certificated by several tests, in which we show comparisons with the corresponding analytic solutions. Based on the hydrodynamical data in our collapsar simulation, we estimate the annihilation rates in a post-processing manner. Increasing the Kerr parameter from 0 to 1, it is found that the general relativistic effect can increase the local energy deposition rate by about one order of magnitude, and the net energy deposition rate by several tens of percents. After the accretion disk settles into a stationary state (typically later than $\sim 9$ s from the onset of gravitational collapse), we point out that the neutrino-heating timescale in the vicinity of the polar funnel region can be shorter than the dynamical timescale. Our results suggest the neutrino pair annihilation has a potential importance equal to the conventional magnetohydrodynamic mechanism for igniting the GRB fireballs.

NEUTRINO PAIR ANNIHILATION IN COLLAPSARS: A RAY-TRACING METHOD IN SPECIAL RELATIVITY

We develop a numerical scheme and code for estimating the energy and momentum transfer via neutrino pair annihilation ($\nu + {\bar \nu} \to e^{-}+ e^{+}$), bearing in mind the application to the collapsar models of gamma-ray bursts (GRBs). To calculate the neutrino flux illuminated from the accretion disk, we perform a ray-tracing calculation in the framework of special relativity. The numerical accuracy of the developed code is certificated by several tests, in which we show comparisons with the corresponding analytical solutions. Using hydrodynamical data in our collapsar simulation, we estimate the annihilation rates in a post-processing manner. We show that the neutrino energy deposition and momentum transfers are strongest near the inner edge of the accretion disk. The beaming effects of special relativity are found to change the annihilation rates by several factors in the polar funnel region. After the accretion disk settles into a stationary state (typically later than $\sim 9$ s from the onset of gravitational collapse), we find that the neutrino-heating timescale in the vicinity of the polar funnel ($\lesssim 80$ km) can become shorter than the hydrodynamical timescale, indicating that the neutrino-heated outflows can be launched there. We point out that the momentum transfer can play as important role as the energy deposition for the efficient acceleration of neutrino-driven outflows. Our results suggest that the neutrino pair annihilation has a potential importance equal to the conventional magnetohydrodynamic mechanism for igniting the GRB fireballs.

The distribution of ND2H in LDN 1689N

Finding tracers of the innermost regions of prestellar cores is important for understanding their chemical and dynamical evolution before the onset of gravitational collapse. While classical molecular tracers, such as CO and CS, have been shown to be strongly depleted in cold, dense gas by condensation on grain mantles, it has been a subject of discussion to what extent nitrogen-bearing species, such as ammonia, are affected by this process. As deuterium fractionation is efficient in cold, dense gas, deuterated species are excellent tracers of prestellar cores. A comparison of the spatial distribution of neutral and ionized deuterated species with the dust continuum emission can thus provide important insights into the physical and chemical structure of such regions. We study the spatial distribution of the ground-state 335.5 GHz line of ND2H in LDN1689N, using APEX, and compare it with the distribution of the DCO+(3--2) line, as well as the 350 micron dust continuum emission observed with the SHARC~II bolometer camera at CSO. While the distribution of the ND2H emission in LDN1689N is generally similar to that of the 350 microns dust continuum emission, the peak of the ND2H emission is offset by ~10'' to the East from the dust continuum and DCO+ emission peak. ND2H and ND3 share the same spatial distribution. The observed offset between the ND2H and DCO+ emission is consistent with the hypothesis that the deuterium peak in LDN1689N is an interaction region between the outflow shock from IRAS16293--2422 and the dense ambient gas. We detect the J = 4 - 3 line of H13CO+ at 346.998 GHz in the image side band serendipitously. This line shows the same spatial distribution as DCO+(3--2), and peaks close to the 350 mic emission maximum which provides further support for the shock interaction scenario.

Strongly induced collapse in the Class 0 protostar NGC 1333 IRAS 4A

Context. The onset of gravitational collapse in cluster-forming clouds is still poorly known. Aims. Our goal is to use the Class 0 protostar IRAS 4A, which is undergoing collapse in the active molecular cloud NGC 1333, to set constraints on this process. In particular we want to measure the mass infall rate and investigate whether the collapse could have been triggered by a strong external perturbation. Methods. We analyze existing continuum observations to derive the density structure of the envelope, and use our new molecular line observations with the IRAM 30 m telescope to probe its velocity structure. We perform a detailed comparison of this set of data with a numerical model of collapse triggered by a fast external compression. Results. Both the density and velocity structures of the envelope can be well fitted by this model of collapse induced by a fast external compression for a time of 1-2 x 10 4 yr after the formation of the central protostar. We deduce a high mass infall rate of 0.7-2 x 10 -4 M ⊙ yr -1 . The momentum required for the perturbation to produce this high mass infall rate is on the same order as the momenta measured for the NGC 1333 numerous outflows. Our analysis also shows that the turbulence is highly non uniform in the envelope, dropping from supersonic to subsonic values toward the center. The inner subsonic turbulence is most likely a relic of the conditions prevailing in the dense core before the onset of collapse. Conclusions. The vigorous collapse undergone by IRAS 4A was triggered by a fast external compression, probably related to the expansion of a nearby cavity, which could have simultaneously triggered the collapse of the nearby Class 0 protostar IRAS 4B. This cavity could have been generated by an outflow but we have not found a good protostellar candidate yet.

Radiation-driven implosion of molecular cloud cores

We present the first three-dimensional calculations of Radiation Driven Implosion of Molecular Clouds including self-gravity and ionization. We discuss the effects of initial density perturbations on the dynamics of ionizing globules and show that the onset of gravitational collapse can be significantly delayed for a multiple of the implosion timescale. We demonstrate that Radiation Driven Implosion could be an efficient process for injecting disordered kinetic energy into molecular clouds in the vicinity of massive stars.