We present a set of new numerical methods that are relevant to calculating radiation pressure terms in hydrodynamics calculations, with a particular focus on massive star formation. The radiation ...force is determined from a Monte Carlo estimator and enables a complete treatment of the detailed microphysics, including polychromatic radiation and anisotropic scattering, in both the free-streaming and optically thick limits. Since the new method is computationally demanding we have developed two new methods that speed up the algorithm. The first is a photon packet splitting algorithm that enables efficient treatment of the Monte Carlo process in very optically thick regions. The second is a parallelization method that distributes the Monte Carlo workload over many instances of the hydrodynamic domain, resulting in excellent scaling of the radiation step. We also describe the implementation of a sink particle method that enables us to follow the accretion on to, and the growth of, the protostars. We detail the results of extensive testing and benchmarking of the new algorithms.
Abstract
We present a numerical simulation of the formation of a massive star using Monte Carlo-based radiation hydrodynamics (RHD). The star forms via stochastic disc accretion and produces fast, ...radiation-driven bipolar cavities. We find that the evolution of the infall rate (considered to be the mass flux across a 1500 au spherical boundary) and the accretion rate on to the protostar, are broadly consistent with observational constraints. After 35 kyr the star has a mass of 25 M⊙ and is surrounded by a disc of mass 7 M⊙ and 1500 au radius, and we find that the velocity field of the disc is close to Keplerian. Once again these results are consistent with those from recent high-resolution studies of discs around forming massive stars. Synthetic imaging of the RHD model shows good agreement with observations in the near- and far-IR, but may be in conflict with observations that suggest that massive young stellar objects are typically circularly symmetric in the sky at 24.5 μm. Molecular line simulations of a CH3CN transition compare well with observations in terms of surface brightness and line width, and indicate that it should be possible to reliably extract the protostellar mass from such observations.
The ancient heritage of water ice in the solar system Cleeves, L. Ilsedore; Bergin, Edwin A.; Alexander, Conel M. O’D. ...
Science (American Association for the Advancement of Science),
09/2014, Letnik:
345, Številka:
6204
Journal Article
Recenzirano
Odprti dostop
Identifying the source of Earth’s water is central to understanding the origins of life-fostering environments and to assessing the prevalence of such environments in space. Water throughout the ...solar system exhibits deuterium-to-hydrogen enrichments, a fossil relic of low-temperature, ion-derived chemistry within either (i) the parent molecular cloud or (ii) the solar nebula protoplanetary disk. Using a comprehensive treatment of disk ionization, we find that ion-driven deuterium pathways are inefficient, which curtails the disk’s deuterated water formation and its viability as the sole source for the solar system’s water. This finding implies that, if the solar system’s formation was typical, abundant interstellar ices are available to all nascent planetary systems.
We investigate the effect of including diffuse field radiation when modelling the radiatively driven implosion of a Bonnor-Ebert sphere (BES). Radiation-hydrodynamical calculations are performed by ...using operator splitting to combine Monte Carlo photoionization with grid-based Eulerian hydrodynamics that includes self-gravity. It is found that the diffuse field has a significant effect on the nature of radiatively driven collapse which is strongly coupled to the strength of the driving shock that is established before impacting the BES. This can result in either slower or more rapid star formation than expected using the on-the-spot approximation depending on the distance of the BES from the source object. As well as directly compressing the BES, stronger shocks increase the thickness and density in the shell of accumulated material, which leads to short, strong, photoevaporative ejections that reinforce the compression whenever it slows. This happens particularly effectively when the diffuse field is included as rocket motion is induced over a larger area of the shell surface. The formation and evolution of 'elephant trunks' via instability is also found to vary significantly when the diffuse field is included. Since the perturbations that seed instabilities are smeared out elephant trunks form less readily and, once formed, are exposed to enhanced thermal compression.
Abstract
We simulate a self-gravitating, turbulent cloud of 1000 M⊙ with photoionization and radiation pressure feedback from a 34 M⊙ star. We use a detailed Monte Carlo radiative transfer scheme ...alongside the hydrodynamics to compute photoionization and thermal equilibrium with dust grains and multiple atomic species. Using these gas temperatures, dust temperatures, and ionization fractions, we produce self-consistent synthetic observations of line and continuum emission. We find that all material is dispersed from the (15.5 pc)3 grid within 1.6 Myr or 0.74 free-fall times. Mass exits with a peak flux of 2 × 10−3 M⊙ yr−1, showing efficient gas dispersal. The model without radiation pressure has a slight delay in the breakthrough of ionization, but overall its effects are negligible. 85 per cent of the volume, and 40 per cent of the mass, become ionized – dense filaments resist ionization and are swept up into spherical cores with pillars that point radially away from the ionizing star. We use free–free emission at 20 cm to estimate the production rate of ionizing photons. This is almost always underestimated: by a factor of a few at early stages, then by orders of magnitude as mass leaves the volume. We also test the ratio of dust continuum surface brightnesses at 450 and 850 µm to probe dust temperatures. This underestimates the actual temperature by more than a factor of 2 in areas of low column density or high line-of-sight temperature dispersion; the $\rm {H\, {\small II}}$ region cavity is particularly prone to this discrepancy. However, the probe is accurate in dense locations such as filaments.
A new Monte Carlo algorithm for calculating time-dependent radiative transfer under the assumption of local thermodynamic equilibrium is presented. Unlike flux-limited diffusion, the method is ...polychromatic, includes scattering, and is able to treat the optically-thick and free-streaming regimes simultaneously. The algorithm is tested on a variety of 1D and 2D problems, and good agreement with benchmark solutions is found. The method is used to calculate the time-varying spectral energy distribution from a circumstellar disc illuminated by a protostar whose accretion luminosity is varying. It is shown that the time-lag between the optical variability and the infrared variability results from a combination of the photon traveltime and the thermal response in the disc, and that the lag is an approximately linear function of wavelength.
Abstract
Dust evolution in protoplanetary disks from small dust grains to pebbles is key to the planet formation process. The gas in protoplanetary disks should influence the vertical distribution of ...small dust grains (∼1
μ
m) in the disk. Utilizing archival near-infrared polarized light and millimeter observations, we can measure the scale height and flare parameter
β
of the small dust grain scattering surface and
12
CO gas emission surface for three protoplanetary disks: IM Lup, HD 163296, and HD 97048 (CU Cha). For two systems, IM Lup and HD 163296, the
12
CO gas and small dust grains at small radii from the star have similar heights, but at larger radii (>100 au), the dust grain scattering surface height is lower than the
12
CO gas emission surface height. In the case of HD 97048, the small dust grain scattering surface has similar heights to the
12
CO gas emission surface at all radii. We ran a protoplanetary disk radiative transfer model of a generic protoplanetary disk with TORUS and showed that there is no difference between the observed scattering surface and
12
CO emission surface. We also performed analytical modeling of the system and found that gas-to-dust ratios larger than 100 could explain the observed difference in IM Lup and HD 163296. This is the first direct comparison of observations of gas and small dust grain height distribution in protoplanetary disks. Future observations of gas emission and near-infrared scattered-light instruments are needed to look for similar trends in other protoplanetary disks.
We present the first images of the transition disk around the close binary system HD 34700A in polarized scattered light using the Gemini Planet Imager instrument on Gemini South. The J- and H-band ...images reveal multiple spiral-arm structures outside a large (R = 0 49 = 175 au) cavity, along with a bluish spiral structure inside the cavity. The cavity wall shows a strong discontinuity, and we clearly see significant non-azimuthal polarization Uφ, consistent with multiple scattering within a disk at an inferred inclination ∼42°. Radiative transfer modeling along with a new Gaia distance suggest HD 37400A is a young (∼5 Myr) system consisting of two intermediate-mass (∼2 M ) stars surrounded by a transitional disk and not a solar-mass binary with a debris disk, as previously classified. Conventional assumptions of the dust-to-gas ratio would rule out a gravitational instability origin to the spirals while hydrodynamical models using the known external companion or a hypothetical massive protoplanet in the cavity both have trouble reproducing the relatively large spiral-arm pitch angles (∼30°) without fine-tuning of gas temperature. We explore the possibility that material surrounding a massive protoplanet could explain the rim discontinuity after also considering effects of shadowing by an inner disk. Analysis of archival Hubble Space Telescope data suggests the disk is rotating counterclockwise as expected from the spiral-arm structure and revealed a new low-mass companion at 6 45 separation. We include an appendix that sets out clear definitions of Q, U, Qφ, Uφ, correcting some confusion and errors in the literature.
We investigate how the detectability of signatures of self-gravity in a protoplanetary disk depends on its temporal evolution. We run a one-dimensional model for secular timescales to follow the disk ...mass as a function of time. We then combine this with three-dimensional global hydrodynamics simulations that employ a hybrid radiative transfer method to approximate realistic heating and cooling. We simulate ALMA continuum observations of these systems and find that structures induced by the gravitational instability (GI) are readily detectable when q = Mdisk/M* 0.25 and Router 100 au. The high accretion rate generated by gravito-turbulence in such a massive disk drains its mass to below the detection threshold in ∼104 years, or approximately 1% of the typical disk lifetime. Therefore, disks with spiral arms detected in ALMA dust observations, if generated by self-gravity, must either be still receiving infall to maintain a high q value, or have just emerged from their natal envelope. Detection of substructure in systems with lower q is possible, but would require a specialist integration with the most extended configuration over several days. This disfavors the possibility of GI-caused spiral structure in systems with q < 0.25 being detected in relatively short integration times, such as those found in the DSHARP ALMA survey. We find no temporal dependence of detectability on dynamical timescales.
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