ALBERT

Description: Radiation hydrodynamics (RHD) simulations are used to study many astrophysical phenomena; however, they require the use of simplified radiation transport and thermal prescriptions to reduce computational cost. In this paper, we present a systematic study of the importance of microphysical processes in RHD simulations using the example of D-type H ii region expansion. We compare the simplest hydrogen-only models with those that include: ionization of H, He, C, N, O, S and Ne, different gas metallicity, non-LTE metal-line-blanketed stellar spectral models of varying metallicity, radiation pressure, dust and treatment of photodissociation regions. Each of these processes is explicitly treated using modern numerical methods rather than parametrization. In line with expectations, changes due to microphysics in either the effective number of ionizing photons or the thermal structure of the gas lead to differences in D-type expansion. In general, we find that more realistic calculations lead to the onset of D-type expansion at smaller radii and a slower subsequent expansion. Simulations of star-forming regions using simplified microphysics are therefore likely overestimating the strength of radiative feedback. We find that both variations in gas metallicity and the inclusion of dust can affect the ionization front evolution at the 10–20 per cent level over 500 kyr, which could substantially modify the results of simplified 3D models including feedback. Stellar metallicity, radiation pressure and the inclusion of photodissociation regions are all less-significant effects at the 1 per cent level or less, rendering them of minor importance in the modelling the dynamical evolution of H ii regions.

Description: We use a simple, self-consistent, self-gravitating semi-analytic disc model to conduct an examination of the parameter space in which self-gravitating discs may exist. We then use Monte Carlo radiative transfer to generate synthetic Atacama Large Millimeter/submillimeter Array (ALMA) images of these self-gravitating discs to determine the subset of this parameter space in which they generate non-axisymmetric structure that is potentially detectable by ALMA. Recently, several transition discs have been observed to have non-axisymmetric structure that extends out to large radii. It has been suggested that one possible origin of these asymmetries could be spiral density waves induced by disc self-gravity. We use our simple model to see if these discs exist in the region of parameter space where self-gravity could feasibly explain these spiral features. We find that for self-gravity to play a role in these systems typically requires a disc mass around an order of magnitude higher than the observed disc masses for the systems. The spiral amplitudes produced by self-gravity in the local approximation are relatively weak when compared to amplitudes produced by tidal interactions, or spirals launched at Lindblad resonances due to embedded planets in the disc. As such, we ultimately caution against diagnosing spiral features as being due to self-gravity, unless the disc exists in the very narrow region of parameter space where the spiral wave amplitudes are large enough to produce detectable features, but not so large as to cause the disc to fragment.

Description: The interaction of ionizing and far-ultraviolet radiation with the interstellar medium is of great importance. It results in the formation of regions in which the gas is ionized, beyond which are photodissociation regions (PDRs) in which the gas transitions to its atomic and molecular form. Several numerical codes have been implemented to study these two main phases of the interstellar medium either dynamically or chemically. In this paper we present torus-3dpdr , a new self-consistent code for treating the chemistry of three-dimensional photoionization and photodissociation regions. It is an integrated code coupling the two codes torus , a hydrodynamics and Monte Carlo radiation transport code, and 3d-pdr , a PDRs code. The new code uses a Monte Carlo radiative transfer scheme to account for the propagation of the ionizing radiation including the diffusive component as well as a ray-tracing scheme based on the healpix package in order to account for the escape probability and column density calculations. Here, we present the numerical techniques we followed and we show the capabilities of the new code in modelling three-dimensional objects including single or multiple sources. We discuss the effects introduced by the diffusive component of the ultraviolet field in determining the thermal balance of PDRs as well as the effects introduced by a multiple sources treatment of the radiation field. With this new code, three-dimensional synthetic observations for the major cooling lines are possible, for making feasible a detailed comparison between hydrodynamical simulations and observations.

Description: We assess the multiwavelength observable properties of the bow shock around a runaway early-type star using a combination of hydrodynamical modelling, radiative transfer calculations and synthetic imaging. Instabilities associated with the forward shock produce dense knots of material which are warm, ionized and contain dust. These knots of material are responsible for the majority of emission at far infrared, H α and radio wavelengths. The large-scale bow shock morphology is very similar and differences are primarily due to variations in the assumed spatial resolution. However infrared intensity slices (at 22 microns and 12 microns) show that the effects of a temperature gradient can be resolved at a realistic spatial resolution for an object at a distance of 1 kpc.

Description: 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.

Description: We present photometric and spectroscopic models of the Classical T Tauri star AA Tau. Photometric and spectroscopic variability present in observations of AA Tau is attributed to a magnetically induced warp in the accretion disc, periodically occulting the photosphere on an 8.2 d time-scale. Emission line profiles show signatures of both infall, attributed to magnetospherically accreting material, and outflow. Using the radiative transfer code torus , we have investigated the geometry and kinematics of AA Tau's circumstellar disc and outflow, which is modelled here as a disc wind. Photometric models have been used to constrain the aspect ratio of the disc, the offset angle of the magnetosphere dipole with respect to the stellar rotation axis, and the inner radius of the circumstellar disc. Spectroscopic models have been used to constrain the wind and magnetosphere temperatures, wind acceleration parameter, and mass-loss rate. We find that observations are best fitted by models with a mass accretion rate of 5 10 –9 M  yr –1 , a dipole offset of between 10° and 20°, a magnetosphere that truncates the disc from 5.2 to 8.8 R * , a mass-loss-rate to accretion-rate ratio of ~0.1, a magnetosphere temperature of 8500–9000 K, and a disc-wind temperature of 8000 K.

Description: We investigate observational signatures of triggered star formation in bright rimmed clouds (BRCs) by using molecular line transfer calculations based on radiation-hydrodynamic radiatively driven implosion models. We find that for BRCs the separation in velocity between the line profile peak of an optically thick and an optically thin line is determined by both the observer viewing angle and the density of the shell driving into the cloud. In agreement with observations, we find that most BRC line profiles are symmetric and that asymmetries can be either red or blue, in contrast to the blue dominance expected for a collapsing cloud. Asymmetries in the line profiles arise when an optically thick line is dominated by the shell and an optically thin line is dominated by the cloud interior to the shell. The asymmetries are red or blue depending on whether the shell is moving towards or away from the observer, respectively. Using the known motions of the molecular gas in our models we rule out the ‘envelope expansion with core collapse’ mechanism as the cause of the lack of blue-asymmetry in our simulated observations. We show that the absence of a strong photon-dominated region (PDR) around a BRC may not rule out the presence of triggered star formation: if the BRC line profile has a strong blue component then the shell is expected to be driving towards the observer, suggesting that the cloud is being viewed from behind and the PDR is obstructed. This could explain why BRCs such as SFO 80, 81 and 86 have a blue secondary peak and only a weak PDR inferred at 8 μm. Finally we also test the use of 12 CO, 13 CO and C 18 O as diagnostics of cloud mass, temperature and column density. We find that the inferred conditions are in reasonable agreement with those from the models. Calculating the cloud mass assuming spherical symmetry is shown to introduce an error of an order of magnitude whereas integrating the column density over a given region is found to introduce an error of up to a factor of 2.