Computer Science
Scientific paper
Dec 1998
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1998phdt........16r&link_type=abstract
Ph.D. thesis, Julius-Maximilians-Univ. Würzburg, (1998)
Computer Science
1
Scientific paper
Circumstellar disks are a phenomenon frequently observed during the formation process of stars and planets. Young massive stars produce hydrogen-ionizing photons (EUV photons with energies h ν >= 13.6; eV) and are able to photoevaporate their own disks and the disks of low-mass objects in their immediate environment. Radiation hydrodynamical simulations are used to examine the interaction of circumstellar disks with the EUV photons and the stellar wind of the central star as well as the interaction of circumstellar disks with the EUV photons of an external source. An improved radiation hydrodynamics code solves the hydrodynamical equations including turbulent viscosity and self-gravity for a mixture of gas and dust. A continuum radiation transfer subroutine determines the dust temperature and the radiative acceleration. The transfer of EUV photons is calculated along radial lines of sight. This part of the code includes an implicit solution algorithm for the energy equation which solves the time-dependent rate equation for the degree of ionization of hydrogen and considers heating and cooling mechanisms most important for EUV-dominated regions. In addition, the transfer of diffuse EUV radiation is calculated for photons resulting from the recombination of hydrogen into the ground state and from scattering on dust grains. The calculations start with star-disk systems resulting from collapse simulations. The EUV photons of the central object of a star-disk system continuously photoevaporate the disk which leads to the formation of an ultracompact HII (UCHII) region. The lifetime of the UCHII region is determined by the photoevaporation rate of the disk. The simulations show that the diffuse EUV radiation field resulting from scattering on dust grains is important for the evolution of photoevaporating disks. Depending on the absorption and scattering properties of the dust particles the photoevaporation rate varies an order of magnitude. In addition, the code was used to examine systematically the dependence of the photoevaporation rate on the parameters of the ionizing star. The dependence of the photoevaporation rate .Mph on the stellar EUV photon rate S is consistent with semi-analytical calculations (.Mph ~ S0.58). The influence of the velocity and mass loss rate of the stellar wind is strongly dependent on the modeling of the wind and the structure of the disk. Hydrodynamical collimated bipolar outflows are created without invoking magnetic fields. The interaction of circumstellar disks with the EUV photons of an external star is calculated for a 0.58 Modot star surrounded by a 0.40 Modot disk. The external EUV flux is chosen to match the situation of the objects close to the Trapezium star θ1 Ori C in the Orion Nebula called proplyds. Cometary tails develop and break off into filaments which leave the immediate vicinity of the disk with the evaporating flow. The total mass of the disk fragments which break off during the relatively short cometary phase is of order 10% of the disk mass. After several 104 yr the disk is completely enveloped by the ionization front. With decreasing distance the densest parts of the disk remnant are more strongly disturbed. Applying a diagnostic ray-tracing procedure to the resulting structures shows that the disks in the cometary phase reproduce the typical head-tail structure of the proplyds. However, observations reveal a stand-off of the ionization front from the disk at several disk radii. This property of the proplyds can be explained by a non-negligible flux of FUV photons (6 eV <= hν <= 13.6 eV). A FUV module was developed which calculates the transfer of direct FUV photons, the transfer of diffuse FUV photons resulting from scattering on dust grains, the ionization of carbon and additional heating and cooling functions. Simulations including the FUV module show that FUV photons heat the region between the disk surface and the ionization front to temperatures up to 1 500 K and generate a wind of neutral material which prevents the ionization front from reaching the disk surface. Part of the neutral wind is redirected to the side of the disk opposed to the ionizing star. The redirection rate is ~20% of the photoevaporation rate. Due to this process the region behind the disk is continuously filled up with neutral material causing long-living tails (~106 yr). Simulations with warmer (~5000 K) neutral disk winds, with more powerful stellar winds and different star-disk models would help to retrieve further information on the nature of proplyds.
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