Other
Scientific paper
May 2002
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2002phdt.........8v&link_type=abstract
Ph.D. thesis, Leiden University, May 2002.
Other
4
Ism: Molecules, Stars: Pre-Main Sequence, Stars: Circumstellar Matter, Stars: Planetary Systems: Protoplanetary Disks, Radiative Transfer, Line: Profiles
Scientific paper
In order to understand better the nature of circumstellar disks around (proto)stars and their position in an evolutionary sense towards planetary systems, different approaches have been used in this thesis. All these approaches start with a model containing information on the surface density and temperature structure, taken from the literature or calculated, and are eventually compared to molecular line observations.
Radiative transfer is one of the key instruments used in this thesis. In Chapter 2, a new two dimensional radiative transfer code for the UV continuum radiation is introduced. With the use of this code the stellar and interstellar radiation field can be traced inside any cylindrical symmetric medium. This can be used both for the calculation of the dissociation and ionization rates of molecules and atoms and for the calculation of the gas and dust temperature in disks. In this thesis only the first of these possibilities is used. The line radiative transfer code by Hogerheijde & van der Tak (2000) is used to calculate the rotational level populations of the atom/ion/molecule of interest. In this thesis, this code is tested against seven other line radiative transfer codes using a one-dimensional problem.
The physical conditions of different disk modesl are tested by line ratios of CO, HCO+, HCN and their isotopomers The different line ratios and optical depths indicate that most of the observed line emission arises from an intermediate disk layer with high densities of 1e6 -1e8 and moderately warm temperatures in the outer regions. The density and temperatures found prefer flaring disk models over non flaring models. The 2-D UV code described in Chapter 2 is used to calculate the chemistry more self-consistently in Chapter 6. The use of this code enabled us to take into account different stellar UV fields and study the influence arising from this on the emission from radicals like CN.
In the last Chapter the focus is centered on the optically thin regime, where low-mass disks around A-stars are discussed. The most prominent of these disks are Vega and -Pictoris, two nearby A-stars. The line emission for these disks is calculated and compared to the few available observed molecular tracers. Predictions are made for observations using future telescope facilities. It is proposed that the combined C and C+ emission observations can determine whether these disks are true proto-planetary disks or gaspoor debris disks. Care must be taken, however, in subtracting any background C+ emission.
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