C. M. Leung

CSDUST3: A radiation transport code for a dusty medium with 1-d planar, spherical or cylindrical geometry

Abstract The program solves the radiation transport problem in a dusty medium with one-dimensional planar, spherical or cylindrical geometry. It determines self-consistently the effects of multiple scattering, absorption, and re-emission of photons on the temperature of dust grains and the characteristics of the internal radiation field. The program can treat radiation field anisotropy, linear anisotropic scattering, and multi-grain components. The program output consists of the dust temperature distribution, flux spectrum, surface brightness at each frequency and the observed intensities (involving a convolution with a telescope beam pattern).

Testing star formation theories - VLA observations of H2CO in the BOK globule B335

The H2CO 6-cm line was observed with the VLA toward B335. The line is seen in absorption against the cosmic background radiation in a ring-shaped region centered on the infrared source. The data were analyzed with both LVG and microturbulent radiative transfer models. The two models agree qualitatively but differ quantitatively. The microturbulent model, which is physically more appropriate for this source, yields a best-fit power-law density profile of the form n proportional to 1/r exp alpha with alpha = 1.5-2.0, a temperature of 13-15 K, and an H2CO abundance of 2 x 10 to the -9th. Comparison of observations with theoretical models for low-mass star formation in the subcritical and superumbral regime (Shu et al., 1987) shows good agreement. The density estimates are considerably lower than previous density estimates from dust emission. By considering the effects of temperature and density gradients, it is possible to produce a better ageement between the estimates from dust and H2CO. 38 refs.

Grain formation around carbon stars. 1: Stationary outflow models

Asymptotic giant branch (AGB) stars are known to be sites of dust formation and undergo significant mass loss. The outflow is believed to be driven by radiation pressure on grains and momentum coupling between the grains and gas. While the physics of shell dynamics and grain formation are closely coupled, most previous models of circumstellar shells have treated the problem separately. Studies of shell dynamics typically assume the existence of grains needed to drive the outflow, while most grain formation models assume a constant veolcity wind in which grains form. Furthermore, models of grain formation have relied primarily on classical nucleation theory instead of using a more realistic approach based on chemical kinetics. To model grain formation in carbon-rich AGB stars, we have coupled the kinetic equations governing small cluster growth to moment equations which determine the growth of large particles. Phenomenological models assuming stationary outflow are presented to demonstrate the differences between the classical nucleation approach and the kinetic equation method. It is found that classical nucleation theory predicts nucleation at a lower supersaturation ratio than is predicted by the kinetic equations, resulting in significant differences in grain properties. Coagulation of clusters larger than monomers is unimportant for grain formation in high mass-loss models but becomes more important to grain growth in low mass-loss situations. The properties of the dust grains are altered considerably if differential drift velocities are ignored in modeling grain formation. The effect of stellar temperature, stellar luminosity, and different outflow velocities are investigated. The models indicate that changing the stellar temperature while keeping the stellar luminosity constant has little effect on the physical parameters of the dust shell formed. Increasing the stellar luminosity while keeping the stellar temperature constant results in large differences in grain properties. For small outflow velocities, grains form at lower supersaturation ratios and close to the stellar photosphere, resulting in larger but fewer grains. The reverse is true when grains form under high outflow velocities, i.e., they form at higher supersaturation ratios, farther from the star, and are much smaller but at larger quantities.