We have developed a Monte Carlo code, which solves the problem of radiative transfer in anisotropically scattering atmosphere. The radiative code is flexible in handlings of the system geometry, the distribution of scattering particles, and the source-particle geometry. This code treats the case of highly forward throwing scattering. As performance tests, we have compared the result of Monte Carlo calculations with that of Quasi-Diffusion method for a spherically symmetric cloud model.

Infrared emissions from spherical dust, clouds are calculated using quasi-diffusion method. We have employed graphite-silicate mixture with power-law size distribution for the dust model. The grains are assumed to be heated and cooled by radiative processes only. The primary heating source is diffuse interstellar radiation field. hut the cases with an embedded source are also considered. Since graphite grains have higher temperature than silicate grains, the observed IR emission is mainly due to graphite grains, unless the fraction of graphite grains is negligibly small. The color temperature of Bok globules obtained from IRAS 60 and 100 μ m data are found to be consistent with the dust cloud with graphite-silicate mixture exposed to average interstellar radiation field. The color temperature is sensitive to the external radiation field, but rather insensitive to the size distribution of the grains. We found that the density distribution can be recovered outside the beam size using the inversion technique that assumes negligible optical depth. However, the information within the beam size is lost for if beam convolved intensity distributions are used in deriving density profile.

We have examined how sensitively the extinction value determined by the method of star-count depends on such factors as the plate limit, the size of counting reseau, the non-linearity in the number distribution of stars with magnitude, and the angular resolution demanded by the given problem. We let the Poisson distribution portray the statistical nature of the countings, and chose the region containing the globule Barnard 361 as an example field. Uncertainties due to various combinations of the factors are presented in graphic forms: (1) Dynamic range in the extinction measurements is evaluated as a function of reseau size for varying plate limits. (2) Statistical errors involved in the star-count are analized in terms of the signal-to-noise ratio, the plate limit and the reseau size. (3) Systematic error due to the non-linearity in the number distribution are thoroughly analized. (4) Finally, a methodology is presented for correcting the systematic error in the observed radial density gradient. These graphs are meant to be used in selecting proper size of the reseau and in estimating errors inherent to the star-count analysis.

By inverting the brightness integral for the dust-scattered continuum, we have determined how the dust grains are distributed inside the Orion nebula. The scattering characteristics of the Orion dust at a given wavelength is kept constant within the nebula, and the geometry of the nebula is assumed to have a hemispherical shape. The resulting radial distance dependence of the distribution of dust number density, N d ( r ) , shows that the dust grains are depleted at the central region of the Orion nebula and concentrated in the region 5 ′ ∼ 6 ′ away from the Trapezium stars. The scattering characteristics of the Orion dust are of moderately forward throwing nature, and the Orion dust has low values of albedo.

Radial distribution of internal density has been determined for thirteen subclouds in the three giant molecular cloud complexes accompanying Mon OB1, Mon OB2 and CMa OB1 associations, We modeled their radial density structures with the density distribution of isothermal gas spheres. Most of the subclouds, nine out of the thirteen, are well described by isothermal spheres of single component; while the rest four require an additional component. Total mass and potential energy of each subcloud are also derived from the radial density structure; thermal energy and internal velocity dispersion required for sustaining the density structure are deduced from the isothermal gas model. Our derived masses of the clouds are comparable to the values determined by Blitz (1978) under LTE assumption. This agreement suggests that the correction factor for non-LTE effect on mass-estimate is not far from unity. The ratio of the gravitational potential energy to the kinetic energy of thermal motion is as large as 250; hence the thermal motion alone cannot support these clouds against the gravity. Being supported by turbulence motion with velocities of six to seven times the thermal velocity, the clouds of one-component type seem to be in equilibrium with the gravity; while the clouds of two-component type are likely to be in the stage of gravitational collapse.

Effect of magnetic field on the thermal instability is studied in the radiatively cooling region behind an interstellar shock of moderate propagation velocity ( ∼ 10 k m / s e c ). It is shown that the presence of interstellar magnetic field of a few micro gauss is very effective in preventing the thermal instability from building-up density concentration. In the absence of magnetic field, the shock-induced thermal instability amplifies preshock density inhomogeneity by more than an order of magnitude. However, in the presence of magnetic field, the amplified density contrast is shown to be only a factor 2.

Analyses of an integrated form N ( τ ) = ∫ ∞ τ n ( τ ) d τ of the distribution of cluster ages, rather than its differential form n ( τ ) , demonstrate that the observed distribution has clusters older than about 500 million years in a significant excess over theoretical model distributions. Considerations on cluster disruption processes show that a single disruption time-scale, frequently employed by current theoretical models, is no longer an adequate parameter for describing survival probability of clusters over wide age range, because different initial conditions of these clusters produce corresponding spreads in their lifetimes. To take into account for the spread in initial conditions, we have introduced an age-dependent disruption time, and deduced its age-dependence from the present-day age distribution of clusters. Results show a distinct two-stage variation: The newly introduced disruption time stays constant at about 50 million years for clusters younger than about 100 million years, while for clusters older than that it increases monotonically with the cluster age. This leads us to conclude that clusters experience different types of disrupting causes as they get old.

To derive the distributions of electron density, temperature and gas-phase metal abundances within the Orion Nebula, we have performed a non-LTE analysis to the radio observations of hydrogen recombination lines and continuum flux over the frequency range from 0.1GHz to 100GHz. We have explicitly included the thermal balance condition in our analysis, hence our derived distributions have their internal consistencies. This enables us to derive the radial distribution of Oxygen and Nitrogen. The gas-phase concentrations of these cooling agents show about the solar values at the very central part of the nebula, then, decrease slowly outward, and finally become about one quarter of the solar values in the outer extended envelope. Such an outward decrease is interpreted as an outward increase of dust concentrations in the Orion.

Various assumptions used in interpreting the observations of hydrogen recombination lines are critically assessed to confirm the gradient of electron temperature with distance from the galactic center. The total temperature increase from 5 to 13 kpc is about 2,500 K. Among many suggestions, we have singled out the decrease of trace dement abundances with the galactoccntric distance as the most viable cause for the temperature gradient. This will impose an important constraint on evolutionary models of the Galaxy.

Temperature history of very small interstellar dust particles is followed under diffuse interstellar radiation. Because of extremely small thermal capacities of these grains with sizes ranging from a few tens to hundred Angstroms in radii, they are to experience strong fluctuations in temperature whenever they are hit by interstellar ultraviolet photons. Fluctuating temperature can inhibit these smaller component of interstellar dust from growing into core-mantle particles of submicron sizes by continuously evaporating atoms and molecules adsorbed on their surface. This is interpreted as a possible physical reason for the bimodal nature in grain size distribution. A brief discussion is also given to the far infrared emission properties of such small grains in diffuse interstellar dust clouds.