We report results of multi-transition observations and modeling of the hydrocarbon ring molecule cyclopropenylidene (C$\sb3$H$\sb2$). From a survey of the 1$\sb{10}$-1$\sb{01}$ (18 GHz) and 2$\sb{12}$-1$\sb{01}$ (85 GHz) transitions in the Galaxy, we have found C$\sb3$H$\sb2$ present in a variety of sources including cold, dark clouds, giant molecular clouds, the envelope of a carbon star, and diffuse clouds. Up to 10 transitions of C$\sb3$H$\sb2$ ranging in wavelength from 1.3 cm to 1.3 mm were observed in the dark clouds L1498, L134N, B335 and toward several positions in TMC-1. The Large Velocity Gradient (LVG) approximation was used to model the observations. Optical depth values of C$\sb3$H$\sb2$, estimated from C$\sp{13}$C$\sb2$H$\sb2$ observations, are necessary to constrain the results since the range in excitation energies of the observed C$\sb3$H$\sb2$ transitions does not contrast sufficiently. The molecular hydrogen density in TMC-1 is estimated to be 3.7 $\times$ 10$\sp4$ cm$\sp{-3}$, while the fractional abundance of C${\sb3}$H$\sb2$ relative to H$\sb2$ is 5.7 $\times$ 10$\sp{-9}$. Previous estimates assuming LTE conditions overestimate the abundance of C$\sb3$H$\sb2$. The abundance in the ridge component in Orion is estimated to be approximately 8 $\times$ 10$\sp{-10}$ cm $\sp{-2}$. Gas phase chemical models can reproduce the high C$\sb3$H$\sb2$ abundance found in dark clouds under assumptions such as steady state conditions with (C) / (O) / $>$ 1.0, conditions of earlier evolutionary time, or 'optimistic' rate coefficients. However, large deuteration ratios (0.05 to 0.15) create difficulties for gas phase models.

To improve our understanding about giant molecular clouds (GMC) associated with R-associations, a $\sp{12}CO\ J=1-0$ map of 167,000 spectra with 45$\sp{\prime\prime}$ resolution and 25$\sp{\prime\prime}$ spacing, a $\sp{13}CO\ J=1-0$ map of $\sim$40,000 spectra with 1.5$\sp\prime$ resolution and 1$\sp\prime$ spacing, IRAS BIGMAP images, and maps of high density molecular tracers for the dense cores are obtained for the GMC Monoceros R2 ($D=830\pm 50\ pc$). These data reveal that the large-scale structure of Mon R2 is dominated by an expanding bubble shell ($\sim$30 pc) with front side moving towards us at a radial velocity of $\sim$4-5 km $s\sp{-1}$. Distortions of this shell are obvious, suggesting of the inhomogeneity of the cloud before the formation of the bubble. There is no evidence for red-shifted shell at the far side of the bubble. There are at least two generations of star formation in Mon R2. The older generation of stars with an age of $6-10\times 10\sp6$ years are represented mostly by reflection nebulae. The younger generation of stars with an age of $\sim$10$\sp5$ years are represented mostly by IRAS point sources. It is proposed that the large-scale expanding bubble shell is the result of combined effects of ionizing flux and stellar winds originating from the older generation of young stellar objects, but perhaps dominated by O type stars which either are obscured or left main sequence. It is suggested that the formation of the younger generation of stars has been triggered by the older generation of stars. The main and the GGD12-15 cores are located on the large-scale expanding shell, and their harboring both generations of stars can be explained were the cores preexisting clumps. Our CO data reveal an eggplant-shaped bipolar outflow shell, whose shape can be satisfactorily modeled with radially directed stellar winds sweeping up ambient material with momentum conservation. An inversion method is implemented for analyzing dust emission spectra at FIR wavelengths in terms of a continuous dust temperature distribution, and has been applied to IRAS BIGMAP images of Mon R2. Computer programs are developed for identifying clumps and determining their properties in molecular emission maps. The results support the reality of size-linewidth and mass-linewidth relations for clumps in individual GMCs.