Emission from interstellar dust in L 1641 and Mon R2 was mapped with three arcminute resolution at 400 microns wavelength from White Mountain, California at 3.9 km elevation. Dust brightness distributions and 13CO antenna temperatures are shown below with equal linear scales, emphasizing that the core of Mon R2 is considerably larger and more massive than the core of the Orion region containing OMC-1 and OMC-2. Dust and 13CO brightness distributions for each object are similar, but some of the differences may be real. The differences become accentuated when mass distributions are compared.

We discuss the origin, lifetime, destruction, spatial distribution and relation to star formation of giant molecular clouds. A coagulation model including the effects of spiral density wave shocks is described. We explore implications for CO observations of external galaxies. The collective effects of OB star winds and supernova remnants in disrupting clouds are considered.

Giant molecular clouds can be destroyed in several ways: by conversion of gas into stars; by dissociation of H2 into HI; by ionization and formation of HII regions; and by dispersal of the cloud into cloudets. The relative importance of these processes can be assessed by observations of evolved HII regions and associated molecular clouds. A number of such complexes have been studied in 12CO at the Five College Radio Astronomy Observatory (FCRAO) as part of an extensive investigation of clouds associated with about 60 Sharpless HII regions. A particularly clear example of cloud disruption is found in the case of W80, the North America and Pelican Nebulae in Cygnus (Bally and Scoville 1980).

A model of the W3 molecular cloud derived from molecular observations is presented and the evolution of the cloud is discussed.

Cold HI gas appears as self-absorption dips in the 21-cm line profiles in and around the giant molecular cloud near W3 and W4. The cold HI cloud is ∼150 pc long and extends along the galactic plane. It consists of several fragments, each of which is typically ∼25 pc in diameter and (1 − 4) × 104 M⊙ in mass. The [H2]/[HI] ratio is estimated to be 15 – 50. The mass of the entire HI cloud amounts to ∼105 M⊙, which is comparable to that observed in CO emission.

Tenorio-Tagle (1979) first proposed the idea of a third dynamical phase, the champagne phase, following the formation and expansion phases of an HII region; the idea was further explored by Bodenheimer et al. (1979) and Tenorio-Tagle et al. (1979). The champagne phase begins when the high pressure gas of an HII region formed inside a molecular cloud reaches the edge of the cloud and bursts into the lower pressure, low density, intercloud medium. One important implication of the model is the prediction of an enormous enhancement of the rate of erosion of the molecular cloud by the ionising radiation of hot stars, which begins as soon as the process of the decrease of the gas density between the star and the cloud is started. The proportion of hydrogen molecules eroded by ionising photons may reach about 10-2. The mass eroded may exceed the mass of the ionised gas in the case where the ionisation front reaching the edge of the cloud is of D-type. Additional mechanisms (for instance stellar winds), if at work, may even increase the efficiency of the mechanism.

Winds from T-Tauri stars may provide an important dynamical input into cold molecular clouds. If the frequency of T-Tauri stars exceeds 20 pc-3, wind-driven shells collide and form ram pressure confined clumps. The supersonic clump motions can account for cloud line widths. Clumps collide inelastically, coalescing and eventually becoming Jeans unstable. For characteristic dark cloud temperatures low mass stars form, and we speculate that in this manner clouds can be self-sustaining for 107 − 108 yr. Only when either the gas supply is exhausted or an external trigger stimulates massive star formation (for example, by heating the cloud or enhancing the clump collision rate), will the cloud eventually be disrupted. A natural consequence of this model is that dark cloud lifetimes are identified with the duration of low mass star formation, inferred to exceed 107 yr from studies of nearby star clusters. Other implications include the prediction of the existence of embedded low mass stars in turbulent cloud cores, the presence of an internal source of radiation in dark clouds, and a clumpy structure for cold molecular clouds.

Observations of the J=1-0 lines of 12CO have been made to determine the extent and nature of the broad secondary velocity feature in L1551 (S239) first detected by Knapp et al. (1976). L1551 is associated with an extended Herbig-Haro object, HH102 (Strom et al. 1974), and two compact HH objects, HH28 and HH29 (Herbig 1974). Two nearby stars, HL Tau and XZ Tau, show extreme T-Tauri characteristics. The broad CO velocity component is seen toward HH102. Strom et al. (1976) have detected an infrared source near the HH objects which has a luminosity and infrared colors consistent with a late B star reddened by roughly 20 magnitudes of visual extinction (Snell 1979). The infrared source lies in the direction of the core of L1551, which has a density of 105 cm-3 (Loren et al. 1979; Snell 1979). The infrared source is likely to be embedded in this dense core.

A study in NH3 was conducted towards H-H objects. Cloud fragmentation appears to have occurred in the regions mapped. Rotation is present with velocity gradients of 1-2 km s−1 pc−1. We suggest that in the regions containing H-H objects, formation of stars of different spectral types may be taking place.

It is customary to convert observed brightness temperatures of molecular lines to molecular column densities by assuming some excitation temperature to describe the population of the energy levels of the molecule. Observers often speak about “abundances” after dividing these column densities by the dimension of the cloud that is being studied. This procedure can be misleading because the level populations are usually not well characterized by one single excitation temperature and because clouds are probably quite centrally condensed. Some molecules are formed preferentially at high densities in the opaque cores of clouds, others reach their maximum abundance in the moderately shielded much less dense outer regions of clouds.

Three young stars are still embedded in the small (0.7 pc wide) and densest region of the Rho Ophiuchi dark cloud. One of these stars coincides with the continuum source Oph 4 (Brown and Zuckerman, 1975). Their existence is indicated by the following: i) thermal continuum sources detected at 21 cm and 6 cm with the Westerbork Synthesis Radiotelescope (Netherlands) which display the presence of compact HII regions surrounding each of three stars; ii) peaks of CO emission (J = 1 → 0 transition) detected with the MWO antenna of MacDonald (Texas) which are associated with asymmetrically shaped lines; iii) C 158α (and S 158α for Oph 4) line emission mapped with the Nancay Radiotelescope. All these observations imply that the three stars have B3 - B2 spectral types.

We have mapped an extensive molecular cloud in Perseus in the 115 GHz line of 12CO. Observations were made every 10′ in right ascension and declination over most of the cloud, and every 2′ in the regions of most intense emission, near the open cluster IC 348 and near the reflection nebula NGC 1333. We also obtained 110 GHz 13CO data every 2′ in the latter regions, as well as every 10′ in several long strips across the cloud. A total of 812 positions were observed in 12CO, and 200 in 13CO. This work was done using the 5 m antenna of the Millimeter Wave Observatory of the University of Texas. The half-power beam size was 2!6.

Sato and Fukui (1978, hereafter SF) studied the HI 21-cm line in the giant molecular cloud near M17 and showed that the self-absorption of the 21-cm line is a useful tool for probing star-forming clouds. The giant molecular cloud at V(LSR) ∼ 20 km s-1 has been found to extend 4° (∼170 pc) southwest of M17 (Elmegreen et al. 1979). To gain a better knowledge of its physical conditions, we used the data from the Maryland-Green Bank Survey (Westerhout 1973) to examine the self-absorption features of the 21-cm line in and around the whole molecular cloud except the two fragments adjoining M17. The HI profiles at every in galactic latitude and longitude in the region were analyzed in almost the same manner as in SF by assuming that the spin temperature of the cold HI gas is equal to 20 K. The distance was taken to be 2.5 kpc.

The relationship between dense molecular clouds and diffuse clouds, as well as the mechanisms connected with the formation of molecules in diffuse clouds, may be studied using HI 21-cm line observations and molecular line observations in the same directions. For this purpose we previously studied the OH 18-cm main lines (Kazès et al., 1977) and the 2.6-mm CO lines (Crovisier and Kazès, 1977) in directions where strong 21-cm absorption features had been detected in the Nancay survey (Crovisier et al., 1978). Liszt and Burton (1979) also measured CO lines toward 19 directions observed in the Arecibo 21-cm emission/absorption survey (Dickey et al., 1978). This paper presents preliminary results of a more comprehensive search for 12CO in directions previously studied in the Nancay survey.

An optical study of four spiral galaxies, combined with radiative transfer models for transmitted and scattered light, has led to a determination of the opacities and masses of numerous dark patches and dust lanes that outline spiral structure. The observed compression factors for the spiral-like dust lanes are in accord with our expectations from the theory of gas flow in spiral density waves. Several low density (102 cm-3) clouds containing 106 to 107 M⊙ were also studied. We discuss these results in terms of recent theoretical models of cloud and star formation in spiral galaxies. The long-term evolution of giant molecular clouds is shown to have important consequences for the positions and ages of star formation sites in spiral arms.

A 1.2-meter millimeter-wave telescope has been used to survey CO in the constellations of Orion and Monoceros. Many new molecular clouds have been found. The distribution of molecular material shows two striking characteristics: 1) Most of the molecular clouds in this region appear to be connected by continuous extensions and filaments. To judge from continuity in radial velocity, most of these connections appear to be real, and are not merely the result of projection along the line of sight. 2) There are at least two slender filamentary features longer than 10° in angular extent. These filaments may connect the molecular clouds lying well out of the Galactic plane to clouds lying in the plane. Their shape and orientation suggest that magnetic fields may play a role in their evolution. The observed velocity gradients may be explained by accelerated gas flow along the filament.

We have recently completed at Columbia University a survey with a 1.2 meter telescope of 2.6 mm CO emission from the first galactic quadrant. The beamwidth of this telescope − 7.5′ at 2.6 mm - is optimal for a large-scale galactic survey: it is as small as that of the best 21-cm surveys but not so small that undersampling is required to finish a survey in a few hundred days of observing. The survey contains more than 3000 spectra between longitudes 12°and 60°and latitudes −1°and +1°. Spectra were taken every beamwidth for |b|≤0.25° and every two beamwidths elsewhere. Integration times, which averaged about 8 minutes, were adjusted to give an rms noise level of 0.3 K per 0.5 MHz spectral channel.

Molecular hydrogen of ≳108M⊙ exists in the galactic center region, as has been revealed by recent observations of molecular emission lines (see e.g. Oort 1977). In the inner region of most of the dominant emission features concentrate at and 0 km s-1 ≲v ≲100 km s-1 extremely unevenly with respect to the galactic center (see Fig. 1). As a model of the molecular complex we propose a fan of 360-pc radius whose pivot is at the center. The vertical angle of the fan is about 50° and the central line of the fan makes an angle of about 60° to the line of sight. Molecules in the fan are flowing out radially from the center with a velocity of 110-140 km s-1. The ℓ-v pattern of the fan model is superposed on the CO map in Fig. 1. The model can explain the whole structure of the molecular complex as well as several fine details such as asymmetry in emission line profiles (Fukui et al. 1979). As for Sgr A and Sgr B2, numerical calculations of molecular line profiles have been made by using the large velocity gradient approximation. The calculations show that the broad and asymmetric line profiles in the complex are well reproduced by the fan model. Further, an isotope effect on line shape is predicted, which will be useful as an observational check of the fan model. Additionally, the carbon isotope ratio 12C/13C in HCN and CO was estimated to be 10-20 in the Sgr A +50-km s-1 cloud.

The major stumbling block in the determination of a rotation curve beyond the solar circle has been the lack of a suitable set of objects with well defined and independently measured distances and velocities which can be observed to large galactocentric radii. Two things have changed this situation. The first was the realization that essentially all local HII regions have associated molecular material. The second was the acquisition of reliable distances to the stars exciting a sizable number of HII regions at large galactocentric radii (Moffat, FitzGerald, and Jackson 1979). Because the velocity of the associated molecular gas can be measured very accurately by means of radio observations of CO, we have been able to overcome the past difficulties and have measured the rotation curve of the Galaxy to a galactocentric distance of 18 kpc.

Because of its abundance in molecular clouds, the hydrogen molecule is the most important collision partner in a discussion of thermal equilibrium or non-equilibrium in most interstellar molecules. The purpose of this talk is to summarize the properties of the H2 molecule as a collision partner based on laboratory experiments on NH3-H2 collisions.