There have been several recent changes in perspective on the diffuse interstellar environment, including recognition of a thick disk of warm gas, cosmic rays, and magnetic field. In addition, evidence for a pervasive hot phase driven by supernova disruption has weakened to the point that a quasihomogeneous warm intercloud gas may occupy most of the interstellar volume at midplane, with individual bubbles created by supernovae and OB associations occupying perhaps 10 and 20 per cent respectively. The bubble population is sufficient to explain the high stage ions (0 VI, N V, C IV, perhaps Si IV) found in the disk, though possibly not those found at higher z. The estimated midplane pressure has increased, leaving the thermal pressure inside clouds almost negligible. The reduced porosity of the medium, its greater thickness, and its larger pressure all act to suppress fountain activity, either arising from the disk generally, or from the blowout of superbubbles. Finally, there appears to be a peculiar coincidence between the cloud heating mechanism and the activity determining the interstellar pressure.

Molecular gas in the interior of the Orion superbubble consists of sheets, filaments, and partial shells in which the active star forming dense cloud cores are embedded. The main body of the Orion A and B clouds and at least 14 smaller clouds in Orion region are cometary in appearance suggesting strong interaction with massive stars in the Orion OB association. While the small scale (< 1 pc) structure of the clouds may be determined primarily by internal magnetic fields, gravity, and the effects of outflows from young stellar objects, the large scale morphology and kinematics is affected by the energy injected by massive stars. Supernovae, stellar winds, and radiation have compressed, accelerated, ablated, and dispersed molecular gas over the last 107 years. Most GMC/OB star complexes in the Solar neighborhood exhibit morphological and kinematic properties similar to the Orion region. We argue that energy injection by massive stars plays a vital role in the evolution of the ISM and may be responsible for much of the observed large-scale structure and kinematics of molecular clouds.

The age distribution of stars and stellar groupings was studied in the galactic large-scale star-gas complexes (SGCs).

the CS J=2 →1 molecular line at 98 GHz, a normally optically thin line requiring high densities to be excited, has been detected with SEST (Swedish ESO Submillimeter Telescope) toward 294 IRAS pointlike sources having the characteristic FIR colors of embedded stellar objects and apparently associated with the largest molecular cloud complexes in the southern Milky Way. We present here their Galactocentric radial distribution and a correlation between their FIR and CS luminosities.

The IRAS 60 and 100 μm flux from cirrus clouds are commonly explained by dust continuum emission. But this explanation in some cases requires unexpectedly high dust temperatures.

We argue that the contribution of fine structure emission of neutral oxygen, O°(63 μm), can be significant in the IRAS 60 μm band. The 0°(63 μm) line emission together with the dust continuum emission offers a plausible explanation of the observed flux ratio I(60)/I(100), as well as of its variation across individual clouds. We also discuss the clumpy/filamentary structure of these clouds.

The presence of HI in the interstellar medium is ubiquitous. HI is the principal actor in the majority of the physical processes at work in our Galaxy. Restricting ourselves to the topics of this symposium, atomic hydrogen is involved with the formation of molecular clouds and is one of the byproducts of their destruction by young stars. HI has different roles during a molecular cloud's life. I will discuss here a case of coexisting HI and H2 at large scale and the origin of HI in star forming regions. For completeness' sake, it should be mentionned that there are at least three other aspects of HI involvement: HI envelopes around molecular clouds, the impact of SNRs (see work on IC 443), and the role of HI in quiescent dark clouds (see van der Werf's work).

This paper concentrates on reviewing magnetic fields in dense regions; see Heiles (1988, 90) for a review of fields in diffuse regions. The past few years have increased our observational knowledge of magnetic fields in dense regions by an enormous factor—not because there are many measurements, but because we started from zero. The observable is polarization, which is small and subject to systematic errors. The fact that the advances have occurred only recently is a result of several factors: technological development; the interest and commitment of experimentally-minded astronomers; and the maturing of molecular and infrared astronomy to the point that really new results require either new insights or more difficult techniques.

BIMA molecular-line observations show evidence for an expanding molecular ring around IRS 4, a newly formed massive star at the center of the bipolar nebula S106. VLA observations of the Zeeman effect in the OH 1665 MHz line show that the magnetic field strength is about 1 mG and that it reverses direction from one lobe of the bipolar nebula to the other.

A new method of investigation of the hierarchical structure of the interstellar magnetic fields is discussed. Synchrotron intensity fluctuations are used. Statistical characteristics of the irregular magnetic field are available. The method was successfully tested.

The magnetic field and the angular momentum are major obstacles against cloud contraction. I will review recent results on magnetic flux loss rate and angular momentum loss rate and will investigate a gross feature of cloud contraction.

Magnetic fields are believed to play an important role in the star formation process. Correlations in the velocity field in molecular filaments are indicative of dynamical interactions between clouds and parts within a cloud. The magnetic field is a likely candidate as the vector of such interactions. Perturbations of the field at large scales can feed the velocity dispersion within condensations at small scale. This mechanism is discussed in the framework of two simple analytical approximations describing transverse waves fed into plane parallel slabs.

in this paper I first review some of the simple structural concepts associated with compressible turbulence. In particular the hierarchical or self-similar fractal structure to be expected is formulated in a manner readily compared to the observations, and to previous work. In the next section I present the first results of a wavelet analysis on molecular clouds, which seem to comfirm the hierarchical scaling. I conclude with an extention of the theory to include magnetic fields. This latter theory represents an alternative to the more conventional dynamo theory.

Chemical considerations and simplified dynamical modeling suggest that dark cloud cores may be incessantly evolving such that the time spent at high core densities decreases as the density increases. After reaching a high density, gravitationally contracting dark cloud cores may either form stars or expand to states of lower densities. Cloud mass and initial density are amongst the factors that may control the evolutionary fate of the core. This view is diametrically opposite of the common belief that dense cores may be in near mechanical equilibrium. Mutually consistent end-to-end modeling of the spectral line profiles and intensities is needed to discern the reality.

We give an overview of the use of numerical simulations in the modeling of turbulence in molecular clouds.

We present an ensemble of recent observational results on molecular clouds which, taken separately, could all be understood by invoking various unrelated physical processes, but taken all together form a coherent ensemble stressing the imprints of turbulence in the physics of the cold interstellar medium. These results are first, the existence of wings in the molecular line profiles, which can be interpreted on statistical grounds as the signature of the intermittency of the velocity field in turbulent flows, second the fractal geometry of the cloud edges, with properties reminiscent of those of various surfaces studied in turbulent laboratory flows, and third, the fact that the dense gas fills only a very small fraction of the space. The last points are supported by CO multitransition observations of a few fields in nearby molecular clouds. They show that the excitation conditions are the same for the gas emitting in the linewings and in the linecores and are also remarkably uniform over a large range (factor 10) of column densities. An attractive interpretation of the molecular line data is that most of the 12CO(J=2—1) and (J=3—2) emissions arise in cold (Tk ≥ 10K) and dense (nH2 ∼ 104 cm —3 or more) structures distributed on a fractal set with no characteristic scale size greater than about 1000 AU.

The small, thin diffuse and translucent molecular clouds are excellent laboratories for studying the ways in which small—scale structure and interstellar chemistry affect each other. Variations of density or column density and chemical stratification can be found on scales as small as 0.01 pc. The origin of such structures and the evolutionary states of small clouds remain elusive.

IRAS images of nearby molecular clouds show that the mid-IR emission from small particles in the size range 102 to 105 atoms is distributed very differently from the 100 μm emission from large dust grains. Variations in color ratios by as much as one order of magnitude are seen on all angular scales. We summarize observational properties of the color variations and argue that neither their large amplitude nor their morphology can be explained by changes of the excitation by the UV radiation field only. The color variations reflect considerable inhomogeneities in the abundance of small particles. We suggest that the abundance variations are related to the cycling of interstellar matter between the gas phase and dust grains. This interpretation entails that clouds with distinct IR colors differ in their density and velocity structure and that cycling of matter between gas phase and dust grains is more ubiquitous and rapid that generally thought.