An outstanding feature of the water maser emission from star formation regions is the strong time variability, which was recognised soon after the discovery of interstellar H2O masers. It is likely that the observed considerable variations of the H2O line profiles as well as of the flux density reflect the disturbancies in the protostellar nebule probably associated with the outflow from the newly formed stars.

The observed distribution of ionised carbon with depth into the edge-on photodissociation region (PDR) in M17SW is not consistent with results from chemical models which assume a constant or smoothly varying density. These observations have been interpreted to indicate that the PDR actually consists of many clumps embedded in a low density interclump medium (1). A chemical model is described which is designed to simulate such a clumpy PDR.

In interstellar clouds, where “thermal” /nonmaser/ OH emission in the 18-cm lines is observed, prominent anomalies of the intensities of the two satellite lines, 1612 and 1720 MHz, are often seen. If a cloud is observed against a background source of radio continuum, then one satellite may be in strong absorption, while the other one is in emission almost as strong as that in the main lines, 1665 and 1667 MHz. In those clouds, for which several positions were observed in the OH lines, the roles of the satellites are sometimes interchanged with the displacement along the cloud. At the same time, the main lines usually have the intensity ratio close to the equilibrium one. This behaviour indicates to deviations of the level populations of OH molecules from the Boltzmann ones.

Ammonia towards the UCHII—region NGC 7538—IRS 1 shows an extremely hot component observed in absorption in several transitions and a so far unexplained (9,6) maser (see Fig. 1). This source is the only known source in the Galaxy where the 141 GHz line (1450 K above ground) of vibrationally excited Ammonia is seen in absorption (Schilke et al. 1990). An additional unique feature is the 15NH3(3,3) maser observed by Mauersberger et al. (1986) in Effelsberg and by Johnston et al. (1989) with the VLA. There are theoretical models to explain such a maser (see e.g. Flower, Offer and Schilke 1990) in low density regions where ortho-H2 is underabundant. In this study, our aim was to investigate 15NH3 in more detail. We succeeded in observing the (1,1) and (2,2) lines of 15NH3 in absorption and, surprisingly, the (4,3) and (4,4) lines in emission (see Fig. 1).

We have recently discovered a large-scale (200″) outflow system in the core of OMC-1 (fig. 1), centered about 100″ South of IRc2 and extending over some 120″ (red lobe) resp. 60″ (blue) along a position angle of —31° (Schmid-Burgk et al. 1990). The blue lobe which might actually protrude into the HII region M42 is poorly defined in CO 2-1, but the red lobe reveals a number of remarkable properties which we summarize here:

The outflow is very straight and smooth. Over the full length of 120″, the center of any cross scan deviates by not more than about 1″ from a straight line. This line passes to within 2″ the peak of the submm source FIR4 of OMC-1 (Mezger, Wink and Zylka 1990) and the mm continuum peak CS3 (Mundy et al. 1986); it also cuts across the red and blue SiO-outflow lobes recently discovered some 5-10″ to either side of FIR4 (Ziurys, Wilson and Mauersberger 1990). It thus seems that the “base” of our large-scale CO jet can be seen as well.

The dense core of the W49A molecular cloud (Miyawaki et al. 1986, Schloerb et al. 1987) has been mapped in 5 different transitions of CS and C34S, in order to determine its density structure. The three lower frequency transitions (CS J=3-2, CS J=5-4 and C34S J=5-4) were observed with the IRAM 30m telescope, and the two highest frequency transitions (CS J=7-6 and C34S J=7-6) with the Caltech Submillimeter Observatory. The beamsizes were in the range 12″ to 20″. As a calibration check, the CS J=7-6 line was observed with both telescopes, and was found to give a consistent temperature scale. The spectra at the peak of the emission are shown in Fig. 1.

Molecular clouds are characterized by large density contrasts and pronounced internal motions. Some information on their internal physics has been acquired through relatively simple data analysis and through theoretical modelling, mainly hydrodynamical calculations. More detailed radiative transfer models have been limited to spherical cloud models.

A related but less recognized problem is the classification of molecular clouds, or otherwise stated, can we compress the information in the observed two-dimensional maps for various line parameters in such a way that the physical differences are retained, while the random component has been removed? This is a more fundamental formulation of the special problem of comparing the appearance of two molecular clouds, or of the problems of quantifying the velocity correlation within the clouds.

We present preliminary results of a survey of bright-rimmed clouds associated with IRAS point sources.

Bright-rimmed globules associated with old HII regions have long been suspected as a potential site for star formation. Physical conditions of such clouds seem to well match to models of radiation-driven implosion, which have been studied as an effective process for induced star formation (e.g. Klein et al. 1985, Bertoldi 1989). Bright-rimmed globules associated with IRAS point sources are good candidates for the sites of induced star formation. Three well-established cases of radiation-driven implosions in bright-rimmed globules (Ori I-2, IC1396-n, and L1206) were reported (Sugitani et al. 1989). Similar examples were also reported in HH46/47 (Olberg et al. 1989) and in GN21.38.9 (Duvert et al. 1990). However, the samples are still not numerous enough to establish comprehensive understanding of star formation by such implosion process.

We mapped 5 dense cores in the Cepheus molecular cloud in the optically thin C18O (J = 1 — 0) line using the 45-m telescope at Nobeyama and derived density profiles around those cores assuming spherical symmetry. Cloud cores are selected from the 13CO (J = 1 — 0) map obtained through the unbiased survey program with the 4-m telescope at Nagoya university.

We made 13CO and C180 (both J = 1 — 0) maps of the 4′ × 4′ (0.2 pc × 0.2 pc) area toward TMC1-C and ρ-Oph core with the 45-m telescope of the Nobeyama radio Observatory in order to find small scale velocity fluctuation in molecular clouds based on the analysis described by Kleiner and Dickman (1987).

We have developed a model which simulates the radiative transfer of molecular line emission through clumpy molecular clouds. The dynamical structure of the model cloud is based on the work of Kwan and Sanders (1986). The model incorporates the existence of an intense source of UV photons at the surface of the cloud. The UV source heats the clumps and creates kinetic temperature and CO abundance gradients within them. The amount of heating depends on the intensity of the UV field, which decreases from the surface to the core of the cloud due to attenuation by dust. We treat in detail the photochemistry and self-shielding properties of CO as a function of UV intensity and gas density in order to obtain the CO line intensities emerging from each clump. The line intensity emerging from the cloud is obtained by integrating the emission from all clumps along the line of sight, weighted by an area covering factor, and attenuated by the opacity of intervening clumps. The effects of the heating are significantly noticeable on the line intensities of CO transitions arising from levels with J between ∼ 3 and ∼ 7. We apply our model to the case of the Orion A molecular cloud, and in particular to observations of the J=3 →2 12CO and 13CO lines. The model is in general agreement with the observed enhanced intensity of the 12CO J=3 →2 transition relative to the J=1 →0 transition throughout the central ∼ 10′ region of Orion. It also produces centrally peaked spectral lines whose intensity is maximum in a shell-like distribution centered on the Trapezium HII region, as is observed.

Two topological tools for studying the global structure of molecular clouds, the genus and the contour-crossing statistic, are discussed. Preliminary results for the Taurus molecular cloud complex are presented.

To study the structure of interstellar clouds we used the so-called perimeter-area relation to estimate fractal dimensions. We studied the reliability of the method by applying it to artificial fractals and discuss some of the problems and pitfalls. Results for two different cloud types (high-velocity clouds (HVCs) and infrared cirrus) are summarized. We find dimensions 1.2<D<1.55, somewhat higher than found in previous, similar studies.

IRAS Calibrated Raw Detector Data (CRDD) are presented of a part of the Cygnus-X region, incorporating W75, DR21 and W75N, and a previously unknown loop of dust emission is observed. This loop is interpreted as a spherical shell-like shock front, and two alternative explanations for its origin are explored — a wind-blown bubble around an OB association, and an old supernova remnant. The arguments for each are outlined, and it is deduced that there are insufficient OB stars old enough to have formed the loop by combined stellar wind action, although a SNR appears consistent with the data.

IC 1396 is an H II region in Cepheus excited by the massive 06.5 star HD 206267. The distance is about 750 pc. The region exhibits a number of bright-rimmed molecular clouds in which outflows have been detected (Sugitani et al. 1989, Duvert et al. 1990).

We argue that if gravitational interactions play an important role in the amplification and transfer of turbulent energy between sub-clouds of different scales in a GMC, then the Second Law of Thermodynamics requires that the transfer occur from smaller to larger scales, and hence that the seed-energy be injected on relatively small scales.

We show that the radiative cooling properties of the interstellar gas lead naturally to a scaling law for the magnetic field of the form B ∝ n 1/2, if the magnetic field is amplified by a vigorous injection of turbulent energy due to expanding HII regions, stellar winds, supernova explosions, stellar jets, etc.

Supersonic head-on collisions between quiescent clouds produce flattened sheets of shocked gas. We derive the condition which the cooling law must satisfy if this sheet is to fragment into protostellar condensations (i.e. gravitationally unstable lumps). If this condition is not satisfied, colliding clouds are likely to be disrupted and dispersed. We show that under the conditions obtaining in GMCs, most cloud-cloud collisions probably do not result in fragmentation.

I propose and briefly elaborate on a major new mechanism for the formation of wide, low-mass binary stars: the fragmentation of a collapsing, initially elongated dense molecular core rotating end over end. This initial structure will develop into two independent gravitationally bound stellar condensations orbiting each other in a rather eccentric orbit.