A discussion is presented of the chemistry of quiescent molecular clouds, and the effects of the presence of polycyclic aromatic hydrocarbon molecules and of cosmic-ray induced ultraviolet photons are examined. A comparison is made with the chemistry occurring in molecular clouds that are subjected to shocks and the differences between dissociative and non-dissociative shocks are described. The changes in composition caused by intense cosmic ray fluxes or intense ultraviolet radiation fields are explored.

Dark clouds in the southern skies have been observed with CO J = 1-0 line. The mm-wave telescope of 4-m diameter was used. The beam width was 2’.7 at 2.6 mm wavelength and beam efficiency 0.52. The authors have developed a high-resolution acousto-optical spectrograph with a velocity resolution of 0.07 kms-1 for this purpose. The 115.27 GHz line was detected over an area of 27’ x 21’ including the dark cloud of DC 303.6+0.9 in Crux. Most of the positions, out of a total number of 63, show a double peak line structure. Some positions also show red and blue shifted wings. An average profile shows a higher peak at the radial velocity of -2.8 kms-1 with a calibrated radiation temperature of 5.5 K, a half line width of 1.5 kms-1, and another peak in relative red at a velocity of -0.8 kms-1 with a temperature of 4.3 K, a half line widh of 1.0 kms-1. Profiles distributed over this area show a bipolar diagram. It hints that a certain systematic motions may exist in this region.

Since 1984 the molecular line search for dark clouds, mainly TMC1, for wide frequency ranges in the short cm- and mm- waverength regions has been proceeded at Nobeyama Radio Observatory using the 45-m telescope. The main collaborators are: H.Suzuki, M. Ohishi, S. Saito, Y.Yamamoto, K.Kawaguchi, and N.Kaifu.

We review new observational and theoretical developments of the understanding of the H2 infrared emission in the last 5 years since the discovery of the fluorescent emission in NGC 2023. An excitation analysis of H2 in a variety of Galactic sources has revealed that in many sources the excitation is expressed as a mixture of thermal and fluorescent components. This finding is in good agreement with theories of photodissociation regions, in which the population of H2 changes its character from pure fluorescence to thermal as the density of the region increases. The ortho/para abundance ratio of the fluorescent H2 is observed to lie within a limited range of 1.1 — 1.8 which is well reproduced by depth-dependent model calculations of the ultraviolet excitation and dissociation of H2 molecules. This may be understood as due to the independent self shielding of each of the ortho- and para-H2, rather than the ortho/para abundance ratio of the predissociated H2, a low formation temperature of H2 on grains, or gas phase interchange reactions. A laser emission of molecular hydrogen discovered in the planetary nebula NGC7027 further demonstrates the nonthermal nature of the H2 emission in photodissociation regions.

Ultraviolet photons are created in the interior of dense interstellar clouds by the impact excitation of molecular hydrogen by secondary electrons generated by cosmic ray ionization. The resulting photodissociation and photoionization rates of a wide range of interstellar molecules are calculated. The effects on the equilibrium chemical composition of dense clouds are briefly discussed.

Evidence is presented to suggest that the shocked molecular hydrogen emission in the brightest part of the Orion outflow is produced in a J-shock and not a C-shock; that this is true throughout the entire flow; that it may be true in many outflow sources; and that this exacerbates problems with current explanations of the very wide velocity profiles observed in molecular hydrogen emission.

A central problem in the theory of star formation is to understand the spectrum of masses, or Initial Mass Function, with which stars are formed. The fundamental role of the IMF in galactic evolution has been described by Tinsley (1980), and an extensive review of evidence concerning the IMF and its possible variability has been presented by Scalo (1986). Although the IMF derived from the observations is subject to many uncertainties, two basic features seem reasonably well established. One is that the typical stellar mass, defined such that equal amounts of matter condense into stars above and below this mass, is within a factor of 3 of one solar mass. A theory of star formation should therefore be able to explain why most stars are formed with masses of order one solar mass. The second apparently universal feature is that the IMF for relatively massive stars can be approximated by a power law with a slope not greatly different from that originally proposed by Salpeter (1955). Thus we also need to understand why the IMF always has a similar power-law tail toward higher masses.

Judging from the poster that the Organizing Committee has selected to announce the celebration of Guido Munch Jubilee, one can easily conclude that the main characteristics of the process of star formation as emerged in recent years through the combined efforts of multiwavelengths studies of molecular clouds, were already known, here in Granada, several centuries ago to the masters who built and enriched the enigmatic palace of the Alhambra. As we can appreciate from a quick inspection of the picture, it is rather obvious to infer that stars are the byproduct of a quite complex series of phenomena, each connected to, and somewhat dependent on, the others. Also, stars do not form in isolation, but rather in clusters or associations, with a strong tendency for the largest ones, also the most massive ones, to sit in the middle of the distribution. Moreover, smaller and less massive stars outnumber their massive counterparts, apparently obeying a power-law distribution. Finally, but with the benefit of doubt, it appears that the idea that the whole process reflects an intrinsic fractal nature was also put forward at the time. With this background in mind, let us now turn to the new emerging aspects of the study of star formation.

Above masses of the order of lOO M⊙, molecular clouds have masses and sizes which scale like those of self-gravitating polytropes bounded by an external pervading pressure. It is unlikely that this scaling is due to mere observational bias. But the physics underlying this behaviour is far from being understood. In particular, the possible contribution of turbulence to both the ambient pressure and the internal pressure (whose dependence with the density would mimic a polytropic behavior) is a difficult and much debated issue. The clouds mass, size and internal velocity dispersion are such that they are observed to be in approximate virial balance between their self-gravity, the surface energy term due to the ambient pressure and their internal energy. The latter is dominated by the kinetic energy of disordered internal motions. However, there has been little evidence so far that these motions are actually turbulent rather than simply disordered. The transition to turbulence in a flow occurs when the non linear advection term in the momentum equation, v.Δv, considerably exceeds the viscous dissipation term, vΔv (where v is the kinematic viscosity). Non linearities therefore dominate the physics of a turbulent flow and the velocities are not randomly distributed. Most of the previous attempts to determine a well-defined correlation length in the velocity field (Kleiner and Dickman 1985, a and b; Scalo 1984), which is predicted to be close to the scale at which the energy is injected, or to characterize the expected hierarchical structure (Pérault et al. 1986) have been plagued by the lack of dynamical range in the data set and the range of scales over which the correlation functions have been computed. The most plausible determination, that of Kleiner and Dickman (1987) who claim to have found a correlation length of 0.2 pc in the Taurus cloud, gives a result which is so close to the angular resolution of the observations that it is doubtful.

The Fill regions morphology have suggested, from the beginning of this century, the evidence of internal motions, very few gas radial velocities (RV) on the brightest “nebulae” had been obtained, because of obvious technical difficulties. The modern Fabry-Perot interferometers and the new integral Field Spectrograph provide abundant RV fields at various scales in the interstellar gas. New ways in imagery, owing to these new instruments, lead to a better analyse of both, the velocities field and their precise morphology free of stellar, dusty, and non thermal continua. The richness and the guallty of RV data, a better understanding of the expansion phenomena authorize again a rebirth of the turbulence observations of the ionized interstellar gas.

The main features of HII regions expanding in spherical and disk-like clouds with density gradients are reviewed. The spherical cases assume power-law density stratifications, r~w, and the disk-like cases include exponential, gaussian, and sech2 distributions. For power-law profiles, there is a critical exponent, wcrit = 3/2, above which the ionization front cannot be “trapped” and the cloud becomes fully ionized. For clouds with w < 3/2, the radius of the ionized region grows as t4/(7-2w) and drives a shock front into the ambient neutral medium. For w = wcrit = 3/2 the shock wave cannot detach from the ionization front and the two move together with a constant speed equal to about 2ci, where ci is the sound speed in the ionized gas. For w > 3/2 the expansion corresponds to the “champagne phase”, and two regimes, fast and slow, are apparent: between 3/2 < w ≤ 3, the slow regime, the inner region drives a weak shock moving with almost constant velocity through the cloud, and for w > 3, the fast regime, the shock becomes strong and accelerates with time.

For the case of disk-like clouds, which are assumed cylindrically symmetric, the dimensions of the initial HII regions along each azimuthal angle, θ, are described in terms of the Strömgren radius for the midplane density, Ro, and the disk scale height, H. For yo = Rosin(θ)/H ≤ α (where α is a constant dependent on the assumed density distribution) the whole HII region is contained within the disk, and for yo > α a conical section of the disk becomes totally ionized. The critical azimuthal angle above which the HII region becomes unbounded is defined by θcrit =sin-1(αH/Ro). The expansion of initially unbounded HII regions (i.e. with yo > α) proceeds along the z-axis and, if the disk column density remains constant during the evolution, the ionization front eventually recedes from infinity to become trapped within the expanding disk. For clouds threaded by a B-field oriented parallel to the symmetry axis, as expected in magnetically dominated clouds, this effect can be very prominent. The expanding gas overtaken by the receding ionization front maintains its linear momentum after recombination and is transformed into a high-velocity neutral outflow. In the absence of magnetic fields, the trapping has only a short duration.

Initial CO observations of the Cepheus OB cloud by Sargent (1977), identified the condensation known as Cepheus A. Other observations have shown that it has the normal empirical indications of a star forming region. In the radio continuum, using the WSRT, it consists of two thermal components (Hughes and Wou-terloot 1982). The West component is associated with the Herbig-Haro object GGD-37, and will not be considered further here. The East component when observed further with the VLA, is seen to consist of two strings of about 14 compact HII regions, and is the subject of the present paper.

Star forming regions are one of the active components of the interstellar medium and as such play an important role in the galactic “ecosystem”. When massive young stars are borned they have a strong impact on their environment through their radiation flux and stellar wind. We are then facing complicated interplays between gas in different states (ionized, atomic and molecular), dust particles and the young stars. The understanding of these interplays can only be done using multifrequency observations. Such an endaveour is already in progress (see Joncas et al. 1988 and Kömpe et al. 1989). We will describe here the young star forming region S187 (Sharpless 1959). This gas complex is nearby (≈1 kpc) thus permitting high spatial resolution with medium size instruments. It contains a faint optically visible HII region ionized by an unidentified BO or BO.5 star. The associated molecular cloud, discovered by Blair et al. (1975), contains a molecular outflow (Bally and Lada 1983) to which an H2O maser is associated (Henkel et al. 1986).

Turbulent mixing between an ionization bounded HII region and a hot shocked stellar wind (HSSW), which keeps it under pressure, is examined. Recently we have shown that acoustic disturbances can grow there to finite amplitude in a time scale which is comparable to the sound crossing time in the HII layer. The resulting turbulence will then stretch fluid elements and the frozen-in magnetic field. A condition under which turbulence can decay down to the viscous scale, where mixing is very efficient, is derived. For a uniform and plane parallel magnetic field Bo and a constant density ρo of the ambient medium, we find that efficient mixing takes place near the polar regions. Subsequently the rate of mass addition to the hot bubble is calculated and it is shown that catastrophic cooling is likely to occur. In the case of NGC 6334(A) it seems that this has just happened and we predict an upper limit for Bo of 4 x 10-5 gauss there. This model may also explain the existence of highly ionized species (e.g. OVI), soft X-rays and high velocity flows of the order of 100 km/s.

We compare observed and simulated images of the accretion flows associated with high mass star formation in the regions G10.6-0.4 and DR21. We describe, as a result of the comparison, the temperature, density, and velocity fields. Our results indicate that the G10.6-0.4 cloud core is strongly condensed and has approximately equal velocities in rotation and infall at its current evolutionary state. The rapid collapse and lack of rotational support suggests that significant angular momentum transfer is occuring over scales at least as large as those observed (0.5 pc). A milligauss magnetic field would have sufficient energy to supply the required braking torque of 1047 ergs. The DR21 core shows approximately spherically symmetric radial accretion with no detectable rotation. Unlike the G10.6-0.4 core, the DR21 core does not contain an embedded HII region. Thus this core may represent a molecular cloud condensation undergoing gravitational collapse and accretion just prior to the formation of massive stars.

The NGC 2071 molecular cloud has been studied with a broad array of techniques, including a large scale study of CS emission, high resolution scans in the far-infrared, N H3 studies with the VLA, and near-infrarred imaging. The far-infrared emission constrains the density distribution to fall off approximately as r−1. The NH3 data strongly supports the presence of a disk oriented perpendicular to the molecular outflow, while the CS emission indicates the presence of dense gas in the region. The results will be combined into a coherent picture of this region of current star formation and molecular outflow.

NGC 7129 is a region where many signs of active star formation have been observed. Three water vapor masers, five Herbig-Haro objects, two FIR sources, two molecular outflows and several PMS stars have been detected. We have obtained very deep CCD images of the GGD nebulosities found in the region. The observations have been carried out with the 3.5 m. telescope at the Calar Alto Observatory (Almeria, Spain). The main conclusions inferred from the data are: (1) The Herbig-Haro nature of GGD 32, GGD 34, GGD 35, HH 103 and HH 105 is confirmed, however GGD 33 has shown to be a PMS star, GGD 33a, with a cometary nebulosity, GGD 33b; (2) GGD 32 and HH 103 are embedded in an extended structure (at least as large as 0.465x1.36 pc), where another Herbig-Haro object and an arc connecting GGD 32 and HH 103 have been detected. The whole ensemble seems to be linked to a third molecular outflow which is not well resolved in the available radiomaps; (3) The arc connecting GGD 32 and HH 103 contains low excitation material and probably traces a low density cavity within the molecular cloud; (4) GGD 34 is constituted by three condensations aligned in the East-West direction, the western one is a bow-shock where the cooling region is resolved and has a size of around 2000 AU; (5) the PMS star associated to GGD 35 has been identified. The mass outflow is highly collimated close to the star, but at a distance of 0.072 pc from the star it is deflected by an external agent which deviates it in the direction of the molecular outflow associated to LkHo 234.

Molecular clouds are clumpy, and the mass spectrum (dN/dM) of clumps scales with the clump mass M to the -1.5 power, as determined from CO observations of molecular clouds (Blitz 1988, Stutzki et al. 1989). The basic idea is to translate the mass spectrum of clumps into a mass spectrum of stars (that are assumed to form from these clumps) by virtue of a clump-star (i.e. initial-final) mass relation.