Our interferometric and single-dish observations of the continuum emission from L1551-IRS 5 show that, at millimeter wavelengths, there are two distinct components to the source, an envelope with a radius ≥ 2000 AU, and a compact core with a radius ≤ 64 AU. The compact core has a large optical depth, indicating a high column density (∼ 1000 g cm—2). By modeling the temperature in the region of the compact core, we show that its size must lie in the range 45 ± 20 AU. The compact core is most plausibly identified with an accretion, or preplanetary, disk around the star, although the present observations do not have sufficient angular resolution to rule out other structures.

A supersonic wind from a young star will produce regions of strong magnetic field in the stellar environment. The associated shocks compress the molecular gas, increasing the density n, pressure p, and field B. Crucially, the Alfvén speed, v A ∝ B/n1/2, is also increased since the total shock compression is approximately of the form B ∝ n. But is there any evidence for such high v A - or ‘active cloud’ - regions within bipolar outflows? We indicate below one implication which has important observable consequences: fast shocks of low Alfvén number (v/v A ) now arise. With a low ionization level, the C-shock structure is qualitatively different from the high Alfvén number flows which are common to ‘quiescent cloud’ conditions. The magnetic-field cushioning now allows molecular hydrogen to survive very fast shocks and broad H2 lines are feasible. We display results which show that the resolved broad lines and line ratio properties in the OMC-1 outflow can be explained with fast bow shocks moving through such active regions.

This Symposium on fragmentation and star formation has dealt with the heart of the study of molecular clouds, which is how they form stars. This problem is one of the most profound and challenging problems in all of astrophysics. The complexity of the interstellar medium adds to its difficulty and we cannot expect a quick and easy solution. Nonetheless, the reports presented at this Symposium indicate that substantial progress is being made in this field.

On the main axis of two bipolar outflows (L1448 and HH7-11), we have discovered molecular clumps (“bullets”) moving at extremely high-velocities. The observations were made in CO lines (mainly J = 2 — 1) with the IRAM 30-m telescope at Pico Veleta, near Granada (Spain). We present in Figure 1 some profiles observed toward the axis of the two outflows. In addition to the ambient line and to the extremely high-velocity wings, we observe several very well defined peaks at high velocities. Our maps show that these features are well delimited in the space, indicating that they arise in small high-velocity clumps or “bullets”.

The gravitational stability of molecular cloud clumps before and after the onset of massive star formation is discussed. We suggest that the most massive clumps are magnetically supercritical but gravitationally stabilized by the hydromagnetic turbulence caused by FUV photoionization-regulated low-mass star formation in their interiors. The ionizing radiation of an O star can trigger star formation in initially sub- and supercritical clumps.

The penetration of visible and UV continuum radiation is a governing factor for many processes inside interstellar clouds. It determines for instance: 1) the overall chemical equilibrium (formation/destruction of molecules and neutral or ionized species, fractional ionization of the gas which is directly related to the coupling with the magnetic field); 2) the overall energy balance (heating of the dust, heating of the gas through collisions with electrons extracted from grains by the photoelectric effect or with grains; 3) cooling of the gas due to fine structure line emission (OI,CI,CII).

Recent numerical works on self-gravitating turbulence (Bonazzola et al., 1987; Léorat, Passot, Pouquet 1990) tend to show that turbulent velocity fields could act as a “turbulent” pressure, i.e. inhibit the collapse of self-gravitating clouds. This effect is efficient when the caracteristic scale of the turbulent flow is small compared to the Jeans's length. When these two scales are comparable, we show (Chantry, Grappin, Léorat, 1990) that the turbulence do no more inhibit the collapse, but seems to trigger it.

B209. Fragments are detected in the outer parts of the B209 molecular cloud in a region of density 1000 cm−3 using 12CO, 13CO, H2CO, IRAS 60 and 100 μm images, discernable as velocity components, and have characteristics: <Rfragment>∼ 0.4 pc; <Mfragment>∼ 2 M⊙; <MVRfragment>∼ 1.6 M⊙ km/s pc; Tgas ∼ 11 K; Tdust> ∼ 16 K??. The system of fragments has <separation>∼ 0.5 pc and <MVR>∼ 1.6 M⊙ km/s pc. These are illustrated in Figure 1, which contains an IRAS 100 μm map made with Geisha and attendant U. Köln Gornergrat CO spectra for each respective fragment. The differing characteristic velocities of each fragment are readily apparent.

The observed widths of molecular lines indicate that molecular clouds are in a highly turbulent state with supersonic internal velocities. At the same time molecular clouds are highly fragmented into substructures exhibiting large density contrasts. These substructures seem to be arranged in a hierarchical order scheme where one can think of newly formed stars representing the smallest spatial scale of ordering. Stars, protostars and even dense clumps can be regarded as being hydrodynamically decoupled from the ambient gas. Thus, in an idealized way, one can think of molecular clouds as being a gravitationally coupled system consisting of gas (continuous component) and ‘point-masses’ (particle component) randomly moving through the gas. The random motion of these ‘point-masses’ is accompanied by spatial and temporal fluctuations of their gravitational potential which, in turn, induce velocity and density fluctuations of the gas.

The properties of the interstellar magnetic clouds, their hierarchy, turbulence and their origin in a processes of the rising of MHD-instabilities are discussed.

In order to study the excitation conditions in the Orion-A region, we applied an LVG code to 12CO, 13CO and C18O data obtained with the AT&T Bell Laboratories 7-meter telescope in USA (CO isotopes: J = 1 — 0, CS: J = 2 — 1) and the radiotelescope of the "Groupe d'Astrophysique de Grenoble" in France (CO isotopes: J = 2 — 1).

The observed flattening of the initial stellar mass function at low mass can be accounted for in terms of the different interstellar cloud size-mass scaling and different ambipolar diffusion time scaling for small, thermally-supported clouds and larger clouds supported primarily by turbulent pressure.

Two molecular clouds associated with well known reflection nebulosities have been studied in order to stablish the effects of the UV radiation on the surrounding gas and dust. Observations of the J=2 →1 and J=1 →0 transitions of CO and 13CO have been carried out with the telescopes POM 1 and POM 2 towards the clouds associated with LKHα 198 and HD200775. The IRAS maps at 12, 25, 60 and 100 μm towards these globules have been also examined.

IRAS has provided infrared maps of a large number of galaxies. However the size of IRAS survey detectors is quite large (∼5′ in the cross-scan direction) and there is a need for improving the resolution of these maps. In order to understand the spatial structure of galaxies, we have generated high resolution maps of 18 large galaxies by deconvolving the IRAS pointed observations from survey detectors as well as from Chopped Photometric Channel (CPC) using Maximum Entropy Method (MEM).

12CO and 13CO J = 1 → 0 and J = 2 → 1 observations of a 1 deg × 2 deg region centered on the BN/KL nebula in Orion A showed almost everywhere surprising intensity ratios. According to the standard interpretation of CO lines the 13CO TA (2 → 1)/TA (1 → 0) ratio meant optically thick and thermalized 13CO emission whereas at the same positions the 12CO/13CO intensity ratios indicated optically thin 13CO (Castets et al. 1989). Castets et al. (1990) suggested that temperature gradients in the observed clumps caused by external UV heating could be responsible for these results.

A careful analysis of the data available in the literature has been carried out to determine the global properties of star formation in Taurus. Three main trends arise in the data:

1. (1) the magnetic field in Taurus is distorted in the direction of the proper motions of the T Tauri Stars (TIS) (see below).

2. (2) the groups with different proper motions seem to have different stellar contents. An analysis of the stellar population based on the equivalent width of Hα, W(Hα), shows that groups I and III (Jones and Herbig, 1979) have approximately the same population (TTS show either large or small W(Hα)). However in group II (central part of the cloud) there are also a large number of stars with intermediate W(Hα) (Gomez de Castro and Pudritz, 1990).

3. (3) the mass function of the NH3 cores (thought to be "protostellar cores") is dN/dM< M∝ with α=−1.5, in contrast with the IMF of the TTS given by Cohen and Kuhi (1979), α=−2.5.

We discuss the use of polarization maps of background starlight in studying the structure of the interstellar magnetic field. We make the assumption that the polarization observed is due to magnetically aligned dust grains associated with interstellar clouds along the line of sight, and that the position angle (ΘE) of polarization observed gives the direction (modulo 180°) of , the plane-of-the-sky projection of the (line-of-sight-averaged) magnetic field.

There are two basic points in this paper. (1.) Out of context, the projected orientation of an elongated dark cloud may appear special (e.g. roughly "parallel" or “perpendicular”) in relation to the local (plane-of-the-sky) field direction given by polarization observations, but, when the view is expanded to include an entire complex of dark clouds, the shape and orientation of clouds within a complex often appears unrelated to the field structure. (2.) the dispersion in the postion angle of polarization observed in a region of the sky contains information about the ratio of the strength of the uniform (straight) component of the local magnetic field as compared to the dispersion (nonuniform component) in the field. Furthermore, in a region where Zeeman measurements covering the same region as polarization observations have been made, the uniform-to-nonuniform ratio deduced for the field from polarization data, can be combined with information about the line-of-sight field and an estimate of the correlation length of the field, to describe the magnetic field in three dimensions. We discuss the results of such an analysis for the dark cloud Lynds 204 (L204).

Submillimeter and far-infrared observations of carbon monoxide (Jaffe, Harris, and Genzel 1987; Genzel, Poglitsch, and Stacey 1988; Schmid-Burgk et al. 1989; Boreiko, Betz, and Zmuidzinas 1989) have indicated the presence of warm, dense molecular gas near regions of recent star forming activity. Estimates based on the comparison of mid-J (submm) and high-J (far-IR) 12CO lines in M17 and S106 (Harris et al. 1987a) gave a lower limit of ≈1018 cm−2 (τ(12CO 7 →6) ≈ 1) to the CO column density of quiescent (Δv ≤ 10 km/s) gas at temperatures of at least 100 K and H2 densities of 104 to 106 cm−3. The mid-J 12CO lines are likely to be optically thick in most sources. In order to obtain a better estimate of the column densities, it is thus of great interest to observe isotopic mid-J CO lines, which are likely to be optically thin.

We obtain the mass distribution and the age distribution of the young stars associated with Chamaeleon I and Rho Ophiuchi, two nearby sites of star formation. Our method consists in determining the temperature and the luminosity of each object in order to locate it on the HR diagram, and then comparing the position on the HR diagram with the evolutionary tracks and isochrones presented by Cohen and Kuhi (1979). The star-formation process is found to have started more recently in ρ Oph than in Cham I.

In order to search for ultra-dense gas condensations as possible sites of future star formation, observations with highest possible angular resolution in high-gas density probing molecular species are required. Here we report first results from two-telescope (IRAM 30-m & CSO 10.4-m) multi-transition excitation studies of the linear molecules CS and HCN, both with critical densities n c ∼107 cm−3. We obtained maps towards a number of galactic cloud cores in the CS(J=5−4;7−6) and the HCN(3-2;4-3) mm/submm transitions and those of their optically thin isotopic species (C34S; H13CN) accessible from ground. For a proper excitation analysis, the gas temperature has been determined independently from the symmetric top molecules NH3 or CH3CN. Here we present first results obtained towards the NGC 2024 molecular cloud core (Schulz et al. 1990).