A survey for the molecules C2H and HC3N in a variety of interstellar clouds has been completed. Both molecules are very widespread, in cold dark clouds as well as in hot clouds. C2H emission has been mapped in L1534. In cold clouds the fractional abundance X(C2H) is found to be 2-6×10−9. The ratio of abundances X(C2H)/X(HC3N) falls in the range 6-10, consistent with some gas-phase reaction schemes for these molecules.

Ammonia is a favoured molecule for the study of molecular clouds since several important parameters of the cloud can be deduced from simple observations of the J,K=1,1 and 2,2 inversion doublet transitions and the hyperfine structure in the (1,1) line. With the additional knowledge of the kinetic temperature Tk from observations of CO, for example, it is possible to compute the excitation temperature of the (1,1) line (T11), the rotational temperature between the (1,1) and (2,2) levels (T21), the molecular hydrogen density n(H2) and ammonia column density N(NH3) (see, for example, Martin and Barrett, 1978).

Maps of the J=1,K=1 inversion transition of interstellar ammonia are presented and compared with observations of carbon monoxide and formaldehyde.

The region surrounding the optical nebula S106 has been mapped in the J=1,K=1 (1,1) transition of ammonia with a 2.2 arc min. beam. The source is observed as two components with the HII region sandwiched between them. It is likely that the asymmetry of the optical nebula is produced by preferential escape of u.v. radiation from the central source of excitation in directions of reduced density within the molecular condensation. Observations of the (1,1) and (2,2) transitions yield a kinetic temperature for the NH3 molecules in the range 18-22K. There is evidence for “clumpiness” in the distribution of matter.

Simultaneous observation of the J,K=1,1 and 2,2 inversion transitions of ammonia (NH3) with high spatial resolution (≲1 arc min) offers a powerful method of probing the core region of interstellar clouds for evidence of molecular clumping and of prevailing physical conditions which could lead to star formation. We have therefore used the Effelsberg 100-m radiotelescope to make an extensive study of the central region of the nearby dark dust cloud L183 (also known as L134N) in the NH3 (1,1) transition; the spatial resolution was 40 arcsec. The core region as mapped in the NH3 (1,1) transition with a velocity resolution of 0.08 km s-1 consists of two elongated condensations separated by about 2 arcmin in north-south direction (see Fig. 1). The central part of the NH3 cloud has an approximate dimension of 6′ (N-S) by 2′ (E-W) corresponding to a linear extent of 0.17 × 0.06 pc at an assumed distance of 100 pc. The measured velocity structure of the NH3 cloud seems to reflect the double peaked nature of the cloud in that it increases from 2.30 km s-1 in the south to about 2.5 km s-1 at the northern end of the southern NH3 peak, and then decreases again to 2.3 km s-1 towards the north. The intrinsic linewidths of NH3 (corrected for hyperfine blending) do not vary significantly with position and are between 0.2 and 0.3 km s-1. The two ammonia peaks are part of a central molecular ridge from which we have observed NH3 (2,2) emission at 9 positions (see Fig. 1). The rotation temperature T21 as determined from the optical depths of the (1,1) and (2,2) transitions is ∼9K for all positions, and hence the kinetic temperature Tkin seems close to this value as well, i.e. ∼10K throughout the central part of L183. A more detailed account is being publsihed elsewhere (Ungerechts, Walmsley and Winnewisser).

Figure 1 shows the (J,K) = (1,1), (2,2) and (3,3) spectra of NH3 observed toward the continuum peak of DR21 and four positions offset 50″ from the peak. These observations were made with the Effelsberg 100-m radio telescope, which has a half-power beam width of ∼43″ at these frequencies.

We mapped the distribution of 6 cm H2CO (110→111) absorption against the HII regions DR21 and W58 with an angular resolution of 6″8 (RA) and a velocity resolution of 0.73 km s-1. The Westerbork SRT was used with the newly completed 5120 channel digital line receiver. With all 14 telescopes, a maximum baseline of 1440 m, both linear polarizations and a bandwidth per channel of 10 kHz the rms noise in the channel maps was about 7 K. The goal of this work is to measure scale sizes of H2CO in molecular clouds near HII regions and to study the kinematics of the clouds in the molecular line.

The 4.8 GHz H2CO absorption line was mapped in the directions of the dark clouds in M17 and NGC 2024. The observations were made with a beamwidth of 6.6 arc min and a velocity resolution of 0.12 km s-1.

The ratio H2CO/H2 has been determined for Heiles Cloud 2 (HC2) (Sherwood and Wilson, 1980, and references therein). Rather surprisingly, the ratio was much larger than that found by Kutner (1973) in a cloud KC only 3 degrees away. In both ratios the H2 density was determined from a relation between star counts and dust column density (see Sherwood and Wilson, 1980). Two other clouds have been studied in a similar manner: Khavtassi 3 (K3) (Myers, 1975) and ρ Oph (Myers et al., 1978). The ratio H2CO/H2 varies among the four clouds as does the minimum visual extinction apparently required before H2CO can form. The data are summarized in the Table. In all cases linear fits were used to determine H2CO/H2.

The formaldehyde molecule is an excellent probe of physical conditions inside interstellar clouds. We illustrate this by presenting results of our recent series of observations using the MPIfR Effelsberg 100-m radio telescope for the 6-cm transition, the NRAO Green Bank 43-m radio telescope for the 2-cm transition and the NRAO Kitt Peak 11-m mm-wave telescope for the 2-mm transition.

3.4-mm line of HCO+ has been mapped toward the galactic center. The telescope used was the 6-m mm-wave telescope at the Tokyo Astronomical Observatory, equipped with a GaAs Schottky barrier diode mixer and a 256-ch filter bank of 1-MHz resolution. Fig. 1 shows the observed positions on the antenna temperature contour map of HCN(Fukui et al. 1977). Intensive observations were made of the line SR where the HCN emission is strongest. Fig. 2 shows the HCO+ profile as well as the HCN profile, both of which are averages along SR().

The two-dimensional distribution of molecular clouds in the galactic center region has been investigated in the CO 115 GHz line and in the OH 1665 and 1667 MHz lines. As the former is an emission line, we can find molecular clouds without the unavoidable bias to continuum sources which is inherent in a survey of OH absorption lines. Because the CO line is usually optically thick, the brightness temperature of the line is directly related to the kinetic temperature of the cloud. On the other hand, the real optical depth of the OH line can be obtained from the intensity ratio between 1665 and 1667 MHz lines (assuming LTE). From this point of view we have compared the CO and OH observational results.

Existing surveys of our galaxy are able to detect CO clouds with an excitation temperature Tex ≳ 5 K. We have made observations to determine if a substantial molecular component is colder than ∼ 5 K. We compared CO emission features with H2CO absorption against 17 continuum sources near the galactic plane. After elimination of features in the spectra that are probably associated with known H II regions, there were 39 clouds remaining, most of which show an excellent kinematic agreement between H2CO and CO. The results do not suggest the existence of a large amount of mass in ultra-cold molecular clouds.

We used the new OVRO 10 meter millimeter telescope together with the Estec heterodyne receiver and backend to observe 12CO (J=2-1) emission in the direction of several galactic HII regions. At a frequency of 230 GHz our angular resolution was 30 arcsec and our velocity resolutions were 0.3 and 1.3 km s-1. We observed the cores of 16 molecular clouds associated mainly with compact and subcompact HII regions: S88, S106, S157, S158 (NGC 7538), S159, S187, S228, S255 (IC 2162), S269, ON-1, G45.5-0.1, Wl (center), W58 (K3-50), Cep A and Mon R2. Observations of all sources were in a cross pattern of at least five points. Four sources - Cep A, Mon R2, S158 and W58 - were mapped in more detail. Results for the latter two are shown in Figures 1 and 2.

We have made carefully calibrated measurements of the J=1→0 and J=2→1 transitions of CS in 32 molecular sources in order to obtain density and fractional abundance information from the excitation of this molecule. The antennas used provided beams of nearly equal size. The line intensities, which fall between 1 and 5K, have typically been determined with a (1σ) uncertainty of ∼10%. The two transitions show nearly equal intensities and linewidths which are, respectively, about one tenth and one half of the corresponding quantities for CO. Our observations are seen to be in good agreement with the results of excitation calculations involving either a velocity gradient model or a purely micro-turbulent model since in both cases optical depths are small (0.3 to 3.0). Values obtained for n(H2) and X(CS)/dv/dr from the velocity gradient model fall in the range 2×104 to 2×105 cm−3 and 3×10−11 to 3×10−10 (km s−1pc−1)−1 respectively.

CO maps of areas around HII regions show many secondary hotspots. J=2-1 observations of 2 dark clouds are discussed.

We used the ESO 3.6 m telescope at La Silla (Chile) in conjunction with the Estec heterodyne receiver to observe CO (J=2-1) emission from southern sources. The telescope HPBW was 2.2 arcmin, the beam efficiency 43 per cent. The receiver used backward-wave oscillators and Schottky barrier diode mixers. It had a single-sideband noise temperature of 4000 K at 230 GHz. The backend was a 256 channel filterbank of 1 MHz (1.3 km s−1) bandwidth per channel. Calibration was done in the usual way.

We have investigated under which physical conditions (kinetic temperature, H2 density, CO column density) the CO molecule shows suprathermal excitation and population inversion. The computations are based on a model in which the excitation is basically due to collisions with H2 molecules. The collision cross-sections were taken from Green and Thaddeus (1976). The radiative transport in the molecular lines is treated in an on-the-spot approximation (see e.g. Kegel 1979)

J = S(1 - e-τ) + Ibge-τ

in order to keep the computing time low enough to permit the investigation of a wide range of parameters. Our approximation of the radiative transfer is mathematically equivalent to a formalism involving an escape probability β = e-τ. This escape probability arises from the fact that the radiative transfer in the line wings is neglected.

High surface brightness 1.0 mm continuum emission has been mapped in nine southern hemisphere HII/molecular cloud complexes: NGC 6334, RCW 38, RCW 57, RCW 122, RCW 117, G333.6-0.2, G351.6-1.3, W 33 and W 33A. All of the sources are located in the inner part of the Galaxy near the galactic plane. This paper presents new 1.0 mm continuum mapping results with 65 arc sec resolution. A more detailed discussion of the observations is given by Cheung et al. (1978, 1979).

We have determined the extinction efficiency factor Q for the objects in the Table by means of maps, scans, or photometry at 1 mm using a composite bolometer at the prime focus of the ESO 3.6m telescope (Arnold et al., 1978; Arnold, 1979). The beam is 2′ in diameter.