Marc Leslie Kutner

Origin and structure of isolated dark globules

High resolution carbon monoxide observations of over a dozen isolated dark globules indicate that a power-law correlation exists between the internal velocity dispersion and cloud size. The power-law relation implies the presence of supersonic turbulence that is similar to that found in other diffuse clouds and molecular cloud complexes. Thus the observed motions in globules may be all part of a common hierarchy of interstellar tubular motions. Most of the gobules are found to be gravitationally bound and in virial equilibrium. The average gas density seems to decrease with increasing cloud size, suggesting that gravitational contraction may account for the difference in gas density. The effects of gravitational contraction, which tends to increase the central gas density, coupled with the more rapid dissipation of turbulent motions in high density regions may account for the observed difference in velocity dispersion in different parts of a cloud. The results are consistent with the scenario that isolated dark globules represent condensed fragments from nearby filamentary cloud complexes, the formation of which depends strongly on the properties of interstellar turbulence.

Molecular clouds in the outer Galaxy. III. CO studies of individual clouds

Maps of the CO (J = 1 to 0) line have been obtained in order to explore the physical conditions and star-forming activities in 31 molecular clouds in the outer Galaxy. On a large scale, the clouds are found to follow the H I warp. Antenna temperatures (corrected for atmospheric attenuation, ohmic losses, and rear and forward spillover) are shown to decrease somewhat with distance. The results suggest low kinetic temperatures in the clouds and that the CO line is optically thick. Most of the clouds lie between R = 11.5 kpc and R = 14 kpc, are about 40 pc in the longest dimension, and have masses in the 10 to the 4th to 10 to the 5th solar mass range. 33 references.

The nature of the radio and infrared sources in S140

Observations of the S140 molecular cloud have been obtained with the VLA at 6 and 2 cm and with the KPNO infrared camera at 1.2, 1.65, and 2.2 microns. Radio continuum emission is seen from all three mid-infrared sources, IRS 1, 2, and 3. The near-infrared images show IRS 1 and 3 very clearly, but IRS 2 is not detected. Two additional radio sources not associated with mid-infrared sources, but coincident with strong, scattered, near-infrared emission, are probably clumps of shock-ionized gas, whose associated dust is illuminated by photons escaping through holes in the dense material around IRS 1. The near-infrared emission from IRS 1 and 3 is too strong to come from the stellar photospheres; instead it must be emission from hot dust grains. The absence of IRS 2 from the near-infrared images implies that the extinction toward IRS 2 must be much larger than the average. A substantial number of weaker sources are seen at 2 microns in the vicinity of IRS 1, indicating that many lower mass stars are also forming in the region. 38 refs.

CS multitransitional study of density distribution in star-forming regions. 2: The S140 region

The S140 molecular cloud was observed in five transitions of CS with resolutions of 11 to 45 arcsec. The data were analyzed with both the LVG and microturbulent models of radiative transfer to derive the density structure. It was found that the CS emission comes from three components of gas: a spherical component centered on the infrared cluster, an arc component along the ionization front between the S140 H II region and the dense molecular cloud core, and a high-velocity component from the dense part of a molecular outflow. The spherical component contributes most to the CS emission and was analyzed in more detail than the other components. Using a temperature distribution derived from an analysis of the dust emission from S140, we fit a power-law density distribution of n(r) = n(sub i)(r/r(sub i))(exp -alpha) to the spherical component. The best fit was for n(sub i) = 1.4 x 10(exp 6) (density at r(sub i) = 0.026 pc) and alpha = 0.8. The density (n(sub i)) was found to be greater than or equal to the density required to account for the dust emission, depending on the dust opacity laws adopted. The presence of optical emission (Dinerstein, Lester, & Rank 1979) suggests a clumpy structure for the dense gas. Considerations of the virial mass and the lowest amount of column density required to produce dust emission put the volume filling factor (f(sub nu)) of the dense gas at approximately 0.14-0.5. We compared S140 with other regions of star formation where the density structure has been derived from excitation analysis. Source-source variations in density gradients and clumpiness clearly exist, ranging from alpha = 2 and f(sub nu) approximately 1 in B335 to alpha approximately 0, f(sub nu) approximately 0.1 in M17. There is a tendency for more massive star-forming regions to have a flatter density distribution, a more clumpy structure, and a large number of young stars. The implications of this tendency are discussed.

Molecular clouds in the outer Galaxy. IV. Studies of star formation

Star formation has been studied in 17 outer Galaxy molecular clouds using the VLA to perform 6 and 20 cm continuum observations to search for H II regions as evidence of massive star formation. IRAS data are used to measure the far-IR luminosity as an indicator of the total star formation rate. H II regions are found to be associated with each of the clouds. The number of ionizing photons required by radio luminosity ranges from 6 x 10 to the 45th/s to 5.5 x 10 to the 49th/s. Far-IR emission was detected from each of the clouds with the range of luminosities being 500 to one million solar. From the data, it is concluded that there is no strong correlation between star formation activity and cloud mass. 39 refs.

A comparison of CO(J = 1 yields 0) and CO(J = 2 yields 1) emission in the Milky Way molecular ring

We have carried out a CO (J = 2 right arrow 1) survey of the Scutum Arm region of the Milky Way molecular ring. Our goals are to compare CO (J = 2 right arrow 1) maps of individual Galactic clouds with the large-scale CO (J = 2 right arrow 1) emission from the Galactic plane, and to predict the CO (J = 2 right arrow 1) appearance of a Galactic cloud ensemble in an external galaxy. The angular resolution and spatial coverage of our survey are compatible with the existing CO (J = 1 right arrow 0) survey of this region by Sanders et al., which we use for comparison. We identify 34 molecular clouds in our map region; their relationships between size and line width and between virial mass and luminosity are consistent with the relationships seen in CO (J = 1 right arrow 0) emission. However, we note that previous studies have shown considerable variation in these relationships; we attribute much of this variation to differences in their cloud definition algorithm and statistical method. We find that the median ratio of integrated emission in the two lowest CO transitions for the clouds in our sample is I(2 right arrow 1)/ I(1 right arrow 0) = 0.69, implying that the typical emitting region in the line of sight contains cold gas that is not actively star-forming. Our conclusion that the molecular ring emission is not dominated by star-forming regions is consistent with other large-scale studies of the Milky Way. Our distributions of cloud size and temperature also imply that relatively massive molecular clouds that lac k star formation, such as Maddalenas cloud, are not rare in the inner Galaxy.

Results of the SEST Key Programme : CO in the Magellanic Clouds. VIII. The giant molecular complex No. 37 of the LMC

We report observations of the CO(1!0), CO(2!1) and 13 CO(1!0) line emission from the giant molec- ular complex No. 37 of the Large Magellanic Cloud, made with linear resolutions between 6 and 12 pc. The obser- vations were undertaken with the Swedish-ESO Submillimetre Telescope (SEST) as part of the Key Programme: CO in the Magellanic Clouds. We nd that the CO(1!0) emission arises from six large, distinct, molecular clouds, with CO luminosities in the range 1 10 4 to 5 10 4 Kk m s 1 pc 2 and sizes between 22 and 38 pc, and seven smaller clumps, with CO luminosities in the range between 7 10 2 and 2 10 3 Kk m s 1 pc 2 . The opacities in the CO(1!0) line at the peak position of the large clouds are remarkably smaller than those derived for Galactic molecular clouds. Relationships between line width, size and CO luminosities are discussed. The total CO lumi- nosity of the complex determined from the SEST observations, of 1:810 5 Kk m s 1 pc 2 , is in excellent agreement with that determined from the low spatial resolution (140 pc) observations of Cohen et al. On the other hand, the total mass of molecular gas in the complex derived from the SEST observations, assuming that the individual clouds are virialized, is 2:4 10 6 M, which is a factor of 6 lower than that estimated by Cohen et al. under the same assumption. We conclude that the value of the velocity integrated CO emission to H2 column density conversion factor in the LMC determined from low angular resolution observations has been overestimated by a factor of 3. We derive a conversion factor for clouds in Complex-37 of 6 10 20 cm 2 K 1 km 1 s, which is similar to that for clouds in the outer Galaxy.

A precise measurement of the cosmic background radiation at 1. 32 millimeters

A precise value of the cosmic background radiation temperature at 1.32 mm has been determined from the interstellar absorption lines of the CN molecule toward HD 154368. Equivalent widths for CN and (C-13)N in the main cloud are reported. A very weak secondary cloud has been discovered. Millimeter observations at 2.64 mm revealed weak CN emission from the main cloud with T-asterisk(R) = 19.0 + or - 5.1 mK. Thus there is some local excitation of CN. Correcting for this yields T(CBR) = 2.832 + or - 0.072 K at 1.32 mm which when combined with the result of Meyer et al. (1989) gives the T(CBR) = 2.831 + or - 0.056 K at 1.32 mm. At 2.64 mm, T(CBR) = 2.834 + or - 0.085 K is found. 25 refs.

Molecular clouds associated with reflection nebulae. I. A survey of carbon monoxide emission

We present 2.6 mm wavelength CO and /sup 13/CO observations of 130 molecular clouds associated with reflection nebulae. Enhanced CO emission was found in the vicinity of the illuminating star in about half the objects studied. There is a tendency for the CO peak to be slightly displaced from the star. We find many examples of peaks that appear to result from heating of the cloud by the nearby star, while others appear to be associated with independent concentrations of material.

VLA observations of H/sub 2/CO in absorption against the cosmic background radiation - clumps in the S140 molecular cloud

The present 6-cm H/sub 2/CO observations of the S140 molecular clouds with the VLA note absorption over an about 4 x 3 arcmin region that does not lie in the continuum emission direction, implying absorption of the cosmic background radiation. It is possible to construct a consistent model in which the absorption primarily arises in the lower density envelopes of the dense clumps inferred from multitransition studies of this region. Application of the virial theorem to these clumps yields a total mass of 560-700 solar masses for the region, with about 40 solar masses/clump. 23 references.