Massive Molecular Clouds Research Articles

Giant molecular clouds in the galaxy: distribution, mass, size and age

Millimeter wave observations of emission from the CO molecule have become, over the past eight years, the dominant method for determining the physical properties of dense interstellar clouds, composed primarily of molecular hydrogen and for exploring the structure and kinematics of the galactic disk. In this paper we briefly review the CO survey results in the literature (Section 2) and then present new results (Section 3-7) of an extensive 13CO and 12CO survey of the galactic distribution, size, mass and age of molecular clouds. The interpretation of this survey leads to a new picture of the interstellar medium dominated by very massive stable long-lived clouds which we refer to as Giant Molecular Clouds. We find that Giant Molecular Clouds (GMC's) with M = 105–3 × 106M⊙ are a major constituent of the galactic disk, the dominant component of the interstellar medium in the galaxy interior to the sun and the most massive objects in the galaxy. We find that the interstellar medium and star formation are dominated by massive gravitationally bound clouds in which stars and associations are forming but at a very low rate in comparison to the free fall time. The galactic distribution of the molecules as traced by CO emission is interpreted as the distribution of GMC's. As the most massive objects in the galaxy they are also basic to the dynamics of the disk.

The erosion and dispersal of massive molecular clouds by young stars

We evaluate the efficiency with which O stars disperse the molecular clouds from which they form. An O star near the surface of an extended molecular cloud ionizes a H II region which is radiation-bounded on the inner side (towards the cloud centre) and density-bounded on the outer side; consequently the ionized gas can stream away into space on the outer side, and this in turn enables the ionization front on the inner side to advance faster into the cloud, thus eroding a large cavity around the O star. This process may be very effective in destroying molecular clouds; if ∼ 4 per cent of a cloud&s material were converted into new stars with a Salpeter Initial Mass Function, the remaining 96 per cent could be broken up and dispersed by the ionizing radiation from the O stars

Far-infrared emission of molecular clouds

view Abstract Citations (33) References (42) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Far-infrared emission of molecular clouds. Ryter, C. E. ; Puget, J. L. Abstract The far-IR power released by interstellar matter on a galactic scale is evaluated using available IR observations of massive molecular clouds and of the diffuse galactic radiation along with the distribution of interstellar matter deduced from 2.6-mm CO observations. An IR luminosity that is normalized per hydrogen atom is defined, an interstellar-grain model is outlined, and an attempt is made to relate the far-IR brightness of the 'cold' envelope of nine IR sources to the column density of that envelope. An average normalized IR luminosity of approximately 2.1 by 10 to the -30th power W per H atom is obtained for these sources, and the average value for the whole Galaxy is suggested to be of the same order of magnitude. It is concluded that the galactic plane should exhibit a color temperature of the order of 23 K and that most molecular clouds are associated with very powerful sources, the most likely being newly formed stars within the clouds. A global star-formation rate per unit mass of interstellar matter is computed and compared with rates derived from star counts. Publication: The Astrophysical Journal Pub Date: August 1977 DOI: 10.1086/155412 Bibcode: 1977ApJ...215..775R Keywords: Far Infrared Radiation; Hydrogen Clouds; Infrared Astronomy; Interstellar Gas; Stellar Evolution; Carbon Monoxide; Cosmic Dust; Galactic Radiation; Gas Density; H Lines; Radiant Flux Density; Star Formation Rate; Stellar Luminosity; Astrophysics full text sources ADS |

Observations of extragalactic molecules. II - HCN and CS

view Abstract Citations (36) References (6) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Observations of extragalactic molecules. II. HCN and CS. Rickard, L. J. ; Palmer, P. ; Turner, B. E. ; Morris, M. ; Zuckerman, B. Abstract Results are reported for searches for HCN and CS emission from NGC 253 and M82. The features observed in the J = 1-0 spectra of HCN show rough agreements of shape and velocity centroid with (C-12)O profiles, and imply HCN/CO abundance ratios that are close to Galactic values. The presence of HCN emission would support the hypothesis that massive molecular clouds (rather than small dust clouds) are the major contributors to the CO emission observed in these galaxies. No features were found in the J = 2-1 spectra of CS. Publication: The Astrophysical Journal Pub Date: June 1977 DOI: 10.1086/155261 Bibcode: 1977ApJ...214..390R Keywords: Carbon Compounds; Hydrocyanic Acid; Interstellar Matter; Radio Astronomy; Spectrum Analysis; Sulfides; Carbon Monoxide; Cassegrain Optics; Error Analysis; Radial Velocity; Astrophysics full text sources ADS | data products NED (2)

The structure of W49 A as deduced from molecular and recombination line observations

view Abstract Citations (58) References (44) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS The structure of W49 A as deduced from molecular and recombination line observations. Mufson, S. L. ; Liszt, H. S. Abstract The molecular cloud complex W49 A has been observed in the 2.6-mm CO line, the 2-cm H2CO line, and the H76-alpha recombination line. These observations are most consistent with a picture of the complex in which a large H II region lies between two massive molecular clouds. Using a simple hydrodynamic model coupled with calculations of radiative transfer in the recombination lines, it is found that many of the spectral-line and continuum observations obtained on this region can be naturally explained, including: (1) the multiple structure observed in the molecular lines and the single structure in the recombination lines; (2) the local standard of rest (LSR) velocities of both the molecular and recombination lines; (3) the velocity gradients observed in the lines; (4) the continuum emission from the H II region; and (5) the line widths and line-to-continuum ratios of the recombination lines. Publication: The Astrophysical Journal Pub Date: March 1977 DOI: 10.1086/155089 Bibcode: 1977ApJ...212..664M Keywords: Astronomical Spectroscopy; Carbon Monoxide; Formaldehyde; Interstellar Gas; Radio Sources (Astronomy); Spectrum Analysis; Astronomical Maps; Cold Gas; Fine Structure; H Alpha Line; H Ii Regions; Ionized Gases; Nebulae; Radiative Recombination; Spectral Line Width; Astrophysics full text sources ADS |

Distribution of NH3 clouds in the galactic center region

The galactic center has been surveyed in three transitions of NH3 along the galactic plane at b = - 2' in the longitude range 3580 < 1 < 2?3. The velocity-longitude contour map of the (1, 1) inversion transition shows a remarkably asymmetric distribution of dense molecular clouds with respect to the galactic center. Strong NH3 sources are concentrated in the region of 0 <% 1 <% 20 and 0 km 1 <% Vr <% 100 km 1 Also weak, extended features have been found in this region and in regions corresponding to the nuclear disk. The total mass of the dense molecular clouds in the northern part of the galactic center region (00 < 1 < 20) is estimated to be > 6 x l0 M0. Subject headings: galactic nuclei - interstellar matter - molecules, interstellar

Models of massive molecular clouds

view Abstract Citations (353) References (8) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Models of Massive Molecular Clouds Zuckerman, B. ; Evans, N. J., II Abstract Observations of carbon monoxide lines from massive molecular clouds, such as those near Orion A and Sgr B2, have recently been interpreted in terms of large-scale motions, generally collapse or expansion, that imply rapid evolution of these clouds. An alternative explanation is suggested - namely, the clouds are not dominated by large-scale systematic motions and the CO profiles are determined primarily by local motions. Therefore, it is generally not possible to see through these clouds in optically thick lines of molecules such as CO and HCN. Some consequences and tests of this model are briefly discussed. Publication: The Astrophysical Journal Pub Date: September 1974 DOI: 10.1086/181613 Bibcode: 1974ApJ...192L.149Z Keywords: Astronomical Models; Carbon Monoxide; Hydrogen Clouds; Interstellar Matter; Line Spectra; Carbon Isotopes; Cloud Physics; Hydrocyanic Acid; Line Shape; Optical Thickness; Astrophysics full text sources ADS | data products SIMBAD (10)