Galactic Molecular Clouds Research Articles

A new method for determining the CO to H2 conversion factor for translucent clouds

In this paper we describe a new technique for obtaining the conversion factor between the molecular hydrogen column density and the CO(J = 1-0) integrated antenna temperature. This factor, typically known as X(sub CO) is often to be of order a few times 10(exp 20)/sq cm/K km/s) for the molecular clouds in the Galaxy and is one of the primary means of determining the molecular cloud mass from CO observations. However, for the low-extinction interstellar clouds known as the translucent molecular clouds, estimates of X(sub CO) vary by up to a factor of 60 depending on the object and techniques employed to calibrate X(sub CO). Since the cloud mass is directly proportional to X(sub CO) uncertainties in mass estimates of translucent clouds can be more than an order of magnitude. We calibrate the H2 content in translucent clouds by using the linear relationship between the CH and H2 column densities. The CH column density is readily determined from observations of the CH ground-state hyperfine main-line transition at 3335 MHz. Using CH as a surrogate tracer for H2 and CO(J = 1-0) observations of a sample of translucent and dark molecular clouds, we find a wide variation in values for X(sub CO). For translucent clouds, X(sub CO) ranges from 0.3 to 6.8 x 10(exp 20) and for dark clouds the values range from 0.8 to 8.6. Although the average values for both types of cloud are similar to the canonical value determined for the Galactic molecular cloud ensemble (2-4 x 10(exp 20)), the scatter in individual X(sub CO) values may indicate that X(sub CO) for a given translucent cloud cannot be determined a priori and must be obtained for each cloud so that a reliable mass determination may be made.

The Growth of Solids, Destruction of Molecules, and Shielding of Radiation in the Young Stellar Disk of HD 45677

view Abstract Citations (9) References (62) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS The Growth of Solids, Destruction of Molecules, and Shielding of Radiation in the Young Stellar Disk of HD 45677 Brown, Thomas M. ; Buss, Richard, Jr. ; Grady, Carol ; Bjorkman, Karen Abstract Ultraviolet spectra (912-3300 Å) from the Astro-1 and lUE space missions of two Herbig Be stars, HD 45677 and HD 200775, show that the HD 200775 intrinsic continuum shape generally coincides with a T = 20,000 K Kurucz model and that the HD 45677 continuum exhibits additional line blanketing, extinction, and emission from gas and dust in its disk and bipolar wind. With log N(H I) = 21.4±0.1 cm-1, our measured upper limit on the fraction of HD 45677 disk H atoms in molecules (f < 1.5 × 10-2) is much less than that seen in general Galactic molecular clouds (f ≍ 0.5). Also, our derived HD 45677 circumstellar dust extinction for λ > 1400 Å is produced by an inhomogeneous disk of material: during periods of moderate visual extinction (Av ≍ 1.0±0.3 mag) the disk as a whole causes mid-UV extinction with an Rv ≍ 4-5 (values found in Galactic molecular clouds), yet at the thickest optical depths, there are regions with larger grains (inferred Rv > 7) than those in molecular clouds (Rv < 5.6). Alternately, we find in the FUV (1300-1000 A) a strong, steeply curved circumstellar extinction for HD 45677 that has a very similar shape as extinction produced by interstellar polycyclic aromatic hydrocarbon molecules (PAHs) in an illuminated molecular cloud (NGC 2023). The PAHs around HD 45677, like NGC 2023, are more abundant relative to the large grains (Aλ/Av) than predicted by the extinction parameter Rv, because HD 45677 has evaporated the PAHs off large grains. With a measured FUV depth corresponding to τλ ≍ 1.4-3.5, the excess PAHs around HD 45677 partly shield the outer disk from FUV radiation. Both the disk gas and dust have evolved greatly since the formation of the mid-mass star HD 45677 from molecular clouds: in some regions, the disk grains have grown to micron sizes suitable for the formation of planetesimals, the ISM H_{2 }has been photodissociated into H I in the disk, and CO (McGregor, Hyland, & Hillier 1988) has disappeared. In addition, the molecular PAHs that are condensed onto grains in molecular clouds have photoevaporated off grains into the gaseous disk of HD 45677, becoming dehydrogenated and ionized. Thus, because of strong FUV radiation from HD 45677 and moderate FUV shielding, the disk around this intermediate-mass star differs dramatically from the condensate-rich, molecular gas around low-mass stars and would seem to lead to a different chemical and physical evolution toward planets than those around low-mass stars such as our Sun. Publication: The Astrophysical Journal Pub Date: February 1995 DOI: 10.1086/175323 Bibcode: 1995ApJ...440..865B Keywords: STARS: CIRCUMSTELLAR MATTER; ISM: DUST; EXTINCTION; STARS: EMISSION-LINE; BE; ULTRAVIOLET: STARS; STARS: INDIVIDUAL HENRY DRAPER NUMBER: HD 45677 full text sources ADS | data products SIMBAD (5) MAST (2)

High energy gamma ray observations with the egret instrument on the compton gamma ray observatory

The Energetic Gamma Ray Experiment Telescope (EGRET) on the Compton Gamma Ray Observatory has, since launch in April 1991, observed the entire sky with a exposure of at least 10 8 cm 2 s and has detected a wide variety of gamma ray sources in addition to the diffuse galactic gamma-ray emission. The spectra and time variability of these sources, which include AGN, pulsars, gamma ray bursts, and solar flares, are discussed along with the diffuse emission from the galactic plane and molecular clouds.

First observations of individual molecular clouds in the irregular galaxy NGC 6822

view Abstract Citations (37) References (30) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS First Observations of Individual Molecular Clouds in the Irregular Galaxy NGC 6822 Wilson, Christine D. Abstract Three molecular clouds have been mapped in the nearby metal-poor dwarf irregular galaxy NGC 6822 using the Owens Valley Millimeter-Wave Interferometer. These molecular clouds are similar to molecular clouds recently studied in the Small Magellanic Cloud, and are somewhat less massive on average than molecular clouds in Local Group spiral galaxies. Based on upper limits to the virial masses of the clouds, the CO-to-H_2_ conversion factor in NGC 6822 is < 2.2 +/- 1.3 times larger than the Galactic value [3 x 10^20^ cm^-2^ (K km s^-1^)^-1^]. Conversion factors obtained for Galactic molecular clouds, NGC 6822, and the SMC are consistent with the variation in the CO-to-H_2_ conversion factor being linearly proportional to the oxygen abundance of the galaxy. However, the uncertainties are sufficiently large that equal conversion factors in NGC 6822 and the Galaxy cannot be ruled out. The gas depletion time obtained for the H II region Hubble V is comparable to those of giant H II regions in M33. Thus high-efficiency star formation is not limited to the giant H II regions seen spiral galaxies, but can also occur in a small group of relatively low-mass (a few times 10^4^ M_sun_) molecular clouds. Publication: The Astrophysical Journal Pub Date: October 1994 DOI: 10.1086/187560 arXiv: arXiv:astro-ph/9407070 Bibcode: 1994ApJ...434L..11W Keywords: Abundance; Irregular Galaxies; Local Group (Astronomy); Molecular Clouds; Astronomical Interferometry; Brightness Temperature; Galactic Mass; Oxygen; Star Formation; Astrophysics; GALAXIES: INDIVIDUAL NGC NUMBER: NGC 6822; GALAXIES: IRREGULAR; GALAXIES: ISM; GALAXIES: LOCAL GROUP; ISM: MOLECULES; Astrophysics E-Print: 8 pages + 5 figures, uuencoded compressed postscript file, CDW-94-2 full text sources arXiv | ADS | data products SIMBAD (8) NED (1)

CO Emission from Molecular Clouds in the Central Region of M31

We detected 12 CO (J=1-0) line emission from a complex of dark cloud located at ∼250 pc from M31's center, which is the most conspicuous extinction feature within the central few hundred parsecs as observed with B, V, R, and I band CCD observations. The darkest cloud has a 30 pc size, and the virial mass is estimated to be ∼8.7×10 5 M ○. , which is the most massive molecular cloud in the bulge of M31. The conversion factor from CO intensity to H 2 column mass is found to be by an order of magnitude greater than that for galactic molecular clouds. The hydrogen mass-to-color excess ratio is also found to be anomalously large

Photometric and spectroscopic analysis of high Galactic latitude molecular clouds. I - Distances and extinctions of stars toward 25 selected regions

New photoelectric photometry in the UBV, uvby, and Hβ systems is presented for 320 stars at high Galactic latitudes, selected for their proximity to the densest portions of a variety of high Galactic latitude atomic and molecular clouds. Low-resolution digital spectra are used to classify 115 of the stars, using both stellar line strengths and statistical tests against a library of standard stars of all spectral types. Several new techniques are developed for automatic digital spectral classification, and for dereddening Stromgren uvbyβ and Johnson UBV, Hβ photometry

Nitrogen Sulfide (NS) in Star Forming Regions

Gas phase models of ion molecule chemistry have been rather successful in matching the observed abundances of small interstellar molecules containing carbon, hydrogen, and oxygen. However, the situation is somewhat less clear for nitrogen-containing species, partly because the important initiating reaction N+ + H2 is slightly endothermic; and for sulfur-containing molecules, where it remains uncertain whether it is necessary to invoke surface reactions on grains to match the observed abundances. As a relatively simple species, the abundance of nitrogen sulfide should provide a good test of the models of the coupled chemistry of nitrogen and sulfur. Until very recently only two molecules containing both these elements were known in the interstellar medium, NS and HNCS, and both have been observed only in Sgr B2. We have therefore undertaken a survey for interstellar NS in Galactic molecular clouds using the FCRAO 14-meter telescope. The 2Π1/2, J = 5/2 → 3/2, transition has in fact been detected in many regions of massive star formation (see table).

AST/RO: A submillimetre-wave telescope for the South Pole

The Antarctic submillimetre Telescope and Remote Observatory (AST/RO), a 1.7 m offset Gregorian, is scheduled for installation at the South Pole in November 1993. It is a collaboration including the Smithsonian Astrophysical Observatory, AT&amp;T Bell Laboratories, Boston University, the University of Illinois, the University of Colorado, and the University of Cologne. Observational tests (Pajot 1990; Dragovan et al 1990) and modeling (Bally 1990) of atmospheric transparency over the Pole indicate that it is the best accessible submillimetre-wave site in the world. The immediate scientific goals are heterodyne spectroscopy of galactic molecular clouds and molecular lines in the earth’s stratosphere at wavelengths near 600 μm. Two early observational programs are a large-scale survey of the CI line at 609 μm and monitoring of the 2.8 mm O3 line arising in the stratosphere. Under the newly-formed Center for Astrophysical Research in Antarctica (centred at Yerkes Observatory), AST/RO will become a general-purpose instrument for the millimetre, sub-millimetre and far-infrared.

Ionized carbon in the Large Magellanic Cloud

The 158 micron 2P3/2-2P1/2 fine-structure transition of C(+) at selected locations in the LMC. The C II emission is most intense toward far-infrared continuum peaks and generally is not seen in positions exhibiting strong CO J = 2-1 radiation. Where both C II and CO emission are detected, the V(LSR) centroids are similar but the C II line is wider. The differences in spatial distribution and spectral shape suggest a more pronounced physical separation between the predominantly neutral atomic and molecular gas regions than is the case in the Galaxy. In the LMC, the intense and extended C II emission near 30 Dor implies a total amount of C(+) several times greater than that of Galactic molecular cloud complexes. An attempt was made to detect the 289 micron J = 9-8 transition of (C-12)O in a few locations. The observed upper intensity limit for N159 implies that moderate density molecular gas fills less than 5 percent of the beam and that most of the low J CO emission comes from lower density gas.

Observations of (C-12)O and (C-13)O J = 9-8 in Galactic molecular clouds

The 12 CO and 13 CO J = 9-8 transitions have been observed at a spectral resolution of 1.5 km s -1 in four Galactic molecular clouds : W3, DR 21, W51, and M17. The line shapes are generally similar to those of low-J CO lines for each species, with the exception that line reversals produced by less excited foreground gas are not evident. The ratio of 12 CO and 13 CO intensities shows that for all sources the J = 9-8 12 CO line is optically thick with τ ∼ 5-20. This high optical depth requires a significant amount of dense gas. The peak brightness temperature of the 12 CO J = 9-8 line is similar to or less than the dust temperature for all sources, and most of the detected J = 9-8 radiation appears to originate in the cool molecular gas which comprises the bulk of the clouds.

Giant molecular clouds and the CO-to-H2 conversion factor in IC 10

Three molecular clouds have been mapped in the nearby metal-poor dwarf irregular galaxy IC 10 using the Owens Valley Millimeter-Wave Interferometer. The clouds have velocity widths, diameters, temperatures, and masses that are very similar to those of Galactic giant molecular clouds. However, the virial masses are substantially larger than the molecular masses. Two of these clouds obey the size-line width relation established for molecular clouds in the Galaxy and, thus, are probably gravitationally bound. Assuming these two clouds are self-gravitating and adopting a distance of 1 Mpc, the CO-to-H2 conversion factor of IC 10 is 6 x 10 to the 20th/sq cm per (K km/s), twice as large as the Galactic value. Thus, for a given mass of molecular gas, IC 10 is underluminous in CO by only a factor of 2 relative to the Galaxy. This observational result disagrees with recent theoretical predictions that the CO-to-H2 conversion factor should be a strong function of metallicity. 21 refs.

Molecular gas in the Small Magellanic Cloud

We summarize the results of observations of molecular gas from the Small Magellanic Cloud (SMC) made with low angular resolution (8'.8). These observations show that the CO emission is weak (TA˜ 0.04K) and that the CO luminosities of the Clouds are low compared to those of Galactic molecular clouds. The factor to convert the CO luminosity to molecular hydrogen column density for the SMC is ˜20 and three times larger than those derived for clouds in our Galaxy and in the Large Magellanic Cloud (LMC) respectively. In addition, we present preliminary results of high resolution (40″) observations of SMC molecular clouds made with the SEST telescope.

Small Galactic H II regions. II - The molecular clouds and star formation

CO maps of molecular clouds associated with 11 small Galactic H II regions are presented and compared with IR images obtained by IRAS. The molecular masses of the clouds are computed and compared with the masses of the stellar content. The mapped clouds have masses of 1000-60,000 solar and are typical of the more numerous, smaller Galactic molecular clouds. All of the clouds have recently made massive OB stars, and many have complex spatial and kinematic structures. The coincidence of IRAS sources and CO peaks suggests that many of the clouds have sites of star formation other than the optically visible H II region. Star-formation efficiencies are uncertain, with values for the clouds ranging from 0.02 to 0.6 with an average value of 0.2. There is no trend of the upper stellar mass limit with Galactic radius and with molecular cloud mass. 53 refs.

Molecular absorption by the dust lane and nuclear region of Centaurus A

view Abstract Citations (23) References (26) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Molecular Absorption by the Dust Lane and Nuclear Region of Centaurus A Seaquist, E. R. ; Bell, M. B. Abstract We present VLA observations of the absorption spectra of OH (1.667 GHz), H_2_CO (both 4.83 and 14.5 GHz), and NH3 (23.7 GHz) against the nuclear continuum radio source in Centaurus A. H_2_CO was also detected against the jet at 4.83 GHz. The nuclear features detected are compared with previous detections of other molecules and neutral hydrogen against the same source. We conclude that the absorption at the systemic velocity of 550 km s^-1^, attributed to the dark dust lane which crosses the bulge of the galaxy, is produced by either one or more modest-sized molecular clouds or the edge of a large cloud. The same appears to be true in the direction of the jet. A comparison of the new H_2_CO transitions permits an estimate to be made of the rotational temperature. We obtain a figure of 3.9 K (or lower) which is at the extreme low end of the range for galactic molecular clouds. We have detected a counterpart in OH to the H I features at 576 and 596 km s^-1^ attributed to gas falling toward the nucleus of Cen A. This feature, centered at about 591 km s^-1^, is not detected for any other molecule, but there is an extension at the red edge of the H_2_CO line at 14.5 GHz (and possibly 4.83 GHz) which may also be produced by infalling gas. There is also weak evidence for 4.83 GHz absorption against the jet at 610 km s^-1^. Thus, the infalling gas appears to have a distinctive molecular composition and contains fewer molecules than the gas at the systemic velocity. Either the molecular abundances are modified by shock heating in the infall region, or high excitation by the microwave (and other) radiation fields in the nuclear region has reduced the opacity of some molecules. Publication: The Astrophysical Journal Pub Date: November 1990 DOI: 10.1086/169388 Bibcode: 1990ApJ...364...94S Keywords: Cosmic Dust; Galactic Nuclei; Molecular Absorption; Radio Sources (Astronomy); Ammonia; Centaurus Constellation; Formaldehyde; Hydroxyl Emission; Very Large Array (Vla); Astrophysics; GALAXIES: INDIVIDUAL NAME: CENTAURUS A; GALAXIES: NUCLEI; GALAXIES: THE GALAXY; INTERSTELLAR: MOLECULES; NEBULAE: INTERNAL MOTIONS full text sources ADS | data products SIMBAD (4) NED (1)

The molecular hydrogen content of NGC 604 and other M33 H II region complexes

We have searched six bright H II regions in M33 for H2 line emission. Previously detected fluorescent H2 emission in the first-ranked M33 H II region, NGC 604, has been mapped, allowing us to estimate the mass of its molecular cloud (of order |$3\,\times \,{10}^{6}\,{M}_{\odot}$|⁠). This result implies a ratio |$\text{I}_\text{co}/\text{N}(\text{H}_{2})$| lower by an order of magnitude than that of galactic molecular clouds. The lack of shocked H2 in NGC 604 implies that the ionized stellar wind bubbles seen in the nebula are not interacting strongly with the molecular material. The dynamics of NGC 604 appear to be dominated by gravitational interaction. A 2-µ spectrum of NGC 604 also shows H I and He I recombination lines from the ionized gas, as well as CO absorption in the photospheres of late-type stars, which must dominate the 2-µ continuum. With the exception of IC 133, the other H II regions yielded only upper limits for H2 emission. IC 133 appears to contain an unusually small amount of gas and dust. The lack of H2 in the other regions is briefly discussed.

Are molecular clouds in virial equilibrium?

A correlation between CO luminosities and virial theorem masses of Galactic molecular clouds has been claimed to prove conclusively that molecular clouds are in gravitational virial equilibrium. This correlation is an artifact of the size-line width relation exhibited by molecular clouds, which does not require clouds to be virialized. Virial theorem masses do not provide an estimate of N(H2)/I(co). Comparison of the virial theorem-derived values of N(H2)/I(co) with the gamma-ray value shows that the masses of lower luminosity clouds are considerably overestimated by virial theorem analyses; these clouds are probably strongly influenced by nongravitational forces. 24 refs.

Photoionization-regulated star formation and the structure of molecular clouds

A model for the rate of low-mass star formation in Galactic molecular clouds and for the influence of this star formation on the structure and evolution of the clouds is presented. The rate of energy injection by newly formed stars is estimated, and the effect of this energy injection on the size of the cloud is determined. It is shown that the observed rate of star formation appears adequate to support the observed clouds against gravitational collapse. The rate of photoionization-regulated star formation is estimated and it is shown to be in agreement with estimates of the observed rate of star formation if the observed molecular cloud parameters are used. The mean cloud extinction and the Galactic star formation rate per unit mass of molecular gas are predicted theoretically from the condition that photionization-regulated star formation be in equilibrium. A simple model for the evolution of isolated molecular clouds is developed.

Detection of strong carbon monoxide emission from the host galaxy of the quasar I ZW 1

view Abstract Citations (68) References (27) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Detection of Strong Carbon Monoxide Emission from the Host Galaxy of the Quasar I ZW 1 Barvainis, Richard ; Alloin, Danielle ; Antonucci, Robert Abstract We have observed the radio-quiet quasar I Zw 1 in the J = 1-0 and J = 2-1 lines of CO and obtained clear detections in both transitions. The J = 1-0 line appears much more luminous than than the J = 2-1 line. If the CO is optically thick and thermalized, as it is in galactic molecular clouds, this implies that the CO is extended on the scale of the larger J = 1-0 beam size (26 kpc). This is roughly the same scale as the optically visible disk of the host galaxy of I Zw 1, so the CO emission must originate throughout the host galaxy's disk. This is substantiated by the CO profile shape, which is double-horned and symmetric like those seen in the rotating disks of the spiral galaxies. In a plot of CO luminosity versus far-infrared luminosity, I Zw 1 lies well within the distribution of luminous IRAS galaxies, demonstrating the plausibility of the hypothesis that the far-infrared emission in I Zw 1 is thermal. If more than about half of the far-infrared is attributed to a nonthermal mechanism, I Zw 1 moves into an unoccupied zone on the diagram. Therefore, despite the quasi-power-law infrared spectrum, the far-infrared may be dominated by dust emission. Publication: The Astrophysical Journal Pub Date: February 1989 DOI: 10.1086/185380 Bibcode: 1989ApJ...337L..69B Keywords: Carbon Monoxide; Galactic Radiation; Infrared Sources (Astronomy); Quasars; Disk Galaxies; Far Infrared Radiation; Luminosity; Molecular Clouds; Astrophysics; INFRARED: SOURCES; INTERSTELLAR: MOLECULES; QUASARS full text sources ADS | data products SIMBAD (4) NED (4)

Gamma-ray line production in astronomical objects

This review focuses on the conditions for γ-ray line production in the most interesting astronomical objects, in light of the planned experiments: ‘Gamma-1’, ‘GRO’, ‘Sigma’, ‘GRASP’, and others. Among these objects are the Sun, the galactic center region, molecular and dust clouds, accreting and exploding stars.

Aperture synthesis observations of individual molecular clouds in M33

Aperture synthesis maps of six fields in the nearby spiral galaxy M33 have revealed the presence of a population of molecular clouds whose masses, sizes, velocity widths, and brightness temperatures are similar to Galactic giant molecular clouds. The masses of the clouds obtained from the virial theorem and from the integrated CO flux are in relatively good agreement, suggesting that the conversion factor from CO flux to molecular mass in M33 is unlikely to be much different from the value measured in our own Galaxy. The star formation properties of these regions are under study with deep optical CCD imaging and near-infrared data.