George F. Mitchell

Large Area Mapping at 850 Microns. III. Analysis of the Clump Distribution in the Orion B Molecular Cloud

We present results from a survey of a 900 arcmin2 region of the Orion B molecular cloud, including NGC 2068, NGC 2071, and HH 24/25/26, at 850 μm using the Submillimeter Common-User Bolometer Array (SCUBA) on the James Clerk Maxwell Telescope. Following the techniques developed by Johnstone et al., we identify 75 independent objects and compute size, flux, and degree of central concentration. Comparison with isothermal, pressure-confined, self-gravitating Bonnor-Ebert spheres implies that the clumps have internal temperatures of 20-40 K and surface pressures 5.5 1.0 M☉. Significant incompleteness makes determination of the slope at lower masses difficult. The two-point correlation function of the clump separations is measured revealing clustering on size scales r < 1.5 × 105 AU with a radial power-law exponent γ = 0.75.

Low upper limits on the O2 abundance from the Odin satellite

For the first time, a search has been conducted in our Galaxy for the 119 GHz transition connecting to the ground state of O2, using the Odin satellite. Equipped with a sensitive 3 mm receiver (Tsy ...

Hot and cold gas toward young stellar objects

High-resolution M band spectra are presented for the seven embedded IR sources W3 IRS 5, S140 IRS1, NGC 7538 IRS 1, NGC 7538 IRS 9, GL 2136, LkH-alpha 101, and MWC 349A, and the data are combined with previously published work for W33A and GL 2591. Cold CO is seen toward all nine sources, with temperatures from 11 K to 66 K. Column densities of cold CO are presented. Hot gas is seen toward eight of the nine objects with temperatures from 120 K to 1010 K. New lower limits to the hot gas density are obtained. The hot gas toward GL 2591, GL 2136, W3 IRS 5, and S140 IRS 1 is probably very near the central source and heated via gas-grain collisions. The optical depth in the silicate feature is strongly correlated with the (C-13)O column density, confirming that silicate optical depth is a useful measure of gas column density. The ratio of solid-to-gaseous CO is obtained for seven sources. 43 refs.

Large area mapping at 850 μm. IV. Analysis of the clump distribution in the orion B south molecular cloud

We present results from a 2300 arcmin2 survey of the Orion A molecular cloud at 450 and 850 μm using the Submillimeter Common-User Bolometer Array (SCUBA) on the James Clerk Maxwell Telescope. The region mapped lies directly south of the OMC 1 cloud core and includes OMC 4, OMC 5, HH 1/2, HH 34, and L1641N. We identify 71 independent clumps in the 850 μm map and compute size, flux, and degree of central concentration in each. Comparison with isothermal, pressure-confined, self-gravitating Bonnor-Ebert spheres implies that the clumps have internal temperatures Td ~ 22 ± 5 K and surface pressures log(k-1 P cm-3 K) = 6.0 ± 0.2. The clump masses span the range 0.3-22 M☉ assuming a dust temperature Td ~ 20 K and a dust emissivity κ850 = 0.02 cm2 g-1. The distribution of clump masses is well characterized by a power law N(M) M-α with α = 2.0 ± 0.5 for M > 3.0 M☉, indicating a clump mass function steeper than the stellar initial mass function. Significant incompleteness makes determination of the slope at lower masses difficult. A comparison of the submillimeter emission map with an H2 2.122 μm survey of the same region is performed. Several new Class 0 sources are revealed and a correlation is found between both the column density and degree of concentration of the submillimeter sources and the likelihood of coincident H2 shock emission.

A CO J = 2 - 1 study of the outflow sources GL 490, GL 2591, M8E-IR, and W3 IRS 5

The CO J = 2→1 emission from the high-velocity sources GL 490, GL 2591, M8E-IR, and W3 IRS 5 was mapped, at 21″ resolution. New bipolar outflows are detected from GL 2591, M8E-IR, and W3 IRS 5. The new W3 IRS 5 bipolar outflow (with a lobe separation of 30″) has the same orientation as a very small-scale bipolar outflow (lobe separation of 2.5″) previously found by aperture synthesis, suggesting two episodes of outflow acivity from IRS 5. Observations of 13 CO J = 2→1 are combined with the 12 CO data to determine the mass and other physical properties of the outflowing gas

The Missing Interstellar Medium of NGC 147 and M32

We present the results of a search for CO 1 ? 0 emission from NGC 147 and M32, two of the four dwarf elliptical companions of M31. Return of gas from evolved stars to the interstellar medium of these galaxies should have resulted in detections, but we find instead upper limits of 4100 and 5100 M? of H2 for NGC 147 and M32, respectively. Including and earlier H I limit, we find that the gaseous interstellar medium (ISM) of NGC 147 comprises less than 2% of what is expected. The large published H I mass limit for M32 prevents us from reaching a similarly extreme conclusion for this galaxy. These results stand in stark contrast to what is seen in NGC 185 and NGC 205, where the observed gas is approximately what is expected from stellar mass return, although some of the gas in NGC 205 probably had an external origin. There are no obvious differences in masses or luminosities that would explain the results. The proposal that differences may be related to the recent interaction histories of the galaxies with M31 is rendered moot by the lack of orbital information but does not seem to be viable for NGC 147 and NGC 185. We can offer no convincing explanation for these puzzling results, although they may point to a fundamental gap in our understanding of galaxy evolution.

Episodic outflows from high-mass protostars

This paper examines the kinematics and physical properties of the outflowing gas from seven luminous deeply embedded young stellar objects or protostars: M8E-IR, GL 490, GL 2591, W3 IRS 5, NGC 7538 IRS 1, NGC 7538 IRS 9, and S140 IRS 1. The outflows are seen as blueshifted absorption features in lines of the fundamental band of CO. The CO lines seen in absorption are compared with CO lines seen in emission at mm wavelengths. New CO J = 2-1 emission-line data are presented for the first five of the sources. 60 refs.

The Puzzling Features of the Interstellar Medium in NGC 205

We present CO 1 → 0 and 2 → 1 observations of NGC 205, which is a dwarf elliptical galaxy and a companion to M31. It has long been suggested that NGC 205 has interacted with M31 in the past. The total mass of gas (molecular, atomic, and X-ray emitting) associated with NGC 205 is ~106 M☉, which is 1 order of magnitude less than the mass that should have been returned to its ISM by evolved stars during its lifetime. A burst of star formation began in NGC 205 ~5 × 108 yr ago, ending a few million yr ago; while some of the mass that was returned to the ISM would be used up by the starburst, star formation is a notoriously inefficient process. Moreover, the gas in NGC 205 is rotating, while the stars are not, so any gas returned to the ISM should have zero net angular momentum. We suggest that the angular momentum was added when the preexisting gas interacted with an external gas cloud, either in the disk of M31 or, perhaps less likely, in the region surrounding it. The recent starburst could have been triggered by the interaction, and the blast waves from ensuing supernovae would then have removed most of the remaining gas. Sufficient time has passed since these events for planetary nebulae to have contributed most of the gas we see in this galaxy.

The gas environment of the young stellar object GL 2591 studied by infrared spectroscopy

High-resolution M band (4.6 microns) spectroscopy of GL 2591 is presented. Physical structures noted include an absorption feature with an outflow velocity of about 17 km/s, cold gas (identified with the core of the molecular cloud within which the object is embedded), and very broad C-12O lines formed in a neutral wind. The detection of hot low-velocity gas together with warm high-velocity gas suggests the scenario of a warm neutral wind accelerating from an accretion disk. 32 refs.

First detection of NH3 (10 -g 00) from a low mass cloud core. On the low ammonia abundance of the rho Oph A core

Odin has successfully observed the molecular core rho Oph A in the 572.5 GHz rotational ground state line of ammonia, NH3 (JK = 10 -> 00). The interpretation of this result makes use of compleme ...