George Mitchell | Saint Mary's University (Canada) (original) (raw)

Papers by George Mitchell

We present results from a survey of a 1300 arcmin^2 region of the Orion B South molecular cloud, ... more We present results from a survey of a 1300 arcmin^2 region of the Orion B South molecular cloud, including NGC 2024, NGC 2023, and the Horsehead Nebula (B33), obtained using the Submillimetre Common-User Bolometer Array (SCUBA) on the James Clerk Maxwell Telescope. Submillimeter continuum observations at 450 microns and 850 microns are discussed. Using an automated algorithm, 57 discrete emission features (``clumps'') are identified in the 850 micron map. The physical conditions within these clumps are investigated under the assumption that the objects are in quasi-hydrostatic equilibrium. The best fit dust temperature for the clumps is found to be T_d = 18 +/- 4 K, with the exception of those associated with the few known far infrared sources residing in NGC 2024. The latter internally heated sources are found to be much warmer. In the region surrounding NGC 2023, the clump dust temperatures agree with clump gas temperatures determined from molecular line excitation measure...

Journal of the Royal Astronomical Society of Canada, Sep 29, 1984

High Resolution Infrared Spectroscopy in Astronomy, 2006

ABSTRACT We report the carbon isotope ratio in nearby molecular clouds LkHα 101, AFGL 490, and Mo... more ABSTRACT We report the carbon isotope ratio in nearby molecular clouds LkHα 101, AFGL 490, and Mon R2 IRS 3. The vibrational transition bands of 12CO ν = 2 ← 0 and 13CO ν = 1 ← 0 were observed with high resolution near-infrared spectroscopy (R = 23,000) to measure the relative abundance of 13CO to 12CO. The isotopic ratios are 12CO/13CO = 137±9 (LkHα 101), 86±49 (AFGL 490), and 158 (Mon R2 IRS 3), which is twice higher than in the solar neighborhood. The molecular clouds are with high visible extinction (AV = 10–70 mag), well shielded from destructive FUV field. It is questionable that the selective photo-destruction of 13CO plays a major role in biasing isotope ratio. Uncertainty in the Doppler parameters of the unresolved absorption lines, and possible emission filling of fundamental transitions are suspected to account for the high 12CO/13CO ratio. Higher resolution spectroscopy (R~100,000) is the key to go for the accurate measurement of isotope ratio.

Astrochemistry of Cosmic Phenomena, 1992

Using high-resolution infrared CO spectroscopy new results have been obtained on the environment ... more Using high-resolution infrared CO spectroscopy new results have been obtained on the environment of high-mass young stellar objects (YSOs). In particular, a new class of neutral, warm and dense outflows has been discovered. The infrared outflows appear a general phenomenon of the activity of high-mass YSOs. With well-defined and often multiple velocities they seem to correspond to episodic and violent mass-loss events from the central source. In addition, a shell of warm, quiescent gas, is formed near the massive star. All these dynamical elements influence the chemistry inside the giant molecular clouds. Beside CO in solid and in gas phase, detection in the infrared has been attempted of simple molecules like CH4, C2H2, H2O and ion H^" toward few of these sources.

Millimeter and Submillimeter Detectors for Astronomy II, 2004

The Astrophysical Journal, 1989

High-resolution M band (4.6 microns) spectroscopy of GL 2591 is presented. Physical structures no... more 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.

The Astrophysical Journal, 2001

Using SCUBA on the James Clerk Maxwell Telescope, we obtained a map of 850 km continuum emission ... more Using SCUBA on the James Clerk Maxwell Telescope, we obtained a map of 850 km continuum emission from the Orion B molecular cloud. The map is 20@ ] 40@ in extent and covers much of the northern half of the giant molecular cloud. A total of 67 discrete continuum sources, or clumps, have been identiÐed, many of which are grouped in three regions, near NGC 2071IR, NGC 2068, and HH 24/25/26. Masses of the sources range from 0.2 to 12 About half of the area of our 850 km map is M _. covered by the current release of the 2MASS infrared survey. Of 40 clumps covered by the 2MASS, 14 have associated infrared sources detected in J, H, and K. Maps of 13CO J \ 2È1 and C18O J \ 2È1 line emission were obtained for two regions in order to Ðnd the gas column density. Formaldehyde spectra were obtained toward eight of the continuum clumps to determine the gas kinetic temperature. Three of the clumps with measured temperature are hot K) while the other Ðve are cold K). (T kin º 80 (T kin ¹ 20 The gas-to-dust ratios di †er substantially between the two regions mapped in CO. In the NGC 2068 region we Ðnd close to constant ratios of dust-togas emission, except in one compact source. However, in the HH 24/25/26 region the dust-togas emission ratio varies substantially with some of the brightest dust continuum sources almost absent in CO emission. One explanation is that CO molecules have frozen onto grains in the dense cores. Why this freeze-out should happen in the HH 24/25/26 cores but not in the NGC 2068 cores remains unexplained. A 12CO J \ 3È2 map of the NGC 2068 region shows patches of high-velocity gas associated with Ðve of the compact continuum sources. The presence of out-Ñows provides strong evidence that the group of sources south of NGC 2068 is actively forming stars.

The Astrophysical Journal, 1995

ABSTRACT We analyze spectra of the J = 4-3 and J = 3-2 transitions of HCO(+) and H(13)CO(+) in NG... more ABSTRACT We analyze spectra of the J = 4-3 and J = 3-2 transitions of HCO(+) and H(13)CO(+) in NGC 7358 IRS 9 and GL 490. The spectra show a narrow core and broad wings. We identify the narrow component with quiescent gas and the wings with the outflowing gas seen in CO in these sources. In both sources, the molecular hydrogen density for the narrow components is 106 to 108/cm. The gas density for the wing component is less well constrained but is at least about 1 x 105/cm. The abundance of HCO(+) relative to CO is estimated forthe wing and narrow components of the HCO(+) spectra in the two sources. The HCO(+) to CO ratio in the quiescent gas in the two sources is almost normal about 1 x 10-4, provided the gas is warm (about 30 K), but can be 50-100 times enhenced if the HCO(+) emission originates from hot gas (100-200 K). The estimates for the HCO(+) to CO ratio in the outflowing gas do not contradict normal values of order 1 x 10-4, although the results are dependent on gas temperature and source size. More complete observations and analyses are necessary before a substantially high or low abundance of HCO(+) for molecular outflows can be concluded.

The Astrophysical Journal, 1992

Results are presented of the mapping, at 21-arcsec resolution, of the CO J = 2 - 1 emission from ... more Results are presented of the mapping, at 21-arcsec resolution, of the CO J = 2 - 1 emission from four high-velocity sources. Observations of (C-13) J = 2 - 1 are combined with the (C-12)O data to determine the mass, dynamical times scales, and mass-loss rates of the outflowing gas. A comparison is made between the momentum and mass-loss rates

The Astrophysical Journal, 2006

We present results from a 2300 arcmin 2 survey of the Orion A molecular cloud at 450 and 850 µm u... more We present results from a 2300 arcmin 2 survey of the Orion A molecular cloud at 450 and 850 µm using the Submillimetre Common-User Bolometer Array (SCUBA) on the James Clerk Maxwell Telescope. The region mapped lies directly south of the OMC1 cloud core and includes OMC4, OMC5, HH1/2, HH34, 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 T d ∼ 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 T d ∼ 20 K and a dust emissivity κ 850 = 0.02 cm 2 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 H 2 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 H 2 shock emission.

The Astrophysical Journal, 1991

ABSTRACT

The Astrophysical Journal, 1988

... FROM MSE-IR GEORGE F. MITCHELL' Saint Mary's University, Halifax MARK ALLEN,' REI... more ... FROM MSE-IR GEORGE F. MITCHELL' Saint Mary's University, Halifax MARK ALLEN,' REINHARD BEER,' RICHARD DEKANY, AND WESLEY HUNTRESS' Jet ... developed for the variation of dust temperature with distance from a star (eg, Kwok, Bell, and Feldman 1981 ; Jiang ...

The Astrophysical Journal, 1998

We present observations of molecular gas made with the 15-m James Clark Maxwell Telescope toward ... more We present observations of molecular gas made with the 15-m James Clark Maxwell Telescope toward the sites of OH(1720 MHz) masers in three supernova remnants: W 28, W 44 and 3C 391. Maps made in the 12 CO J = 3 − 2 line reveal that the OH masers are preferentially located along the edges of thin filaments or clumps of molecular gas. There is a strong correlation between the morphology of the molecular gas and the relativistic gas traced by synchrotron emission at centimeter wavelengths. Broad CO line widths (∆V=30-50 km s −1) are seen along these gaseous ridges, while narrow lines are seen off the ridges. The ratio of H 2 CO line strengths is used to determine temperatures in the broad-line gas of 80 K, and the 13 CO J = 3 − 2 column density suggests densities of 10 4-10 5 cm −3. These observations support the hypothesis that the OH(1720 MHz) masers originate in post-shock gas, heated by the passage of a supernova remnant shock through dense molecular gas. From the observational constraints on the density, velocity and magnetic field we examine the physical properties of the shock and discuss the shock-production of OH. These OH(1720 MHz) masers are useful "signposts", which point to the most promising locations to study supernova remnant/molecular cloud interactions.

ESA Special …, 2005

365 SURVEYS OF THE GALAXY WITH THE JCMT Michel Fich', James Di Francesco2, Jason Fiege3, Rac... more 365 SURVEYS OF THE GALAXY WITH THE JCMT Michel Fich', James Di Francesco2, Jason Fiege3, Rachel Friesen4, Wayne Holland5, Gilles Joncas6, Doug Johnstone24, Helen Kirk4, Brenda Matthews2, Henry Matthews7, George Mitchell8, Rene Plume9, and Christine ...

The Astrophysical Journal, 2001

We present results from a survey of a 1300 arcmin^2 region of the Orion B South molecular cloud, ... more We present results from a survey of a 1300 arcmin^2 region of the Orion B South molecular cloud, including NGC 2024, NGC 2023, and the Horsehead Nebula (B33), obtained using the Submillimetre Common-User Bolometer Array (SCUBA) on the James Clerk Maxwell Telescope. Submillimeter continuum observations at 450 microns and 850 microns are discussed. Using an automated algorithm, 57 discrete emission features (``clumps'') are identified in the 850 micron map. The physical conditions within these clumps are investigated under the assumption that the objects are in quasi-hydrostatic equilibrium. The best fit dust temperature for the clumps is found to be T_d = 18 +/- 4 K, with the exception of those associated with the few known far infrared sources residing in NGC 2024. The latter internally heated sources are found to be much warmer. In the region surrounding NGC 2023, the clump dust temperatures agree with clump gas temperatures determined from molecular line excitation measure...

Journal of the Royal Astronomical Society of Canada, Sep 29, 1984

High Resolution Infrared Spectroscopy in Astronomy, 2006

ABSTRACT We report the carbon isotope ratio in nearby molecular clouds LkHα 101, AFGL 490, and Mo... more ABSTRACT We report the carbon isotope ratio in nearby molecular clouds LkHα 101, AFGL 490, and Mon R2 IRS 3. The vibrational transition bands of 12CO ν = 2 ← 0 and 13CO ν = 1 ← 0 were observed with high resolution near-infrared spectroscopy (R = 23,000) to measure the relative abundance of 13CO to 12CO. The isotopic ratios are 12CO/13CO = 137±9 (LkHα 101), 86±49 (AFGL 490), and 158 (Mon R2 IRS 3), which is twice higher than in the solar neighborhood. The molecular clouds are with high visible extinction (AV = 10–70 mag), well shielded from destructive FUV field. It is questionable that the selective photo-destruction of 13CO plays a major role in biasing isotope ratio. Uncertainty in the Doppler parameters of the unresolved absorption lines, and possible emission filling of fundamental transitions are suspected to account for the high 12CO/13CO ratio. Higher resolution spectroscopy (R~100,000) is the key to go for the accurate measurement of isotope ratio.

Astrochemistry of Cosmic Phenomena, 1992

Using high-resolution infrared CO spectroscopy new results have been obtained on the environment ... more Using high-resolution infrared CO spectroscopy new results have been obtained on the environment of high-mass young stellar objects (YSOs). In particular, a new class of neutral, warm and dense outflows has been discovered. The infrared outflows appear a general phenomenon of the activity of high-mass YSOs. With well-defined and often multiple velocities they seem to correspond to episodic and violent mass-loss events from the central source. In addition, a shell of warm, quiescent gas, is formed near the massive star. All these dynamical elements influence the chemistry inside the giant molecular clouds. Beside CO in solid and in gas phase, detection in the infrared has been attempted of simple molecules like CH4, C2H2, H2O and ion H^" toward few of these sources.

Millimeter and Submillimeter Detectors for Astronomy II, 2004

The Astrophysical Journal, 1989

High-resolution M band (4.6 microns) spectroscopy of GL 2591 is presented. Physical structures no... more 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.

The Astrophysical Journal, 2001

Using SCUBA on the James Clerk Maxwell Telescope, we obtained a map of 850 km continuum emission ... more Using SCUBA on the James Clerk Maxwell Telescope, we obtained a map of 850 km continuum emission from the Orion B molecular cloud. The map is 20@ ] 40@ in extent and covers much of the northern half of the giant molecular cloud. A total of 67 discrete continuum sources, or clumps, have been identiÐed, many of which are grouped in three regions, near NGC 2071IR, NGC 2068, and HH 24/25/26. Masses of the sources range from 0.2 to 12 About half of the area of our 850 km map is M _. covered by the current release of the 2MASS infrared survey. Of 40 clumps covered by the 2MASS, 14 have associated infrared sources detected in J, H, and K. Maps of 13CO J \ 2È1 and C18O J \ 2È1 line emission were obtained for two regions in order to Ðnd the gas column density. Formaldehyde spectra were obtained toward eight of the continuum clumps to determine the gas kinetic temperature. Three of the clumps with measured temperature are hot K) while the other Ðve are cold K). (T kin º 80 (T kin ¹ 20 The gas-to-dust ratios di †er substantially between the two regions mapped in CO. In the NGC 2068 region we Ðnd close to constant ratios of dust-togas emission, except in one compact source. However, in the HH 24/25/26 region the dust-togas emission ratio varies substantially with some of the brightest dust continuum sources almost absent in CO emission. One explanation is that CO molecules have frozen onto grains in the dense cores. Why this freeze-out should happen in the HH 24/25/26 cores but not in the NGC 2068 cores remains unexplained. A 12CO J \ 3È2 map of the NGC 2068 region shows patches of high-velocity gas associated with Ðve of the compact continuum sources. The presence of out-Ñows provides strong evidence that the group of sources south of NGC 2068 is actively forming stars.

The Astrophysical Journal, 1995

ABSTRACT We analyze spectra of the J = 4-3 and J = 3-2 transitions of HCO(+) and H(13)CO(+) in NG... more ABSTRACT We analyze spectra of the J = 4-3 and J = 3-2 transitions of HCO(+) and H(13)CO(+) in NGC 7358 IRS 9 and GL 490. The spectra show a narrow core and broad wings. We identify the narrow component with quiescent gas and the wings with the outflowing gas seen in CO in these sources. In both sources, the molecular hydrogen density for the narrow components is 106 to 108/cm. The gas density for the wing component is less well constrained but is at least about 1 x 105/cm. The abundance of HCO(+) relative to CO is estimated forthe wing and narrow components of the HCO(+) spectra in the two sources. The HCO(+) to CO ratio in the quiescent gas in the two sources is almost normal about 1 x 10-4, provided the gas is warm (about 30 K), but can be 50-100 times enhenced if the HCO(+) emission originates from hot gas (100-200 K). The estimates for the HCO(+) to CO ratio in the outflowing gas do not contradict normal values of order 1 x 10-4, although the results are dependent on gas temperature and source size. More complete observations and analyses are necessary before a substantially high or low abundance of HCO(+) for molecular outflows can be concluded.

The Astrophysical Journal, 1992

Results are presented of the mapping, at 21-arcsec resolution, of the CO J = 2 - 1 emission from ... more Results are presented of the mapping, at 21-arcsec resolution, of the CO J = 2 - 1 emission from four high-velocity sources. Observations of (C-13) J = 2 - 1 are combined with the (C-12)O data to determine the mass, dynamical times scales, and mass-loss rates of the outflowing gas. A comparison is made between the momentum and mass-loss rates

The Astrophysical Journal, 2006

We present results from a 2300 arcmin 2 survey of the Orion A molecular cloud at 450 and 850 µm u... more We present results from a 2300 arcmin 2 survey of the Orion A molecular cloud at 450 and 850 µm using the Submillimetre Common-User Bolometer Array (SCUBA) on the James Clerk Maxwell Telescope. The region mapped lies directly south of the OMC1 cloud core and includes OMC4, OMC5, HH1/2, HH34, 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 T d ∼ 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 T d ∼ 20 K and a dust emissivity κ 850 = 0.02 cm 2 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 H 2 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 H 2 shock emission.

The Astrophysical Journal, 1991

ABSTRACT

The Astrophysical Journal, 1988

... FROM MSE-IR GEORGE F. MITCHELL' Saint Mary's University, Halifax MARK ALLEN,' REI... more ... FROM MSE-IR GEORGE F. MITCHELL' Saint Mary's University, Halifax MARK ALLEN,' REINHARD BEER,' RICHARD DEKANY, AND WESLEY HUNTRESS' Jet ... developed for the variation of dust temperature with distance from a star (eg, Kwok, Bell, and Feldman 1981 ; Jiang ...

The Astrophysical Journal, 1998

We present observations of molecular gas made with the 15-m James Clark Maxwell Telescope toward ... more We present observations of molecular gas made with the 15-m James Clark Maxwell Telescope toward the sites of OH(1720 MHz) masers in three supernova remnants: W 28, W 44 and 3C 391. Maps made in the 12 CO J = 3 − 2 line reveal that the OH masers are preferentially located along the edges of thin filaments or clumps of molecular gas. There is a strong correlation between the morphology of the molecular gas and the relativistic gas traced by synchrotron emission at centimeter wavelengths. Broad CO line widths (∆V=30-50 km s −1) are seen along these gaseous ridges, while narrow lines are seen off the ridges. The ratio of H 2 CO line strengths is used to determine temperatures in the broad-line gas of 80 K, and the 13 CO J = 3 − 2 column density suggests densities of 10 4-10 5 cm −3. These observations support the hypothesis that the OH(1720 MHz) masers originate in post-shock gas, heated by the passage of a supernova remnant shock through dense molecular gas. From the observational constraints on the density, velocity and magnetic field we examine the physical properties of the shock and discuss the shock-production of OH. These OH(1720 MHz) masers are useful "signposts", which point to the most promising locations to study supernova remnant/molecular cloud interactions.

ESA Special …, 2005

365 SURVEYS OF THE GALAXY WITH THE JCMT Michel Fich', James Di Francesco2, Jason Fiege3, Rac... more 365 SURVEYS OF THE GALAXY WITH THE JCMT Michel Fich', James Di Francesco2, Jason Fiege3, Rachel Friesen4, Wayne Holland5, Gilles Joncas6, Doug Johnstone24, Helen Kirk4, Brenda Matthews2, Henry Matthews7, George Mitchell8, Rene Plume9, and Christine ...

The Astrophysical Journal, 2001