Snell, Ronald

Job Title

Professor, Department of Astronomy

Last Name

Snell

First Name

Ronald

Discipline

Astrophysics and Astronomy

Expertise

Molecular clouds and star formation
Radio astronomy

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Now showing 1 - 10 of 36

Submillimeter Wave Astronomy Satellite observations of Comet 9P/Tempel 1 and Deep Impact
(2006-01) Bensch, F; Melnick, GJ; Neufeld, DA; Harwit, M; Snell, Ronald L.; Patten, BM; Tolls, V
On 4 July 2005 at 5:52 UT the Deep Impact mission successfully completed its goal to hit the nucleus of 9P/Tempel 1 with an impactor, forming a crater on the nucleus and ejecting material into the coma of the comet. NASA's Submillimeter Wave Astronomy Satellite (SWAS) observed the 110–101 ortho-water ground-state rotational transition in Comet 9P/Tempel 1 before, during, and after the impact. No excess emission from the impact was detected by SWAS and we derive an upper limit of 1.8×107 kg on the water ice evaporated by the impact. However, the water production rate of the comet showed large natural variations of more than a factor of three during the weeks before and after the impact. Episodes of increased activity with alternated with periods with low outgassing (). We estimate that 9P/Tempel 1 vaporized a total of N4.5×1034 water molecules (1.3×109 kg) during June–September 2005. Our observations indicate that only a small fraction of the nucleus of Tempel 1 appears to be covered with active areas. Water vapor is expected to emanate predominantly from topographic features periodically facing the Sun as the comet rotates. We calculate that appreciable asymmetries of these features could lead to a spin-down or spin-up of the nucleus at observable rates.
The Organic Chemistry of Nearby Galaxies Measured with a New, Very Broadband Receiver
(2009-01) Narayanan, G; Snell, Ronald L.; Erickson, NR; Chung, A; Heyer, M; Min, Y; Irvine, WM
Millimeter-wavelength spectra of a number of nearby galaxies have been obtained at the Five College Radio Astronomy Observatory (FCRAO) in Massachusetts using a new, very broadband receiver (Erickson et al., 2007). This instrument, which we call the redshift search receiver (RSR), has an instantaneous bandwidth of 36 GHz and operates from 74 to 110.5 GHz, permitting the measurement of most of the 3 mm spectrum with a single receiver setting. The receiver has been built at UMass/FCRAO to be part of the initial instrumentation for the Large Millimeter Telescope (LMT), a 50-m diameter millimeterwavelength single-dish telescope being built jointly by UMass and the Instituto Nacional de Astrofísica, Óptica y Electrónica in Mexico (Perez-Grovas et al., 2006). The LMT is sited at 4,600 m elevation at latitude 19° in the Mexican state of Puebla, permitting good access to the southern sky. It is designed for operation in the 0.85–4 mm wavelength band. The new receiver is intended for determination of the redshift and hence distance of distant, dust-obscured galaxies, but it can also be used to investigate the chemistry of galaxies. Since the LMT is not yet complete (we are hoping for initial 3 mm commissioning this year), the receiver is being tested on the FCRAO 14 m by measuring the 3 mm spectra of a number of nearby galaxies. There are interesting differences in the chemistry of these objects, e.g., in the relative strength of emission lines from HCN, HNC, HCO+, CH3OH, 13CO, CS and N2H+ (a proxy for N2).
Water abundance in molecular cloud cores
(2000-01) Snell, Ronald L.; Howe, JE; Ashby, MLN; Bergin, EA; Chin, G; Erickson, NR; Goldsmith, PF; Harwit, M; Kleiner, SC; Koch, DG; Neufeld, DA; Patten, BM; Plume, R; Schieder, R; Stauffer, JR; Tolls, V; Wang, Z; Winnewisser, G; Zhang, YF; Melnick, GJ
We present Submillimeter Wave Astronomy Satellite (SWAS) observations of the 110 → 101 transition of ortho-H2O at 557 GHz toward 12 molecular cloud cores. The water emission was detected in NGC 7538, ρ Oph A, NGC 2024, CRL 2591, W3, W3OH, Mon R2, and W33 and was not detected in TMC-1, L134N, and B335. We also present a small map of the H2O emission in S140. Observations of the H218O line were obtained toward S140 and NGC 7538, but no emission was detected. The abundance of ortho-H2O relative to H2 in the giant molecular cloud cores was found to vary between 6 × 10-10 and 1 × 10-8. Five of the cloud cores in our sample have previous H2O detections; however, in all cases the emission is thought to arise from hot cores with small angular extents. The H2O abundance estimated for the hot core gas is at least 100 times larger than in the gas probed by SWAS. The most stringent upper limit on the ortho-H2O abundance in dark clouds is provided in TMC-1, where the 3 σ upper limit on the ortho-H2O fractional abundance is 7 × 10-8.
O-2 in interstellar molecular clouds
(2000-01) Goldsmith, PF; Melnick, GJ; Bergin, EA; Howe, JE; Snell, Ronald L.; Neufeld, DA; Harwit, M; Ashby, MLN; Patten, BM; Kleiner, SC; Plume, R; Stauffer, JR; Tolls, V; Wang, Z; Zhang, YF; Erickson, NR; Koch, DG; Schieder, R; Winnewisser, G; Chin, G
We have used the Submillimeter Wave Astronomy Satellite (SWAS) to carry out deep integrations on the NJ = 33 → 12 transition of O2 in a variety of Galactic molecular clouds. We here report no convincing detection in an initial set of observations of 20 sources. We compare O2 integrated intensities with those of C18O in a similarly sized beam and obtain 3 σ upper limits for the O2/C18O abundance ratio ≤ 2.3 in four clouds and ≤ 3.6 in five additional clouds. Our lowest individual limit corresponds to N(O2)/N(H2) < 2.6 × 10-7 (3 σ). A combination of data from nine sources yields N(O2)/N(H2) = [0.33 ± 1.6 (3 σ)] × 10-7. These low limits, characterizing a variety of clouds in different environments at different Galactocentric radii, indicate that O2 is not a major constituent of molecular clouds and is not an important coolant. The abundance of O2 is significantly lower than predicted by steady state single-component chemical models. The present results are best understood in the context of cloud chemical and dynamical models that include the interaction of gas-phase molecules and grain surfaces and/or circulation of material between well-shielded and essentially unshielded regions. This circulation may be powered by turbulence or other driving forces that effectively keep molecular clouds chemically unevolved.
Spitzer observations of hydrogen deuteride
(2006-01) Neufeld, DA; Green, JD; Hollenbach, DJ; Sonnentrucker, P; Melnick, GJ; Bergin, EA; Snell, Ronald L.; Forrest, WJ; Watson, DM; Kaufman, MJ
The distribution of water emission in M17SW
(2000-01) Snell, Ronald L.; Howe, JE; Ashby, MLN; Bergin, EA; Chin, G; Erickson, NR; Goldsmith, PF; Harwit, M; Kleiner, SC; Koch, DG; Neufeld, DA; Patten, BM; Plume, R; Schieder, R; Stauffer, JR; Tolls, V; Wang, Z; Winnewisser, G; Zhang, YF; Melnick, GJ
We present a 17-point map of the M17SW cloud core in the 110 → 101 transition of ortho-H2O at 557 GHz obtained with Submillimeter Wave Astronomy Satellite. Water emission was detected in 11 of the 17 observed positions. The line widths of the H2O emission vary between 4 and 9 km s-1 and are similar to other emission lines that arise in the M17SW core. A direct comparison is made between the spatial extent of the H2O emission and the 13CO J = 5 → 4 emission; the good agreement suggests that the H2O emission arises in the same warm, dense gas as the 13CO emission. A spectrum of the H218O line was also obtained at the center position of the cloud core, but no emission was detected. We estimate that the average abundance of ortho-H2O relative to H2 within the M17 dense core is approximately 1 × 10-9, 30 times smaller than the average for the Orion core. Toward the H II region/molecular cloud interface in M17SW the ortho-H2O abundance may be about 5 times larger than in the dense core.
THE REDSHIFT SEARCH RECEIVER 3 MM WAVELENGTH SPECTRA OF 10 GALAXIES
(2011-01) Snell, Ronald L.; Narayanan, Gopal; Yun, Min; Heyer, M; Chung, A; Irvine, William M.; Erickson, NR; Liu, G
The 3 mm wavelength spectra of 10 galaxies have been obtained at the Five College Radio Astronomy Observatory using a new, very broadband receiver and spectrometer, called the Redshift Search Receiver (RSR). The RSR has an instantaneous bandwidth of 37 GHz covering frequencies from 74 to 111 GHz and has a spectral resolution of 31 MHz (~100 km s–1). During tests of the RSR on the FCRAO 14 m telescope the complete 3 mm spectra of the central regions of NGC 253, Maffei 2, NGC1068, IC 342, M82, NGC 3079, NGC 3690, NGC 4258, Arp 220, and NGC 6240 were obtained. Within the wavelength band covered by the RSR, 20 spectral lines from 14 different atomic and molecular species were detected. Based on simultaneous fits to the spectrum of each galaxy, a number of key molecular line ratios are derived. A simple model which assumes the emission arises from an ensemble of Milky Way like Giant Molecular Cloud cores can adequately fit the observed line ratios using molecular abundances based on Galactic molecular cloud cores. Variations seen in some line ratios, such as 13CO/HCN and HCO+/HCN, can be explained if the mean density of the molecular gas varies from galaxy to galaxy. However, NGC 3690, NGC 4258, and NGC 6240 show very large HCO+/HCN ratios and require significant abundance enhancement of HCO+ over HCN, possible due to the proximity to active galactic nucleus activity. Finally, the mass of dense molecular gas is estimated and we infer that 25%-85% of the total molecular gas in the central regions of these galaxies must have densities greater than 104 cm–3.
Discovery of a Molecular Outflow in the Haro 6-10 Star-forming Region
(2008-01) Stojimirović, Irena; Narayanan, Gopal; Snell, Ronald L.
We present high sensitivity 12CO and 13CO J=1!0 molecular line maps covering the full extent of the parsec scale Haro 6-10 Herbig-Haro (HH) flow. We report the discovery of a molecular CO outflow along the axis of parsec-scale HH flow. Previous molecular studies missed the identification of the outflow probably due to their smaller mapping area and the confusing spectral features present towards the object. Our detailed molecular line study of the full 1.6 pc extent of the optical flow shows evidence for both blueshifted and redshifted gas set in motion by Haro 6-10 activity. The molecular outflow is centered at Haro 6-10, with redshifted gas being clumpy and directed towards the northeast, while blueshifted gas is in the southwest direction. The molecular gas terminates well within the cloud, short of the most distant HH objects of the optical flow. Contamination from an unrelated cloud along the same line of sight prevents a thorough study of the blueshifted outflow lobe and the mass distribution at the lowest velocities in both lobes. The cloud core in which Haro 6-10 is embedded is filamentary and flattened in the east-west direction. The total cloud mass is calculated from 13CO J=1--0 to be ~ 200 M⊙. The lower limit of the mass associated with the outflow is ~0.25 M⊙.
Entrainment Mechanisms for Outflows in the L1551 Star-Forming Region
(2006-01) Stojimirovív, Irena; Narayanan, Gopal; Snell, Ronald L.; Bally, John
We present high sensitivity 12/13CO(1-0) molecular line maps covering the full extent of the parsec scale L1551 molecular outflow, including the redshifted east-west (EW) flow. We also present 12CO(3-2) data that extends over a good fraction of the area mapped in the 1-0 transition. We compare the molecular data to widefield, narrow-band optical emission in Hα. While there are multiple outflows in the L1551 cloud, the main outflow is oriented at 50\arcdeg position angle and appears to be driven by embedded source(s) in the central IRS 5 region. The 3-2 data indicate that there may be molecular emission associated with the L1551 NE jet, within the redshifted lobe of main outflow. We have also better defined the previously known EW flow and believe we have identified its blueshifted counterpart. We further speculate that the origin of the EW outflow lies near HH 102. We use velocity dependent opacity correction to estimate the mass and the energy of the outflow. The resulting mass spectral indices from our analysis, are systematically lower (less steep) than the power law indices obtained towards other outflows in several recent studies that use a similar opacity correction method. We show that systematic errors and biases in the analysis procedures for deriving mass spectra could result in errors in the determination of the power-law indices. The mass spectral indices, the morphological appearance of the position-velocity plots and integrated intensity emission maps of the molecular data, compared with the optical, suggest that jet-driven bow-shock entrainment is the best explanation for the driving mechanism of outflows in L1551. The kinetic energy of the outflows is found to be comparable to the binding energy of the cloud and sufficient to maintain the turbulence in the L1551 cloud.
A survey of 557 GHz water vapor emission in the NGC 1333 molecular cloud
(2003-01) Bergin, EA; Kaufman, MJ; Melnick, GJ; Snell, Ronald L.; Howe, JE
Using NASA's Submillimeter Wave Astronomy Satellite (SWAS), we have examined the production of water in quiescent and shocked molecular gas through a survey of the 556.936 GHz 110-101 transition of ortho-H2O in the NGC 1333 molecular core. These observations reveal broad emission lines associated with the IRAS 2, IRAS 4, IRAS 7, and HH 7-11 outflows. Toward three positions we detect narrow (Δv ~ 2-3 km s-1) emission lines clearly associated with the ambient gas. The SWAS observations, with a resolution of ~4', are supplemented with observations from the Infrared Space Observatory (ISO) and by an unbiased survey of a ~17' × 15' area, with ~50'' resolution, in the low-J transitions of CO, 13CO, C18O, N2H+, CH3OH, and SiO. Using these combined data sets, with consistent assumptions, we find beam-averaged ortho-H2O abundances of greater than 10-6 relative to H2 for all four outflows. A comparison of SWAS and ISO water data is consistent with nondissociative shock models, provided the majority of the ortho-H2O (110-101) emission arises from cool postshock material with enhanced abundances. In the ambient gas the ortho-H2O abundance is found to lie between 0.1 × 10-7 and 1 × 10-7 relative to H2 and is enhanced when compared to cold prestellar molecular cores. A comparison of the water emission with tracers of dense condensations and shock chemistry finds no clear correlation. However, the water emission appears to be associated with the presence of luminous external heating sources that power the reflection nebula and the photodissociation region (PDR). Simple PDR models are capable of reproducing the water and high-J 13CO emission, suggesting that a PDR may account for the excitation of water in low-density undepleted gas, as suggested by Spaans & van Dishoeck.