Snell, Ronald
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Professor, Department of Astronomy
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Snell
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Ronald
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Astrophysics and Astronomy
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Molecular clouds and star formation
Radio astronomy
Radio astronomy
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Publication Open Access A Survey Of 557 Ghz Water Vapor Emission In The Ngc 1333 Molecular Cloud(2003) Bergin, EA; Kaufman, MJ; Melnick, GJ; Snell, Ronald L.; Howe, JEUsing 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.Publication Metadata only The Organic Chemistry Of Nearby Galaxies Measured With A New, Very Broadband Receiver(2009) Narayanan, G; Snell, Ronald L.; Erickson, NR; Chung, A; Heyer, M; Min, Y; Irvine, WMMillimeter-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).Publication Open Access Embedded Stellar Clusters In The W3/w4/w5 Molecular Cloud Complex(2000) Carpenter, JM; Heyer, MH; Snell, Ronald L.We analyze the embedded stellar content in the vicinity of the W3/W4/W5 H II regions using the FCRAO Outer Galaxy 12CO (1-0) Survey, the IRAS Point Source Catalog, published radio continuum surveys, and new near-infrared and molecular-line observations. Thirty-four IRAS point sources are identified that have far-infrared colors characteristic of embedded star forming regions, and we have obtained K' mosaics and 13CO (1-0) maps for 32 of them. Ten of the IRAS sources are associated with an OB star and 19 with a stellar cluster, although three OB stars are not identified with a cluster. Half of the embedded stellar population identified in the K' images is found in just the five richest clusters, and 61% is contained in IRAS sources associated with an embedded OB star. Thus, rich clusters around OB stars contribute substantially to the stellar population currently forming in the W3/W4/W5 region. Approximately 39% of the cluster population is embedded in small clouds with an average mass of 130 M that are located as far as 100 pc from the W3/W4/W5 cloud complex. We speculate that these small clouds are fragments of a cloud complex dispersed by previous episodes of massive star formation. Finally, we find that four of the five known embedded massive star forming sites in the W3 molecular cloud are found along the interface with the W4 H II region despite the fact that most of the molecular mass is contained in the interior regions of the cloud. These observations are consistent with the classical notion that the W4 H II region has triggered massive star formation along the eastern edge of the W3 molecular cloud.Publication Metadata only An Analysis Of Water Line Profiles In Star Formation Regions Observed By The Submillimeter Wave Astronomy Satellite(2000) Ashby, MLN; Bergin, EA; Plume, R; Carpenter, JM; Neufeld, DA; Chin, G; Erickson, NR; Goldsmith, PF; Harwit, M; Howe, JE; Kleiner, SC; Koch, DG; Patten, BM; Schieder, R; Snell, Ronald L.; Stauffer, JR; Tolls, V; Wang, Z; Winnewisser, G; Zhang, YF; Melnick, GJWe present spectral line profiles for the 557 GHz 110 → 101 ground-state rotational transition of ortho-H216O for 18 Galactic star formation regions observed by the Submillimeter Wave Astronomy Satellite. Water is unambiguously detected in every source. The line profiles exhibit a wide variety of shapes, including single-peaked spectra and self-reversed profiles. We interpret these profiles using a Monte Carlo code to model the radiative transport. The observed variations in the line profiles can be explained by variations in the relative strengths of the bulk flow and small-scale turbulent motions within the clouds. Bulk flow (infall, outflow) must be present in some cloud cores, and in certain cases this bulk flow dominates the turbulent motions.Publication Metadata only Observations Of Water Vapor Toward Orion Bn/kl(2000) Melnick, GJ; Ashby, MLN; Plume, R; Bergin, EA; Neufeld, DA; Chin, G; Erickson, NR; Goldsmith, PE; Harwit, M; Howe, JE; Kleiner, SC; Koch, DG; Patten, BM; Schieder, R; Snell, Ronald L.; Stauffer, JR; Tolls, V; Wang, Z; Winnewisser, C; Zhang, YFWe have obtained spectra of the rotational ground-state 110-101 556.936 GHz ortho-H216O and 110-101 547.676 GHz ortho-H218O transitions toward Orion BN/KL using the Submillimeter Wave Astronomy Satellite (SWAS). The ortho-H216O spectrum shows strong evidence for both a broad (Δv 48 km s-1) and a narrow (Δv 7.5 km s-1) component, while the ortho-H218O shows evidence for only a broad (Δv 24 km s-1) component. The broad component emission in both ortho-H216O and ortho-H218O arises primarily from gas heated within the low- and high-velocity outflows and shocked gas surrounding IRc2 in which the ortho-H216O and ortho-H218O fractional abundances are estimated to be 3.5 × 10-4 and 7 × 10-7, respectively. This finding provides further confirmation that water is efficiently and abundantly produced within warm shock-heated gas. We estimate that the hot core plus the compact ridge contribute 10% to the ortho-H216O integrated intensity within the SWAS beam. The narrow component seen in the ortho-H216O spectrum is best fitted by ortho-water emission from the extended ridge (ER) and the higher temperature core of the extended ridge (CER) with a common fractional abundance of 3.3 × 10-8. The absence of any discernible narrow component in the ortho-H218O spectrum is used to set 3 σ upper limits on the ortho-water fractional abundance within the ER of 7 × 10-8 and within the CER of 5.2 × 10-7. This implies that within the dense extended quiescent region, gas-phase water is neither a major repository of oxygen nor a major coolant in Orion BN/KL.Publication Open Access Submillimeter Wave Astronomy Satellite Observations Of Comet 9p/tempel 1 And Deep Impact(2006) Bensch, F; Melnick, GJ; Neufeld, DA; Harwit, M; Snell, Ronald L.; Patten, BM; Tolls, VOn 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.Publication Metadata only O-2 In Interstellar Molecular Clouds(2000) 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, GWe 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.Publication Metadata only A Study Of The Physics And Chemistry Of Tmc-1(1997) Pratap, P; Dickens, JE; Snell, Ronald L.; Miralles, MP; Bergin, EA; Irvine, William M.; Schloerb, FPWe present a comprehensive study of the physical and chemical conditions along the TMC-1 ridge. Temperatures were estimated from observations of CH3CCH, NH3, and CO. Densities were obtained from a multitransition study of HC3N. The values of the density and temperature allow column densities for 13 molecular species to be estimated from statistical equilibrium calculations, using observations of rarer isotopomers where possible, to minimize opacity effects. The most striking abundance variations relative to HCO+ along the ridge were seen for HC3N, CH3CCH, and SO, while smaller variations were seen in CS, C2H, and HCN. On the other hand, the NH3, HNC, and N2H+ abundances relative to HCO+ were determined to be constant, indicating that the so-called NH3 peak in TMC-1 is probably a peak in the ammonia column density rather than a relative abundance peak. In contrast, the well-studied cyanopolyyne peak is most likely due to an enhancement in the abundance of long-chain carbon species. Comparisons of the derived abundances to the results of time-dependent chemical models show good overall agreement for chemical timescales around 105 yr. We find that the observed abundance gradients can be explained either by a small variation in the chemical timescale from 1.2 × 105 to 1.8 × 105 yr or by a factor of 2 change in the density along the ridge. Alternatively, a variation in the C/O ratio from 0.4 to 0.5 along the ridge produces an abundance gradient similar to that observed.Publication Metadata only Chemical And Physical Gradients Along The Omc-1 Ridge(1997) Ungerechts, H; Bergin, EA; Goldsmith, PF; Irvine, William M.; Schloerb, FP; Snell, Ronald L.We present a survey of the distribution of 20 chemical and isotopic molecular species along the central ridge of the Orion molecular cloud from 6' north to 6' south of BN-KL observed with the QUARRY focal plane array on the FCRAO 14 m telescope, which provides an angular resolution of 50'' in the 3 mm wavelength region. We use standard tools of multivariate analysis for a systematic investigation of the similarities and differences among the maps of integrated intensities of the 32 lines observed. The maps fall in three broad classes: first, those strongly peaked toward BN-KL; second, those having rather flat distributions along the ridge; and third, those with a clear north-south gradient or contrast. We identify six positions or regions where we calculate relative abundances. Line velocities and line widths indicate that the optically thin lines generally trace the same volume of dense gas, except in the molecular bar, where C18O, C34S, H13CO+, CN, C2H, SO, and C3H2 have velocities characteristic of the bar itself, whereas the emission from other detected species is dominated by the background cloud. The strongest abundance variations in our data are the well-known enhancements seen in HCN, CH3OH, HC3N, and SO toward BN-KL and, less strongly, toward the Orion-South outflow 13S. The principal result of this study is that along the extended quiescent ridge the chemical abundances, within factors of 3-4, exhibit an impressive degree of uniformity. The northern part of the ridge has a chemistry closest to that found in quiescent dense clouds. While temperature and density are similar around the northern radical-ion peak near 35N and in the southern core near 42S, some abundances, in particular, those of the ions HCO+ and N2H+, are significantly lower toward 42S. The areas near 42S and the molecular bar itself around (17E, 24S) stand out with peculiar and similar properties—probably caused by stronger UV fields penetrating deeper into the clumpy molecular gas. This leads to higher electron abundances and thereby reduced abundances of the ions, as well as a lack of complex molecules.Publication Open Access Distribution Ofwater Vapor In Molecular Clouds(2011) Melrick, G; Tolls, V; Snell, Ronald L.; Bergin, E; Hollenbach, D; Kaufman, M; Li, D; Neufeld, DWe report the results of a large-area study of water vapor along the Orion Molecular Cloud ridge, the purpose of which was to determine the depth-dependent distribution of gas-phase water in dense molecular clouds. We find that the water vapor measured toward 77 spatial positions along the face-on Orion ridge, excluding positions surrounding the outflow associated with BN/KL and IRc2, display integrated intensities that correlate strongly with known cloud surface tracers such as CN, C2H, 13CO J = 5-4, and HCN, and less well with the volume tracer N2H+. Moreover, at total column densities corresponding to A V< 15 mag, the ratio of H2O to C18O integrated intensities shows a clear rise approaching the cloud surface. We show that this behavior cannot be accounted for by either optical depth or excitation effects, but suggests that gas-phase water abundances fall at large A V. These results are important as they affect measures of the true water-vapor abundance in molecular clouds by highlighting the limitations of comparing measured water-vapor column densities with such traditional cloud tracers as 13CO or C18O. These results also support cloud models that incorporate freeze out of molecules as a critical component in determining the depth-dependent abundance of water vapor.