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

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, 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.
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, 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.
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Observations Of Water Vapor Toward Orion Bn/kl
(2000) Melnick, GJ; Ashby, MLN; Plume, R; Bergin, EA; 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, C; Zhang, YF
We 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.
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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, GJ
We 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.
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.
Open Access
Spitzer Spectral Line Mapping Of Supernova Remnants. I. Basic Data And Principal Component Analysis
(2007) Neufeld, DA; Hollenbach, DJ; Kaufman, MJ; Snell, Ronald L.; Melnick, GJ; Bergin, EA; Sonnentrucker, P
We report the results of spectroscopic mapping observations carried out toward small (1' × 1') regions within the supernova remnants W44, W28, IC 443, and 3C 391 using the Infrared Spectrograph (IRS) of the Spitzer Space Telescope. These observations, covering the 5.2-37 μm spectral region, have led to the detection of a total of 15 fine-structure transitions of Ne+, Ne++, Si+, P+, S, S++, Cl+, Fe+, and Fe++; the S(0)-S(7) pure rotational lines of molecular hydrogen; and the R(3) and R(4) transitions of hydrogen deuteride. In addition to these 25 spectral lines, the 6.2, 7.7, 8.6, 11.3, and 12.6 μm PAH emission bands were also observed. Most of the detected line transitions have proven strong enough to map in several sources, providing a comprehensive picture of the relative distribution of the various line emissions observable in the Spitzer IRS bandpass. A principal component analysis of the spectral-line maps reveals that the observed emission lines fall into five distinct groups, each of which may exhibit a distinct spatial distribution: (1) lines of S and H2(J > 2); (2) the H2 S(0) line; (3) lines of ions with appearance potentials less than 13.6 eV; (4) lines of ions with appearance potentials greater than 13.6 eV, not including S++; (5) lines of S++. Lines of group 1 likely originate in molecular material subject to a slow, nondissociative shock that is driven by the overpressure within the supernova remnant, and lines in groups 3-5 are associated primarily with dissociative shock fronts with a range of (larger) shock velocities. The H2 S(0) line shows a low-density diffuse emission component and, in some sources, a shock-excited component.
Open Access
Swas Observations Of Water In Molecular Outflows
(2008) Franklin, J; Snell, Ronald L.; Kaufman, MJ; Melnick, GJ; Neufeld, DA; Hollenbach, DJ; Bergin, EA
We present detections of the ground-state 110→ 101 transition of ortho-H2O at 557 GHz in 18 molecular outflows based on data from the Submillimeter Wave Astronomy Satellite (SWAS). These results are combined with ground-based observations of the J = 1–0 transitions of 12CO and 13CO obtained at the Five College Radio Astronomy Observatory (FCRAO). Data from Infrared Space Observatory (ISO) for a subset of the outflows are also discussed. Assuming that the SWAS water-line emission originates from the same gas traced by CO emission, we find that the outflowing gas in most outflows has an ortho-H2O abundance relative to H2 of between ~10−7 and 10−6. Analysis of the water abundance as a function of outflow velocity reveals a strong dependence. The abundance of ortho-H2O increases with velocity, and at the highest outflow velocities some of the outflows have relative ortho-H2O abundances of order 10−4. However, the mass of very high velocity gas with such elevated H2O abundances represents less than 1% of the total outflow gas mass. The ISO LWS observations of high-J rotational lines of CO and the 179.5 μm transition of ortho-H2O provide evidence for a warmer outflow component than required to produce either the SWAS or FCRAO lines. The ISO line-flux ratios can be reproduced with C-shock models with shock velocities of order 25 km s−1 and preshock densities of order 105 cm−3; these C-shocks have postshock relative water abundances greater than 10−4. The mass associated with the ISO emission is also quite small compared with the total outflow mass and is similar to that responsible for the highest velocity water emission detected by SWAS. Although the gas responsible for the ISO emission has elevated levels of water, the bulk of the outflowing gas has an abundance of ortho-H2O well below what would be expected if the gas has passed through a C-shock with shock velocities greater than 10 km s−1. Gas-phase water can be depleted in the postshock gas due to freezeout onto grain mantles; however, the rate of freezeout is too slow to explain our results. Therefore, we believe that only a small fraction of the outflowing molecular gas has passed through shocks strong enough to fully convert the gas-phase oxygen to water. This result has implications for the acceleration mechanism of the molecular gas in these outflows.
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A Survey Of The Chemical Properties Of The M17 And Cepheus A Cloud Cores
(1997) Bergin, EA; Ungerechts, H; Goldsmith, PF; Snell, Ronald L.; Irvine, William M.; Schloerb, FP
We present the results of a systematic survey of the chemical properties of two giant molecular cloud (GMC) cores in M17 and Cepheus A. In all, we have mapped the emission from 32 molecular transitions of 13 molecules and seven isotopic variants over a 4' × 5' region in each core. Each map includes known sites of massive star formation, as well as the more extended quiescent material. In M17 most molecules have emission peaks away from the H II region/molecular cloud interface, while two species, HC3N and CH3C2H, deviate from this structure with sharp maxima closer to this interface. In Cepheus A the core is influenced by a compact high-velocity molecular outflow and a more extended low-velocity flow. The molecular emission distributions in this source are generally quite similar, with most molecules peaking near the center of the core to the east of the compact H II region HW 2. A few molecules, SO, CH3OH, H13CN, and C18O, have more extended emission. Only two molecules, CO and HCO+, appear to trace the high- and low-velocity outflows; all other species are tracing the quiescent core. We have used the results of previous studies of the density and temperature of the dense gas in the same cloud cores to derive accurate abundances relative to CO for several positions in each core. The principal result is that the chemical composition of all the cores we have surveyed (which include OMC-1 as well as M17 and Cepheus A) show remarkable similarity, both within a given core and among the cores. This suggests that the chemical processes are similar in quiescent GMC core material. In M17 the lack of variation of molecular abundances is remarkable because the radiation field and the gas temperature are known to vary appreciably throughout the surveyed region, suggesting that the bulk of the emission arises from gas that is well shielded from radiation.
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The Submillimeter Wave Astronomy Satellite: Science Objectives And Instrument Description
(2000) Melnick, GJ; Stauffer, JR; Ashby, MLN; Bergin, EA; Chin, G; Erickson, NR; Goldsmith, PF; Harwit, M; Howe, JE; Kleiner, SC; Koch, DG; Neufeld, DA; Patten, BM; Plume, R; Schieder, R; Snell, Ronald L.; Tolls, V; Wang, Z; Winnewisser, G; Zhang, YF
The Submillimeter Wave Astronomy Satellite (SWAS), launched in 1998 December, is a NASA mission dedicated to the study of star formation through direct measurements of (1) molecular cloud composition and chemistry, (2) the cooling mechanisms that facilitate cloud collapse, and (3) the large-scale structure of the UV-illuminated cloud surfaces. To achieve these goals, SWAS is conducting pointed observations of dense [n(H2) > 103 cm-3] molecular clouds throughout our Galaxy in either the ground state or a low-lying transition of five astrophysically important species: H2O, H218O, O2, C I, and 13CO. By observing these lines SWAS is (1) testing long-standing theories that predict that these species are the dominant coolants of molecular clouds during the early stages of their collapse to form stars and planets and (2) supplying previously missing information about the abundance of key species central to the chemical models of dense interstellar gas. SWAS carries two independent Schottky barrier diode mixers—passively cooled to ~175 K—coupled to a 54 × 68 cm off-axis Cassegrain antenna with an aggregate surface error ~11 μm rms. During its baseline 3 yr mission, SWAS is observing giant and dark cloud cores with the goal of detecting or setting an upper limit on the water and molecular oxygen abundance of 3 × 10-6 (relative to H2). In addition, advantage is being taken of SWAS's relatively large beam size of 33 × 45 at 553 GHz and 35 × 50 at 490 GHz to obtain large-area (~1° × 1°) maps of giant and dark clouds in the 13CO and C I lines. With the use of a 1.4 GHz bandwidth acousto-optical spectrometer, SWAS has the ability to simultaneously observe either the H2O, O2, C I, and 13CO lines or the H218O, O2, and C I lines. All measurements are being conducted with a velocity resolution less than 1 km s-1.
Open Access
Extended [c I] And (co)-c-13 (5 -> 4) Emission In M17sw
(2000) Howe, JE; Ashby, MLN; Bergin, EA; Chin, G; Erickson, NR; Goldsmith, PF; Harwit, M; Hollenbach, DJ; Kaufman, MJ; Kleiner, SC; Koch, DG; Neufeld, DA; Patten, BM; Plume, R; Schieder, R; Snell, Ronald L.; Stauffer, JR; Tolls, V; Wang, Z; Winnewisser, G; Zhang, YF; Melnick, GJ
We mapped a 13 × 22 pc region in emission from 492 GHz [C I] and, for the first time, 551 GHz 13CO (5 → 4) in the giant molecular cloud M17SW. The morphologies of the [C I] and 13CO emission are strikingly similar. The extent and intensity of the [C I] and 13CO (5 → 4) emission is explained as arising from photodissociation regions on the surfaces of embedded molecular clumps. Modeling of the 13CO (5 → 4) emission in comparison to 13CO (1 → 0) indicates a temperature gradient across the cloud, peaking to at least 63 K near the M17 ionization front and decreasing to at least 20 K at the western edge of the cloud. We see no correlation between gas density and column density. The beam-averaged column density of C I in the core is 1 × 1018 cm-2, and the mean column density ratio N(C I)/N(CO) is about 0.4. The variations of N(C I)/N(CO) with position in M17SW indicate a similar clump size distribution throughout the cloud.