Publications (88)326.88 Total impact
-
Article: Extended [C I] and 13CO (5 → 4) Emission in M17SW
[show abstract] [hide abstract]
ABSTRACT: 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.The Astrophysical Journal 12/2008; 539(2):L137. · 6.02 Impact Factor -
Article: An Analysis of Water Line Profiles in Star Formation Regions Observed by the Submillimeter Wave Astronomy Satellite
[show abstract] [hide abstract]
ABSTRACT: 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.The Astrophysical Journal 12/2008; 539(2):L115. · 6.02 Impact Factor -
Article: Observations of Interstellar Water Vapor in Outflow Regions
[show abstract] [hide abstract]
ABSTRACT: We have observed the 110-101 transition of interstellar water vapor near 557 GHz toward the three outflow sources NGC 2071, L1157, and NGC 1333 IRAS 4 using the Submillimeter Wave Astronomy Satellite. In each case, the observations reveal broad emission from water in the outflow accompanied by a narrow absorption feature at the velocity of the quiescent gas. Our estimates of the average ortho-water abundance within the outflowing gas lie in the range (0.5-1.6) × 10-6 relative to H2.The Astrophysical Journal 12/2008; 539(2):L107. · 6.02 Impact Factor -
Article: Large-scale 13CO J = 5 → 4 and [C I] Mapping of Orion A
[show abstract] [hide abstract]
ABSTRACT: We present maps of the 13CO J = 5 → 4 (551 GHz) and [C I] 3P1 → 3P0 (492 GHz) emission in the Orion A molecular cloud, covering a 05 × 2° area. A large velocity gradient (LVG) analysis of 13CO J = 5 → 4 and J = 1 → 0 suggests that the gas temperatures in the northern part of OMC-1 (north of Δδ = -20') are ~45 K and are, on average, at least 20 K higher than those to the south. The average 13CO column density is log(N/cm-2) = 16.4 ± 0.3 and is fairly constant throughout the cloud, even in the low-temperature region south of BN/KL. LVG modeling of the [C I] emission shows a typical C0 column density of 2 × 1017 cm-2, which yields a C/CO abundance ratio in the cloud of ~0.1 (rising to levels in excess of 0.5 at the cloud edges). Comparison of the Δ-variance (which measures spatial structure in a manner similar to a power spectrum) of the Submillimeter Wave Astronomy Satellite [C I], Five College Radio Astronomy Observatory 13CO J = 1 → 0, and CS J = 1 → 0 velocity-integrated maps suggests that the [C I] and 13CO emission arise from the same gas. In contrast, the CS emission likely originates in gas that is considerably more clumpy.The Astrophysical Journal 12/2008; 539(2):L133. · 6.02 Impact Factor -
Article: Observations of Absorption by Water Vapor toward Sagittarius B2
[show abstract] [hide abstract]
ABSTRACT: We have observed the 110-101 pure rotational transitions of both H216O and H218O toward Sagittarius B2 using the Submillimeter Wave Astronomy Satellite. The spectra thereby obtained show a complex pattern of absorption and—in the case of H216O—emission, with numerous features covering a wide range of LSR velocities (-130 to 130 km s-1) and representing absorption both in gas associated with Sgr B2 as well as by several components of foreground gas along the line of sight. The ortho-water abundance derived for the absorbing foreground gas is ~6 × 10-7 relative to H2.The Astrophysical Journal 12/2008; 539(2):L111. · 6.02 Impact Factor -
Article: Submillimeter Wave Astronomy Satellite Observations of Extended Water Emission in Orion
[show abstract] [hide abstract]
ABSTRACT: We have used the Submillimeter Wave Astronomy Satellite to map the ground-state 110 → 101 transition of ortho-H2O at 557 GHz in the Orion molecular cloud. H2O emission was detected in Orion over an angular extent of about 20', or nearly 3 pc. The water emission is relatively weak, with line widths (3-6 km s-1) and VLSR velocities (9-11 km s-1) consistent with an origin in the cold gas of the molecular ridge. We find that the ortho-H2O abundance relative to H2 in the extended gas in Orion varies between 1 and 8 × 10-8, with an average of 3 × 10-8. The absence of detectable narrow-line ortho-H218O emission is used to set a 3 σ upper limit on the relative ortho-H2O abundance of 7 × 10-8.The Astrophysical Journal 12/2008; 539(2):L93. · 6.02 Impact Factor -
Article: Detection of Water in the Shocked Gas Associated with IC 443: Constraints on Shock Models
[show abstract] [hide abstract]
ABSTRACT: We have used the Submillimeter Wave Astronomy Satellite (SWAS) to observe the ground-state 110 → 101 transition of ortho-H2O at 557 GHz in three of the shocked molecular clumps associated with the supernova remnant IC 443. We also observed simultaneously the 487 GHz line (3,1 → 3,2) of O2, the 492 GHz line (3P1 → 3P0) of C I, and the 550 GHz line (J = 5 → 4) of 13CO. We detected the H2O, C I, and 13CO lines toward the shocked clumps B, C, and G. In addition, ground-based observations of the J = 1 → 0 transitions of CO and HCO+ were obtained. Assuming that the shocked gas has a temperature of 100 K and a density of 5 × 105 cm-3, we derive SWAS beam-averaged ortho-H2O column densities of 3.2 × 1013, 1.8 × 1013, and 3.9 × 1013 cm-2 in clumps B, C, and G, respectively. Combining the SWAS results with our ground-based observations, we derive a relative abundance of ortho-H2O to CO in the postshock gas of between 2 × 10-4 and 3 × 10-3. On the basis of our results for H2O, published results of numerous atomic and molecular shock tracers, and archival Infrared Space Observatory (ISO) observations, we conclude that no single shock type can explain these observations. However, a combination of fast J-type shocks (~100 km s-1) and slow C-type shocks (~12 km s-1) or, more likely, slow J-type shocks (~12-25 km s-1) can most naturally explain the postshock velocities and the emission seen in various atomic and molecular tracers. Such a superposition of shocks might be expected as the supernova remnant overtakes a clumpy interstellar medium. The fast J-type shocks provide a strong source of ultraviolet radiation, which photodissociates the H2O in the cooling (T ≤ 300 K) gas behind the slow shocks and strongly affects the slow C-type shock structure by enhancing the fractional ionization. At these high ionization fractions, C-type shocks break down at speeds ~10-12 km s-1, while faster flows will produce J-type shocks. Our model favors a preshock gas-phase abundance of oxygen not in CO that is depleted by a least a factor of 2, presumably as water ice on grain surfaces. Both freezeout of H2O and photodissociation of H2O in the postshock gas must be significant to explain the weak H2O emission seen by SWAS and ISO from the shocked and postshock gas.The Astrophysical Journal 12/2008; 620(2):758. · 6.02 Impact Factor -
Article: Tentative Detection of Molecular Oxygen in the ρ Ophiuchi Cloud
[show abstract] [hide abstract]
ABSTRACT: We report the tentative detection of molecular oxygen in the interstellar medium. Deep integrations using the Submillimeter Wave Astronomy (SWAS) satellite of the ρ Oph A cloud have resulted in a detection of an emission feature consistent with the NJ = 33 → 12 transition of O2 at a local standard of rest velocity of 6.0 km s-1. The line width and velocity are suggestive of the redshifted wing emission seen in species that trace the molecular outflows seen in this region. The fractional abundance of O2 relative to H2 in this high-velocity gas is approximately 10-5. The fractional abundance of ortho-H2O in this material is 8 × 10-8, somewhat higher than found in quiescent material by earlier SWAS observations but less than that found in younger outflows. We suggest a unified scenario that explains the low O2 abundance in cloud cores previously reported [X(O2) ≤ few × 10-7], along with the much enhanced abundance in the outflow region. In this picture, quiescent clouds are characterized by significant depletion of gas-phase oxygen, which ends up largely as water ice on the dust grains. A shock associated with the outflow results in almost all oxygen being processed into gas-phase water. In the cool postshock gas, the standard gas-phase chemistry reasserts itself, and after an elapsed time of a few times 105 yr, the O2 abundance has increased to the point that this species is one of the main repositories of oxygen atoms. At the same time, the gas-phase water abundance drops drastically, in agreement with the SWAS observations that we report here. On a somewhat longer timescale, depletion again dominates and the gas-phase abundance of O2 drops by several orders of magnitude, with a significant fraction of oxygen remaining in atomic form.The Astrophysical Journal 12/2008; 576(2):814. · 6.02 Impact Factor -
Article: Water Abundance in Molecular Cloud Cores
[show abstract] [hide abstract]
ABSTRACT: 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.The Astrophysical Journal 12/2008; 539(2):L101. · 6.02 Impact Factor -
Article: O2 in Interstellar Molecular Clouds
[show abstract] [hide abstract]
ABSTRACT: 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.The Astrophysical Journal 12/2008; 539(2):L123. · 6.02 Impact Factor -
Article: Submillimeter Wave Astronomy Satellite Observations of Jupiter and Saturn:Detection of 557 GHz Water Emission from the Upper Atmosphere
[show abstract] [hide abstract]
ABSTRACT: We have used the Submillimeter Wave Astronomy Satellite to carry out observations on Jupiter and Saturn in two bands centered at 489 and 553 GHz. We detect spectrally resolved 557 GHz H2O emission on both planets, constraining for the first time the residence levels of external water vapor in Jupiter's and Saturn's stratosphere. For both planets, the line appears to be formed at maximum pressures of about 5 mbar. For Jupiter, the data further show that water is not uniformly mixed but increases with altitude above the condensation level. In each planet, the amount of water implied by the data is 1.5-2.5 times larger than inferred from Infrared Space Observatory data. In addition, our observations provide new whole-disk brightness measurements of Jupiter and Saturn near 489 and 553 GHz.The Astrophysical Journal 12/2008; 539(2):L147. · 6.02 Impact Factor -
Article: Submillimeter Wave Astronomy Satellite Performance on the ground and in orbit
[show abstract] [hide abstract]
ABSTRACT: The Submillimeter Wave Astronomy Satellite (SWAS), which was launched in 1998 December, is a NASA mission dedicated to the study of interstellar chemistry and star formation. SWAS is conducting pointed observations of molecular clouds throughout our Galaxy in either the ground state or a low-lying transition of five astrophysically important species: O2, C I, H218O, 13CO, and H216O at approximately 487, 492, 548, 551, and 557 GHz, respectively. The SWAS instrument is comprised of a 54 cm × 68 cm off-axis Cassegrain telescope feeding two independent heterodyne receivers with Schottky barrier diode mixers, passively cooled to about 175 K. An Acousto-Optical Spectrometer (AOS) provides ~1 MHz (0.6 km s-1) frequency resolution and 1400 MHz (840 km s-1) total bandwidth with 350 MHz (210 km s-1) per line for spectral analysis. SWAS was fully characterized during ground-based testing, and all performance parameters were verified on-orbit. During its on-orbit operation, SWAS observed more than 200 astronomical objects with more than 5000 lines of sight. This paper describes the tests conducted and compares the ground-based test results with the on-orbit test results.The Astrophysical Journal Supplement Series 12/2008; 152(1):137. · 13.46 Impact Factor -
Article: Water Abundance and Velocity Structure in S140, ρ Oph A, and B335
[show abstract] [hide abstract]
ABSTRACT: We present Monte Carlo radiative transfer models for B335, S140, and ρ Ophiuchi A that simulate the emission line arising from the 557 GHz 110 → 101 ground-state transition of ortho-H216O. These models reproduce the line profiles with a combination of turbulent and macroscopic motions, and are consistent with gravitational infall in ρ Oph A. We cannot, however, discriminate infall from outflow for S140. Our models are consistent with ortho-H2O abundances relative to H2 of 2 × 10-8 and 4.5 × 10-9 in S140 and ρ Oph A, respectively, and an upper limit of 4 × 10-6 (3 σ) in B335, assuming the ratio ortho-H2/para-H2 = 0.1. We quantify the impact of dust continuum emission on the derived abundances.The Astrophysical Journal 12/2008; 539(2):L119. · 6.02 Impact Factor -
Article: A Study of the Physics and Chemistry of L134N
[show abstract] [hide abstract]
ABSTRACT: We have carried out a comprehensive and self-consistent study of the physical and chemical state of the core of the dark cloud L134N (L183), whose molecular abundances provide a standard against which chemical models may be compared. We used observations of the NH3(1, 1) and (2, 2) rotation-inversion transitions to estimate the kinetic temperature, which was found to be consistent with 10 K and not varying with position. Densities were determined from multitransition statistical equilibrium calculations for HC3N, N2H+, and CS. The average density toward all lines of sight was 2 × 104 cm-3. As found by previous studies, the emission from various molecular species peaks in different positions: SO and SO2 peak west of the central position, which is the location of the strongest emission from (e.g.) N2H+ and CH3OH, with a second peak occurring for NH3 and HC3N to the north of the center. The most striking abundance variations occur in a north-south cut through the core center for HC3N, C2H, CS, SO, and SO2. A north to south decrease in the abundance of HC3N and CS and a dramatic change in the CS/SO ratio, which has been shown to be a sensitive tracer of chemical evolutionary state, suggests that the north is at a younger evolutionary state than the south. Despite the "youth" of the N position, the CS/SO ratio suggests that it is still as "old" as or older than the most evolved region in TMC-1 (the northwest end of the ridge).The Astrophysical Journal 12/2008; 542(2):870. · 6.02 Impact Factor -
Article: Observations of Water Vapor toward Orion BN/KL
[show abstract] [hide abstract]
ABSTRACT: 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.The Astrophysical Journal 12/2008; 539(2):L87. · 6.02 Impact Factor -
Article: Submillimeter Wave Astronomy Satellite Observations of Water Vapor toward Comet C/1999 H1 (Lee)
[show abstract] [hide abstract]
ABSTRACT: We have detected the 110-101 pure rotational transition of water vapor toward comet C/1999 H1 using the Submillimeter Wave Astronomy Satellite. Over the period 1999 May 19.01-23.69 UT, the average integrated antenna temperature was 1.79 ± 0.03 K km s-1 within a 33 × 45 (FWHM) elliptical beam. For an assumed ortho-to-para ratio of 3, we estimate the total water production rate as 8 × 1028 s-1. This value lies approximately 50% above the value estimated by Biver et al. from contemporaneous radio observations of hydroxyl molecules. The observed line width of 1.8 km s-1 (FWHM) is broader than the instrumental profile and suggests an intrinsic line width of about 1.4 km s-1 (FWHM). The data, taken during a portion of every 97 minute spacecraft orbit over a 4.68 day period, provide no evidence for variability.The Astrophysical Journal 12/2008; 539(2):L151. · 6.02 Impact Factor -
Article: Submillimeter Wave Astronomy Satellite Observations of the Martian Atmosphere: Temperature and Vertical Distribution of Water Vapor
[show abstract] [hide abstract]
ABSTRACT: We report the first detections of absorption features in the submillimeter spectrum of Mars that are due to the H2O (110-101) and 13CO (5-4) rotational transitions. Observations were obtained over several days near the planet's closest approach to Earth in 1999 April. These observations simultaneously provide us with an opportunity to derive the atmospheric temperature structure and to measure directly the distribution of water vapor with altitude. The Martian atmosphere is found to be relatively cool, consistent with results found from ground-based millimeter observations of CO. The distribution of water in the Martian atmosphere matches a profile of constant, 100% saturation from 10 to 45 km altitude.The Astrophysical Journal 12/2008; 539(2):L143. · 6.02 Impact Factor -
Article: The Distribution of Water Emission in M17SW
[show abstract] [hide abstract]
ABSTRACT: 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 Astrophysical Journal 12/2008; 539(2):L97. · 6.02 Impact Factor -
Article: Implications of Submillimeter Wave Astronomy Satellite Observations for Interstellar Chemistry and Star Formation
[show abstract] [hide abstract]
ABSTRACT: A long-standing prediction of steady state gas-phase chemical theory is that H2O and O2 are important reservoirs of elemental oxygen and major coolants of the interstellar medium. Analysis of Submillimeter Wave Astronomy Satellite (SWAS) observations has set sensitive upper limits on the abundance of O2 and has provided H2O abundances toward a variety of star-forming regions. Based on these results, we show that gaseous H2O and O2 are not dominant carriers of elemental oxygen in molecular clouds. Instead, the available oxygen is presumably frozen on dust grains in the form of molecular ices, with a significant portion potentially remaining in atomic form, along with CO, in the gas phase. H2O and O2 are also not significant coolants for quiescent molecular gas. In the case of H2O, a number of known chemical processes can locally elevate its abundance in regions with enhanced temperatures, such as warm regions surrounding young stars or in hot shocked gas. Thus, water can be a locally important coolant. The new information provided by SWAS, when combined with recent results from the Infrared Space Observatory, also provides several hard observational constraints for theoretical models of the chemistry in molecular clouds, and we discuss various models that satisfy these conditions.The Astrophysical Journal 12/2008; 539(2):L129. · 6.02 Impact Factor -
Chapter: Submillimeter Wave Astronomy Satellite Observations of Comet 9P/Tempel 1
[show abstract] [hide abstract]
ABSTRACT: We present observations of the 110-1011_{10}-1_{01} ortho-water ground-state rotational transition in comet 9P/Tempel 1 made with the NASA Submillimeter Wave Astronomy Satellite (SWAS). The SWAS monitoring covered a 3-month period from June to August 2005 and included observations of the comet’s activity during the collision of the Deep Impact spacecraft with the nucleus on 4 July 2005. No excess emission from the material ejected by the impactor was detected and we derive an upper limit of 1.8× 1071.8× 10^7 kg on the vaporized water ice. However, the water production rate of the comet shows large natural variations of up to a factor of three throughout the 3-month monitoring period. The total amount of water vaporized between 5 June and 1 September 2005 is 4.5× 10 < sup > 34 < /sup >4.5× 1034 molecules ($∼ 1.3× 10^9$∼ 1.3× 10^9 kg) and we estimate that 9P/Tempel 1 has lost as much as $N∼ 1035$N∼ 1035 molecules during its 2005 apparition.10/2008: pages 221-226;
Top co-authors
Top Journals
Institutions
-
1980–2008
-
University of Massachusetts Amherst
- Department of Astronomy
Amherst Center, MA, USA
-
-
2005
-
University of Hawaiʻi at Mānoa
- Institute for Astronomy
Honolulu, HI, USA
-
-
1999
-
Cornell University
- Department of Astronomy
Ithaca, NY, USA
-
-
1989
-
Queen's University
Kingston, Ontario, Canada
-
-
1984–1986
-
National Radio Astronomy Observatory
Charlottesville, VA, USA
-
-
1981
-
Princeton University
Princeton, NJ, USA
-
