83 research outputs found
SWAS observations of comet 9P/Tempel 1 and Deep Impact
On 4 July 2005 at 1:52 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. The 370 kg
impactor collided with the sunlit side of the nucleus with a relative velocity
of 10.2 km/s. NASA's Submillimeter Wave Astronomy Satellite (SWAS) observed the
1(10)-1(01) 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. However, the water production rate of the comet showed large
natural variations of more than a factor of three during the weeks before the
impact.Comment: to appear in the proceedings of the IAU Symposium No. 231:
"Astrochemistry - Recent Successes and Current Callenges". Typo corrected in
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A survey of 557 GHz water vapor emission in the NGC 1333 molecular cloud
Using NASA\u27s 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\u27, are supplemented with observations from the Infrared Space Observatory (ISO) and by an unbiased survey of a ~17\u27 × 15\u27 area, with ~50\u27\u27 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
Spitzer spectral line mapping of supernova remnants: I. Basic data and principal component analysis
We report the results of spectroscopic mapping observations carried out
toward small (1 x 1 arcmin) regions within the supernova remnants W44, W28,
IC443, and 3C391 using the Infrared Spectrograph of the Spitzer Space
Telescope. These observations, covering the 5.2 - 37 micron 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 micron 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.Comment: 43 pages, including 21 figures. Accepted for publication in Ap
Distribution of Water Vapor in Molecular Clouds
We 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 Av < 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 Av. 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
Herschel Search for O_2 toward the Orion Bar
We report the results of a search for molecular oxygen (O_2) toward the Orion Bar, a prominent photodissociation region at the southern edge of the H II region created by the luminous Trapezium stars. We observed the spectral region around the frequency of the O_2 NJ = 33-12 transition at 487 GHz and the 5_(4)-3_(4) transition at 774 GHz using the Heterodyne Instrument for the Far-Infrared on the Herschel Space Observatory. Neither line was detected, but the 3σ upper limits established here translate to a total line-of-sight O2 column density <1.5 × 10^(16) cm^(–2) for an emitting region whose temperature is between 30 K and 250 K, or <1 × 10^(16) cm^(–2) if the O_2 emitting region is primarily at a temperature of ≲100 K. Because the Orion Bar is oriented nearly edge-on relative to our line of sight, the observed column density is enhanced by a factor estimated to be between 4 and 20 relative to the face-on value. Our upper limits imply that the face-on O_2 column density is less than 4 × 10^(15) cm^(–2), a value that is below, and possibly well below, model predictions for gas with a density of 10^(4)-10^(5) cm^(–3) exposed to a far-ultraviolet flux 10^4 times the local value, conditions inferred from previous observations of the Orion Bar. The discrepancy might be resolved if (1) the adsorption energy of O atoms to ice is greater than 800 K; (2) the total face-on A V of the Bar is less than required for O_2 to reach peak abundance; (3) the O_2 emission arises within dense clumps with a small beam filling factor; or (4) the face-on depth into the Bar where O_2 reaches its peak abundance, which is density dependent, corresponds to a sky position different from that sampled by our Herschel beams
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