6,287 research outputs found
Discovery of Water Vapor in the High-redshift Quasar APM 08279+5255 at z = 3.91
We report a detection of the excited 2_(20)-2_(11) rotational transition of para-H_2O in APM 08279+5255 using the IRAM Plateau de Bure Interferometer. At z = 3.91, this is the highest-redshift detection of interstellar water to date. From large velocity gradient modeling, we conclude that this transition is predominantly radiatively pumped and on its own does not provide a good estimate of the water abundance. However, additional water transitions are predicted to be detectable in this source, which would lead to an improved excitation model. We also present a sensitive upper limit for the hydrogen fluoride (HF) J = 1-0 absorption toward APM 08279+5255. While the face-on geometry of this source is not favorable for absorption studies, the lack of HF absorption is still puzzling and may be indicative of a lower fluorine abundance at z = 3.91 compared with the Galactic interstellar medium
Guidance on Carbon Capture Readiness and Applications under Section 36 of the Electricity Act 1989: consultation response
On the structure of hot absorption spectra of polyatomic molecules: Solvent effect on the transition energy gap.
Models for Dense Molecular Cloud Cores
We present a detailed model for the thermal balance, chemistry, and radiative
transfer within quiescent dense molecular cloud cores that contain a central
protostar. Large variations in the gas temperature are expected to affect the
gas-phase chemistry dramatically; with the predicted H2O abundance varying by
more than a factor of 1000 within cloud cores. Based on our predicitions for
the thermal and chemical structure of the cores, we have constructed
self-consistent radiative transfer models to compute line strengths and
profiles for transitions of various isotopomers of CO, H2O, and OI. We predict
the high lying transitions of water to be in absorption, and low gain maser
emission at 183 GHz. We predict the 63 micron line of OI to be in absorption
against the continuum for many sources. Finally, our model can also account
successfully for recent ISO observations of absorption in rovibrational
transitions of water toward the source AFGL 2591.Comment: 36 pages, 18 figures, LaTex, Accepted for publication by ApJ (11/97
A Herschel/HIFI Legacy Survey of HF and H2O in the Galaxy: Probing Diffuse Molecular Cloud Chemistry
We combine Herschel observations of a total of 12 sources to construct the
most uniform survey of HF and H2O in our Galactic disk. Both molecules are
detected in absorption along all sight lines. The high spectral resolution of
the Heterodyne Instrument for the Far-Infrared (HIFI) allows us to compare the
HF and H2O distributions in 47 diffuse cloud components sampling the disk. We
find that the HF and H2O velocity distributions follow each other almost
perfectly and establish that HF and H2O probe the same gas-phase volume. Our
observations corroborate theoretical predictions that HF is a sensitive tracer
of H2 in diffuse clouds, down to molecular fractions of only a few percent.
Using HF to trace H2 in our sample, we find that the N(H2O)-to-N(HF) ratio
shows a narrow distribution with a median value of 1.51. Our results further
suggest that H2O might be used as a tracer of H2 -within a factor 2.5- in the
diffuse interstellar medium. We show that the measured factor of ~2.5 variation
around the median is driven by true local variations in the H2O abundance
relative to H2 throughout the disk. The latter variability allows us to test
our theoretical understanding of the chemistry of oxygen-bearing molecules in
the diffuse gas. We show that both gas-phase and grain-surface chemistry are
required to reproduce our H2O observations. This survey thus confirms that
grain surface reactions can play a significant role in the chemistry occurring
in the diffuse interstellar medium n_H < 1000 cm^-3.Comment: 53 pages; 12 figures, accepted for publication in ApJ main journa
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