449 research outputs found
Water in the Near IR spectrum of Comet 8P/Tuttle
High resolution spectra of Comet 8P/Tuttle were obtained in the frequency
range 3440.6-3462.6 cm-1 on 3 January 2008 UT using CGS4 with echelle grating
on UKIRT. In addition to recording strong solar pumped fluorescent (SPF) lines
of H2O, the long integration time (152 miutes on target) enabled eight weaker
H2O features to be assigned, most of which had not previously been identified
in cometary spectra. These transitions, which are from higher energy upper
states, are similar in character to the so-called 'SH' lines recorded in the
post Deep Impact spectrum of comet Tempel 1 (Barber et al., 2007). We have
identified certain characteristics that these lines have in common, and which
in addition to helping to define this new class of cometary line, give some
clues to the physical processes involved in their production. Finally, we
derive an H2O rotational temperature of 62+/- K and a water production rate of
(1.4+/-0.3)E28 molecules/s.Comment: Paper has been accepted for publication by MNRAS (11/06/09
A comprehensive study of infrared OH prompt emission in two comets. I. Observations and effective g-factors
We present high-dispersion infrared spectra of hydroxyl (OH) in comets C/2000 WM1 (LINEAR) and C/2004 Q2 (Machholz), acquired with the Near Infrared Echelle Spectrograph at the Keck Observatory atop Mauna Kea, Hawaii. Most of these rovibrational transitions result from photodissociative excitation of H_2O giving rise to OH "prompt" emission. We present calibrated emission efficiencies (equivalent g-factors, measured in OH photons s^(-1) [H_2O molecule]^(-1)) for more than 20 OH lines sampled in these two comets. The OH transitions analyzed cover a broad range of rotational excitation. This infrared database for OH can be used in two principal ways: (1) as an indirect tool for obtaining water production in comets simultaneously with the production of other parent volatiles, even when direct detections of H_2O are not available; and (2) as an observational constraint to models predicting the rotational distribution of rovibrationally excited OH produced by water photolysis
Comet C/2004 Q2 (MACHHOLZ): Parent Volatiles, a Search for Deuterated Methane, and Constraint on the CH4 Spin Temperature
High-dispersion (l/dl ~ 25,000) infrared spectra of Comet C/2004 Q2
(Machholz) were acquired on Nov. 28-29, 2004, and Jan. 19, 2005 (UT dates) with
NIRSPEC at the Keck-2 telescope on Mauna Kea. We detected H2O, CH4, C2H2, C2H6,
CO, H2CO, CH3OH, HCN, and NH3 and we conducted a sensitive search for CH3D. We
report rotational temperatures, production rates, and mixing ratios (with
respect to H2O) at heliocentric distances of 1.49 AU (Nov. 2004) and 1.21 AU
(Jan. 2005). We highlight three principal results: (1) The mixing ratios of
parent volatiles measured at 1.49 AU and 1.21 AU agree within confidence
limits, consistent with homogeneous composition in the mean volatile release
from the nucleus of C/2004 Q2. Notably, the relative abundance of C2H6/C2H2 is
substantially higher than those measured in other comets, while the mixing
ratios C2H6/H2O, CH3OH/H2O, and HCN/H2O are similar to those observed in
comets, referred to as "organics-normal". (2) The spin temperature of CH4 is >
35-38 K, an estimate consistent with the more robust spin temperature found for
H2O. (3) We obtained a 3s upper limit of CH3D/CH4 < 0.020 (D/H < 0.005). This
limit suggests that methane released from the nucleus of C/2004 Q2 is not
dominated by a component formed in extremely cold (near 10 K) environments.
Formation pathways of both interstellar and nebular origin consistent with the
measured D/H in methane are discussed. Evaluating the relative contributions of
these pathways requires further modeling of chemistry including both gas-phase
and gas-grain processes in the natal interstellar cloud and in the
protoplanetary disk.Comment: Accepted by The Astrophysical Journa
Infrared heterodyne spectroscopy of astronomical and laboratory sources at 8.5 micron
The first infrared heterodyne spectrometer using tuneable semiconductor (PbSe) diode lasers has been constructed and was used near 8.5 micron to measure absorption line profiles of N2O in the laboratory and black body emission from the Moon and from Mars. Spectral information was recorded over a 200 MHz bandwidth using an 8-channel filter bank. The resolution was 25 MHz and the minimum detectable (black body) power was 1 x 10 to the minus 16th power watts for 8 minutes of integration. The results demonstrate the usefulness of heterodyne spectroscopy for the study of remote and local sources in the infrared
Ethyl cyanide on Titan: Spectroscopic detection and mapping using ALMA
We report the first spectroscopic detection of ethyl cyanide (CHCN)
in Titan's atmosphere, obtained using spectrally and spatially resolved
observations of multiple emission lines with the Atacama Large
Millimeter/submillimeter array (ALMA). The presence of CHCN in Titan's
ionosphere was previously inferred from Cassini ion mass spectrometry
measurements of CHCNH. Here we report the detection of 27
rotational lines from CHCN (in 19 separate emission features detected
at confidence), in the frequency range 222-241 GHz. Simultaneous
detections of multiple emission lines from HCN, CHCN and CHCCH were
also obtained. In contrast to HCN, CHCN and CHCCH, which peak in
Titan's northern (spring) hemisphere, the emission from CHCN is found
to be concentrated in the southern (autumn) hemisphere, suggesting a distinctly
different chemistry for this species, consistent with a relatively short
chemical lifetime for CHCN. Radiative transfer models show that most of
the CHCN is concentrated at altitudes 300-600 km, suggesting production
predominantly in the mesosphere and above. Vertical column densities are found
to be in the range (2-5) cm.Comment: Published in 2015, ApJL, 800, L1
Astrophysical Lasers Operating in optical Fe II Lines Lines in Stellar Ejecta of Eta Carinae
After the discovery of space masers based on OH radicals (Weaver et al, 1965)
and H2O (Cheung et al, 1969) such microwave lasers have been found to work in
more than 100 molecular species (Elitzur, 1992; Townes, 1997), as well as in
highly excited H atoms (Strelnitski et al,1996). In the IR region (10 microns),
the effect of stimulated emission of radiation in the CO2 molecule has been
discovered in the Martian and Venus' atmospheres (Betz et al, 1976; Mumma et
al, 1981). We report here on the discovery of laser action in the range 0.9-2
micr. in several spectral lines of Fe II, which are associated with transitions
from "pseudo-metastable" states populated by spontaneous transitions from
Ly-alpha pumped Fe II levels. The intense Ly-alpha radiation is formed in the
HII region of gas condensations close to the star Eta Carinae. The laser
transitions form together with spontaneous transitions closed radiative cycles,
one of which includes the extremely bright 2507/09 A lines. Closed radiative
cycles, together with an accidental mixing of energy levels, may provide an
explanation of the abnormal intensities of these UV non-lasing lines. Using the
complicated energy level diagram of Fe II we present those peculiar features,
which are essential for the inverted population and laser effect: the pumping,
the level mixing, and the "bottle neck" for spontaneous decay. The laser action
is a new indicator of non-equilibrium and spatially non-homogeneous physical
conditions as well as a high brightness temperature of Ly-alpha in ejecta from
eruptive stars. Such conditions are very difficult to probe by existing
methods, and we propose some future experiments. The fact, that the lasing
near-IR lines appear in the spectrum with about the same inten- sity as
non-lasing lines is discussed and compared with the situation in masers.Comment: 27 pages, 11 figures; to be published in A&A; also available at
http://130.235.102.158/blackhole/astrolaser.pd
The vertical transport of methane from different potential emission types on Mars
The contrasting evolutionary behavior of the vertical profile of methane from three potential release scenarios is analysed using a global circulation model with assimilated temperature profiles. Understanding the evolving methane distribution is essential for interpretation of future retrievals of the methane vertical profile taken by instruments on the ExoMars Trace Gas Orbiter spacecraft. We show that at methane release rates constrained by previous observations and modelling studies, discriminating whether the methane source is a sustained or instantaneous surface emission requires at least ten sols of tracking the emission. A methane source must also be observed within five to ten sols of the initial emission to distinguish whether the emission occurs directly at the surface or within the atmosphere via destabilization of metastable clathrates. Assimilation of thermal data is shown to be critical for the most accurate back-tracking of an observed methane plume to its origin
Temporal and Spatial Aspects of Gas Release During the 2010 Apparition of Comet 103P/Hartley-2
We report measurements of eight primary volatiles (H2O, HCN, CH4, C2H6,
CH3OH, C2H2, H2CO, and NH3) and two product species (OH and NH2) in comet
103P/Hartley-2 using high dispersion infrared spectroscopy. We quantified the
long- and short-term behavior of volatile release over a three-month interval
that encompassed the comet's close approach to Earth, its perihelion passage,
and flyby of the comet by the Deep Impact spacecraft during the EPOXI mission.
We present production rates for individual species, their mixing ratios
relative to water, and their spatial distributions in the coma on multiple
dates. The production rates for water, ethane, HCN, and methanol vary in a
manner consistent with independent measures of nucleus rotation, but mixing
ratios for HCN, C2H6, & CH3OH are independent of rotational phase. Our results
demonstrate that the ensemble average composition of gas released from the
nucleus is well defined, and relatively constant over the three-month interval
(September 18 through December 17). If individual vents vary in composition,
enough diverse vents must be active simultaneously to approximate (in sum) the
bulk composition of the nucleus. The released primary volatiles exhibit diverse
spatial properties which favor the presence of separate polar and apolar ice
phases in the nucleus, establish dust and gas release from icy clumps (and
also, directly from the nucleus), and provide insights into the driver for the
cyanogen (CN) polar jet. The spatial distributions of C2H6 & HCN along the
near-polar jet (UT 19.5 October) and nearly orthogonal to it (UT 22.5 October)
are discussed relative to the origin of CN. The ortho-para ratio (OPR) of water
was 2.85 \pm 0.20; the lower bound (2.65) defines Tspin > 32 K. These values
are consistent with results returned from ISO in 1997.Comment: 18 pages, 3 figures, to be published in: Astrophysical Journal
Letter
A Search for Variation in the H_2O Ortho-Para Ratio and Rotational Temperature in the Inner Coma of Comet C/2004 Q2 (Machholz)
We present spatially resolved measurements of the rotational temperature and ortho-para ratio for H_2O in the inner coma of the Oort Cloud comet C/2004 Q2 (Machholz). Our results are based on direct simultaneous detections of ortho-H_2O and para-H_2O via "hot-band" fluorescence near 2.9 μm. We find a well-defined decline in rotational temperature with increasing nucleocentric distance (up to ~1000 km). The ortho-para ratio remains constant (within stochastic uncertainty) with increasing nucleocentric distance and is close to the statistical equilibrium value of 3.0 (2.86 ± 0.06 [0.17], including, respectively, stochastic [systematic] uncertainty), resulting in spin temperature T_(spin) ≥ 34 K. We compare the present results with those reported for other comets and discuss the difficulties in interpreting spin temperatures deduced from measured ortho-para ratios. Improved understanding of the special conditions that enable nuclear spin conversion would test the extent to which derived spin temperatures reflect the formative history or the processing record of cometary ices
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