130 research outputs found
First Time-dependent Study of H2 and H3+ Ortho-Para Chemistry in the Diffuse Interstellar Medium: Observations Meet Theoretical Predictions
The chemistry in the diffuse interstellar medium initiates the gradual
increase of molecular complexity during the life cycle of matter. A key
molecule that enables build-up of new molecular bonds and new molecules via
proton-donation is H3+. Its evolution is tightly related to molecular hydrogen
and thought to be well understood. However, recent observations of ortho and
para lines of H2 and H3+ in the diffuse ISM showed a puzzling discrepancy in
nuclear spin excitation temperatures and populations between these two key
species. H3+, unlike H2, seems to be out of thermal equilibrium, contrary to
the predictions of modern astrochemical models. We conduct the first
time-dependent modeling of the para-fractions of H2 and H3+ in the diffuse ISM
and compare our results to a set of line-of-sight observations, including new
measurements presented in this study. We isolate a set of key reactions for H3+
and find that the destruction of the lowest rotational states of H3+ by
dissociative recombination largely control its ortho/para ratio. A plausible
agreement with observations cannot be achieved unless a ratio larger than 1:5
for the destruction of (1,1)- and (1,0)-states of H3+ is assumed. Additionally,
an increased CR ionization rate to 10(-15) 1/s further improves the fit whereas
variations of other individual physical parameters, such as density and
chemical age, have only a minor effect on the predicted ortho/para ratios. Thus
our study calls for new laboratory measurements of the dissociative
recombination rate and branching ratio of the key ion H3+ under interstellar
conditions.Comment: 27 pages, 6 figures, 3 table
Fundamental Vibrational Transitions of HCl Detected in CRL 2136
We would like to understand the chemistry of dense clouds and their hot cores
more quantitatively by obtaining more complete knowledge of the chemical
species present in them. We have obtained high-resolution infrared absorption
spectroscopy at 3-4 um toward the bright infrared source CRL 2136. The
fundamental vibration-rotation band of HCl has been detected within a dense
cloud for the first time. The HCl is probably located in the warm compact
circumstellar envelope or disk of CRL 2136. The fractional abundance of HCl is
(4.9-8.7)e-8, indicating that approximately 20 % of the elemental chlorine is
in gaseous HCl. The kinetic temperature of the absorbing gas is 250 K, half the
value determined from infrared spectroscopy of 13CO and water. The percentage
of chlorine in HCl is approximately that expected for gas at this temperature.
The reason for the difference in temperatures between the various molecular
species is unknown.Comment: 6 pages, 3 figures, A&A in pres
Absorption Line Survey of H3+ toward the Galactic Center Sources III. Extent of the Warm and Diffuse Clouds
We present follow-up observations to those of Geballe & Oka (2010), who found
high column densities of H3+ ~100 pc off of the Galactic center (GC) on the
lines of sight to 2MASS J17432173-2951430 (J1743) and 2MASS J17470898-2829561
(J1747). The wavelength coverages on these sightlines have been extended in
order to observe two key transitions of H3+, R(3,3)l and R(2,2)l, that
constrain the temperatures and densities of the environments. The profiles of
the H3+ R(3,3)l line, which is due only to gas in the GC, closely matches the
differences between the H3+ R(1,1)l and CO line profiles, just as it does for
previously studied sightlines in the GC. Absorption in the R(2,2)l line of H3+
is present in J1747 at velocities between -60 and +100 km/s. This is the second
clear detection of this line in the interstellar medium after GCIRS 3 in the
Central Cluster. The temperature of the absorbing gas in this velocity range is
350 K, significantly warmer than in the diffuse clouds in other parts of the
Central Molecular Zone. This indicates that the absorbing gas is local to Sgr B
molecular cloud complex. The warm and diffuse gas revealed by Oka et al. (2005)
apparently extends to ~100 pc, but there is a hint that its temperature is
somewhat lower in the line of sight to J1743 than elsewhere in the GC. The
observation of H3+ toward J1747 is compared with the recent Herschel
observation of H2O+ toward Sgr B2 and their chemical relationship and
remarkably similar velocity profiles are discussed.Comment: 6 pages, 3 figures, 2 tables, Accepted for publication in
Publications of the Astronomical Society of Japa
SOFIA/EXES Observations of Water Absorption in the Protostar AFGL 2591 at High Spectral Resolution
We present high spectral resolution (~3 km/s) observations of the nu_2
ro-vibrational band of H2O in the 6.086--6.135 micron range toward the massive
protostar AFGL 2591 using the Echelon-Cross-Echelle Spectrograph (EXES) on the
Stratospheric Observatory for Infrared Astronomy (SOFIA). Ten absorption
features are detected in total, with seven caused by transitions in the nu_2
band of H2O, two by transitions in the first vibrationally excited nu_2 band of
H2O, and one by a transition in the nu_2 band of H2{18}O. Among the detected
transitions is the nu_2 1(1,1)--0(0,0) line which probes the lowest lying
rotational level of para-H2O. The stronger transitions appear to be optically
thick, but reach maximum absorption at a depth of about 25%, suggesting that
the background source is only partially covered by the absorbing gas, or that
the absorption arises within the 6 micron emitting photosphere. Assuming a
covering fraction of 25%, the H2O column density and rotational temperature
that best fit the observed absorption lines are N(H2O)=(1.3+-0.3)*10^{19}
cm^{-2} and T=640+-80 K.Comment: 6 pages, 3 figures, 1 table, accepted for publication in ApJ
On the Ortho:Para Ratio of H3+ in Diffuse Molecular Clouds
The excitation temperature T_01 derived from the relative intensities of the
J = 0 (para) and J = 1 (ortho) rotational levels of H2 has been assumed to be
an accurate measure of the kinetic temperature in interstellar environments. In
diffuse molecular clouds, the average value of T_01 is ~70 K. However, the
excitation temperature T(H3+) derived from the (J,K) = (1,1) (para) and (1,0)
(ortho) rotational levels of H3+ has been observed to be ~30 K in the same
types of environments. In this work, we present observations of H3+ in three
additional diffuse cloud sight lines for which H2 measurements are available,
showing that in 4 of 5 cases T_01 and T(H3+) are discrepant. We then examine
the thermalization mechanisms for the ortho:para ratios of H3+ and H2,
concluding that indeed T_01 is an accurate measure of the cloud kinetic
temperature, while the ortho:para ratio of H3+ need not be thermal. By
constructing a steady-state chemical model taking into account the
nuclear-spindependence of reactions involving H3+, we show that the ortho:para
ratio of H3+ in diffuse molecular clouds is likely governed by a competition
between dissociative recombination with electrons and thermalization via
reactive collisions with H2.Comment: 13 pages, 8 figures, 5 tables, accepted for publication in Ap
Herschel observations of interstellar chloronium. II - Detections toward G29.96-0.02, W49N, W51, and W3(OH), and determinations of the ortho-to-para and Cl/Cl isotopic ratios
We report additional detections of the chloronium molecular ion, HCl,
toward four bright submillimeter continuum sources: G29.96, W49N, W51, and
W3(OH). With the use of the HIFI instrument on the Herschel Space Observatory,
we observed the transition of ortho-HCl at 781.627
GHz in absorption toward all four sources. Much of the detected absorption
arises in diffuse foreground clouds that are unassociated with the background
continuum sources and in which our best estimates of the ratio lie in the range .
These chloronium abundances relative to atomic hydrogen can exceed the
predictions of current astrochemical models by up to a factor of 5. Toward
W49N, we have also detected the transition of
ortho-HCl at 780.053 GHz and the transition of
para-HCl at 485.418 GHz. These observations imply column density ratios that are consistent with the
solar system Cl/Cl isotopic ratio of 3.1, and chloronium
ortho-to-para ratios consistent with 3, the ratio of spin statistical weights.Comment: 31 pages, including 7 figures. Accepted for publication in the Ap
Herschel Observations Reveal Anomalous Molecular Abundances toward the Galactic Center
We report the Herschel detections of hydrogen fluoride (HF) and para-water (p-H_(2)O) in gas intercepting the sight lines to two well-studied molecular clouds in the vicinity of the Sgr A complex: G–0.02–0.07 (the "+50 km s^(–1) cloud") and G–0.13–0.08 (the "+20 km s^(–1) cloud"). Toward both sight lines, HF and water absorption components are detected over a wide range of velocities covering ~250 km s^(–1). For all velocity components with V_LSR > –85 km s^(–1), we find that the HF and water abundances are consistent with those measured toward other sight lines probing the Galactic disk gas. The velocity components with V LSR ≤ –85 km s^(–1), which are known to trace gas residing within ~200 pc of the Galactic center, however, exhibit water vapor abundances with respect to HF at least a factor three higher than those found in the Galactic disk gas. Comparison with CH data indicates that our observations are consistent with a picture where HF and a fraction of the H_(2)O absorption arise in diffuse molecular clouds showing Galactic disk-like abundances while the bulk of the water absorption arises in warmer (T ≥ 400 K) diffuse molecular gas for V LSR ≤ –85 km s^(–1). This diffuse Interstellar Medium (ISM) phase has also been recently revealed through observations of CO, HF, H^(+)_3, and H_(3)O^+ absorption toward other sight lines probing the Galactic center inner region
Sensitivity Analysis of Grain Surface Chemistry to Binding Energies of Ice Species
Advanced telescopes, such as ALMA and the James Webb Space Telescope, are likely to show that the chemical universe may be even more complex than currently observed, requiring astrochemical modelers to improve their models to account for the impact of new data. However, essential input information for gas‑grain models, such as binding energies of molecules to the surface, have been derived experimentally only for a handful of species, leaving hundreds of species with highly uncertain estimates. We present in this paper a systematic study of the effect of uncertainties in the binding energies on an astrochemical two-phase model of a dark molecular cloud, using the rate equations approach. A list of recommended binding energy values based on a literature search of published data is presented. Thousands of simulations of dark cloud models were run, and in each simulation a value for the binding energy of hundreds of species was randomly chosen from a normal distribution. Our results show that the binding energy of H2 is critical for the surface chemistry. For high binding energies, H2 freezes out on the grain forming an H2 ice. This is not physically realistic, and we suggest a change in the rate equations. The abundance ranges found are in reasonable agreement with astronomical ice observations. Pearson correlation coefficients revealed that the binding energy of HCO, HNO, CH2, and C correlate most strongly with the abundance of dominant ice species. Finally, the formation route of complex organic molecules was found to be sensitive to the branching ratios of H2CO hydrogenation
Rotationally Warm Molecular Hydrogen in the Orion Bar
The Orion Bar is one of the nearest and best-studied photodissociation or
photon-dominated regions (PDRs). Observations reveal the presence of H2 lines
from vibrationally or rotationally excited upper levels that suggest warm gas
temperatures (400 to 700 K). However, standard models of PDRs are unable to
reproduce such warm rotational temperatures. In this paper we attempt to
explain these observations with new comprehensive models which extend from the
H+ region through the Bar and include the magnetic field in the equation of
state. We adopt the model parameters from our previous paper which successfully
reproduced a wide variety of spectral observations across the Bar. In this
model the local cosmic-ray density is enhanced above the galactic background,
as is the magnetic field, and which increases the cosmic-ray heating elevating
the temperature in the molecular region. The pressure is further enhanced above
the gas pressure in the H+ region by the momentum transferred from the absorbed
starlight. Here we investigate whether the observed H2 lines can be reproduced
with standard assumptions concerning the grain photoelectric emission. We also
explore the effects due to the inclusion of recently computed H2 + H2, H2 + H
and H2 + He collisional rate coefficients.Comment: Accepted for publication in ApJ (34 pages, including 16 figures
The HO Spectrum of the Massive Protostar AFGL 2136 IRS 1 from 2 to 13 m at High Resolution: Probing the Circumstellar Disk
We have observed the massive protostar AFGL 2136 IRS 1 in multiple wavelength
windows in the near-to-mid-infrared at high ( km s) spectral
resolution using VLT+CRIRES, SOFIA+EXES, and Gemini North+TEXES. There is an
abundance of HO absorption lines from the and vibrational
bands at 2.7 m, from the vibrational band at 6.1 m, and from
pure rotational transitions near 10-13 m. Analysis of state-specific
column densities derived from the resolved absorption features reveals that an
isothermal absorbing slab model is incapable of explaining the relative depths
of different absorption features. In particular, the strongest absorption
features are much weaker than expected, indicating optical depth effects
resulting from the absorbing gas being well-mixed with the warm dust that
serves as the "background" continuum source at all observed wavelengths. The
velocity at which the strongest HO absorption occurs coincides with the
velocity centroid along the minor axis of the compact disk in Keplerian
rotation recently observed in HO emission with ALMA. We postulate that the
warm regions of this dust disk dominate the continuum emission at near-to-mid
infrared wavelengths, and that HO and several other molecules observed in
absorption are probing this disk. Absorption line profiles are not symmetric,
possibly indicating that the warm dust in the disk that produces the infrared
continuum has a non-uniform distribution similar to the substructure observed
in 1.3 mm continuum emission.Comment: 30 pages, 12 figures, 3 tables, ApJ accepte
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