43 research outputs found
Recommended Thermal Rate Coefficients for the C + H Reaction and Some Astrochemical Implications
We have incorporated our experimentally derived thermal rate coefficients for
C + H forming CH and CH into a commonly used astrochemical
model. We find that the Arrhenius-Kooij equation typically used in chemical
models does not accurately fit our data and use instead a more versatile
fitting formula. At a temperature of 10 K and a density of 10 cm, we
find no significant differences in the predicted chemical abundances, but at
higher temperatures of 50, 100, and 300 K we find up to factor of 2 changes.
Additionally, we find that the relatively small error on our thermal rate
coefficients, , significantly reduces the uncertainties on the
predicted abundances compared to those obtained using the currently implemented
Langevin rate coefficient with its estimated factor of 2 uncertainty.Comment: 19 pages, 5 figures. Accepted for publication in Ap
Merged-beams Reaction Studies of O + H_3^+
We have measured the reaction of O + H3+ forming OH+ and H2O+. This is one of
the key gas-phase astrochemical processes initiating the formation of water
molecules in dense molecular clouds. For this work, we have used a novel merged
fast-beams apparatus which overlaps a beam of H3+ onto a beam of ground-term
neutral O. Here, we present cross section data for forming OH+ and H2O+ at
relative energies from \approx 3.5 meV to \approx 15.5 and 0.13 eV,
respectively. Measurements were performed for statistically populated O(3PJ) in
the ground term reacting with hot H3+ (with an internal temperature of \approx
2500-3000 K). From these data, we have derived rate coefficients for
translational temperatures from \approx 25 K to \approx 10^5 and 10^3 K,
respectively. Using state-of-the-art theoretical methods as a guide, we have
converted these results to a thermal rate coefficient for forming either OH+ or
H2O+, thereby accounting for the temperature dependence of the O fine-structure
levels. Our results are in good agreement with two independent flowing
afterglow measurements at a temperature of \approx 300 K, and with a
corresponding level of H3+ internal excitation. This good agreement strongly
suggests that the internal excitation of the H3+ does not play a significant
role in this reaction. The Langevin rate coefficient is in reasonable agreement
with the experimental results at 10 K but a factor of \approx 2 larger at 300
K. The two published classical trajectory studies using quantum mechanical
potential energy surfaces lie a factor of \approx 1.5 above our experimental
results over this 10-300 K range.Comment: 43 pages, 11 figures. Submitted to the Astrophysical Journa
An open-source framework to plan and interpret observations of atmospheric escape in exoplanets
Stars and planetary system
Recommended from our members
Recommended Thermal Rate Coefficients for the C + H3+ Reaction and Some Astrochemical Implications
We incorporate our experimentally derived thermal rate coefficients for C + forming CH+ and CH2 + into a commonly used astrochemical model. We find that the ArrheniusâKooij equation typically used in chemical models does not accurately fit our data and instead we use a more versatile fitting formula. At a temperature of 10 K and a density of 104 cmâ3, we find no significant differences in the predicted chemical abundances, but at higher temperatures of 50, 100, and 300 K we find up to factor of 2 changes. In addition, we find that the relatively small error on our thermal rate coefficients, ~15%, significantly reduces the uncertainties on the predicted abundances compared to those obtained using the currently implemented Langevin rate coefficient with its estimated factor of 2 uncertainty
Recommended from our members
Recommended Thermal Rate Coefficients for the C + H3+ Reaction and Some Astrochemical Implications
We incorporate our experimentally derived thermal rate coefficients for C + forming CH+ and CH2 + into a commonly used astrochemical model. We find that the ArrheniusâKooij equation typically used in chemical models does not accurately fit our data and instead we use a more versatile fitting formula. At a temperature of 10 K and a density of 104 cmâ3, we find no significant differences in the predicted chemical abundances, but at higher temperatures of 50, 100, and 300 K we find up to factor of 2 changes. In addition, we find that the relatively small error on our thermal rate coefficients, ~15%, significantly reduces the uncertainties on the predicted abundances compared to those obtained using the currently implemented Langevin rate coefficient with its estimated factor of 2 uncertainty
p-winds: an open-source Python code to model planetary outflows and upper atmospheres
Computational astrophysic
Confronting compositional confusion through the characterisation of the sub-Neptune orbiting HD 77946
We report on the detailed characterization of the HD 77946 planetary system. HD 77946 is an F5 ( = 1.17 M, = 1.31 R) star, which hosts a transiting planet recently discovered by NASA's Transiting Exoplanet Survey Satellite (TESS), classified as TOI-1778 b. Using TESS photometry, high-resolution spectroscopic data from HARPS-N, and photometry from CHEOPS, we measure the radius and mass from the transit and RV observations, and find that the planet, HD 77946 b, orbits with period = d, has a mass of M, and a radius of R. From the combination of mass and radius measurements, and the stellar chemical composition, the planet properties suggest that HD 77946 b is a sub-Neptune with a 1\% H/He atmosphere. However, a degeneracy still exists between water-world and silicate/iron-hydrogen models, and even though interior structure modelling of this planet favours a sub-Neptune with a H/He layer that makes up a significant fraction of its radius, a water-world composition cannot be ruled out, as with K, water may be in a supercritical state. The characterisation of HD 77946 b, adding to the small sample of well-characterised sub-Neptunes, is an important step forwards on our journey to understanding planetary formation and evolution pathways. Furthermore, HD 77946 b has one of the highest transmission spectroscopic metrics for small planets orbiting hot stars, thus transmission spectroscopy of this key planet could prove vital for constraining the compositional confusion that currently surrounds small exoplanets
Methanol ice co-desorption as a mechanism to explain cold methanol in the gas-phase
Context. Methanol is formed via surface reactions on icy dust grains. Methanol is also detected in the gas-phase at temperatures below its thermal desorption temperature and at levels higher than can be explained by pure gas-phase chemistry. The process that controls the transition from solid state to gas-phase methanol in cold environments is not understood.
Aims. The goal of this work is to investigate whether thermal CO desorption provides an indirect pathway for methanol to co-desorb at low temperatures.
Methods. Mixed CHâOH:CO/CHâ ices were heated under ultra-high vacuum conditions and ice contents are traced using RAIRS (reflection absorption IR spectroscopy), while desorbing species were detected mass spectrometrically. An updated gas-grain chemical network was used to test the impact of the results of these experiments. The physical model used is applicable for TW Hya, a protoplanetary disk in which cold gas-phase methanol has recently been detected.
Results. Methanol release together with thermal CO desorption is found to be an ineffective process in the experiments, resulting in an upper limit of †7.3 Ă 10â7 CHâOH molecules per CO molecule over all ice mixtures considered. Chemical modelling based on the upper limits shows that co-desorption rates as low as 10â6 CHâOH molecules per CO molecule are high enough to release substantial amounts of methanol to the gas-phase at and around the location of the CO thermal desorption front in a protoplanetary disk. The impact of thermal co-desorption of CHâOH with CO as a grain-gas bridge mechanism is compared with that of UV induced photodesorption and chemisorption