82 research outputs found
Evaporation of ices near massive stars: models based on laboratory TPD data
Hot cores and their precursors contain an integrated record of the physics of
the collapse process in the chemistry of the ices deposited during that
collapse. In this paper, we present results from a new model of the chemistry
near high mass stars in which the desorption of each species in the ice mixture
is described as indicated by new experimental results obtained under conditions
similar to those hot cores. Our models show that provided there is a monotonic
increase in the temperature of the gas and dust surrounding the protostar, the
changes in the chemical evolution of each species due to differential
desorption are important. The species HS, SO, SO, OCS, HCS, CS, NS,
CHOH, HCOOCH, CHCO, CHOH show a strong time dependence that
may be a useful signature of time evolution in the warm-up phase as the star
moves on to the Main Sequence. This preliminary study demonstrates the
consequences of incorporating reliable TPD data into chemical models.Comment: 5 pages, accepted by MNRA
Thermal Desorption of Water-Ice in the Interstellar Medium
Water (H2O) ice is an important solid constituent of many astrophysical
environments. To comprehend the role of such ices in the chemistry and
evolution of dense molecular clouds and comets, it is necessary to understand
the freeze-out, potential surface reactivity, and desorption mechanisms of such
molecular systems. Consequently, there is a real need from within the
astronomical modelling community for accurate empirical molecular data
pertaining to these processes. Here we give the first results of a laboratory
programme to provide such data. Measurements of the thermal desorption of H2O
ice, under interstellar conditions, are presented. For ice deposited under
conditions that realistically mimic those in a dense molecular cloud, the
thermal desorption of thin films (~50 molecular layers) is found to occur with
zero order kinetics characterised by a surface binding energy, E_{des}, of 5773
+/- 60 K, and a pre-exponential factor, A, of 10^(30 +/- 2) molecules cm^-2
s^-1. These results imply that, in the dense interstellar medium, thermal
desorption of H2O ice will occur at significantly higher temperatures than has
previously been assumed.Comment: 9 pages, 4 figures, accepted for publication in MNRA
A fibre-coupled UHV-compatible variable angle reflection-absorption UV/visible spectrometer
We present a novel UV/visible reflection-absorption spectrometer for determining the refractive index, n, and thicknesses, d, of ice films. Knowledge of the refractive index of these films is of particular relevance to the astrochemical community, where they can be used to model radiative transfer and spectra of various regions of space. In order to make these models more accurate, values of n need to be recorded under astronomically relevant conditions, that is, under ultra-high vacuum (UHV) and cryogenic cooling. Several design considerations were taken into account to allow UHV compatibility combined with ease of use. The key design feature is a stainless steel rhombus coupled to an external linear drive (z-shift) allowing a variable reflection geometry to be achieved, which is necessary for our analysis. Test data for amorphous benzene ice is presented as a proof of concept, the film thickness, d, was found to vary linearly with surface exposure and a value for n of 1.43 ± 0.07 was determined
Desorption of hot molecules from photon irradiated interstellar ices
We present experimental measurements of photodesorption from ices of
astrophysical relevance. Layers of benzene and water ice were irradiated with a
laser tuned to an electronic transition in the benzene molecule. The
translational energy of desorbed molecules was measured by time-of-flight (ToF)
mass spectrometry. Three distinct photodesorption processes were identified - a
direct adsorbate-mediated desorption producing benzene molecules with a
translational temperature of around 1200 K, an indirect adsorbate-mediated
desorption resulting in water molecules with a translational temperature of
around 450 K, and a substrate-mediated desorption of both benzene and water
producing molecules with translational temperatures of around 530 K and 450 K
respectively. The translational temperature of each population of desorbed
molecules is well above the temperature of the ice matrix. The implications for
gas-phase chemistry in the interstellar medium are discussed.Comment: 23 pages, including 4 figures; submitted to Ap
Laboratory investigations of the interaction between benzene and bare silicate grain surfaces
Experimental results on the thermal desorption of benzene (C6H6) from
amorphous silica (SiO2) are presented. The amorphous SiO2 substrate was imaged
using atomic force microscopy (AFM), revealing a surface morphology reminiscent
of that of interplanetary dust particles (IDPs). Temperature programmed
desorption (TPD) experiments were conducted for a wide range of C6H6 exposures,
yielding information on both C6H6-SiO2 interactions and the C6H6-C6H6
interactions present in the bulk C6H6 ice. The low coverage experiments reveal
complicated desorption behaviour that results both from porosity and roughness
in the SiO2 substrate, and repulsive interactions between C6H6 molecules.
Kinetic parameters were obtained through a combination of direct analysis of
the TPD traces and kinetic modelling, demonstrating the coverage dependence of
both desorption energy and pre-exponential factor. Experiments were also
performed whereby the pores were blocked by pre-exposure of the SiO2 to water
vapour. C6H6 was observed to be adsorbed preferentially on the SiO2 film not
covered by H2O at the temperature at which these experiments were performed.
This observation means that intermolecular repulsion likely becomes important
at smaller C6H6 exposures on grains with a H2O mantle. Kinetic modelling of
C6H6 multilayer desorption yields kinetic parameters in good agreement with
previous studies, with the SiO2 having little impact on the desorption beyond
the first few layers.Comment: 23 pages, including 6 figures and 1 table ; Submitted to MNRA
An Ice Age JWST inventory of dense molecular cloud ices
Icy grain mantles are the main reservoir of the volatile elements that link
chemical processes in dark, interstellar clouds with the formation of planets
and composition of their atmospheres. The initial ice composition is set in the
cold, dense parts of molecular clouds, prior to the onset of star formation.
With the exquisite sensitivity of JWST, this critical stage of ice evolution is
now accessible for detailed study. Here we show the first results of the Early
Release Science program "Ice Age" that reveal the rich composition of these
dense cloud ices. Weak ices, including, CO, OCN, CO, OCS,
and COMs functional groups are now detected along two pre-stellar lines of
sight. The CO ice profile indicates modest growth of the icy grains.
Column densities of the major and minor ice species indicate that ices
contribute between 2 and 19% of the bulk budgets of the key C, O, N, and S
elements. Our results suggest that the formation of simple and complex
molecules could begin early in a water-ice rich environment.Comment: To appear in Nature Astronomy on January 23rd, 2023. 33 pages, 16
figures, 3 tables; includes extended and supplemental data sections. Part of
the JWST Ice Age Early Release Science program's science enabling products.
Enhanced spectra downloadable on Zenodo at the following DOI:
10.5281/zenodo.750123
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