85 research outputs found
CO diffusion and desorption kinetics in CO ices
Diffusion of species in icy dust grain mantles is a fundamental process that
shapes the chemistry of interstellar regions; yet measurements of diffusion in
interstellar ice analogs are scarce. Here we present measurements of CO
diffusion into CO ice at low temperatures (T=11--23~K) using CO
longitudinal optical (LO) phonon modes to monitor the level of mixing of
initially layered ices. We model the diffusion kinetics using Fick's second law
and find the temperature dependent diffusion coefficients are well fit by an
Arrhenius equation giving a diffusion barrier of 300 40 K. The low
barrier along with the diffusion kinetics through isotopically labeled layers
suggest that CO diffuses through CO along pore surfaces rather than through
bulk diffusion. In complementary experiments, we measure the desorption energy
of CO from CO ices deposited at 11-50 K by temperature-programmed
desorption (TPD) and find that the desorption barrier ranges from 1240 90
K to 1410 70 K depending on the CO deposition temperature and
resultant ice porosity. The measured CO-CO desorption barriers demonstrate
that CO binds equally well to CO and HO ices when both are compact. The
CO-CO diffusion-desorption barrier ratio ranges from 0.21-0.24 dependent on
the binding environment during diffusion. The diffusion-desorption ratio is
consistent with the above hypothesis that the observed diffusion is a surface
process and adds to previous experimental evidence on diffusion in water ice
that suggests surface diffusion is important to the mobility of molecules
within interstellar ices
Interstellar Comets from Post-Main Sequence Systems as Tracers of Extrasolar Oort Clouds
Interstellar small bodies are unique probes into the histories of
exoplanetary systems. One hypothesized class of interlopers are "Jurads,"
exo-comets released into the Milky Way during the post-main sequence as the
thermally-pulsing asymptotic giant branch (AGB) host stars lose mass. In this
study, we assess the prospects for the Legacy Survey of Space and Time (LSST)
to detect a Jurad and examine whether such an interloper would be
observationally distinguishable from exo-comets ejected during the (pre-)main
sequence. Using analytic and numerical methods, we estimate the fraction of
exo-Oort Cloud objects that are released from 1-8 solar mass stars during
post-main sequence evolution. We quantify the extent to which small bodies are
altered by the increased luminosity and stellar outflows during the AGB,
finding that some Jurads may lack hypervolatiles and that stellar winds could
deposit dust that covers the entire exo-comet surface. Next, we construct
models of the interstellar small body reservoir for various size-frequency
distribution slopes, characteristic sizes, and the total mass sequestered in
the minor planets of exo-Oort Clouds. Even with the LSST's increased search
volume compared to contemporary surveys, we find that detecting a Jurad is
unlikely but not infeasible given the current understanding of (exo)planet
formation.Comment: 28 pages, 13 figures; accepted to PS
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