152 research outputs found
CO Infrared Phonon Modes in Interstellar Ice Mixtures
CO ice is an important reservoir of carbon and oxygen in star and planet
forming regions. Together with water and CO, CO sets the physical and
chemical characteristics of interstellar icy grain mantles, including
desorption and diffusion energies for other ice constituents. A detailed
understanding of CO ice spectroscopy is a prerequisite to characterize
CO interactions with other volatiles both in interstellar ices and in
laboratory experiments of interstellar ice analogs. We report laboratory
spectra of the CO longitudinal optical (LO) phonon mode in pure CO ice
and in CO ice mixtures with HO, CO, O components. We show that the
LO phonon mode position is sensitive to the mixing ratio of various ice
components of astronomical interest. In the era of JWST, this characteristic
could be used to constrain interstellar ice compositions and morphologies. More
immediately, LO phonon mode spectroscopy provides a sensitive probe of ice
mixing in the laboratory and should thus enable diffusion measurements with
higher precision than has been previously possible
Evidence for Multiple Pathways to Deuterium Enhancements in Protoplanetary Disks
The distributions of deuterated molecules in protoplanetary disks are
expected to depend on the molecular formation pathways. We use observations of
spatially resolved DCN emission from the disk around TW Hya, acquired during
ALMA Science verification with a ~3" synthesized beam, together with comparable
DCO+ observations from the Submillimeter Array, to investigate differences in
the radial distributions of these species and hence differences in their
formation chemistry. In contrast to DCO+, which shows an increasing column
density with radius, DCN is better fit by a model that is centrally peaked. We
infer that DCN forms at a smaller radii and thus at higher temperatures than
DCO+. This is consistent with chemical network model predictions of DCO+
formation from H2D+ at T<30 K and DCN formation from additional pathways
involving CH2D+ at higher temperatures. We estimate a DCN/HCN abundance ratio
of ~0.017, similar to the DCO+/HCO+ abundance ratio. Deuterium fractionation
appears to be efficient at a range of temperatures in this protoplanetary disk.
These results suggest caution in interpreting the range of deuterium fractions
observed in Solar System bodies, as multiple formation pathways should be taken
into account.Comment: accepted for publication in Ap
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
SO and SiS Emission Tracing an Embedded Planet and Compact CO and CO Counterparts in the HD 169142 Disk
Planets form in dusty, gas-rich disks around young stars, while at the same
time, the planet formation process alters the physical and chemical structure
of the disk itself. Embedded planets will locally heat the disk and sublimate
volatile-rich ices, or in extreme cases, result in shocks that sputter heavy
atoms such as Si from dust grains. This should cause chemical asymmetries
detectable in molecular gas observations. Using high-angular-resolution ALMA
archival data of the HD 169142 disk, we identify compact SO J=8-7 and
SiS J=19-18 emission coincident with the position of a 2 M
planet seen as a localized, Keplerian NIR feature within a gas-depleted,
annular dust gap at 38 au. The SiS emission is located along an
azimuthal arc and has a similar morphology as a known CO kinematic
excess. This is the first tentative detection of SiS emission in a
protoplanetary disk and suggests that the planet is driving sufficiently strong
shocks to produce gas-phase SiS. We also report the discovery of compact
CO and CO J=3-2 emission coincident with the planet location.
Taken together, a planet-driven outflow provides the best explanation for the
properties of the observed chemical asymmetries. We also resolve a bright,
azimuthally-asymmetric SO ring at 24 au. While most of this SO
emission originates from ice sublimation, its asymmetric distribution implies
azimuthal temperature variations driven by a misaligned inner disk or
planet-disk interactions. Overall, the HD 169142 disk shows several distinct
chemical signatures related to giant planet formation and presents a powerful
template for future searches of planet-related chemical asymmetries in
protoplanetary disks.Comment: 22 pages, 12 figures, accepted for publication in ApJ
The ancient heritage of water ice in the solar system
Identifying the source of Earth's water is central to understanding the
origins of life-fostering environments and to assessing the prevalence of such
environments in space. Water throughout the solar system exhibits
deuterium-to-hydrogen enrichments, a fossil relic of low-temperature,
ion-derived chemistry within either (i) the parent molecular cloud or (ii) the
solar nebula protoplanetary disk. Utilizing a comprehensive treatment of disk
ionization, we find that ion-driven deuterium pathways are inefficient,
curtailing the disk's deuterated water formation and its viability as the sole
source for the solar system's water. This finding implies that if the solar
system's formation was typical, abundant interstellar ices are available to all
nascent planetary systems.Comment: 33 pages, 7 figures including main text and supplementary materials.
Published in Scienc
Exploring the Origins of Deuterium Enrichments in Solar Nebular Organics
Deuterium-to-hydrogen (D/H) enrichments in molecular species provide clues
about their original formation environment. The organic materials in primitive
solar system bodies have generally higher D/H ratios and show greater D/H
variation when compared to D/H in solar system water. We propose this
difference arises at least in part due to 1) the availability of additional
chemical fractionation pathways for organics beyond that for water, and 2) the
higher volatility of key carbon reservoirs compared to oxygen. We test this
hypothesis using detailed disk models, including a sophisticated, new disk
ionization treatment with a low cosmic ray ionization rate, and find that disk
chemistry leads to higher deuterium enrichment in organics compared to water,
helped especially by fractionation via the precursors CHD/CH. We
also find that the D/H ratio in individual species varies significantly
depending on their particular formation pathways. For example, from
AU, CH can reach , while D/H in CHOH
remains locally unaltered. Finally, while the global organic D/H in our models
can reproduce intermediately elevated D/H in the bulk hydrocarbon reservoir,
our models are unable to reproduce the most deuterium-enriched organic
materials in the solar system, and thus our model requires some inheritance
from the cold interstellar medium from which the Sun formed.Comment: 11 pages, 7 figures, accepted for publication in Ap
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