2,563 research outputs found
Mass inventory of the giant-planet formation zone in a solar nebula analog
The initial mass distribution in the solar nebula is a critical input to
planet formation models that seek to reproduce today's Solar System.
Traditionally, constraints on the gas mass distribution are derived from
observations of the dust emission from disks, but this approach suffers from
large uncertainties in grain growth and gas-to-dust ratio. On the other hand,
previous observations of gas tracers only probe surface layers above the bulk
mass reservoir. Here we present the first partially spatially resolved
observations of the CO J=3-2 line emission in the closest
protoplanetary disk, TW Hya, a gas tracer that probes the bulk mass
distribution. Combining it with the CO J=3-2 emission and the previously
detected HD J=1-0 flux, we directly constrain the mid-plane temperature and
optical depths of gas and dust emission. We report a gas mass distribution of
13(R/20.5AU) g cm in the
expected formation zone of gas and ice giants (5-21AU). We find the total
gas/millimeter-sized dust mass ratio is 140 in this region, suggesting that at
least 2.4M_earth of dust aggregates have grown to >centimeter sizes (and
perhaps much larger). The radial distribution of gas mass is consistent with a
self-similar viscous disk profile but much flatter than the posterior
extrapolation of mass distribution in our own and extrasolar planetary systems.Comment: Definitive version of the manuscript is published in Nature
Astronomy, 10.1038/s41550-017-0130. This is the authors' versio
Direct Identification of Acetaldehyde Formation and Characterization of the Active Site in the [VPO4].+/C2H4 Couple by GasâPhase Vibrational Spectroscopy
The gasâphase reaction of the heteronuclear oxide cluster [VPO4].+ with C2H4 is studied under multiple collision conditions at 150â
K using cryogenic ionâtrap vibrational spectroscopy combined with electronic structure calculations. The exclusive formation of acetaldehyde is directly identified spectroscopically and discussed in the context of the underlying reaction mechanism. In line with computational predictions it is the terminal P=O and not the V=O unit that provides the oxygen atom in the barrierâfree thermal C2H4âCH3CHO conversion. Interestingly, in the course of the reaction, the emerging CH3CHO product undergoes a rather complex intramolecular migration, coordinating eventually to the vanadium center prior to its liberation. Moreover, the spectroscopic structural characterization of neutral C2H4O deserves special mentioning as in most, if not all, ion/molecule reactions, the neutral product is usually only indirectly identified.DFG, 390540038, EXC 2008: UniSysCatDFG, 234149247, SFB 1109: Molekulare Einblicke in Metalloxid-Wasser-Systeme: Strukturelle Evolution, GrenzflĂ€chen und AuflösungTU Berlin, Open-Access-Mittel - 201
Hepatoma cell density promotes claudin-1 and scavenger receptor BI expression and hepatitis C virus internalization.
Hepatitis C virus (HCV) entry occurs via a pH- and clathrin-dependent endocytic pathway and requires a number of cellular factors, including CD81, the tight-junction proteins claudin 1 (CLDN1) and occludin, and scavenger receptor class B member I (SR-BI). HCV tropism is restricted to the liver, where hepatocytes are tightly packed. Here, we demonstrate that SR-BI and CLDN1 expression is modulated in confluent human hepatoma cells, with both receptors being enriched at cell-cell junctions. Cellular contact increased HCV pseudoparticle (HCVpp) and HCV particle (HCVcc) infection and accelerated the internalization of cell-bound HCVcc, suggesting that the cell contact modulation of receptor levels may facilitate the assembly of receptor complexes required for virus internalization. CLDN1 overexpression in subconfluent cells was unable to recapitulate this effect, whereas increased SR-BI expression enhanced HCVpp entry and HCVcc internalization, demonstrating a rate-limiting role for SR-BI in HCV internalization
Rapid Evolution of Volatile CO from the Protostellar Disk Stage to the Protoplanetary Disk Stage
Recent observations show that the CO gas abundance, relative to H, in
many 1-10 Myr old protoplanetary disks may be heavily depleted, by a factor of
10-100 compared to the canonical interstellar medium value of 10. When
and how this depletion happens can significantly affect compositions of
planetesimals and atmospheres of giant planets. It is therefore important to
constrain if the depletion occurs already at the earliest protostellar disk
stage. Here we present spatially resolved observations of CO, CO,
and CO =2-1 lines in three protostellar disks. We show that
the CO line emits from both the disk and the inner envelope, while
CO and CO lines are consistent with a disk origin. The
line ratios indicate that both CO and CO lines are optically
thick in the disk region, and only CO line is optically thin. The
line profiles of the CO emissions are best reproduced by
Keplerian gaseous disks at similar sizes as their mm-continuum emissions,
suggesting small radial separations between the gas and mm-sized grains in
these disks, in contrast to the large separation commonly seen in
protoplanetary disks. Assuming a gas-to-dust ratio of 100, we find that the CO
gas abundances in these protostellar disks are consistent with the ISM
abundance within a factor of 2, nearly one order of magnitude higher than the
average value of 1-10 Myr old disks. These results suggest that there is a
fast, 1 Myr, evolution of the abundance of CO gas from the protostellar
disk stage to the protoplanetary disk stage.Comment: 10 pages, 3 figures, 2 tables. Accepted for publication in ApJ
Unlocking CO Depletion in Protoplanetary Disks II. Primordial C/H Predictions Inside the CO Snowline
CO is thought to be the main reservoir of volatile carbon in protoplanetary
disks, and thus the primary initial source of carbon in the atmospheres of
forming giant planets. However, recent observations of protoplanetary disks
point towards low volatile carbon abundances in many systems, including at
radii interior to the CO snowline. One potential explanation is that gas phase
carbon is chemically reprocessed into less volatile species, which are frozen
on dust grain surfaces as ice. This mechanism has the potential to change the
primordial C/H ratio in the gas. However, current observations primarily probe
the upper layers of the disk. It is not clear if the low volatile carbon
abundances extend to the midplane, where planets form. We have run a grid of
198 chemical models, exploring how the chemical reprocessing of CO depends on
disk mass, dust grain size distribution, temperature, cosmic ray and X-ray
ionization rate, and initial water abundance. Building on our previous work
focusing on the warm molecular layer, here we analyze the results for our grid
of models in the disk midplane at 12 au. We find that either an ISM level
cosmic ray ionization rate or the presence of UV photons due to a low dust
surface density are needed to chemically reduce the midplane CO gas abundance
by at least an order of magnitude within 1 Myr. In the majority of our models
CO does not undergo substantial reprocessing by in situ chemistry and there is
little change in the gas phase C/H and C/O ratios over the lifetime of the
typical disk. However, in the small sub-set of disks where the disk midplane is
subject to a source of ionization or photolysis, the gas phase C/O ratio
increases by up to nearly 9 orders of magnitude due to conversion of CO into
volatile hydrocarbons.Comment: Accepted for publication in ApJ, 15 pages, 10 figures, 3 table
Systematic Variations of CO Gas Abundance with Radius in Gas-rich Protoplanetary Disks
CO is the most widely used gas tracer of protoplanetary disks. Its abundance
is usually assumed to be an interstellar ratio throughout the warm molecular
layer of the disk. But recent observations of low CO gas abundance in many
protoplanetary disks challenge our understanding of physical and chemical
evolutions in disks. Here we investigate the CO abundance structures in four
well-studied disks and compare their structures with predictions of chemical
processing of CO and transport of CO ice-coated dust grains in disks. We use
spatially resolved CO isotopologue line observations and detailed
thermo-chemical models to derive CO abundance structures. We find that the CO
abundance varies with radius by an order of magnitude in these disks. We show
that although chemical processes can efficiently reduce the total column of CO
gas within 1 Myr under an ISM level of cosmic-ray ionization rate, the
depletion mostly occurs at the deep region of a disk. Without sufficient
vertical mixing, the surface layer is not depleted enough to reproduce weak CO
emissions observed. The radial profiles of CO depletion in three disks are
qualitatively consistent with predictions of pebble formation, settling, and
drifting in disks. But the dust evolution alone cannot fully explain the high
depletion observed in some disks. These results suggest that dust evolution may
play a significant role in transporting volatile materials and a coupled
chemical-dynamical study is necessary to understand what raw materials are
available for planet formation at different distances from the central star.Comment: 17 pages, 8 figures, accepted for publication in the Ap
CO Depletion in Protoplanetary Disks: A Unified Picture Combining Physical Sequestration and Chemical Processing
The gas-phase CO abundance (relative to hydrogen) in protoplanetary disks
decreases by up to 2 orders of magnitude from its ISM value ,
even after accounting for freeze-out and photo-dissociation. Previous studies
have shown that while local chemical processing of CO and the sequestration of
CO ice on solids in the midplane can both contribute, neither of these
processes appears capable of consistently reaching the observed depletion
factors on the relevant timescale of . In this study, we
model these processes simultaneously by including a compact chemical network
(centered on carbon and oxygen) to 2D () simulations of the outer
() disk regions that include turbulent diffusion, pebble
formation, and pebble dynamics. In general, we find that the CO/H abundance
is a complex function of time and location. Focusing on CO in the warm
molecular layer, we find that only the most complete model (with chemistry and
pebble evolution included) can reach depletion factors consistent with
observations. In the absence of pressure traps, highly-efficient planetesimal
formation, or high cosmic ray ionization rates, this model also predicts a
resurgence of CO vapor interior to the CO snowline. We show the impact of
physical and chemical processes on the elemental (C/O) and (C/H) ratios (in the
gas and ice phases), discuss the use of CO as a disk mass tracer, and, finally,
connect our predicted pebble ice compositions to those of pristine
planetesimals as found in the Cold Classical Kuiper Belt and debris disks.Comment: Accepted for publication in The Astrophysical Journa
On the Commonality of 10-30AU Sized Axisymmetric Dust Structures in Protoplanetary Disks
An unsolved problem in step-wise core-accretion planet formation is that rapid radial drift in gas-rich protoplanetary disks should drive millimeter-/meter-sized particles inward to the central star before large bodies can form. One promising solution is to confine solids within small-scale structures. Here, we investigate dust structures in the (sub)millimeter continuum emission of four disks (TW Hya, HL Tau, HD 163296, and DM Tau), a sample of disks with the highest spatial resolution Atacama Large Millimeter/submillimeter Array observations to date. We retrieve the surface brightness distributions using synthesized images and fitting visibilities with analytical functions. We find that the continuum emission of the four disks is ~axisymmetric but rich in 10â30 AU-sized radial structures, possibly due to physical gaps, surface density enhancements, or localized dust opacity variations within the disks. These results suggest that small-scale axisymmetric dust structures are likely to be common, as a result of ubiquitous processes in disk evolution and planet formation. Compared with recent spatially resolved observations of CO snow lines in these same disks, all four systems show enhanced continuum emission from regions just beyond the CO condensation fronts, potentially suggesting a causal relationship between dust growth/trapping and snow lines
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