6,079 research outputs found
Photodissociation and photoionisation of atoms and molecules of astrophysical interest
A new collection of photodissociation and photoionisation cross sections for
102 atoms and molecules of astrochemical interest has been assembled, along
with a brief review of the basic processes involved. These have been used to
calculate dissociation and ionisation rates, with uncertainties, in a standard
ultraviolet interstellar radiation field (ISRF) and wavelength-dependent
radiation fields. The new ISRF rates generally agree within 30% with our
previous compilations, with a few notable exceptions. The reduction of rates in
shielded regions was calculated as a function of dust, molecular and atomic
hydrogen, atomic C, and self-shielding column densities. The relative
importance of shielding types depends on the species in question and the dust
optical properties. The new data are publicly available from the Leiden
photodissociation and ionisation database.
Sensitivity of rates to variation of temperature and isotope, and cross
section uncertainties, are tested. Tests were conducted with an
interstellar-cloud chemical model, and find general agreement (within a factor
of two) with the previous iteration of the Leiden database for the ISRF, and
order-of-magnitude variations assuming various kinds of stellar radiation. The
newly parameterised dust-shielding factors makes a factor-of-two difference to
many atomic and molecular abundances relative to parameters currently in the
UDfA and KIDA astrochemical reaction databases. The newly-calculated cosmic-ray
induced photodissociation and ionisation rates differ from current standard
values up to a factor of 5. Under high temperature and cosmic-ray-flux
conditions the new rates alter the equilibrium abundances of abundant dark
cloud abundances by up to a factor of two. The partial cross sections for H2O
and NH3 photodissociation forming OH, O, NH2 and NH are also evaluated and lead
to radiation-field-dependent branching ratios.Comment: Corrected some inconsistent table/figure data. Significant change: Zn
photoionisation rate corrected. Accepted for publication by A&
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Efficiency of radial transport of ices in protoplanetary disks probed with infrared observations: the case of CO
The efficiency of radial transport of icy solid material from outer disk to
the inner disk is currently unconstrained. Efficient radial transport of icy
dust grains could significantly alter the composition of the gas in the inner
disk. Our aim is to model the gaseous CO abundance in the inner disk and
use this to probe the efficiency of icy dust transport in a viscous disk.
Features in the simulated CO spectra are investigated for their dust flux
tracing potential. We have developed a 1D viscous disk model that includes gas
and grain motions as well as dust growth, sublimation and freeze-out and a
parametrisation of the CO chemistry. The thermo-chemical code DALI was used
to model the mid-infrared spectrum of CO, as can be observed with
JWST-MIRI. CO ice sublimating at the iceline increases the gaseous CO
abundance to levels equal to the CO ice abundance of , which
is three orders of magnitude more than the gaseous CO abundances of observed by Spitzer. Grain growth and radial drift further increase
the gaseous CO abundance. A CO destruction rate of at least
s is needed to reconcile model prediction with observations. This rate
is at least two orders of magnitude higher than the fastest known chemical
destruction rate. A range of potential physical mechanisms to explain the low
observed CO abundances are discussed. Transport processes in disks can have
profound effects on the abundances of species in the inner disk. The
discrepancy between our model and observations either suggests frequent shocks
in the inner 10 AU that destroy CO, or that the abundant midplane CO is
hidden from our view by an optically thick column of low abundance CO in to
the disk surface XDR/PDR. Other molecules, such as CH or NH, can give
further handles on the rate of mass transport.Comment: Accepted for publication in A&A, 18 pages, 13 figures, abstract
abridge
The dry and carbon poor inner disk of TW Hya: evidence for a massive icy dust trap
Gas giants accrete their envelopes from the gas and dust of proto-planetary
disks, so it is important to determine the composition of the inner few AU,
where most giant planets are expected to form. We aim to constrain the
elemental carbon and oxygen abundance in the inner disk (2.3 AU) of TW Hya
and compare with the outer disk ( AU) where carbon and oxygen appear
underabundant by a factor of 50. Archival infrared observations of TW Hya
are compared with a detailed thermo-chemical model, DALI. The inner disk gas
mass and elemental C and O abundances are varied to fit the infrared CO, H
and HO line fluxes. Best fitting models have an inner disk that has a gas
mass of with C/H and O/H
. The elemental oxygen and carbon abundances of the
inner disk are times underabundant compared to the ISM and are
consistent with those found in the outer disk. The uniformly low volatile
abundances imply that the inner disk is not enriched by ices on drifting bodies
that evaporate. This indicates that drifting grains are stopped in a dust trap
outside the water ice line. Such a dust trap would also form a cavity as seen
in high resolution sub-millimeter continuum observations. If CO is the major
carbon carrier in the ices, dust needs to be trapped efficiently outside the CO
ice line of 20 AU. This would imply that the shallow sub-millimeter rings
in the TW Hya disk outside of 20 AU correspond to very efficient dust traps.
The more likely scenario is that more than 98\% of the CO has been converted
into less volatile species, e.g. CO and CHOH. A giant planet forming in
the inner disk would be accreting gas with low carbon and oxygen abundances as
well as very little icy dust, potentially leading to a planet atmosphere with
strongly substellar C/H and O/H ratios.Comment: 6 pages, 3 figures, accepted to A&A letter
Reionization history constraints from neural network based predictions of high-redshift quasar continua
Observations of the early Universe suggest that reionization was complete by
, however, the exact history of this process is still unknown. One
method for measuring the evolution of the neutral fraction throughout this
epoch is via observing the Ly damping wings of high-redshift quasars.
In order to constrain the neutral fraction from quasar observations, one needs
an accurate model of the quasar spectrum around Ly, after the spectrum
has been processed by its host galaxy but before it is altered by absorption
and damping in the intervening IGM. In this paper, we present a novel machine
learning approach, using artificial neural networks, to reconstruct quasar
continua around Ly. Our QSANNdRA algorithm improves the error in this
reconstruction compared to the state-of-the-art PCA-based model in the
literature by 14.2% on average, and provides an improvement of 6.1% on average
when compared to an extension thereof. In comparison with the extended PCA
model, QSANNdRA further achieves an improvement of 22.1% and 16.8% when
evaluated on low-redshift quasars most similar to the two high-redshift quasars
under consideration, ULAS J1120+0641 at and ULAS J1342+0928 at
, respectively. Using our more accurate reconstructions of these two
quasars, we estimate the neutral fraction of the IGM using a homogeneous
reionization model and find at
and at . Our
results are consistent with the literature and favour a rapid end to
reionization
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