147 research outputs found
A Coupled Dynamical and Chemical Model of Starless Cores of Magnetized Molecular Clouds: I. Formulation and Initial Results
We develop a detailed chemical model for the starless cores of strongly
magnetized molecular clouds, with the ambipolar diffusion-driven dynamic
evolution of the clouds coupled to the chemistry through ion abundances. We
concentrate on two representative model clouds in this initial study, one with
magnetic fields and the other without. The model predictions on the peak values
and spatial distributions of the column densities of CO, CCS, NH and
HCO are compared with those observationally inferred for the well-studied
starless core L1544, which is thought to be on the verge of star formation. We
find that the magnetic model, in which the cloud is magnetically supported for
several million years before collapsing dynamically, provides a reasonable
overall fit to the available data on L1544; the fit is significantly worse for
the non-magnetic model, in which the cloud collapses promptly. The observed
large peak column density for NH and clear central depression for CCS
favor the magnetically-retarded collapse over the free-fall collapse. A
relatively high abundance of CCS is found in the magnetic model, resulting most
likely from an interplay of depletion and late-time hydrocarbon chemistry
enhanced by CO depletion. These initial results lend some support to the
standard picture of dense core formation in strongly magnetized clouds through
ambipolar diffusion. They are at variance with those of Aikawa et al. (2001)
who considered a set of models somewhat different from ours and preferred one
in which the cloud collapses more or less freely for L1544.Comment: 25 pages, 7 figures, accepted to Ap
Dust Emission from Herbig Ae/Be stars - Evidence for Disks and Envelopes
IR and mm-wave emission from Herbig Ae/Be stars has produced conflicting
conclusions regarding the dust geometry in these objects. We show that the
compact dimensions of the mm-wave emitting regions are a decisive indication
for disks. But a disk cannot explain the spectral energy distribution (SED)
unless it is embedded in an extended envelope that (1) dominates the IR
emission and (2) provides additional disk heating on top of the direct stellar
radiation. Detailed radiative transfer calculations based on the simplest model
for envelope-embedded disks successfully fit the data from UV to mm wavelengths
and show that the disks have central holes. This model also resolves naturally
some puzzling results of IR imaging.Comment: 9 pages, 2 figures. accepted to ApJ
Energetic oxygen atoms in the polar geocorona
peer reviewedThe role of the auroral sources induced by the electron and proton precipitation in the formation of the hot oxygen corona in the polar upper atmosphere is studied. It is found that both electron precipitation through exothermic chemistry and proton precipitation through atmospheric sputtering significantly contribute to the population of the hot oxygen geocorona. It is also found that only atmospheric sputtering results in the formation of the escape flux of energetic oxygen atoms, providing an important source of heavy atoms for the magnetosphere. The exothermic chemistry induced by the electron precipitation and/or by the absorption of the solar UV radiation is operating continuously in the polar upper atmosphere and results in a steady population of the very near-Earth environment by suprathermal oxygen atoms with energies below a few eV. By contrast, atmospheric sputtering by magnetospheric protons provides a more variable contribution, strongly coupled with the cusp region. It produces the more energetic oxygen atoms that populate the external regions of the hot oxygen geocorona. The results of calculations are in a good agreement with the analysis of the low-latitude perigee Low Energy Neutral Atom (LENA) images showing that the instrument signal consists of low to medium energy (5-30 eV) oxygen atoms produced in and near the cusp region. The more energetic (>30 eV) fraction of energetic oxygen atoms produced by the ion-induced atmospheric sputtering could be responsible for the energetic neutrals observed by the instrument far away from the cusp or oval regions. The total escape flux of oxygen atoms associated with atmospheric sputtering by protons is found about 8 × 10[SUP]23[/SUP] s[SUP]-1[/SUP] therefore this mechanism may provide a substantial contribution to the magnetospheric oxygen population
On the master equation approach to diffusive grain-surface chemistry: the H, O, CO system
We have used the master equation approach to study a moderately complex
network of diffusive reactions occurring on the surfaces of interstellar dust
particles. This network is meant to apply to dense clouds in which a large
portion of the gas-phase carbon has already been converted to carbon monoxide.
Hydrogen atoms, oxygen atoms, and CO molecules are allowed to accrete onto dust
particles and their chemistry is followed. The stable molecules produced are
oxygen, hydrogen, water, carbon dioxide (CO2), formaldehyde (H2CO), and
methanol (CH3OH). The surface abundances calculated via the master equation
approach are in good agreement with those obtained via a Monte Carlo method but
can differ considerably from those obtained with standard rate equations.Comment: 13 pages, 5 figure
An inversion method for cometary atmospheres
Remote observation of cometary atmospheres produces a measurement of the cometary emissions integrated along the line of sight. This integration is the so-called Abel transform of the local emission rate. The observation is generally interpreted under the hypothesis of spherical symmetry of the coma. Under that hypothesis, the Abel transform can be inverted. We derive a numerical inversion method adapted to cometary atmospheres using both analytical results and least squares fitting techniques. This method, derived under the usual hypothesis of spherical symmetry, allows us to retrieve the radial distribution of the emission rate of any unabsorbed emission, which is the fundamental, physically meaningful quantity governing the observation. A Tikhonov regularization technique is also applied to reduce the possibly deleterious effects of the noise present in the observation and to warrant that the problem remains well posed. Standard error propagation techniques are included in order to estimate the uncertainties affecting the retrieved emission rate. Several theoretical tests of the inversion techniques are carried out to show its validity and robustness. In particular, we show that the Abel inversion of real data is only weakly sensitive to an offset applied to the input flux, which implies that the method, applied to the study of a cometary atmosphere, is only weakly dependent on uncertainties on the sky background which has to be subtracted from the raw observations of the coma. We apply the method to observations of three different comets observed using the TRAPPIST telescope: 103P/ Hartley 2, F6/ Lemmon and A1/ Siding Spring. We show that the method retrieves realistic emission rates, and that characteristic lengths and production rates can be derived from the emission rate for both CN and C2 molecules. We show that the retrieved characteristic lengths can differ from those obtained from a direct least squares fitting over the observed flux of radiation, and that discrepancies can be reconciled for by correcting this flux by an offset (to which the inverse Abel transform is nearly not sensitive). The A1/Siding Spring observations were obtained very shortly after the comet produced an outburst, and we show that the emission rate derived from the observed flux of CN emission at 387 nm and from the C2 emission at 514.1 nm both present an easily-identifiable shoulder that corresponds to the separation between pre- and post-outburst gas. As a general result, we show that diagnosing properties and features of the coma using the emission rate is easier than directly using the observed flux, because the Abel transform produces a smoothing that blurs the signatures left by features present in the coma. We also determine the parameters of a Haser model fitting the inverted data and fitting the line-of-sight integrated observation, for which we provide the exact analytical expression of the line-of-sight integration of the Haser model
Estimates of non-thermal atmospheric loss of exoplanet GJ 436b due to dissociation processes H2
Оценен вклад процессов диссоциации молекулярного водорода жестким ультрафиолетовым (УФ) излучением и сопутствующим потоком фотоэлектронов в образование фракции надтеплового атомарного водорода в переходной H2 −→ H области и формирование нетеплового потока убегания из протяженной верхней атмосферы экзопланеты — горячего нептуна GJ 436b. Рассчитаны скорость образования и энергетический спектр атомов водорода, образующихся с избытком кинетической энергии при диссоциации H2.The contribution of the processes of dissociation of molecular hydrogen by hard ultraviolet (UV) radiation and the accompanying flux of photoelectrons to the formation of the fraction of suprathermal atomic hydrogen in the transition H2 −→ H region and the formation of the non-thermal escape flux from the extended upper atmosphere of the exoplanet — hot neptune GJ 436b — is estimated. The rate of formation and the energy spectrum of hydrogen atoms formed with an excess of kinetic energy during the dissociation of H2 are calculated.Исследование выполнено в рамках проекта № 075-15-2020-780 «Теоретические и экспериментальные исследования формирования и эволюции внесолнечных планетных систем и характеристик экзопланет» Министерства науки и высшего образования РФ
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