23 research outputs found
Circumstellar chemistry
The study of the outer envelopes of cool evolved stars has become an active area of research. The physical properties of CS envelopes are presented. Observations of many wavelengths bands are relevant. A summary of observations and a discussion of theoretical considerations concerning the chemistry are summarized. Recent theoretical considerations show that the thermal equilibrium model is of limited use for understanding the chemistry of the outer CS envelopes. The theoretical modeling of the chemistry of CS envelopes provides a quantitive test of chemical concepts which have a broader interest than the envelopes themselves
X-ray Ionization of Heavy Elements Applied to Protoplanetary Disks
The consequences of the Auger effect on the population of heavy-element ions are analyzed for the case of relatively cool gas irradiated by keV X-rays with intended applications to the accretion disks of young stellar objects. Highly charged ions are rapidly reduced to the doubly charged state in neutral gas, so the aim here is to derive the production rates for these singly and doubly charged ions and to specify their transformation by recombination, charge transfer, and molecular reactions. The theory is illustrated by calculations of the abundance
Fine-Structure Line Emission from the Outflows of Young Stellar Objects
The flux and line shape of the fine-structure transitions of \NeII\ and
\NeIII\ at 12.8 and 15.55\,m and of the forbidden transitions of \OI\
are calculated for young stellar objects with a range of
mass-loss rates and X-ray luminosities using the X-wind model of jets and the
associated wide-angle winds. For moderate and high accretion rates, the
calculated \NeII\ line luminosity is comparable to or much larger than produced
in X-ray irradiated disk models. All of the line luminosities correlate well
with the main parameter in the X-wind model, the mass-loss rate, and also with
the assumed X-ray luminosity --- and with one another. The line shapes of an
approaching jet are broad and have strong blue-shifted peaks near the effective
terminal velocity of the jet. They serve as a characteristic and testable
aspect of jet production of the neon fine-structure lines and the \OI\
forbidden transitions.Comment: 8 pages, 5 figures, published in Ap
Formation of Organic Molecules and Water in Warm Disk Atmospheres
Observations from Spitzer and ground-based infrared spectroscopy reveal
significant diversity in the molecular emission from the inner few AU of T
Tauri disks. We explore theoretically the possible origin of this diversity by
expanding on our earlier thermal-chemical model of disk atmospheres. We
consider how variations in grain settling, X-ray irradiation, accretion-related
mechanical heating, and the oxygen-to-carbon ratio can affect the thermal and
chemical properties of the atmosphere at 0.25-40 AU. We find that these model
parameters can account for many properties of the detected molecular emission.
The column density of the warm (200-2000K) molecular atmosphere is sensitive to
grain settling and the efficiency of accretion-related heating, which may
account, at least in part, for the large range in molecular emission fluxes
that have been observed. The dependence of the atmospheric properties on the
model parameters may also help to explain trends that have been reported in the
literature between molecular emission strength and mid-infrared color, stellar
accretion rate, and disk mass. We discuss whether some of the differences
between our model results and the observations (e.g., for water) indicate a
role for vertical transport and freeze-out in the disk midplane. We also
discuss how planetesimal formation in the outer disk (beyond the snowline) may
imprint a chemical signature on the inner few AU of the disk and speculate on
possible observational tracers of this process.Comment: 5 figures, accepted for publication in Ap
Neon Fine-Structure Line Emission By X-ray Irradiated Protoplanetary Disks
Using a thermal-chemical model for the generic T-Tauri disk of D'Alessio et
al. (1999), we estimate the strength of the fine-structure emission lines of
NeII and NeIII at 12.81 and 15.55 microns that arise from the warm atmosphere
of the disk exposed to hard stellar X-rays. The Ne ions are produced by the
absorption of keV X-rays from the K shell of neutral Ne, followed by the Auger
ejection of several additional electrons. The recombination cascade of the Ne
ions is slow because of weak charge transfer with atomic hydrogen in the case
of Ne2+ and by essentially no charge transfer for Ne+. For a distance of 140pc,
the 12.81 micron line of Ne II has a flux of 1e-14 erg/cm2s, which should be
observable with the Spitzer Infrared Spectrometer and suitable ground based
instrumentation. The detection of these fine-structure lines would clearly
demonstrate the effects of X-rays on the physical and chemical properties of
the disks of young stellar objects and provide a diagnostic of the warm gas in
protoplanetary disk atmospheres. They would complement the observed H2 and CO
emission by probing vertical heights above the molecular transition layer and
larger radial distances that include the location of terrestrial and giant
planets.Comment: 24 pages, 5 figure
Cosmic-ray and X-ray Heating of Interstellar Clouds and Protoplanetary Disks
Cosmic-ray and X-ray heating are derived from the electron energy loss
calculations of Dalgarno, Yan and Liu for hydrogen-helium gas mixtures. These
authors treated the heating from elastic scattering and collisional
de-excitation of rotationally excited hydrogen molecules. Here we consider the
heating that can arise from all ionization and excitation processes, with
particular emphasis on the reactions of cosmic-ray and X-ray generated ions
with the heavy neutral species, which we refer to as chemical heating. In
molecular regions, chemical heating dominates and can account for 50 per cent
of the energy expended in the creation of an ion pair. The heating per ion pair
ranges in the limit of negligible electron fraction from about 4.3 eV for
diffuse atomic gas, to about 13 eV for the moderately dense regions of
molecular clouds and to about 18 eV for the very dense regions of
protoplanetary disks. An important general conclusion of this study is that
cosmic-ray and X-ray heating depends on the physical properties of the medium,
i.e., on the molecular and electron fractions, the total density of hydrogen
nuclei, and to a lesser extent on the temperature. It is also noted that
chemical heating, the dominant process for cosmic-ray and X-ray heating, plays
a role in UV irradiated molecular gas.Comment: 39 pages, accepted for publication in the Astrophysical Journa