130 research outputs found
Cooling of Dense Gas by H2O Line Emission and an Assessment of its Effects in Chondrule-Forming Shocks
We consider gas at densities appropriate to protoplanetary disks and
calculate its ability to cool due to line radiation emitted by H2O molecules
within the gas. Our work follows that of Neufeld & Kaufman (1993; ApJ, 418,
263), expanding on their work in several key aspects, including use of a much
expanded line database, an improved escape probability formulism, and the
inclusion of dust grains, which can absorb line photons. Although the escape
probabilities formally depend on a complicated combination of optical depth in
the lines and in the dust grains, we show that the cooling rate including dust
is well approximated by the dust-free cooling rate multiplied by a simple
function of the dust optical depth. We apply the resultant cooling rate of a
dust-gas mixture to the case of a solar nebula shock pertinent to the formation
of chondrules, millimeter-sized melt droplets found in meteorites. Our aim is
to assess whether line cooling can be neglected in chondrule-forming shocks or
if it must be included. We find that for typical parameters, H2O line cooling
shuts off a few minutes past the shock front; line photons that might otherwise
escape the shocked region and cool the gas will be absorbed by dust grains.
During the first minute or so past the shock, however, line photons will cool
the gas at rates ~ 10,000 K/hr, dropping the temperature of the gas (and most
likely the chondrules within the gas) by several hundred K. Inclusion of H2O
line cooling therefore must be included in models of chondrule formation by
nebular shocks.Comment: Accepted for publication in The Astrophysical Journa
The shape and composition of interstellar silicate grains
We investigate the composition and shape distribution of silicate dust grains
in the interstellar medium. The effect of the amount of magnesium in the
silicate lattice is studied. We fit the spectral shape of the interstellar 10
mu extinction feature as observed towards the galactic center. We use very
irregularly shaped coated and non-coated porous Gaussian Random Field particles
as well as a statistical approach to model shape effects. For the dust
materials we use amorphous and crystalline silicates with various composition
and SiC. The results of our analysis of the 10 mu feature are used to compute
the shape of the 20 mu silicate feature and to compare this with observations.
By using realistic particle shapes we are, for the first time, able to derive
the magnesium fraction in interstellar silicates. We find that the interstellar
silicates are highly magnesium rich (Mg/(Fe+Mg)>0.9) and that the stoichiometry
lies between pyroxene and olivine type silicates. This composition is not
consistent with that of the glassy material found in GEMS in interplanetary
dust particles indicating that these are, in general, not unprocessed remnants
from the interstellar medium. Also, we find a significant fraction of SiC
(~3%). We discuss the implications of our results for the formation and
evolutionary history of cometary and circumstellar dust. We argue that the fact
that crystalline silicates in cometary and circumstellar grains are almost
purely magnesium silicates is a natural consequence of our findings that the
amorphous silicates from which they were formed were already magnesium rich.Comment: Accepted for publication in A&
Dust and the spectral energy distribution of the OH/IR star OH 127.8+0.0: Evidence for circumstellar metallic iron
We present a fit to the spectral energy distribution of OH 127.8+0.0, a
typical asymptotic giant branch star with an optically thick circumstellar dust
shell. The fit to the dust spectrum is achieved using non-spherical grains
consisting of metallic iron, amorphous and crystalline silicates and water ice.
Previous similar attempts have not resulted in a satisfactory fit to the
observed spectral energy distributions, mainly because of an apparent lack of
opacity in the 3--8 micron region of the spectrum. Non-spherical metallic iron
grains provide an identification for the missing source of opacity in the
near-infrared. Using the derived dust composition, we have calculated spectra
for a range of mass-loss rates in order to perform a consistency check by
comparison with other evolved stars. The L-[12 micron] colours of these models
correctly predict the mass-loss rate of a sample of AGB stars, strengthening
our conclusion that the metallic iron grains dominate the near-infrared flux.
We discuss a formation mechanism for non-spherical metallic iron grains.Comment: 10 pages, 6 figures, accepted for publication by A&
The effect of the regular solution model in the condensation of protoplanetary dust
We utilize a chemical equilibrium code in order to study the condensation
process which occurs in protoplanetary discs during the formation of the first
solids. The model specifically focuses on the thermodynamic behaviour on the
solid species assuming the regular solution model. For each solution, we
establish the relationship between the activity of the species, the composition
and the temperature using experimental data from the literature. We then apply
the Gibbs free energy minimization method and study the resulting condensation
sequence for a range of temperatures and pressures within a protoplanetary
disc. Our results using the regular solution model show that grains condense
over a large temperature range and therefore throughout a large portion of the
disc. In the high temperature region (T > 1400 K) Ca-Al compounds dominate and
the formation of corundum is sensitive to the pressure. The mid-temperature
region is dominated by Fe(s) and silicates such as Mg2SiO4 and MgSiO3 . The
chemistry of forsterite and enstatite are strictly related, and our simulations
show a sequence of forsterite-enstatite-forsterite with decreasing temperature.
In the low temperature regions (T < 600 K) a range of iron compounds and
sulfides form. We also run simulations using the ideal solution model and see
clear differences in the resulting condensation sequences with changing
solution model In particular, we find that the turning point in which
forsterite replaces enstatite in the low temperature region is sensitive to the
solution model. Our results show that the ideal solution model is often a poor
approximation to experimental data at most temperatures important in
protoplanetary discs. We find some important differences in the resulting
condensation sequences when using the regular solution model, and suggest that
this model should provide a more realistic condensation sequence.Comment: MNRAS: Accepted 2011 February 16. Received 2011 February 14; in
original form 2010 July 2
Invasive Disease Caused by Nontuberculous Mycobacteria, Tanzania
Data on nontuberculous mycobacterial (NTM) disease in sub-Saharan Africa are limited. During 2006–2008, we identified 3 HIV-infected patients in northern Tanzania who had invasive NTM; 2 were infected with “Mycobacterium sherrisii” and 1 with M. avium complex sequevar MAC-D. Invasive NTM disease is present in HIV-infected patients in sub-Saharan Africa
The Spitzer Spectroscopic Survey of S-type Stars
S-type AGB stars are thought to be in the transitional phase between M-type
and C-type AGB stars. Because of their peculiar chemical composition, one may
expect a strong influence of the stellar C/O ratio on the molecular chemistry
and the mineralogy of the circumstellar dust. In this paper, we present a large
sample of 87 intrinsic galactic S-type AGB stars, observed at infrared
wavelengths with the Spitzer Space Telescope, and supplemented with
ground-based optical data. On the one hand, we derive the stellar parameters
from the optical spectroscopy and photometry, using a grid of model
atmospheres. On the other, we decompose the infrared spectra to quantify the
flux-contributions from the different dust species. Finally, we compare the
independently determined stellar parameters and dust properties. For the stars
without significant dust emission, we detect a strict relation between the
presence of SiS absorption in the Spitzer spectra and the C/O ratio of the
stellar atmosphere. These absorption bands can thus be used as an additional
diagnostic for the C/O ratio. For stars with significant dust emission, we
define three groups, based on the relative contribution of certain dust species
to the infrared flux. We find a strong link between group-membership and C/O
ratio. We show that these groups can be explained by assuming that the
dust-condensation can be cut short before silicates are produced, while the
remaining free atoms and molecules can then form the observed magnesium
sulfides or the carriers of the unidentified 13 and 20 micron features.
Finally, we present the detection of emission features attributed to molecules
and dust characteristic to C-type stars, such as molecular SiS, hydrocarbons
and magnesium sulfide grains. We show that we often detect magnesium sulfides
together with molecular SiS and we propose that it is formed by a reaction of
SiS molecules with Mg.Comment: Accepted for publication in A&
Experiments on the Photophoretic Motion of Chondrules and Dust Aggregates - Indications for the Transport of Matter in Protoplanetary Disks
In a set of 16 drop tower experiments the motion of sub-mm to mm-sized
particles under microgravity was observed. Illumination by a halogen lamp
induced acceleration of the particles due to photophoresis. Photophoresis on
dust-free chondrules, on chondrules, glass spheres and metal spheres covered
with SiC dust and on pure SiC dust aggregates was studied. This is the first
time that photophoretic motion of mm-sized particles has been studied
experimentally. The absolute values for the photophoretic force are consistent
with theoretical expectations for spherical particles. The strength of the
photophoretic force varies for chondrules, dust covered particles and pure dust
from low to strong, respectively. The measurements support the idea that
photophoresis in the early Solar System can be efficient to transport solid
particles outward
Setting the volatile composition of (exo)planet-building material. Does chemical evolution in disk midplanes matter?
Context. The atmospheres of extrasolar planets are thought to be built largely through accretion of pebbles and planetesimals. Such pebbles are also the building blocks of comets. The chemical composition of their volatiles are usually taken to be inherited from the ices in the collapsing cloud. However, chemistry in the protoplanetary disk midplane can modify the composition of ices and gases. Aims. To investigate if and how chemical evolution affects the abundances and distributions of key volatile species in the midplane of a protoplanetary disk in the 0.2–30 AU range. Methods. A disk model used in planet population synthesis models is adopted, providing temperature, density and ionisation rate at different radial distances in the disk midplane. A full chemical network including gas-phase, gas-grain interactions and grain-surface chemistry is used to evolve chemistry in time, for 1 Myr. Both molecular (inheritance from the parent cloud) and atomic (chemical reset) initial conditions are investigated. Results. Great diversity is observed in the relative abundance ratios of the main considered species: H2O, CO, CO2, CH4, O2, NH3 and N2. The choice of ionisation level, the choice of initial abundances, as well as the extent of chemical reaction types included are all factors that affect the chemical evolution. The only exception is the inheritance scenario with a low ionisation level, which results in negligible changes compared with the initial abundances, regardless of whether or not grain-surface chemistry is included. The grain temperature plays an important role, especially in the critical 20–28 K region where atomic H no longer sticks long enough to the surface to react, but atomic O does. Above 28 K, efficient grain-surface production of CO2 ice is seen, as well as O2 gas and ice under certain conditions, at the expense of H2O and CO. H2O ice is produced on grain surfaces only below 28 K. For high ionisation levels at intermediate disk radii, CH4 gas is destroyed and converted into CO and CO2 (in contrast with previous models), and similarly NH3 gas is converted into N2. At large radii around 30 AU, CH4 ice is enhanced leading to a low gaseous CO abundance. As a result, the overall C/O ratios for gas and ice change significantly with radius and with model assumptions. For high ionisation levels, chemical processing becomes significant after a few times 105 yr. Conclusions. Chemistry in the disk midplane needs to be considered in the determination of the volatile composition of planetesimals. In the inner <30 AU disk, interstellar ice abundances are preserved only if the ionisation level is low, or if these species are included in larger bodies within 105 yr
Pixie Dust: The Silicate Features in the Diffuse Interstellar Medium
We have analyzed the 9.7 and ``18'' micron interstellar silicate absorption
features along the line of sight toward four heavily extincted galactic WC-type
Wolf-Rayet (WR) stars. We construct two interstellar extinction curves from
1.25 to 25 micron using near-IR extinction measurements from the literature
along with the silicate profiles of WR 98a (representing the local ISM) and GCS
3 (representing the Galactic Center). We have investigated the mineralogy of
the interstellar silicates by comparing extinction profiles for amorphous
silicates with olivine and pyroxene stochiometry to the 9.7 and ``18'' micron
absorption features in the WR 98a spectrum. In this analysis, we have
considered solid and porous spheres and a continuous distribution of
ellipsoids. While it is not possible to simultaneously provide a perfect match
to both profiles, we find the best match requires a mixture of these two types
of compounds. We also consider iron oxides, aluminosilicates and silicate
carbide (SiC) as grain components. Iron oxides cannot be accommodated in the
observed spectrum, while the amount of Si in SiC is limited to <4%. Finally, we
discuss the cosmic elemental abundance constraints on the silicate mineralogy,
grain shape and porosity.Comment: 34 pages, 8 figures, 7 table
Morphological effects on IR band profiles: Experimental spectroscopic analysis with application to observed spectra of oxygen-rich AGB stars
To trace the source of the unique 13, 19.5, and 28 m emission features
in the spectra of oxygen-rich circumstellar shells around AGB stars, we have
compared dust extinction spectra obtained by aerosol measurements. We have
measured the extinction spectra for 19 oxide powder samples of eight different
types, such as Ti-compounds (TiO, TiO, TiO, TiO,
AlTiO, CaTiO), -, -,
---AlO, and MgAlO in the infrared region
(10 - 50 m) paying special attention to the morphological (size, shape,
and agglomeration) effects and the differences in crystal structure. Anatase
(TiO) particles with rounded edges are the possible 13, 19.5 and 28 m
band carriers as the main contributor in the spectra of AGB stars, and
spherically shaped nano-sized spinel and AlTiO dust grains are possibly
associated with the anatase, enhancing the prominence of the 13 m feature
and providing additional features at 28 m. The extinction data sets
obtained by the aerosol and CsI pellet measurements have been made available
for public use at http://elbe.astro.uni-jena.deComment: 17 pages, 8 figures, Accepted 24 March 2009 for publication in A&
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