6,344 research outputs found
Laboratory mid-IR spectra of equilibrated and igneous meteorites. Searching for observables of planetesimal debris
Meteorites contain minerals from Solar System asteroids with different
properties (like size, presence of water, core formation). We provide new
mid-IR transmission spectra of powdered meteorites to obtain templates of how
mid-IR spectra of asteroidal debris would look like. This is essential for
interpreting mid-IR spectra of past and future space observatories, like the
James Webb Space Telescope. We show that the transmission spectra of wet and
dry chondrites, carbonaceous and ordinary chondrites and achondrite and
chondrite meteorites are distinctly different in a way one can distinguish in
astronomical mid-IR spectra. The two observables that spectroscopically
separate the different meteorites groups (and thus the different types of
parent bodies) are the pyroxene-olivine feature strength ratio and the peak
shift of the olivine spectral features due to an increase in the iron
concentration of the olivine
The lunar phases of dust grains orbiting Fomalhaut
Optical images of the nearby star Fomalhaut show a ring of dust orbiting the
central star. This dust is in many respects expected to be similar to the
zodiacal dust in the solar system. The ring displays a clear brightness
asymmetry, attributed to asymmetric scattering of the central starlight by the
circumstellar dust grains. Recent measurements show that the bright side of the
Fomalhaut ring is oriented away from us. This implies that the grains in this
system scatter most of the light in the backward direction, in sharp contrast
to the forward-scattering nature of the grains in the solar system. In this
letter, we show that grains considerably larger than those dominating the solar
system zodiacal dust cloud provide a natural explanation for the apparent
backward scattering behavior. In fact, we see the phases of the dust grains in
the same way as we can observe the phases of the Moon and other large solar
system bodies. We outline how the theory of the scattering behavior of
planetesimals can be used to explain the Fomalhaut dust properties. This
indicates that the Fomalhaut dust ring is dominated by very large grains. The
material orbiting Fomalhaut, which is at the transition between dust and
planetesimals, can, with respect to their optical behavior, best be described
as micro-asteroids.Comment: Accepted for publication in A&
Crystallinity versus mass-loss rate in Asymptotic Giant Branch stars
Infrared Space Observatory (ISO) observations have shown that O-rich
Asymptotic Giant Branch (AGB) stars exhibit crystalline silicate features in
their spectra only if their mass-loss rate is higher than a certain threshold
value. Usually, this is interpreted as evidence that crystalline silicates are
not present in the dust shells of low mass-loss rate objects. In this study,
radiative transfer calculations have been performed to search for an
alternative explanation to the lack of crystalline silicate features in the
spectrum of low mass-loss rate AGB stars. It is shown that due to a temperature
difference between amorphous and crystalline silicates it is possible to
include up to 40% of crystalline silicate material in the circumstellar dust
shell, without the spectra showing the characteristic spectral features. Since
this implies that low mass-loss rate AGB stars might also form crystalline
silicates and deposit them into the Interstellar Medium (ISM), the described
observational selection effect may put the process of dust formation around AGB
stars and the composition of the predominantly amorphous dust in the
Interstellar Medium in a different light. Our model calculations result in a
diagnostic tool to determine the crystallinity of an AGB star with a known
mass-loss rate.Comment: accepted by A&A, 10 pages, 11 figure
The effects of dust evolution on disks in the mid-IR
In this paper, we couple together the dust evolution code two-pop-py with the
thermochemical disk modelling code ProDiMo. We create a series of
thermochemical disk models that simulate the evolution of dust over time from
0.018 Myr to 10 Myr, including the radial drift, growth, and settling of dust
grains. We examine the effects of this dust evolution on the mid-infrared gas
emission, focussing on the mid-infrared spectral lines of C2H2, CO2, HCN, NH3,
OH, and H2O that are readily observable with Spitzer and the upcoming E-ELT and
JWST.
The addition of dust evolution acts to increase line fluxes by reducing the
population of small dust grains. We find that the spectral lines of all species
except C2H2 respond strongly to dust evolution, with line fluxes increasing by
more than an order of magnitude across the model series as the density of small
dust grains decreases over time. The C2H2 line fluxes are extremely low due to
a lack of abundance in the infrared line-emitting regions, despite C2H2 being
commonly detected with Spitzer, suggesting that warm chemistry in the inner
disk may need further investigation. Finally, we find that the CO2 flux
densities increase more rapidly than the other species as the dust disk
evolves. This suggests that the flux ratios of CO2 to other species may be
lower in disks with less-evolved dust populations.Comment: 13 pages, 9 figures, accepted in A&
The Thermal Structure of the Circumstellar Disk Surrounding the Classical Be Star gamma Cassiopeia
We have computed radiative equilibrium models for the gas in the
circumstellar envelope surrounding the hot, classical Be star Cassiopeia. This calculation is performed using a code that incorporates a
number of improvements over previous treatments of the disk's thermal structure
by \citet{mil98} and \citet{jon04}; most importantly, heating and cooling rates
are computed with atomic models for H, He, CNO, Mg, Si, Ca, & Fe and their
relevant ions. Thus, for the first time, the thermal structure of a Be disk is
computed for a gas with a solar chemical composition as opposed to assuming a
pure hydrogen envelope. We compare the predicted average disk temperature, the
total energy loss in H, and the near-IR excess with observations and
find that all can be accounted for by a disk that is in vertical hydrostatic
equilibrium with a density in the equatorial plane of to
. We also discuss the changes in
the disk's thermal structure that result from the additional heating and
cooling processes available to a gas with a solar chemical composition over
those available to a pure hydrogen plasma.Comment: 11 pages, 8 figures high resolution figures available at
http://inverse.astro.uwo.ca/sig_jon07.htm
The circumstellar envelope of AFGL 4106
We present new imaging and spectroscopy of the post-red supergiant binary
AFGL 4106. Coronographic imaging in H-alpha reveals the shape and extent of the
ionized region in the circumstellar envelope (CSE). Echelle spectroscopy with
the slit covering almost the entire extent of the CSE is used to derive the
physical conditions in the ionized region and the optical depth of the dust
contained within the CSE.
The dust shell around AFGL 4106 is clumpy and mixed with ionized gas. H-alpha
and [N II] emission is brightest from a thin bow-shaped layer just outside of
the detached dust shell. On-going mass loss is traced by [Ca II] emission and
blue-shifted absorption in lines of low-ionization species. A simple model is
used to interpret the spatial distribution of the circumstellar extinction and
the dust emission in a consistent way.Comment: 10 pages, 11 figures. Accepted for publication in Astronomy &
Astrophysics Main Journa
Theory of high-energy emission from the pulsar/Be-star system PSR 125963 I: radiation mechanisms and interaction geometry
We study the physical processes of the PSR B1259-63 system containing a 47 ms
pulsar orbiting around a Be star in a highly eccentric orbit. Motivated by the
results of a multiwavelength campaign during the January 1994 periastron
passage of PSR B1259-63, we discuss several issues regarding the mechanism of
high-energy emission. Unpulsed power law emission from the this system was
detected near periastron in the energy range 1-200 keV. We find that the
observed high energy emission from the PSR B1259-63 system is not compatible
with accretion or propeller-powered emission. Shock-powered high-energy
emission produced by the pulsar/outflow interaction is consistent with all high
energy observations. By studying the evolution of the pulsar cavity we
constrain the magnitude and geometry of the mass outflow outflow of the Be
star. The pulsar/outflow interaction is most likely mediated by a collisionless
shock at the internal boundary of the pulsar cavity. The system shows all the
characteristics of a {\it binary plerion} being {\it diffuse} and {\it compact}
near apastron and periastron, respectively. The PSR B1259-63 cavity is subject
to different radiative regimes depending on whether synchrotron or inverse
Compton (IC) cooling dominates the radiation of electron/positron pairs
advected away from the inner boundary of the pulsar cavity. The highly
non-thermal nature of the observed X-ray/gamma-ray emission near periastron
establishes the existence of an efficient particle acceleration mechanism
within a timescale shown to be less than s. A synchrotron/IC
model of emission of e\pm-pairs accelerated at the inner shock front of the
pulsar cavity and adiabatically expanding in the MHD flow provides an excellent
explanation of the observed time variableX-ray flux and spectrum from the PSRComment: 68 pages, accepted for publication in the Astrophys. J. on Aug. 26,
199
Variability and nature of the binary in the Red Rectangle Nebula
We present new observations of the central binary inside the Red Rectangle
nebula. The detection of zinc in the optical spectrum confirms that the
peculiar photospheric abundances are due to accretion of circumstellar gas.
Grey brightness variations with the orbital period are observed. They are
interpreted as being due to the variation of the scattering angle with orbital
phase. The small orbital separation of the system is not compatible with
previous normal evolution of the primary on the AGB. We point out the
similarity of the orbital history of this and other similar systems with those
of some close Barium stars and suggest that the nonzero eccentricity of the
orbit is the result of tidal interaction with the circumbinary disk.Comment: 4 pages, 3 figures, A&A Letters accepte
The absence of the 10 um silicate feature in the isolated Herbig Ae star HD 100453
We analyse the optical and IR spectra, as well as the spectral energy
distribution (UV to mm) of the candidate Herbig Ae star HD100453. This star is
particular, as it shows an energy distribution similar to that of other
isolated Herbig Ae/Be stars (HAEBEs), but unlike most of them, it does not have
a silicate emission feature at 10 um, as is shown in Meeus (2001). We confirm
the HAEBE nature of HD100453 through an analysis of its optical spectrum and
derived location in the H-R diagram. The IR spectrum of HD100453 is modelled by
an optically thin radiative transfer code, from which we derive constraints on
the composition, grain-size and temperature distribution of the circumstellar
dust. We show that it is both possible to explain the lack of the silicate
feature as (1) a grain-size effect - lack of small silicate grains, and (2) a
temperature effect - lack of small, hot silicates, as proposed by Dullemond
(2001), and discuss both possibilities.Comment: 9 pages, 7 figures; accepted by A&
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