125 research outputs found
Dust Emission from IRC+10216
Infrared emission from the dust shell around IRC+10216 is analysed in detail,
employing a self-consistent model for radiatively driven winds around late-type
stars that couples the equations of motion and radiative transfer in the dust.
The resulting model provides agreement with the wealth of available data,
including the spectral energy distribution in the range 0.5--1000 \mic, and
visibility and array observations. Previous conclusions about two dust shells,
derived from modelling the data with a few single-temperature components of
different radii, are not supported by our results. The extended, continuous
temperature and density distributions derived from our model obviate the need
for such discrete shells. The IR properties vary with the stellar phase,
reflecting changes in both the dust condensation radius and the overall
optical depth -- as the luminosity increases from minimum to maximum,
increases while decreases. We find that the angular size of the
dust condensation zone varies from 0.3 arcsec at minimum light to 0.5 arcsec at
maximum. The shortage of flux at short wavelengths encountered in previous
studies is resolved by employing a grain size distribution that includes grains
larger than \about\ 0.1 \mic, required also for the visibility fits. This
distribution is in agreement with the one recently proposed by Jura in a study
that probed the outer regions of the envelope. Since our constraints on the
size distribution mostly reflect the envelope's inner regions, the agreement of
these independent studies is evidence against significant changes in grain
sizes through effects like sputtering or grain growth after the initial
formation at the dust condensation zone.Comment: LaTeX with 3 figures, requires MNRAS mn.sty; figures and/or complete
PS or PS.Z preprint (7 pages) available by anonymous ftp at
ftp://asta.pa.uky.edu/ivezic/irc10216/irc10216.ps (or fig1.ps, fig2.ps,
fig3.ps
Self-similarity and scaling behavior of IR emission from radiatively heated dust: I. Theory
Dust infrared emission possesses scaling properties. Overall luminosity is
never an input parameter of the radiative transfer problem, spectral shape is
the only relevant property of the heating radiation when the inner boundary of
the dusty region is controlled by dust sublimation. Similarly, the absolute
scales of densities and distances are irrelevant; the geometry enters only
through angles, relative thicknesses and aspect ratios, and the actual
magnitudes of densities and distances enter only through one independent
parameter, the overall optical depth. Dust properties enter only through
dimensionless, normalized distributions that describe the spatial variation of
density and the wavelength dependence of scattering and absorption
efficiencies. Scaling enables a systematic approach to modeling and
classification of IR spectra. We develop a new, fully scale-free method for
solving radiative transfer, present exact numerical results, and derive
approximate analytical solutions for spherical geometry, covering the entire
range of parameter space relevant to observations. Scaling implies tight
correlations among the SEDs of various members of the same class of sources
such as young stellar objects, late-type stars, etc. In particular, all members
of the same class occupy common, well defined regions in color-color diagrams.
The observational data corroborate the existence of these correlations.Comment: 14 pages, 10 Postscript figures (included), uses mn.sty. To appear in
Monthly Notices of the Royal Astronomical Societ
Infrared Imaging of Late-Type Stars
Infrared imaging properties of dusty winds around late-type stars are
investigated in detail, employing a self-consistent model that couples the
equations of motion and radiative transfer. Because of general scaling
properties, the angular profiles of surface brightness are self-similar. In any
given star, the profile shape is determined essentially by overall optical
depth at each wavelength and it is self-similarly scaled by the size of the
dust condensation zone. We find that the mid-IR is the best wavelength range to
measure directly the angular size of this zone, and from {\it IRAS} data we
identify the 15 best candidates for such future observations. We also show that
the visibility function at short wavelengths (\la 2 \mic) directly determines
the scattering optical depth, and produce theoretical visibility curves for
various characteristic wavelengths and the entire parameter range relevant to
late-type stars. The infrared emission should display time variability because
of cyclical changes in overall optical depth, reflecting luminosity-induced
movement of the dust condensation point. Calculations of the wavelength
dependence of photometric amplitudes and time variability of envelope sizes are
in agreement with observations; envelopes are bigger and bluer at maximum
light.Comment: LaTeX with 2 figures, requires MNRAS mn.sty; figures and/or complete
PS or PS.Z preprint (8 pages) available by anonymous ftp at
ftp://asta.pa.uky.edu/ivezic/imaging/imaging.ps (or fig1.ps, fig2.ps
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