247,438 research outputs found
Resolving Stellar Atmospheres I: The H alpha line and comparisons to microlensing observations
We present work on H alpha spectral line characteristics in PHOENIX stellar
model atmospheres and their comparison to microlensing observations. We examine
in detail the H alpha equivalent width (EW) and the line shape characteristics
for effective temperatures of 4500K< Teff < 5600K where H alpha is a strong
spectral feature. We find that H alpha EW in models calculated under the
assumption of local thermodynamic equilibrium (LTE) is up to 15% smaller than
in models without this assumption, non-LTE models (NLTE) and that line shapes
vary significantly for the two model types. A comparison with available high
quality microlensing data, capable of tracing H alpha absorption across the
face of one G5III giant, shows that the LTE model that fits the EW best is
about 100K hotter than and the best-fitting NLTE model has a similar Teff as
predicted by the spectral type analysis of the observed star but agree within
the uncertainties of the observationally derived temperature. Neither LTE nor
NLTE models fit the line shape well. We suspect unmodelled chromospheric
emission. Line shape diagnostics suggest lower gravities than derived for the
star and are unacceptable low in the case of the LTE models. We show that EW
alone is insufficient for comparison to stellar model atmospheres, but combined
with a new shape parameter we define is promising. In stellar parameter ranges
where the H alpha line is strong, a NLTE approach of modeling stellar
atmospheres is not only beneficial but mandatory.Comment: 11 pages, 9 figures, accepted to Astronomy & Astrophysic
Absorption Efficiencies of Forsterite. I: DDA Explorations in Grain Shape and Size
We compute the absorption efficiency (Qabs) of forsterite using the discrete
dipole approximation (DDA) in order to identify and describe what
characteristics of crystal grain shape and size are important to the shape,
peak location, and relative strength of spectral features in the 8-40 {\mu}m
wavelength range. Using the DDSCAT code, we compute Qabs for non-spherical
polyhedral grain shapes with a_eff = 0.1 {\mu}m. The shape characteristics
identified are: 1) elongation/reduction along one of three crystallographic
axes; 2) asymmetry, such that all three crystallographic axes are of different
lengths; and 3) the presence of crystalline faces that are not parallel to a
specific crystallographic axis, e.g., non-rectangular prisms and (di)pyramids.
Elongation/reduction dominates the locations and shapes of spectral features
near 10, 11, 16, 23.5, 27, and 33.5 {\mu}m, while asymmetry and tips are
secondary shape effects. Increasing grain sizes (0.1-1.0 {\mu}m) shifts the 10,
11 {\mu}m features systematically towards longer wavelengths and relative to
the 11 {\mu}m feature increases the strengths and slightly broadens the longer
wavelength features. Seven spectral shape classes are established for
crystallographic a-, b-, and c-axes and include columnar and platelet shapes
plus non-elongated or equant grain shapes. The spectral shape classes and the
effects of grain size have practical application in identifying or excluding
columnar, platelet or equant forsterite grain shapes in astrophysical environs.
Identification of the shape characteristics of forsterite from 8-40 {\mu}m
spectra provides a potential means to probe the temperatures at which
forsterite formed.Comment: 55 pages, 15 figure
The Mid-Infrared Spectrum of the Zodiacal and Exozodiacal Light
The zodiacal light is the dominant source of the mid-infrared sky brightness
seen from Earth, and exozodiacal light is the dominant emission from planetary
and debris systems around other stars. We observed the zodiacal light spectrum
with ISOCAM over 5-16 over a wide range of orientations relative to the Sun and
the ecliptic. We present theoretical models for a wide range of particle size
distributions and compositions. The observed temperature is as expected for
large (>10 um radius), low-albedo (< 0.08), rapidly-rotating, grey particles 1
AU from the Sun. In addition to the continuum, we detect a weak excess in the
9-11 um range, with an amplitude of 6% of the continuum. The shape of the
feature can be matched by a mixture of silicates: amorphous forsterite/olivine,
dirty crystalline olivine, and a hydrous silicate (montmorillonite). The
presence of hydrous silicate suggests the parent bodies of those particles were
formed in the inner solar nebula. Large particles dominate the size
distribution, but at least some small particles (radii ~1 um) are required to
produce the silicate emission feature. To compare the properties of zodiacal
dust to dust around other main sequence stars, we reanalyzed the exozodiacal
light spectrum for Beta Pic. The exozodiacal spectra are dominated by cold
dust, with emission peaking in the far-infrared, while the zodiacal spectrum
peaks around 20 um. The shape of the silicate feature from Beta Pic is nearly
identical to that derived from the ISO spectrum of 51 Oph; both exozodiacal
features are very different from that of the zodiacal light. The exozodiacal
features are roughly triangular, peaking at 10.3 um while the zodiacal feature
is more boxy.Comment: accepted to Icaru
A temporal study of oxygen-rich pulsating variable AGB star, T Cep: Investigation on dust formation and dust properties
Pulsation is believed to be the leading cause of dusty mass loss from
Asymptotic Giant Branch (AGB) stars. We present a temporal study of T Cep, a
long-period Mira variable, using seven ISO SWS spectra, covering a 16-month
period over a single pulsation cycle. The observed spectral dust features
change over the pulsation cycle of this Mira. In general, the overall apparent
changes in spectral features can be attributed to changes in the dust
temperature, resulting from the intrinsic pulsation cycle of the central star.
However, not all feature changes are so easily explained. Based on direct
comparison with laboratory spectra of several potential minerals, the dust is
best explained by crystalline iron-rich silicates. These findings contradict
the currently favored dust formation hypotheses.Comment: This article has been accepted for publication in The Astrophysical
Journal. It is tentatively scheduled for the May 10, 2011, v732 -2 issu
Mid-infrared spectra of late-type stars: Long-term evolution
Recent ground-based mid-infrared spectra of 29 late-type stars, most with
substantial dust shells, are compared to ground-based spectra of these stars
from the 1960s and 1970s and to IRAS-LRS spectra obtained in 1983. The spectra
of about half the stars show no detectable changes, implying that their
distributions of circumstellar material and associated dust grain properties
have changed little over this time interval. However, many of the stars with
strong silicate features showed marked changes. In nearly all cases the
silicate peak has strengthened with respect to the underlying continuum,
although there is one case (VY~CMa) in which the silicate feature has almost
completely disappeared. This suggests that, in general, an oxygen-rich star
experiences long periods of gradual silicate feature strengthening, punctuated
by relatively rare periods when the feature weakens. We discuss various
mechanisms for producing the changes, favoring the slow evolution of the
intrinsic dust properties (i.e., the chemical composition or grain structure).
Although most IRAS spectra agree well with ground-based spectra, there are a
number of cases where they fall well outside the expected range of uncertainty.
In almost all such cases the slopes of the red and blue LRS spectra do not
match in their region of overlap.Comment: Accepted in ApJ, 20 pages, 5 figures, 1 tabl
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