7,244 research outputs found
Water in Emission in the ISO Spectrum of the Early M Supergiant Star mu Cephei
We report a detection of water in emission in the spectrum of the M2
supergiant atar mu Cep (M2Ia) observed by the Short Wavelength Spectrometer
(SWS) aboard Infrared Space Observatory (ISO) and now released as the ISO
Archives. The emission first appears in the 6 micron region (nu2 fundamental)
and then in the 40 micron region (pure rotation lines) despite the rather
strong dust emission. The intensity ratios of the emission features are far
from those of the optically thin gaseous emission. Instead, we could reproduce
the major observed emission features by an optically thick water sphere of the
inner radius about two stellar radii (1300Rsun), Tex = 1500K, and Ncol (H2O) =
3.0E+20/cm2. This model also accounts for the H2O absorption bands in the near
infrared (1.4, 1.9, and 2.7 micron) as well. The detection of water in emission
provides strong constraints on the nature of water in the early M supergiant
stars, and especially its origin in the outer atmosphere is confirmed against
other models such as the large convective cell model. We finally confirm that
the early M supergiant star is surrounded by a huge optically thick sphere of
the warm water vapor, which may be referred to as MOLsphere for simplicity.
Thus, the outer atmosphere of M supergiant stars should have a complicated
hierarchical and/or hybrid structure with at least three major constituents
including the warm MOLsphere (T about 1.0E+3K) together with the previously
known hot chromosphere (T about 1.0E+4K) and cool expanding gas-dust envelope
(T about 1.0E+2K).Comment: 14 pages, 5 postscript figures, to appear in ApJ
A short note on the nested-sweep polarized traces method for the 2D Helmholtz equation
We present a variant of the solver in Zepeda-N\'u\~nez and Demanet (2014),
for the 2D high-frequency Helmholtz equation in heterogeneous acoustic media.
By changing the domain decomposition from a layered to a grid-like partition,
this variant yields improved asymptotic online and offline runtimes and a lower
memory footprint. The solver has online parallel complexity that scales
\emph{sub linearly} as , where is
the number of volume unknowns, and is the number of processors, provided
that . The variant in Zepeda-N\'u\~nez and Demanet
(2014) only afforded . Algorithmic scalability is a
prime requirement for wave simulation in regimes of interest for geophysical
imaging.Comment: 5 pages, 5 figure
SDSS J080531.84+481233.0: An Unresolved L Dwarf/T Dwarf Binary
SDSS J080531.84+481233.0 is a peculiar L-type dwarf that exhibits unusually
blue near-infrared and mid-infrared colors and divergent optical (L4) and
near-infrared (L9.5) spectral classifications. These peculiar spectral traits
have been variously attributed to condensate cloud effects or subsolar
metallicity. Here I present an improved near-infrared spectrum of this source
which further demonstrates the presence of weak CH4 absorption at 1.6 micron
but no corresponding band at 2.2 micron. It is shown that these features can be
collectively reproduced by the combined light spectrum of a binary with L4.5
and T5 components, as deduced by spectral template matching. Thus, SDSS
J080531.84+481233.0 appears to be a new low-mass binary straddling the L
dwarf/T dwarf transition, an evolutionary phase for brown dwarfs that remains
poorly understood by current theoretical models. The case of SDSS
J080531.84+481233.0 further illustrates how a select range of L dwarf/T dwarf
binaries could be identified and characterized without the need for high
angular resolution imaging or radial velocity monitoring, potentially
alleviating some of the detection biases and limitations inherent to such
techniques.Comment: 11 pages, 4 figures, accepted by A
Detectability of dirty dust grains in brown dwarf atmospheres
Dust clouds influence the atmospheric structure of brown dwarfs, and they
affect the heat transfer and change the gas-phase chemistry. However, the
physics of their formation and evolution is not well understood. In this
letter, we predict dust signatures and propose a potential observational test
of the physics of dust formation in brown dwarf atmosphere based on the
spectral features of the different solid components predicted by dust formation
theory. A momentum method for the formation of dirty dust grains (nucleation,
growth, evaporation, drift) is used in application to a static brown dwarf
atmosphere structure to compute the dust grain properties, in particular the
heterogeneous grain composition and the grain size. Effective medium and Mie
theory are used to compute the extinction of these spherical grains. Dust
formation results in grains whose composition differs from that of grains
formed at equilibrium. Our kinetic model predicts that solid amorphous SiO2[s]
(silica) is one of the most abundant solid component followed by amorphous
MgSiO4[s] and MgSiO3[s], while SiO2[s] is absent in equilibrium models
because it is a metastable solid. Solid amorphous SiO2[s] possesses a strong
broad absorption feature centered at 8.7mum, while amorphous
Mg2SiO4[s]/MgSiO3[s] absorb at 9.7mum beside other absorption features at
longer wavelength. Those features at lambda < 15mum are detectable in
absorption if grains are small (radius < 0.2mum) in the upper atmosphere as
suggested by our model. We suggest that the detection of a feature at 8.7mum in
deep infrared spectra could provide evidence for non-equilibrium dust formation
that yields grains composed of metastable solids in brown dwarf atmospheres.
This feature will shift towards 10mum and broaden if silicates (e.g. fosterite)
are much more abundant.Comment: A&A Letter, accepte
The Theorem of Jentzsch--Szeg\H{o} on an analytic curve. Application to the irreducibility of truncations of power series
The theorem of Jentzsch--Szeg\H{o} describes the limit measure of a sequence
of discrete measures associated to the zeroes of a sequence of polynomials in
one variable. Following the presentation of this result by Andrievskii and
Blatt in their book, we extend this theorem to compact Riemann surfaces, then
to analytic curves over an ultrametric field. The particular case of the
projective line over an ultrametric field gives as corollaries information
about the irreducibility of the truncations of a power series in one variable.Comment: 16 pages; the application to irreducibility and the final example
have been correcte
Comparison of cloud models for Brown Dwarfs
A test case comparison is presented for different dust cloud model approaches
applied in brown dwarfs and giant gas planets. We aim to achieve more
transparency in evaluating the uncertainty inherent to theoretical modelling.
We show in how far model results for characteristic dust quantities vary due to
different assumptions. We also demonstrate differences in the spectral energy
distributions resulting from our individual cloud modelling in 1D substellar
atmosphere simulationsComment: 5 pages, Proceeding to "Exoplantes: Detection, Formation, Dynamics",
eds. Ferraz-Mello et
A comparison of chemistry and dust cloud formation in ultracool dwarf model atmospheres
The atmospheres of substellar objects contain clouds of oxides, iron,
silicates, and other refractory condensates. Water clouds are expected in the
coolest objects. The opacity of these `dust' clouds strongly affects both the
atmospheric temperature-pressure profile and the emergent flux. Thus any
attempt to model the spectra of these atmospheres must incorporate a cloud
model. However the diversity of cloud models in atmospheric simulations is
large and it is not always clear how the underlying physics of the various
models compare. Likewise the observational consequences of different modeling
approaches can be masked by other model differences, making objective
comparisons challenging. In order to clarify the current state of the modeling
approaches, this paper compares five different cloud models in two sets of
tests. Test case 1 tests the dust cloud models for a prescribed L, L--T, and
T-dwarf atmospheric (temperature T, pressure p, convective velocity
vconv)-structures. Test case 2 compares complete model atmosphere results for
given (effective temperature Teff, surface gravity log g). All models agree on
the global cloud structure but differ in opacity-relevant details like grain
size, amount of dust, dust and gas-phase composition. Comparisons of synthetic
photometric fluxes translate into an modelling uncertainty in apparent
magnitudes for our L-dwarf (T-dwarf) test case of 0.25 < \Delta m < 0.875 (0.1
< \Delta m M 1.375) taking into account the 2MASS, the UKIRT WFCAM, the Spitzer
IRAC, and VLT VISIR filters with UKIRT WFCAM being the most challenging for the
models. (abr.)Comment: 22 pages, 17 figures, MNRAS 2008, accepted, (minor grammar/typo
corrections
Infrared Spectra and Visibilities as Probes of the Outer Atmospheres of Red Supergiant Stars
In the light of the recent results of the stellar interferometry, we examine
the nature of the extra molecular layer outside the photosphere of red super-
giant stars, so far studied mostly with the use of the infrared spectra.
Although the visibility data are more direct probes of the spatial structure of
the outer atmosphere, it is essential that they are analyzed in combination
with the spectral data of a wide spectral coverage. In the case of the M2
supergiant mu Cephei, several sets of data, both spectra and visibilities,
strongly suggested the presence of an extra-molecular layer, and its basic
parameters are estimated to be: excitation temperature T_ex = 1600 K, column
densities of CO and H2O N_col = 3.0d+20/cm2, and inner radius R_in = 2.0R*. The
result shows reasonable agreement with the one based on the infrared spectra
alone, and this may be because the infrared spectra already include some
information on the spatial structure of the outer atmosphere. It is important,
however, that the model inferred from the spectra is now fully supported with
the recent visibility data. In the case of the M2 supergiant alpha Orionis, the
infrared spectra and visibilities show a consistent picture in that its
molecular layer is closer to the photosphere (R_in = 1.3R*) with higher gas
temperature (T_ex = 2250 K) and lower gas column density (N_col = 1.0d+20/cm2),
compared with that of mu Cephei. Some controversy on the interpretation of the
mid infrared data of alpha Orionis can be reconciled.Comment: 47 pages, 14 Postscript figures, to be published in the Astrophysical
Journa
Quantitative estimates of discrete harmonic measures
A theorem of Bourgain states that the harmonic measure for a domain in
is supported on a set of Hausdorff dimension strictly less than
\cite{Bourgain}. We apply Bourgain's method to the discrete case, i.e., to the
distribution of the first entrance point of a random walk into a subset of , . By refining the argument, we prove that for all \b>0 there
exists \rho (d,\b)N(d,\b), any , and any | \{y\in\Z^d\colon \nu_{A,x}(y)
\geq n^{-\b} \}| \leq n^{\rho(d,\b)}, where denotes the
probability that is the first entrance point of the simple random walk
starting at into . Furthermore, must converge to as \b \to
\infty.Comment: 16 pages, 2 figures. Part (B) of the theorem is ne
Resolved Spectroscopy of M Dwarf/L Dwarf Binaries. II. 2MASS J 17072343-0558249AB
We present IRTF SpeX observations of the M/L binary system 2MASS
J17072343-0558249. SpeX imaging resolves the system into a 1"01+/-0.17 visual
binary in which both components have red near infrared colors. Resolved
low-resolution (R~150) 0.8-2.5 micron spectroscopy reveals strong H2O, CO and
FeH bands and alkali lines in the spectra of both components, characteristic of
late-type M and L dwarfs. A comparison to a sample of late-type field dwarf
spectra indicates spectral types M9 and L3. Despite the small proper motion of
the system (0"100+/-0"009 yr^{-1}), imaging observations over 2.5 yr provide
strong evidence that the two components share common proper motion. Physical
association is also likely due to the small spatial volume occupied by the two
components (based on spectrophotometric distances estimates of 15+/-1 pc) as
compared to the relatively low spatial density of low mass field stars. The
projected separation of the system is 15+/-3 AU, similar to other late-type M
and L binaries. Assuming a system age of 0.5-5 Gyr, we estimate the masses of
the binary components to be 0.072-0.083 and 0.064-0.077 M_sun, with an orbital
period of roughly 150-300 yr. While this is nominally too long a baseline for
astrometric mass measurements, the proximity and relatively wide angular
separation of the 2MASS J1707-0558AB pair makes it an ideal system for studying
the M dwarf/L dwarf transition at a fixed age and metallicity
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