10,625 research outputs found
Dust in the Photospheric Environment II. Effect on the Near Infrared Spectra of L and T Dwarfs
We report an attempt to interpret the spectra of L and T dwarfs with the use
of the Unified Cloudy Model (UCM). For this purpose, we extend the grid of the
UCMs to the cases of log g = 4.5 and 5.5. The dust column density relative to
the gas column density in the observable photosphere is larger at the higher
gravities, and molecular line intensity is generally smaller at the higher
gravities. The overall spectral energy distributions (SEDs) are f_{J} < f_{H} <
f_{K} in middle and late L dwarfs, f_{J} f_{K} in early T dwarfs (L/T
transition objects), and finally f_{J} > f_{H} > f_{K} in middle and late T
dwarfs, where f_{J}, f_{H}, and f_{K} are the peak fluxes at J, H, and K bands,
respectively, in f_{nu} unit. This tendency is the opposite to what is expected
for the temperature effect, but can be accounted for as the effect of thin dust
clouds formed deep in the photosphere together with the effect of the gaseous
opacities including H_2 (CIA), H_2O, CH_4, and K I. Although the UCMs are
semi-empirical models based on a simple assumption that thin dust clouds form
in the region of T_{cr} < T < T_{cond} (T_{cr} = 1800K is an only empirical
parameter while T_{cond} about 2000K is fixed by the thermodynamical data), the
major observations including the overall SEDs as well as the strengths of the
major spectral features are consistently accounted for throughout L and T
dwarfs. In view of the formidable complexities of the cloud formation, we hope
that our UCM can be of some use as a guide for future modelings of the
ultracool dwarfs as well as for interpretation of observed data of L and T
dwarfs.Comment: 43 pages, 13 figures, to appear in Astrophys. J. (May 20, 2004) Some
minor corrections including the address of our web site, which is now read
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
Study of K^0 \to pi^- e^+ nu_e e^+ e^- in chiral perturbation theory
K^0 \to pi^- e^+ nu_e e^+ e^- decay is studied up to the next-to-leading
order O^4 in chiral perturbation theory. It is found that the O^4 terms
appreciably modifiy the shape of the invariant mass distribution of the leptons
and the energy spectrum of the neutrino.Comment: 17 pages, 7 figures, figures and formula are adde
Water vapor on supergiants. The 12 micron TEXES spectra of mu Cephei
Several recent papers have argued for warm, semi-detached, molecular layers
surrounding red giant and supergiant stars, a concept known as a MOLsphere.
Spectroscopic and interferometric analyses have often corroborated this general
picture. Here, we present high-resolution spectroscopic data of pure rotational
lines of water vapor at 12 microns for the supergiant mu Cephei. This star has
often been used to test the concept of molecular layers around supergiants.
Given the prediction of an isothermal, optically thick water-vapor layer in
Local Thermodynamic Equilibrium around the star (MOLsphere), we expected the 12
micron lines to be in emission or at least in absorption but filled in by
emission from the molecular layer around the star. Our data, however, show the
contrary; we find definite absorption. Thus, our data do not easily fit into
the suggested isothermal MOLsphere scenario. The 12 micron lines, therefore,
put new, strong constraints on the MOLsphere concept and on the nature of water
seen in signatures across the spectra of early M supergiants. We also find that
the absorption is even stronger than that calculated from a standard,
spherically symmetric model photosphere without any surrounding layers. A cool
model photosphere, representing cool outer layers is, however, able to
reproduce the lines, but this model does not account for water vapor emission
at 6 microns. Thus, a unified model for water vapor on mu Cephei appears to be
lacking. It does seem necessary to model the underlying photospheres of these
supergiants in their whole complexity. The strong water vapor lines clearly
reveal inadequacies of classical model atmospheres.Comment: Accepted for publication in the Astrophysical Journa
Photophoretic Structuring of Circumstellar Dust Disks
We study dust accumulation by photophoresis in optically thin gas disks.
Using formulae of the photophoretic force that are applicable for the free
molecular regime and for the slip-flow regime, we calculate dust accumulation
distances as a function of the particle size. It is found that photophoresis
pushes particles (smaller than 10 cm) outward. For a Sun-like star, these
particles are transported to 0.1-100 AU, depending on the particle size, and
forms an inner disk. Radiation pressure pushes out small particles (< 1 mm)
further and forms an extended outer disk. Consequently, an inner hole opens
inside ~0.1 AU. The radius of the inner hole is determined by the condition
that the mean free path of the gas molecules equals the maximum size of the
particles that photophoresis effectively works on (100 micron - 10 cm,
depending on the dust property). The dust disk structure formed by
photophoresis can be distinguished from the structure of gas-free dust disk
models, because the particle sizes of the outer disks are larger, and the inner
hole radius depends on the gas density.Comment: 15 pages, 9 figures, Accepted by ApJ; corrected a typo in the author
nam
The K giant star Arcturus: the hybrid nature of its infrared spectrum
We study infrared spectrum of Arcturus to clarify the nature of the cool
component of its atmosphere, referred to as the CO-mosphere, with the use of
the IR spectral atlas by Hinkle et al.(1995). The nature of CO lines shows an
abrupt change at logW/nu = -4.75, and the lines stronger than this limit can no
longer be analyzed by the classical line-formation theory. A more simple
manifestation of this fact is that the curves-of-growth (CG) of CO lines show
an unpredictable upturn at logW/nu = -4.75. Similar unusual behaviors of
empirical CG are confirmed in other red (super)giant stars, and it looks as if
the CG is a hybrid of at least two components of different origins. Although
strong lines of the CO fundamentals observed in Arcturus show strengthening
compared with the predicted photospheric spectrum, the weaker lines show slight
weakening, and we interpret these results as due to absorption/emission by the
molecular clouds formed in the extended atmosphere. Now do clouds exist in
stellar atmospheres? It is by no means easy to answer this question by
spectroscopic observations alone, but we find several arguments in favor of
such a possibility in Arcturus by analyzing the CO lines. In cooler
(super)giant stars in which CO lines show similar unusual behaviors as in
Arcturus, the presence of molecular clouds in the outer atmospheres was
demonstrated by direct observations with spatial interferometry. We suggest
that the formation of molecular clouds is a general feature in cool luminous
stars from early K to late M (super)giant stars.Comment: 18 pages, 15 Postscript figures, 1 Table in electronic form, Accepted
for publication in Astronomy and Astrophysic
Chemical Equilibrium Abundances in Brown Dwarf and Extrasolar Giant Planet Atmospheres
We calculate detailed chemical abundance profiles for a variety of brown
dwarf and extrasolar giant planet atmosphere models, focusing in particular on
Gliese 229B, and derive the systematics of the changes in the dominant
reservoirs of the major elements with altitude and temperature. We assume an
Anders and Grevesse (1989) solar composition of 27 chemical elements and track
330 gas--phase species, including the monatomic forms of the elements, as well
as about 120 condensates. We address the issue of the formation and composition
of clouds in the cool atmospheres of substellar objects and explore the rain
out and depletion of refractories. We conclude that the opacity of clouds of
low--temperature (900 K), small--radius condensibles (specific chlorides
and sulfides), may be responsible for the steep spectrum of Gliese 229B
observed in the near infrared below 1 \mic. Furthermore, we assemble a
temperature sequence of chemical transitions in substellar atmospheres that may
be used to anchor and define a sequence of spectral types for substellar
objects with Ts from 2200 K to 100 K.Comment: 57 pages total, LaTeX, 14 figures, 5 tables, also available in
uuencoded, gzipped, and tarred form via anonymous ftp at
www.astrophysics.arizona.edu (cd to pub/burrows/chem), submitted to Ap.
Dust in the Photospheric Environment: Unified Cloudy Models of M, L, and T Dwarfs
We address the problem of how dust forms and how it could be sustained in the
static photospheres of cool dwarfs for a long time. In the cool and dense gas,
dust forms easily at the condensation temperature, T_cond, and the dust can be
in detailed balance with the ambient gas so long as it remains smaller than the
critical radius, r_cr. However, dust will grow larger and segregate from the
gas when it will be larger than r_cr somewhere at the lower temperature, which
we refer to as the critical temperature, T_cr. Then, the large dust grains will
precipitate below the photosphere and only the small dust grains in the region
of T_cr < T < T_cond can be sustained in the photosphere. Thus a dust cloud is
formed. Incorporating the dust cloud, non-grey model photo- spheres in
radiative-convective equilibrium are extended to T_eff as low as 800K. Observed
colors and spectra of cool dwarfs can consistently be accounted for by a single
grid of our cloudy models. This fact in turn can be regarded as supporting
evidence for our basic assumption on the cloud formation.Comment: 50 pages with 14 postscript figures, to be published in Astrophys.
Backward Clusters, Hierarchy and Wild Sums for a Hard Sphere System in a Low-Density Regime
We study the statistics of backward clusters in a gas of hard spheres at low
density. A backward cluster is defined as the group of particles involved
directly or indirectly in the backwards-in-time dynamics of a given tagged
sphere. We derive upper and lower bounds on the average size of clusters by
using the theory of the homogeneous Boltzmann equation combined with suitable
hierarchical expansions. These representations are known in the easier context
of Maxwellian molecules (Wild sums). We test our results with a numerical
experiment based on molecular dynamics simulations
Spectral Classification and Effective Temperatures of L and T Dwarfs Based of Near-Infrared Spectra
We have obtained near-infrared spectra of L dwarfs, L/T transition objects
and T dwarfs using Subaru. Resulting spectra are examined in detail to see
their dependence on the spectral types. We have obtained bolometric
luminosities of the objects with known parallaxes in our sample, first by
integrating the spectra and second by K band bolometric correction. We derive
the relation between effective temperature and spectral type.Comment: To appear in May 20, 2004 issue of ApJ There is a companion paper by
Tsuji, Nakajima and Yanagisaw
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