10,518 research outputs found
A Study on Surrender of Security by Creditor : Focusing on Art.504 of the Japanese Civil Code and the Legislative Process
A Study on Surrender of Security by Creditor : Focusing on Case Law and Scholarly Opinion (4)
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
Photoinduced insulator-metal transition and nonlinear optical response of correlated electrons -- a DMFT analysis
We investigate a photoinduced insulator-metal transition in the
Falicov-Kimball model with the dynamical mean-field theory (DMFT) extended to
nonequilibrium systems in periodic modulations in time. When the photon energy
of the pump light is (: the interaction strength), a
Drude-like peak is found to grow in the optical conductivity spectrum, which is
an evidence that the system is driven into a metallic state. During the
transition, the band gap does not collapse, whereas the distribution function
exhibits a non-monotonic behaviour away from the Fermi distribution. This
indicates that the transition cannot be accounted for by heating effects, but
creation of photo-carriers is responsible.Comment: 3pages, 1 figure, Proceedings of Yamada Conference LXIII on
Photo-induced Phase Transitions and Cooperative Phenomena (PIPT 2008
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.
The non-uniform, dynamic atmosphere of Betelgeuse observed at mid-infrared wavelengths
We present an interferometric study of the continuum surface of the red
supergiant star Betelgeuse at 11.15 microns wavelength, using data obtained
with the Berkeley Infrared Spatial Interferometer each year between 2006 and
2010. These data allow an investigation of an optically thick layer within 1.4
stellar radii of the photosphere. The layer has an optical depth of ~1 at 11.15
microns, and varies in temperature between 1900 K and 2800 K and in outer
radius between 1.16 and 1.36 stellar radii. Electron-hydrogen atom collisions
contribute significantly to the opacity of the layer. The layer has a
non-uniform intensity distribution that changes between observing epochs. These
results indicate that large-scale surface convective activity strongly
influences the dynamics of the inner atmosphere of Betelgeuse, and mass-loss
processes.Comment: 13 pages, 5 figures, in press (ApJ
The Evolution of L and T Dwarfs in Color-Magnitude Diagrams
We present new evolution sequences for very low mass stars, brown dwarfs and
giant planets and use them to explore a variety of influences on the evolution
of these objects. We compare our results with previous work and discuss the
causes of the differences and argue for the importance of the surface boundary
condition provided by atmosphere models including clouds.
The L- to T-type ultracool dwarf transition can be accommodated within the
Ackerman & Marley (2001) cloud model by varying the cloud sedimentation
parameter. We develop a simple model for the evolution across the L/T
transition. By combining the evolution calculation and our atmosphere models,
we generate colors and magnitudes of synthetic populations of ultracool dwarfs
in the field and in galactic clusters. We focus on near infrared color-
magnitude diagrams (CMDs) and on the nature of the ``second parameter'' that is
responsible for the scatter of colors along the Teff sequence. Variations in
metallicity and cloud parameters, unresolved binaries and possibly a relatively
young population all play a role in defining the spread of brown dwarfs along
the cooling sequence. We find that the transition from cloudy L dwarfs to
cloudless T dwarfs slows down the evolution and causes a pile up of substellar
objects in the transition region, in contradiction with previous studies. We
apply the same model to the Pleiades brown dwarf sequence. Taken at face value,
the Pleiades data suggest that the L/T transition occurs at lower Teff for
lower gravity objects. The simulated populations of brown dwarfs also reveal
that the phase of deuterium burning produces a distinctive feature in CMDs that
should be detectable in ~50-100 Myr old clusters.Comment: Accepted for publication in the ApJ. 52 pages including 20 figure
The Spectrum of the Brown Dwarf Gliese 229B
We present a spectrum of the cool (T_eff = 900 K) brown dwarf Gliese 229B.
This spectrum, with a relatively high signal-to-noise ratio per spectral
resolution element (> 30), spans the wavelength range from 0.837 microns to 5.0
microns. We identify a total of four different major methane absorption
features, including the fundamental band at 3.3 microns, at least four steam
bands, and two neutral cesium features. We confirm the recent detection of
carbon monoxide (CO) in excess of what is predicted by thermochemical
equilibrium calculations. Carbon is primarily involved in a chemical balance
between methane and CO at the temperatures and pressures present in the outer
parts of a brown dwarf. At lower temperatures, the balance favors methane,
while in the deeper, hotter regions, the reaction reverses to convert methane
into CO. The presence of CO in the observable part of the atmosphere is
therefore a sensitive indicator of vertical flows. The high signal-to-noise
ratio in the 1 to 2.5 microns region permits us to place constraints on the
quantity of dust in the atmosphere of the brown dwarf. We are unable to
reconcile the observed spectrum with synthetic spectra that include the
presences of dust. The presence of CO but lack of dust may be a clue to the
location of the boundaries of the outer convective region of the atmosphere:
The lack of dust may mean that it is not being conveyed into the photosphere by
convection, or that it exists in patchy clouds. If the dust is not in clouds,
but rather sits below the outer convective region, we estimate that the
boundary between outer convective and inner radiative layers is between 1250 K
and 1600 K, in agreement with recent models.Comment: 15 pages, 8 figure
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