8,468 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
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
Extrasolar Giant Planets under Strong Stellar Irradiation
We investigate the effects on extrasolar giant planets [EGPs] of intense
irradiation by their parent stars, describing the issues involved in treating
the model atmosphere problem correctly. We treat the radiative transfer in
detail, allowing the flux from the parent star to interact with all relevant
depths of the planetary atmosphere, with no need for a pre-assumed albedo. We
present a low-resolution optical and near-IR spectrum of a close-in EGP,
focusing on the differences from an isolated planet.
In our dust-free planetary atmospheres we find that Rayleigh scattering
increases the EGP's flux by orders of magnitude shortward of the CaII H&K
doublet (393 nm), and the spectral features of the parent star are exactly
reflected. In the optical and near-IR the thermal absorption of the planet
takes over, but the absorption features are changed by the irradiation. The
inclusion of dust increases the reflected flux in the blue; the stellar
spectral lines can be seen blueward of H-beta (486 nm).Comment: 14 pages, 4 figures, LaTex, accepted in ApJ
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 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
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
Coefficient of Restitution for Viscoelastic Spheres: The Effect of Delayed Recovery
The coefficient of normal restitution of colliding viscoelastic spheres is
computed as a function of the material properties and the impact velocity. From
simple arguments it becomes clear that in a collision of purely repulsively
interacting particles, the particles loose contact slightly before the distance
of the centers of the spheres reaches the sum of the radii, that is, the
particles recover their shape only after they lose contact with their collision
partner. This effect was neglected in earlier calculations which leads
erroneously to attractive forces and, thus, to an underestimation of the
coefficient of restitution. As a result we find a novel dependence of the
coefficient of restitution on the impact rate.Comment: 11 pages, 2 figure
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
Increasing the Adaptive Capacity of Indigenous People to Environmental Change: The Potential Use of an Innovative, Web-Based, Collaborative-Geomatics Informatics Tool to Reduce the Degree of Exposure of First Nations Cree to Hazardous Travel Routes
The arctic and subarctic regions of Canada are experiencing amplified climate change impacts, which are disproportionately impacting Canadian indigenous populationsâ ability to safely travel on land to acquire resources. Less predictable and more dangerous travel conditions are impacting not only the health and safety of individuals but also the traditional lifestyles that are vital to the cultural well-being of these indigenous communities. The University of Waterlooâs Computer Systems Group has developed a novel decision-support tool termed âCollaborative-Geomatics.â This web-based informatics tool can allow for the community to monitor, in real-time, the safety of travel routes. Using handheld GPS tracking systems, the utility of the geomatics system to present real-time travel conditions was carried out in a Canadian First Nations community, located along the Western James Bay coast. The results of this study showed that the collaborative-geomatics tool offers the potential to monitor and store information on the safety of travel routes, helping to promote adaptive capacity and aid in knowledge transfer within arctic and subarctic indigenous communities
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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