2,268 research outputs found
Planetary host stars: Evaluating uncertainties in ultra-cool model atmospheres
M-dwarfs are emerging in the literature as promising targets for detecting
low-mass, Earth-like planets. An important step in this process is to determine
the stellar parameters of the M-dwarf host star as accurately as possible.
Different well-tested stellar model atmosphere simulations from different
groups are widely applied to undertake this task. This paper provides a
comparison of different model atmosphere families to allow a better estimate of
systematic errors on host-star stellar parameter introduced by the use of one
specific model atmosphere family only. We present a comparison of the ATLAS9,
MARCS, Phoenix and Drift-Phoenix model atmosphere families including the
M-dwarf parameter space (TK4000K,
log(g)=3.05.0, [M/H]=). We examine the differences in
the (T, p)-structures, in synthetic photometric fluxes
and in colour indices. Model atmospheres results for higher log(g) deviate
considerably less between different models families than those for lower log(g)
for all TK4000K examined. We compiled the broad-band
synthetic photometric fluxes for all available model atmospheres (incl.
M-dwarfs and brown dwarfs) for the UKIRT WFCAM ZYJHK, 2MASS JHKs and Johnson
UBVRI filters, and calculated related colour indices. Synthetic colours in the
IR wavelengths diverge by no more than 0.15 dex amongst all model families. For
all spectral bands considered, model discrepancies in colour diminish for
higher T atmosphere simulations. We notice differences in synthetic
colours between all model families and observed example data (incl. Kepler 42
and GJ1214).Comment: accepted for publication in MNRA
The Influence of Dust Formation Modelling on Na I and K I Line Profiles in Substellar Atmospheres
We aim to understand the correlation between cloud formation and alkali line
formation in substellar atmospheres.We perform line profile calculations for Na
I and K I based on the coupling of our kinetic model for the formation and
composition of dust grains with 1D radiative transfer calculations in
atmosphere models for brown dwarfs and giant gas planets. The Na I and K I line
profiles sensibly depend on the way clouds are treated in substellar atmosphere
simulations. The kinetic dust formation model results in the highest
pseudo-continuum compared to the limiting cases.Comment: 5 pages, Accepted for publication in MNRA
Law and Diversity Program: A Method for Attracting, Retaining, and Preparing Diverse Students for Law School
Law and Diversity Program: A Method for Attracting, Retaining, and Preparing Diverse Students for Law School
Groups of Fibonacci type revisited
This article concerns a class of groups of Fibonacci type introduced by Johnson and Mawdesley that includes Conway?s Fibonacci groups, the Sieradski groups, and the Gilbert-Howie groups. This class of groups provides an interesting focus for developing the theory of cyclically presented groups and, following questions by Bardakov and Vesnin and by Cavicchioli, Hegenbarth, and Repov?s, they have enjoyed renewed interest in recent years. We survey results concerning their algebraic properties, such as isomorphisms within the class, the classification of the finite groups, small cancellation properties, abelianizations, asphericity, connections with Labelled Oriented Graph groups, and the semigroups of Fibonacci type. Further, we present a new method of proving the classification of the finite groups that deals with all but three groups
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
Hamiltonian and physical Hilbert space in polymer quantum mechanics
In this paper, a version of polymer quantum mechanics, which is inspired by
loop quantum gravity, is considered and shown to be equivalent, in a precise
sense, to the standard, experimentally tested, Schroedinger quantum mechanics.
The kinematical cornerstone of our framework is the so called polymer
representation of the Heisenberg-Weyl (H-W) algebra, which is the starting
point of the construction. The dynamics is constructed as a continuum limit of
effective theories characterized by a scale, and requires a renormalization of
the inner product. The result is a physical Hilbert space in which the
continuum Hamiltonian can be represented and that is unitarily equivalent to
the Schroedinger representation of quantum mechanics. As a concrete
implementation of our formalism, the simple harmonic oscillator is fully
developed.Comment: 19 pages, 2 figures. Comments and references added. Version to be
published in CQ
Prospects for Detection of Exoplanet Magnetic Fields Through Bow-Shock Observations During Transits
An asymmetry between the ingress and egress times was observed in the near-UV
light curve of the transit planet WASP-12b. Such asymmetry led us to suggest
that the early ingress in the UV light curve of WASP-12b, compared to the
optical observations, is caused by a shock around the planet, and that shocks
should be a common feature in transiting systems. Here, we classify all the
transiting systems known to date according to their potential for producing
shocks that could cause observable light curve asymmetries. We found that 36/92
of known transiting systems would lie above a reasonable detection threshold
and that the most promising candidates to present shocks are: WASP-19b,
WASP-4b, WASP-18b, CoRoT-7b, HAT-P-7b, CoRoT-1b, TrES-3, and WASP-5b. For
prograde planets orbiting outside the co-rotation radius of fast rotating
stars, the shock position, instead of being ahead of the planetary motion as in
WASP-12b, trails the planet. In this case, we predict that the light curve of
the planet should present a late-egress asymmetry. We show that CoRoT-11b is a
potential candidate to host such a behind shock and show a late egress. If
observed, these asymmetries can provide constraints on planetary magnetic
fields. For instance, for a planet that has a magnetic field intensity similar
to Jupiter's field (~ 14 G) orbiting a star whose magnetic field is between 1
and 100G, the stand-off distance between the shock and the planet, which we
take to be the size of the planet's magnetosphere, ranges from 1 to 40
planetary radii.Comment: 7 pages (including the complete version of Table 1), 2 Tables, 3
Figures. Accepted by MNRAS Letter
Polymer state approximations of Schroedinger wave functions
It is shown how states of a quantum mechanical particle in the Schroedinger
representation can be approximated by states in the so-called polymer
representation. The result may shed some light on the semiclassical limit of
loop quantum gravity.Comment: 11 pages, 1 figure, Conclusions section adde
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