2,025 research outputs found
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
Hubble Space Telescope times-series photometry of the planetary transit of HD189733: no moon, no rings, starspots
We monitored three transits of the giant gas planet around the nearby K dwarf
HD 189733 with the ACS camera on the Hubble Space Telescope. The resulting
very-high accuracy lightcurve (signal-to-noise ratio near 15000 on individual
measurements, 35000 on 10-minute averages) allows a direct geometric
measurement of the orbital inclination, radius ratio and scale of the system: i
= 85.68 +- 0.04, Rpl/R*=0.1572 +- 0.0004, a/R*=8.92 +- 0.09. We derive improved
values for the stellar and planetary radius, R*=0.755+- 0.011 Rsol, Rpl=1.154
+- 0.017 RJ, and the transit ephemerides, Ttr=2453931.12048 +- 0.00002 + n
2.218581 +- 0.000002$. The HST data also reveal clear evidence of the planet
occulting spots on the surface of the star. At least one large spot complex
(>80000 km) is required to explain the observed flux residuals and their colour
evolution. This feature is compatible in amplitude and phase with the
variability observed simultaneously from the ground. No evidence for satellites
or rings around HD 189733b is seen in the HST lightcurve. This allows us to
exlude with a high probability the presence of Earth-sized moons and
Saturn-type debris rings around this planet. The timing of the three transits
sampled is stable to the level of a few seconds, excluding a massive second
planet in outer 2:1 resonance.Comment: revised version. Significant updates and new figures; to appear in
Astronomy and Astrophysic
The dynamical structure factor in topologically disordered systems
A computation of the dynamical structure factor of topologically disordered
systems, where the disorder can be described in terms of euclidean random
matrices, is presented. Among others, structural glasses and supercooled
liquids belong to that class of systems. The computation describes their
relevant spectral features in the region of the high frequency sound. The
analytical results are tested with numerical simulations and are found to be in
very good agreement with them. Our results may explain the findings of
inelastic X-ray scattering experiments in various glassy systems.Comment: Version to be published in J. Chem. Phy
Stellar Oscillations Network Group
Stellar Oscillations Network Group (SONG) is an initiative aimed at designing
and building a network of 1m-class telescopes dedicated to asteroseismology and
planet hunting. SONG will have 8 identical telescope nodes each equipped with a
high-resolution spectrograph and an iodine cell for obtaining precision radial
velocities and a CCD camera for guiding and imaging purposes. The main
asteroseismology targets for the network are the brightest (V<6) stars. In
order to improve performance and reduce maintenance costs the instrumentation
will only have very few modes of operation. In this contribution we describe
the motivations for establishing a network, the basic outline of SONG and the
expected performance.Comment: Proc. Vienna Workshop on the Future of Asteroseismology, 20 - 22
September 2006. Comm. in Asteroseismology, Vol. 150, in the pres
On the high-density expansion for Euclidean Random Matrices
Diagrammatic techniques to compute perturbatively the spectral properties of
Euclidean Random Matrices in the high-density regime are introduced and
discussed in detail. Such techniques are developed in two alternative and very
different formulations of the mathematical problem and are shown to give
identical results up to second order in the perturbative expansion. One method,
based on writing the so-called resolvent function as a Taylor series, allows to
group the diagrams in a small number of topological classes, providing a simple
way to determine the infrared (small momenta) behavior of the theory up to
third order, which is of interest for the comparison with experiments. The
other method, which reformulates the problem as a field theory, can instead be
used to study the infrared behaviour at any perturbative order.Comment: 29 page
Nature of vibrational eigenmodes in topologically disordered solids
We use a local projectional analysis method to investigate the effect of
topological disorder on the vibrational dynamics in a model glass simulated by
molecular dynamics. Evidence is presented that the vibrational eigenmodes in
the glass are generically related to the corresponding eigenmodes of its
crystalline counterpart via disorder-induced level-repelling and hybridization
effects. It is argued that the effect of topological disorder in the glass on
the dynamical matrix can be simulated by introducing positional disorder in a
crystalline counterpart.Comment: 7 pages, 6 figures, PRB, to be publishe
Vibrational spectrum of topologically disordered systems
The topological nature of the disorder of glasses and supercooled liquids
strongly affects their high-frequency dynamics. In order to understand its main
features, we analytically studied a simple topologically disordered model,
where the particles oscillate around randomly distributed centers, interacting
through a generic pair potential. We present results of a resummation of the
perturbative expansion in the inverse particle density for the dynamic
structure factor and density of states. This gives accurate results for the
range of densities found in real systems.Comment: Completely rewritten version, accepted in Physical Review Letter
de Sitter relativity: a natural scenario for an evolving Lambda
The dispersion relation of de Sitter special relativity is obtained in a
simple and compact form, which is formally similar to the dispersion relation
of ordinary special relativity. It is manifestly invariant under change of
scale of mass, energy and momentum, and can thus be applied at any energy
scale. When applied to the universe as a whole, the de Sitter special
relativity is found to provide a natural scenario for the existence of an
evolving cosmological term, and agrees in particular with the present-day
observed value. It is furthermore consistent with a conformal cyclic view of
the universe, in which the transition between two consecutive eras occurs
through a conformal invariant spacetime.Comment: V1: 11 pages. V2: Presentation changes, new discussion added, 13
page
Characterization of the K2-18 multi-planetary system with HARPS: A habitable zone super-Earth and discovery of a second, warm super-Earth on a non-coplanar orbit
The bright M dwarf K2-18 at 34 pc is known to host a transiting
super-Earth-sized planet orbiting within the star's habitable zone; K2-18b.
Given the superlative nature of this system for studying an exoplanetary
atmosphere receiving similar levels of insolation as the Earth, we aim to
characterize the planet's mass which is required to interpret atmospheric
properties and infer the planet's bulk composition. We obtain precision radial
velocity measurements with the HARPS spectrograph and couple those measurements
with the K2 photometry to jointly model the observed radial velocity variation
with planetary signals and a radial velocity jitter model based on Gaussian
process regression. We measure the mass of K2-18b to be
M with a bulk density of g/cm which may correspond
to a predominantly rocky planet with a significant gaseous envelope or an ocean
planet with a water mass fraction %. We also find strong evidence
for a second, warm super-Earth K2-18c at days with a semi-major axis
2.4 times smaller than the transiting K2-18b. After re-analyzing the available
light curves of K2-18 we conclude that K2-18c is not detected in transit and
therefore likely has an orbit that is non-coplanar with K2-18b. A suite of
dynamical integrations with varying simulated orbital eccentricities of the two
planets are used to further constrain each planet's eccentricity posterior from
which we measure and at 99% confidence. The discovery
of the inner planet K2-18c further emphasizes the prevalence of multi-planet
systems around M dwarfs. The characterization of the density of K2-18b reveals
that the planet likely has a thick gaseous envelope which along with its
proximity to the Solar system makes the K2-18 planetary system an interesting
target for the atmospheric study of an exoplanet receiving Earth-like
insolation.Comment: 13 pages, 8 figures including 4 interactive figures best viewed in
Adobe Acrobat. Submitted to Astronomy & Astrophysics. Comments welcom
Line Broadening in Field Metal-poor Red Giant and Red Horizontal Branch Stars
We report 349 radial velocities for 45 metal-poor field red giant and red
horizontal branch stars. We have have identified one new spectroscopic binary,
HD 4306, and one possible such system, HD 184711. We also report 57 radial
velocities for 11 of the 91 stars reported on previously by Carney et al.
(2003). As was found in the previous study, radial velocity "jitter" is present
in many of the most luminous stars. Excluding stars showing spectroscopic
binary orbital motion, all 7 of the red giants with M(V) <= -2.0 display
jitter, as well as 3 of the 14 stars with -2.0 <= M(V) <= -1.4. We have also
measured line broadening in all of the new spectra, using synthetic spectra as
templates. The most luminous red giants show significant line broadening, as do
many of the red horizontal branch stars, and we discuss briefly possible
causes.Comment: To appear in the Astronomical Journa
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