142 research outputs found
Hysteresis and beats in loop quantum cosmology
Differences in pressure during expansion and contraction stages in cosmic
evolution can result in a hysteresis-like phenomena in non-singular cyclic
models sourced with scalar fields. We discuss this phenomena for spatially
closed isotropic spacetime in loop quantum cosmology (LQC) for a quadratic and
a cosh-like potential, with and without a negative cosmological constant using
effective spacetime description of the underlying quantum geometry. Two
inequivalent loop quantizations of this spacetime, one based on holonomies of
the Ashtekar-Barbero connection using closed loops, and another based on the
connection operator, are discussed. Due to quantum geometric effects, both
models avoid classical singularities, but unlike the holonomy based
quantization, connection based quantization results in two quantum bounces. In
spite of differences in non-singular effective dynamics of both the models, the
phenomena of hysteresis is found to be robust for the potential.
Quasi-periodic beats exist for a cosh-like potential, irrespective of the
nature of classical recollapse whether by spatial curvature, or a negative
cosmological constant. An interplay of negative cosmological constant and
spatial curvature in presence of potentials results in rich features such as
islands of cluster of bounces separated by accelerated expansion, and a
universe which either undergoes a step like expansion with multiple turnarounds
or quasi-periodic beats depending on a "tuning" of the steepness parameter of
the potential.Comment: 12 pages, 17 figure
Implications of quantum ambiguities in k = 1 loop quantum cosmology: Distinct quantum turnarounds and the super-Planckian regime
The spatially closed Friedmann-Lema\^{i}tre-Robertson-Walker model in loop
quantum cosmology admits two inequivalent consistent quantizations: one based
on expressing the field strength in terms of the holonomies over closed loops,
and, another using a connection operator and open holonomies. Using the
effective dynamics, we investigate the phenomenological differences between the
two quantizations for the single fluid and the two fluid scenarios with various
equations of state, including the phantom matter. We show that a striking
difference between the two quantizations is the existence of two distinct
quantum turnarounds, either bounces or recollapses, in the connection
quantization, in contrast to a single distinct quantum bounce or a recollapse
in the holonomy quantization. These results generalize an earlier result on the
existence of two distinct quantum bounces for stiff matter by Corichi and
Karami. However, we find that in certain situations two distinct quantum
turnarounds can become virtually indistinguishable. And depending on the
initial conditions, a pure quantum cyclic universe can also exist undergoing a
quantum bounce and a quantum recollapse. We show that for various equations of
states, connection based quantization leads to super-Planckian values of the
energy density and the expansion scalar at quantum turnarounds. Interestingly,
we find that very extreme energy densities can also occur for the holonomy
quantization, breaching the maximum allowed density in the spatially flat loop
quantized model. However, the expansion scalar in all these cases is bounded by
a universal value.Comment: Revised version with expanded results and discussion on the role of
inverse volume effects. Four figures added. To appear in Phys. Rev.
Larval culture of the calico scallop, Argopecten gibbus
Mature calico scallops, Argopecten gibbus, collected from the grounds off Cape Kennedy, Florida, were induced to spawn in the laboratory. Fertilized eggs were reared to postlarvae in sea water of 23° C ± 2.0° C at a salinity of 35 %o. The external morphology of eggs and developing larval stages are described
NLTT 41135: a field M-dwarf + brown dwarf eclipsing binary in a triple system, discovered by the MEarth observatory
We report the discovery of an eclipsing companion to NLTT 41135, a nearby M5
dwarf that was already known to have a wider, slightly more massive common
proper motion companion, NLTT 41136, at 2.4 arcsec separation. Analysis of
combined-light and radial velocity curves of the system indicates that NLTT
41135B is a 31-34 +/- 3 MJup brown dwarf (where the range depends on the
unknown metallicity of the host star) on a circular orbit. The visual M-dwarf
pair appears to be physically bound, so the system forms a hierarchical triple,
with masses approximately in the ratio 8:6:1. The eclipses are grazing,
preventing an unambiguous measurement of the secondary radius, but follow-up
observations of the secondary eclipse (e.g. with the James Webb Space
Telescope) could permit measurements of the surface brightness ratio between
the two objects, and thus place constraints on models of brown dwarfs.Comment: 15 pages, 6 figures, 10 tables, emulateapj format. Accepted for
publication in Ap
Identification of a Wide, Low-Mass Multiple System Containing the Brown Dwarf 2MASS J0850359+105716
We report our discovery of NLTT 20346 as an M5+M6 companion system to the
tight binary (or triple) L dwarf 2MASS J0850359+105716. This nearby (~31 pc),
widely separated (~7700 AU) quadruple system was identified through a
cross-match of proper motion catalogs. Follow-up imaging and spectroscopy of
NLTT 20346 revealed it to be a magnetically active M5+M6 binary with components
separated by ~2" (50-80 AU). Optical spectroscopy of the components show only
moderate Halpha emission corresponding to a statistical age of ~5 - 7 Gyr for
both M dwarfs. However NLTT 20346 is associated with the XMM-Newton source
J085018.9+105644, and based on X-ray activity the age of NLTT 20346 is between
250-450 Myr. Strong Li absorption in the optical spectrum of 2MASS J0850+1057
indicates an upper age limit of 0.8 - 1.5 Gyr favoring the younger age for the
primary. Using evolutionary models in combination with an adopted system age of
0.25-1.5 Gyr indicates a total mass for 2MASS J0850+1057 of 0.07+/-0.02 Msun if
it is a binary. NLTT 20346/2MASS J0850+1057 joins a growing list of
hierarchical systems containing brown dwarf binaries and is among the lowest
binding energy associations found in the field. Formation simulations via
gravitational fragmentation of massive extended disks have successfully
produced a specific analog to this system.Comment: 13 pages, accepted for publication to A
Clouds in the Coldest Brown Dwarfs: FIRE Spectroscopy of Ross 458C
Condensate clouds are a salient feature of L dwarf atmospheres, but have been
assumed to play little role in shaping the spectra of the coldest T-type brown
dwarfs. Here we report evidence of condensate opacity in the near-infrared
spectrum of the brown dwarf candidate Ross 458C, obtained with the Folded-Port
Infrared Echellette (FIRE) spectrograph at the Magellan Telescopes. These data
verify the low-temperature nature of this source, indicating a T8 spectral
classification, log Lbol/Lsun = -5.62+/-0.03, Teff = 650+/-25 K, and a mass at
or below the deuterium burning limit. The data also reveal enhanced emission at
K-band associated with youth (low surface gravity) and supersolar metallicity,
reflecting the properties of the Ross 458 system (age = 150-800 Myr, [Fe/H] =
+0.2 to +0.3). We present fits of FIRE data for Ross 458C, the T9 dwarf ULAS
J133553.45+113005.2, and the blue T7.5 dwarf SDSS J141624.08+134826.7B, to
cloudless and cloudy spectral models from Saumon & Marley. For Ross 458C we
confirm a low surface gravity and supersolar metallicity, while the temperature
differs depending on the presence (635 [+25,-35] K) or absence (760 [+70,-45]
K) of cloud extinction. ULAS J1335+1130 and SDSS J1416+1348B have similar
temperatures (595 [+25,-45] K), but distinct surface gravities (log g = 4.0-4.5
cgs versus 5.0-5.5 cgs) and metallicities ([M/H] ~ +0.2 versus -0.2). In all
three cases, cloudy models provide better fits to the spectral data,
significantly so for Ross 458C. These results indicate that clouds are an
important opacity source in the spectra of young cold T dwarfs, and should be
considered when characterizing the spectra of planetary-mass objects in young
clusters and directly-imaged exoplanets. The characteristics of Ross 458C
suggest it could itself be regarded as a planet, albeit one whose cosmogony
does not conform with current planet formation theories.Comment: Accepted for publication to ApJ: 18 pages, 11 figures in emulateapj
forma
Aerosol properties associated with air masses arriving into the North East Atlantic during the 2008 Mace Head EUCAARI intensive observing period: an overview
As part of the EUCAARI Intensive Observing Period, a 4-week campaign to measure aerosol physical, chemical and optical properties, atmospheric structure, and cloud microphysics was conducted from mid-May to mid-June, 2008 at the Mace Head Atmospheric Research Station, located at the interface of Western Europe and the N. E. Atlantic and centered on the west Irish coastline. During the campaign, continental air masses comprising both young and aged continental plumes were encountered, along with polar, Arctic and tropical air masses. Polluted-continental aerosol concentrations were of the order of 3000 cm(-3), while background marine air aerosol concentrations were between 400-600 cm(-3). The highest marine air concentrations occurred in polar air masses in which a 15 nm nucleation mode, with concentration of 1100 cm(-3), was observed and attributed to open ocean particle formation. Continental air submicron chemical composition (excluding refractory sea salt) was dominated by organic matter, closely followed by sulphate mass. Although the concentrations and size distribution spectral shape were almost identical for the young and aged continental cases, hygroscopic growth factors (GF) and cloud condensation nuclei (CCN) to total condensation nuclei (CN) concentration ratios were significantly less in the younger pollution plume, indicating a more oxidized organic component to the aged continental plume. The difference in chemical composition and hygroscopic growth factor appear to result in a 40-50% impact on aerosol scattering coefficients and Aerosol Optical Depth, despite almost identical aerosol microphysical properties in both cases, with the higher values been recorded for the more aged case. For the CCN/CN ratio, the highest ratios were seen in the more age plume. In marine air, sulphate mass dominated the sub-micron component, followed by water soluble organic carbon, which, in turn, was dominated by methanesulphonic acid (MSA). Sulphate concentrations were highest in marine tropical air - even higher than in continental air. MSA was present at twice the concentrations of previously-reported concentrations at the same location and the same season. Both continental and marine air exhibited aerosol GFs significantly less than ammonium sulphate aerosol pointing to a significant organic contribution to all air mass aerosol properties
First Light LBT AO Images of HR 8799 bcde at 1.65 and 3.3 Microns: New Discrepancies between Young Planets and Old Brown Dwarfs
As the only directly imaged multiple planet system, HR 8799 provides a unique
opportunity to study the physical properties of several planets in parallel. In
this paper, we image all four of the HR 8799 planets at H-band and 3.3 microns
with the new LBT adaptive optics system, PISCES, and LBTI/LMIRCam. Our images
offer an unprecedented view of the system, allowing us to obtain H and 3.3$
micron photometry of the innermost planet (for the first time) and put strong
upper-limits on the presence of a hypothetical fifth companion. We find that
all four planets are unexpectedly bright at 3.3 microns compared to the
equilibrium chemistry models used for field brown dwarfs, which predict that
planets should be faint at 3.3 microns due to CH4 opacity. We attempt to model
the planets with thick-cloudy, non-equilibrium chemistry atmospheres, but find
that removing CH4 to fit the 3.3 micron photometry increases the predicted L'
(3.8 microns) flux enough that it is inconsistent with observations. In an
effort to fit the SED of the HR 8799 planets, we construct mixtures of cloudy
atmospheres, which are intended to represent planets covered by clouds of
varying opacity. In this scenario, regions with low opacity look hot and
bright, while regions with high opacity look faint, similar to the patchy cloud
structures on Jupiter and L/T transition brown-dwarfs. Our mixed cloud models
reproduce all of the available data, but self-consistent models are still
necessary to demonstrate their viability.Comment: Accepted to Ap
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