1,779 research outputs found
Probing Spatial Variation Of The Fine-Structure Constant Using The CMB
The fine-structure constant, α, controls the strength of the electromagnetic interaction. There are extensions of the standard model in which α is dynamical on cosmological length and time scales. The physics of the cosmic microwave background (CMB) depends on the value of α. The effects of spatial variation in α on the CMB are similar to those produced by weak lensing: smoothing of the power spectrum, and generation of non-Gaussian features. These would induce a bias to estimates of the weak-lensing potential power spectrum of the CMB. Using this effect, Planck measurements of the temperature and polarization power spectrum, as well as estimates of CMB lensing, are used to place limits (95% C.L.) on the amplitude of a scale-invariant angular power spectrum of α fluctuations relative to the mean value (CαL=AαSI/[L(L+1)]) of AαSI≤1.6×10−5. The limits depend on the assumed shape of the α-fluctuation power spectrum. For example, for a white-noise angular power spectrum (CαL=AαWN), the limit is AαWN≤2.3×10−8. It is found that the response of the CMB to α fluctuations depends on a separate-universe approximation, such that theoretical predictions are only reliable for α multipoles with L≲100. An optimal trispectrum estimator can be constructed and it is found that it is only marginally more sensitive than lensing techniques for Planck but significantly more sensitive when considering the next generation of experiments. For a future CMB experiment with cosmic-variance limited polarization sensitivity (e.g., CMB-S4), the optimal estimator could detect α fluctuations with AαSI\u3e1.9×10−6 and AαWN\u3e1.4×10−9
Hintergrund und erste Ergebnisse der vulkanologisch-petrologischen Untersuchungen von SO246 (CHATHAM RISE): Kompressions-, Extensions- und Abbruchmechanismen eines submarinen Kontinentalplateaus
Discovery of a High Proper Motion L Dwarf Binary: 2MASS J15200224-4422419AB
We report the discovery of the wide L1.5+L4.5 binary 2MASS
J15200224-4422419AB, identified during spectroscopic followup of high proper
motion sources selected from the Two Micron All Sky Survey. This source was
independently identified by Kendall et al. in the SuperCOSMOS Sky Survey.
Resolved JHK photometry and low resolution near-infrared spectroscopy
demonstrate that this system is composed of two well-separated (1"174+/-0"016)
L dwarfs. Component classifications are derived using both spectral ratios and
comparison to near-infrared spectra of previously classified field L dwarfs.
Physical association for the pair is deduced from the large (mu = 0"73+/-0"03
/yr) common proper motion of the components and their similar
spectrophotometric distances (19+/-2 pc). The projected separation of the
binary, 22+/-2 AU, is consistent with maximum separation/total system mass
trends for very low mass binaries. The 2MASS J1520-4422 system exhibits both
large tangential (66+/-7 km/s) and radial velocities (-70+/-18 km/s), and its
motion in the local standard of rest suggests that it is an old member of the
Galactic disk population. This system joins a growing list of well-separated
(>0"5), very low mass binaries, and is an excellent target for resolved optical
spectroscopy to constrain its age as well as trace activity/rotation trends
near the hydrogen-burning limit.Comment: 35 pages, 8 figures; accepted for publication to ApJ; see also
Kendall et al. astro-ph/060939
South Carolina Marine Game Fish Tagging Program 1974-1992
South Carolina's Marine Game Fish Tagging Program began in 1974. This tagging program has proven to be valuable not only for the information gathered on different species of game fish but also for promoting conservation among saltwater anglers. From 1974 through 1992, cooperating anglers tagged over 42,000 marine fishes of 96 species representing 35 families
Radii of 88 M subdwarfs and updated radius relations for low-metallicity M-dwarf stars
M subdwarfs are low-metallicity M dwarfs that typically inhabit the halo population of the Galaxy. Metallicity controls the opacity of stellar atmospheres; in metal-poor stars, hydrostatic equilibrium is reached at a smaller radius, leading to smaller radii for a given effective temperature. We compile a sample of 88 stars that span spectral classes K7 to M6 and include stars with metallicity classes from solar-metallicity dwarf stars to the lowest metallicity ultra subdwarfs to test how metallicity changes the stellar radius. We fit models to Palomar Double Spectrograph (DBSP) optical spectra to derive effective temperatures (T_ eff) and we measure bolometric luminosities (L_ bol) by combining broad wavelength-coverage photometry with Gaia parallaxes. Radii are then computed by combining the T_ eff and L_ bol using the Stefan–Boltzman law. We find that for a given temperature, ultra subdwarfs can be as much as five times smaller than their solar-metallicity counterparts. We present color-radius and color-surface brightness relations that extend down to [Fe/H] of −2.0 dex, in order to aid the radius determination of M subdwarfs, which will be especially important for the WFIRST exoplanetary microlensing survey.Published versio
Substellar Companions to Main Sequence Stars: No Brown Dwarf Desert at Wide Separations
We use three field L and T dwarfs which were discovered to be wide companions
to known stars by the Two Micron All-Sky Survey (2MASS) to derive a preliminary
brown dwarf companion frequency. Observed L and T dwarfs indicate that brown
dwarfs are not unusually rare as wide (Delta >1000 A.U.) systems to F-M0
main-sequence stars (M>0.5M_sun, M_V<9.5), even though they are rare at close
separation (Delta <3 A.U.), the ``brown dwarf desert.'' Stellar companions in
these separation ranges are equally frequent, but brown dwarfs are >~ 10 times
as frequent for wide than close separations. A brown dwarf wide-companion
frequency as low as the 0.5% seen in the brown dwarf desert is ruled out by
currently-available observations.Comment: ApJL, in pres
A Cross-Match of 2MASS and SDSS: Newly-Found L and T Dwarfs and an Estimate of the Space Densitfy of T Dwarfs
We report new L and T dwarfs found in a cross-match of the SDSS Data Release
1 and 2MASS. Our simultaneous search of the two databases effectively allows us
to relax the criteria for object detection in either survey and to explore the
combined databases to a greater completeness level. We find two new T dwarfs in
addition to the 13 already known in the SDSS DR1 footprint. We also identify 22
new candidate and bona-fide L dwarfs, including a new young L2 dwarf and a
peculiar L2 dwarf with unusually blue near-IR colors: potentially the result of
mildly sub-solar metallicity. These discoveries underscore the utility of
simultaneous database cross-correlation in searching for rare objects. Our
cross-match completes the census of T dwarfs within the joint SDSS and 2MASS
flux limits to the 97% level. Hence, we are able to accurately infer the space
density of T dwarfs. We employ Monte Carlo tools to simulate the observed
population of SDSS DR1 T dwarfs with 2MASS counterparts and find that the space
density of T0-T8 dwarf systems is 0.0070 (-0.0030; +0.0032) per cubic parsec
(95% confidence interval), i.e., about one per 140 cubic parsecs. Compared to
predictions for the T dwarf space density that depend on various assumptions
for the sub-stellar mass function, this result is most consistent with models
that assume a flat sub-stellar mass function dN/dM ~ M^0. No >T8 dwarfs were
discovered in the present cross-match, though less than one was expected in the
limited area (2099 sq. degrees) of SDSS DR1.Comment: To appear in ApJ, Feb 10, 2008 issue. 37 pages, including 12 figures
and 14 table
Variance of transmitted power in multichannel dissipative ergodic structures invariant under time reversal
We use random matrix theory (RMT) to study the first two moments of the wave
power transmitted in time reversal invariant systems having ergodic motion.
Dissipation is modeled by a number of loss channels of variable coupling
strength. To make a connection with ultrasonic experiments on ergodic
elastodynamic billiards, the channels injecting and collecting the waves are
assumed to be negligibly coupled to the medium, and to contribute essentially
no dissipation. Within the RMT model we calculate the quantities of interest
exactly, employing the supersymmetry technique. This approach is found to be
more accurate than another method based on simplifying naive assumptions for
the statistics of the eigenfrequencies and the eigenfunctions. The results of
the supersymmetric method are confirmed by Monte Carlo numerical simulation and
are used to reveal a possible source of the disagreement between the
predictions of the naive theory and ultrasonic measurements.Comment: 10 pages, 2 figure
Three-dimensional subwavelength confinement of light with dielectric microspheres
International audienceDielectric microspheres are shown to be capable of confining light in a three-dimensional region of subwavelength dimensions when they are illuminated by tightly focused Gaussian beams. We show that a simple configuration, not involving resonances, permits one to reach an effective volume as small as 0.6 (l/n)3. It is shown that this three-dimensional confinement arises from interferences between the field scattered by the sphere and the high angular components of the incident Gaussian beam passing aside the sphere
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