523 research outputs found
Into the third dimension: stochastic measurements of Stokes parameters within the Poincar\'e sphere
Inspired by recent use of polarimetry to study the Cosmic Microwave
Background and extragalatic supernovae, a foray into the statistical properties
of Stokes parameters expressed in spherical coordinates is began, allowing
circular polarization and linear polarization to be treated in a unified
manner. The use of spherical coordinates is quite necessary as it permits a
Stokes polarization state to be expressed in terms of the customary
polarization angles and degree of polarization usually needed for human
interpretation. As shall be demonstrated, circular and linear polarization are
not statistically independent quantities but intertwined in a way that is
especially important, for instance, at low signal-to-noise. New distributions,
classical estimators, and marginalizations are presented for this
"three-dimensional" polarization problem including a generalization of the Rice
distribution. The paper concludes with discussion regarding the potential
pitfalls of a lower dimensional analysis.Comment: 8 pages, 3 figure
Adaptive Mesh Refinement Simulations of the Ionization Structure and Kinematics of Damped Ly Systems with Self-consistent Radiative Transfer
We use high resolution Eulerian hydrodynamics simulations to study kinematic
properties of the low ionization species in damped Ly-alpha systems at redshift
z=3. Our adaptive mesh refinement simulations include most key ingredients
relevant for modeling neutral gas in high-column density absorbers:
hydrodynamics, gravitational collapse, continuum radiative transfer and gas
chemistry, but no star formation. We model high-resolution Keck spectra with
unsaturated low ion transitions in two Si II lines (1526 and 1808 A), and
compare simulated line profiles to the data from the SDSS DLA survey.
We find that with increasing grid resolution the models show a trend in
convergence towards the observed distribution of HI column densities. While in
our highest resolution model we recover the cumulative number of DLAs per unit
absorption distance, none of our models predicts DLA velocity widths as high as
indicated by the data, suggesting that feedback from star formation might be
important. At z=3 a non-negligible fraction of DLAs with column densities below
10^21 cm^-2 is caused by tidal tails due to galaxy-galaxy interactions in more
massive halo environments. Lower column density absorbers with N_HI < 10^21.4
cm^-2 are sensitive to changes in the UV background resulting in a 10%
reduction of the cumulative number of DLAs for twice the quasar background
relative to the fiducial value. We find that the mass cut-off below which a
large fraction of dwarf galaxies cannot retain gas after reionization is 7*10^7
msun, lower than the previous estimates. Finally, we show that models with
self-shielding commonly used in the literature produce significantly lower DLA
velocity widths than the full radiative transfer runs.Comment: 13 pages, 11 figures, updated version, accepted to Ap
A Light Echo from Type Ia SN 1995E?
We identify a light echo candidate from Hubble Space Telescope (HST) imaging of NGC 2441, the host galaxy of the Type Ia supernova 1995E. From the echo's angular size and the estimated distance to the host galaxy, we find a distance of 207 ± 35 pc betwe
Orbits for the Impatient: A Bayesian Rejection Sampling Method for Quickly Fitting the Orbits of Long-Period Exoplanets
We describe a Bayesian rejection sampling algorithm designed to efficiently
compute posterior distributions of orbital elements for data covering short
fractions of long-period exoplanet orbits. Our implementation of this method,
Orbits for the Impatient (OFTI), converges up to several orders of magnitude
faster than two implementations of MCMC in this regime. We illustrate the
efficiency of our approach by showing that OFTI calculates accurate posteriors
for all existing astrometry of the exoplanet 51 Eri b up to 100 times faster
than a Metropolis-Hastings MCMC. We demonstrate the accuracy of OFTI by
comparing our results for several orbiting systems with those of various MCMC
implementations, finding the output posteriors to be identical within shot
noise. We also describe how our algorithm was used to successfully predict the
location of 51 Eri b six months in the future based on less than three months
of astrometry. Finally, we apply OFTI to ten long-period exoplanets and brown
dwarfs, all but one of which have been monitored over less than 3% of their
orbits, producing fits to their orbits from astrometric records in the
literature.Comment: 32 pages, 28 figures, Accepted to A
Testing the methodological utility of trace element analysis for detetecting dietary differences in fossil fauna from Turkana
International audienc
Gemini Planet Imager Observational Calibrations VI: Photometric and Spectroscopic Calibration for the Integral Field Spectrograph
The Gemini Planet Imager (GPI) is a new facility instrument for the Gemini
Observatory designed to provide direct detection and characterization of
planets and debris disks around stars in the solar neighborhood. In addition to
its extreme adaptive optics and corona graphic systems which give access to
high angular resolution and high-contrast imaging capabilities, GPI contains an
integral field spectrograph providing low resolution spectroscopy across five
bands between 0.95 and 2.5 m. This paper describes the sequence of
processing steps required for the spectro-photometric calibration of GPI
science data, and the necessary calibration files. Based on calibration
observations of the white dwarf HD 8049B we estimate that the systematic error
in spectra extracted from GPI observations is less than 5%. The flux ratio of
the occulted star and fiducial satellite spots within coronagraphic GPI
observations, required to estimate the magnitude difference between a target
and any resolved companions, was measured in the -band to be in laboratory measurements and using
on-sky observations. Laboratory measurements for the , , and
filters are also presented. The total throughput of GPI, Gemini South and the
atmosphere of the Earth was also measured in each photometric passband, with a
typical throughput in -band of 18% in the non-coronagraphic mode, with some
variation observed over the six-month period for which observations were
available. We also report ongoing development and improvement of the data cube
extraction algorithm.Comment: 15 pages, 6 figures. Proceedings of the SPIE, 9147-30
A First Comparison of Kepler Planet Candidates in Single and Multiple Systems
In this letter we present an overview of the rich population of systems with
multiple candidate transiting planets found in the first four months of Kepler
data. The census of multiples includes 115 targets that show 2 candidate
planets, 45 with 3, 8 with 4, and 1 each with 5 and 6, for a total of 170
systems with 408 candidates. When compared to the 827 systems with only one
candidate, the multiples account for 17 percent of the total number of systems,
and a third of all the planet candidates. We compare the characteristics of
candidates found in multiples with those found in singles. False positives due
to eclipsing binaries are much less common for the multiples, as expected.
Singles and multiples are both dominated by planets smaller than Neptune; 69
+2/-3 percent for singles and 86 +2/-5 percent for multiples. This result, that
systems with multiple transiting planets are less likely to include a
transiting giant planet, suggests that close-in giant planets tend to disrupt
the orbital inclinations of small planets in flat systems, or maybe even to
prevent the formation of such systems in the first place.Comment: 13 pages, 13 figures, submitted to ApJ Letter
A Light Echo from Type Ia Supernova 1995E?
We identify a light echo candidate from Hubble Space Telescope (HST) imaging
of NGC 2441, the host galaxy of the Type Ia supernova 1995E. From the echo's
angular size and the estimated distance to the host galaxy, we find a distance
of 207 +/- 35 pc between the dust and the site of the supernova. If confirmed,
this echo brings the total number of observed non-historical Type Ia light
echoes to three -- the others being SN 1991T and SN 1998bu -- suggesting they
are not uncommon. We compare the properties of the known Type Ia supernova
echoes and test models of light echoes developed by Patat et al. (2005). HST
photometry of the SN 1991T echo shows a fading which is consistent with
scattering by dust distributed in a sphere or shell around the supernova. Light
echoes have the potential to answer questions about the progenitors of Type Ia
supernovae and more effort should be made for their detection given the
importance of Type Ia supernovae to measurements of dark energy.Comment: 19 pages, 4 postscript figures, 3 tables, accepted for publication in
the Ap
Rapid UBVRI Follow-up of the Highly Collimated Optical Afterglow of GRB010222
(Abridged) We present the earliest optical observations of the optical
counterpart to the GRB 010222, obtained with the FLWO 1.2-m telescope in UBVRI
passbands, starting 3.64 hours after the burst. We also present late R-band
observations of the afterglow obtained with the 1.8-m VATT ~25 days after the
burst. The temporal analysis of our data joined with published data indicates a
steepening decay, independent of wavelength, asymptotically approaching
t^{-0.80+/-0.05} at early times (t << 1 day) and t^{-1.30+/-0.05} at late
times, with a sharp break at t_b=0.72+/-0.1 days. This is the second earliest
observed break of any afterglow (after GRB 980519), which clearly indicates the
importance of rapid multi-band follow-up for GRB afterglow research. The
optical spectral energy distribution, corrected for small Galactic reddening,
can be fit fairly well by a single power-law with a slope of -1.07+/-0.09.
However, when we fit using our BVRI data only, we obtain a shallower slope of
-0.88+/- 0.1, in excellent agreement with the slope derived from our
low-resolution spectrum (-0.89 +/- 0.03). The spectral slope and light curve
decay slopes we derive are not consistent with a jet model despite the presence
of a temporal break. Significant host dust extinction with a star-burst
reddening law would flatten the spectral index to match jet predictions and
still be consistent with the observed spectral energy distribution. We derive
an opening angle of 2.1 deg, smaller than any listed in the recent compilation
of Frail et al. The total beamed energy corrected for the jet geometry is
4x10^50 erg, very close to the ``standard'' value of 5x10^50 erg found by Frail
et al. for a number of other bursts with light-curve breaks.Comment: revised version (minor changes only) to be published in the ApJ Part
1, 12 pages, 4 figures; all data used for the fits and our CCD frames
available at ftp://cfa-ftp.harvard.edu/pub/kstanek/GRB010222/ and through WWW
at http://cfa-www.harvard.edu/cfa/oir/Research/GRB
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