3,510 research outputs found
Do Social Connections Reduce Moral Hazard? Evidence from the New York City Taxi Industry
This study investigates the role of social networks in aligning the incentives of economic agents in settings with incomplete contracts. We study the New York City taxi industry where taxis are often leased and lessee-drivers have worse driving outcomes than owner-drivers as a result of a moral hazard associated with incomplete leasing contracts. Using instrumental variables and fixed-effects analyses, we find that: (1) drivers leasing from members of their country-of-birth community exhibit significantly reduced effects of moral hazard; (2) network effects appear to operate primarily via social sanctions; and (3) network benefits can help to explain the organization of the industry in terms of which drivers and owners form business relationships.
Theoretical Transmission Spectra During Extrasolar Giant Planet Transits
The recent transit observation of HD 209458 b - an extrasolar planet orbiting
a sun-like star - confirmed that it is a gas giant and determined that its
orbital inclination is 85 degrees. This inclination makes possible
investigations of the planet atmosphere. In this paper we discuss the planet
transmission spectra during a transit. The basic tenet of the method is that
the planet atmosphere absorption features will be superimposed on the stellar
flux as the stellar flux passes through the planet atmosphere above the limb.
The ratio of the planet's transparent atmosphere area to the star area is
small, approximately 10^{-3} to 10^{-4}; for this method to work very strong
planet spectral features are necessary. We use our models of close-in
extrasolar giant planets to estimate promising absorption signatures: the
alkali metal lines, in particular the Na I and K I resonance doublets, and the
He I - triplet line at 1083.0 nm. If successful, observations
will constrain the line-of-sight temperature, pressure, and density. The most
important point is that observations will constrain the cloud depth, which in
turn will distinguish between different atmosphere models. We also discuss the
potential of this method for EGPs at different orbital distances and orbiting
non-solar-type stars.Comment: revised to agree with accepted paper, ApJ, in press. 12 page
Weak lensing mass reconstructions of the ESO Distant Cluster Survey
We present weak lensing mass reconstructions for the 20 high-redshift
clusters i n the ESO Distant Cluster Survey. The weak lensing analysis was
performed on deep, 3-color optical images taken with VLT/FORS2, using a
composite galaxy catalog with separate shape estimators measured in each
passband. We find that the EDisCS sample is composed primarily of clusters that
are less massive than t hose in current X-ray selected samples at similar
redshifts, but that all of the fields are likely to contain massive clusters
rather than superpositions of low mass groups. We find that 7 of the 20 fields
have additional massive structures which are not associated with the clusters
and which can affect the weak lensing mass determination. We compare the mass
measurements of the remaining 13 clusters with luminosity measurements from
cluster galaxies selected using photometric redshifts and find evidence of a
dependence of the cluster mass-to-light ratio with redshift. Finally we
determine the noise level in the shear measurements for the fields as a
function of exposure time and seeing and demonstrate that future ground-based
surveys which plan to perform deep optical imaging for use in weak lensing
measurements must achieve point-spread functions smaller than a median of 0.6"
FWHM.Comment: 35 pages, 24 figures, accepted to A&A, a version with better figure
resolution can be found at http://www.mpa-garching.mpg.de/ediscs/papers.htm
Unbiased Cosmological Parameter Estimation from Emission Line Surveys with Interlopers
The galaxy catalogs generated from low-resolution emission line surveys often
contain both foreground and background interlopers due to line
misidentification, which can bias the cosmological parameter estimation. In
this paper, we present a method for correcting the interloper bias by using the
joint-analysis of auto- and cross-power spectra of the main and the interloper
samples. In particular, we can measure the interloper fractions from the
cross-correlation between the interlopers and survey galaxies, because the true
cross-correlation must be negligibly small. The estimated interloper fractions,
in turn, remove the interloper bias in the cosmological parameter estimation.
For example, in the Hobby-Eberly Telescope Dark Energy Experiment (HETDEX)
low-redshift () [O II] {\AA} emitters contaminate
high-redshift () Lyman- line emitters. We demonstrate that
the joint-analysis method yields a high signal-to-noise ratio measurement of
the interloper fractions while only marginally increasing the uncertainties in
the cosmological parameters relative to the case without interlopers. We also
show the same is true for the high-latitude spectroscopic survey of Wide-Field
Infrared Survey Telescope (WFIRST) mission where contamination occurs between
the Balmer- line emitters at lower redshifts () and Oxygen
([O III] {\AA}) line emitters at higher redshifts ().Comment: 36 pages, 26 figure
Screening of yeasts for production of xylitol fromd-xylose and some factors which affect xylitol yield inCandida guilliermondii
The ability to convertd-xylose to xylitol was screened in 44 yeasts from five genera. All but two of the strains produced some xylitol with varying rates and yields. The best xylitol producers were localized largely in the speciesCandida guilliermondii andC. tropicalis. Factors affecting xylitol production by a selectedC. guilliermondii strain, FTI-20037, were investigated. The results showed that xylitol yield by this strain was affected by the nitrogen source. Yield was highest at 30â35°C, and could be increased with decreasing aeration rate. Using high cell density and a defined medium under aerobic conditions, xylitol yield byC. guilliermondii FTI-20037 from 104 g/ld-xylose was found to be 77.2 g/l. This represented a yield of 81% of the theoretical value, which was computed to be 0.9 mol xylitol per mold-xylose
Nonlocalized modulation of periodic reaction diffusion waves: The Whitham equation
In a companion paper, we established nonlinear stability with detailed
diffusive rates of decay of spectrally stable periodic traveling-wave solutions
of reaction diffusion systems under small perturbations consisting of a
nonlocalized modulation plus a localized perturbation. Here, we determine
time-asymptotic behavior under such perturbations, showing that solutions
consist to leading order of a modulation whose parameter evolution is governed
by an associated Whitham averaged equation
Observations of Mass Loss from the Transiting Exoplanet HD 209458b
Using the new Cosmic Origins Spectrograph (COS) on the {\it Hubble Space
Telescope (HST)}, we obtained moderate-resolution, high signal/noise
ultraviolet spectra of HD 209458 and its exoplanet HD 209458b during transit,
both orbital quadratures, and secondary eclipse. We compare transit spectra
with spectra obtained at non-transit phases to identify spectral features due
to the exoplanet's expanding atmosphere. We find that the mean flux decreased
by % for the C II 1334.5323\AA\ and 1335.6854\AA\ lines and by
% for the Si III 1206.500\AA\ line during transit compared to
non-transit times in the velocity interval --50 to +50 km s. Comparison
of the C II and Si III line depths and transit/non-transit line ratios shows
deeper absorption features near --10 and +15 km s and less certain
features near --40 and +30--70 km s, but future observations are needed
to verify this first detection of velocity structure in the expanding
atmosphere of an exoplanet. Our results for the C II lines and the
non-detection of Si IV 1394.76\AA\ absorption are in agreement with
\citet{Vidal-Madjar2004}, but we find absorption during transit in the Si III
line contrary to the earlier result. The % obscuration of the star
during transit is far larger than the 1.5% obscuration by the exoplanet's disk.
Absorption during transit at velocities between --50 and +50 km s in the
C II and Si III lines requires high-velocity ion absorbers, but models that
assume that the absorbers are high-temperature thermal ions are inconsistent
with the COS spectra. Assuming hydrodynamic model values for the gas
temperature and outflow velocity at the limb of the outflow as seen in the C II
lines, we find mass-loss rates in the range (8--40) g s.Comment: 25 pages, 4 figures, Astrophysical Journal in pres
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