2,254 research outputs found
Estimating acreage by double sampling using LANDSAT data
Double sampling techniques employing LANDSAT data for estimating the acreage of corn and soybeans was investigated and evaluated. The evaluation was based on estimated costs and correlations between two existing procedures having differing cost/variance characteristics, and included consideration of their individual merits when coupled with a fictional 'perfect' procedure of zero bias and variance. Two features of the analysis are: (1) the simultaneous estimation of two or more crops; and (2) the imposition of linear cost constraints among two or more types of resource. A reasonably realistic operational scenario was postulated. The costs were estimated from current experience with the measurement procedures involved, and the correlations were estimated from a set of 39 LACIE-type sample segments located in the U.S. Corn Belt. For a fixed variance of the estimate, double sampling with the two existing LANDSAT measurement procedures can result in a 25% or 50% cost reduction. Double sampling which included the fictional perfect procedure results in a more cost effective combination when it is used with the lower cost/higher variance representative of the existing procedures
Accuracy of stellar parameters of exoplanet-host stars determined from asteroseismology
In the context of the space-based mission CoRoT, devoted to asteroseismology
and search for planet transits, we analyse the accuracy of fundamental stellar
parameters (mass, radius, luminosity) that can be obtained from
asteroseismological data.} Our work is motivated by the large uncertainties on
planetary radius determination of transiting planets which are mainly due to
uncertainties on the stellar parameters. Our goal is to analyse uncertainties
of fundamental stellar parameters for a given accuracy of oscillation frequency
determination. We generate grids of equilibrium models of stars and compute
their pulsation spectra based on a linear nonadiabatic stability analysis.
Using differents methods of comparison of oscillation mode spectra, we derive
uncertainties on fundamental stellar parameters and analyse the effect of
varying the number of considered modes.} The limits obtained depend strongly on
the adapted method to compare spectra. We find a degeneracy in the stellar
parameter solutions, up to a few % in mass (from less than 1% to more than 7%
depending on the method used and the number of considered modes), luminosity
(from 2% to more than 10%) or radius (from less than 1% to 3%), for a given
pulsation spectrum.Comment: 7 pages, 9 figure
The ARAUCARIA project. Discovery of Cepheid Variables in NGC 300 from a Wide-Field Imaging Survey
Based on observations of NGC 300, obtained with the Wide-Field Camera at the
2.2 m ESO/MPI telescope during 29 nights spread over a 5.3 month interval, 117
Cepheids and 12 Cepheid candidates were found which cover the period range from
115 to 5.4 days. We present a catalog which provides equatorial coordinates,
period, time of maximum brightness, and intensity mean B and V magnitudes for
each variable, and we show phased B and V light curves for all the Cepheids
found. We also present the individual B and V observations for each Cepheid in
our catalog. We find very good agreement between our photometry and that
obtained by Freedman et al. from ground-based CCD data for common stars. The
Cepheids delineate the spiral arms of NGC 300, and a couple of them were
detected very close to the center of the galaxy. From the color-magnitude
diagram of NGC 300 constructed from our data, we expect that our Cepheid
detection is near-complete for variables with periods larger than about 10
days. We present plots of the PL relations in the B and V bands obtained from
our data, which clearly demonstrate the presence of a Malmquist bias for
periods below about 10 days. A thorough discussion of the distance to NGC 300
will be presented in a forthcoming paper which will include the analysis of
photometry in longer-wavelength bands.Comment: 26 pages, Latex. Astronomical Journal in pres
Surface Brightness and Stellar Populations at the Outer Edge of the Large Magellanic Cloud: No Stellar Halo Yet
We present a high quality CMD for a 36'x 36' field located 8 degrees (7 kpc)
from the LMC center, as well as a precise determination of the LMC surface
brightness derived from the resolved stellar population out to this large
galactocentric radius. This deep CMD shows for the first time the detailed age
distribution at this position, where the surface brightness is V=26.5 mag/sq".
At a radius R=474' the main sequence is well populated from the oldest turnoff
at I=21.5 to the 2.5 Gyr turnoff at I=19.5. Beyond this radius, a relatively
strong gradient in the density of stars with ages in the 2.5-4 Gyr range is
apparent. There are some stars brighter and bluer than the main population,
quite uniformly distributed over the whole area surveyed, which are well
matched by a 1.5 Gyr isochrone and may be indicative of a relatively recent
star formation, or merger, event. The surface brightness profile of the LMC
remains exponential to this large galactocentric radius and shows no evidence
of disk truncation. Combining the information on surface brightness and stellar
population we conclude that the LMC disk extends (and dominates over a possible
stellar halo) out to a distance of at least 7 kpc. These results confirm that
the absence of blue stars in the relatively shallow off-center CMDs of dIrr
galaxies is not necessarily evidence for an exclusively old stellar population
resembling the halo of the Milky Way.Comment: ApJLett, in press 13 pages including 3 color figure
A transiting planet among 23 new near-threshold candidates from the OGLE survey - OGLE-TR-182
By re-processing the data of the second season of the OGLE survey for
planetary transits and adding new mesurements on the same fields gathered in
subsequent years with the OGLE telescope, we have identified 23 new transit
candidates, recorded as OGLE-TR-178 to OGLE-TR-200. We studied the nature of
these objects with the FLAMES/UVES multi-fiber spectrograph on the VLT. One of
the candidates, OGLE-TR-182, was confirmed as a transiting gas giant planet on
a 4-day orbit. We characterised it with further observations using the FORS1
camera and UVES spectrograph on the VLT. OGLE-TR-182b is a typical ``hot
Jupiter'' with an orbital period of 3.98 days, a mass of 1.01 +- 0.15 MJup and
a radius of 1.13 (+0.24-0.08) RJup. Confirming this transiting planet required
a large investment in telescope time with the best instruments available, and
we comment on the difficulty of the confirmation process for transiting planets
in the OGLE survey. We delienate the zone were confirmation is difficult or
impossible, and discuss the implications for the Corot space mission in its
quest for transiting telluric planets.Comment: 7 pages, submitted to Astronomy and Astrophysic
A new look at NICMOS transmission spectroscopy of HD189733, GJ-436 and XO-1: no conclusive evidence for molecular features
We present a re-analysis of archival HST/NICMOS transmission spectroscopy of
three exoplanet systems; HD 189733, GJ-436 and XO-1. Detections of several
molecules, including H20, CH4 and CO2, have been claimed for HD 189733 and
XO-1, but similarly sized features are attributed to systematic noise for
GJ-436. The data consist of time-series grism spectra covering a planetary
transit. After extracting light curves in independent wavelength channels, we
use a linear decorrelation technique account for instrumental systematics
(which is becoming standard in the field), and measure the planet-to-star
radius ratio as a function of wavelength. For HD 189733, the uncertainties in
the transmission spectrum are significantly larger than those previously
reported. We also find the transmission spectrum is considerably altered when
using different out-of-transit orbits to remove the systematics, when some
parameters are left out of the decorrelation procedure, or when we perform the
decorrelation with quadratic functions rather than linear functions. Given that
there is no physical reason to believe the baseline flux should be modelled as
a linear function of any particular set of parameters, we interpret this as
evidence that the linear decorrelation technique is not a robust method to
remove systematic effects from the light curves for each wavelength channel.
For XO-1, the parameters measured to decorrelate the light curves would require
extrapolation to the in-transit orbit to remove the systematics, and we cannot
reproduce the previously reported results. We conclude that the resulting
NICMOS transmission spectra are too dependent on the method used to remove
systematics to be considered robust detections of molecular species in
planetary atmospheres, although the presence of these molecules is not ruled
out.Comment: 17 pages, 28 figures, accepted in MNRA
Criminalization of True Anonymity in Cyberspace, The
The question of whether a state or the federal government can create a narrowly tailored restriction on cyberspace anonymity without violating the First Amendment remains unresolved[...]The Supreme Court has not directly addressed the issue, but it may soon consider the constitutionality of criminalizing certain kinds of cyber-anonymity in light of the unique nature of cyberspace. This comment explores the various forms of anonymity, examines the First Amendment status of anonymity in and outside of cyberspace, analyzes relevant scholarly commentary, and concludes that a narrowly tailored legislative restriction on true anonymity in cyberspace would not violate the First Amendment
Criminalization of True Anonymity in Cyberspace, The
The question of whether a state or the federal government can create a narrowly tailored restriction on cyberspace anonymity without violating the First Amendment remains unresolved[...]The Supreme Court has not directly addressed the issue, but it may soon consider the constitutionality of criminalizing certain kinds of cyber-anonymity in light of the unique nature of cyberspace. This comment explores the various forms of anonymity, examines the First Amendment status of anonymity in and outside of cyberspace, analyzes relevant scholarly commentary, and concludes that a narrowly tailored legislative restriction on true anonymity in cyberspace would not violate the First Amendment
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