2,082,864 research outputs found
Probability of Detecting a Planetary Companion during a Microlensing Event
The probability of detecting a planetary companion of a lensing star during a
microlensing event toward the Galactic center, averaged over all relevant event
and galactic parameters, when the planet-star mass ratio has a
maximum exceeding 10% at an orbit semimajor axis near 1.5 AU for a uniform
distribution of impact parameters. The maximum probability is raised to more
than 20% for a distribution of source-lens impact parameters that is determined
by the efficiency of event detection. The averaging procedures are carefully
defined, and they determinine the dependence of the detection probabilities on
several properties of the Galaxy. The probabilities scale approximately as
. A planet is assumed detectable if the perturbation of the single
lens light curve exceeds for at least 20 consecutive photometric
points sometime during the event. Two meter telescopes with 60 second
integrations in I-band with high time resolution photometry throughout the
duration of an ongoing event are assumed. The probabilities are derived as a
function of , where they remain significant for AU. Dependence of
the detection probabilities on the lens mass function, luminosity function of
the source stars as modified by extinction, distribution of source-lens impact
parameters, and the line of sight to the source are also determined, and the
probabilities are averaged over the distribution of the projected planet
position, the lens mass function, the distribution of impact parameters, the
lens and source distances as weighted by their distributions along the line of
sight and over the -band apparent luminosity function of the sources. The
extraction of the probabilility as a function of for a particular from
empirical data is indicated.Comment: 32 pages, 20 figures, In Press, ApJ, Latex format with aas2pp4 forma
Newly discovered brown dwarfs not seen in microlensing time scale frequency distribution?
The 2-Micron All Sky Survey (2MASS) (Skrutskie et al. 1997) and the DEep Near
Infrared Survey of the southern sky (DENIS) (Epchtein et al. 1997) have
revealed a heretofore unknown population of free brown dwarfs that has extended
the local mass function down to as small as 0.01M_sun (Reid et al. 1999). If
this local proportion of brown dwarfs extends throughout the Galaxy---in
particular in the Galactic bulge---one expects an increase in the predicted
fraction of short time scale microlensing events in directions toward the
Galactic bulge. Zhao et al.(1996) have indicated that a mass function with
30-60% of the lens mass in brown dwarfs is not consistent with empirical
microlensing data. Here we show that even the much lower mass fraction (~ 10%)
of brown dwarfs inferred from the new discoveries appears inconsistent with the
data. The added brown dwarfs do indeed increase the expected number of short
time scale events, but they appear to drive the peak in the time scale
frequency distribution to time scales smaller than that observed, and do not
otherwise match the observed distribution. A reasonably good match to the
empirical data (Alcock et al. 1996) is obtained by increasing the fraction of
stars in the range 0.08<m<0.7M_sun considerably above that deduced from several
star counts. However, all inferences from microlensing about the appropriate
stellar mass function must be qualified by the meagerness of the microlensing
data and the uncertainties in the Galactic model.Comment: 5 pages, 1 figure. PS file using aas2pp4.sty. To appear in ApJ
Letter
Electronic structures of ZnCoO using photoemission and x-ray absorption spectroscopy
Electronic structures of ZnCoO have been investigated using
photoemission spectroscopy (PES) and x-ray absorption spectroscopy (XAS). The
Co 3d states are found to lie near the top of the O valence band, with a
peak around eV binding energy. The Co XAS spectrum provides
evidence that the Co ions in ZnCoO are in the divalent Co
() states under the tetrahedral symmetry. Our finding indicates that the
properly substituted Co ions for Zn sites will not produce the diluted
ferromagnetic semiconductor property.Comment: 3 pages, 2 figure
Mix-and-match compatibility in asymmetric system markets
This paper shows that the private incentive for mix-and-match compatibility in system markets diverges from the social planner's incentive if competing suppliers are asymmetric in production cost or product quality. There can be too much or too little compatibility when the market is served by fully integrated system suppliers. Also, the market outcome involves socially too much incompatibility in the form of exclusive technological alliances when the market is composed of independent component suppliers. These results contrast with the standard one obtained in the symmetric setup and shed new light on public policy towards compatibility, technological alliances, and bundling practices in system markets
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