64,598 research outputs found
Comparison of the INRIM and PTB lattice-spacing standards
To base the kilogram definition on the atomic mass of the silicon 28 atom,
the present relative uncertainty of the silicon 28 lattice parameter must
lowered to 3E-9. To achieve this goal, a new experimental apparatus capable of
a centimetre measurement-baseline has been made at the INRIM. The comparison
between the determinations of the lattice parameter of crystals MO*4 of INRIM
and WASO4.2a of PTB is intended to verify the measurement capabilities and to
assess the limits of this experiment.Comment: 10 pages, 8 figures, submitted to Metrologi
A Conversation with Leo Goodman
Leo A. Goodman was born on August 7, 1928 in New York City. He received his
A.B. degree, summa cum laude, in 1948 from Syracuse University, majoring in
mathematics and sociology. He went on to pursue graduate studies in
mathematics, with an emphasis on mathematical statistics, in the Mathematics
Department at Princeton University, and in 1950 he was awarded the M.A. and
Ph.D. degrees. His statistics professors at Princeton were the late Sam Wilks
and John Tukey. Goodman then began his academic career as a statistician, and
also as a statistician bridging sociology and statistics, with an appointment
in 1950 as assistant professor in the Statistics Department and the Sociology
Department at the University of Chicago, where he remained, except for various
leaves, until 1987. He was promoted to associate professor in 1953, and to
professor in 1955. Goodman was at Cambridge University in 1953--1954 and
1959--1960 as visiting professor at Clare College and in the Statistical
Laboratory. And he spent 1960--1961 as a visiting professor of mathematical
statistics and sociology at Columbia University. He was also a research
associate in the University of Chicago Population Research Center from 1967 to
1987. In 1970 he was appointed the Charles L. Hutchinson Distinguished Service
Professor at the University of Chicago, a title that he held until 1987. He
spent 1984--1985 at the Center for Advanced Study in the Behavioral Sciences in
Stanford. In 1987 he was appointed the Class of 1938 Professor at the
University of California, Berkeley, in the Sociology Department and the
Statistics Department. Goodman's numerous honors include honorary D.Sc. degrees
from the University of Michigan and Syracuse University, and membership in the
National Academy of Sciences, the American Academy of Arts and Sciences, and
the American Philosophical Society.Comment: Published in at http://dx.doi.org/10.1214/08-STS276 the Statistical
Science (http://www.imstat.org/sts/) by the Institute of Mathematical
Statistics (http://www.imstat.org
HST Proper Motion confirms the optical identification of the nearby pulsar PSR 1929+10
We report on the proper motion measurement of the proposed optical
counterpart of the X-ray/radio pulsar PSR 1929+10. Using images obtained with
the HST/STIS (average epoch 2001.73) we computed a yearly displacement of +97
+/- 1 mas yr^{-1} in RA and +46 +/- 1 mas yr^{-1} in Dec since the epoch
(1994.52) of the original HST/FOC detection. Both the magnitude and direction
of the optical proper motion components are found to be fully consistent with
the most recent VLBA radio measurements. This result provides an unambiguous
confirmation of the pulsar optical identification. In addition, we have used
the combined STIS/FOC datasets to derive information on the pulsar spectrum,
which seems characterized by a power law component, apparently unrelated to the
X-ray emission.Comment: 12 pages, 3 figures, submitted to ApJ Letter
Pointless Hyperelliptic Curves
In this paper we consider the question of whether there exists a hyperelliptic curve of genus g which is defined over but has no rational points over for various pairs . As an example of such a result, we show that if p is a prime such that is also prime then there will be pointless hyperelliptic curves over of every genus
Atmospheric Heat Redistribution on Hot Jupiters
Infrared lightcurves of transiting hot Jupiters present a trend in which the
atmospheres of the hottest planets are less efficient at redistributing the
stellar energy absorbed on their daysides---and thus have a larger day-night
temperature contrast---than colder planets. No predictive atmospheric model has
been published that identifies which dynamical mechanisms determine the
atmospheric heat redistribution efficiency on tidally locked exoplanets. Here
we present a two-layer shallow water model of the atmospheric dynamics on
synchronously rotating planets that explains the observed trend. Our model
shows that planets with weak friction and weak irradiation exhibit a banded
zonal flow with minimal day-night temperature differences, while models with
strong irradiation and/or strong friction exhibit a day-night flow pattern with
order-unity fractional day-night temperature differences. To interpret the
model, we develop a scaling theory that shows that the timescale for gravity
waves to propagate horizontally over planetary scales, t_wave, plays a dominant
role in controlling the transition from small to large temperature contrasts.
This implies that heat redistribution is governed by a wave-like process,
similar to the one responsible for the weak temperature gradients in the
Earth's tropics. When atmospheric drag can be neglected, the transition from
small to large day-night temperature contrasts occurs when t_wave ~
sqrt(t_rad/Omega), where t_rad is the radiative relaxation time and Omega is
the planetary rotation frequency. Alternatively, this transition criterion can
be expressed as t_rad ~ t_vert, where t_vert is the timescale for a fluid
parcel to move vertically over the difference in day-night thickness. These
results subsume the commonly used timescale comparison for estimating heat
redistribution efficiency between t_rad and the global horizontal advection
timescale, t_adv.Comment: Accepted to ApJ with minor edits compared to version 1; 17 pages, 11
figure
Cyclic Critical Groups of Graphs
In this note, we describe a construction that leads to families of graphs whose critical groups are cyclic. For some of these families we are able to give a formula for the number of spanning trees of the graph, which then determines the group exactly
The Deep Lens Survey Transient Search I : Short Timescale and Astrometric Variability
We report on the methodology and first results from the Deep Lens Survey
transient search. We utilize image subtraction on survey data to yield all
sources of optical variability down to 24th magnitude. Images are analyzed
immediately after acquisition, at the telescope and in near-real time, to allow
for followup in the case of time-critical events. All classes of transients are
posted to the web upon detection. Our observing strategy allows sensitivity to
variability over several decades in timescale. The DLS is the first survey to
classify and report all types of photometric and astrometric variability
detected, including solar system objects, variable stars, supernovae, and short
timescale phenomena. Three unusual optical transient events were detected,
flaring on thousand-second timescales. All three events were seen in the B
passband, suggesting blue color indices for the phenomena. One event (OT
20020115) is determined to be from a flaring Galactic dwarf star of spectral
type dM4. From the remaining two events, we find an overall rate of \eta = 1.4
events deg-2 day-1 on thousand-second timescales, with a 95% confidence limit
of \eta < 4.3. One of these events (OT 20010326) originated from a compact
precursor in the field of galaxy cluster Abell 1836, and its nature is
uncertain. For the second (OT 20030305) we find strong evidence for an extended
extragalactic host. A dearth of such events in the R passband yields an upper
95% confidence limit on short timescale astronomical variability between 19.5 <
R < 23.4 of \eta_R < 5.2. We report also on our ensemble of astrometrically
variable objects, as well as an example of photometric variability with an
undetected precursor.Comment: 24 pages, 12 figures, 3 tables. Accepted for publication in ApJ.
Variability data available at http://dls.bell-labs.com/transients.htm
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