3,265 research outputs found
Coefficients and terms of the liquid drop model and mass formula
The coefficients of different combinations of terms of the liquid drop model
have been determined by a least square fitting procedure to the experimental
atomic masses. The nuclear masses can also be reproduced using a Coulomb radius
taking into account the increase of the ratio with increasing
mass, the fitted surface energy coefficient remaining around 18 MeV
Test results of Spacelab 2 infrared telescope focal plane
The small helium cooled infrared telescope for Spacelab 2 is designed for sensitive mapping of extended, low-surface-brightness celestial sources as well as highly sensitive investigations of the shuttle contamination environment (FPA) for this mission is described as well as the design for a thermally isolated, self-heated J-FET transimpedance amplifier. This amplifier is Johnson noise limited for feedback resistances from less than 10 to the 8th power Omega to greater than 2 x 10 to the 10th power Omega at T = 4.2K. Work on the focal plane array is complete. Performance testing for qualification of the flight hardware is discussed, and results are presented. All infrared data channels are measured to be background limited by the expected level of zodiacal emission
Dynamical Mean Field Theory of the Antiferromagnetic Metal to Antiferromagnetic Insulator Transition
We study the antiferromagnetic metal to antiferromagnetic insulator using
dynamical mean field theory and exact diagonalization methods. We find two
qualitatively different behaviors depending on the degree of magnetic
correlations. For strong correlations combined with magnetic frustration, the
transition can be described in terms of a renormalized slater theory, with a
continuous gap closure driven by the magnetism but strongly renormalized by
correlations. For weak magnetic correlations, the transition is weakly first
order.Comment: 4 pages, uses epsfig,4 figures,notational errors rectifie
Caltech Faint Field Galaxy Redshift Survey IX: Source detection and photometry in the Hubble Deep Field Region
Detection and photometry of sources in the U_n, G, R, and K_s bands in a 9x9
arcmin^2 region of the sky, centered on the Hubble Deep Field, are described.
The data permit construction of complete photometric catalogs to roughly
U_n=25, G=26, R=25.5 and K_s=20 mag, and significant photometric measurements
somewhat fainter. The galaxy number density is 1.3x10^5 deg^{-2} to R=25.0 mag.
Galaxy number counts have slopes dlog N/dm=0.42, 0.33, 0.27 and 0.31 in the
U_n, G, R and K_s bands, consistent with previous studies and the trend that
fainter galaxies are, on average, bluer. Galaxy catalogs selected in the R and
K_s bands are presented, containing 3607 and 488 sources, in field areas of
74.8 and 59.4 arcmin^2, to R=25.5 and and K_s=20 mag.Comment: Accepted for publication in ApJS; some tables and slightly nicer
figures available at http://www.sns.ias.edu/~hogg/deep
AVHRR and VISSR satellite instrument calibration results for both Cirrus and marine stratocumulus IFO periods
Accurate characterizations of some cloud parameters are dependent upon the absolute accuracy of satellite radiance measurements. Visible wavelength measurements from both the AVHRR and VISSR instruments are often used to study cloud characteristics. Both of these instruments were radiometrically calibrated prior to launch, but neither has an onboard device to monitor degradation after launch. During the FIRE/SRB cirrus Intensive Field Operation (IFO), a special effort was made to monitor calibration of these two instruments onboard the NOAA-9 and GOES-6 spacecraft. In addition, several research groups have combined their efforts to assess the long-term performance of both instruments. These results are presented, and a limited comparison is made with the ERBE calibration standard
IRS Spectra of Solar-Type Stars: \break A Search for Asteroid Belt Analogs
We report the results of a spectroscopic search for debris disks surrounding
41 nearby solar type stars, including 8 planet-bearing stars, using the {\it
Spitzer Space Telescope}. With accurate relative photometry using the Infrared
Spectrometer (IRS) between 7-34 \micron we are able to look for excesses as
small as 2% of photospheric levels with particular sensitivity to weak
spectral features. For stars with no excess, the upper limit in a
band at 30-34 m corresponds to 75 times the brightness of our
zodiacal dust cloud. Comparable limits at 8.5-13 m correspond to
1,400 times the brightness of our zodiacal dust cloud. These limits correspond
to material located within the 1 to 5 AU region that, in our solar
system, originates from debris associated with the asteroid belt. We find
excess emission longward of 25 m from five stars of which four also
show excess emission at 70 m. This emitting dust must be located around
5-10 AU. One star has 70 micron emission but no IRS excess. In this case, the
emitting region must begin outside 10 AU; this star has a known radial velocity
planet. Only two stars of the five show emission shortward of 25 \micron
where spectral features reveal the presence of a population of small, hot dust
grains emitting in the 7-20 m band. The data presented here strengthen the
results of previous studies to show that excesses at 25 \micron and shorter
are rare: only 1 star out of 40 stars older than 1 Gyr or % shows an
excess. Asteroid belts 10-30 times more massive than our own appear are rare
among mature, solar-type stars
New Debris Disks Around Nearby Main Sequence Stars: Impact on The Direct Detection of Planets
Using the MIPS instrument on the Spitzer telescope, we have searched for
infrared excesses around a sample of 82 stars, mostly F, G, and K main-sequence
field stars, along with a small number of nearby M stars. These stars were
selected for their suitability for future observations by a variety of
planet-finding techniques. These observations provide information on the
asteroidal and cometary material orbiting these stars - data that can be
correlated with any planets that may eventually be found. We have found
significant excess 70um emission toward 12 stars. Combined with an earlier
study, we find an overall 70um excess detection rate of % for mature
cool stars. Unlike the trend for planets to be found preferentially toward
stars with high metallicity, the incidence of debris disks is uncorrelated with
metallicity. By newly identifying 4 of these stars as having weak 24um excesses
(fluxes 10% above the stellar photosphere), we confirm a trend found in
earlier studies wherein a weak 24um excess is associated with a strong 70um
excess. Interestingly, we find no evidence for debris disks around 23 stars
cooler than K1, a result that is bolstered by a lack of excess around any of
the 38 K1-M6 stars in 2 companion surveys. One motivation for this study is the
fact that strong zodiacal emission can make it hard or impossible to detect
planets directly with future observatories like the {\it Terrestrial Planet
Finder (TPF)}. The observations reported here exclude a few stars with very
high levels of emission, 1,000 times the emission of our zodiacal cloud,
from direct planet searches. For the remainder of the sample, we set relatively
high limits on dust emission from asteroid belt counterparts
The Spitzer Space Telescope Mission
The Spitzer Space Telescope, NASA's Great Observatory for infrared astronomy,
was launched 2003 August 25 and is returning excellent scientific data from its
Earth-trailing solar orbit. Spitzer combines the intrinsic sensitivity
achievable with a cryogenic telescope in space with the great imaging and
spectroscopic power of modern detector arrays to provide the user community
with huge gains in capability for exploration of the cosmos in the infrared.
The observatory systems are largely performing as expected and the projected
cryogenic lifetime is in excess of 5 years. This paper summarizes the on-orbit
scientific, technical and operational performance of Spitzer. Subsequent papers
in this special issue describe the Spitzer instruments in detail and highlight
many of the exciting scientific results obtained during the first six months of
the Spitzer mission.Comment: Accepted for publication in the Astrophyscial Journal Supplement
Spitzer Special Issue, 22 pages, 3 figures. Higher resolution versions of the
figures are available at http://ssc.spitzer.caltech.edu/pubs/journal2004.htm
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