2,764 research outputs found
Final Calibration of the Berkeley Extreme and Far-Ultraviolet Spectrometer on the ORFEUS-SPAS I and II Missions
The Berkeley Extreme and Far-Ultraviolet Spectrometer (BEFS) flew as part of
the ORFEUS telescope on the ORFEUS-SPAS I and II space-shuttle missions in 1993
and 1996, respectively. The data obtained by this instrument have now entered
the public domain. To facilitate their use by the astronomical community, we
have re-extracted and re-calibrated both data sets, converted them into a
standard (FITS) format, and placed them in the Multimission Archive at Space
Telescope (MAST). Our final calibration yields improved wavelength scales and
effective-area curves for both data sets.Comment: To appear in the January 2002 issue of the PASP. 17 pages with 9
embedded postscript figures; uses emulateapj5.st
A Search for EUV Emission from Comets with the Cosmic Hot Interstellar Plasma Spectrometer (CHIPS)
We have obtained EUV spectra between 90 and 255 \AA of the cometsC/2002 T7
(LINEAR), C/2001 Q4 (NEAT), and C/2004 Q2 (Machholz) near their perihelion
passages in 2004 with the Cosmic Hot Interstellar Plasma Spectrometer (CHIPS).
We obtained contemporaneous data on Comet NEAT Q4 with the X-ray
Observatory ACIS instrument, marking the first simultaneous EUV and X-ray
spectral observations of a comet. The total CHIPS/EUV observing times were 337
ks for Q4, 234 ks for T7, and 483 ks for Machholz and for both CHIPS and
we calculate we have captured all the comet flux in the instrument
field of view. We set upper limits on solar wind charge exchange emission lines
of O, C, N, Ne and Fe occurring in the spectral bandpass of CHIPS. The spectrum
of Q4 obtained with can be reproduced by modeling emission lines of
C, N O, Mg, Fe, Si, S, and Ne solar wind ions. The measured X-ray emission line
intensities are consistent with our predictions from a solar wind charge
exchange model. The model predictions for the EUV emission line intensities are
determined from the intensity ratios of the cascading X-ray and EUV photons
arising in the charge exchange processes. They are compatible with the measured
limits on the intensities of the EUV lines. For comet Q4, we measured a total
X-ray flux of 3.7 ergs cm s, and derive from
model predictions a total EUV flux of 1.5 erg cm
s. The CHIPS observations occurred predominantly while the satellite was
on the dayside of Earth. For much of the observing time, CHIPS performed
observations at smaller solar angles than it was designed for and EUV emission
from the Sun scattered into the instrument limited the sensitivity of the EUV
measurements.Comment: 28 pages total, 4 tables, 7 figures. Accepted by The Astrophysical
Journa
How to relate the oscillator and Coulomb systems on spheres and pseudospheres?
We show that the oscillators on a sphere and pseudosphere are related, by the
so-called Bohlin transformation, with the Coulomb systems on the pseudosphere:
the even states of an oscillator yields the conventional Coulomb system on
pseudosphere, while the odd states yield the Coulomb system on pseudosphere in
the presence of magnetic flux tube generating half spin. In the higher
dimensions the oscillator and Coulomb(-like) systems are connected in the
similar way. In particular, applying the Kustaanheimo-Stiefel transformation to
the oscillators on sphere and pseudosphere, we obtained the preudospherical
generalization of MIC-Kepler problem describing three-dimensional charge-dyon
system.Comment: 12 pages, Based on talk given at XXIII Colloquium on Group
Theoretical Methods in Physics (July 31-August 5, 2000, Dubna
Enhancement of Cavity Cooling of a Micromechanical Mirror Using Parametric Interactions
It is shown that an optical parametric amplifier inside a cavity can
considerably improve the cooling of the micromechanical mirror by radiation
pressure. The micromechanical mirror can be cooled from room temperature 300 K
to sub-Kelvin temperatures, which is much lower than what is achievable in the
absence of the parametric amplifier. Further if in case of a precooled mirror
one can reach millikelvin temperatures starting with about 1 K. Our work
demonstrates the fundamental dependence of radiation pressure effects on photon
statistics.Comment: 14 pages, 7 figure
A New Measurement of the Average FUV Extinction Curve
We have measured the extinction curve in the far-ultraviolet wavelength
region of (900 -- 1200 A) using spectra obtained with the Berkeley EUV/FUV
spectrometer during the ORFEUS-I and the ORFEUS-II missions in 1993 and 1996.
From the complete sample of early-type stars observed during these missions,
we have selected pairs of stars with the same spectral type but different
reddenings to measure the differential FUV extinction. We model the effects of
molecular hydrogen absorption and exclude affected regions of the spectrum to
determine the extinction from dust alone. We minimize errors from inaccuracies
in the cataloged spectral types of the stars by making our own determinations
of spectral types based on their IUE spectra. We find substantial scatter in
the curves of individual star pairs and present a detailed examination of the
uncertainties and their effects on each extinction curve. We find that, given
the potentially large uncertainties inherent in using the pair method at FUV
wavelengths, a careful analysis of measurement uncertainties is critical to
assessing the true dust extinction. We present a new measurement of the average
far-ultraviolet extinction curve to the Lyman limit; our new measurement is
consistent with an extrapolation of the standard extinction curve of Savage &
Mathis (1979).Comment: 13 pages text, 7 figures 4 tables. Sent as gzipped tar, with ms.tex
and 7 figure
Local Pheromone Release from Dynamic Polarity Sites Underlies Cell-Cell Pairing during Yeast Mating.
Cell pairing is central for many processes, including immune defense, neuronal connection, hyphal fusion, and sexual reproduction. How does a cell orient toward a partner, especially when faced with multiple choices? Fission yeast Schizosaccharomyces pombe P and M cells, which respectively express P and M factor pheromones [1, 2], pair during the mating process induced by nitrogen starvation. Engagement of pheromone receptors Map3 and Mam2 [3, 4] with their cognate pheromone ligands leads to activation of the Gα protein Gpa1 to signal sexual differentiation [3, 5, 6]. Prior to cell pairing, the Cdc42 GTPase, a central regulator of cell polarization, forms dynamic zones of activity at the cell periphery at distinct locations over time [7]. Here we show that Cdc42-GTP polarization sites contain the M factor transporter Mam1, the general secretion machinery, which underlies P factor secretion, and Gpa1, suggesting that these are sub-cellular zones of pheromone secretion and signaling. Zone lifetimes scale with pheromone concentration. Computational simulations of pair formation through a fluctuating zone show that the combination of local pheromone release and sensing, short pheromone decay length, and pheromone-dependent zone stabilization leads to efficient pair formation. Consistently, pairing efficiency is reduced in the absence of the P factor protease. Similarly, zone stabilization at reduced pheromone levels, which occurs in the absence of the predicted GTPase-activating protein for Ras, leads to reduction in pairing efficiency. We propose that efficient cell pairing relies on fluctuating local signal emission and perception, which become locked into place through stimulation
Structured matrices, continued fractions, and root localization of polynomials
We give a detailed account of various connections between several classes of
objects: Hankel, Hurwitz, Toeplitz, Vandermonde and other structured matrices,
Stietjes and Jacobi-type continued fractions, Cauchy indices, moment problems,
total positivity, and root localization of univariate polynomials. Along with a
survey of many classical facts, we provide a number of new results.Comment: 79 pages; new material added to the Introductio
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