468 research outputs found
Signal-to-noise ratio of the bispectral analysis of speckle interferometry
Monte Carlo simulations of an atmospheric phase screen, based on a Kolmogorov spectrum of phase fluctuations,
were performed. Speckle patterns produced from the phase screens were used to derive statistical properties of
power spectra and bispectra of speckle interferograms. We present the bispectral modulation transfer function and
its signal-to-noise ratio at high light levels. The results confirm the validity of a heuristic treatment based on an
interferometric picture of speckle pattern formation in deriving the attenuation factor and the signal-to-noise ratio
of the bispectral modulation transfer function in the mid-spatial-frequency range. The derived modulation
transfer function is also interpreted in terms of the signal-to-noise ratio at low light levels. A general expression of
the signal-to-noise ratio of the bispectrum is derived as a function of the transfer functions of the telescope, the
number of speckles, and the mean photon counts in the mid-spatial-frequency range
Near Infrared Spectroscopy of M Dwarfs. I. CO Molecule as an Abundance Indicator of Carbon
Based on the near infrared spectra of 42 M dwarfs, carbon abundances are
determined from the ro-vibrational lines of CO 2-0 band. We use Teff values
based on the angular diameters if available or apply a logTeff - M3.4 (the
absolute magnitude at 3.4 micron based on the WISE W1 flux and the Hipparcos
parallax) relation to estimate Teff values of objects for which angular
diameters are unknown. Also, we discuss briefly the HR diagram of low mass
stars. On the observed spectrum of M dwarf, the continuum is depressed by the
numerous weak lines of H2O and only the depressed continuum or the pseudo-
continuum can be seen. On the theoretical spectrum of M dwarfs, we find that
the pseudo-continuum can be evaluated accurately thanks to the recent H2O line
database. Then quantitative analysis of the spectrum of M dwarf can be done by
referring to the pseudo-continua both on the observed and theoretical spectra.
Since the basic principle of the spectroscopic analysis should be the same
whether the true- or pseudo-continuum is referred to, the difficulty related to
the continuum in cool stars can in principle be overcome. Then, the numerous CO
lines can be excellent abundance indicators of carbon, since almost all the
carbon atoms are in stable CO molecules whose abundance remains almost
unchanged for the changes of physical condition in the photosphere and,
somewhat unexpectedly, carbon abundances in late-type stars can best be
determined in M dwarfs rather than in solar type stars. The resulting C/Fe
ratios for most M dwarfs are nearly constant at about the solar value based on
the classical high carbon abundance rather than on the recently revised lower
value. This result implies that the solar carbon abundance is atypical for its
metallicity among the stellar objects in the solar neighborhood if the downward
revised solar carbon abundance is correct.Comment: 33 pages, 17 figures, 13 tables + 1 table for electronic version
only, minor changes in abstract, errors corrected in Tables 5, 8, and 10,
corrected typos, accepted by PAS
An Interpretation of Flat Density Cores of Clusters of Galaxies by Degeneracy Pressure of Fermionic Dark Matter: A Case Study of Abell 1689
Flat density cores have been obtained for a limited number of clusters of
galaxies by strong gravitational lensing. This paper explores the possibility
that the degeneracy pressure of fermionic dark matter accounts for the flat top
density profiles. This is a case study of A1689 for which the density profile
has been obtained from the inner region out to 1Mpc by the combination of
strong and weak lensing. In the case that the dark matter consists of the
mixture of degenerate relic neutrinos and collisionless cold dark matter
particles, the acceptable mass range for relic neutrinos is between 1 and 2 eV,
if the ratio of the two kinds of dark matter particles is fixed to its cosmic
value.Comment: Accepted for Publication in ApJ. Companion paper to astro-ph/060709
Noise in optical synthesis images. II. Sensitivity of an ^nC_2 interferometer with bispectrum imaging
We study the imaging sensitivity of a ground-based optical array of n apertures in which the beams are combined pairwise, as in radio-interferometric arrays, onto n(n - 1)/2 detectors, the so-called ^nC_2 interferometer. Groundbased operation forces the use of the fringe power and the bispectrum phasor as the primary observables rather than the simpler and superior observable, the Michelson fringe phasor. At high photon rates we find that bispectral imaging suffers no loss of sensitivity compared with an ideal array (space based) that directly uses the Michelson fringe phasor. In the opposite limit, when the number of photons per spatial coherence area per coherence time drops below unity, the sensitivity of the array drops rapidly relative to an ideal array. In this regime the sensitivity is independent of n, and hence it may be efficient to have many smaller arrays, each operating separately and simultaneously
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