2,813 research outputs found
Impact cratering and the Oort cloud
We calculate the expected flux profile of comets into the planetary system
from the Oort cloud arising from Galactic tides and encounters with molecular
clouds. We find that both periodic and sporadic bombardment episodes, with
amplitudes an order of magnitude above background, occur on characteristic
timescales ~25-35 Myr.
Bombardment episodes occurring preferentially during spiral arm crossings may
be responsible both for mass extinctions of life and the transfer of viable
microorganisms from the bombarded Earth into the disturbing nebulae. Good
agreement is found between the theoretical expectations and the age
distribution of large, well-dated terrestrial impact craters of the past 250
million years.
A weak periodicity of ~36 Myr in the cratering record is consistent with the
Sun's recent passage through the Galactic plane, and implies a central plane
density ~0.15 M_Sun pc^(-3). This leaves little room for a significant dark
matter component in the disc
Radio Observations of SN 1980K: Evidence for Rapid Presupernova Evolution
New observations of SN 1980K made with the VLA at 20 and 6 cm from 1994 April
through 1996 October show that the supernova (SN) has undergone a significant
change in its radio emission evolution, dropping by a factor of ~2 below the
flux density S \propto t^{-0.73} power-law decline with time t observed
earlier. However, although S at all observed frequencies has decreased
significantly, its current spectral index of \alpha= -0.42\pm0.15 (S \propto
\nu^{+\alpha}) is consistent with the previous spectral index of
\alpha=-0.60_{-0.07}^{+0.04}.
It is suggested that this decrease in emission may be due to the SN shock
entering a new region of the circumstellar material which has a lower density
than that expected for a constant speed (w), constant mass-loss rate (Mdot)
wind from the progenitor. If such an interpretation is correct, the difference
in wind and shock speeds appears to indicate a significant evolution in the
mass-loss history of the SN progenitor ~10^4 years before explosion, with a
change in circumstellar density (\propto Mdot/w) occurring over a time span of
\lesssim 4 kyr. Such features could be explained in terms of a fast
``blue-loop'' evolutionary phase of a relatively massive pre-SN progenitor
star. If so, we may, for the first time, provide a stringent constraint on the
mass of the SN progenitor based solely on the SN's radio emission.Comment: 22 pages, 3 figures, to appear in Ap
On the investigations of galaxy redshift periodicity
In this article we present a historical review of study of the redshift
periodicity of galaxies, starting from the first works performed in the
seventies of the twentieth century until the present day. We discuss the
observational data and methods used, showing in which cases the discretization
of redshifts was observed. We conclude that galaxy redshift periodisation is an
effect which can really exist. We also discussed the redshift discretization in
two different structures: the Local Group of galaxies and the Hercules
Supercluster. Contrary to the previous studies we consider all galaxies which
can be regarded as a structure member disregarding the accuracy of velocity
measurements. We applied the power spectrum analysis using the Hann function
for weighting, together with the jackknife error estimator. In both the
structures we found weak effects of redshift periodisation.Comment: 10 pages, 4 figures, to be published in Part. and Nucl. Lett. 200
The 74MHz System on the Very Large Array
The Naval Research Laboratory and the National Radio Astronomy Observatory
completed implementation of a low frequency capability on the VLA at 73.8 MHz
in 1998. This frequency band offers unprecedented sensitivity (~25 mJy/beam)
and resolution (~25 arcsec) for low-frequency observations. We review the
hardware, the calibration and imaging strategies, comparing them to those at
higher frequencies, including aspects of interference excision and wide-field
imaging. Ionospheric phase fluctuations pose the major difficulty in
calibrating the array. Over restricted fields of view or at times of extremely
quiescent ionospheric ``weather'', an angle-invariant calibration strategy can
be used. In this approach a single phase correction is devised for each
antenna, typically via self-calibration. Over larger fields of view or at times
of more normal ionospheric ``weather'' when the ionospheric isoplanatic patch
size is smaller than the field of view, we adopt a field-based strategy in
which the phase correction depends upon location within the field of view. This
second calibration strategy was implemented by modeling the ionosphere above
the array using Zernike polynomials. Images of 3C sources of moderate strength
are provided as examples of routine, angle-invariant calibration and imaging.
Flux density measurements indicate that the 74 MHz flux scale at the VLA is
stable to a few percent, and tied to the Baars et al. value of Cygnus A at the
5 percent level. We also present an example of a wide-field image, devoid of
bright objects and containing hundreds of weaker sources, constructed from the
field-based calibration. We close with a summary of lessons the 74 MHz system
offers as a model for new and developing low-frequency telescopes. (Abridged)Comment: 73 pages, 46 jpeg figures, to appear in ApJ
Departures From Axisymmetric Morphology and Dynamics in Spiral Galaxies
New HI synthesis data have been obtained for six face-on galaxies with the
Very Large Array. These data and reanalyses of three additional data sets make
up a sample of nine face-on galaxies analyzed for deviations from axisymmetry
in morphology and dynamics. This sample represents a subsample of galaxies
already analyzed for morphological symmetry properties in the R-band. Four
quantitative measures of dynamical nonaxisymmetry are compared to one another
and to the quantitative measures of morphological asymmetry in HI and R-band to
investigate the relationships between nonaxisymmetric morphology and dynamics.
We find no significant relationship between asymmetric morphology and most of
the dynamical measures in our sample. A possible relationship is found,
however, between morphology and dynamical position angle differences between
approaching and receding sides of the galaxy.Comment: 24 pages, 19 figures, AASTeX, accepted for publication in AJ,
postscript figures available at
ftp://culebra.tn.cornell.edu/pub/david/figures.tar.g
A Search for Molecular Gas in GHz Peaked Spectrum Radio Sources
We present searches for molecular gas (CO, OH, CS, and Ammonia) in six GHz
Peaked Spectrum (GPS) radio sources. We do not detect gas in any source and
place upper limits on the mass of molecular gas which are generally in the
range 1E9 to a few times 1E10 solar masses. These limits are consistent with
the following interpretations: (1) GPS sources do not require very dense gas in
their hosts, and (2) The GPS sources are unlikely to be confined by dense gas
and will evolve to become larger radio sources
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