1,704 research outputs found
Cosmic Needles versus Cosmic Microwave Background Radiation
It has been suggested by a number of authors that the 2.7K cosmic microwave
background (CMB) radiation might have arisen from the radiation from Population
III objects thermalized by conducting cosmic graphite/iron needle-shaped dust.
Due to lack of an accurate solution to the absorption properties of exceedingly
elongated grains, in existing literature which studies the CMB thermalizing
process they are generally modelled as (1) needle-like spheroids in terms of
the Rayleigh approximation; (2) infinite cylinders; and (3) the antenna theory.
We show here that the Rayleigh approximation is not valid since the Rayleigh
criterion is not satisfied for highly conducting needles. We also show that the
available intergalactic iron dust, if modelled as infinite cylinders, is not
sufficient to supply the required opacity at long wavelengths to obtain the
observed isotropy and Planckian nature of the CMB. If appealing to the antenna
theory, conducting iron needles with exceedingly large elongations (10^4)
appear able to provide sufficient opacity to thermalize the CMB within the iron
density limit. But the applicability of the antenna theory to exceedingly thin
needles of nanometer/micrometer in thickness needs to be justified.Comment: 13 pages, 4 figures; submitted to ApJ
A control theorem for -adic automorphic forms and Teitelbaum's -invariant
In this article, we describe an efficient method for computing Teitelbaum's
-adic -invariant. These invariants are realized as the
eigenvalues of the -operator acting on a space of harmonic
cocycles on the Bruhat-Tits tree , which is computable by the
methods of Franc and Masdeu described in \cite{fm}. The main difficulty in
computing the -operator is the efficient computation of the
-adic Coleman integrals in its definition. To solve this problem, we use
overconvergent methods, first developed by Darmon, Greenberg, Pollack and
Stevens. In order to make these methods applicable to our setting, we prove a
control theorem for -adic automorphic forms of arbitrary even weight.
Moreover, we give computational evidence for relations between slopes of
-invariants of different levels and weights for .Comment: 26 page
Eccentric discs in binaries with intermediate mass ratios: Superhumps in the VY Sculptoris stars
We investigate the role of the eccentric disc resonance in systems with mass
ratios q greater than 1/4, and demonstrate the effects that changes in the mass
flux from the secondary star have upon the disc radius and structure. The
addition of material with low specific angular momentum to its outer edge
restricts a disc radially. Should the mass flux from the secondary be reduced,
it is possible for the disc in a system with mass ratio as large as 1/3 to
expand to the 3:1 eccentric inner Lindblad resonance and for superhumps to be
excited.Comment: 6 pages with 7 figures, accepted by MNRA
Finite Source Sizes and the Information Content of MACHO-Type Lens Search Light Curves
If the dark halo matter is primarily composed of MACHOs toward the lower end
of the possible detection range ( ) a fraction of the
lens detection events should involve the lens crossing directly in front of the
disk of the background star. Previously, Nemiroff (1987) has shown that each
crossing would create an inflection point in the light curve of the MACHO
event. Such inflection points would allow a measure of the time it took for the
gravitational lens to cross the stellar disk. Given an independent estimate of
the stellar radius by other methods, one could then obtain a more accurate
estimate of the velocity of the lens. This velocity could then, in turn, be
used to obtain a more accurate estimate of the mass range for the MACHO or disk
star doing the lensing.Comment: in press: ApJ (Lett.), 10 pages in Plain TeX version 3.0, 1 figure
available by FA
The open cluster initial-final mass relationship and the high-mass tail of the white dwarf distribution
Recent studies of white dwarfs in open clusters have provided new constraints
on the initial - final mass relationship (IFMR) for main sequence stars with
masses in the range 2.5 - 6.5 Mo. We re-evaluate the ensemble of data that
determines the IFMR and argue that the IFMR can be characterised by a mean
initial-final mass relationship about which there is an intrinsic scatter. We
investigate the consequences of the IFMR for the observed mass distribution of
field white dwarfs using population synthesis calculations. We show that while
a linear IFMR predicts a mass distribution that is in reasonable agreement with
the recent results from the PG survey, the data are better fitted by an IFMR
with some curvature. Our calculations indicate that a significant (~28%)
percentage of white dwarfs originating from single star evolution have masses
in excess of ~0.8 Mo, obviating the necessity for postulating the existence of
a dominant population of high-mass white dwarfs that arise from binary star
mergers.Comment: 5 pages, 2 color Postscript figures. Accepted for publication in
MNRA
The effects of tidally induced disc structure on white dwarf accretion in intermediate polars
We investigate the effects of tidally induced asymmetric disc structure on
accretion onto the white dwarf in intermediate polars. Using numerical
simulation, we show that it is possible for tidally induced spiral waves to
propagate sufficiently far into the disc of an intermediate polar that
accretion onto the central white dwarf could be modulated as a result. We
suggest that accretion from the resulting asymmetric inner disc may contribute
to the observed X-ray and optical periodicities in the light curves of these
systems. In contrast to the stream-fed accretion model for these periodicities,
the tidal picture predicts that modulation can exist even for systems with
weaker magnetic fields where the magnetospheric radius is smaller than the
radius of periastron of the mass transfer stream. We also predict that
additional periodic components should exist in the emission from low mass ratio
intermediate polars displaying superhumps.Comment: 9 pages, 5 figures, accepted for publication in MNRA
Galactic Escape Speeds in Mirror and Cold Dark Matter Models
The mirror dark matter (MDM) model of Berezhiani et al. has been shown to
reproduce observed galactic rotational curves for a variety of spiral galaxies,
and has been presented as an alternative to cold dark matter (CDM) models. We
investigate possible additional tests involving the properties of stellar
orbits, which may be used to discriminate between the two models. We
demonstrate that in MDM and CDM models fitted equally well to a galactic
rotational curve, one generally expects predictable differences in escape
speeds from the disc. The recent radial velocity (RAVE) survey of the Milky Way
has pinned down the escape speed from the solar neighbourhood to
km s, placing an additional constraint on dark
matter models. We have constructed an MDM model for the Milky Way based on its
rotational curve, and find an escape speed that is just consistent with the
observed value given the current errors, which lends credence to the viability
of the MDM model. The Gaia-ESO spectroscopic survey is expected to lead to an
even more precise estimate of the escape speed that will further constrain dark
matter models. However, the largest differences in stellar escape speeds
between both models are predicted for dark matter dominated dwarf galaxies such
as DDO 154, and kinematical studies of such galaxies could prove key in
establishing, or abolishing, the validity of the MDM model.Comment: Accepted for publication in the European Physical Journal
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