635 research outputs found
An elementary approach to toy models for D. H. Lehmer's conjecture
In 1947, Lehmer conjectured that the Ramanujan's tau function
never vanishes for all positive integers , where is the -th
Fourier coefficient of the cusp form of weight 12. The theory of
spherical -design is closely related to Lehmer's conjecture because it is
shown, by Venkov, de la Harpe, and Pache, that is equivalent to
the fact that the shell of norm of the -lattice is a spherical
8-design. So, Lehmer's conjecture is reformulated in terms of spherical
-design.
Lehmer's conjecture is difficult to prove, and still remains open. However,
Bannai-Miezaki showed that none of the nonempty shells of the integer lattice
\ZZ^2 in \RR^2 is a spherical 4-design, and that none of the nonempty
shells of the hexagonal lattice is a spherical 6-design. Moreover, none
of the nonempty shells of the integer lattices associated to the algebraic
integers of imaginary quadratic fields whose class number is either 1 or 2,
except for \QQ(\sqrt{-1}) and \QQ(\sqrt{-3}) is a spherical 2-design. In
the proof, the theory of modular forms played an important role.
Recently, Yudin found an elementary proof for the case of \ZZ^{2}-lattice
which does not use the theory of modular forms but uses the recent results of
Calcut. In this paper, we give the elementary (i.e., modular form free) proof
and discuss the relation between Calcut's results and the theory of imaginary
quadratic fields.Comment: 18 page
Imaging Polarimetric Observations of a New Circumstellar Disk System
Few circumstellar disks have been directly observed. Here we use sensitive
differential polarimetric techniques to overcome atmospheric speckle noise in
order to image the circumstellar material around HD 169142. The detected
envelope or disk is considerably smaller than expectations based on the
measured strength of the far-IR excess from this system
UBVJHKLM photometry and modeling of R Coronae Borealis
We present the results of UBVJHKLM photometry of R CrB spanning the period
from 1976 to 2001. Studies of the optical light curve have shown no evidence of
any stable harmonics in the variations of the stellar emission. In the L band
we found semi-regular oscillations with the two main periods of ~3.3 yr and
11.9 yr and the full amplitude of ~0.8 mag and ~0.6 mag, respectively. The
colors of the warm dust shell (resolved by Ohnaka et al. 2001) are found to be
remarkably stable in contrast to its brightness. This indicates that the inner
radius is a constant, time-independent characteristic of the dust shell. The
observed behavior of the IR light curve is mainly caused by the variation of
the optical thickness of the dust shell within the interval \tau(V)= 0.2-0.4.
Anticorrelated changes of the optical brightness (in particular with P ~ 3.3
yr) have not been found. Their absence suggests that the stellar wind of R CrB
deviates from spherical symmetry. The light curves suggest that the stellar
wind is variable. The variability of the stellar wind and the creation of dust
clouds may be caused by some kind of activity on the stellar surface. With some
time lag, periods of increased mass-loss cause an increase in the dust
formation rate at the inner boundary of the extended dust shell and an increase
in its IR brightness. We have derived the following parameters of the dust
shell (at mean brightness) by radiative transfer modeling: inner dust shell
radius r_in ~ 110 R_*, temperature T_dust(r_in) ~ 860 K, dust density
\rho_dust(r_in) ~ 1.1x10^{-20} g cm^-3, optical depth \tau(V) ~ 0.32 at 0.55
micron, mean dust formation rate [dM/dt]_dust ~ 3.1x10^-9 M_sun / yr, mass-loss
rate [dM/dt]_gas ~ 2.1x10^-7 M_sun / yr, size of the amorphous carbon grains
<(~) 0.01 micron, and B-V ~ -0.28.Comment: 9 pages, 6 figures, accepted for publication in A&
Correlation dynamics between electrons and ions in the fragmentation of D molecules by short laser pulses
We studied the recollision dynamics between the electrons and D ions
following the tunneling ionization of D molecules in an intense short pulse
laser field. The returning electron collisionally excites the D ion to
excited electronic states from there D can dissociate or be further
ionized by the laser field, resulting in D + D or D + D,
respectively. We modeled the fragmentation dynamics and calculated the
resulting kinetic energy spectrum of D to compare with recent experiments.
Since the recollision time is locked to the tunneling ionization time which
occurs only within fraction of an optical cycle, the peaks in the D kinetic
energy spectra provides a measure of the time when the recollision occurs. This
collision dynamics forms the basis of the molecular clock where the clock can
be read with attosecond precision, as first proposed by Corkum and coworkers.
By analyzing each of the elementary processes leading to the fragmentation
quantitatively, we identified how the molecular clock is to be read from the
measured kinetic energy spectra of D and what laser parameters be used in
order to measure the clock more accurately.Comment: 13 pages with 14 figure
Unified ab initio treatment of attosecond photoionization and Compton scattering
We present a new theoretical approach to attosecond laser-assisted photo- and
Compton ionization. Attosecond x-ray absorption and scattering are described by
\hat{\mathrsfs{S}}^{(1,2)}-matrices, which are coherent superpositions of
"monochromatic" -matrices in a laser-modified Furry
representation. Besides refining the existing theory of the soft x-ray
photoelectron attosecond streak camera and spectral phase interferometry (ASC
and ASPI), we formulate a theory of hard x-ray photoelectron and Compton ASC
and ASPI. The resulting scheme has a simple structure and leads to closed-form
expressions for ionization amplitudes. We investigate Compton electron
interference in the separable Coulomb-Volkov continuum with both Coulomb and
laser fields treated non-perturbatively. We find that at laser-field
intensities below 10 Wcm normalized Compton lines almost coincide
with the lines obtained in the laser-free regime. At higher intensities,
attosecond interferences survive integration over electron momenta, and feature
prominently in the Compton lines themselves. We define a regime where the
electron ground-state density can be measured with controllable accuracy in an
attosecond time interval. The new theory provides a firm basis for extracting
photo- and Compton electron phases and atomic and molecular wavefunctions from
experimental data.Comment: 15 pages, 5 figure
Lithium in the Symbiotic Mira V407 Cyg
We report an identification of the lithium resonance doublet LiI 6708A in the
spectrum of V407 Cyg, a symbiotic Mira with a pulsation period of about 745
days. The resolution of the spectra used was R~18500 and the measured
equivalent width of the line is ~0.34A. It is suggested that the lithium
enrichment is due to hot bottom burning in the intermediate mass AGB variable,
although other possible origins cannot be totally ruled out. In contrast to
lithium-rich AGB stars in the Magellanic clouds, ZrO 5551A, 6474A absorption
bands were not found in the spectrum of V407Cyg. These are the bands used to
classify the S-type stars at low-resolution. Although we identified weak ZrO
5718A, 6412A these are not visible in the low-resolution spectra, and we
therefore classify the Mira in V407 Cyg as an M type. This, together with other
published work, suggests lithium enrichment can precede the third dredge up of
s-process enriched material in galactic AGB stars.Comment: 4 pages, 2 figures, to be published in MNRA
Speckle interferometry and radiative transfer modelling of the Wolf-Rayet star WR 118
WR 118 is a highly evolved Wolf-Rayet star of the WC10 subtype surrounded by
a permanent dust shell absorbing and re-emitting in the infrared a considerable
fraction of the stellar luminosity. We present the first diffraction-limited
2.13micron speckle interferometric observations of WR 118 with 73 mas
resolution. The speckle interferograms were obtained with the 6m telescope at
the Special Astrophysical Observatory. The two-dimensional visibility function
of the object does not show any significant deviation from circular symmetry.
The visibility curve declines towards the diffraction cut-off frequency to 0.66
and can be approximated by a linear function. Radiative transfer calculations
have been carried out to model the spectral energy distribution, given in the
range of 0.5-25micron, and our 2.13micron visibility function, assuming
spherical symmetry of the dust shell. Both can be fitted with a model
containing double-sized grains (``small'' and ``large'') with the radii of a =
0.05micron and 0.38micron, and a mass fraction of the large grains greater than
65%. Alternatively, a good match can be obtained with the grain size
distribution function n(a)~a^-3, with a ranging between 0.005micron and
0.6micron. At the inner boundary of the modelled dust shell (angular diameter
(17 +/- 1)mas), the temperature of the smallest grains and the dust shell
density are 1750K +/- 100K and (1 +/- 0.2)x10^-19 g/cm^3, respectively. The
dust formation rate is found to be (1.3 +/- 0.5)x10^-7 Msol/yr assuming Vwind =
1200 km/s.Comment: 6 pages including 4 PostScript figures, also available from
http://www.mpifr-bonn.mpg.de/div/ir-interferometry/publications.html;
accepted for publication in Astronomy & Astrophysic
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