2,468 research outputs found
Envelope tomography of long-period variable stars: I. The Schwarzschild mechanism and the Balmer emission lines
This paper is the first one in a series devoted to the study of the dynamics
of the atmospheres of long-period variable stars. Results from a two-month-long
monitoring of the Mira variables RT Cyg and X Oph around maximum light with the
ELODIE spectrograph at the Haute-Provence Observatory are presented. The
monitoring covers phases 0.80 to 1.16 for RT Cyg and phases 0.83 to 1.04 for X
Oph. The cross-correlation profile of the spectrum of RT Cyg with a K0 III mask
confirms that the absorption lines of RT Cyg in the optical domain appear
double around maximum light. No line doubling was found in the optical spectrum
of X Oph around maximum light, indicating that this feature is not common to
all long-period variables. This paper also presents the application to RT Cyg
of a new tomographic technique deriving the velocity field across the
atmosphere by cross-correlating the optical spectrum with numerical masks
constructed from synthetic spectra and probing layers of increasing depths.
This technique reveals that both the temporal evolution of the line doubling,
and its variation with depth in the atmosphere of RT Cyg, are consistent with
the ``Schwarzschild scenario''. This scenario relates the temporal evolution of
the red and blue peaks of the double absorption lines to the progression of a
shock wave in the atmosphere. The temporal evolution of the Balmer Halpha, H
beta, Hgamma and Hdelta emission lines around maximum light is also presented
for RT Cyg and X Oph. The velocity variations of Halpha and of the absorption
lines are discussed in the framework of two competing models for the formation
of Balmer emission lines in long-period variable stars.Comment: 11 pages, 8 figures, Latex, accepted for publication in Astronomy and
Astrophysics main journal. Also available at
http://www-astro.ulb.ac.be/Html/ps.htm
Quicksort with unreliable comparisons: a probabilistic analysis
We provide a probabilistic analysis of the output of Quicksort when
comparisons can err.Comment: 29 pages, 3 figure
Melting and Pressure-Induced Amorphization of Quartz
It has recently been shown that amorphization and melting of ice were
intimately linked. In this letter, we infer from molecular dynamics simulations
on the SiO2 system that the extension of the quartz melting line in the
metastable pressure-temperature domain is the pressure-induced amorphization
line. It seems therefore likely that melting is the physical phenomenon
responsible for pressure induced amorphization. Moreover, we show that the
structure of a "pressure glass" is similar to that of a very rapidly (1e+13 to
1e+14 kelvins per second) quenched thermal glass.Comment: 9 pages, 4 figures, LaTeX2
Semipurity of tempered Deligne cohomology
In this paper we define the formal and tempered Deligne cohomology groups,
that are obtained by applying the Deligne complex functor to the complexes of
formal differential forms and tempered currents respectively. We then prove the
existence of a duality between them, a vanishing theorem for the former and a
semipurity property for the latter. The motivation of these results comes from
the study of covariant arithmetic Chow groups. The semi-purity property of
tempered Deligne cohomology implies, in particular, that several definitions of
covariant arithmetic Chow groups agree for projective arithmetic varieties
Search for surface magnetic fields in Mira stars. First detection in chi Cyg
In order to complete the knowledge of the magnetic field and of its influence
during the transition from Asymptotic Giant Branch to Planetary Nebulae stages,
we have undertaken a search for magnetic fields at the surface of Mira stars.
We used spectropolarimetric observations, collected with the Narval instrument
at TBL, in order to detect - with Least Squares Deconvolution method - a Zeeman
signature in the visible part of the spectrum. We present the first
spectropolarimetric observations of the S-type Mira star chi Cyg, performed
around its maximum light. We have detected a polarimetric signal in the Stokes
V spectra and we have established its Zeeman origin. We claim that it is likely
to be related to a weak magnetic field present at the photospheric level and in
the lower part of the stellar atmosphere. We have estimated the strength of its
longitudinal component to about 2-3 Gauss. This result favors a 1/r law for the
variation of the magnetic field strength across the circumstellar envelope of
chi Cyg. This is the first detection of a weak magnetic field at the stellar
surface of a Mira star and we discuss its origin in the framework of shock
waves periodically propagating throughout the atmosphere of these radially
pulsating stars. At the date of our observations of chi Cyg, the shock wave
reaches its maximum intensity, and it is likely that the shock amplifies a weak
stellar magnetic field during its passage through the atmosphere. Without such
an amplification by the shock, the magnetic field strength would have been too
low to be detected. For the first time, we also report strong Stokes Q and U
signatures (linear polarization) centered onto the zero velocity (i.e., at the
shock front position). They seem to indicate that the radial direction would be
favored by the shock during its propagation throughout the atmosphere.Comment: 9 pages, 4 figures accepted by Astronomy and Astrophysics (21
November 2013
Envelope tomography of long-period variable stars III. Line-doubling frequency among Mira stars
This paper presents statistics of the line-doubling phenomenon in a sample of
81 long-period variable (LPV) stars of various periods, spectral types and
brightness ranges. When correlated with a mask mimicking a K0III spectrum, 54%
of the sample stars clearly showed a double-peaked cross-correlation profile
around maximum light, reflecting double absorption lines. Several pieces of
evidence are presented that point towards the double absorption lines as being
caused by the propagation of a shock wave through the photosphere. The
observation of the Balmer lines appearing in emission around maximum light in
these stars corroborates the presence of a shock wave. The observed velocity
discontinuities, ranging between 10 and 25 km/s, are not correlated with the
brightness ranges. A comparison with the center-of-mass (COM) velocity obtained
from submm CO lines originating in the circumstellar envelope reveals that the
median velocity between the red and blue peaks is blueshifted with respect to
the COM velocity, as expected if the shock moves upwards.Comment: Accepted by Astronomy & Astrophysics (21 pages, 15 figures
Temperature dependence of the electronic structure of semiconductors and insulators
The renormalization of electronic eigenenergies due to electron-phonon
coupling is sizable in many materials with light atoms. This effect, often
neglected in ab-initio calculations, can be computed using the
perturbation-based Allen-Heine-Cardona theory in the adiabatic or non-adiabatic
harmonic approximation. After a short description of the numerous recent
progresses in this field, and a brief overview of the theory, we focus on the
issue of phonon wavevector sampling convergence, until now poorly understood.
Indeed, the renormalization is obtained numerically through a q-point sampling
inside the BZ. For q-points close to G, we show that a divergence due to
non-zero Born effective charge appears in the electron-phonon matrix elements,
leading to a divergence of the integral over the BZ for band extrema. Although
it should vanish for non-polar materials, unphysical residual Born effective
charges are usually present in ab-initio calculations. Here, we propose a
solution that improves the coupled q-point convergence dramatically. For polar
materials, the problem is more severe: the divergence of the integral does not
disappear in the adiabatic harmonic approximation, but only in the
non-adiabatic harmonic approximation. In all cases, we study in detail the
convergence behavior of the renormalization as the q-point sampling goes to
infinity and the imaginary broadening parameter goes to zero. This allows
extrapolation, thus enabling a systematic way to converge the renormalization
for both polar and non-polar materials. Finally, the adiabatic and
non-adiabatic theory, with corrections for the divergence problem, are applied
to the study of five semiconductors and insulators: a-AlN, b-AlN, BN, diamond
and silicon. For these five materials, we present the zero-point
renormalization, temperature dependence, phonon-induced lifetime broadening and
the renormalized electronic bandstructure.Comment: 27 pages and 26 figure
Big Line Bundles over Arithmetic Varieties
We prove a Hilbert-Samuel type result of arithmetic big line bundles in
Arakelov geometry, which is an analogue of a classical theorem of Siu. An
application of this result gives equidistribution of small points over
algebraic dynamical systems, following the work of Szpiro-Ullmo-Zhang. We also
generalize Chambert-Loir's non-archimedean equidistribution
High-pressure investigations of CaTiO3 up to 60 GPa using X-ray diffraction and Raman spectroscopy
In this work, we investigate calcium titanate (CaTiO3 - CTO) using X-ray
diffraction and Raman spectroscopy up to 60 and 55 GPa respectively. Both
experiments show that the orthorhombic Pnma structure remains stable up to the
highest pressures measured, in contradiction to ab-initio predictions. A fit of
the compression data with a second-order Birch-Murnaghan equation of state
yields a bulk modulus K0 of 181.0(6) GPa. The orthorhombic distortion is found
to increase slightly with pressure, in agreement with previous experiments at
lower pressures and the general rules for the evolution of perovskites under
pressure. High-pressure polarized Raman spectra also enable us to clarify the
Raman mode assignment of CTO and identify the modes corresponding to rigid
rotation of the octahedra, A-cation shifts and Ti-O bond stretching. The Raman
signature is then discussed in terms of compression mechanisms.Comment: 11 pages, 6 figures, 4 table
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