4,583 research outputs found
Inversion of stellar fundamental parameters from Espadons and Narval high-resolution spectra
The general context of this study is the inversion of stellar fundamental
parameters from high-resolution Echelle spectra. We aim indeed at developing a
fast and reliable tool for the post-processing of spectra produced by Espadons
and Narval spectropolarimeters. Our inversion tool relies on principal
component analysis. It allows reduction of dimensionality and the definition of
a specific metric for the search of nearest neighbours between an observed
spectrum and a set of observed spectra taken from the Elodie stellar library.
Effective temperature, surface gravity, total metallicity and projected
rotational velocity are derived. Various tests presented in this study, and
done from the sole information coming from a spectral band centered around the
Mg I b-triplet and with spectra from FGK stars are very promising.Comment: 13 pages, 8 figures (accepted A&A). arXiv admin note: text overlap
with arXiv:1401.108
Full O(alpha) Radiative Corrections to High-Energy Compton Scattering
Using computer-algebraic methods we derive compact analytical expressions for
the virtual electroweak radiative corrections to polarized Compton scattering.
Moreover the helicity amplitudes for double Compton scattering, which prove to
be extremely simple in terms of Weyl-van der Waerden spinor products, are
presented for massless electrons. The inclusion of a finite electron mass is
described, too. Finally numerical results both for the purely photonic and the
full O(alpha) electroweak corrections, which turn out to be of the order of
5-10%, are discussed for energies ranging from 10GeV to 2TeV.Comment: 20 pages, LaTex (uuencoded figures appended on tex-file), Bi-TP 93/6
Spectroscopic monitoring of the Herbig Ae star HD 104237. II. Non-radial pulsations, mode analysis and fundamental stellar parameters
Herbig Ae/Be stars are intermediate-mass pre-main sequence (PMS) stars
showing signs of intense activity and strong stellar winds, whose origin is not
yet understood in the frame of current theoretical models of stellar evolution
for young stars. The evolutionary tracks of the earlier Herbig Ae stars cross a
recently discovered PMS instability strip. Many of these stars exhibit
pulsations of delta Scuti type. HD 104237 is a well-known pulsating Herbig Ae
star. In this article, we reinvestigated an extensive high-resolution
quasi-continuous spectroscopic data set in order to search for very faint
indications of non-radial pulsations in the line profile. To do this, we worked
on dynamical spectra of equivalent photospheric (LSD) profiles of HD 104237. A
2D Fourier analysis (F2D) was performed of the entire profile and the temporal
variation of the central depth of the line was studied with the time-series
analysis tools Period04 and SigSpec. We present a mode identification
corresponding to the detected dominant frequency. We perform a new accurate
determination of the fundamental stellar parameters in view of a forthcoming
asteroseismic modeling. Following the previous studies on this star, our
analysis of the dynamical spectrum of recentered LSD profiles corresponding to
the 22nd -25th of April 1999 nights spectra has confirmed the presence of
multiple oscillation modes of low-degree l in HD 104237 and led to the first
direct detection of a non-radial pulsation mode in this star: the dominant mode
F1 was identified by the Fourier 2D method having a degree l value comprised
between 1 and 2, the symmetry of the pattern variation indicating an azimuthal
order of +1 or -1. The detailed study of the fundamental stellar parameters has
provided a Teff, log g and iron abundance of 8550 +/- 150K, 3.9 +/- 0.3 and
-4.38 +/- 0.19 (i.e. [Fe/H]=+0.16 +/- 0.19), respectively
p3d: a general data-reduction tool for fiber-fed integral-field spectrographs
The reduction of integral-field spectrograph (IFS) data is demanding work.
Many repetitive operations are required in order to convert raw data into,
typically a large number of, spectra. This effort can be markedly simplified
through the use of a tool or pipeline, which is designed to complete many of
the repetitive operations without human interaction. Here we present our
semi-automatic data-reduction tool p3d that is designed to be used with
fiber-fed IFSs. Important components of p3d include a novel algorithm for
automatic finding and tracing of spectra on the detector, and two methods of
optimal spectrum extraction in addition to standard aperture extraction. p3d
also provides tools to combine several images, perform wavelength calibration
and flat field data. p3d is at the moment configured for four IFSs. In order to
evaluate its performance we have tested the different components of the tool.
For these tests we used both simulated and observational data. We demonstrate
that for three of the IFSs a correction for so-called cross-talk due to
overlapping spectra on the detector is required. Without such a correction
spectra will be inaccurate, in particular if there is a significant intensity
gradient across the object. Our tests showed that p3d is able to produce
accurate results. p3d is a highly general and freely available tool. It is
easily extended to include improved algorithms, new visualization tools and
support for additional instruments. The program code can be downloaded from the
p3d-project web site http://p3d.sourceforge.netComment: 18 pages, 15 figures, 3 tables, accepted for publication in A&
Complex collective states in a one-dimensional two-atom system
We consider a pair of identical two-level atoms interacting with a scalar
field in one dimension, separated by a distance . We restrict our
attention to states where one atom is excited and the other is in the ground
state, in symmetric or anti-symmetric combinations. We obtain exact collective
decaying states, belonging to a complex spectral representation of the
Hamiltonian. The imaginary parts of the eigenvalues give the decay rates, and
the real parts give the average energy of the collective states. In one
dimension there is strong interference between the fields emitted by the atoms,
leading to long-range cooperative effects. The decay rates and the energy
oscillate with the distance . Depending on , the decay rates
will either decrease, vanish or increase as compared with the one-atom decay
rate. We have sub- and super-radiance at periodic intervals. Our model may be
used to study two-cavity electron wave-guides. The vanishing of the collective
decay rates then suggests the possibility of obtaining stable configurations,
where an electron is trapped inside the two cavities.Comment: 14 pages, 14 figures, submitted to Phys. Rev.
Long-term magnetic field stability of Vega
We present new spectropolarimetric observations of the normal A-type star
Vega, obtained during the summer of 2010 with NARVAL at T\'elescope Bernard
Lyot (Pic du Midi Observatory). This new time-series is constituted of 615
spectra collected over 6 different nights. We use the
Least-Square-Deconvolution technique to compute, from each spectrum, a mean
line profile with a signal-to-noise ratio close to 20,000. After averaging all
615 polarized observations, we detect a circularly polarized Zeeman signature
consistent in shape and amplitude with the signatures previously reported from
our observations of 2008 and 2009. The surface magnetic geometry of the star,
reconstructed using the technique of Zeeman-Doppler Imaging, agrees with the
maps obtained in 2008 and 2009, showing that most recognizable features of the
photospheric field of Vega are only weakly distorted by large-scale surface
flows (differential rotation or meridional circulation).Comment: Proceedings of the conference "Stellar polarimetry: from birth to
death", 2011 Jun 27-30, Madiso
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