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 $x_{21}$. 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 $x_{21}$. Depending on $x_{21}$, 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