Showing papers by "David Mouillet published in 2004"

Photospheric magnetic field and surface differential rotation of the FK Com star HD 199178

Abstract: We present spectropolarimetric observations of the FK Com star HD 199178 obtained between 1998 December and 2003 August at the Telescope Bernard Lyot (Observatoire du Pic du Midi, France). We report the detection of a photospheric magnetic field and reconstruct its distribution by means of Zeeman–Doppler imaging. We observe large regions where the magnetic field is mainly azimuthal, suggesting that the dynamo processes generating the magnetic activity of HD 199178 may be active very close to the stellar surface. We investigate the rapid evolution of surface brightness and magnetic structures from a continuous monitoring of the star over several weeks in 2002 and 2003. We report that significant changes occur in the distribution of cool-spots and magnetic regions on typical time-scales of the order of two weeks. Our spectropolarimetric observations also suggest that the surface of HD 199178 is sheared by differential rotation, with a difference in rotation rate between equatorial and polar regions of the order of 1.5 times that of the Sun.

Photospheric magnetic field and surface differential rotation of the FK Com star HD 199178

Abstract: We present spectropolarimetric observations of the FK Com star HD 199178 obtained between 1998 December and 2003 August at the Telescope Bernard Lyot (Observatoire du Pic du Midi, France). We report the detection of a photospheric magnetic field and reconstruct its distribution by means of Zeeman-Doppler Imaging. We observe large regions where the magnetic field is mainly azimuthal, suggesting that the dynamo processes generating the magnetic activity of HD 199178 may be active very close to the stellar surface. We investigate the rapid evolution of surface brightness and magnetic structures from a continuous monitoring of the star over several weeks in 2002 and 2003. We report that significant changes occur in the distribution of cool spots and magnetic regions on typical timescales of the order of 2 weeks. Our spectropolarimetric observations also suggest that the surface of HD 199178 is sheared by differential rotation, with a difference in rotation rate between equatorial and polar regions of the order of 1.5 times that of the Sun.

Hot Very Small dust Grains in NGC 1068 seen in jet induced structures thanks to VLT/NACO adaptive optics

Abstract: We present K, L and M diffraction-limited images of NGC 1068 obtained with NAOS+CONICA at VLT/YEPUN over a 3.5" field around the central engine. Hot dust (Tcol = 550-650 K) is found in three different regions : (a) in the true nucleus, seen as a slightly NS elongated, core of extremely hot dust, "resolved" in K and L with respective diameters of ~5 pc and 8.5 pc ; (b) along the NS direction, as a "spiral arm" and a southern tongue ; (c) as a set of parallel elongatednodules ("wave-like") bracketting the jet. Several structures observed on radio maps, mid-IR or HST UV-visible maps are seen, so that a precise registration can be done from UV to 6 cm. These results dosupport the current interpretion that source (a) corresponds to emission from dust near sublimation temperature delimiting the walls of the cavity in the central obscuring torus. Structure (b) is thought tobe a mixture of hot dust and active star forming regions along a micro spiral structure that could trace the tidal mechanism bringing matter to the central engine. Structure c)which was not known, exhibits too high a temperature for "classical'' grains ; it is most probably the signature of transiently heated very small dust grains (VSG) : "nano-diamonds", whichare resistant and can form in strong UV field or in shocks, are very attractive candidates. The "waves'' can be condensations triggered by jet induced shocks, as predicted by recent models. First estimates, based on a simple VSG model and on a detailed radiative transfer model, do agree with those interpretations,both qualitatively and quantitatively.

VLT/NACO infrared adaptive optics images of small scale structures in OMC1 ⋆

Abstract: Near-infrared observations of line emission from excited H 2 and in the continuum are reported in the direction of the Orion molecular cloud OMC1 , using the European Southern Observatory Very Large Telescope UT4 , equipped with the NAOS adaptive optics system on the CONICA infrared array camera. Spatial resolution has been achieved at close to the diffraction limit of the telescope (0. 08 −0. 12) and images show a wealth of morphological detail. Structure is not fractal but shows two preferred scale sizes of 2. (1100 AU) and 1. 2 (540 AU) , where the larger scale may be associated with star formation. Key words. ISM : individual objects : OMC1 – ISM : circumstellar matter – ISM : kinematics and dynamics – ISM : molecules – infrared : ISM

NACO performance: status after 2 years of operation

Abstract: NACO is a VLT/Yepun instrument which provides adaptive optics corrected images in the near and thermal infrared. It is composed of the NAOS adaptive optics system and of an infrared imager CONICA. NACO has been operating since October 2001 and has already delivered a large amount of scientific results in various fields, eg the Solar System (Titan), the Interstellar Medium (outflows in Orion-OMC1), the Galactic Center, the central regions of AGN and ULIRG, ... We present the instrument performance in terms of image quality after two years of operation at Paranal. We first remind the system performance obtained from simulations, design, tests and compare them to the original specifications. We point out the telescope vibrations as a source of performance degradation. We then evaluate the impact of these vibrations on the Strehl ratio. We eventually analyze studies of the telescope vibrations to identify the systems that could excite the telescope vibration modes.

A survey of the weakest-field magnetic Ap stars: discovery of a threshold magnetic field strength?

Abstract: We are conducting a magnetic survey of a sample of about 30 spectroscopically identified Ap stars, with weak or previously undetected magnetic fields. For 28 studied stars, we have obtained 25 detections of Stokes V Zeeman signatures. Our results suggest that all Ap stars are magnetic. Further there may exist a minimum field strength for which Ap-type characteristics are produced.