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Optical spectropolarimetry with incomplete data sets
Context: Linear spectropolarimetry is a “photon-hungry”observing technique, requiring a specific sequence of observations todetermine the Stokes Q and U parameters. For dual-beamspectropolarimeters, the Q and U Stokes parameters can be ideallydetermined using observations at N=2 retarder plate positions. Theadditional polarization signal introduced by instrumental effectsrequires the redundancy of N=4 observations to correct for these effectsand to accurately measure the linear polarization of astronomicalobjects. Aims: We wish to determine if the “instrumentalsignature corrections” for the Stokes Q and U parameters,?Q and ?U, are identical forobservations with dual-beam spectropolarimeters. For instances whenobservations were only acquired at N=3 retarder plate angles, we wish todetermine if the complete measurement of one Stokes parameter and theassociated instrumental signature correction can be used to determinethe other Stokes parameter. Methods: We constructed analyticaland Monte Carlo models of a general dual-beam spectropolarimeter tostudy the factors affecting the assumption?Q=?U and the uncertainty thereon.We compared these models with VLT FORS1 linear spectropolarimetryobservations. Results: We find that, in general,?Q-?U?0, with the variancearound zero (?(?Q-?U)) beingdirectly related to the signal-to-noise ratio of the observations.Observations of a polarized standard star, observed under identicalinstrumental conditions over the period of 2002-2007, show that theassumption of ?Q-?U=0 is generallytrue over a long period, although the absolute values of?Q and ?U vary betweenobservational epochs. While the variance of?Q-?U is not dependent on thepolarization angle, significant deviations from?Q-?U=0 arise when p ? 20%.Conclusions: Incomplete VLT FORS1 spectropolarimetry datasets, forwhich observations at only N=3 retarder plate position angles have beenacquired, can be analyzed under the assumption that?Q??U. The uncertaintyassociated with this assumption is directly related to thesignal-to-noise ratio of the observations. This property of the analysisof spectropolarimetry, with dual beam spectropolarimeters, can be usedto test for the presence of artifacts affecting individual observationsand to assess the quality of the data reduction, when observations atall four retarder plate angles have been acquired.
| Circular Polarization of Starlight
Of the 7500 stars cited in the Catalog of starlight polarization, thosewhich satisfy the condition P obs % and A V 0m.5 are selected. It ispresumed that the selected stars (n=216) have circularly polarizedlight.
| Vitesses radiales. Catalogue WEB: Wilson Evans Batten. Subtittle: Radial velocities: The Wilson-Evans-Batten catalogue.
We give a common version of the two catalogues of Mean Radial Velocitiesby Wilson (1963) and Evans (1978) to which we have added the catalogueof spectroscopic binary systems (Batten et al. 1989). For each star,when possible, we give: 1) an acronym to enter SIMBAD (Set ofIdentifications Measurements and Bibliography for Astronomical Data) ofthe CDS (Centre de Donnees Astronomiques de Strasbourg). 2) the numberHIC of the HIPPARCOS catalogue (Turon 1992). 3) the CCDM number(Catalogue des Composantes des etoiles Doubles et Multiples) byDommanget & Nys (1994). For the cluster stars, a precise study hasbeen done, on the identificator numbers. Numerous remarks point out theproblems we have had to deal with.
| Interstellar polarization from observations of A and F stars in high and intermediate galactic latitudes, and from stars in the Mathewson and Ford polarization catalogue
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1986A&AS...64..487K&db_key=AST
| Multicolor photometry of metallic-line stars. III. A photometric catalogue
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1974RMxAA...1..175M&db_key=AST
| Photoelectric UBVRI observations of AM stars
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1974AJ.....79.1290F&db_key=AST
| A finding list of early-type stars near the south galactic pole.
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1971AJ.....76..338S&db_key=AST
| Catalogue et bibliographie des étoiles A à spectre particulier - Deuxième supplément
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