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Accurate magnetic field measurements of Vega-like stars and Herbig Ae/Be stars
We obtained accurate circular spectropolarimetric observations of asample of Vega-like and Herbig Ae/Be stars with FORS 1 at the VLT in anattempt to detect their magnetic fields. No magnetic field could bediagnosed in any Vega-like star. The most accurate determination of amagnetic field, at 2.6 σ level, was performed for the Vega-likestar ι Cen, for which we measured =-77±30 G. Inthe prototype of Vega-like stars, the star β Pictoris, which showsconspicuous signs of chromospheric activity, a longitudinal magneticfield is measured only at ~1.5 σ level. We diagnosed alongitudinal magnetic field for the first time at a level higher than 3σ for the two Herbig Ae stars HD 31648 and HD 144432 and confirmthe existence of a previously detected magnetic field in a third HerbigAe star, HD 139614. Finally, we discuss the discovery of distinctiveZeeman features in the unusual Herbig Ae star HD 190073, where the Ca IIdoublet displays several components in both H and K lines. From themeasurement of circular polarization in all Balmer lines from Hβ toH8, we obtain =+26±34 G. However, using only the Ca IIH and K lines for the measurement of circular polarization, we are ableto diagnose a longitudinal magnetic field at 2.8 σ level,=+84±30 G.
| New periodic variables from the Hipparcos epoch photometry
Two selection statistics are used to extract new candidate periodicvariables from the epoch photometry of the Hipparcos catalogue. Theprimary selection criterion is a signal-to-noise ratio. The dependenceof this statistic on the number of observations is calibrated usingabout 30000 randomly permuted Hipparcos data sets. A significance levelof 0.1 per cent is used to extract a first batch of candidate variables.The second criterion requires that the optimal frequency be unaffectedif the data are de-trended by low-order polynomials. We find 2675 newcandidate periodic variables, of which the majority (2082) are from theHipparcos`unsolved' variables. Potential problems with theinterpretation of the data (e.g. aliasing) are discussed.
| Candidate Main-Sequence Stars with Debris Disks: A New Sample of Vega-like Sources
Vega-like sources are main-sequence stars that exhibit IR fluxes inexcess of expectations for stellar photospheres, most likely due toreradiation of stellar emission intercepted by orbiting dust grains. Wehave identified a large sample of main-sequence stars with possibleexcess IR radiation by cross-correlating the Michigan Catalog ofTwo-dimensional Spectral Types for the HD Stars with the IRAS FaintSource Survey Catalog. Some 60 of these Vega-like sources were not foundduring previous surveys of the IRAS database, the majority of whichemployed the lower sensitivity Point Source Catalog. Here, we providedetails of our search strategy, together with a preliminary examinationof the full sample of Vega-like sources.
| SAO stars with infrared excess in the IRAS Point Source Catalog
We have undertaken a search for SAO stars with infrared excess in theIRAS Point Source Catalog. In contrast to previous searches, the entireIRAS (12)-(25)-(60) color-color diagram was used. This selection yieldeda sample of 462 stars, of which a significant number are stars withcircumstellar material. The stars selected can be identified aspre-main-sequence stars, Be stars, protoplanetary systems, post-AGBstars, etc. A number of objects are (visual) binary stars.Characteristic temperatures and IR excesses are calculated and theirrelations to spectral type are investigated.
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