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Are magnetic hot stars intrinsic X-ray sources?
Context: . X-ray surveys carried out with the Einstein and ROSATsatellites have resulted in rather unexpected detections of X-rayemission from late B-type and early A-type stars. These stars possessneither winds like early-type stars nor convective envelopes aslate-type stars, so that the origin and production mechanism of thisX-emission is unclear. Aims: . We investigate whether thepresence of large magnetic fields is related to the observed X-rayemission. Methods: . We carried out Chandra high-angularresolution observations of a sample of late B-type and A-type stars withmeasured magnetic fields in the range from 0.2-17 kG. Out of theselected 10 sample stars, 6 objects had been previously detected asX-ray sources, some of them, however, with high positional uncertaintyand a low signal-to-noise ratio, while 4 of our sample stars do havelarge magnetic fields but no previous detections of X-ray emission. Results: . Our Chandra data confirm all previous ROSAT detectionswith an extremely high significance, and the limits of the offsetsbetween X-ray and optical positions are greatly improved. In particular,HD 215441, known as Babcock's star with the strongestmagnetic field by far (17 kG) of our sample stars, a rather faint andsomewhat marginal ROSAT source, can clearly be detected. However, noneof the 4 ROSAT non-detections could be detected with the new Chandraobservations. Conclusions: . The pure existence of a magneticfield of kiloGauss strength on a late B-type or A-type star is thereforenot necessarily a prerequisite for finding X-ray emission among thesestars. Understanding the observed X-ray emission from Babcock's star isa challenge for observational and theoretical astrophysics.
| The Vienna-KPNO search for Doppler-imaging candidate stars. I. A catalog of stellar-activity indicators for 1058 late-type Hipparcos stars
We present the results from a spectroscopic Ca ii H&K survey of 1058late-type stars selected from a color-limited subsample of the Hipparcoscatalog. Out of these 1058 stars, 371 stars were found to showsignificant H&K emission, most of them previously unknown; 23% withstrong emission, 36% with moderate emission, and 41% with weak emission.These spectra are used to determine absolute H&K emission-linefluxes, radial velocities, and equivalent widths of theluminosity-sensitive Sr ii line at 4077 Ä. Red-wavelengthspectroscopic and Strömgren y photometric follow-up observations ofthe 371 stars with H&K emission are used to additionally determinethe absolute Hα -core flux, the lithium abundance from the Li i6708 Å equivalent width, the rotational velocity vsin i, theradial velocity, and the light variations and its periodicity. Thelatter is interpreted as the stellar rotation period due to aninhomogeneous surface brightness distribution. 156 stars were found withphotometric periods between 0.29 and 64 days, 11 additional systemsshowed quasi-periodic variations possibly in excess of ~50 days. Further54 stars had variations but no unique period was found, and four starswere essentially constant. Altogether, 170 new variable stars werediscovered. Additionally, we found 17 new SB1 (plus 16 new candidates)and 19 new SB2 systems, as well as one definite and two possible new SB3systems. Finally, we present a list of 21 stars that we think are mostsuitable candidates for a detailed study with the Doppler-imagingtechnique. Tables A1--A3 are only available in electronic form at theCDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/Abstract.html
| UBVRI Photometry of FKSZ Stars - Part One
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1987A&AS...70..369C&db_key=AST
| Photoelectric observations of lunar occultations. XI
Occultation observations of 466 events are presented. These includediameter measures of Aldebaran under very bad conditions and a gooddiameter measure of 82 Vir. The list includes 91 reappearances andobservations of 34 double or multiple stars, both new and previouslyknown. The inconsistencies of the observations of Kui 88 are discussed.Further observations are most desirable.
| A study of stellar rotation. II. The origin of Ap and Am stars and other slowly rotating A and B-type main-sequence stars
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