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Multifrequency observations of AB Doradus. X-ray flaring and rotational modulation of a young star
X-ray observations of AB Doradus, performed by the Large Area Counter(LAC) instrument of the GINGA satellite on January 1990, are reported.The observations covered 5 rotations of the star (2.6 days) during which4 flares were detected. When added to the previously observed EINSTEINand EXOSAT flares, a total of 7 X-ray flares in AB Dor have beenobserved so far. The flares seem to cluster around rotational phases0.1-0.25 and 0.6-0.75 although the statistics are poor. The mean flareenergies were around (1-3) x 1034 erg with peak luminosities(4-6) x 1030 ergs/s. The flaring loops were compact(ne = 1012/cu cm) and extended (1-2) x1010 cm above the surface. Flare masses (1018 g)and frequencies (two per day) are similar to prominence-like cloudformations discovered previously in the star. The flare spectra can bebest-fitted either by thermal Bremstrahlung with kT = 3-6 keV or with apower-law, with photon index gamma = 2.2-2.6. During the strongest flarepeaks AB Dor is a 10 mCrab source with a Crab-like spectrum. The 3 sigmaupper limit for the 6.7 keV iron line during the flares is somewhatsmaller than predicted by thin plasma models. We discuss the possibilityof lowering the equivalent width by an extra non-thermal continuum dueto mildly relativistic electrons. Simultaneous 8.4 GHz observationsduring flare No. 1 gave only a marginal detection, constraining themagnetic field strength to less than 50 Gauss if the total X-raycontinuum is non-thermal in origin. The sensitivity was not good enoughto detect any clear modulation in the X-ray light curve, folded over the0.514 d rotation period. Simultaneous 8.4 GHz observations wereperformed with the 64 m antenna of the Australia Telescope NationalFacility at Parkes and reveal a clear variability with two maxima atphases 0.0 (spot A) and 0.5 (spot B). Nearly simultaneous opticalphotometry can be modeled by a cool extended photospheric spot at thephase 0.0 (spot A). Simultaneous H-alpha photometry revealed a transientH-alpha absorption feature above the spot A, trapped in co-rotation withthe stellar magnetic field. All these multifrequency data can beunderstood by a geometrical two spot model. The geometry seems to havebeen permanent during the past 10 years, although the relativecontributions of the two poles vary and the spots are not necessarilyfixed into 180 degrees apart all the time.
| Einstein Observatory magnitude-limited X-ray survey of late-type giant and supergiant stars
Results are presented of an extensive X-ray survey of 380 giant andsupergiant stars of spectral types from F to M, carried out with theEinstein Observatory. It was found that the observed F giants orsubgiants (slightly evolved stars with a mass M less than about 2 solarmasses) are X-ray emitters at the same level of main-sequence stars ofsimilar spectral type. The G giants show a range of emissions more than3 orders of magnitude wide; some single G giants exist with X-rayluminosities comparable to RS CVn systems, while some nearby large Ggiants have upper limits on the X-ray emission below typical solarvalues. The K giants have an observed X-ray emission level significantlylower than F and F giants. None of the 29 M giants were detected, exceptfor one spectroscopic binary.
| Radial velocities of southern stars obtained with the photoelectric scanner CORAVEL. III - 790 late-type bright stars
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1985A&AS...59...15A&db_key=AST
| Narrow-Band and Broad-Band Photometry of Red Stars. III. Southern Giants
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1970ApJ...161..199E&db_key=AST
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