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Disk-Loss and Disk-Renewal Phases in Classical Be Stars. I. Analysis of Long-Term Spectropolarimetric Data
Classical Be stars are known to occasionally transition from having agaseous circumstellar disk ("Be phase") to a state in which allobservational evidence for the presence of these disks disappears("normal B-star phase"). We present one of the most comprehensivespectropolarimetric views to date of such a transition for two Be stars,? Aquarii and 60 Cygni.The disk-loss episode of 60 Cyg wascharacterized by a generally monotonic decrease in emission strengthover a timescale of ~1000 days from the maximum V-band polarization tothe minimum H? equivalent width, consistent with the viscoustimescale of the disk, assuming ?~0.14. ? Aqr's disk loss wasepisodic in nature and occurred over a timescale of ~2440 days. Anobserved time lag between the behavior of the polarization and H?in both stars indicates the disk clearing proceeded in an "inside-out"manner. We determine the position angle of the intrinsic polarization tobe 166fdg7 ± 0fdg1 for ? Aqr and 107fdg7 ± 0fdg4 for 60Cyg, and model the wavelength dependence of the observed polarizationduring the quiescent diskless phase of each star to determine theinterstellar polarization along the line of sight. Minor outburstsobserved during the quiescent phase of each star shared similarlifetimes as those previously reported for ? Cen, suggesting that theoutbursts represent the injection and subsequent viscous dissipation ofindividual blobs of material into the inner circumstellar environmentsof these stars. We also observe deviations from the mean intrinsicpolarization position angle during polarization outbursts in each star,indicating deviations from axisymmetry. We propose that these deviationsmight be indicative of the injection (and subsequent circularization) ofnew blobs into the inner disk, either in the plane of the bulk of thedisk material or in a slightly inclined (non-coplanar) orbit.
| Spectropolarimetry of the Type II Supernovae 1997ds, 1998A, and 1999gi
We present single-epoch spectropolarimetry of the Type II supernovae(SNe II) 1997ds, 1998A, and 1999gi. SN 1997ds and SN 1998A were bothobserved during the early photospheric phase, less than 50 days afterexplosion, while spectropolarimetry of SN 1999gi was obtained near thestart of the transition to the nebular phase, about 110 days afterexplosion. Uncorrected for interstellar polarization (ISP), SN 1997ds ischaracterized by pV=0.85%+/-0.02%, SN 1998A haspV=0.24%+/-0.05%, and SN 1999gi is polarized atpV=5.72%+/-0.01%. SN 1997ds and SN 1999gi exhibit distinctpolarization modulations (up to Δptot=1.6% in SN 1997dsand Δptot=1.0% in SN 1999gi) at the wavelengths of thestrongest spectral line features. While no spectral polarizationfeatures were observed in SN 1998A, the data are insensitive topolarization features at the levels confirmed in the other two objects.The low continuum polarization inferred for SN 1997ds and SN 1998A andthe amplitude of (or limits on) the polarization modulations areconsistent with those measured at similar epochs for SN 1987A and theType II plateau SN 1999em and supports the growing consensus thatcore-collapse events with hydrogen envelopes substantially intact at thetime of explosion are not significantly aspherical during the earlyphotospheric phase. The spectral shape of the high continuumpolarization of SN 1999gi closely resembles a ``Serkowski'' ISP curve(characterized by pmax=5.8%, θ=154deg, andλmax=5300 Å) and is inconsistent with thewavelength-independent nature of electron scattering expected for anaspherical SN atmosphere. Since Galactic reddening is minimal along thisline of sight, the majority of the observed polarization in SN 1999gi isbelieved to be due to ISP of the host galaxy, although significant (upto p~2%) intrinsic polarization cannot be ruled out. The potential powerof SN spectropolarimetry to study the properties of interstellar dust inexternal galaxies is described and applied to the SN 1999gi data, whereit is shown that if the polarization is indeed predominantlyinterstellar in origin, then RV=3.0+/-0.2 for the dust alongthis line of sight in NGC 3184.
| The HR-diagram from HIPPARCOS data. Absolute magnitudes and kinematics of BP - AP stars
The HR-diagram of about 1000 Bp - Ap stars in the solar neighbourhoodhas been constructed using astrometric data from Hipparcos satellite aswell as photometric and radial velocity data. The LM method\cite{luri95,luri96} allows the use of proper motion and radial velocitydata in addition to the trigonometric parallaxes to obtain luminositycalibrations and improved distances estimates. Six types of Bp - Apstars have been examined: He-rich, He-weak, HgMn, Si, Si+ and SrCrEu.Most Bp - Ap stars lie on the main sequence occupying the whole width ofit (about 2 mag), just like normal stars in the same range of spectraltypes. Their kinematic behaviour is typical of thin disk stars youngerthan about 1 Gyr. A few stars found to be high above the galactic planeor to have a high velocity are briefly discussed. Based on data from theESA Hipparcos astrometry satellite and photometric data collected in theGeneva system at ESO, La Silla (Chile) and at Jungfraujoch andGornergrat Observatories (Switzerland). Tables 3 and 4 are onlyavailable in electronic form at the CDS via anonymous ftp tocdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/Abstract.html
| The galactic reddening law - The evidence from uvby-beta photometry of B stars
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1985A&A...142..189T&db_key=AST
| Objective-prism discoveries in the declination zone 0 deg to -20 deg
An inspection of 130 10-deg-prism plates taken for the University ofMichigan Southern Spectral Survey has yielded 154 new peculiar orotherwise interesting stars. The regions surveyed are mainly at fairlyhigh galactic latitudes; this fills a gap in previous work of this type.About three-quarters of the objects will eventually be reclassified byHouk in a systematic work on all of the Henry Draper Catalogue stars,but many of these are of sufficient interest to justify earlypublication.
| Four-colour and H beta photometry of southern B stars at high galactic latitudes
Four-color and H beta photometry has been obtained for 105 early-type HDstars with galactic latitudes less than -45 deg. They are mostly late Bstars of luminosity class V to III. Two new Am stars and several Ap orBp stars are detected photometrically. Absolute magnitudes aredetermined from the photometry and from MK types where available. Theinterstellar reddening of many of the more distant stars is very small,suggesting either the existence of undetected peculiar stars in thesample or that there are areas of effectively zero reddening at highsouthern galactic latitudes.
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| Takýmyýldýz: | Kova |
| Sað Açýklýk: | 22h14m54.06s |
| Yükselim: | -06°44'07.7" |
| Görünürdeki Parlaklýk: | 7.624 |
| Uzaklýk: | 454.545 parsek |
| özdevim Sað Açýklýk: | 12.2 |
| özdevim Yükselim: | 1 |
| B-T magnitude: | 7.508 |
| V-T magnitude: | 7.615 |
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