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Hubble Space Telescope and Ground-based Observations of I ZW 1 and MRK 486 and the Variability of Polarization in Radio-quiet Active Galactic Nuclei
Hubble Space Telescope UV spectropolarimetry and new ground-basedoptical observations of the Seyfert 1/QSO I Zw 1 and highly polarizedSeyfert 1 Mrk 486 are presented. I Zw 1 is found to be a polarimetricvariable. Our data show that the polarization position angle has changedby ~50 deg compared to measurements obtained 10--15 yr ago. There isevidence that the degree of optical linear polarization may be variableas well, but for lambda >~ 4000 Angstroms, P remained less than 1%.Optical/UV synchrotron radiation can be ruled out as the cause of theobserved polarized flux and variations because the permitted lines arepolarized. The optical polarization increases dramatically to the blue.In contrast, the UV polarization is roughly constant with wavelength at~1% and falls far below the polarization level expected fromground-based observations. The overall wavelength dependence of theoptical and UV polarization can be reconciled if the effect of starlightfrom the luminous host galaxy is taken into account. The strong increasein the optical polarization with decreasing wavelength for Mrk 486continues into the UV and reaches nearly 8% by 2000 Angstroms. Thissuggests that dust scattering may be the dominant polarizing mechanismin this object. Spectropolarimetry of H alpha and H beta reveals complexpolarization structure across the lines (mostly in the polarizationposition angle), leading to the conclusion that at least some of thescattering material is located within the broad-line region (BLR). Theseresults are consistent with the BLR being composed of two distinctline-emitting regions: (1) a region of width ~1500 km s-1- (FWHM)polarized at P.A. 150 deg--160 deg, and (2) a region of width ~3000 kms-1- polarized at P.A. 100 deg--110 deg. Identification of thesedistinct regions in the BLR by their differing polarizationsdemonstrates the existence of an intermediate-line region in alow-luminosity AGN. Associated C IV absorption is detected in the UVspectrum of Mrk 486, in keeping with the general trend that highpolarization in radio-quiet AGNs is somehow linked to the presence ofabsorbing gas. The object also shows evidence for variations inpolarization. Observations acquired at two separate telescopes during1994--1995 reveal a decrease of ~10 deg in polarization P.A. comparedwith earlier measurements. The polarization variability of these twoobjects, along with the deduced proximity of the scattering region tothe nucleus of Mrk 486, argues that the structure of the scatteringand/or emission-line regions in at least some radio-quiet AGNs evolve onrelatively short time scales (a few years or less).
| 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.
| Faint blue stars in the region near the South Galactic Pole.
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