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Limb polarization of Uranus and Neptune. I. Imaging polarimetry and comparison with analytic models
Imaging polarimetry of Uranus and Neptune in the R, i, and z bands arepresented. In all observations a radial limb polarization on the orderof 1% was detected with a position angle perpendicular to the limb. Thepolarization is higher in both planets for the shorter wavelength bands.As a first approximation, the polarization seems to be equally strongalong the entire limb. This is unlike Jupiter and Saturn, wheresignificant limb polarization is only observed at the poles. Wedetermined flux-weighted averages of the limb polarization and radiallimb polarization profiles, and investigated the degradation andcancellation effects in the polarization signal due to theseeing-limited spatial resolution of our observations. Taking this intoaccount we derived corrected values for the limb polarization in Uranusand Neptune. The results are compared with analytic models for Rayleighscattering atmospheres for the semi-infinite case and finite layers withground albedo. The comparison shows that the detected polarization iscompatible with expectations. This indicates that limb-polarizationmeasurements offer a powerful diagnostic tool for investigating theproperties of scattering particles in the upper atmospheres of Uranusand Neptune, in particular if more sophisticated numerical modeling ofthe limb polarization becomes available. It is also concluded from theoverall strength of the limb polarization that the disk-integratedpolarization of Uranus and Neptune for large phase angles is high(p>20%). This is of interest for future polarimetric detections ofextra-solar planets with atmospheric properties similar to Uranus andNeptune.
| New optical polarization measurements of quasi-stellar objects. The data
New linear polarization measurements (mainly in the V band) arepresented for 203 quasi-stellar objects (QSOs). The sample is made up of94 QSOs located in the North Galactic Pole (NGP) region and of 109 QSOsin the South Galactic Pole (SGP) region. First time measurements havebeen obtained for 184 QSOs. Among them, 109 known radio-emitters, 42known Broad Absorption Line (BAL) QSOs, and 1 gravitationally lensedquasi-stellar object. We found high polarization levels (p > 3%) for12 QSOs, including the BAL QSO SDSS J1409+0048. For10 objects, measurements obtained at different epochs do exist. Two ofthem show evidence for variability: the highly polarized BL Laccandidate PKS 1216-010 and the radio sourcePKS 1222+037.Based on observations collected at the European Southern Observatory(ESO, La Silla and Paranal). Table 4 is only available in electronicform at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5)or via http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/433/757
| Polarimetric evolution of V838 Monocerotis
We present the results of our polarimetric and spectropolarimetricmonitoring of V838 Monocerotis, performed at the Asiago and Crimeanobservatories during and after the multiple outbursts that occurred inJanuary-March 2002. The polarization of the object is mainly due tointerstellar polarization (P 2.48%). Intrinsic polarization up to 0.7% at 5000 Å is present during the second maximum of theobject (February 2002). This intrinsic component increases towardshorter wavelengths but our limited spectral coverage (5000-7500Å) does not allow conclusive inferences about its origin. A strongdepolarization across the Hα profile is observed. Theinterstellar polarization gives a lower limit to the reddening of E(B-V)>0.28, with E(B-V) 0.5 being the most probable value. A normalratio of total to selective absorption (RV=3.22±0.17)was derived from the wavelength of maximum interstellar polarization.This suggests a low (if any) contribution by circumstellar material witha peculiar dust to gas ratio. A polarimetric map of a portion of thelight echo shows a complex polarization distribution reachingPmax=45%.Based on observations collected at Padua Astronomical Observatory atAsiago and Crimean Astrophysical Observatory.
| Optical Spectropolarimetry of the GRB 020813 Afterglow
The optical afterglow of gamma-ray burst GRB 020813 was observed for 3hr with the LRIS spectropolarimeter at the Keck I telescope, beginning4.7 hr after the burst was detected by the HETE-2. The spectrum revealsnumerous metal absorption lines that we identify with two systems atz=1.223 and z=1.255. We also detect an [O II] λ3727 emission lineat z=1.255, and we identify this galaxy as the likely host of the GRB.After a correction for Galactic interstellar polarization, the opticalafterglow has a linear polarization of 1.8%-2.4% during 4.7-7.9 hr afterthe burst. A measurement of p=0.80%+/-0.16% on the following night byCovino et al. demonstrates significant polarization variability over thenext 14 hr. The lack of strong variability in the position angle oflinear polarization indicates that the magnetic field in the jet islikely to be globally ordered rather than composed of a number ofrandomly oriented cells. Within the framework of afterglow models withcollimated flows, the relatively low observed polarization suggests thatthe magnetic field components perpendicular and parallel to the shockfront are only different by about 20%.
| Distances and Metallicities of High- and Intermediate-Velocity Clouds
A table is presented that summarizes published absorption linemeasurements for the high- and intermediate-velocity clouds (HVCs andIVCs). New values are derived for N(H I) in the direction of observedprobes, in order to arrive at reliable abundances and abundance limits(the H I data are described in Paper II). Distances to stellar probesare revisited and calculated consistently, in order to derive distancebrackets or limits for many of the clouds, taking care to properlyinterpret nondetections. The main conclusions are the following. (1)Absolute abundances have been measured using lines of S II, N I, and OI, with the following resulting values: ~0.1 solar for one HVC (complexC), ~0.3 solar for the Magellanic Stream, ~0.5 solar for a southern IVC,and ~solar for two northern IVCs (the IV Arch and LLIV Arch). Finally,approximate values in the range 0.5-2 solar are found for three moreIVCs. (2) Depletion patterns in IVCs are like those in warm disk or halogas. (3) Most distance limits are based on strong UV lines of C II, SiII, and Mg II, a few on Ca II. Distance limits for major HVCs aregreater than 5 kpc, while distance brackets for several IVCs are in therange 0.5-2 kpc. (4) Mass limits for major IVCs are0.5-8×105 Msolar, but for major HVCs theyare more than 106 Msolar. (5) The Ca II/H I ratiovaries by up to a factor 2-5 within a single cloud, somewhat morebetween clouds. (6) The Na I/H I ratio varies by a factor of more than10 within a cloud, and even more between clouds. Thus, Ca II can beuseful for determining both lower and upper distance limits, but Na Ionly yields upper limits.
| The Structure of the Galactic Magnetic Field toward the High-Latitude Clouds
We present the results of an optical polarization survey toward thegalactic anticenter, in the area 5h >= alpha >= 2h and 6 deg <=delta <= 12 deg. This region is characterized by the presence of astream of high-velocity H I as well as high galactic latitude molecularclouds. We used our polarization data together with 100 mu m IRAS mapsof the region to study the relation between the dust distribution andthe geometry of the magnetic field. We find that there is a correlationbetween the percent polarization and the 100 mu m flux such that P(%)<= (0.16 +/- 0.05)F100. When the IRAS flux is converted into H Icolumn densities this becomes P(%) <= (0.13 +/- 0.03)N20 - 0.22,which is consistent with previous interstellar medium studies on therelation between reddening and polarization. This correlation indicatesthat our survey is as deep as IRAS and that the magnetic field geometrydoes not change strongly with the optical depth in the lines of sightthat we have studied. The implied lower limit to the distance of oursurvey is 500--700 pc at galactic latitudes b = -20 deg and 100 pc at b= -50 deg. Our main finding is that the magnetic field is perpendicularto the H I high-velocity stream as well as the molecular cloud MBM 16 inthe high-latitude region. The field is also perpendicular to thevelocity gradient in the stream. Closer to the plane, the magnetic fieldis parallel to the dust filaments that extend like a plume toward thehalo. Our observations indicate that the galactic magnetic field towardthe high-latitude clouds is toroidal. We propose a model in which fluxtubes that rise out of the galactic plane become force-free andpredominantly toroidal at high latitudes. The high-latitude clouds maybe gas streams that are falling back toward the galactic plane withinsuch buoyant braided ropes of magnetic flux.
| Imaging polarimetry of the comet P/Swift-Tuttle
We develop a novel method of imaging polarimetry based on simultaneousrecording of two-dimensional polarization information and compensationfor atmospheric effects. We present wide-angle imaging polarimetry ofthe comet P/Swift-Tuttle at one pre-perihelion date near phase 45 deg.8.Our images show extended activity in the near-nucleus region, resultingprobably from a single dusty jet which emerges from the rotatingnucleus. We find that the polarization is higher than in typical `dusty'comets and interpret this as the contribution of fresh dust injectedinto the coma from the nucleus. We derive an independent value of 660 ms^-1 for the ejection velocity and compare it with that from molecularradio spectroscopy.
| Optical and Ultraviolet Spectroscopy Towards Stars in the Direction of the Cohen High-Velocity HI Stream
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1994MNRAS.270..597K&db_key=AST
| The Hubble Space Telescope Northern-Hemisphere grid of stellar polarimetric standards
Results are presented from a comprehensive program of opticalpolarimetry of strongly polarized and null-polarization standard stars.Potential sources of systematic error either instrumental in origin ordue to absolute calibration are critically discussed. Although theproject emphasized the requirements of a spaceborne optical-UVobservatory (HST), the resulting grid of standards represents a distinctimprovement over previous such tabulations and offers an opportunity toreference future multiwavelength polarimetry to a common system.
| An atlas of Hubble Space Telescope photometric, spectrophotometric, and polarimetric calibration objects
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1990AJ.....99.1243T&db_key=AST
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Observation and Astrometry data
| Constellation: | Κήτος |
| Right ascension: | 02h16m45.19s |
| Declination: | +07°41'10.7" |
| Apparent magnitude: | 9.061 |
| Proper motion RA: | 12.3 |
| Proper motion Dec: | -9.6 |
| B-T magnitude: | 9.254 |
| V-T magnitude: | 9.077 |
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