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Optical Polarization Mapping Toward the Interface Between the Local Cavity and Loop I
The Sun is located inside an extremely low density and quite irregularvolume of the interstellar medium, known as the Local Cavity (LC). Ithas been widely believed that some kind of interaction could beoccurring between the LC and Loop I, a nearby superbubble seen in thedirection of the Galactic center. As a result of such interaction, awall of neutral and dense material, surrounded by a ring-shaped feature,would be formed at the interaction zone. Evidence of this structure waspreviously observed by analyzing the soft X-ray emission in thedirection of Loop I. Our goal is to investigate the distance of theproposed annular region and map the geometry of the Galactic magneticfield in these directions. On that account, we have conducted an opticalpolarization survey of 878 stars from the Hipparcos catalog. Our resultssuggest that the structure is highly twisted and fragmented, showingvery discrepant distances along the annular region: ?100 pc on theleft side and 250 pc on the right side, independently confirming theindication from a previous photometric analysis. In addition, thepolarization vectors' orientation pattern along the ring also shows awidely different behavior toward both sides of the studied feature,running parallel to the ring contour on the left side and showing norelation to its direction on the right side. Altogether, these evidencessuggest a highly irregular nature, casting some doubt on the existenceof a unique large-scale ring-like structure.
| Improved Astrometry and Photometry for the Luyten Catalog. II. Faint Stars and the Revised Catalog
We complete construction of a catalog containing improved astrometry andnew optical/infrared photometry for the vast majority of NLTT starslying in the overlap of regions covered by POSS I and by the secondincremental Two Micron All Sky Survey (2MASS) release, approximately 44%of the sky. The epoch 2000 positions are typically accurate to 130 mas,the proper motions to 5.5 mas yr-1, and the V-J colors to0.25 mag. Relative proper motions of binary components are measured to 3mas yr-1. The false-identification rate is ~1% for11<~V<~18 and substantially less at brighter magnitudes. Theseimprovements permit the construction of a reduced proper-motion diagramthat, for the first time, allows one to classify NLTT stars intomain-sequence (MS) stars, subdwarfs (SDs), and white dwarfs (WDs). We inturn use this diagram to analyze the properties of both our catalog andthe NLTT catalog on which it is based. In sharp contrast to popularbelief, we find that NLTT incompleteness in the plane is almostcompletely concentrated in MS stars, and that SDs and WDs are detectedalmost uniformly over the sky δ>-33deg. Our catalogwill therefore provide a powerful tool to probe these populationsstatistically, as well as to reliably identify individual SDs and WDs.
| Subdwarf studies. I - UBVRI photometry of NLTT stars
UBVRI photometry is presented for a sample of 1656 southern stars,including 1211 that were previously unmeasured, drown from the NLTTproper-motion catalog. The catalog is shown to be a rich source ofsubdwarfs. The normalized ultraviolet excess delta (U - B)0.6,photometric parallax, and interstellar reddening are calculated for eachstar when possible. Photometric parallaxes are compared withtrigonometric parallaxes from the literature. It is found that theformer do not have systematic errors greater than about 25 percent. Inagreement with other studies, the bluest subdwarfs are found at B - V =0.35. The selection of the program stars on the basis of large reducedproper motions restricted subgiant contamination of the sample to about5 percent and increased the discovery fraction of halo stars relative todisk stars. The claim is made here that the sample can be used toinvestigate the abundance distribution of the halo. The sample includesstars with ultraviolet excesses characteristic of disk abundances butwith velocities up to 150 km/s. These are believed to be stars that,quite expectedly, reside in the high-velocity tail of the disk velocitydistribution.
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Observation and Astrometry data
| Constellation: | Centaurus |
| Right ascension: | 11h14m55.26s |
| Declination: | -40°06'16.6" |
| Apparent magnitude: | 10.226 |
| Proper motion RA: | 150.6 |
| Proper motion Dec: | -205.5 |
| B-T magnitude: | 11.449 |
| V-T magnitude: | 10.327 |
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