Astronomy and Astrophysics – Astronomy
Scientific paper
Jun 2006
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2006rmxac..26r.185v&link_type=abstract
XI IAU Regional Latin American Meeting of Astronomy (Eds. L. Infante & M. Rubio) Revista Mexicana de Astronomía y Astrofísica (S
Astronomy and Astrophysics
Astronomy
Scientific paper
Low extinction windows towards the Galactic bulge provide a good opportunity to use stellar proper motions to probe its kinematics and study the gravitational potential and mass distribution at this location. In particular, Plaut's window (A_V ˜ 0.8 mag) located at (l, b) = (0, -8), offers the opportunity to study the transition zone between the bulge and the Galactic disk. With this is mind, we performed an astrometric reduction on a set of 43 scanned photographic visual and blue plates, taken at this target with five different telescopes: the 20 arcsec Double Astrograph at the Yale Southern Observatory (Argentina), the 84 arcsec at KPNO (USA), the 100 arcsec du Pont at Las Campanas (Chile), the 4-m at CTIO (Chile), and the 200 arcsec Hale at Palomar (USA). These plates cover ˜ 30'×30' on the sky, with seeing from 1 arcsec to 3 arcsec and V_{lim} ˜ 22 so they reach below the bulge main-sequence turn off. These plates span over a 21-year baseline, adequate for a proper motion study, but the distortion introduced by each telescope and the crowded fields represent a challenge in the astrometric reduction. Centroids of the stars were initially computed by SExtractor and then refined with a Yale centering routine. A reduction scheme was adopted to maximize the ``free-of-distortion'' area on the plate, and to give more weight to the better quality plates. High order terms were required to model the distortion of the plates in most cases. Photometric calibration was done on ˜ 60,000 stars, based on 29 photoelectric standards in the field, and was used to include a color term in the astrometric reduction, when necessary. Typical photometric errors are ˜ 0.2 mag in each band. The magnitude equation was also corrected. The final catalogue covers 20'×20' and has 21660 stars, all of them with calibrated V and B-V, proper motions (μ_{l}, μ_{b}) and errors (eμ_{l}, eμ_{b}) in μas/yr (galactic coordinates). Proper motion errors are mostly around 1.0 mas and systematic errors are all below 0.2 mas/yr. With the purpose of selecting bulge red giant stars, we cross-referenced our catalogue with the 2MASS Catalogue and used the infrared photometry to cleanly identify these stars. From the color-magnitude diagram, K vs. J-K, we get a final sample of 537 stars. This sample shows the following dispersion in p.m.: (σμl, σμb) = (3.36, 2.80)+/- (0.11, 0.10) mas/yr. Assuming that the Sun-bulge distance is 8.541 kpc, then these values translate to a (ςl, ςb) = (135.37, 112.81) +/- (4.43, 4.03) km/s. Compared to previous studies, our results agree very well.
Girard TA
Mendez Rafael A.
van Altena William
Vieira Katherine
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