Proper motion and densification of the International Celestial Reference Frame in the direction of the Galactic bulge^⋆,⋆⋆

¹ Instituto de Astronomia, Geofísica e Ciências Atmosféricas, Universidade de São Paulo, Rua do Matão, 1226 – Cidade Universitária, 05508-900 São Paulo - SP, Brazil
e-mail: teixeira@astro.iag.usp.br
² Observatoire Aquitaine des Sciences de l’Univers, CNRS-UMR 5804, BP 89, 33270 Floirac, France
³ Faculdade de Ciências, Universidade de Lisboa, Ed. C8, Campo Grande, 1749-016 Lisboa, Portugal
⁴ Institute of Astronomy and Astrophysics, National Observatory of Athens, I. Metaxa & Bas. Pavlou, 152 36 Palea Penteli, Greece

Received: 7 July 2011
Accepted: 30 August 2011

Abstract

Context. The definition, construction and realization of a reference system is one of the oldest and most fundamental tasks of astronomy. Currently, the International Celestial Reference System (ICRS), realized by the International Celestial Reference Frame (ICRF) or ICRF2, is the reference system adopted by the IAU. It is based on the very precise VLBI positions of a few hundred compact extragalactic radio sources. Despite its excellent precision, the ICRF is far from providing a reference system that is available for the entire sky and accessible to all observers. The ICRF has to be densified and extended to other wavelengths, mainly to the optical domain where the astronomical activities are more intense. For this reason, the IAU has recommended and encouraged works in this direction over the years. Many of them were developed to give sometimes good positions and proper motions, but they are very limited in magnitude, while others are extremely dense and deep in magnitude but have low accuracy, mainly for the proper motions. Nevertheless, all these contributions are very important because they are complementary.

Aims. We present a homogeneous and precise optical astrometric catalog that extends the ICRF in the direction of 12 low-extinction windows of the Galactic bulge and provides at the same time a useful database for kinematic studies.

Methods. The catalog was constructed on a total of 9 observational epochs spread over 15 years of observations with the CCD meridian circle of the Abrahão de Moraes Observatory (Valinhos – Brazil). The Galactic Bulge Valinhos Observatory Catalog (GBVOC) contains only those stars with at least 6 observational epochs and a time base of 10 years or more. We also imposed a minimum of 3 observations per star in each observational epoch and 20 observations in total.

Results. The catalog provides positions and proper motions for about 11 600 stars with a mean internal precision of about 22 mas and 4.0 mas/yr, which ensures a high quality and reliable extension of the ICRF in this direction. The GBVOC is extremely dense with more than 3500 stars per square degree. Comparison of the GBVOC with other densification works allows us to estimate the external precision of the GBVOC to be better than 42.0 mas and 5.5 mas/yr in position and proper motion.