Astrometry
Imants Platais, Terry Girard, Vera Kozhurina-Platais, Bill van Altena
Primary telescopes and CCD cameras are the same as for the photometric effort (WIYN 0.9-m and 3.5-m at Kitt Peak); however, for deep astrometry we use the KPNO Mayall and CTIO Blanco 4-m telescopes. The latter offer a FOV of 36 x 36 arcmin and a scale of 0.26 arcsec/pix. In addition, we use old photographic plates obtained with the 4-m telescopes and the other smaller specialized telescopes such as the Cesco Observatory and Lick 51 cm double astrographs. Until recently we used the Yale 2020G PDS microdensitometer equipped with laser-interferometer encoders to digitize the images on photographic plates. A single beam scanner like a PDS, has an excellent dynamical range, but is nevertheless slow in dense star fields. Hence we are experimenting with the USNO StarScan measuring machine based upon CCD technology, which promises much shorter scanning times for the entire plate. Precision astrometry with CCD mosaics is not a straightforward endeavor. Besides a time-consuming PSF fit to the images on each individual chip, considerable effort is required to obtain the distortion-free world coordinate system over the entire CCD mosaic. Our extensive experience with such mosaics is detailed in Platais et al. 2002 (WOCS VIII).
NGC 188: This 7-Gyr old cluster is a classical representative of the Galactic old disk. It is also a WOCS cornerstone cluster. We have obtained very precise proper motions and reliable membership probabilities down toV=21 which is about 5 magnitudes deeper than the existing studies of proper motions (WOCS XVII). One of the main results of this study is demonstrating the feasibility of combining archival photographic plates with CCD mosaic images.
NGC 2451:. This 60-Myr old cluster is among the nearest dozen open clusters and, yet, until recently has not received as much attention as it deserves. The WOCS astrometry and BV photometry provide new extensive and complete membership down to Mv=+8 (WOCS VII). It has been shown that its isochone fitting distance is in excellent agreement with the distance derived from Hipparcos parallaxes.
NGC 6882/5: This double cluster was selected because of a possibility of the nearby NGC 6882 (d=600 pc) being older than 1 Gyr. Our astrometric study (AAS Meeting 203, #14.11) reveals a very different picture. NGC 6882 is a rather distant (d=1350 pc), reddened E(B-V)=0.57, and young (100 Myr) open cluster. Another cluster, NGC 6885, most likely is just part of a larger OB association.
a Car open cluster: In 1998 we published a list of suspected new open clusters and OB associations from the Hipparcos data (Platais et al. 1998, AJ,116,2433). One of them, around the star a Car, appears to be the best candidate in an open cluster located approximately at the Pleiades distance. However, new proper motions of over 20,000 stars around a Car, photographic photometry and radial velocities of possible new members do not add re-assuring evidence on cluster's existence. Most likely it is part of a huge complex of young stars in Carina.
IC 2391 and Blanco 1: These two young southern open clusters have our preliminary proper motions and will be completed after obtaining new radial velocities with the ESO 2.2-m telescope and FEROS spectrograph in February-March 2004.
Inspired by the success of NGC 188 and the clearly complementary nature of proper motions and radial velocities (see Spectroscopy section), we will derive these kinematic parameters for five 2-8 Gyr old open clusters:
NGC 2506, 2682, 6791, 6819, 7789: In these clusters the faint limit of proper motions is expected to be V = 20-22 and multi-epoch radial-velocities to V = 17-18. The studies of these highly evolved stellar systems should provide highly reliable three-dimensional membership information for the entire evolved cluster population and a complete census for cluster members to masses as low as 0.5 solar masses.