Red giants in open clusters*
X. NGC 1817
Institut d'Astronomie de l'Université de Lausanne, 1290 Chavannes-des-Bois, Switzerland
2 Harvard-Smithsonian Center for Astrophysics, 60 Graden Street, Cambridge, Massachusetts 02138, USA
3 Sternberg Astronomical Institute, Universitetskii pr. 13, Moscow 119899, Russia
4 Ulugh Beg Astronomical Institute, Academy of Sciences of Uzbekistan, Astronomicheskaya str. 33, Tashkent 700052, Uzbekistan
5 School of physics and Astronomy, University of St Andrews, St Andrews, Fife, KY169SS, UK
Corresponding author: J.-C. Mermilliod, Jean-Claude.Mermilliod@obs.unige.ch
Accepted: 12 November 2002
Radial-velocity and BV CCD observations of 29 red-giant candidates in the central part of the intermediate-age open cluster NGC 1817 have confirmed the membership of 21 stars. In addition, we have used radial velocities and new UBVR photoelectric photometry to investigate the membership of 59 new candidates located in an outer corona surrounding NGC 1817, identified from the POSS plates. We have confirmed 18 new members out to a distance of 27′ from the centre, thus doubling the previous cluster radius. Ten definite spectroscopic-binary members have been discovered, corresponding to a binary frequency of 25.6% and four orbits have been determined so far. Three additional stars have been identified as possible spectroscopic-binary members. We derive a cluster mean velocity of +65.33 ± 0.09 (0.52 s.e.) km s-1 based on 29 members with no detected velocity variations plus the 3 binary members with orbital solutions. The radial distribution of the 39 red-giant members shows a strong concentration of spectroscopic binaries in the inner part of the cluster, similar to that observed in M 67, although NGC 1817 is much younger.
Key words: open clusters and associations: individual: NGC 1817 / stars: binaries: spectroscopic / techniques: radial velocities / stars: AGB and post-AGB
Based on observations collected at the Haute-Provence Observatory (France), and with the Jacobus Kapteyn Telescope, operated on the island of La Palma by PPARC at the Spanish Observatorio del Roque de los Muchachos. Some of the radial-velocity observations were obtained with the Multiple Mirror Telescope, a joint facility of the Smithsonian Institution and the University of Arizona.
© ESO, 2003