A Photometric and radial-velocity study of the open cluster NGC 7789
Date
1998
Authors
Gim, Munhwan
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Abstract
A (V, V - I)-diagram, reaching V ~ 21 (Mv ~ 9), has been derived for the intermediate-age open cluster NGC 7789 from CCD observations of more than 15,000 stars within ~ 18 arcmin of the cluster center. A prominent clump of core helium-burning stars is evident at V = 13.0 and the upper end of the main sequence shows a fairly pronounced curvature to the red, which indicates significant convective core overshooting. Interestingly, if sufficient overshooting is assumed in order to match the main-sequence data, it is not possible to reproduce the cluster's extended giant branch unless the cluster age is at least 1.6 Gyr (assuming a metallicity in the range -0.2 ~ [Fe/ H] :::; 0.0). This, in turn, requires that the cluster have an apparent distance modulus (m - M)v :::; 12.2. We infer a reddening 0.35 :::; E(V - I) :::; 0.38.
A total of 597 radial-velocity observations for 112 giant stars, which have been obtained since 1979 with the radial velocity spectrometer at the Dominion Astrophysical Observatory, shows that the mean radial velocity is -54.9 ± 0.12 km s-1 and the dispersion is 0.86 km s-1 , estimated from 50 constant-velocity stars selected as members. Twenty-five stars (32%) among 78 members are possible radial-velocity variable stars, but no orbits are determined because of the sparse sampling. Seventeen stars are radial-velocity non-members, while membership estimates for six stars are uncertain. There is a hint that the observed velocity dispersion falls off at large radius. The known radial-velocity variables also seem to be more centrally-concentrated than members with constant velocity, suggesting that mass segregation is being detected.