An Unusual Old Open Cluster: Berkeley 20

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Session 110 -- Open Clusters
Display presentation, Saturday, January 15, 9:30-6:45, Salons I/II Room (Crystal Gateway)

[110.06] An Unusual Old Open Cluster: Berkeley 20

D. MacMinn (UCSC and MMO), R.L. Phelps (Phillips Lab), K.A. Janes (Boston U.), E.D. Friel (MMO)

We present CCD $VI_{c}$ photometry for Berkeley 20, a heretofore unstudied old open cluster found over 2 kpc below the galactic plane. The data were gathered with both the KPNO 2.1m and the CTIO 0.9m during 1992 as part of an ongoing survey of old open clusters. Based on the best fits of VandenBerg's 1985 isochrones to the $V,V-I$ CMD, we estimate an age of 6 to 8 Gyr, metallicity of [Fe/H] $\approx$ $-$0.25 to $-$0.50 dex, $(m-M)_{v}$ of 14.8 to 15.1 and $E(V-I) (\approx 1.3E(B-V))$ of 0.12 to 0.21 mag. It is one of the most distant clusters known at roughly 8 kpc from the Sun, and the current galactocentric distance champion at $R_{gc} \cong 15.6$ kpc. Its great distance below the plane poses difficult questions for any picture of disk formation and structure; Be 20 may also be related to the galactic warp.

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