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1 Acta Astronomica

1 2001

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Search for Variable Stars in Two Old Open Clusters: NGC 2506 and NGC 2420

Kim S.-L., Chum M.-Y., Park B.-G., Kim S. C., Lee S. H., Lee M. G., Ann H. B., Sung H., Jeon Y.-B., Yuk I.-S.

Acta Astronomica

2001

Vol. 51, No. 1

49--63

We present results of photometric observations of two old open clusters: NGC 2506 and NGC 2420. V-band time-series and UBVI absolute CCD photometric observations were carried out to search for variable stars and to investigate their physical properties. From the UBVI photometry of NGC 2506, we obtain interstellar reddening of E(B-V)=0.04±0.03 mag, distance modulus (V-M_V)0=12.5±0.1 mag, and age log(t/yr)=9.25. From monitoring of 590 stars with 304 CCD frames taken for ten nights, three δ Sct stars and one eclipsing binary star are discovered in NGC 2506. For the other cluster, NGC 2420, we have examined light variations of 505 stars using 347 time-series data obtained for five nights, finding no variable star. It should be noted that we have found no γ Dor star among main-sequence stars near F0 in these two old open clusters, which is consistent with the suggestion that γ Dor-type phenomenon occurs in stars younger than log(t/yr)=8.4. On the other hand, it has been known that color-magnitude diagrams of these two clusters show well-established binary sequences, implying high incidence of binary systems: ≥20% for NGC 2506 and ≈50% for NGC 2420. However, only one eclipsing binary star was found in these two clusters.