Photometric study of the double cluster h & χ Persei

Abstract

We present uvbyβ CCD photometry of the central region of the double cluster h & χ Persei. We identify ≈350 stars, of which 214 were not included in Oosterhof's catalogue. Our magnitude limit V = 16.5 allows us to reach early F spectral type and obtain very accurate fits to the ZAMS. We derive reddening values of E(b - y) = 0.44 ± 0.02 for h Persei and E(b - y) = 0.39 × 0.05 for χ Persei. From the ZAMS fitting, we derive distance moduli Vo - MV = 11.66 ± 0.20 and Vo - MV = 11.56 ± 0.20 for h and χ Persei respectively. These values are perfectly compatible with both clusters being placed at the same distance and having identical reddenings. The shift in the main-sequence turnoff and isochronr fitting, however, show that there is a significant age difference between both clusters, with the bulk of stars in h Persei being older than χ Persei. There is, however, a significant population of stars in h Persei which are younger than χ Persei. All this argues for at least three different epochs of star formation, corresponding approximately to log t = 7.0, 7.15 and 7.3.