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Issue A&A
Volume 372, Number 2, June III 2001
Page(s) 477 - 494
Section Stellar clusters and associations
DOI http://dx.doi.org/10.1051/0004-6361:20010441



A&A 372, 477-494 (2001)
DOI: 10.1051/0004-6361:20010441

Photometric study of the double cluster $\vec h$ & $\mathsf{\chi}$ Persei

A. Marco and G. Bernabeu

Dpto. de Física, Ingeniería de Sistemas y Teoría de la Señal, Universidad de Alicante, Aptdo. de Correos 99, 03080, Alicante, Spain

(Received 20 July 2000 / Accepted 28 January 2001 )

Abstract
We present $uvby\beta$ CCD photometry of the central region of the double cluster h & $\chi$ Persei. We identify $\approx$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 $E(b-y) = 0.44\pm0.02$ for h Persei and $E(b-y) = 0.39\pm0.05$ for $\chi$ Persei. From the ZAMS fitting, we derive distance moduli $V_{0}-M_{V} = 11.66\pm0.20$ and $V_{0}-M_{V} = 11.56\pm0.20$ for h and $\chi$ 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 isochrone fitting, however, show that there is a significant age difference between both clusters, with the bulk of stars in h Persei being older than $\chi$ Persei. There is, however, a significant population of stars in h Persei which are younger than $\chi$ 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.


Key words: techniques: photometric -- Galaxy: open clusters and associations: individual: h Persei, $\chi$ Persei -- stars: evolution -- emission-line, Be -- formation

Offprint request: A. Marco, amparo@astronomia.disc.ua.es

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