A complete photometric study of the open cluster NGC 7790 containing Cepheid variables

Send offprint request: A.C. Gupta, Astronomy and Astrophysics Division, Physical Research Laboratory, Navarangpura, Ahmedabad 380 009, India.

1 - Department of Physics, University of Gorakhpur, Gorakhpur 273 009, India
e-mail: acgupta@prl.ernet.in
2 - U.P. State Observatory, Manora Peak, Naini Tal 263 129, India
3 - Indian Institute of Astrophysics, Bangalore 560034, India
4 - Department of Physics, University of Leeds, Leeds LS2 9JT, UK

Abstract:

CCD photometry of the northern open star cluster NGC 7790 has been carried out in BVI photometric passbands down to $V \sim$ 21 mag for 1150 stars of which for $\sim$ 700 stars, the data is obtained for the first time. We derive the most reliable parameters for this cluster using all the available photometric, spectroscopic and proper motion data. The interstellar extinction over the cluster area is uniform and normal with $E(B-V)=0.51\,\pm\,0.03$ mag. We determine a distance of 3.3 $\,\pm\,$ 0.23 Kpc to the cluster. The theoretical isochrone fittings to the Cepheid variables as well as the evolved part of the main-sequence near turn-off point indicate that the cluster is 120 $\,\pm\,$ 20 Myr old. We estimate the cluster radius to be 3 $\hbox {$.\mkern -4mu^\prime $ }$ 7 using radial stellar density profile. Both the distance and age determined using period luminosity/age relations for the Cepheid variables are consistent with their membership of the cluster. This unique opportunity has therefore been used to refine the zero-point of the period-luminosity relation for the Galactic Cepheids.

Key words: Galaxy: open clusters: individual: NGC 7790 -- stars: evolution -- HR diagram