Photometric multi-site campaign on the open cluster NGC 884^*

I. Detection of the variable stars

¹ Instituut voor Sterrenkunde, Katholieke Universiteit Leuven, Celestijnenlaan 200 D, 3001 Leuven, Belgium e-mail: sophie@ster.kuleuven.be
² Instytut Astronomiczny Uniwersytetu Wrocławskiego, Kopernika 11, 51-622 Wrocław, Poland
³ Department of Astrophysics, Radboud University Nijmegen, PO Box 9010, 6500 GL Nijmegen, The Netherlands
⁴ Institut für Astronomie, Universität Wien, Türkenschanzstrasse 17, 1180 Wien, Austria
⁵ Department of Astronomy, Beijing Normal University, Beijing 100875, PR China
⁶ National Astronomical Observatories, Chinese Academy of Sciences, Beijing 100012, PR China
⁷ INAF – Osservatorio Astronomico di Brera, via Bianchi 46, 23807 Merate, Italy
⁸ Laboratoire AIM, CEA/DSM-CNRS-Université Paris Diderot, CEA, IRFU, SAp, Centre de Saclay, 91191 Gif-sur-Yvette, France
⁹ Department of Physics, Astronomy, & Materials Science, Missouri State University, 901 S. National, Springfield, MO 65897, USA
¹⁰ Institute of Astronomy, University of Cambridge, Madingley Road, Cambridge CB3 0HA, UK
¹¹ Department of Astronomy & Space Sciences, Ege University, 35100 Izmir, Turkey
¹² Izmir Institute of Technology, Department of Physics, 35430 Izmir, Turkey
¹³ Observatoire de Genève, Université de Genève, Chemin des Maillettes 51, 1290 Sauverny, Switzerland
¹⁴ Copernicus Astronomical Centre, Bartycka 18, 00-716 Warsaw, Poland
¹⁵ Department of Astrophysics, Astronomy and Mechanics, National and Kapodistrian University of Athens, Panepistimiopolis, 157 84 Zografos, Athens, Greece
¹⁶ Korea Astronomy and Space Science Institute, Daejeon 305-348, South Korea

Received: 3 September 2009
Accepted: 9 December 2009

Abstract

Context. Recent progress in the seismic interpretation of field β Cep stars has resulted in improvements of the physics in the stellar structure and evolution models of massive stars. Further asteroseismic constraints can be obtained from studying ensembles of stars in a young open cluster, which all have similar age, distance and chemical composition.

Aims. To improve our comprehension of the β Cep stars, we studied the young open cluster NGC 884 to discover new B-type pulsators, besides the two known β Cep stars, and other variable stars.

Methods. An extensive multi-site campaign was set up to gather accurate CCD photometry time series in four filters (U, B, V, I) of a field of NGC 884. Fifteen different instruments collected almost 77 500 CCD images in 1286 h. The images were calibrated and reduced to transform the CCD frames into interpretable differential light curves. Various variability indicators and frequency analyses were applied to detect variable stars in the field. Absolute photometry was taken to deduce some general cluster and stellar properties.

Results. We achieved an accuracy for the brightest stars of 5.7 mmag in V, 6.9 mmag in B, 5.0 mmag in I and 5.3 mmag in U. The noise level in the amplitude spectra is 50 μmag in the V band. Our campaign confirms the previously known pulsators, and we report more than one hundred new multi- and mono-periodic B-, A- and F-type stars. Their interpretation in terms of classical instability domains is not straightforward, pointing to imperfections in theoretical instability computations. In addition, we have discovered six new eclipsing binaries and four candidates as well as other irregular variable stars in the observed field.