Nainital Microlensing Survey-detection of short period Cepheids in the disk of M31

Abstract

Context: Cepheids are the primary distance indicators for the external galaxies, so discovery of large number of Cepheid variables in far-off galaxies offers a unique opportunity to determine the accurate distance of the host galaxy through their period-luminosity relation. Aims. The main purpose of this study is to identify short-period and relatively faint Cepheids in the crowded field of M31 disk, which was observed as part of the Nainital Microlensing Survey. Methods. The Cousins R and I band photometric observations were carried out in the direction of M31 with the aim of detecting microlensing events. The data was obtained with a 1-m telescope on more than 150 nights over the period between November 1998 and January 2002. The data was analysed using the pixel technique and the mean magnitudes of the Cepheids were determined by correlating their pixel fluxes with the corresponding PSF-fitted photometric magnitudes. Results. In the present study we report identification of short-period Cepheid variables in the M31 disk. We present a catalogue of 39 short-period (P < 15 days) Cepheids in a ~13' x 13' region of the M31 disk and give positions and pulsation periods along with their R and I bands photometric magnitudes wherever possible. Most of the Cepheids are found with R (mean) ~20-21 mag, and the dense phase coverage of our observations enabled us to identify Cepheids with periods as short as 3.4 days. The period distribution of these Cepheids peaks at log P ~ 0.9 and 1.1 days. Conclusions. We demonstrate that using the pixel method, faint and short-period Cepheids in M31 can even be detected with small-size telescopes and moderate observing conditions.