Nainital Microlensing Survey – detection of short period Cepheids in the disk of M 31

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 M 31 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 M 31 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 M 31 disk. We present a catalogue of 39 short-period (P < 15 days) Cepheids in a ∼13 × 13 region of the M 31 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 M 31 can even be detected with smallsize telescopes and moderate observing conditions.

Population I Cepheids and star formation history of the Large Magellanic Cloud

“…Unless a homogeneous and near complete sample of Cepheids are obtained as well as identification of the pulsation modes, for different galaxies of the Local group, as has been done in the LMC by the OGLE survey, no satisfactorily conclusion can be drawn. Nevertheless, it is conspicuous from these studies (e.g, Antonello et al 2002, Vilardell et al 2007, Joshi et al 2010) along with the present one that while metal poor galaxies (e.g, LMC) have peaks at smaller periods, metal rich galaxies (e.g, Milky way, M31) have peaks at longer periods and it is evident that maximum frequency in the period distribution shifted towards higher period with increase in the metallicity.…”

“…They found a peak at log P ∼ 1.0, however, on a closer inspection at their period distribution, we noticed a second peak at log P ∼ 0.3. It is worth mentioning here that Antonello et al (2002) and Macri (2004) using data from the DIRECT survey, and Joshi et al (2010) using data from the Nainital Microlensing Survey, found two peaks at log P ∼ 0.9 and 1.1 in the Cepheids period distribution in M 31. For Milky Way Cepheids, Vilardell et al (2007) and Joshi et al (2010) found two peaks at log P = 0.7 and 1.1.…”

“…We found only 110 Cepheids which pulsate with log P > 1, all of them in FU mode. It is well known that the number of Cepheids in any galaxy is not uniformly distributed over all the pulsation periods, and period distribution depends upon chemical composition along with many other factors like initial mass function, structure and evolutionary time scales of stars during their transit through the instability strip, etc (Alcock et al 1999, Joshi et al 2010). To understand the period distribution of CCs in the LMC, we plot the period distribution in Figure 1, with a bin width of ∆ log P = 0.05.…”

Long-term photometric study of a faint W UMa binary in the direction of M31

“…Several surveys aimed to find microlensing events towards M31 have already unearthed thousands of variable stars as a major by-product, most of which were previously unknown (Ansari et al 2004, Darnley et al 2004, Fliri et al 2006, Lee et al 2012, Soszyńksi et al 2016. Apart from detecting microlensing events in the NMS data (Joshi et al 2005), we have detected Cepheids (Joshi et al 2003(Joshi et al , 2010 and classical novae (Joshi et al 2004) in M31. In continuation of our efforts to identify variables in the archival data taken under the survey, we searched for eclipsing binary stars in the target field of M31.…”

“…Unlike our earlier study (Joshi et al 2003(Joshi et al , 2005(Joshi et al , 2010, we have not combined the frames on a nightly basis in the present analysis but carried out photometry on each individual frame in order to increase W UMa W UMa Fig. 1 A 13 ′ × 13 ′ finding chart centered at (0:42:59.33, +41:06:29.3).…”