Search for low-mass objects in the globular cluster M4. I: Detection of variable stars (original) (raw)

With every new discovery of an extra-solar planet, the absence of planets in globular clusters (GCs) becomes more and more conspicuous. Null detection of transiting hot Jupiters in globular clusters 47 Tuc, omega Cen and NGC6397 presents an important puzzle, raising questions about the role played by cluster metallicity and environment on formation and survival of planetary systems in densely populated stellar clusters. GCs were postulated to have many free-floating planets, for which the microlensing (ML) is an established tool for detection. Dense environment, well-constrained distances and kinematics of lenses and sources, and photometry of thousands of stars simultaneously make GCs the ideal targets to search for the microlensing. We present first results of a multi-site, 69-nights long campaign to search for ML signatures of low-mass objects in the globular cluster M4, which was chosen due to its proximity, location and an actual existence of a planet. M4 was observed in R and I bands by two telescopes, 1-m T40 and 18-inch C18, of WISE Observatory, Tel Aviv, Israel, from April to July 2011. Observations on 1-m telescope were carried out in service mode, gathering 6 to 18 20-sec exposures a night for a total of 69 nights. C18 observations were done for about 4 hrs a night for 7 nights in May 2011. We employ the semi-automated pipeline to calibrate and reduce the images to the light curves that our group is developing for this purpose which includes the differential photometry package DIAPL, written by Wozniak and modified by W. Pych. Several different diagnostics are employed for search of variability/transients. While no high-significance ML event was found in this observational run, we have detected more than twenty new variables and variable candidates in M4 field, which we present here.