Search for low-mass objects in the globular cluster M4. I. detection of variable stars

Abstract

With every new discovery of an extrasolar planet, the absence of planets in globular clusters ( GCs ) becomes more and more conspicuous. Null detection of transiting hot Jupiters in GCs 47 Tuc, ω Cen, and NGC 6397 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- fl oating planets, for which microlensing ( ML ) is an established tool for detection. Dense environments, well-constrained distances and kinematics of lenses and sources, and photometry of thousands of stars simultaneously make GCs the ideal targets to search for ML. We present fi rst results of a multisite, 69-night-long campaign to search for ML signatures of low-mass objects in the GC M4, which was chosen because of its proximity, location, and the actual existence of a planet. M4 was observed in R and I bands by two telescopes, 1

m T40 and 18-inch C18, of the Wise

Observatory, Tel Aviv, Israel, from 2011 April to July. Observations on the 1

m telescope were carried out in serice mode, gathering 12 to 48 20

s exposres per

night for a total of 69 nights. C18 observations were done for about 4 hr a night for six

nights in 2011 May. We employ a semiautomated 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 modi fi ed by W. Pych. Several different diagnostics are employed for search of variability / transients. While no high-signi fi cance ML event was found in this observational run, we have detected more than 20

new variables and variable candidates in the M4 fi eld, which we present here.