Exoplanets & Stellar Populations Group

M4 Large Program (GO-12911, P.I. Bedin)

Bedin et al. (2013) AN, 334, 1062

The Hubble Space Telescope (HST) Large Program entitled: “A search for binaries with massive companions in the core of the closest globular cluster M 4” (program GO-12911, PI: Bedin, for which 120 orbits have been allocated during HST's Cycle 20), has finished to collect data last September 15th 2013 (Bedin et al. (2013) AN, 334, 1062, Milone et al. (2014) MNRAS, 439, 1588, Nascimbeni et al. (2014) MNRAS, 442, 2381).
The project takes advantage of the exquisitely high spatial resolution and astrometric capabilities of HST to measure the “wobble” (i.e., back and forth motions, at the 50 micro-arcsec level) of luminous companions orbiting around massive dark-remnant primaries in the core of a Galactic globular cluster (GC), where massive binaries are expected to have sunk because of mass segregation.
The target GC is M 4 (NGC 6121), the cluster geometrically closest to us and known to be rich in binaries.
This project also has a spectroscopic counter-part involving about 10,000 FLAMES@VLT spectra aimed at searching for and characterizing the binary population in M 4, by measuring radial-velocity variations, mainly outside the core region (Sommariva et al. 2009 and Malavolta et al. 2013, the latter spectra available here).
The final data-set of program GO-12911 will consist of 840 HST images (720 WFC3/UVIS primary and 120 ACS/WFC parallels) and will have many unique characteristics that were not available in any of the previous HST programs.
The target and cadence of this data set enable a large number of studies, and our team has identified our top priorities. Our focus will be on: a search for binaries with massive and dark companions (the main project); a search for exo-planets; a search for stellar variability and X-ray \ optical counterparts; the measurement of the cluster absolute proper motion and of the cluster parallax; the measurement of internal motions of stars in the cluster core; the search for an intermediate-mass black hole; and the study of the multiple stellar populations in the cluster. All of these sub-projects together will produce improved constraints for our dynamical modelling of the present-day M 4 and of its dynamical evolution history.
This unique data-set will also allow us to perform a number of technical studies and instrument calibrations, which we will make publicly available. One of the major outcomes of this project will be a catalog of sources in the center of M 4 of great richness.

Related papers to GO-12911:

• Paper I: Bedin et al. (2013) AN, 334, 1062
• Paper II: Milone et al. (2014) MNRAS, 439, 1588
• Paper III: Nascimbeni et al. (2014) MNRAS, 442, 2381
• Paper IV: Malavolta et al. (2015) MNRAS, 454, 2621