HUNTING FOR OPTICAL COMPANIONS TO BINARY MSPS IN GLOBULAR CLUSTERS

HUNTING FOR OPTICAL COMPANIONS TO BINARY MSPS IN GLOBULAR CLUSTERS Here we present a proposal which exploits the re-newed potential of HST after the Service Mission 4 for probing the population of binary Millisecond Pulsars (MSPs) in Globular Clusters. In particular we intend to: (1) extend the search for optical counterparts in Terzan 5, by pushing the performance of the WFC3 IR channel to sample the entire MS extension down to M=0.1 Mo; (2) perform a deep multi-band search of MSP companions with the WFC3, in 3 clusters (namely NGC6440, M28 and M5), where recent radio observations have found particularly interesting objects; (3) derive an accurate radial velocity (with STIS) of the puzzling optical companion COM6266B recently discovered by our group, to firmly assess its cluster membership. This program is the result of a large collaboration among the three major groups (lead by Freire, Ransom and Possenti) which are performing extensive MSP search in GCs in the radio bands, and our group which has a large experience in performing accurate stellar photometry in crowded environments. This collaboration has produced a number of outstanding discoveries. In fact, three of the 6 optical counterparts to binary MSP companions known to date in GCs have been discovered by our group. The observations here proposed would easily double/triple the existing sample of known MSP companions, allowing the first meaningful approach to the study of the formation, evolution and recycling process of pulsar in GCs. Moreover, since most of binary MSPs in GCs are thought to form via stellar interactions in the high density core regions, the determination of the nature of the companion and the incidence of this collisionally induced population has a significant impact on our knowledge of the cluster dynamics. Even more interesting, the study of the optical companions to NSs in GCs allows one to derive tighter constraints (than those obtainable for NS binaries in the Galactic field) on the system properties. This has, in turn, an intrisic importance for fundamental physics, since it offers the opportunity of measuring the mass of the NS and hence constraining the equation of state of matter at the nuclear equilibrium density.

The proposal is organized in 4 visits, one for each programme cluster. Visit 1 (6 orbits) is devoted to secure deep IR observations of Terzan 5 by using the WFC3/IR. Deep exposures in the F160W and F125W filters are planned. Parallel observations with the ACS/WFC (in the F606W,F814W and F435W) are also included. Visit 2 (6 orbits) is devoted to multiband deep imaging of M28, by using the WFC3/UVIS channel. Deep exposures in the F390W, F606W, F814W and F656N (Halpha) filters are secured. Parallel observations are dropped because the long buffer dump times of both the ACS and WFC3 prevent to properly plan the primary WFC3 exposures. In order to reduce the overheads caused by the required short V and I exposures, we decided to organized the visit in an appropriate sequence of short and long exposures thus to allow "parallel" buffer dumps. Visit 3 (4 orbits) is devoted to multiband deep imaging of M5, by using the WFC3/UVIS channel. Deep exposures in the F390W, F606W, F814W and F656N (Halpha) filters are secured. Parallel observations are dropped because the long buffer dump times of both the ACS and WFC3 prevent to properly plan the primary WFC3 exposures. In order to reduce the overheads caused by the required short V and I exposures, we decided to organized the visit in an appropriate sequence of short and long exposures thus to allow "parallel" buffer dumps. Visit 4 (2 orbits) is devoted to perform STIS spectroscopy of a faint object suspected to be the optical companion of a MSP in NGC6266. The two orbits are organised as follows: an ACQ exposure to image the reference star and a set of 6 long exposures dithered along the slit direction. We use the 52X0.2 aperture and the G750L spectral element. A fringe flat field exposure is also planned at the end of the second orbit. Parallel ACS/WFC observations are also planned. RESOLVED STELLAR POPULATIONS 12 false 0.0 0.0 false false false false false SUPPORTED STELLAR CLUSTER GLOBULAR CLUSTER This object was generated by the targetselector and retrieved from the NED database. J2000 17 11 13.2000 -27 11 38.00 J2000 STELLAR CLUSTER GLOBULAR CLUSTER This object was generated by the targetselector and retrieved from the NED database. J2000 18 24 32.9000 -24 52 11.00 J2000 STELLAR CLUSTER GLOBULAR CLUSTER This object was generated by the targetselector and retrieved from the NED database. J2000 15 18 33.7000 +02 04 58.00 J2000 STELLAR CLUSTER GLOBULAR CLUSTER J2000 17 01 12.6900 -30 06 48.61 J2000 STELLAR CLUSTER GLOBULAR CLUSTER J2000 00 00 0.0000 +00 00 0.00 J2000 WFC3/IR Imaging No No F125W WFC3/IR Imaging No No F160W WFC3/IR Imaging No No F125W WFC3/IR Imaging No No F160W WFC3/UVIS Imaging No No F606W WFC3/UVIS Imaging No No F814W WFC3/UVIS Imaging No No F390W WFC3/UVIS Imaging No No F606W WFC3/UVIS Imaging No No F814W WFC3/UVIS Imaging No No F390W WFC3/UVIS Imaging No No F606W WFC3/UVIS Imaging No No F814W STIS/CCD Spectroscopic No No G750L ACS/WFC Imaging No No F435W ACS/WFC Imaging No No F606W ACS/WFC Imaging No No F814W ACS/WFC Imaging No No F435W ACS/WFC Imaging No No F606W ACS/WFC Imaging No No F814W ACS/WFC Imaging No No F435W ACS/WFC Imaging No No F606W ACS/WFC Imaging No No F814W ACS/WFC Imaging No No F435W ACS/WFC Imaging No No F606W ACS/WFC Imaging No No F814W WFC3/UVIS Imaging No No F606W WFC3/UVIS Imaging No No F814W WFC3/UVIS Imaging No No F438W POS-TARG DITHER 0.15 LINE false false 3 90.0 STIS-ALONG-SLIT 5

false

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