Hubble Space Telescope /NICMOS Observations of Terzan 5: Stellar Content and Structure of the Core

Abstract

We report results from Hubble SpaceTelescope/NICMOS imaging of the extremely dense core of the globular clusterTerzan 5. This highly obscured bulge cluster contains a low-mass X-raybinary and at least two millisecond pulsars, with evidence for a largemillisecond pulsar population. It has been estimated to have one of thehighest collision rates of any Galactic globular cluster, makingits core a particularly conducive environment for the production ofinteracting binary systems. We have reconstructed high-resolution images ofthe central 19 '' �19 '' region of Terzan 5 byapplication of the drizzle algorithm to dithered NIC2 images in the F110W,F187W, and F187N near-infrared filters. We have used a DAOPHOT/ALLSTARanalysis of these images to produce the deepest color-magnitude diagram(CMD) yet obtained for the core of Terzan 5. We have also analyzedthe parallel 11 '' �11 '' NIC1 field, centered30 '' from the cluster center and imaged in F110W and F160W,and an additional NIC2 field that is immediately adjacent to thecentral field. This imaging results in a clean detection of the redgiant branch and horizontal branch in the central NIC2 field, and thedetection of these plus the main-sequence turnoff and the upper mainsequence in the NIC1 field. We have constructed an H versus J-H CMDfor the NIC1 field, which provides a measurement of the infraredreddening, E(J-H) = 0.72 [corresponding to E(B-V) = 2.16]. We obtaina new distance estimate of 8.7 kpc, which places Terzan 5 withinless than 1 kpc of the Galactic center. The NIC1 CMD provides anapparent detection of the red giant branch bump, about 1 mag belowthe horizontal branch, indicating that the metallicity is at leastsolar and possibly somewhat higher. A number of blue objects aredetected in the central NIC2 field and the NIC1 field, which hint atthe presence of a blue horizontal branch. We have also determined acentral surface-density profile that results in a maximum likelihoodestimate of 7.9"+/-0.6" for the cluster core radius. We discuss theimplications of these results for the dynamical state of Terzan 5.