copy 29 to AB -> AD ----- Submission 6 (Wed May 31 14:36:21 GMT 2006) ----- ----- Submission 7 (Thu Jun 29 19:29:42 GMT 2006) ----- ----- Submission 8 (Fri Jul 14 12:38:04 GMT 2006) ----- d We propose to conduct an ACS/WFC imaging survey of Galactic globular clusters. We will construct the most extensive and deepest set of photometry and astrometry to-date for these systems reaching a main sequence mass of ~0.2 solar mass with S/N >= 10. We will combine these data with archival WFPC2 and STIS images to determine proper motions for the stars in our fields. The resultant cleaned cluster CMDs will allow us to study a variety of scientific questions. These include [but are not limited to] 1) the determination of cluster ages and distances 2) the construction of main sequence mass functions and the issue of mass segregation 3) the internal motions and dynamical evolution of globular clusters, and 4) absolute cluster motions, orbits, and the Milky Way gravitational potential. We anticipate that the unique resource provided by the proposed treasury archive will play a central role in the field of globular cluster studies for decades, with a stature comparable to that of the Hubble Deep Field for high redshift studies.

We propose to image the central regions of Galactic globular clusters (GC) in the F606W (~V) and F814W (~I) passbands with the aim of obtaining S/N=10 photometry at 0.2M_sun along the main sequence. Based on the Girardi et al. (2002) isochrones (Z=0.001), this means attaining our photometric precision at M_v~10.7. The ACS ETC predicts that we can reach this magnitude with the required precision in 1 orbit per filter if the cluster has (m-M)_V<~16.5. More specifically, using the F606W filter with the spectrum of an M0 V star and E(B-V)=0.2, the ETC estimates ~2150 seconds to reach V=27.2 with S/N=10. Similarly, using the F814W filter, the ETC suggests an exposure time of ~2450 seconds to reach I=25.7 with S/N=10 [(V-I)~1.5 for a M0 V star]. Limiting ourselves to clusters with (m-M)_V<~16.5 and E(B-V)<~0.35 to minimize complications associated with differential reddening, we arrive at a list of 55 clusters using the latest version of the Harris (1996) catalog. To this we add NGC5927, NGC6304, NGC6366 because they are metal-rich (thick disk/bulge) clusters and will help us tie down the ages of clusters in that portion of age-metallicity space. We have also added NGC6441 and NGC6388; these are metal-rich clusters with predominantly red horizontal (HB) branches, but with an anomalous blue component to their HBs as well (Rich et al. 1997). As such, we are particularly interested in eliminating field stars from their CMDs so that accurate ages can be determined. Similarly, we have included the 4 GCs that belong to the main body of the Sagittarius dwarf galaxy - NGC6715 (M54), Terzan 7, Terzan 8, and Arp 2. These are 0.5 to 1 mag further than our nominal (m-M)_V<~16.5 limit, but have been included because of their obvious importance in understanding the star formation and dynamical history of the Sgr dwarf (Layden & Sarajedini 2000; Majewski et al. 2003). In fact, because M54 is hypothesized to be the nucleus of Sgr, we have chosen to observe it for 4 orbits to be sure we can adequately study the mass function of its lower main sequence. To further probe the properties of the Sgr dwarf, we have included the cluster Palomar 2 which Majewski et al. (2004) find to be associated with Sgr. Lastly, we note that Frinchaboy et al. (2004) suggest that the globular cluster Lynga 7 is a `transition cluster' with properties between the Galactic GCs and the open clusters so we have included Lynga 7 in our target list as well. This brings our total target list to 66 globular clusters. Note that clusters such as NGC4372, NGC5986, NGC6144, NGC6266, NGC6544, and NGC6638 are not included because they are known to have significant and difficult-to-model differential reddening. We have deliberately included a significant number of metal rich clusters, a sample that has been somewhat problematic in ground based surveys (due to differential reddening and/or field star contamination). Precise photometry from ACS in the cluster core will help to minimize the effects of field contamination (we will be dominated by cluster stars) and the effects of differential reddening (large samples of stars can be measured in a small area). Taking advantage of previous WFPC2 images, we will obtain proper motions for most of these clusters, and therefore clean their CMDs of nonmembers, as has been done by King et al. (1998) and Bedin et al. (2001). We can extend this process to the lower main sequence through ACS observations in later cycles, and, depending on the results from the present observations and on the future health of HST, may pursue those opportunities in future proposals. A typical Galactic GC has a declination of -30 degrees leading to a visibility period of 53 minutes. We propose to take multiple long exposures, totalling ~2150 seconds in F606W and ~2450 seconds in F814W, with dithers between individual exposures to fill in the ACS/WFC inter-CCD gap and provide improved spatial sampling. We will also take at least one short exposure to extend coverage to brighter stars. The exact observing strategy will depend on the visibility of individual clusters, but a typical example in F606W might include 3x700 second exposures coupled with 1x60 second (saturates at V ~ 17) and 1x10 second (saturates at V ~ 15) exposures. We will disable auto-parallels with HRC in order to enable coordinated parallels in the other imaging instruments - WFPC2 and NICMOS. Since our targets are spatially extended, parallel observations with these instruments will yield additional useful data. With WFPC2, we will observe with the F606W (~V) and F814W (~I) filters. With NICMOS, we have specified the F110W (~J) and F222M (~K) filters. Accurate photometry and astrometry will be obtained using both DAOPHOT/ALLFRAME (Stetson 1997) and the effective PSF algorithms developed by Anderson & King (2000, 2004). This will allow us to use positional information about stars measured on multiple dithered exposures to improve the precision and depth of the photometry. The calibration to the standard Cousins VI system will be performed via comparisons with ground-based observations, most of which are already in hand. The ground-based data will also provide many more stars to populate the brighter portions of the clusters' CMDs (RGB and HB). 12 false 0.0 false false false false false SUPPORTED STELLAR CLUSTER GLOBULAR CLUSTER J2000 00 00 0.0000 +00 00 0.00 J2000 STELLAR CLUSTER GLOBULAR CLUSTER J2000 00 00 0.0000 +00 00 0.00 J2000 STELLAR CLUSTER GLOBULAR CLUSTER J2000 00 00 0.0000 +00 00 0.00 J2000 STELLAR CLUSTER GLOBULAR CLUSTER J2000 00 00 0.0000 +00 00 0.00 J2000 STELLAR CLUSTER GLOBULAR CLUSTER J2000 00 00 0.0000 +00 00 0.00 J2000 STELLAR CLUSTER GLOBULAR CLUSTER J2000 00 00 0.0000 +00 00 0.00 J2000 STELLAR CLUSTER GLOBULAR CLUSTER J2000 00 00 0.0000 +00 00 0.00 J2000 STELLAR CLUSTER GLOBULAR CLUSTER J2000 00 00 0.0000 +00 00 0.00 J2000 STELLAR CLUSTER GLOBULAR CLUSTER J2000 00 00 0.0000 +00 00 0.00 J2000 STELLAR CLUSTER GLOBULAR CLUSTER J2000 00 00 0.0000 +00 00 0.00 J2000 STELLAR CLUSTER GLOBULAR CLUSTER J2000 00 00 0.0000 +00 00 0.00 J2000 STELLAR CLUSTER GLOBULAR CLUSTER J2000 00 00 0.0000 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