We propose to obtain deep ACS-WFC images of six globular clusters (five of which have no previous HST photometry) that reside in the Galactic halo, where the second parameter effect is most pronounced. These globular clusters are among the least studied in the Galaxy and yet, from the perspective of the second parameter phenomenon, the most intriguing. With the best available CMDs only reaching the vicinity of the main sequence turn off at present, the unique sensitivity and resolution of ACS-WFC will yield ages of unprecedented precision for these clusters. These data will provide us with new insight into the stellar populations present in the outer Galactic halo and the nature of the second parameter. The second parameter plays a critical role in our understanding of the formation and evolution of the Galaxy and the proposed observations will shed new light on this problem and these exceptional clusters.

We propose to image the central regions of six Galactic globular clusters using the ACS-WFC in the F606W and F814W filters. Our aim is to reach the un-evolved main sequence for the first time in any of these clusters. We wish to achieve a minimum of S/N=10 in both bands at a Mv=8, roughly 4 magnitudes below the MSTO. This will allow important, direct comparisons with the CMDs of inner halo GCs from GO-10775 ("The ACS Survey of Galactic Globular Clusters"). We have used the ACS ETC to estimate exposure times adopting a K2V (Teff=4750 K, log g=4.5) synthetic spectrum which is consistent with Mv=8 in the [Fe/H] = –1.5 isochrones of Dotter et al. (2007). ETC calculations were performed individually for each cluster by adding Mv=8 to the distance modulus and reddening from Harris (1996) and using the resulting apparent V magnitude as normalization. Average levels of background light and other noise were assumed. Our plan is to observe five of the six clusters using 1 orbit per filter, with four long exposures of 500-600s, depending on visibility, and one short exposure at ~10% of the length of the long exposures. Dithers have been placed between each of the long exposures. The faintest and most reddened cluster in our sample, Palomar 15, requires more substantial exposure time to achieve the same depth in the CMD. We have therefore planned on 5 orbits for this cluster, 3 in F814W and 2 in F606W. The exposure times are analogous to the other five clusters. Based on these estimates our total orbit budget is 15. RESOLVED STELLAR POPULATIONS 12 false 0.0 0.0 false false false false false false SUPPORTED STELLAR CLUSTER GLOBULAR CLUSTER This object was generated by the targetselector and retrieved from the SIMBAD database. J2000 09 07 57.8000 -37 13 17.00 J2000 STELLAR CLUSTER GLOBULAR CLUSTER This object was generated by the targetselector and retrieved from the SIMBAD database. J2000 12 38 40.2000 -51 09 1.00 J2000 STELLAR CLUSTER GLOBULAR CLUSTER This object was generated by the targetselector and retrieved from the SIMBAD database. J2000 15 00 18.5700 -82 12 49.60 J2000 STELLAR CLUSTER GLOBULAR CLUSTER This object was generated by the targetselector and retrieved from the SIMBAD database. J2000 16 59 51.0000 -00 32 18.00 J2000 STELLAR CLUSTER GLOBULAR CLUSTER J2000 17 44 54.7000 +03 10 13.00 J2000 STELLAR CLUSTER GLOBULAR CLUSTER This object was generated by the targetselector and retrieved from the SIMBAD database. J2000 21 01 29.4300 +16 11 14.40 J2000 ACS/WFC Imaging No No F606W ACS/WFC Imaging No No F814W ACS/WFC Imaging No No F606W ACS/WFC Imaging No No F814W ACS/WFC Imaging No No F606W ACS/WFC Imaging No No F814W ACS/WFC Imaging No No F606W ACS/WFC Imaging No No F814W ACS/WFC Imaging No No F606W ACS/WFC Imaging No No F814W 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 F606W WFC3/UVIS Imaging No No F814W WFC3/UVIS Imaging No No F606W WFC3/UVIS Imaging No No F814W WFC3/UVIS Imaging No No F606W WFC3/UVIS Imaging No No F814W WFC3/UVIS Imaging No No F606W WFC3/UVIS Imaging No No F814W WFC3/UVIS Imaging No No F606W WFC3/UVIS Imaging No No F814W The scheduling data was generated by CASM 17.2. CASM was run in Phase II mode. Pyxis 1255271856000 256.79541924122714 (Tic-Visit :proposal-id 11586 :visit-id "01" :proposal-version -1 :trans-version "20090130" :date "26-May-2009 14:31:49.0" :trans-errors '(:WARNINGS 0 :ERRORS 0 :HEALTH-AND-SAFETY 0 :FATAL-ERRORS 0 :CONTINUABLE-FATALS 0) :number-of-orbits 2 :saa-model "27" :si-bright-earth-limb-angle 20.0 :data-volume 2842240 :tdrss-contact-duration 0 :coron NIL ) (Tic-Obset :proposal-id 11586 :visit-id "01" :obset-id "01" :pcs-mode "FGS" ) (Tic-Orbit :proposal-id 11586 :visit-id "01" :orbit-number 1 :pcs-time 333 :si-vis-utilization 2975 :al-overhead-at-end 0 :internal-dur-at-end 349 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3310 :pcs-gap 0 ) (Tic-Orbit :proposal-id 11586 :visit-id "01" :orbit-number 2 :pcs-time 195 :si-vis-utilization 3113 :al-overhead-at-end 0 :internal-dur-at-end 349 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3310 :pcs-gap 0 ) true 2 0 The scheduling data was generated by CASM 17.2. CASM was run in Phase II mode. Ruprecht 106 (Tic-Visit :proposal-id 11586 :visit-id "02" :proposal-version -1 :trans-version "20090702" :date "28-Oct-2009 17:30:44.0" :trans-errors '(:WARNINGS 0 :ERRORS 0 :HEALTH-AND-SAFETY 0 :FATAL-ERRORS 0 :CONTINUABLE-FATALS 0) :number-of-orbits 2 :saa-model "27" :si-bright-earth-limb-angle 20.0 :data-volume 2842240 :tdrss-contact-duration 0 :coron NIL ) (Tic-Obset :proposal-id 11586 :visit-id "02" :obset-id "02" :pcs-mode "FGS" ) (Tic-Orbit :proposal-id 11586 :visit-id "02" :orbit-number 1 :pcs-time 333 :si-vis-utilization 3112 :al-overhead-at-end 0 :internal-dur-at-end 349 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3453 :pcs-gap 0 ) (Tic-Orbit :proposal-id 11586 :visit-id "02" :orbit-number 2 :pcs-time 222 :si-vis-utilization 3231 :al-overhead-at-end 0 :internal-dur-at-end 349 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3453 :pcs-gap 0 ) true 2 0 The scheduling data was generated by CASM 17.2. CASM was run in Phase II mode. IC 4499 (Tic-Visit :proposal-id 11586 :visit-id "03" :proposal-version -1 :trans-version "20090702" :date "28-Oct-2009 17:31:38.0" :trans-errors '(:WARNINGS 0 :ERRORS 0 :HEALTH-AND-SAFETY 0 :FATAL-ERRORS 0 :CONTINUABLE-FATALS 0) :number-of-orbits 2 :saa-model "27" :si-bright-earth-limb-angle 20.0 :data-volume 2842240 :tdrss-contact-duration 0 :coron NIL ) (Tic-Obset :proposal-id 11586 :visit-id "03" :obset-id "03" :pcs-mode "FGS" ) (Tic-Orbit :proposal-id 11586 :visit-id "03" :orbit-number 1 :pcs-time 333 :si-vis-utilization 3329 :al-overhead-at-end 0 :internal-dur-at-end 349 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3662 :pcs-gap 0 ) (Tic-Orbit :proposal-id 11586 :visit-id "03" :orbit-number 2 :pcs-time 222 :si-vis-utilization 3440 :al-overhead-at-end 0 :internal-dur-at-end 349 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3662 :pcs-gap 0 ) true 2 0 The scheduling data was generated by CASM 17.2. CASM was run in Phase II mode. NGC 6426 1249395429000 114.67059555869628 (Tic-Visit :proposal-id 11586 :visit-id "04" :proposal-version -1 :trans-version "20090130" :date "26-May-2009 14:32:13.0" :trans-errors '(:WARNINGS 0 :ERRORS 0 :HEALTH-AND-SAFETY 0 :FATAL-ERRORS 0 :CONTINUABLE-FATALS 0) :number-of-orbits 2 :saa-model "27" :si-bright-earth-limb-angle 20.0 :data-volume 2842240 :tdrss-contact-duration 0 :coron NIL ) (Tic-Obset :proposal-id 11586 :visit-id "04" :obset-id "04" :pcs-mode "FGS" ) (Tic-Orbit :proposal-id 11586 :visit-id "04" :orbit-number 1 :pcs-time 333 :si-vis-utilization 2902 :al-overhead-at-end 0 :internal-dur-at-end 349 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3236 :pcs-gap 0 ) (Tic-Orbit :proposal-id 11586 :visit-id "04" :orbit-number 2 :pcs-time 195 :si-vis-utilization 3040 :al-overhead-at-end 0 :internal-dur-at-end 349 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3236 :pcs-gap 0 ) true 2 0 The scheduling data was generated by CASM 17.2. CASM was run in Phase II mode. NGC 7006 1254757298000 95.01312598204208 (Tic-Visit :proposal-id 11586 :visit-id "05" :proposal-version -1 :trans-version "20090130" :date "26-May-2009 14:32:19.0" :trans-errors '(:WARNINGS 0 :ERRORS 0 :HEALTH-AND-SAFETY 0 :FATAL-ERRORS 0 :CONTINUABLE-FATALS 0) :number-of-orbits 2 :saa-model "27" :si-bright-earth-limb-angle 20.0 :data-volume 2842240 :tdrss-contact-duration 0 :coron NIL ) (Tic-Obset :proposal-id 11586 :visit-id "05" :obset-id "05" :pcs-mode "FGS" ) (Tic-Orbit :proposal-id 11586 :visit-id "05" :orbit-number 1 :pcs-time 333 :si-vis-utilization 2922 :al-overhead-at-end 0 :internal-dur-at-end 349 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3257 :pcs-gap 0 ) (Tic-Orbit :proposal-id 11586 :visit-id "05" :orbit-number 2 :pcs-time 195 :si-vis-utilization 3060 :al-overhead-at-end 0 :internal-dur-at-end 349 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3257 :pcs-gap 0 ) true 2 0 The scheduling data was generated by CASM 17.2. CASM was run in Phase II mode. Palomar 15 1255655135000 81.25488745971228 (Tic-Visit :proposal-id 11586 :visit-id "06" :proposal-version -1 :trans-version "20090130" :date "26-May-2009 14:31:36.0" :trans-errors '(:WARNINGS 0 :ERRORS 0 :HEALTH-AND-SAFETY 0 :FATAL-ERRORS 0 :CONTINUABLE-FATALS 0) :number-of-orbits 5 :saa-model "27" :si-bright-earth-limb-angle 20.0 :data-volume 7105600 :tdrss-contact-duration 0 :coron NIL ) (Tic-Obset :proposal-id 11586 :visit-id "06" :obset-id "06" :pcs-mode "FGS" ) (Tic-Orbit :proposal-id 11586 :visit-id "06" :orbit-number 1 :pcs-time 333 :si-vis-utilization 2899 :al-overhead-at-end 0 :internal-dur-at-end 349 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3236 :pcs-gap 0 ) (Tic-Orbit :proposal-id 11586 :visit-id "06" :orbit-number 2 :pcs-time 195 :si-vis-utilization 3040 :al-overhead-at-end 0 :internal-dur-at-end 349 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3236 :pcs-gap 0 ) (Tic-Orbit :proposal-id 11586 :visit-id "06" :orbit-number 3 :pcs-time 195 :si-vis-utilization 3040 :al-overhead-at-end 0 :internal-dur-at-end 349 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3236 :pcs-gap 0 ) (Tic-Orbit :proposal-id 11586 :visit-id "06" :orbit-number 4 :pcs-time 195 :si-vis-utilization 3040 :al-overhead-at-end 0 :internal-dur-at-end 349 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3236 :pcs-gap 0 ) (Tic-Orbit :proposal-id 11586 :visit-id "06" :orbit-number 5 :pcs-time 195 :si-vis-utilization 3040 :al-overhead-at-end 0 :internal-dur-at-end 349 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3236 :pcs-gap 0 ) false 5 0