We propose to obtain second epoch imaging of the globular cluster NGC 6397, which has the deepest ACS/WFC dataset (126 orbits in Cycle 13) ever obtained in a globular cluster. These additional 14 orbits would enable the construction of a proper motion - cleaned white dwarf cooling sequence reaching fainter than the observed truncation point of the white dwarf luminosity function; a byproduct will be absolute proper motions (relative to the extragalactic reference frame) of the rich spheroid field population. Our data shows the long sought ``blue hook'', a feature in the WD cooling sequence predicted in theoretical white dwarf models; the improved photometry and statistics afforded by the second epoch observations are needed to confirm the blue hook and to test other aspects of white dwarf cooling models that contribute to a 0.5 Gyr or greater uncertainty in the age. The proposed observations will provide formal constraints on the age and formation timescale of NGC 6397 that will be smaller than 1 Gyr and place the cluster's formation epoch relative to the end of reionization. The final proper motion-purified white dwarf sequence will be a powerful constraint for white dwarf cooling models and atmospheres, the basis for white dwarf-based globular cluster age determinations, and also constrain the fraction of binary white dwarfs.

second epoch observations to do proper motion cleaning of the cluster sample. HOT STARS 12 false 0.0 0.0 false false false false false SUPPORTED STELLAR CLUSTER GLOBULAR CLUSTER I=28.0 +/- 1 J2000 17 41 2.6960 -53 44 20.79 J2000 ACS/WFC Imaging No No F814W WFC3/UVIS Imaging No No F218W WFC3/UVIS Imaging No No F555W The scheduling data was generated by CASM 17.2. CASM was run in Phase II mode. (Tic-Visit :proposal-id 11633 :visit-id "01" :proposal-version -1 :trans-version "20080624" :date "29-Sep-2008 15:47:31.0" :trans-errors '(:WARNINGS 0 :ERRORS 0 :HEALTH-AND-SAFETY 0 :FATAL-ERRORS 0 :CONTINUABLE-FATALS 0) :number-of-orbits 3 :saa-model "29" :si-bright-earth-limb-angle 20.0 :data-volume 6022960 :tdrss-contact-duration 0 :coron NIL ) (Tic-Obset :proposal-id 11633 :visit-id "01" :obset-id "01" :pcs-mode "FGS" ) (Tic-Orbit :proposal-id 11633 :visit-id "01" :orbit-number 1 :pcs-time 333 :si-vis-utilization 3120 :al-overhead-at-end 104 :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 11633 :visit-id "01" :orbit-number 2 :pcs-time 195 :si-vis-utilization 3258 :al-overhead-at-end 73 :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 11633 :visit-id "01" :orbit-number 3 :pcs-time 195 :si-vis-utilization 3253 :al-overhead-at-end 73 :internal-dur-at-end 703 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3453 :pcs-gap 0 ) false 3 0 The scheduling data was generated by CASM 17.2. CASM was run in Phase II mode. (Tic-Visit :proposal-id 11633 :visit-id "02" :proposal-version -1 :trans-version "20080624" :date "29-Sep-2008 15:47:35.0" :trans-errors '(:WARNINGS 0 :ERRORS 0 :HEALTH-AND-SAFETY 0 :FATAL-ERRORS 0 :CONTINUABLE-FATALS 0) :number-of-orbits 3 :saa-model "29" :si-bright-earth-limb-angle 20.0 :data-volume 6022960 :tdrss-contact-duration 0 :coron NIL ) (Tic-Obset :proposal-id 11633 :visit-id "02" :obset-id "02" :pcs-mode "FGS" ) (Tic-Orbit :proposal-id 11633 :visit-id "02" :orbit-number 1 :pcs-time 333 :si-vis-utilization 3120 :al-overhead-at-end 104 :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 11633 :visit-id "02" :orbit-number 2 :pcs-time 195 :si-vis-utilization 3258 :al-overhead-at-end 73 :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 11633 :visit-id "02" :orbit-number 3 :pcs-time 195 :si-vis-utilization 3253 :al-overhead-at-end 73 :internal-dur-at-end 703 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3453 :pcs-gap 0 ) true 3 0 The scheduling data was generated by CASM 17.2. CASM was run in Phase II mode. (Tic-Visit :proposal-id 11633 :visit-id "03" :proposal-version -1 :trans-version "20080624" :date "29-Sep-2008 15:47:38.0" :trans-errors '(:WARNINGS 0 :ERRORS 0 :HEALTH-AND-SAFETY 0 :FATAL-ERRORS 0 :CONTINUABLE-FATALS 0) :number-of-orbits 3 :saa-model "29" :si-bright-earth-limb-angle 20.0 :data-volume 6022960 :tdrss-contact-duration 0 :coron NIL ) (Tic-Obset :proposal-id 11633 :visit-id "03" :obset-id "03" :pcs-mode "FGS" ) (Tic-Orbit :proposal-id 11633 :visit-id "03" :orbit-number 1 :pcs-time 333 :si-vis-utilization 3120 :al-overhead-at-end 104 :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 11633 :visit-id "03" :orbit-number 2 :pcs-time 195 :si-vis-utilization 3258 :al-overhead-at-end 73 :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 11633 :visit-id "03" :orbit-number 3 :pcs-time 195 :si-vis-utilization 3253 :al-overhead-at-end 73 :internal-dur-at-end 703 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3453 :pcs-gap 0 ) true 3 0