0 Assigned ID: 897 ----- Attempting Submission 1 (Fri Apr 10 21:58:03 GMT 2015) ----- HST Phase I Proposal 897 (ngc6397_IR.apt) successfully submitted. Receipt: # 897-1 ----- Attempting Submission 2 (Fri Apr 10 22:21:43 GMT 2015) ----- HST Phase I Proposal 897 (ngc6397_IR.apt) successfully submitted. Receipt: # 897-2 ----- Attempting Submission 3 (Fri Apr 10 22:58:30 GMT 2015) ----- 0 ----- Attempting Submission 1 (Wed Jul 22 22:22:19 GMT 2015) ----- HST Phase II Proposal 14124 (phaseII_ngc6397_v2.apt) successfully submitted. Receipt: # 14124-1 ----- Attempting Submission 2 (Thu Jul 23 21:08:30 GMT 2015) ----- Globular Clusters (GCs) in the Milky Way are the primary laboratories for establishing the ages of the oldest stellar populations and for measuring the color-magnitude relation of stars. The gold standard for these studies has involved high-precision visible light investigations with the Hubble Space Telescope. However, the shape of the color-magnitude relation in the visible bandpasses offers little leverage to disentangle the effects of distance, reddening, and metallicity, and these uncertainties impact our derived age measurements for GCs. Recently, a new feature has been observed in several HST WFC3-IR CMDs of nearby GCs. At low stellar masses, the stellar main sequence in an infrared (IR) CMD exhibits a sharp "kink" (due to opacity effects in M dwarfs), such that lower mass and cooler dwarfs become bluer in the F110W - F160W color baseline and not redder. This inversion of the color-magnitude relation offers a new opportunity to fit GC properties in the IR baseline, and to reduce their uncertainties. Here, we propose a 1 orbit HST WFC3-IR program to obtain the first IR color-magnitude relation of stars for a truly metal-poor GC, NGC 6397. We will establish the most accurate age for the cluster to date, with sub-Gyr precision. Combining this observation with our analysis of publicly available data of 4 other more metal-rich GCs from MAST will establish an independent and sensitive test to the age-metallicity relation of clusters. The target globular cluster NGC6397 is observed with WFC3 using the F110W and F160W broad-band filters. In the target cluster, we expect the absolute magnitude of the kink to occur at M_F110W ~ 6.2 mag and M_F160W ~ 5.5 mag which translates to an apparent magnitude of F110W ~ 18 mag at the distance of the cluster. To accurately map the shape of the main sequence and the color-magnitude relation below the kink we need high signal-to-noise photmetry ~ 4 magnitudes below this limit. To resample the PSF, mitigate hot pixels, and minimize errors from flat fielding we apply a 4 point dither pattern. To ensure that the confusion limit does not truncate the photometric depth reached in the cluster we point at ~ 1.3 arcmin from the cluster center (i.e.: the cluster core is just outside the fov) . RESOLVED STELLAR POPULATIONS 12 false false false false SUPPORTED STELLAR CLUSTER GLOBULAR CLUSTER This object was generated by the targetselector and retrieved from the SIMBAD database. J2000 J2000 WFC3/IR Imaging No No F110W WFC3/IR Imaging No No F160W POS-TARG DITHER 0.572 LINE 0.365 false false 74.653 4 18.528 WFC3-IR-DITHER-BOX-MIN 1 false 1437685528940 /private/var/folders/dm/dc_d1hss43g3x89tg6hc_23c00019q/T/OPVisit1060974466342888089.xml The scheduling data was generated by CASM 23.2. CASM was run in Phase II mode. (Tic-Visit :proposal-id 14124 :visit-id "01" :proposal-version -1 :trans-version "20150609" :date "23-Jul-2015 17:05:28.0" :trans-errors '(:WARNINGS 0 :ERRORS 0 :HEALTH-AND-SAFETY 0 :FATAL-ERRORS 0 :CONTINUABLE-FATALS 0) :number-of-orbits 1 :saa-model "30" :si-bright-earth-limb-angle 20.0 :data-volume 1532000 :tdrss-contact-duration 0 :coron NIL ) (Tic-Obset :proposal-id 14124 :visit-id "01" :obset-id "01" :pcs-mode "FGS" ) (Tic-Orbit :proposal-id 14124 :visit-id "01" :orbit-number 1 :pcs-time 333 :si-vis-utilization 3068 :al-overhead-at-end 0 :internal-dur-at-end 462 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3403 :pcs-gap 0 ) true 1 0