14124( 3) - 24-Jul-2015 16:50:02 - [ 1] HUBBLE SPACE TELESCOPE OBSERVING PROGRAM 14124 Version: 3 Check-in Time: 24-Jul-2015 16:50:02 STScI Edit Number: 0 Title Pushing to Sub-Gyr Globular Cluster Ages: the IR CMD of NGC 6397 ------------------------------------------------------------------------------------ Type Cycle GO 23 ------------------------------------------------------------------------------------ Investigators Contact? PI: Dr. Matteo Correnti Space Telescope Science Institute Y CoI: Dr. Jason S. Kalirai Space Telescope Science Institute N CoI: Dr. Thomas M. Brown Space Telescope Science Institute N CoI: Dr. Mario Gennaro Space Telescope Science Institute N CoI: Dr. Annalisa Calamida Space Telescope Science Institute N ------------------------------------------------------------------------------------ Abstract 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. ------------------------------------------------------------------------------------ Observations Description ------------------------ 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) . ------------------------------------------------------------------------------------ 14124( 3) - 24-Jul-2015 16:50:02 - [ 2] TARGET LIST Fixed Targets ------------------------------------------------------------------------------------------------------------------------------------ Tar| Target | Target | Target |Coord | Radial | Flux data No | Name | Description | Position |Eqnx | Vel. | ------------------------------------------------------------------------------------------------------------------------------------ 1 NGC-6397 STELLAR CLUSTER, RA=17H40M46.4600S, J2000 V = 5.17 GLOBULAR CLUSTER DEC=-53D39'21.00" Reference Frame: Other: SIMBAD Comments: This object was generated by the targetselector and retrieved from the SIMBAD database. ------------------------------------------------------------------------------------------------------------------------------------ 14124( 3) - 24-Jul-2015 16:50:02 - [ 3] Patterns ------------------------------------------------------------------------------------------------------------------------------------ Pattern_Number: 1 Primary_Pattern Secondary_Pattern Pattern_Type WFC3-IR-DITHER-BOX-MIN Pattern_Purpose DITHER Number_Of_Points 4 Point_Spacing 0.572 Line_Spacing 0.365 Coordinate_Frame POS-TARG Pattern_Orient 18.528 Angle_Between_Sides 74.653 Center_Pattern NO ------------------------------------------------------------------------------------------------------------------------------------ 14124( 3) - 24-Jul-2015 16:50:02 - [ 4] Visit: 01 Visit Requirements: On Hold Comments: Additional Comments: Exposures ------------------------------------------------------------------------------------------------------------------------------------ Exp | Target | Instr | Oper. | Aper |Spectral|Central| Optional |Num| Time | Special Num | Name | Config | Mode | or FOV |Element |Waveln.| Parameters |Exp| (Total) | Requirements ------------------------------------------------------------------------------------------------------------------------------------ 1 NGC-6397 WFC3/IR MULTIAC IR-FIX F110W NSAMP=10, SAMP-SEQ=STEP50 1 DEF PATTERN 1 1 CUM (996.928 S) ------------------------------------------------------------------------------------------------------------------------------------ 2 NGC-6397 WFC3/IR MULTIAC IR-FIX F160W NSAMP=10, SAMP-SEQ=STEP100 1 DEF PATTERN 1 2 CUM (1596.927 S) ------------------------------------------------------------------------------------------------------------------------------------ Sub Exposures ------------------------------------------------------------------------------------------------------------------------------------ Target | Exp |Instr | Oper. | Aper |Spectral|Cent.|Primary |Secondary |Iteration |CR-SPLIT |Orbit |Duration Name | Num |Config | Mode | or FOV |Element |Wave.|Pattern Pos |Pattern Pos |Num |Num |Number | ------------------------------------------------------------------------------------------------------------------------------------ NGC-6397 1 WFC3/IR MULTIAC IR-FIX F110W 1 none none none 1 N/A CUM NGC-6397 1 WFC3/IR MULTIAC IR-FIX F110W 2 none none none 1 N/A CUM NGC-6397 1 WFC3/IR MULTIAC IR-FIX F110W 3 none none none 1 N/A CUM NGC-6397 1 WFC3/IR MULTIAC IR-FIX F110W 4 none none none 1 N/A CUM NGC-6397 2 WFC3/IR MULTIAC IR-FIX F160W 1 none none none 1 N/A CUM NGC-6397 2 WFC3/IR MULTIAC IR-FIX F160W 2 none none none 1 N/A CUM NGC-6397 2 WFC3/IR MULTIAC IR-FIX F160W 3 none none none 1 N/A CUM NGC-6397 2 WFC3/IR MULTIAC IR-FIX F160W 4 none none none 1 N/A CUM 14124( 3) - 24-Jul-2015 16:50:02 - [ 5] Summary Form for Proposal 14124 Item Used in this proposal ------------------------------------------------------------------------------------------------------------------------------------ Apertures IR-FIX ------------------------------------------------------------------------------------------------------------------------------------ Configurations WFC3/IR ------------------------------------------------------------------------------------------------------------------------------------ Opmodes MULTIACCUM ------------------------------------------------------------------------------------------------------------------------------------ Optional Parameters NSAMP=10, SAMP-SEQ=STEP100, SAMP-SEQ=STEP50 ------------------------------------------------------------------------------------------------------------------------------------ Special Requirements PATTERN 1 1, PATTERN 1 2 ------------------------------------------------------------------------------------------------------------------------------------ Spectral Elements F110W, F160W ------------------------------------------------------------------------------------------------------------------------------------ Target Names NGC-6397 ------------------------------------------------------------------------------------------------------------------------------------