Resubmitting after advice from instrument scientist Charles Proffitt regarding incorporating patterns, which I have now done. ----- Submission 1 (Fri May 13 03:19:13 GMT 2005) ----- ----- Submission 2 (Wed Jun 29 03:23:57 GMT 2005) ----- Assigned ID: 480 ----- Submission 1 (Fri Jan 21 22:42:28 GMT 2005) ----- White Dwarf Cooling Physics: Calibrating the Clock We know approximate ages for the Galactic disk from white dwarf cooling theory applied to local white dwarfs and for the Galactic halo from main sequence stellar evolutionary theory applied to star clusters. However, the two chronometers are not cross-calibrated to the same absolute scale; our observations will perform this cross-calibration and improve the precision of both chronometers. We propose to use HST/ACS photometry of white dwarfs in five moderately old open cluster (0.6-2.2 Gyr), along with all available up-to-date white dwarf interior and atmosphere models and a powerful new statistical approach, to compare main sequence evolutionary theory and white dwarf cooling theory. This comparison will be done in such a manner as to test white dwarf crystallization and carbon/oxygen phase separation, as well as main sequence models in the range where they are sensitive to the degree of core overshooting and where PP burning transitions to CNO burning. This confrontation is essential before we can accurately and precisely apply white dwarf cosmochronometry to the disk and halo field populations and to globular clusters. Past support by HST for white dwarf ages in globular clusters (123 orbits for M4 and a similarly large scheduled campaign for NGC 6397) will only be fully levereged by ensuring that both stellar chronometers are calibrated to the same age scale. Only then can white dwarf chronometers live up to their potential as fundamental, independent, and new age estimators for the Galaxy.
All five clusters will be observed to the bottom of their WD cooling sequences, which are expected to terminate at V = 24.2-26.0 and I = 23.7-25.2, depending on the cluster. Past experience with HST/WFPC2 has taught us to obtain S/N ~ 15 at the WD terminus, which meets both our photometric and galaxy-resolving requirements. In addition, obtaining reliable photometry ~0.5 mag fainter than the expected WD terminus is essential to demonstrate that the observations have not missed fainter WDs. Depending on the cluster and the zodaical light contribution to the sky during observing, we expect S/N(V) = 11.3-49 and and S/N(I) = 9.5-36 for the faintest cluster WDs. Since these are SNAP observations rather than GO observations, our observing program is guided by the available exposure time in typical single-orbit SNAP windows. Following the STScI recommendations, we set this (flexible) value at 45 minutes and subtract off the expected overheads for our observations. These overheads (guide star acquisition, overheads for 4 exposures, and small dithers between observations in the same filter) total 18.5 minutes. This allows us to obtain a total of 800 sec in each filter, which we split between two exposures (400 sec each) minimally offset. This approach helps remove both cosmic rays and hot pixels. We have attempted to make these observations as simple as possible so that they will be easy to schedule. There are no special constraints, such as timing or roll angle constraints, for instance, and the exposures are simple sequences of 2 x 400s in F555W and 2 x 400s in F814W with the ACS/WFC. Our goal is to obtain deep color-magnitude diagrams for these clusters. The exact pointing is unimportant and if there is a problem with guide stars the target location can be altered somewhat. We have set the pointing near the center of the cluster to maximize the number of white dwarfs observed, and then in some cases adjusted the pointing somewhat to minimize the impact of bright stars, which will saturate and leave less of the area of the ACS/WFC available for deep photometry. We have used a pattern to offset the second F555W and the second F814W exposures for each target in order to create a dither that will remove cosmic rays and hot pixels. The goal of the POS-TARG between the F555W pair and the F814 pair is to avoid the overhead and extra complexity of a spacecraft dither offset, since this isn't necessary when the filters change. HOT STARS 12 false 0.0 false false false false false SUPPORTED STELLAR CLUSTER OPEN CLUSTER V=13-26, I=12-25.2 J2000 03 14 40.0000 +47 15 0.00 J2000 STELLAR CLUSTER OPEN CLUSTER V=13-24.5, I=12-23.7 J2000 07 17 45.0000 -15 37 30.00 J2000 STELLAR CLUSTER OPEN CLUSTER V=13-25.8, I=12-24.8 J2000 08 42 15.0000 -47 09 0.00 J2000 STELLAR CLUSTER OPEN CLUSTER V=13-24.2, I=12-24.0 J2000 11 50 50.0000 -55 42 0.00 J2000 STELLAR CLUSTER OPEN CLUSTER V=13-25.2, I=12-24.5 J2000 23 57 0.0000 +56 44 0.00 J2000 ACS/WFC Imaging No No F555W ACS/WFC Imaging No No F814W ACS/WFC Imaging No No F555W ACS/WFC Imaging No No F814W ACS/WFC Imaging No No F555W ACS/WFC Imaging No No F814W ACS/WFC Imaging No No F555W ACS/WFC Imaging No No F814W ACS/WFC Imaging No No F555W ACS/WFC Imaging No No F814W POS-TARG DITHER 0.145 Using this pattern for hot pixel and CR removal with only 2 exposures per filter. 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