9444( 4) - 08/01/02 11:20 - [ 1] PROPOSAL FOR HUBBLE SPACE TELESCOPE OBSERVATIONS ST ScI Use Only ID: 9444 Version: 4 Check-in Date: 01-Aug-2002 11:19:43 1.Proposal Title: The Region of the Hydrogen-Burning Limit in Omega Centauri and 47 Tucanae ------------------------------------------------------------------------------------ 2. Proposal For 3. Cycle GO 11 ------------------------------------------------------------------------------------ 4. Investigators Contact? PI: Ivan King University of Washington CoI: Dr. Jay Anderson University of California, Berkeley N CoI: Prof. Adrienne Cool San Francisco State University N CoI: Prof. Giampaolo Piotto University of Padua N CoI: Dr. Luigi Bedin University of Padua N ------------------------------------------------------------------------------------ 5. Abstract We propose a photometric study of the lower main sequences of Omega Cen and 47 Tuc, down to the region of the H-burning limit, which the deeper faintness limit of ACS will allow us to reach. For the faintest stars, proper-motion separation of cluster from field is essential; hence we include Cycle 13 observations. The resulting color--magnitude diagrams (CMDs) and luminosity functions (LFs) will allow study of stars in a mass regime and metallicity that have never been accessible before, and will serve as an important check on theories of the structure of low-mass stars. Our CMDs will check the luminosity--radius relation, while the faint end of the LF can be used to check the mass--luminosity relation. With 47 Tuc we extend these checks to a higher metallicity than before, while in Omega Cen we investigate the effect of a range of metallicity within a single cluster. In both clusters we will produce the faintest existing MS and WD sequences, and the faintest LF. With this proposal we initiate high-precision astrometry with ACS using the state-of-the-art techniques that we have developed for WFPC2. A separate proposal addresses the geometric distortion of ACS, but we can do well in the present work even with approximate distortion corrections, as we can if necessary confine our measurements to relative positions of closely neighboring stars. ------------------------------------------------------------------------------------ 9444( 4) - 08/01/02 11:20 - [ 2] Observations Description ------------------------ For maximum throughput and a good color baseline, we will use F606W and F814W. As discussed in the preceding section, at the second epoch F814W is sufficient. RPS2 tells us that in a 3-orbit visit we can get 7x700s + 2x800s; this is what we will use for each filter in Cycle 11 and for F814W in Cycle 13. This is similar, in exposure length and number of orbits, to what we have used in other clusters with WFPC2. Since ACS should go more than a magnitude fainter than WFPC2 in such exposure times, we have chosen clusters whose apparent distance moduli are within 1.2 mag of that of M4, in which we reached the region of the H-burning limit nicely. Omega Cen and 47 Tuc are the most interesting of these clusters scientifically. The choice of field to observe in each cluster is an important part of our planning. This choice is a balance between two conflicting requirements. The background of diffuse cluster light must not be so high as to detract seriously from our limiting magnitude; this consideration moves us away from the cluster center. Increasing distance from the center decreases the star density, however, and we need to stay in a region of high enough density, so that we have enough stars to give statistically significant points on the plunging part of the LF that is so important to our aims. Thus our choice will be a compromise. At the present time we have examined, for each cluster, the surface-brightness profiles of Trager, King, & Djorgovski (1995), and the background levels in WFPC2 images of various clusters. Taking into account that the scattered light in ACS is stated by the manual to be greater than that in WFPC2, we have made a tentative choice of field in each cluster. Between the Phase 2 submission date and the time when the observations approach actual scheduling, however, we will revisit this question on the basis of real ACS images that will then be available, in order to make the best- informed choice of field that we can. The small shift in target coordinates in the Phase 2 proposal will not in any way perturb the scheduling process, and we will thus optimize our scientific return. For calibration we have a separate proposal, ``Geometric Distortion of ACS'', that proposes to determine distortion corrections to within a small fraction of a pixel, as we have done already for WFPC2 from archival material. As noted in the preceding section, however, our local astrometric measurements are disturbed rather little by residual distortion errors, and we expect in any case to achieve higher accuracy than with WFPC2. Real Time Justification ----------------------- The Cycle 13 observations will have to be placed identically with those of Cycle 11. Same orientation will restrict the scheduling windows in Cycle 13, but not severely. Calibration Justification ------------------------- Additional Comments ------------------- ------------------------------------------------------------------------------------ 9444( 4) - 08/01/02 11:20 - [ 3] TARGET LIST Fixed Targets ------------------------------------------------------------------------------------------------------------------------------------ Tar| Target | Target | Target |Coord | Radial | Flux data No | Name | Description | Position |Eqnx | Vel. | ------------------------------------------------------------------------------------------------------------------------------------ 1 NGC104 STELLAR CLUSTER, RA=0H 22M 40.10S +/- 0.01S, J2000 V = 19.5+/-0.5 GLOBULAR CLUSTER DEC=-72D 3' 51.8" +/- 0.01", PLATE-ID=01X5 Coordinate Source: GSC_SURVEY_PLATE 2 NGC5139 STELLAR CLUSTER, RA=13H 25M 35.5S +/- 0.1S, DEC=- J2000 V = 19.5+/-0.5 GLOBULAR CLUSTER 47D 40' 6.7" +/- 0.1", PLATE- ID=00AK Coordinate Source: GSC_SURVEY_PLATE 9444( 4) - 08/01/02 11:20 - [ 4] Visit: 01 Visit Priority: Visit Requirements: ORIENT 19.34D TO 19.34D BEFORE 15-Nov-2002 On Hold Comments: Additional Comments: Position and orientation chosen to coincide with archival images. Exposures ------------------------------------------------------------------------------------------------------------------------------------ Exposure| Target |Instr | Oper. | Aper |Spectral|Central| Optional |Num| Time | Special Number | Name |Config| Mode |or FOV |Element |Waveln.| Parameters |Exp| | Requirements ------------------------------------------------------------------------------------------------------------------------------------ 1 NGC104 ACS/WF ACCUM WFC F814W CR-SPLIT=NO 1 1390 S C ------------------------------------------------------------------------------------------------------------------------------------ 2 NGC104 ACS/WF ACCUM WFC F814W CR-SPLIT=NO 1 1390 S POS TARG 0.000, 0.253 C ------------------------------------------------------------------------------------------------------------------------------------ 3 NGC104 ACS/WF ACCUM WFC F814W CR-SPLIT=NO 1 1460 S POS TARG 0.253, 0.000 C ------------------------------------------------------------------------------------------------------------------------------------ 4 NGC104 ACS/WF ACCUM WFC F814W CR-SPLIT=NO 1 1460 S POS TARG 0.253, 0.253 C ------------------------------------------------------------------------------------------------------------------------------------ 9444( 4) - 08/01/02 11:20 - [ 5] Visit: 02 Visit Priority: Visit Requirements: ORIENT 90.0D TO 90.0D On Hold Comments: Additional Comments: Pointing and orientation put diffraction spike of a bright star in the ACS gap. If at all possible, schedule this visit in mid-2002, so as to maximize time difference from this to Cycle 13 observations. Exposures ------------------------------------------------------------------------------------------------------------------------------------ Exposure| Target |Instr | Oper. | Aper |Spectral|Central| Optional |Num| Time | Special Number | Name |Config| Mode |or FOV |Element |Waveln.| Parameters |Exp| | Requirements ------------------------------------------------------------------------------------------------------------------------------------ 1 NGC5139 ACS/WF ACCUM WFC F606W CR-SPLIT=NO 1 1300 S C ------------------------------------------------------------------------------------------------------------------------------------ 2 NGC5139 ACS/WF ACCUM WFC F606W CR-SPLIT=NO 1 1300 S POS TARG 0.000, 0.253 C ------------------------------------------------------------------------------------------------------------------------------------ 3 NGC5139 ACS/WF ACCUM WFC F606W CR-SPLIT=NO 1 1375 S POS TARG 0.253, 0.000 C ------------------------------------------------------------------------------------------------------------------------------------ 4 NGC5139 ACS/WF ACCUM WFC F606W CR-SPLIT=NO 1 1375 S POS TARG 0.253, 0.253 C ------------------------------------------------------------------------------------------------------------------------------------ 5 NGC5139 ACS/WF ACCUM WFC F814W CR-SPLIT=NO 1 1340 S C ------------------------------------------------------------------------------------------------------------------------------------ 6 NGC5139 ACS/WF ACCUM WFC F814W CR-SPLIT=NO 1 1340 S POS TARG 0.000, 0.253 C ------------------------------------------------------------------------------------------------------------------------------------ 7 NGC5139 ACS/WF ACCUM WFC F814W CR-SPLIT=NO 1 1375 S POS TARG 0.253, 0.000 C ------------------------------------------------------------------------------------------------------------------------------------ 8 NGC5139 ACS/WF ACCUM WFC F814W CR-SPLIT=NO 1 1375 S POS TARG 0.253, 0.253 C ------------------------------------------------------------------------------------------------------------------------------------ 9444( 4) - 08/01/02 11:20 - [ 6] Summary Form for Proposal 9444 Item Used in this proposal ------------------------------------------------------------------------------------------------------------------------------------ Apertures WFC ------------------------------------------------------------------------------------------------------------------------------------ Configurations ACS/WFC ------------------------------------------------------------------------------------------------------------------------------------ Opmodes ACCUM ------------------------------------------------------------------------------------------------------------------------------------ Optional Parameters CR-SPLIT=NO ------------------------------------------------------------------------------------------------------------------------------------ Proposal Category GO ------------------------------------------------------------------------------------------------------------------------------------ Special Requirements ORIENT 19.34D TO 19.34D BEFORE 15-Nov-2002 POS TARG 0.000, 0.253 POS TARG 0.253, 0.000 POS TARG 0.253, 0.253 ORIENT 90.0D TO 90.0D ------------------------------------------------------------------------------------------------------------------------------------ Spectral Elements F814W F606W ------------------------------------------------------------------------------------------------------------------------------------ Target Names NGC104 NGC5139 ------------------------------------------------------------------------------------------------------------------------------------