5912( 4) - 09/14/95 15:48 - [ 1] PROPOSAL FOR HUBBLE SPACE TELESCOPE OBSERVATIONS ST ScI Use Only ID: 5912 Version: 4 Check-in Date: ********** 1.Proposal Title: Precise Astrometry in the Core of the Globular Cluster 47 Tuc: A Complete Census of High-Velocity Stars ------------------------------------------------------------------------------------ 2. Scientific Category 3. Proposal For 4. Cycle Stellar Populations GO 5 ------------------------------------------------------------------------------------ 5. Investigators Contact? PI: Georges Meylan European Southern Observatory CoI: Dante Minniti European Southern Observatory Y CoI: Sterl Phinney Theoretical Astrophysics, Caltech N CoI: Carlton Pryor Rutgers University, Physics & Astronomy N Department CoI: Bruce Sams Max-Planck-Institut fuer N Extraterrestrische Physik CoI: Christopher Tinney European Southern Observatory N ------------------------------------------------------------------------------------ 6. Abstract Binary stars play an essential role during the late phases of the dynamical evolution of a globular cluster. They transfer energy to passing stars, and so can strongly influence the cluster evolution - enough to delay, halt, and even reverse core collapse. Hard binaries are known to exist in cluster cores, e.g., in the form of millisecond pulsars. The presence of hard binaries may also be revealed by searching for the by- products of close encounters: high-velocity stars. Two such stars were serendipitously discovered in the core of 47 Tuc by Meylan et al. (1991), and similar stars have since been detected by Pryor et al. (1994). This represents the limit of the radial velocity data which can be obtained from the ground. If more progress is to be made in the search for high- velocity stars in 47 Tuc it must be made by obtaining proper motions - a task for which only HST is suitable. We propose to use WFPC2 to obtain deep U (F300W) images of the core of 47 Tuc at three different epochs over two years. This will allow us to measure differential proper motions to a 1 sigma limit of 0.23 mas/yr - this corresponds to a 5 sigma detection of all stars with velocities greater than 22 km/s. The choice of F300W will allow stars to be measured over the whole color-magnitude diagram - from the red-giant branch to well down the main sequence. Such a complete census will provide unique constraints on relaxation processes, collision and ejection rates, and velocity distributions. ------------------------------------------------------------------------------------ 5912( 4) - 09/14/95 15:48 - [ 2] Observations Description ------------------------ CCD astrometry from the ground has shown that three fundamental issues must be addressed in designing the observing program for high-precision relative astrometry. Given that these issues can be addressed, it has been shown that obtaining positions to better than 1 percent can easily be achieved, provided that the required photon counting limits (about 10,000 photons) can be reached. 1. The first requirement is that the astrometry must be carried out in a differential mode. For HST observations then, we need to place a fiducial object back on the same place of the PC1 CCD at each epoch to within a few pixels. Moreover, it imposes the requirement that all observations must be made with an identical roll angle. The overheads on this requirement are high, however the requirement is a stringent one. The field distortion of the PC1 is not known to sufficient precision for 0.4 mas per epoch positions to be determined, therefore differential observations are forced on us. 2. The second requirement is that the PSF must be well sampled by the CCD. Because HST is undersampled by even the PC1, it becomes important to deal with this problem in our program. We have designed a program of multiple will enable us to regain the oversampling of the PC1 images required for high-precision astrometry. 3. The last requirement is for a distant reference frame. Fortuitously, for 47 Tuc this is a simple problem to solve. The background SMC provides a rich reference frame of stars, which can be easily identified by CMDs (see Hesser et al. 1987). These numerous F type stars would appear at U = 21.0-22.0. To summarise, given that we are able to obtain observations with the same roll angle, and shifted in the sub-pixel pattern described above, there is no reason why precisions of 100th of a pixel should not be easily obtained over the course of 3 Cycles, i.e. about two years, allowing the 1-sigma measurement of 0.23 mas/yr proper motions. To minimize telescope time request, we have selected a single filter, viz. F300W, which will reduce the dynamical range in magnitude and prevent heavily saturated pixels by red giants. More or less all stages of evolution in the cluster can be seen. The U filter allows the detection of MS (of a wide mass range), RG, RHB, AGB, BS and WD stars without any of these stages being so bright compared to the others that multiple exposures with different observing times are required. Real Time Justification ----------------------- Our differential approach requires that we observe with the same roll angles and the same filter in different years (cycle 5 through cycle 7). It is important to know that this project was not possible with HST before the refurbishment mission, but present WFPC2 capabilities and stability allow us to measure accurate relative motions. We have an existing catalogue of radial velocities for giants in the core of the cluster. However, from the ground we are limited to measuring radial velocities of the brightest giants, and not all in the core. This represents all that can be done from the ground. These stars represent a clear small subset of the sample which could be acquired by proper motions. However, the ground based velocity distribution will be valuable to model via Monte-Carlo simulations the degree of completeness in detecting the high-velocity tail of the proper motions in the WFPC2 images. Calibration Justification ------------------------- We need subpixel shifts in between frames. Additional Comments ------------------- We need the same roll angle in next cycles. ------------------------------------------------------------------------------------ 5912( 4) - 09/14/95 15:48 - [ 3] Data Distribution Media: 8MM Blocking Factor: 10 Ship To: PI_Address Ship Via: UPS Email: , ------------------------------------------------------------------------------------ 5912( 4) - 09/14/95 15:48 - [ 4] TARGET LIST a) Fixed Targets ------------------------------------------------------------------------------------------------------------------------------------ Tar| Target | Target | Target |Coord | Radial | Flux data No | Name | Description | Position |Eqnx | Vel. | ------------------------------------------------------------------------------------------------------------------------------------ 1 NGC104 STELLAR CLUSTER, RA=0H 21M 52.28S +/- 0.14S, DEC= 1950 V = - V = 4.02 GLOBULAR CLUSTER -72D 21' 27.5" +/- 2.17" 18.8 B-V = 0.88 Comments: Visits 1 Epoch of Position RA proper motion (seconds of time/yr) DEC Proper Motion (arcsec/yr) Annual Parallax (arcsec) 1950 0.0000 0.0000 0.00 5912( 4) - 09/14/95 15:48 - [ 5] Visit: 01 Visit Requirements: CVZ On Hold Comments: Additional Comments: We need to optimize the observing parameters in Cycle 5 in order to be able to reproduce exactly the same configuration (orientation) in cycles 6 and 7. 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 WFPC2 IMAGE PC1 F300W 2911 CR-SPLIT=no 1 350S ------------------------------------------------------------------------------------------------------------------------------------ 2 NGC104 WFPC2 IMAGE PC1 F300W 2911 CR-SPLIT=no 1 350S POSition TARGet 0.500,0.500 ------------------------------------------------------------------------------------------------------------------------------------ 3 NGC104 WFPC2 IMAGE PC1 F300W 2911 CR-SPLIT=no 1 350S POSition TARGet 1.000,1.000 ------------------------------------------------------------------------------------------------------------------------------------ 4 NGC104 WFPC2 IMAGE PC1 F300W 2911 CR-SPLIT=no 1 350S POSition TARGet 0.432,0.432 ------------------------------------------------------------------------------------------------------------------------------------ 5 NGC104 WFPC2 IMAGE PC1 F300W 2911 CR-SPLIT=no 1 350S POSition TARGet 0.932,0.932 ------------------------------------------------------------------------------------------------------------------------------------ 6 NGC104 WFPC2 IMAGE PC1 F300W 2911 CR-SPLIT=no 1 350S POSition TARGet 1.432,1.432 ------------------------------------------------------------------------------------------------------------------------------------ 7 NGC104 WFPC2 IMAGE PC1 F300W 2911 CR-SPLIT=no 1 350S POSition TARGet 0.216,0.216 ------------------------------------------------------------------------------------------------------------------------------------ 8 NGC104 WFPC2 IMAGE PC1 F300W 2911 CR-SPLIT=no 1 350S POSition TARGet 0.716,0.716 ------------------------------------------------------------------------------------------------------------------------------------ 9 NGC104 WFPC2 IMAGE PC1 F300W 2911 CR-SPLIT=no 1 350S POSition TARGet 1.216,1.216 ------------------------------------------------------------------------------------------------------------------------------------ 10 NGC104 WFPC2 IMAGE PC1 F300W 2911 CR-SPLIT=no 1 350S POSition TARGet -0.432,-0.432 ------------------------------------------------------------------------------------------------------------------------------------ 11 NGC104 WFPC2 IMAGE PC1 F300W 2911 CR-SPLIT=no 1 350S POSition TARGet 0.068,0.068 ------------------------------------------------------------------------------------------------------------------------------------ 12 NGC104 WFPC2 IMAGE PC1 F300W 2911 CR-SPLIT=no 1 350S POSition TARGet 0.568,0.568 ------------------------------------------------------------------------------------------------------------------------------------ 13 NGC104 WFPC2 IMAGE PC1 F300W 2911 CR-SPLIT=no 1 350S POSition TARGet 0.432,0.000 ------------------------------------------------------------------------------------------------------------------------------------ 14 NGC104 WFPC2 IMAGE PC1 F300W 2911 CR-SPLIT=no 1 350S POSition TARGet 0.932,0.500 ------------------------------------------------------------------------------------------------------------------------------------ 15 NGC104 WFPC2 IMAGE PC1 F300W 2911 CR-SPLIT=no 1 350S POSition TARGet 1.432,1.000 ------------------------------------------------------------------------------------------------------------------------------------ 5912( 4) - 09/14/95 15:48 - [ 6] Visit: 02 Visit Requirements: CVZ On Hold Comments: Additional Comments: We need to optimize the observing parameters in Cycle 5 in order to be able to reproduce exactly the same configuration (orientation) in cycles 6 and 7. 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 WFPC2 IMAGE PC1 F300W 2911 CR-SPLIT=no 1 350S ------------------------------------------------------------------------------------------------------------------------------------ 2 NGC104 WFPC2 IMAGE PC1 F300W 2911 CR-SPLIT=no 1 350S POSition TARGet 0.500,0.500 ------------------------------------------------------------------------------------------------------------------------------------ 3 NGC104 WFPC2 IMAGE PC1 F300W 2911 CR-SPLIT=no 1 350S POSition TARGet 1.000,1.000 ------------------------------------------------------------------------------------------------------------------------------------ 4 NGC104 WFPC2 IMAGE PC1 F300W 2911 CR-SPLIT=no 1 350S POSition TARGet 0.432,0.432 ------------------------------------------------------------------------------------------------------------------------------------ 5 NGC104 WFPC2 IMAGE PC1 F300W 2911 CR-SPLIT=no 1 350S POSition TARGet 0.932,0.932 ------------------------------------------------------------------------------------------------------------------------------------ 6 NGC104 WFPC2 IMAGE PC1 F300W 2911 CR-SPLIT=no 1 350S POSition TARGet 1.432,1.432 ------------------------------------------------------------------------------------------------------------------------------------ 7 NGC104 WFPC2 IMAGE PC1 F300W 2911 CR-SPLIT=no 1 350S POSition TARGet 0.216,0.216 ------------------------------------------------------------------------------------------------------------------------------------ 8 NGC104 WFPC2 IMAGE PC1 F300W 2911 CR-SPLIT=no 1 350S POSition TARGet 0.716,0.716 ------------------------------------------------------------------------------------------------------------------------------------ 9 NGC104 WFPC2 IMAGE PC1 F300W 2911 CR-SPLIT=no 1 350S POSition TARGet 1.216,1.216 ------------------------------------------------------------------------------------------------------------------------------------ 10 NGC104 WFPC2 IMAGE PC1 F300W 2911 CR-SPLIT=no 1 350S POSition TARGet -0.432,-0.432 ------------------------------------------------------------------------------------------------------------------------------------ 11 NGC104 WFPC2 IMAGE PC1 F300W 2911 CR-SPLIT=no 1 350S POSition TARGet 0.068,0.068 ------------------------------------------------------------------------------------------------------------------------------------ 12 NGC104 WFPC2 IMAGE PC1 F300W 2911 CR-SPLIT=no 1 350S POSition TARGet 0.568,0.568 ------------------------------------------------------------------------------------------------------------------------------------ 13 NGC104 WFPC2 IMAGE PC1 F300W 2911 CR-SPLIT=no 1 350S POSition TARGet 0.432,0.000 ------------------------------------------------------------------------------------------------------------------------------------ 14 NGC104 WFPC2 IMAGE PC1 F300W 2911 CR-SPLIT=no 1 350S POSition TARGet 0.932,0.500 ------------------------------------------------------------------------------------------------------------------------------------ 15 NGC104 WFPC2 IMAGE PC1 F300W 2911 CR-SPLIT=no 1 350S POSition TARGet 1.432,1.000 ------------------------------------------------------------------------------------------------------------------------------------ 5912( 4) - 09/14/95 15:48 - [ 7] Summary Form for Proposal 5912 Item Used in this proposal ------------------------------------------------------------------------------------------------------------------------------------ Configurations WFPC2 ------------------------------------------------------------------------------------------------------------------------------------ Opmodes IMAGE ------------------------------------------------------------------------------------------------------------------------------------ Optional Parameters CR-SPLIT=no ------------------------------------------------------------------------------------------------------------------------------------ Proposal Category GO ------------------------------------------------------------------------------------------------------------------------------------ Scientific Category Stellar Populations ------------------------------------------------------------------------------------------------------------------------------------ Special Requirements CVZ POSition TARGet 0.500,0.500 POSition TARGet 1.000,1.000 POSition TARGet 0.432,0.432 POSition TARGet 0.932,0.932 POSition TARGet 1.432,1.432 POSition TARGet 0.216,0.216 POSition TARGet 0.716,0.716 POSition TARGet 1.216,1.216 POSition TARGet -0.432,-0.432 POSition TARGet 0.068,0.068 POSition TARGet 0.568,0.568 POSition TARGet 0.432,0.000 POSition TARGet 0.932,0.500 POSition TARGet 1.432,1.000 ------------------------------------------------------------------------------------------------------------------------------------ Spectral Elements F300W ------------------------------------------------------------------------------------------------------------------------------------ Target Names NGC104 ------------------------------------------------------------------------------------------------------------------------------------