6467( 5) - 01/21/97 10:42 - [ 0] PROPOSAL FOR HUBBLE SPACE TELESCOPE OBSERVATIONS ST ScI Use Only ID: 6467 Version: 5 Check-in Date: 21-Jan-1997 10:39:48 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 6 ------------------------------------------------------------------------------------ 5. Investigators Contact? PI: Georges Meylan European Southern Observatory CoI: Dante Minniti Lawrence Livermore National Laboratory Y CoI: Carlton Pryor Rutgers University, Physics & Astronomy N Department CoI: Sterl Phinney Theoretical Astrophysics, Caltech N CoI: Bruce Sams Max-Planck-Institut fuer N Extraterrestrische Physik CoI: Christopher Tinney Anglo Australian 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. 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 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 continue (Cycle 5 observations are scheduled for the fall of 1995) 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 distribution, as a function of the stellar mass. ------------------------------------------------------------------------------------ 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\\\\ limits (~ 10,000 photons) can be reached. vspace0.2cm 1. The first requirement is that the astrometry must be carried out in a differential mode. Traditional photographic astrometry has always been limited by the need for a detailed knowledge of field distortions - - if your field is not flat, and you cannot guarantee a priori where your astrometric stars will be placed in that field, it is essential that it be possible to flatten the field to high precision. On the ground, CCDs obviate this problem by allowing one to always place the target objects back in the same place on the CCD to within a few pixels, at which point the field distortions cancel out, and high precisions can be reached. 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. vspace0.2cm 2. The second requirement is that the PSF must be well sampled by the CCD. It has been realised in recent years that CCDs do not have uniform sensitivity across a pixel. In fact in the worst case the sensitivity can vary by as much as 5\\\\ pixel. This obviously has serious implications for a program which aims to measure positions to an accuracy of better than 1\\\\ pixel. Because HST is undersampled by even the PC1, it becomes important to deal with this problem in our program. With the help of R. Hook (ST-ECF), we have designed a program of 10 exposures per epoch (5 per orbit) shifted by subpixel values. This dithering strategy will enable us to regain the oversampling of the PC1 images required for high-precision astrometry (see ST-ECF Newsletter \\\\# 22 and \\\\# 23). vspace0.2cm 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 vspace0.2cm 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. vspace0.5cm 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. The dynamic range needed is only about 5-6 magnitudes, compared with the 10 magnitudes in the optical between the RGB and the WD sequence. The brightest stars are going to be RHB stars and BS, according to the models of Welsh & Code (1980). (Sampling problems are also reduced in the UV compared with optical passbands). We prefer the wide U filter F300W to the Johnson's U filter F336W because we gain about 0.5 magnitudes, critical for the faintest and bluest objects. The red leak of the F300W filter will not be a problem for the proposed science, since color information is available from the archives. vspace0.2cm Since 47 Tuc is in CVZ, 96 minutes are available per orbit. We will obtain per orbit 5 pairs of 6 minute exposures (350- 400 seconds), with cosmic ray splitting, during the 2 complete orbits (for a total of 5 * 6 * 2 * 2 = 120 minutes plus overheads). This procedure must be repeated using the same roll angle in cycles 6 and 7. vspace0.2cm We have kept the request of time to the bare minimum needed: we observe only one cluster, with one filter. At the same time, we have been conservative about the proper motion measurements. Any cut down in the number of orbits will be harmful for the project. Only the full set of observations, taken at three different epochs over two years, will allow, through the extraction of the proper motions, the completion of the scientific goal of this proposal. Null 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), with subpixel shifts in between frames. This project was not possible with HST before the refurbishment mission, but present WFPC2 capabilities and stability allow us to measure accurate relative motions. vspace0.2cm 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. vspace0.2cm Calibration Justification ------------------------- Additional Comments ------------------- ------------------------------------------------------------------------------------ Data Distribution Media: 8MM Blocking Factor: 10 Ship To: PI_Address Ship Via: Email: , ------------------------------------------------------------------------------------ 6467( 5) - 01/21/97 10:42 - [ 1] TARGET LIST a) Fixed Targets ------------------------------------------------------------------------------------------------------------------------------------ Tar| Target | Target | Target |Coord | Radial | Flux data No | Name | Description | Position |Eqnx | Vel. | ------------------------------------------------------------------------------------------------------------------------------------ 1 NGC104 STELLAR CLUSTER, RA=00H 21M 52.28S +/- 0.14S,DEC= 1950 V=-18.8 V = 4.02 GLOBULAR CLUSTER -72D 21' 27.5" +/- 2.17" B-V = 0.88 Epoch of Position RA proper motion (seconds of time/yr) DEC Proper Motion (arcsec/yr) Annual Parallax (arcsec) 1950 0.0004 -0.0016 0.00 6467( 5) - 01/21/97 10:42 - [ 2] Visit: 01 Visit Requirements: ORIENT 23D TO 23D CVZ On Hold Comments: Additional Comments: 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 DITHER-TYPE=BOX 1 400 S ------------------------------------------------------------------------------------------------------------------------------------ 2 NGC104 WFPC2 IMAGE PC1 F300W DITHER-TYPE=BOX 1 400 S ------------------------------------------------------------------------------------------------------------------------------------ 3 NGC104 WFPC2 IMAGE PC1 F300W DITHER-TYPE=BOX 1 400 S ------------------------------------------------------------------------------------------------------------------------------------ 4 NGC104 WFPC2 IMAGE PC1 F300W DITHER-TYPE=BOX 1 500 S ------------------------------------------------------------------------------------------------------------------------------------ 6467( 5) - 01/21/97 10:42 - [ 3] Summary Form for Proposal 6467 Item Used in this proposal ------------------------------------------------------------------------------------------------------------------------------------ Configurations WFPC2 ------------------------------------------------------------------------------------------------------------------------------------ Opmodes IMAGE ------------------------------------------------------------------------------------------------------------------------------------ Optional Parameters DITHER-TYPE=BOX ------------------------------------------------------------------------------------------------------------------------------------ Proposal Category GO ------------------------------------------------------------------------------------------------------------------------------------ Scientific Category STELLAR POPULATIONS ------------------------------------------------------------------------------------------------------------------------------------ Special Requirements ORIENT 23D TO 23D CVZ ------------------------------------------------------------------------------------------------------------------------------------ Spectral Elements F300W ------------------------------------------------------------------------------------------------------------------------------------ Target Names NGC104 ------------------------------------------------------------------------------------------------------------------------------------