Page 1 PROPOSAL FOR HUBBLE SPACE TELESCOPE OBSERVATIONS ST ScI Use Only ID 5439 Report Date: 18-Jul-95:17:23 Version: ********** Check-in Date: ********** 1.Proposal Title: THE CALIBRATION OF THE WHITE DWARF DISTANCE SCALE FOR GLOBULAR CLUSTERS ------------------------------------------------------------------------------------ 2. Scientific Category 3. Proposal For 4. Proposal Type 5. Continuation ID COOL STARS GO Sub Category OLD CLUSTERS ------------------------------------------------------------------------------------ 6. Principal Investigator Institution Country Telephone Alvio Renzini UNIVERSITA' DI BOLOGNA ITALY 39-51-259402 ------------------------------------------------------------------------------------ 7. Abstract Deep WFPC2 UBVI imaging of the nearby Globular Cluster NGC 6752 is proposed in order to use the White Dwarf cooling sequence for the calibration of the globular cluster distance scale, the determination of accurate distances being a prerequisite for the estimate of reliable cluster ages. The goal is to obtain the distance modulus of the cluster with an accuracy better than ~ 0.1 mag, thus reducing the uncertainty in cluster ages from the current, cosmologically ambiguous 25% to less than 10%. Four color UBVI photometry of a carefully selected sample of local calibration WDs with trig parallax will allow to construct Fiducial Cooling Sequences (FCS) in the various color-magnitude diagrams. The observed cooling sequences for the cluster (separately for DA and non-DA WDs) will then be fit to the corresponding FCSs to determine the cluster distance modulus. ------------------------------------------------------------------------------------ ------------------------------------------------------------------------------------ 9. Est obs time (hours) pri: 22.0 par: 0 10. Num targs pri: 6 par: 0 ------------------------------------------------------------------------------------ 11. Instruments requested: WF/PC ------------------------------------------------------------------------------------ ------------------------------------------------------------------------------------ Page 2 I. GENERAL FORM Proposal 5439 PI: Alvio Renzini Proposal Title: THE CALIBRATION OF THE WHITE DWARF DISTANCE SCALE FOR GLOBULAR CLUSTERS ------------------------------------------------------------------------------------ 1. Proposers: Proposers Institution Country ESA ------------------------------------------------------------------------------------ Alvio Renzini 6460 ITALY X Sergio Ortolani 6470 ITALY X Angela Bragaglia 6426 ITALY X Francesco Ferraro 6426 ITALY X Ralph Bohlin 3470 ITALY Roberto Gilmozzi (Esa) 3470 USA X Jay B. Holberg 1214 USA James Liebert 1212 USA Harry Shipman 1840 USA Francois Wesemamel 4648 CANADA ------------------------------------------------------------------------------------ 3. Description of proposed observations. 1) NGC 6752: One WFPC2 pointing 125 arcsec away from the cluster center, for each of the four filters indicated in Table 3, which also gives the exposure times that have been estimated to ensure S/N=~10 for cluster WDs at the indicated V magnitude. The exposures are read at least once per orbit (twice per orbit in V and I), i.e. 14, 5, 5, and 8 times, respectively for the U, B, V, and I, for a total of 26 images. This procedure also ensures that blooming will not reduce the effective WD sampling by more than a few percent. Note that unavoidably WDs coexists with giants in the same WFPC2 frames. Saturation of the central pixel is determined from section 4.2.3 of the WFPC2 handbook. We estimate that the total apparent V magnitude sampled by WFPC2 is ~8.1; with 1200 sec exposures this luminosity is able to fill the capacity (~100,000 electron) of at most ~20,000 pixels, i.e. less than 1% of the total number of pxls in the 4 CCD chips. We estimate that accurate WD photometry is thus possible in ~97% of the field. 2) LOCAL CALIBRATION WDs: The full compliment of four filters observations of each WD will be obtained with a single, multi-exposed WFPC2 frame. Four images (one per filter, and offset by an appropriate amount) will be obtained in each of the four CCDs, and read just once. This would minimize the observational overhead, and the number of images to be read out. In order to avoid contamination from field stars when offsetting, we will specify the Delta x and Delta y offsets for each one of the multiple exposures, offsets will be less than ~ 20 arcsec. Although this sequence of operations is rather complicated, we call to the attention of the TAC that this proposal already successfully passed through Phase-2 in 1989-90 on view of Cycle 1 observations, before being deferred for the spherical aberration problem. Page 3 ------------------------------------------------------------------------------------ 4. Justification of need for HST observations. The HST characteristics exploited in these proposed observations is the high angular resolution coupled with large field of view. This allows the collection of a sufficient number of WD in a frame (~ 100), while maintaining accurate photometry for most of them. Indeed, the Method requires accurately tracing the WD cooling sequence in a cluster. This is done the best with a larger number of usable WDs, each having accurate magnitudes. Observing as close as possible to the cluster center is also required by the need to minimize field contamnation. Some GTO programs include among their scientific goals the discovery and study of globular cluster WDs, and GTO program No. 1112 includes our target cluster. Our proposed observations differ in many respects: a) we intend to point at 125 arcsec from the cluster center, in order to collect a large number of white dwarfs, while preserving photometric accuracy and reducing the relative background contamination. GTO 1112 is supposed to point at 300 arcsec from the cluster center; b) filter selection, as we need four filters in order to separate DAs from non-DAs and get accurate temperatures, and, especially, c) exposure time, as we need a higher photometric accuracy compared to the GTO program which is of mere exploratory nature. These differences are essential in order to achieve the goal of the present program, i.e. the accurate determination of the distance to one reference cluster, which cannot be achieved with the GTO observations as they were not designed for such purpose, nor was it mentioned among the scientific goals of the GTO project. Seemingly, our observations cannot suffice to achieve the scientific goals of GTO 1112, such as the study of the radial gradient in the WD content of the cluster, as we observe at only one distance and therefore we will not have any significan radial gradient information. In summary, our project does not replicate GTO 1112 observations nor interfere with its scientific goals, and we contend that the two projects should be pursued independently, without one being subject to the completion of the other. Attempts to find WDs in GCs (M4, M71, Omega Cen) tend to support the theoretical predictions about their expected number (Chan and Richer 1986, Ap. J. 302, 257; Ortolani and Rosino 1987, A. & A. 185, 102; Richer and Fahlman 1988, Ap. J. 325, 218). A possibly anomalous DA/DB ratio has been also suggested, as well as a higher than expected background contamination. The detections, however, in spite of the state of the art detectors and software, are barely more than qualitative, being the background contamination and the photometric errors quite high (S/N=~ 3-5 at V=24). Since the photometric errors are dominated by crowding (the statistical errors alone are much smaller) it does not seem possible to significantly improve the S/N ratio just increasing the exposure times or using the new super-flat fielding techniques elaborated for low density background fields. Reducing the crowding without increasing the contamination can only be achieved with HST. Finally, contamination by faint background galaxies in the same magnitude range (24