! $Id: 5439,v 22.1 1994/08/04 14:58:28 pepsa Exp $ ! ! Hubble Space Telescope ! Cycle 4 (1993) Phase II Proposal Template ! ! for use with the ! Remote Proposal Submission System (RPSS) !Revised April 1994 COVERPAGE: TITLE_1: THE CALIBRATION OF THE WHITE DWARF DISTANCE SCALE FOR TITLE_2: GLOBULAR CLUSTERS SCI_CAT: COOL STARS SCI_SUBCAT: OLD CLUSTERS PROPOSAL_FOR: GO PI_FNAME: ALVIO PI_MI: ! PI_LNAME: RENZINI PI_INST: UNIVERSITA' DI BOLOGNA PI_COUNTRY: ITALY PI_PHONE: 39-51-259402 HOURS_PRI: 22.0 HOURS_PAR: 0 NUM_PRI: 6 NUM_PAR: 0 ! Indicate which instruments will be used (enter Y or leave blank/delete): WF_PC: Y ! Authorizing Institution Official: OFF_FNAME: ALVIO OFF_LNAME: RENZINI OFF_TITLE: DIRECTOR OFF_INST: 6460 OFF_ADDR_1: DIPARTIMENTO DI ASTRONOMIA OFF_ADDR_2: CP 596 OFF_ADDR_3: ! OFF_CITY: BOLOGNA OFF_STATE: ! OFF_ZIP: I-40100 OFF_COUNTRY: ITALY OFF_PHONE: 39-51-259402 ABSTRACT: ! Enter abstract text: use < 80 characters per line LINE_1: Deep WFPC2 UBVI imaging of the nearby Globular Cluster NGC 6752 is LINE_2: proposed in order to use the White Dwarf cooling sequence for the calibration LINE_3: of the globular cluster distance scale, the determination of accurate distances LINE_4: being a prerequisite for the estimate of reliable cluster ages. The goal is to LINE_5: obtain the distance modulus of the cluster with an accuracy better than ~ LINE_6: 0.1 mag, thus reducing the uncertainty in cluster ages from the current, LINE_7: cosmologically ambiguous 25% to less than 10%. LINE_8: Four color UBVI photometry of a carefully selected sample of local calibration LINE_9: WDs with trig parallax will allow to construct Fiducial Cooling Sequences LINE_10: (FCS) in the various color-magnitude diagrams. The observed cooling sequences LINE_11: for the cluster (separately for DA and non-DA WDs) will then be fit to the LINE_12: corresponding FCSs to determine the cluster distance modulus. GENERAL_FORM_PROPOSERS: ! Replicate the section below for each investigator (PI and CoI's): LNAME: RENZINI ! Last name of investigator, X20 FNAME: ALVIO ! First name of investigator, X20 INST: 6460 ! Institution number COUNTRY: ITALY ! Country, X20 ESA: Y ! Is CoI affiliated with ESA or an ! ESA member-state institution? Y or blank LNAME: ORTOLANI ! Investigator's last name, X20 FNAME: SERGIO ! Investigator's first name, X20 INST: 6470 ! Investigator's institution code number (or full institution name), X60 COUNTRY: ITALY ! Investigator's country, X20 ESA: Y ! Is investigator affiliated with ESA or an ! ESA member-state institution? Y or blank LNAME: BRAGAGLIA ! Investigator's last name, X20 FNAME: ANGELA ! Investigator's first name, X20 INST: 6426 ! Investigator's institution code number (or full institution name), X60 COUNTRY: ITALY ! Investigator's country, X20 ESA: Y ! Is investigator affiliated with ESA or an LNAME: FERRARO ! Investigator's last name, X20 FNAME: FRANCESCO ! Investigator's first name, X20 INST: 6426 ! Investigator's institution code number (or full institution name), X60 COUNTRY: ITALY ! Investigator's country, X20 ESA: Y ! Is investigator affiliated with ESA or an ! ESA member-state institution? Y or blank LNAME: BOHLIN ! Investigator's last name, X20 FNAME: RALPH ! Investigator's first name, X20 INST: 3470 ! Investigator's institution code number (or full institution name), X60 COUNTRY: ITALY ! Investigator's country, X20 ESA: ! Is investigator affiliated with ESA or an ! ESA member-state institution? Y or blank LNAME: GILMOZZI (ESA)! Investigator's last name, X20 FNAME: ROBERTO ! Investigator's first name, X20 INST: 3470 ! Investigator's institution code number (or full institution name), X60 COUNTRY: USA ! Investigator's country, X20 ESA : Y ! Is investigator affiliated with ESA or an ! ESA member-state institution? Y or blank LNAME: HOLBERG ! Investigator's last name, X20 FNAME: JAY B. ! Investigator's first name, X20 INST: 1214 ! Investigator's institution code number (or full institution name), X60 COUNTRY: USA ! Investigator's country, X20 ESA: ! Is investigator affiliated with ESA or an ! ESA member-state institution? Y or blank LNAME: LIEBERT ! Investigator's last name, X20 FNAME: JAMES ! Investigator's first name, X20 INST: 1212 ! Investigator's institution code number (or full institution name), X60 COUNTRY: USA ! Investigator's country, X20 ESA: ! Is investigator affiliated with ESA or an ! ESA member-state institution? Y or blank LNAME: SHIPMAN ! Investigator's last name, X20 FNAME: HARRY ! Investigator's first name, X20 INST: 1840 ! Investigator's institution code number (or full institution name), X60 COUNTRY: USA ! Investigator's country, X20 ESA: ! Is investigator affiliated with ESA or an ! ESA member-state institution? Y or blank LNAME: WESEMAMEL ! Investigator's last name, X20 FNAME: FRANCOIS ! Investigator's first name, X20 INST: 4648 ! Investigator's institution code number (or full institution name), X60 COUNTRY: CANADA ! Investigator's country, X20 ESA: ! Is investigator affiliated with ESA or an ! ESA member-state institution? Y or blank GENERAL_FORM_ADDRESS: ! The PI's address: LNAME: RENZINI ! PI's last name, X20 FNAME: ALVIO ! PI's first name, X20 MI: ! PI's middle initial, X2 CATEGORY: PI ! DO NOT CHANGE INST: 6420 ! PI's institution number, X60 ADDR_1: DIPARTIMENTO DI ASTRONOMIA ! PI's address, X60 ADDR_2: CP 596 ADDR_3: CITY: BOLOGNA ! X30 STATE: ! X2 ZIP: I-40100 ! X9 COUNTRY: ITALY ! X20 PHONE: 39-51-259402 ! PI's telephone number, X20 TELEX: ALMA02::ALVIO, 39-51-259407 ! PI's EMAIL address or FAX number, X60 ! The address of the primary contact for this proposal (if not PI): LNAME: ! Contact's last name, X20 FNAME: ! Contact's first name, X20 MI: ! Contact's middle initial, X2 CATEGORY: CON ! DO NOT CHANGE INST: ! Contact's institution number, X60 ADDR_1: ! Contact's address, X60 ADDR_2: ADDR_3: CITY: ! X30 STATE: ! X2 ZIP: ! X9 COUNTRY: ! X20 PHONE: ! Contact's telephone number, X20 TELEX: ! Contact's EMAIL address or FAX number, X60 GENERAL_FORM_TEXT: QUESTION: 3 ! Description of proposed observations. SECTION: 1 ! Provide a detailed description of the proposed observing plan. ! This description is used during scheduling to ensure that the ! observations are implemented properly, so please answer with care. LINE_1: 1) NGC 6752: One WFPC2 pointing 125 arcsec away from the cluster center, LINE_2: for each of the four filters indicated in Table 3, which also gives the LINE_3: exposure times that have been estimated to ensure S/N=~10 for cluster WDs LINE_4: at the indicated V magnitude. The exposures are read at least once per orbit LINE_5: (twice per orbit in V and I), i.e. 14, 5, 5, and 8 times, respectively for LINE_6: the U, B, V, and I, for a total of 26 images. This procedure also ensures LINE_7: that blooming will not reduce the effective WD sampling by more than a few LINE_8: percent. Note that unavoidably WDs coexists with giants in the same WFPC2 LINE_9: frames. Saturation of the central pixel is determined from section 4.2.3 of LINE_10: the WFPC2 handbook. We estimate that the total apparent V magnitude sampled LINE_11: by WFPC2 is ~8.1; with 1200 sec exposures this luminosity is able to fill LINE_12: the capacity (~100,000 electron) of at most ~20,000 pixels, i.e. less than LINE_13: 1% of the total number of pxls in the 4 CCD chips. We estimate that accurate LINE_14: WD photometry is thus possible in ~97% of the field. LINE_15: 2) LOCAL CALIBRATION WDs: The full compliment of four filters observations of LINE_16: each WD will be obtained with a single, multi-exposed WFPC2 frame. Four LINE_17: images (one per filter, and offset by an appropriate amount) will be obtained LINE_18: in each of the four CCDs, and read just once. This would minimize the LINE_19: observational overhead, and the number of images to be read out. In order LINE_20: to avoid contamination from field stars when offsetting, we will specify the LINE_21: Delta x and Delta y offsets for each one of the multiple exposures, offsets LINE_22: will be less than ~ 20 arcsec. Although this sequence of operations is rather LINE_23: complicated, we call to the attention of the TAC that this proposal already QUESTION: 3 SECTION: 2 ! This is an example of how to append another section to a particular question. ! No more than 23 lines can be included in any one section, and no more than 2 ! sections per question are allowed. Delete any unused LINE_ keywords. LINE_1: successfully passed through Phase-2 in 1989-90 on view of Cycle 1 LINE_2: observations, before being deferred for the spherical aberration problem. QUESTION: 4 ! Justification of need for HST observations. SECTION: 1 LINE_1: The HST characteristics exploited in these proposed observations is the high LINE_2: angular resolution coupled with large field of view. This allows the collection LINE_3: of a sufficient number of WD in a frame (~ 100), while maintaining accurate LINE_4: photometry for most of them. Indeed, the Method requires accurately tracing the LINE_5: WD cooling sequence in a cluster. This is done the best with a larger number of LINE_6: usable WDs, each having accurate magnitudes. Observing as close as possible to LINE_7: the cluster center is also required by the need to minimize field contamnation. LINE_8: Some GTO programs include among their scientific goals the discovery and study LINE_9: of globular cluster WDs, and GTO program No. 1112 includes our target cluster. LINE_10: Our proposed observations differ in many respects: a) we intend to point at 125 LINE_11: arcsec from the cluster center, in order to collect a large number of white LINE_12: dwarfs, while preserving photometric accuracy and reducing the relative LINE_13: background contamination. GTO 1112 is supposed to point at 300 arcsec from the LINE_14: cluster center; b) filter selection, as we need four filters in order to LINE_15: separate DAs from non-DAs and get accurate temperatures, and, especially, c) LINE_16: exposure time, as we need a higher photometric accuracy compared to the LINE_17: GTO program which is of mere exploratory nature. These differences are LINE_18: essential in order to achieve the goal of the present program, i.e. the LINE_19: accurate determination of the distance to one reference cluster, which LINE_20: cannot be achieved with the GTO observations as they were not designed for LINE_21: such purpose, nor was it mentioned among the scientific goals of the GTO LINE_22: project. Seemingly, our observations cannot suffice to achieve the LINE_23: scientific goals of GTO 1112, such as the study of the radial gradient in the QUESTION: 4 SECTION: 2 LINE_1: WD content of the cluster, as we observe at only one distance and therefore we LINE_2: will not have any significan radial gradient information. In summary, our LINE_3: project does not replicate GTO 1112 observations nor interfere with its LINE_4: scientific goals, and we contend that the two projects should be pursued LINE_5: independently, without one being subject to the completion of the other. LINE_6: Attempts to find WDs in GCs (M4, M71, Omega Cen) tend to support the LINE_7: theoretical predictions about their expected number (Chan and Richer 1986, Ap. LINE_8: J. 302, 257; Ortolani and Rosino 1987, A. & A. 185, 102; Richer and Fahlman LINE_9: 1988, Ap. J. 325, 218). A possibly anomalous DA/DB ratio has been also LINE_10: suggested, as well as a higher than expected background contamination. The LINE_11: detections, however, in spite of the state of the art detectors and software, LINE_12: are barely more than qualitative, being the background contamination and the LINE_13: photometric errors quite high (S/N=~ 3-5 at V=24). Since the photometric errors LINE_14: are dominated by crowding (the statistical errors alone are much smaller) it LINE_15: does not seem possible to significantly improve the S/N ratio just increasing LINE_16: the exposure times or using the new super-flat fielding techniques elaborated LINE_17: for low density background fields. Reducing the crowding without increasing LINE_18: the contamination can only be achieved with HST. Finally, contamination by LINE_19: faint background galaxies in the same magnitude range (24