! File: 2495C.PROP ! Database: PEPDB ! Date: 17-FEB-1994:07:26:44 coverpage: title_1: THE AGE(S) OF THE SCULPTOR DWARF SPHEROIDAL GALAXY sci_cat: STELLAR POPULATIONS sci_subcat: OLD FIELD STARS proposal_for: GO pi_title: DR. pi_fname: GARY pi_mi: S. pi_lname: DA COSTA pi_inst: YALE UNIVERSITY pi_country: USA pi_phone: 203-432-3022 keywords_1: DWARF SPHEROIDAL GALAXY, LUMINOSITY FUNCTION, STELLAR keywords_2: POPULATION hours_pri: 4.92 num_pri: 2 wf_pc: Y funds_amount: 21446 funds_length: 12 funds_date: OCT-90 pi_position: ASSOC. PROFESSOR off_fname: PETER off_lname: STOCKMAN off_title: DEPUTY DIRECTOR off_inst: STSCI off_addr_1: 3700 SAN MARTIN DRIVE off_city: BALTIMORE off_state: MD off_zip: 21218 off_country: USA ! end of coverpage abstract: line_1: PC imaging of two fields near the center of the Sculptor dwarf spheroidal line_2: galaxy will be used to construct c-m diagrams and luminosity functions that line_3: reach more than two magnitudes fainter than the turnoff expected for the oldest line_4: possible stars in this system. With these data we will investigate the mean line_5: age, and the age range of the stellar population of this dwarf galaxy. line_6: Knowledge of these quantities is a prerequisite for understanding the line_7: evolutionary history of this galaxy in particular, and of galaxies of this line_8: class in general. The HST is required for its faint limiting magnitude line_9: capability, which enables stars well below the turnoff to be detected, and for line_10: its high resolution capability that will allow such stars to be accurately line_11: measured. ! ! end of abstract general_form_proposers: lname: DA COSTA fname: GARY title: P.I. mi: S. inst: YALE UNIVERSITY country: USA ! lname: SEITZER fname: PATRICK inst: STSCI country: USA ! ! end of general_form_proposers block general_form_text: question: 3 section: 1 line_1: The observations will be carried out with the PC since only it combines line_2: adequate sampling of the image profiles with large enough field size. line_3: The wide-band filters F555W and F791W will be employed since they provide line_4: satisfactory transformations to the ground-based V,I(Cousins) standard line_5: system, and, at their effective wavelengths, quantum efficiency hysteresis line_6: is not a concern. Deferred charge problems will be overcome by pre-flashing line_7: the CCDs by the standard amount. Inclusion of a small position offset line_8: (~=1-2") between exposures will help to minimize uncertainties from the flat line_9: field calibration. The instrumental photometry should then be internally line_10: quite precise. Both fields will be observed with ground-based telescopes line_11: to provide the zeropoint calibration for each individual CCD. Short exposures line_12: with HST will also be obtained of each field to allow measurement of stars line_13: saturated on the long exposures and to increase the magnitude overlap with line_14: the ground-based photometry. line_15: Scaling the number density of stars brighter than V=23 in the field studied line_16: by Da Costa (1984) with the major-axis density profile of Eskridge (1988, line_17: A.J.,95,1706) and the luminosity function of the metal-poor globular cluster line_18: NGC 6752 (Da Costa 1982,A.J.,87,990), indicates that a PC field centered line_19: within the core of this galaxy will contain about 2000 stars brighter than line_20: V~=25.7. Thus such fields will be only moderately crowded, yet there will line_21: be sufficient stars near the turnoff that any age range determined will not line_22: have any significant statistical uncertainty. As noted in the Scientific line_23: Justification, two fields will be imaged. One will be near the center of ! question: 3 section: 2 line_1: the galaxy and the second will be centered on the most prominent of the line_2: density enhancements discovered by Eskridge. Both fields lie within the line_3: galaxy's core. ! question: 4 section: 1 line_1: With considerable effort and only under the very best of conditions, line_2: ground-based imaging can detect faint objects whose magnitudes approach line_3: those that will be routinely achieved with HST. However, detection is line_4: a much less stringent requirement than reasonably precise photometry of line_5: stars. Consequently, for those projects requiring such photometry, line_6: ground-based imaging is limited to V~=24 in uncrowded fields; this limit line_7: rapidly becomes brighter as image crowding becomes significant. The line_8: PI has published the results of faint ground-based imaging of a field in line_9: the outskirts of this galaxy (Da Costa 1984, Ap.J.,285,483). With the line_10: faintest stars measured approaching V~=24, these data represent the best line_11: that can be achieved from the ground for this galaxy. But to answer the line_12: questions raised in the Scientific Justification, this limit is simply line_13: too bright; we must, therefore, use HST. Indeed HST is essential because line_14: only it can provide the required relatively precise (magnitude errors~= line_15: 0.05 mag) photometry at the necessary faint (V~=25.7) magnitude levels. line_16: To calibrate the zeropoint of the HST observations, we will obtain ground- line_17: imaging of both PC fields. We also plan to observe from the ground a line_18: sample of well studied galactic globular clusters, such as M92 and M13 line_19: whose abundances bracket that of Sculptor. The resulting globular line_20: cluster c-m diagrams will cover the same absolute magnitude range as the line_21: HST observations. They are required to aid the interpretation of the line_22: HST observations and are presently not available. ! question: 5 section: 1 line_1: The horizontal branch stars in Sculptor have a mean V magnitude of 20.15 line_2: (Kunkel and Demers 1977,Ap.J.,214,71). Thus a population of stars in this line_3: galaxy that is as old as the galactic globular clusters has a turnoff line_4: magnitude some 3.5 to 3.6 magnitudes fainter (e.g. Sarajedini and King line_5: 1989,A.J.,in press) or V~=23.7. Total exposure times have then been line_6: calculated by requiring a S/N greater than 20 (i.e. minimum magnitude and line_7: color errors of 0.05 and 0.08 mag, respectively) at V=25.9, more than two line_8: magnitudes fainter than the "old" turnoff. This represents a minimum S/N line_9: for achieving the precision necessary to fulfill the scientific objectives. line_10: Assuming 2 subexposures for F555W and 3 for F791W, a preflash of 53 e- line_11: per exposure (Lauer 1989,PASP,101,445), a G0 flux distribution and the mean line_12: sensitivities of the 4 PC chips from the July '88 Newsletter, total line_13: integration times are 2400s (2x1200s) for F555W and 6300s (3x2100s) for line_14: F791W. Short exposure times have been calculated on the basis of requiring line_15: at least one mag of overlap between the short and long exposures at a line_16: S/N > 25. line_17: The amount of unocculted spacecraft time required is detailed on the line_18: attached RPSS Resource Summary Worksheet; the efficiency is 70 percent. ! question: 7 section: 1 line_1: Point-Spread-Function (PSF) fitting codes will be used to produce line_2: instrumental (F555W vs F555W-F791W) c-m diagrams for each field. Both line_3: Da Costa and Seitzer have extensive experience in this type of work. line_4: The possible occurrence of radial variations in the PSF will not pose line_5: any difficulties because they presumably will be stable and well- line_6: calibrated. Ground-based observations of the brighter stars in the PC line_7: fields will be used to set the zeropoints of the instrumental magnitudes. line_8: Thus we will not be directly relying on any HST sensitivity calibration. line_9: The completeness of the photometry, and its errors, will be estimated line_10: as a function of magnitude by the process of finding and measuring line_11: artificial stars, modelled from the PSF, that have been randomly inserted line_12: into the data. Once these incompleteness functions are known, the data line_13: can then be used to construct a luminosity function for each field. line_14: Given the small size of the PC and the high galactic latitude of Sculptor line_15: galactic foreground stars will not be a serious contaminant of these data. line_16: Instead, the principle source of contamination will be faint background line_17: galaxies, but since in general they will be resolved, they can be easily line_18: removed in the reductions. line_19: Once the errors and completeness corrections are established, the analysis line_20: of the c-m diagrams and luminosity functions will proceed in three line_21: distinct steps. First, we will compare the results of the two fields. line_22: This will, of course, immediately reveal whether the stellar populations line_23: of the density enhancements observed by Eskridge (1988, A.J.,96,1336) ! question: 7 section: 2 line_1: are distinct or not. It will therefore provide definitive constraints line_2: on the probable origin of these anomalous high density regions. Second, line_3: we will compare the Sculptor data with the galactic globular clusters. line_4: The c-m diagrams will be overlaid by superposing the main sequences. line_5: Although this analysis is made somewhat complicated by the fact that line_6: Sculptor has a range in abundance (indeed our observations should be of line_7: sufficient accuracy to reveal the intrinsic width of the main sequence line_8: expected from the observed abundance spread), we will, by using clusters line_9: that cover this abundance range, be able to infer whether or not the bulk line_10: of the Sculptor stellar population is younger relative to the galactic line_11: globular clusters as suggested by Da Costa (1984). This comparison line_12: requires the not unreasonable assumption that the individual abundance line_13: ratios, such as \[O/Fe\], are the same in both Sculptor and the galactic line_14: globular clusters of comparable \[Fe/H\]. Third, we will compare the line_15: Sculptor data with the best available theoretical isochrones and line_16: luminosity functions. This technique may allow us, via Monte-Carlo line_17: simulations, to place limits on the age distribution of the stellar line_18: population in addition to providing estimates of the ages of the oldest line_19: and youngest stars present in these fields. ! question: 8 section: 1 line_1: Scheduling should take care to ensure that residual images from previous PC line_2: observations do not contaminate our frames. Our fields have been chosen line_3: as far as practical to avoid the generation of any residual images in our line_4: exposures. ! question: 10 section: 1 line_1: The computational, image display and software resources available at the line_2: Anglo-Australian Observatory (where the PI will be after Jan 1990) and at line_3: STScI will be used for the data reduction and analysis. Virtually all the line_4: necessary hardware and software is installed and tested through use in line_5: numerous ground-based CCD imaging projects. The necessary ground-based line_6: calibration and follow-up observations will also be obtained, at least line_7: in part, through the use of AAO facilities. ! !end of general form text general_form_address: lname: SEITZER fname: PATRICK title: DR. category: CON inst: STSCI addr_1: 3700 SAN MARTIN DRIVE city: BALTIMORE state: MD zip: 21218 country: USA ! lname: DA COSTA fname: GARY mi: S. title: DR. category: PI inst: ANGLO-AUSTRALIAN OBSERVATORY addr_1: 167 VIMIERA ROAD addr_2: EASTWOOD NSW 2122 country: AUSTRALIA from_date: 15-JAN-90 ! ! end of general_form_address records fixed_targets: targnum: 1 name_1: SCULPTOR-F1 descr_1: GALAXY; TYPE=DWARF SPHEROIDAL GALAXY descr_2: LOCAL GROUP; STELLAR POPULATIONS pos_1: PLATE-ID = 024K, pos_2: RA = 0H 59M 46.384S +/- 2", pos_3: DEC = -33D 38' 58.43" +/- 2" equinox: 2000.0 fluxnum_1: 1 fluxval_1: V = 25.9 , TYPE = G0 fluxnum_2: 2 fluxval_2: V = 21.8 , TYPE = G0 ! targnum: 2 name_1: SCULPTOR-F2 descr_1: GALAXY; TYPE=DWARF SPHEROIDAL GALAXY descr_2: LOCAL GROUP; STELLAR POPULATIONS pos_1: PLATE-ID = 024K, pos_2: RA = 1H 00M 19.198S +/- 2", pos_3: DEC = -33D 43' 32.65" +/- 2" equinox: 2000.0 fluxnum_1: 1 fluxval_1: V = 25.9 , TYPE = G0 fluxnum_2: 2 fluxval_2: V = 21.8 , TYPE = G0 ! ! end of fixed targets ! No solar system records found ! No generic target records found exposure_logsheet: linenum: 1.000 sequence_1: DEFINE VS targname: # config: PC opmode: IMAGE aperture: ALL sp_element: F555W num_exp: 1 time_per_exp: 50S s_to_n: 25.3 fluxnum_1: 2 priority: # param_1: PURGE=NO, param_2: CR-SPLIT=NO, param_3: SUM=1X1, param_4: READ=YES, param_5: PRE-FLASH=YES, param_6: CLOCKS=NO ! linenum: 2.000 sequence_1: DEFINE VL targname: # config: PC opmode: IMAGE aperture: ALL sp_element: F555W num_exp: 1 time_per_exp: 1200S s_to_n: 14.8 fluxnum_1: 1 priority: # param_1: PURGE=NO, param_2: CR-SPLIT=NO, param_3: SUM=1X1, param_4: READ=YES, param_5: PRE-FLASH=YES, param_6: CLOCKS=NO ! linenum: 3.000 sequence_1: DEFINE IS targname: # config: PC opmode: IMAGE aperture: ALL sp_element: F791W num_exp: 1 time_per_exp: 100S s_to_n: 24.2 fluxnum_1: 2 priority: # param_1: PURGE=NO, param_2: CR-SPLIT=NO, param_3: SUM=1X1, param_4: READ=YES, param_5: PRE-FLASH=YES, param_6: CLOCKS=NO ! linenum: 4.000 sequence_1: DEFINE IL targname: # config: PC opmode: IMAGE aperture: ALL sp_element: F791W num_exp: 1 time_per_exp: 2100S s_to_n: 11.9 fluxnum_1: 1 priority: # param_1: PURGE=NO, param_2: CR-SPLIT=NO, param_3: SUM=1X1, param_4: READ=YES, param_5: PRE-FLASH=YES, param_6: CLOCKS=NO ! linenum: 5.000 sequence_1: USE VS targname: SCULPTOR-F1 priority: 1 req_1: SEQ 5-11 NO GAP; req_2: POS TARG 0.0,0.0 ! linenum: 6.000 sequence_1: USE IS targname: SCULPTOR-F1 priority: 1 req_1: POS TARG 0.0,0.0 ! linenum: 7.000 sequence_1: USE VL targname: SCULPTOR-F1 priority: 1 req_1: POS TARG 0.0,0.0 ! linenum: 8.000 sequence_1: USE IL targname: SCULPTOR-F1 priority: 1 req_1: POS TARG 0.0,0.0 ! linenum: 9.000 sequence_1: USE IL targname: SCULPTOR-F1 priority: 1 req_1: POS TARG -1.5,1.0 ! linenum: 10.000 sequence_1: USE IL targname: SCULPTOR-F1 priority: 1 req_1: POS TARG 1.5,-1.0 ! linenum: 11.000 sequence_1: USE VL targname: SCULPTOR-F1 priority: 1 req_1: POS TARG 1.5,-1.0 ! linenum: 12.000 sequence_1: USE VS targname: SCULPTOR-F2 priority: 2 req_1: SEQ 12-18 NO GAP; req_2: POS TARG 0.0,0.0 ! linenum: 13.000 sequence_1: USE IS targname: SCULPTOR-F2 priority: 2 req_1: POS TARG 0.0,0.0 ! linenum: 14.000 sequence_1: USE VL targname: SCULPTOR-F2 priority: 2 req_1: POS TARG 0.0,0.0 ! linenum: 15.000 sequence_1: USE IL targname: SCULPTOR-F2 priority: 2 req_1: POS TARG 0.0,0.0 ! linenum: 16.000 sequence_1: USE IL targname: SCULPTOR-F2 priority: 2 req_1: POS TARG -1.5,1.0 ! linenum: 17.000 sequence_1: USE IL targname: SCULPTOR-F2 priority: 2 req_1: POS TARG 1.5,-1.0 ! linenum: 18.000 sequence_1: USE VL targname: SCULPTOR-F2 priority: 2 req_1: POS TARG 1.5,-1.0 ! ! end of exposure logsheet ! No scan data records found