7500( 2) - 02/19/97 19:19 - [ 1] PROPOSAL FOR HUBBLE SPACE TELESCOPE OBSERVATIONS ST ScI Use Only ID: 7500 Version: 2 Check-in Date: 19-Feb-1997 19:16:01 1.Proposal Title: The Horizontal Branch of the M31 Dwarf Spheroidal Companion And III ------------------------------------------------------------------------------------ 2. Scientific Category 3. Proposal For 4. Cycle STELLAR POPULATIONS GO 7 ------------------------------------------------------------------------------------ 5. Investigators Contact? PI: Gary Da Costa MT. STROMLO OBSERVATORY CoI: Taft Armandroff National Optical Astronomy Observatories N CoI: Nelson Caldwell Whipple Observatory, Smithsonian N Institution CoI: Patrick Seitzer University of Michigan N ------------------------------------------------------------------------------------ 6. Abstract The Galaxy's dwarf spheroidal (dSph) companions show a surprising diversity in the epoch of their major star formation episode(s). To aid in understanding the origin of this diversity, we need similar results for the three M31 dSph companions, the nearest set of morphologically similar galaxies. Observations of old main sequence turnoffs in the M31 dSphs are impractical, but with WFPC2 we can establish their HB morphologies, which the observations of galactic dSphs have shown to be a reliable mean age indicator. Cycle 4 observations by this team indicate that the bulk of the star formation in And I took place ~10 Gyr ago, though the presence of blue HB and RR Lyrae stars testify to a minor older component. Observations of And II are scheduled for execution in Cycle 6. Here we seek HST time to complete our study of the M31 dSphs by observing And III, the least luminous (M_V ~ -10.2) and most metal- poor (Fe/H ~ --2.0) of the M31 dSph and dE companions. The primary aim is to determine the HB morphology and thus fix the epoch of the major star formation episode in this galaxy. Other results that will follow include an estimate of the true (unprojected) distance from M31 and limits on the fraction of younger stars, since the data will reveal any main sequence stars younger than 2-3 Gyr. ------------------------------------------------------------------------------------ 7500( 2) - 02/19/97 19:19 - [ 2] Observations Description ------------------------ The required c-m diagrams will be derived from long exposures centered on And III carried out with WFPC2 using the F450W (Wide-B) and F555W (Wide-V) filters. These filters provide adequate transformations to the ground-based B,V system while minimizing HST total exposure times, and have proved satisfactory for the Cycle 4 And I observations. We prefer the F450W filter over an I-band filter such as F814W for the following reasons. 1) It is possible that this dSph may possess a blue HB similar to that seen in Ursa Minor. In such a situation the F450W filter gives considerably better S/N for blue HB stars, in a given total exposure, than does the F814W filter. Conversely, in the case of a red HB, F814W is only slightly more efficient than F450W\\@. 2) RR Lyrae variable stars have larger amplitudes in B than in I so the chances of detecting them is enhanced by use of the F450W filter. 3) With the planned F450W observations we should be able to detect any main sequence population in this dSph if it is younger than ~3 Gyr. F814W observations would require additional integration time to achieve the same limit. The center of And III will be placed on the nominal center of the WFPC2 field. In this configuration the fields imaged by the WF CCDs will fall approximately one core radius from the galaxy center. We require the F450W and the F555W frames to be taken with the same orientation so that the same stars are imaged in both filters. For each filter, the exposures will be made in two sets with an offset of n+0.5 PC pixels (n ~ 20) between each set. This will minimize the effects of small scale uncertainties in the flat fields, hot (high dark current) pixels, pixel defects and undersampling on the final photometry, while still allowing cosmic ray discrimination. This technique was used successfully in the Cycle 4 And I observations. Based on our own ground-based results for And III (Armandroff et al.\\ 1993) and our Cycle 4 results for And I (cf.\\ Fig.\\ 1 above), the horizontal branch in And III is expected to have an apparent V magnitude of mboxV ~ 25.2. Inspection of c-m diagrams for galactic dSphs then shows that if we are to detect a strong blue HB similar to that seen in Ursa Minor, we need to have reasonably precise photometry (i.e.\\ S/N >= 10) at a limit approximately 1 magnitude fainter than the magnitude of the horizontal branch, i.e.\\ mboxV ~ 26.2. At And III's declination, the visibility is 54 minutes per orbit. Allowing for overhead, this visibility allows 2 1200sec exposures to be taken per orbit (no CR-Split required) with a minimum amount of deadtime. The number of 1200sec exposures for each filter has then been calculated by first assuming we will reduce the data in the same way as for And I: photometry with a 2.5pix radius aperture on the PC1 data. The S/N per exposure is then given by equation (6.7) in the latest version of the WFPC2 Handbook used in conjunction with the requirement of S/N >= 10 in both filters for stars with V = 26.2 and spectral type G0. The choice of spectral type G0 is a compromise between the possibility of blue HB stars and the redder giant/subgiant population at this magnitude. With these assumptions, the desired S/N, which is the minimum needed to fill the scientific aims of the program, is achieved with 8 F555W and 16 F450W 1200sec exposures. Thus the program requires 12 orbits to carry out. SAA avoidance requirements, however, mean that two visits are necessary. Each visit will be for 6 orbits and will allow 4 F555W and 8 F450W exposures. The two visits will require use of the GROUP WITHIN 5D and SAME ORIENT special requirements but Cycle 4 and Cycle 6 experience suggests that these requirements do not generate any particular scheduling difficulties. The GROUP WITHIN 5D requirement is in fact a compromise between easing scheduling of the program and minimizing aliasing in the period determination of any RR Lyrae variables found. In these respects the present request is identical to that accepted in Cycle 6 for observing And II. Real Time Justification ----------------------- None We have an on-going ground-based program investigating the transformation of F450W and F555W magnitudes to standard B,V for metal-poor globular cluster giants and horizontal branch stars. This program uses F555W and F450W filters purchased with funds allocated by STScI to the Cycle 4 And I program. It has used telescope time on the 0.9m at Kitt Peak and on the 1m telescope at Siding Spring Observatory where a coated 2k x 2k thick Loral CCD similar to the WFPC2 devices is available. This program will be of benefit to other users of these filters in the WFPC2 community when it is completed. We also have access to the facilities of the Michigan-MIT- Dartmouth Observatory on Kitt Peak and to the facilities of the Harvard-Smithsonian Observatory on Mt.\\ Hopkins if additional ground-based calibration data are required. Observing time in ``WIYN 2-hour queue" was granted by NOAO to verify the mboxAnd I B and V photometric zeropoints; this route is also available for And III calibration verification if required. Calibration Justification ------------------------- Additional Comments ------------------- ------------------------------------------------------------------------------------ 7500( 2) - 02/19/97 19:19 - [ 3] Data Distribution Media: 8MM Blocking Factor: 10 Ship To: Dr. G. Da Costa*Mt Stromlo Observatory*Private Bag*Weston Post Office, ACT 2611*AUSTRALIA Ship Via: Email: gdc@mso.anu.edu.au ------------------------------------------------------------------------------------ 7500( 2) - 02/19/97 19:19 - [ 4] TARGET LIST a) Fixed Targets ------------------------------------------------------------------------------------------------------------------------------------ Tar| Target | Target | Target |Coord | Radial | Flux data No | Name | Description | Position |Eqnx | Vel. | ------------------------------------------------------------------------------------------------------------------------------------ 1 ANDROMEDA- GALAXY, Dwarf RA = 0H 35M 30.32S +/- 2.0", DEC J2000 V = 22.0+/-0.5 III Spheroidal = +36D 30' 20.2" +/- 2.0", PLATE B-V = 1.4+/-0.2 -ID=02S3 Comments: Flux given is for the brightest And III red giants. 7500( 2) - 02/19/97 19:19 - [ 5] Visit: 01 Visit Requirements: ORIENTation 20D TO 70D SAME ORIENTation AS 2 GROUP 1-2 WITHIN 5D On Hold Comments: Additional Comments: Specified orientation range is to ensure that 2 WF CCDs lie close the major axis of this flattened system. Note that ORIENTation 200D to 250D is also acceptable. To ensure the same stars are imaged in both visits, the orientation must be the same for both visits. But the order in which the visits execute (which will set the orientation) does not matter - Visit 2 could execute first. Exposures ------------------------------------------------------------------------------------------------------------------------------------ Exposure| Target |Instr | Oper. | Aper |Spectral|Central| Optional |Num| Time | Special Number | Name |Config| Mode |or FOV |Element |Waveln.| Parameters |Exp| | Requirements ------------------------------------------------------------------------------------------------------------------------------------ 1 ANDROMEDA-I WFPC2 IMAGE WFALL-F F555W CR-SPLIT=NO 4 1200 S POSition TARGet 10.0,10.0 II IX NO SPLIT Comments: POS TARG is to place target coordinates on pyramid apex. ------------------------------------------------------------------------------------------------------------------------------------ 2 ANDROMEDA-I WFPC2 IMAGE WFALL-F F450W CR-SPLIT=NO 8 1300 S SAME POSition AS 1 NO II IX SPLIT ------------------------------------------------------------------------------------------------------------------------------------ 7500( 2) - 02/19/97 19:19 - [ 6] Visit: 02 Visit Requirements: ORIENTation 20D TO 70D SAME ORIENTation AS 1 GROUP 1-2 WITHIN 5D On Hold Comments: Additional Comments: Specified orientation range is to ensure that 2 WF CCDs lie close the major axis of this flattened system. Note that ORIENTation 200D to 250D is also acceptable. To ensure the same stars are imaged in both visits, the orientation must be the same for both visits. But the order in which the visits execute (which will set the orientation) does not matter - Visit 1 could execute first. Exposures ------------------------------------------------------------------------------------------------------------------------------------ Exposure| Target |Instr | Oper. | Aper |Spectral|Central| Optional |Num| Time | Special Number | Name |Config| Mode |or FOV |Element |Waveln.| Parameters |Exp| | Requirements ------------------------------------------------------------------------------------------------------------------------------------ 1 ANDROMEDA-I WFPC2 IMAGE WFALL-F F555W CR-SPLIT=NO 4 1200 S POSition TARGet 10.95, II IX 10.95 NO SPLIT Comments: POS TARG is approximately 9.5 WF pixels in x and y from Visit 1 - aids in photometry error estimation ------------------------------------------------------------------------------------------------------------------------------------ 2 ANDROMEDA-I WFPC2 IMAGE WFALL-F F450W CR-SPLIT=NO 8 1300 S SAME POSition AS 1 NO II IX SPLIT ------------------------------------------------------------------------------------------------------------------------------------ 7500( 2) - 02/19/97 19:19 - [ 7] Summary Form for Proposal 7500 Item Used in this proposal ------------------------------------------------------------------------------------------------------------------------------------ Configurations WFPC2 ------------------------------------------------------------------------------------------------------------------------------------ Opmodes IMAGE ------------------------------------------------------------------------------------------------------------------------------------ Optional Parameters CR-SPLIT=NO ------------------------------------------------------------------------------------------------------------------------------------ Proposal Category GO ------------------------------------------------------------------------------------------------------------------------------------ Scientific Category STELLAR POPULATIONS ------------------------------------------------------------------------------------------------------------------------------------ Special Requirements ORIENTation 20D TO 70D SAME ORIENTation AS 2 GROUP 1-2 WITHIN 5D POSition TARGet 10.0,10.0 NO SPLIT SAME POSition AS 1 NO SPLIT ORIENTation 20D TO 70D SAME ORIENTation AS 1 GROUP 1-2 WITHIN 5D POSition TARGet 10.95,10.95 NO SPLIT ------------------------------------------------------------------------------------------------------------------------------------ Spectral Elements F555W F450W ------------------------------------------------------------------------------------------------------------------------------------ Target Names ANDROMEDA-III ------------------------------------------------------------------------------------------------------------------------------------