8272( 8) - 01/27/00 15:03 - [ 1] PROPOSAL FOR HUBBLE SPACE TELESCOPE OBSERVATIONS ST ScI Use Only ID: 8272 Version: 8 Check-in Date: 27-Jan-2000 15:02:43 1.Proposal Title: The Horizontal Branches of the M31 Dwarf Spheroidal Companions And V & VI ------------------------------------------------------------------------------------ 2. Proposal For 3. Cycle GO 8 ------------------------------------------------------------------------------------ 4. Investigators Contact? PI: Taft Armandroff National Optical Astronomy Observatories, AURA, Inc. CoI: Dr. Gary Da Costa Australian National University N CoI: Dr. George Jacoby Kitt Peak National Observatory N ------------------------------------------------------------------------------------ 5. Abstract The Galaxy's dwarf spheroidal (dSph) companions show a surprising diversity in the epochs of their major star formation episode(s), extending from ~15 Gyr to ~3 Gyr. To aid in understanding the origin of this diversity, we need similar results for M31's 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 horizontal branch (HB) morphologies, which observations of Galactic dSphs have shown to be a reliable mean age indicator. Cycle 4 observations indicate that the bulk of the star formation in And I took place ~10 Gyr ago, though the presence of blue HB and RR Lyr stars testify to a minor older component. Analysis of Cycle 6 observations of And II are nearing completion, and observations of And III are scheduled for execution in Cycle 7. Here we seek HST time to complete our study of the M31 dSphs by observing And V & VI, newly discovered M3 1 dSph companions. The primary aim is to determine the HB morphology and thus fix the epoch of the major star formation episode(s) in these galaxies. 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. ------------------------------------------------------------------------------------ 8272( 8) - 01/27/00 15:03 - [ 2] Observations Description ------------------------ The required c-m diagrams will be derived from long exposures centered on And V & VI 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 and Cycle 6 And II observations. We prefer the F450W filter over an I-band filter such as F814W for the following reasons. 1) It is possible that And V/VI 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 Lyr 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 m ai n sequence population in these d Sphs if it is younger than ~3 Gyr. F814W observations would require additional integration time to achieve the same limit. The centers of And V & VI will be placed on the nominal center of the PC1 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 and Cycle 6 And II observations. Based on our own ground-based results for And V (Armandroff et al. 1998a) and our Cycle 4 results for And I (Da Costa et al. 1996), the hor izontal branch in And V is expected to have an apparent V magnitude of V ~ 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., V ~ 26.2. At And V's declination, the visibility is 55 minutes per orbit. Allowing for overhead, this visibility allows 2 1200-sec exposures to be taken per orbit (no CR-Split required) with a minimum amount of deadtime. The number of 1200-sec 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 = 2 6.2 and spectral type G0. The c hoice 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 1200-sec exposures. Thus the program requires 12 orbits per galaxy to carry out. SAA avoidance requirements, however, mean that two visits per galaxy 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, 6 and 7 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 Lyr variables found. In these respects the present request is nearly iden ti cal to that accepted in Cycle 6 for observing And II and in Cycle 7 for And III. Real Time Justification ----------------------- Calibration Justification ------------------------- Additional Comments ------------------- ------------------------------------------------------------------------------------ 8272( 8) - 01/27/00 15:03 - [ 3] Data Distribution Paper Products: YES Media: 8MM_HI Blocking Factor: 10 Ship To: PI_Address Ship Via: Email: ------------------------------------------------------------------------------------ 8272( 8) - 01/27/00 15:03 - [ 4] TARGET LIST Fixed Targets ------------------------------------------------------------------------------------------------------------------------------------ Tar| Target | Target | Target |Coord | Radial | Flux data No | Name | Description | Position |Eqnx | Vel. | ------------------------------------------------------------------------------------------------------------------------------------ 1 ANDROMEDA-V GALAXY, DWARF RA=1H 10M 17.1S +/- 2", DEC=47D J2000 V = 22.6+/-0.4 SPHEROIDAL 37' 41" +/- 2", PLATE-ID=036I I = 20.9+/-0.4 Comments: Flux given is for the brightest Andromeda V red giants. 2 ANDROMEDA-VI GALAXY, DWARF RA=23H 51M 46.43S +/- 2", J2000 V = 22.2+/-0.4 SPHEROIDAL DEC=24D 34' 59.4" +/- 2", PLATE- I = 20.6+/-0.4 ID=02CZ Comments: Flux given is for the brightest Andromeda VI red giants. 8272( 8) - 01/27/00 15:03 - [ 5] Visit: 01 Visit Priority: Visit Requirements: SAME ORIENT AS 02 GROUP 01, 02 WITHIN 3 D 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 ANDROMEDA-V WFPC2 IMAGE WFALL-F F555W CR-SPLIT=NO 4 1200 S NO SPLIT IX ------------------------------------------------------------------------------------------------------------------------------------ 2 ANDROMEDA-V WFPC2 IMAGE WFALL-F F450W CR-SPLIT=NO 8 1300 S NO SPLIT SAME POS AS 1 IX ------------------------------------------------------------------------------------------------------------------------------------ 8272( 8) - 01/27/00 15:03 - [ 6] Visit: 02 Visit Priority: Visit Requirements: SAME ORIENT AS 01 GROUP 01, 02 WITHIN 3 D 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 ANDROMEDA-V WFPC2 IMAGE WFALL-F F555W CR-SPLIT=NO 4 1200 S NO SPLIT POS TARG -0.95, IX 0.95 ------------------------------------------------------------------------------------------------------------------------------------ 2 ANDROMEDA-V WFPC2 IMAGE WFALL-F F450W CR-SPLIT=NO 8 1300 S SAME POS AS 1 NO SPLIT IX ------------------------------------------------------------------------------------------------------------------------------------ 8272( 8) - 01/27/00 15:03 - [ 7] Visit: 03 Visit Priority: Visit Requirements: SAME ORIENT AS 04 GROUP 03, 04 WITHIN 3 D 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 ANDROMEDA-V WFPC2 IMAGE WFALL-F F555W CR-SPLIT=NO 4 1100 S NO SPLIT I IX ------------------------------------------------------------------------------------------------------------------------------------ 2 ANDROMEDA-V WFPC2 IMAGE WFALL-F F450W CR-TOLERANCE=0.0 4 2600 S SAME POS AS 1 I IX ------------------------------------------------------------------------------------------------------------------------------------ 8272( 8) - 01/27/00 15:03 - [ 8] Visit: 04 Visit Priority: Visit Requirements: SAME ORIENT AS 03 GROUP 03, 04 WITHIN 3 D 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 ANDROMEDA-V WFPC2 IMAGE WFALL-F F555W CR-SPLIT=NO 4 1100 S NO SPLIT POS TARG -0.95, I IX 0.95 ------------------------------------------------------------------------------------------------------------------------------------ 2 ANDROMEDA-V WFPC2 IMAGE WFALL-F F450W CR-TOLERANCE=0.0 4 2600 S SAME POS AS 1 I IX ------------------------------------------------------------------------------------------------------------------------------------ 8272( 8) - 01/27/00 15:03 - [ 9] Visit: 52 Visit Priority: Visit Requirements: ORIENT 105.53D TO 125.53D ORIENT 285.53D TO 305.53D 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 ANDROMEDA-V WFPC2 IMAGE WFALL-F F555W CR-SPLIT=NO 4 1200 S NO SPLIT POS TARG -0.95, IX 0.95 ------------------------------------------------------------------------------------------------------------------------------------ 2 ANDROMEDA-V WFPC2 IMAGE WFALL-F F450W CR-SPLIT=NO 8 1300 S SAME POS AS 1 NO SPLIT IX ------------------------------------------------------------------------------------------------------------------------------------ 8272( 8) - 01/27/00 15:03 - [ 10] Summary Form for Proposal 8272 Item Used in this proposal ------------------------------------------------------------------------------------------------------------------------------------ Apertures WFALL-FIX ------------------------------------------------------------------------------------------------------------------------------------ Configurations WFPC2 ------------------------------------------------------------------------------------------------------------------------------------ Opmodes IMAGE ------------------------------------------------------------------------------------------------------------------------------------ Optional Parameters CR-SPLIT=NO CR-TOLERANCE=0.0 ------------------------------------------------------------------------------------------------------------------------------------ Proposal Category GO ------------------------------------------------------------------------------------------------------------------------------------ Special Requirements SAME ORIENT AS 02 GROUP 01, 02 WITHIN 3 D NO SPLIT NO SPLIT SAME POS AS 1 SAME ORIENT AS 01 GROUP 01, 02 WITHIN 3 D NO SPLIT POS TARG -0.95, 0.95 SAME POS AS 1 NO SPLIT SAME ORIENT AS 04 GROUP 03, 04 WITHIN 3 D SAME POS AS 1 SAME ORIENT AS 03 GROUP 03, 04 WITHIN 3 D ORIENT 105.53D TO 125.53D ORIENT 285.53D TO 305.53D ------------------------------------------------------------------------------------------------------------------------------------ Spectral Elements F555W F450W ------------------------------------------------------------------------------------------------------------------------------------ Target Names ANDROMEDA-V ANDROMEDA-VI ------------------------------------------------------------------------------------------------------------------------------------