! +====================+ ! | PED OUTPUT PRODUCT | ! +====================+ ! ! /data/gandalf1/shea/proposals/6514/6514.prop ! Generated by PREPROCESSOR, version 7.0.1e ! Date: Wed Feb 19 16:01:27 EST 1997 Proposal_Information Title: The Horizontal Branch of the M31 Dwarf Spheroidal Companion And II Proposal_Category: GO Scientific_Category: STELLAR POPULATIONS Cycle: 6 Investigators PI_Name: Dr. Gary Da Costa PI_Institution: Mt Stromlo and Siding Spring Observatories CoI_Name: Dr. Taft Armandroff CoI_Institution: National Optical Astronomy Observatories Contact: CoI_Name: Dr. Nelson Caldwell CoI_Institution: Whipple Observatory, Smithsonian Institution Contact: CoI_Name: Dr. Patrick Seitzer CoI_Institution: University of Michigan Contact: Abstract: The dwarf spheroidal (dSph) companions to the Galaxy show a large diversity in their horizontal branch (HB) morphologies, with many possessing redder HBs than would be expected from their metal abundances. This variety of HB types, which is also seen in the outer halo globular clusters, is often taken as indicating a large age range in the outer galactic halo. Our Cycle 4 results for And I, a dSph in the outer halo of M31, reveal that this dSph, like many of its galactic counterparts, also has a redder HB than expected for its abundance. This new result naturally leads to the speculation that the outer halo of M31 may also contain a significant range of ages, in a similar fashion to our Galaxy. However, determination of the HB morphologies of the two other dSphs in the outer halo of M31, And II and And III, is an essential requirement before this speculation can be pursued. Indeed with results only for And I, we cannot know whether the M31 outer halo contains the same diversity of HB types as the outer galactic halo, or is instead more homogeneous. We therefore sought HST time with WFPC2 primarily in order to determine the HB morphologies of both And II and And III. The TAC, however, granted time for observations of only one system which we have chosen to be And II. Additional results from this project will include comparison of HB and giant-branch- tip distances, improved estimates of abundance dispersions, estimates of the RR Lyrae star content, and limits on the fraction of younger stars since the observations will reveal any main sequence stars younger than 2-3 Gyr. Questions Observing_Description: The required c-m diagrams will be derived from long exposures centered on And II carried out with the 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. In particular, we note that we prefer to use F450W, rather than F439W, because of the 3 times higher throughput of the F450W filter. Further, 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 in a given total exposure than 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) In And II where an intermediate-age population has been identified, with the planned F450W observations we should be able to detect the main sequence progenitor population if it is younger than ~3 Gyr. F814W observations would require additional integration time to achieve the same limit. The positioning and orientation of the WFPC2 has been set to accommodate the fact that there are bright galactic field stars near the center of And II. The field center has then been chosen so that these stars are at least 20" outside the WFPC2 field-of-view. The orientation range has then been specified so that the center of And II will land on one of the WF CCDs. For orientations outside the specified range, there is a chance that the And II center would fall off the PC frame and not be imaged at all. For orientations within the specified range at least one of the other WF CCDs will be centered at approximately one core radius so that we can look for radial gradients. 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. The actual orientation itself, within the specified range does not matter, so either visit can execute first (and set the orientation). 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 (accomplished using two visits). 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 the work of van den Bergh & Pritchet (1987, ApJ, 316, 517) and our own Cycle 4 results for And I, the horizontal branch in the halo of M31 has an apparent V magnitude of V ~ 25.1. 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 ~ 26.1. Given the declination of the target, 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, using the information in the latest version of the WFPC2 Handbook, on the basis of achieving a S/N >= 10 on the PC for stars with V = 26.1 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. In making these calculations, the correction factor applied to the S/N (calculated as required by assuming that all the light falls in a single pixel) has been taken to be the average of the PC-Center and PC-Corner values interpolated to the central wavelengths of the F450W and F555W filters. With these assumptions, the desired S/N, which is the minimum needed to fill the scientific aims of the program, is achieved with 7 F555W and 15 F450W 1200sec exposures per galaxy. Thus the observation of And II requires 11 orbits to carry out. SAA avoidance requirements however, mean that two visits will be required. The first visit will be for 5 orbits and will allow 3 F555W and 7 F450W exposures while the second will be for 6 orbits allowing 4 F555W and 8 F450W exposures. The two visits will require use of the GROUP WITHIN 5D and SAME ORIENT special requirements but the Cycle 4 And I 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. 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" has been granted by NOAO to verify the And I B and V photometric zeropoints; this route is also available for And II and And III calibration verification if required. Calibration_Justification: Additional_Comments: Fixed_Targets Target_Number: 1 Target_Name: ANDROMEDA-II Alternate_Names: Description: GALAXY,DWARF SPHEROIDAL Position: RA=01H 16M 22.24S +/- 1.0",DEC=33D 25' 54.85" +/- 1.0",PLATE-ID=00DV Equinox: J2000 RV_or_Z: RA_PM: Dec_PM: Epoch: Annual_Parallax: Flux: V = 22.0+/-0.5 B-V = 1.4+/-0.2 Comments: Flux given is for brightest And II red giants. Coordinates given are not the center of And II, but are offset to avoid the bright field stars near the center of the Galaxy. Solar_System_Targets Generic_Targets Scan_Data Visits Visit_Number: 01 Visit_Requirements: ORIENT 180D TO 0D GROUP 1-2 WITHIN 5 D On_Hold_Comments: Visit_Comments: Orientation range should be within the range of 180 to 0. This is to ensure that center of And II lands on one of the WF CCDs. For orientations outside this range, possible that the And II center will fall outside the PC field-of-view and off the WF CCDs. If this orientation request produces strong scheduling difficulties, it can be dropped, after consultation with the PI. 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. SAME ORIENTation AS 2 Exposure_Number: 1 Target_Name: ANDROMEDA-II Config: WFPC2 Opmode: IMAGE Aperture: WFALL-FIX Sp_Element: F555W Wavelength: Optional_Parameters: CR-SPLIT=NO Number_of_Iterations: 3 Time_Per_Exposure: 1200 S Special_Requirements: POS TARG 10.0,10.0 NO SPLIT Comments: POS TARG is to place target coordinates on pyramid apex. Target coords have been chosen so that the bright field stars near the center of And II are >20 arcsec outside the field-of-view. Exposure_Number: 2 Target_Name: ANDROMEDA-II Config: WFPC2 Opmode: IMAGE Aperture: WFALL-FIX Sp_Element: F450W Wavelength: Optional_Parameters: CR-SPLIT=NO Number_of_Iterations: 7 Time_Per_Exposure: 1300 S Special_Requirements: SAME POS AS 1 NO SPLIT Comments: Visit_Number: 02 Visit_Requirements: SAME ORIENT AS 1 On_Hold_Comments: Visit_Comments: Orientation range should be within the range of 180 to 0. This is to ensure that center of And II lands on one of the WF CCDs. For orientations outside this range, possible that the And II center will fall outside the PC field-of-view and off the WF CCDs. If this orientation request produces strong scheduling difficulties, it can be dropped, after consultation with the PI. 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. Same orient as 01. Exposure_Number: 1 Target_Name: ANDROMEDA-II Config: WFPC2 Opmode: IMAGE Aperture: WFALL-FIX Sp_Element: F555W Wavelength: Optional_Parameters: CR-SPLIT=NO Number_of_Iterations: 4 Time_Per_Exposure: 1200 S Special_Requirements: POS TARG 10.95,10.95 NO SPLIT Comments: POS TARG is approximately 9.5 WF pixels in x and y from Visit 1 - allows error estimate of photometry Exposure_Number: 2 Target_Name: ANDROMEDA-II Config: WFPC2 Opmode: IMAGE Aperture: WFALL-FIX Sp_Element: F450W Wavelength: Optional_Parameters: CR-SPLIT=NO Number_of_Iterations: 8 Time_Per_Exposure: 1300 S Special_Requirements: SAME POS AS 1 NO SPLIT Comments: Data_Distribution ! Defaults indicated; change if desired Medium: 8MM Ship_To: MSSSO, Private Bag, Weston Post Office, ACT, 2611, AUSTRALIA Recipient_Email: gdc@mso.anu.edu.au ! 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