Page 1 PROPOSAL FOR HUBBLE SPACE TELESCOPE OBSERVATIONS ST ScI Use Only ID 5325 Report Date: 18-Jul-95:17:09 Version: ********** Check-in Date: ********** 1.Proposal Title: THE HORIZONTAL BRANCH OF THE M31 DWARF SPHEROIDAL COMPANION ANDROMEDA-I: CYCLE4 MEDIUM ------------------------------------------------------------------------------------ 2. Scientific Category 3. Proposal For 4. Proposal Type 5. Continuation ID GALAXIES & CLUSTERS GO Sub Category STELLAR POPULATIONS ------------------------------------------------------------------------------------ 6. Principal Investigator Institution Country Telephone Gary S. Da costa ANGLO-AUSTRALIAN OBSERVATORY AUSTRALIA ISD 61 2 372 4 ------------------------------------------------------------------------------------ 7. Abstract WFPC2 imaging of a field near the center of the dwarf spheroidal companion to M31, Andromeda-I, will be used to construct a color- magitude diagram that will reveal for the first time the morphology of the horizontal branch in this system. The dwarf spheroidal companions to the Galaxy show a large diversity of horizontal branch types and many show redder horizontal branches than would be expected from their generally low metal abundances. This trend of red horizontal branches despite low abundances, seen in both the galactic dwarf spheroidals and in some outer galactic halo globular clusters, is frequently taken as an indication that these objects formed later than the inner galactic halo. It is therefore of considerable interest to see if the And I, II and III dwarf spheroidal galaxies, which are objects in the outer halo of M31, show this same effect in their horizontal branch morphologies. The TAC has granted time to observe one object and we have decided to concentrate on And I, the most luminous and least affected by bright galactic field stars of the And systems. Subsidiary results that will also follow from this project include a comparison of horizontal branch and giant-branch-tip distance and an estimate of the internal abundance dispersion free from the complications caused by AGB star contamination on the upper giant branch. ------------------------------------------------------------------------------------ ------------------------------------------------------------------------------------ 9. Est obs time (hours) pri: 9.70 par: 0 10. Num targs pri: 1 par: 0 ------------------------------------------------------------------------------------ 11. Instruments requested: WF/PC ------------------------------------------------------------------------------------ ------------------------------------------------------------------------------------ Page 2 I. GENERAL FORM Proposal 5325 PI: Gary S. Da costa Proposal Title: THE HORIZONTAL BRANCH OF THE M31 DWARF SPHEROIDAL COMPANION ANDROMEDA-I: CYCLE4 MEDIUM ------------------------------------------------------------------------------------ 1. Proposers: Proposers Institution Country ESA ------------------------------------------------------------------------------------ Gary S. Da Costa ANGLO-AUSTRALIAN OBSERVATORY AUSTRALIA Taft E. Armandroff 2902 USA Nelson Caldwell 2168 USA Patrick O. Seitzer 2660 USA ------------------------------------------------------------------------------------ 3. Description of proposed observations. The required color-magnitude diagram will be derived from long exposures centered on the core of Andromeda-I carried out with the WFPC2 using the F450W and F555W filters. These filters should provide satisfactory transformation to the ground-based B,V system while minimizing HST total exposure times. In particular we note that we prefer to use F450W, rather than F439W, because of the higher throughput of the F450W filter despite the fact that the calibration of F439W may be better. The center of Andromeda-I 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. The WF fields will then contain fewer stars per unit area (but more stars in total). Obviously we require the F450W frames and the F555W frames to be taken without a gap so that the orientation of the F450W and F555W frames are similar and the same stars are imaged in both filters. Small offsets (of the order of 1" or approximately 20 PC pixels) will be made between each exposure to minimize the affect of uncertainties in the flat fields on the final photometry. A exposure time of 1800s has been chosen for the long exposures (longer times reduce the effect of readout noise but increase the dark signal and the number of cosmic rays; shorter times give lower dark and fewer cosmic rays but at the expense of more readout noise in a given total exposure); because there will be 7 (F555W) and 12 (F450W) long exposures, it is not necessary to split them for cosmic ray rejection. The number of 1800s exposures required has been calculated by applying the formalism given in the WFPC2 Handbook. In making these calculations, the correction factor applied to the signal-to-noise (calculated as required by assuming that all the light falls in one pixel) has been taken to be the average of the PC-Center and PC-Corner values (Table 6.4) interpolated to the central wavelengths of the F450W and F555W filters. With these corrections, and noting that V=26.1 is one mag fainter than the nominal horizontal branch (HB) magnitude (we need to go at least this faint to ensure measurement of a strong blue HB), 7 1800s exposures in F555W will give a S/N of approx 18 at V=26.1 (SpT A0) (mag error < 0.06) while 12 1800s exposures in F450W will give a S/N approx 19 (again mag error < 0.06). This reasonably precise photometry is the minimum requirement to fill the scientific aims of the project. The photometry zeropoint will be determined from ground based data: CCD frames of Andromeda-I in in V and I are available in the literature and additional ground-based imaging in B and V is planned. Short exposures (3x100s F555W, 3x140s F450W) will also be obtained with HST to allow overlap between the faint HST photometry and the ground based observations. Total short exposure times have been calculated on the basis of requiring at least 1 mag of overlap with the long exposures at a S/N of at least 20. The exposures are split into 3 subexposures to facilitate removal of cosmic rays and to extend the interval of overlap with the ground-based photometry to brighter magnitudes. Page 3 ------------------------------------------------------------------------------------ 4. Justification of need for HST observations. With considerable effort, and only under the very best of conditions, ground- based imaging can detect faint objects whose magnitudes approach those that will be routinely achieved with HST and WFPC2. However, mere detection is a much less stringent requirement than reasonably precise photometry of stars. Consequently, for those problems requiring such photometry, ground-based imaging is limited to approximately V=24 in relatively uncrowded fields; this limit becomes rapidly brighter as image crowding becomes significant. This team has published the results of faint ground-based imaging of the And III dSph (Armandroff et al. 1993, AJ, 106, 986) and a similar paper for And II is in preparation. With the faintest stars measured approaching V=24, these data represent the best that can be achieved from the ground for these galaxies. Similar data for And I have been published by Mould and Kristian (1990, ApJ, 354, 408). But to determine the horizontal branch morphologies in these systems, these limits are simply too bright; we must therefore use HST and WFPC2. Indeed HST and WFPC2 are essential because only they can provide the required relatively precise (errors approximately 0.06 mag) photometry at the necessary faint (V approx 26.1) magnitude levels needed to fulfill the scientific aims of the project. ------------------------------------------------------------------------------------ 5. Description of special scheduling requirements. ------------------------------------------------------------------------------------ 6. Description of special calibration exposures. ------------------------------------------------------------------------------------ 8. Additional comments or special requests. ------------------------------------------------------------------------------------ 9. Description of previous HST work. Da Costa and Seitzer are both Co-Is in GO proposal 2419 "The Chronology of the Formation of the Galactic Halo". Zinn is PI of that proposal and the Co-Is are Carney, Da Costa, Demarque, Heasley, Janes, Olszewski and Seitzer. It is not directly scientifically related to the present proposal. ------------------------------------------------------------------------------------ 10. Resources to be supplied by investigator's institution(s). The computational, image display and software resources available at the Anglo- Australian Observatory, at Kitt Peak National Observatory, at the Whipple Observatory of the Smithsonian Institution, and at the Department of Astronomy, University of Michigan will be used for the data reduction and analysis. The ground-based observations necessary for the calibration will be obtained through the use of the facilities at Kitt Peak National Observatory and at the Michigan- Dartmouth-MIT McGraw-Hill Observatory. ------------------------------------------------------------------------------------ 11. Address Information Name: Gary S. Da Costa Category: PI Institution: Anglo-Australian Observatory Address: Anglo-Australian Observatory Epping Laboratory P.O. Box 296 City: Epping, NSW State: Zip Code: 2121 Country: Australia Telephone: ISD 61 2 372 4800 Telex (or e-mail): gdc@aaoepp.aao.gov.au ------------------------------------------------------------------------------------ Page 4 Name: Category: CON Institution: Address: City: State: Zip Code: Country: Telephone: Telex (or e-mail): ------------------------------------------------------------------------------------ TARGET LIST a) Fixed Targets ID = 5325 [ 5] ------------------------------------------------------------------------------------------------------------------------------------ 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 ------------------------------------------------------------------------------------------------------------------------------------ Tar| Target | Target | Target |Coord | Radial |Acqui|FLX| Flux data No | Name | Description | Position |Eqnx | Vel. |Prblm|REF| | | | | | | | | ------------------------------------------------------------------------------------------------------------------------------------ 1 ANDROMEDA-I E,308 RA = 00H 45M 41.166S +/- 2000.0 1 V=26.1,TYPE=A0 0.1S, 2 V=21.6,TYPE=G0 DEC = +38D 02' 19.63" +/- 1.0" ------------------------------------------------------------------------------------------------------------------------------------ EXPOSURE LOGSHEET ID = 5325 [ 6] ------------------------------------------------------------------------------------------------------------------------------------ 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 |10 | 11 | 12 |13 |14| 15 ------------------------------------------------------------------------------------------------------------------------------------ Line | Seq | Target |Instr | Oper. | Aper |Spectral|Central| Optional |Num| Time | S/N |Flx|Pr| Special Number | Name | Name |Config| Mode |or FOV |Element |Waveln.| Parameters |Exp| |Rel. Time|Ref| | Requirements ------------------------------------------------------------------------------------------------------------------------------------ 1 DEFINE ANDROMEDA-I WFPC2 IMAGE PC1-FIX F555W CR-SPLIT=NO 1 100S 14 2 1 CYCLE 4 VS ------------------------------------------------------------------------------------------------------------------------------------ 2 DEFINE ANDROMEDA-I WFPC2 IMAGE PC1-FIX F555W CR-SPLIT=NO 1 1800S 7 1 1 CYCLE 4 VL ------------------------------------------------------------------------------------------------------------------------------------ 3 DEFINE ANDROMEDA-I WFPC2 IMAGE PC1-FIX F450W CR-SPLIT=NO 1 140S 11.5 2 1 CYCLE 4 BS ------------------------------------------------------------------------------------------------------------------------------------ 4 DEFINE ANDROMEDA-I WFPC2 IMAGE PC1-FIX F450W CR-SPLIT=NO 1 1800S 5.5 1 1 CYCLE 4 BL ------------------------------------------------------------------------------------------------------------------------------------ 5 USE VS CYCLE 4 GROUP 5-29 WITHIN 7D POS TARG 0.0,0.0 ------------------------------------------------------------------------------------------------------------------------------------ 6 USE VS CYCLE 4 POS TARG 1.0,1.0 SAME ORIENT FOR 6- 29 AS 5 ------------------------------------------------------------------------------------------------------------------------------------ 7 USE VS CYCLE 4 POS TARG -1.0,-1.0 ------------------------------------------------------------------------------------------------------------------------------------ 8 USE BS CYCLE 4 POS TARG 0.0,0.0 ------------------------------------------------------------------------------------------------------------------------------------ 9 USE BS CYCLE 4 POS TARG 1.0,1.0 ------------------------------------------------------------------------------------------------------------------------------------ 10 USE BS CYCLE 4 POS TARG -1.0,-1.0 ------------------------------------------------------------------------------------------------------------------------------------ 11 USE VL CYCLE 4 POS TARG 0.0,0.0 ------------------------------------------------------------------------------------------------------------------------------------ 12 USE VL CYCLE 4 POS TARG 0.0,0.0 ------------------------------------------------------------------------------------------------------------------------------------ 13 USE VL CYCLE 4 POS TARG 0.0,0.0 ------------------------------------------------------------------------------------------------------------------------------------ 14 USE VL CYCLE 4 POS TARG 0.0,0.0 ------------------------------------------------------------------------------------------------------------------------------------ 15 USE VL CYCLE 4 POS TARG 0.943, 0.943 ------------------------------------------------------------------------------------------------------------------------------------ EXPOSURE LOGSHEET ID = 5325 [ 7] ------------------------------------------------------------------------------------------------------------------------------------ 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 |10 | 11 | 12 |13 |14| 15 ------------------------------------------------------------------------------------------------------------------------------------ Line | Seq | Target |Instr | Oper. | Aper |Spectral|Central| Optional |Num| Time | S/N |Flx|Pr| Special Number | Name | Name |Config| Mode |or FOV |Element |Waveln.| Parameters |Exp| |Rel. Time|Ref| | Requirements ------------------------------------------------------------------------------------------------------------------------------------ 16 USE VL CYCLE 4 POS TARG 0.943, 0.943 ------------------------------------------------------------------------------------------------------------------------------------ 17 USE VL CYCLE 4 POS TARG 0.943, 0.943 ------------------------------------------------------------------------------------------------------------------------------------ 18 USE BL CYCLE 4 POS TARG 0.0,0.0 ------------------------------------------------------------------------------------------------------------------------------------ 19 USE BL CYCLE 4 POS TARG 0.0,0.0 ------------------------------------------------------------------------------------------------------------------------------------ 20 USE BL CYCLE 4 POS TARG 0.0,0.0 ------------------------------------------------------------------------------------------------------------------------------------ 21 USE BL CYCLE 4 POS TARG 0.0,0.0 ------------------------------------------------------------------------------------------------------------------------------------ 22 USE BL CYCLE 4 POS TARG 0.0,0.0 ------------------------------------------------------------------------------------------------------------------------------------ 23 USE BL CYCLE 4 POS TARG 0.0,0.0 ------------------------------------------------------------------------------------------------------------------------------------ 24 USE BL CYCLE 4 POS TARG 0.943, 0.943 ------------------------------------------------------------------------------------------------------------------------------------ 25 USE BL CYCLE 4 POS TARG 0.943, 0.943 ------------------------------------------------------------------------------------------------------------------------------------ 26 USE BL CYCLE 4 POS TARG 0.943, 0.943 ------------------------------------------------------------------------------------------------------------------------------------ 27 USE BL CYCLE 4 POS TARG 0.943, 0.943 ------------------------------------------------------------------------------------------------------------------------------------ 28 USE BL CYCLE 4 POS TARG 0.943, 0.943 ------------------------------------------------------------------------------------------------------------------------------------ 29 USE BL CYCLE 4 POS TARG 0.943, 0.943 ------------------------------------------------------------------------------------------------------------------------------------ Summary Form for Proposal 5325 [ 8] Item Used in this proposal ------------------------------------------------------------------------------------------------------------------------------------ Configurations WFPC2 ------------------------------------------------------------------------------------------------------------------------------------ Opmodes IMAGE ------------------------------------------------------------------------------------------------------------------------------------ Optional Parameters CR-SPLIT=NO ------------------------------------------------------------------------------------------------------------------------------------ Proposal for GO ------------------------------------------------------------------------------------------------------------------------------------ S/C Hours 9.70 ------------------------------------------------------------------------------------------------------------------------------------ Scientific Category GALAXIES & CLUSTERS ------------------------------------------------------------------------------------------------------------------------------------ Scientific Sub-category STELLAR POPULATIONS ------------------------------------------------------------------------------------------------------------------------------------ Special Requirements CYCLE 4; GROUP 5-29 WITHIN 7D; POS TARG 0.0,0.0 POS TARG 1.0,1.0; SAME ORIENT FOR 6-29 AS 5 POS TARG -1.0,-1.0 POS TARG 1.0,1.0 POS TARG 0.943,0.943 ------------------------------------------------------------------------------------------------------------------------------------ Spectral Elements F555W F450W ------------------------------------------------------------------------------------------------------------------------------------ Target Names ANDROMEDA-I ------------------------------------------------------------------------------------------------------------------------------------