! $Id: 5325,v 8.1 1994/07/27 16:36:47 pepsa Exp $ ! ! coverpage: title_1: THE HORIZONTAL BRANCH OF THE M31 DWARF SPHEROIDAL COMPANION title_2: ANDROMEDA-I: CYCLE4 MEDIUM sci_cat: GALAXIES & CLUSTERS sci_subcat: STELLAR POPULATIONS proposal_for: GO pi_fname: GARY pi_mi: S. pi_lname: DA COSTA pi_inst: ANGLO-AUSTRALIAN OBSERVATORY pi_country: AUSTRALIA pi_phone: ISD 61 2 372 4800 hours_pri: 9.70 num_pri: 1 wf_pc: Y ! end of coverpage abstract: line_1: WFPC2 imaging of a field near the center of the dwarf spheroidal line_2: companion to M31, Andromeda-I, will be used to construct a color- line_3: magitude diagram that will reveal for the first time the morphology line_4: of the horizontal branch in this system. The dwarf spheroidal line_5: companions to the Galaxy show a large diversity of horizontal branch line_6: types and many show redder horizontal branches than would be expected line_7: from their generally low metal abundances. This trend of red line_8: horizontal branches despite low abundances, seen in both the galactic line_9: dwarf spheroidals and in some outer galactic halo globular clusters, line_10: is frequently taken as an indication that these objects formed later line_11: than the inner galactic halo. It is therefore of considerable interest line_12: to see if the And I, II and III dwarf spheroidal galaxies, which are line_13: objects in the outer halo of M31, show this same effect in their line_14: horizontal branch morphologies. The TAC has granted time to observe line_15: one object and we have decided to concentrate on And I, the most line_16: luminous and least affected by bright galactic field stars of the And line_17: systems. Subsidiary results that will also follow from this project line_18: include a comparison of horizontal branch and giant-branch-tip distance line_19: and an estimate of the internal abundance dispersion free from the line_20: complications caused by AGB star contamination on the upper giant branch. ! ! end of abstract general_form_proposers: lname: DA COSTA fname: GARY mi: S. inst: ANGLO-AUSTRALIAN OBSERVATORY country: AUSTRALIA ! lname: ARMANDROFF fname: TAFT mi: E. inst: 2902 country: USA ! lname: CALDWELL fname: NELSON inst: 2168 country: USA ! lname: SEITZER fname: PATRICK mi: O. inst: 2660 country: USA ! ! end of general_form_proposers block general_form_text: question: 3 section: 1 line_1: The required color-magnitude diagram will be derived from long exposures line_2: centered on the core of Andromeda-I carried out with the WFPC2 using the line_3: F450W and F555W filters. These filters should provide satisfactory line_4: transformation to the ground-based B,V system while minimizing HST line_5: total exposure times. In particular we note that we prefer to use F450W, line_6: rather than F439W, because of the higher throughput of the F450W filter line_7: despite the fact that the calibration of F439W may be better. The line_8: center of Andromeda-I will be placed on the nominal center of the PC1 line_9: field; in this configuration the fields imaged by the WF CCDs will fall line_10: approximately one core radius from the galaxy center. The WF fields line_11: will then contain fewer stars per unit area (but more stars in total). line_12: Obviously we require the F450W frames and the F555W frames to be taken line_13: without a gap so that the orientation of the F450W and F555W frames line_14: are similar and the same stars are imaged in both filters. line_15: Small offsets (of the order of 1" or approximately 20 PC pixels) will be line_16: made between each exposure to minimize the affect of uncertainties line_17: in the flat fields on the final photometry. A exposure time of 1800s line_18: has been chosen for the long exposures (longer times reduce the effect line_19: of readout noise but increase the dark signal and the number of line_20: cosmic rays; shorter times give lower dark and fewer cosmic rays but line_21: at the expense of more readout noise in a given total exposure); because line_22: there will be 7 (F555W) and 12 (F450W) long exposures, it is not line_23: necessary to split them for cosmic ray rejection. The number of 1800s ! question: 3 section: 2 line_1: exposures required has been calculated by applying the formalism given line_2: in the WFPC2 Handbook. In making these calculations, the correction line_3: factor applied to the signal-to-noise (calculated as required by line_4: assuming that all the light falls in one pixel) has been taken to be the line_5: average of the PC-Center and PC-Corner values (Table 6.4) interpolated line_6: to the central wavelengths of the F450W and F555W filters. With these line_7: corrections, and noting that V=26.1 is one mag fainter than the nominal line_8: horizontal branch (HB) magnitude (we need to go at least this faint to line_9: ensure measurement of a strong blue HB), 7 1800s exposures in F555W will line_10: give a S/N of approx 18 at V=26.1 (SpT A0) (mag error < 0.06) while 12 line_11: 1800s exposures in F450W will give a S/N approx 19 (again mag error < line_12: 0.06). This reasonably precise photometry is the minimum requirement line_13: to fill the scientific aims of the project. The photometry zeropoint line_14: will be determined from ground based data: CCD frames of Andromeda-I in line_15: in V and I are available in the literature and additional ground-based line_16: imaging in B and V is planned. Short exposures (3x100s F555W, 3x140s line_17: F450W) will also be obtained with HST to allow overlap between the line_18: faint HST photometry and the ground based observations. Total short line_19: exposure times have been calculated on the basis of requiring at least line_20: 1 mag of overlap with the long exposures at a S/N of at least 20. The line_21: exposures are split into 3 subexposures to facilitate removal of cosmic line_22: rays and to extend the interval of overlap with the ground-based line_23: photometry to brighter magnitudes. ! 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 and WFPC2. However, mere line_4: detection is a much less stringent requirement than reasonably precise line_5: photometry of stars. Consequently, for those problems requiring such line_6: photometry, ground-based imaging is limited to approximately V=24 in line_7: relatively uncrowded fields; this limit becomes rapidly brighter as line_8: image crowding becomes significant. This team has published the line_9: results of faint ground-based imaging of the And III dSph (Armandroff line_10: et al. 1993, AJ, 106, 986) and a similar paper for And II is in line_11: preparation. With the faintest stars measured approaching V=24, these line_12: data represent the best that can be achieved from the ground for these line_13: galaxies. Similar data for And I have been published by Mould and line_14: Kristian (1990, ApJ, 354, 408). But to determine the horizontal branch line_15: morphologies in these systems, these limits are simply too bright; we line_16: must therefore use HST and WFPC2. Indeed HST and WFPC2 are essential line_17: because only they can provide the required relatively precise (errors line_18: approximately 0.06 mag) photometry at the necessary faint (V approx line_19: 26.1) magnitude levels needed to fulfill the scientific aims of the line_20: project. ! question: 5 section: 1 line_1: ! question: 6 section: 1 line_1: ! ! ! question: 8 section: 1 line_1: ! question: 9 section: 1 line_1: Da Costa and Seitzer are both Co-Is in GO proposal 2419 "The line_2: Chronology of the Formation of the Galactic Halo". Zinn is PI of that line_3: proposal and the Co-Is are Carney, Da Costa, Demarque, Heasley, Janes, line_4: Olszewski and Seitzer. It is not directly scientifically related to the line_5: present proposal. ! question: 10 section: 1 line_1: The computational, image display and software resources available at the line_2: Anglo-Australian Observatory, at Kitt Peak National Observatory, at the line_3: Whipple Observatory of the Smithsonian Institution, and at the line_4: Department of Astronomy, University of Michigan will be used for the line_5: data reduction and analysis. The ground-based observations necessary line_6: for the calibration will be obtained through the use of the facilities line_7: at Kitt Peak National Observatory and at the Michigan-Dartmouth-MIT line_8: McGraw-Hill Observatory. ! !end of general form text general_form_address: lname: Da Costa fname: Gary mi: S. category: PI inst: Anglo-Australian Observatory addr_1: Anglo-Australian Observatory addr_2: Epping Laboratory addr_3: P.O. Box 296 city: Epping, NSW zip: 2121 country: Australia phone: ISD 61 2 372 4800 telex: gdc@aaoepp.aao.gov.au ! lname: category: CON ! ! end of general_form_address records fixed_targets: targnum: 1 name_1: ANDROMEDA-I descr_1: E,308 pos_1: RA = 00H 45M 41.166S +/- 0.1S, pos_2: DEC = +38D 02' 19.63" +/- 1.0" equinox: 2000.0 fluxnum_1: 1 fluxval_1: V=26.1,TYPE=A0 fluxnum_2: 2 fluxval_2: V=21.6,TYPE=G0 ! ! end of fixed targets ! No solar system records found ! No generic target records found exposure_logsheet: linenum: 1.000 sequence_1: DEFINE sequence_2: VS targname: ANDROMEDA-I config: WFPC2 opmode: IMAGE aperture: PC1-FIX sp_element: F555W num_exp: 1 time_per_exp: 100S s_to_n: 14 fluxnum_1: 2 priority: 1 param_1: CR-SPLIT=NO req_1: CYCLE 4; ! linenum: 2.000 sequence_1: DEFINE sequence_2: VL targname: ANDROMEDA-I config: WFPC2 opmode: IMAGE aperture: PC1-FIX sp_element: F555W num_exp: 1 time_per_exp: 1800S s_to_n: 7 fluxnum_1: 1 priority: 1 param_1: CR-SPLIT=NO req_1: CYCLE 4; ! linenum: 3.000 sequence_1: DEFINE sequence_2: BS targname: ANDROMEDA-I config: WFPC2 opmode: IMAGE aperture: PC1-FIX sp_element: F450W num_exp: 1 time_per_exp: 140S s_to_n: 11.5 fluxnum_1: 2 priority: 1 param_1: CR-SPLIT=NO req_1: CYCLE 4; ! linenum: 4.000 sequence_1: DEFINE sequence_2: BL targname: ANDROMEDA-I config: WFPC2 opmode: IMAGE aperture: PC1-FIX sp_element: F450W num_exp: 1 time_per_exp: 1800S s_to_n: 5.5 fluxnum_1: 1 priority: 1 param_1: CR-SPLIT=NO req_1: CYCLE 4; ! linenum: 5.000 sequence_1: USE sequence_2: VS req_1: CYCLE 4; req_2: GROUP 5-29 WITHIN 7D; req_3: POS TARG 0.0,0.0 ! linenum: 6.000 sequence_1: USE sequence_2: VS req_1: CYCLE 4; req_2: POS TARG 1.0,1.0; req_3: SAME ORIENT FOR 6-29 AS 5 ! linenum: 7.000 sequence_1: USE sequence_2: VS req_1: CYCLE 4; req_2: POS TARG -1.0,-1.0 ! linenum: 8.000 sequence_1: USE sequence_2: BS req_1: CYCLE 4; req_2: POS TARG 0.0,0.0 ! linenum: 9.000 sequence_1: USE sequence_2: BS req_1: CYCLE 4; req_2: POS TARG 1.0,1.0 ! linenum: 10.000 sequence_1: USE sequence_2: BS req_1: CYCLE 4; req_2: POS TARG -1.0,-1.0 ! linenum: 11.000 sequence_1: USE sequence_2: VL req_1: CYCLE 4; req_2: POS TARG 0.0,0.0 ! linenum: 12.000 sequence_1: USE sequence_2: VL req_1: CYCLE 4; req_2: POS TARG 0.0,0.0 ! linenum: 13.000 sequence_1: USE sequence_2: VL req_1: CYCLE 4; req_2: POS TARG 0.0,0.0 ! linenum: 14.000 sequence_1: USE sequence_2: VL req_1: CYCLE 4; req_2: POS TARG 0.0,0.0 ! linenum: 15.000 sequence_1: USE sequence_2: VL req_1: CYCLE 4; req_2: POS TARG 0.943,0.943 ! linenum: 16.000 sequence_1: USE sequence_2: VL req_1: CYCLE 4; req_2: POS TARG 0.943,0.943 ! linenum: 17.000 sequence_1: USE sequence_2: VL req_1: CYCLE 4; req_2: POS TARG 0.943,0.943 ! linenum: 18.000 sequence_1: USE sequence_2: BL req_1: CYCLE 4; req_2: POS TARG 0.0,0.0 ! linenum: 19.000 sequence_1: USE sequence_2: BL req_1: CYCLE 4; req_2: POS TARG 0.0,0.0 ! linenum: 20.000 sequence_1: USE sequence_2: BL req_1: CYCLE 4; req_2: POS TARG 0.0,0.0 ! linenum: 21.000 sequence_1: USE sequence_2: BL req_1: CYCLE 4; req_2: POS TARG 0.0,0.0 ! linenum: 22.000 sequence_1: USE sequence_2: BL req_1: CYCLE 4; req_2: POS TARG 0.0,0.0 ! linenum: 23.000 sequence_1: USE sequence_2: BL req_1: CYCLE 4; req_2: POS TARG 0.0,0.0 ! linenum: 24.000 sequence_1: USE sequence_2: BL req_1: CYCLE 4; req_2: POS TARG 0.943,0.943 ! linenum: 25.000 sequence_1: USE sequence_2: BL req_1: CYCLE 4; req_2: POS TARG 0.943,0.943 ! linenum: 26.000 sequence_1: USE sequence_2: BL req_1: CYCLE 4; req_2: POS TARG 0.943,0.943 ! linenum: 27.000 sequence_1: USE sequence_2: BL req_1: CYCLE 4; req_2: POS TARG 0.943,0.943 ! linenum: 28.000 sequence_1: USE sequence_2: BL req_1: CYCLE 4; req_2: POS TARG 0.943,0.943 ! linenum: 29.000 sequence_1: USE sequence_2: BL req_1: CYCLE 4; req_2: POS TARG 0.943,0.943 ! ! end of exposure logsheet ! No scan data records found