6604( 8) - 03/20/96 10:57 - [ 1] PROPOSAL FOR HUBBLE SPACE TELESCOPE OBSERVATIONS ST ScI Use Only ID: 6604 Version: 8 Check-in Date: 20-Mar-1996 10:49:55 1.Proposal Title: Photometry of the Oldest Field Populations in the Magellanic Clouds ------------------------------------------------------------------------------------ 2. Scientific Category 3. Proposal For 4. Cycle STELLAR POPULATIONS GO 6 ------------------------------------------------------------------------------------ 5. Investigators Contact? PI: Alistair Walker Cerro Tololo Inter-American Observatory CoI: Paul Hodge Department of Astronomy, University of N Washington CoI: Mario Mateo Department of Astronomy, University of N Michigan CoI: Edward Olszewski Steward Observatory, University of Arizona N CoI: Robert Schommer Cerro Tololo Inter-American Observatory N CoI: Nicholas Suntzeff Cerro Tololo Inter-American Observatory N ------------------------------------------------------------------------------------ 6. Abstract We propose to obtain deep V (F555W) and I (F814W) images for one field in the Large Magellanic Cloud (LMC), one field in the Small Magellanic Cloud (SMC), and for the old SMC cluster NGC 121. These observations will allow us to measure ages and population densities of the oldest field stars (t > 5Gyr) in the Magellanic Clouds (MC), and place into context the ages of formation of the Galaxy and the two MC's. In our own Galaxy, the oldest nearby stars appear to belong to two subsystems: an initial population represented by a flattened spheroid with a small amount of rotation, and a non-rotating spherical population which may be a few Gyr younger than the first population. A third old population, the ``metal-weak old disk'', also may exist. In the outer regions of the MC's where we would expect to find populations analogous to the Galactic Population II, we see something much different: a field population that is dominated by stars with Fe/H~ -0.6 and t~ 8Gyr, with small admixtures of a very old metal-poor population analogous to the Galactic halo and a younger (t ~ 5Gyr) system (in the LMC). We propose to measure the ages of the oldest MC field populations and the oldest SMC cluster to compare with the ages of the oldest components of our Galactic halo and disk. The observations will be interpreted using synthetic color-magnitude diagrams and luminosity functions to develop a model for the star formation history in both the MC's over the first 10 Gyr. ------------------------------------------------------------------------------------ 6604( 8) - 03/20/96 10:57 - [ 2] Observations Description ------------------------ We propose to observe one LMC field (requires four HST pointings) and two SMC fields (one requires four pointings, the other, NGC 121, requires a single pointing with the cluster centered on the PC) with the HST WFPC2 instrument and F555W and F814W (VI equivalent) filters. We will observe each of the ``four- pointing'' fields as: 4*500s V + 4*600s I. (LMC, in some cases 3*600 + 1*500 I), 4*600s V and I (SMC). This requires two orbits per pointing, thus 16 orbits total. For the single NGC 121 field we will observe as: 4*400s + 6*40s (V) and 4*500s + 4*20s (I). This also requires 2 orbits. The grand total number of orbits requested is therefore 18. This work will complement our cluster and field star photometry in the LMC bar obtained with HST (N. Suntzeff, PI, Cycle 5 proposal 5916). We have calculated the estimated exposure times as follows: The average magnitude for the LMC horizontal branch is V ~ 19.3 and for the SMC V ~ 19.7 (Walker 1991, IAU Symposium 108). The difference in magnitude between the horizontal branch and the main-sequence turnoff for a typical Galactic globular cluster is Delta V(TO -HB) = 3.55 +/- 0.09 (Buonanno et al 1989, A&A, 216, 80). We estimate that we must reach 2.0 magnitudes below the turnoff with <0.1 accuracy in (V-I) to measure a reliable age and to allow modelling of multiple populations. Thus we need to reach V = 24.85 for the LMC and V = 25.25 for the SMC. Using the WFPC2 Exposure Time Calculator, we find we will reach S/N = 13 in 600s in I and S/N = 14 in 500s in V for V = 24.85, and S/N = 10 and 11 respectively at V = 25.25, for a main sequence MC star. With four exposures per color, we will achieve Sigma (V) = 0.035 and Sigma (V-I) = 0.055 in the LMC, and Sigma(V) = 0.045, Sigma (V-I) = 0.07 in the SMC. For NGC 121 we have slightly shorter exposures, hence Sigma (V) = 0.05, Sigma (V-I) = 0.08. According to information in the WFPC2 Instrument handbook, a 500s exposure with the F555W filter will saturate at V ~ 18.3. For the sparse MC field regions there will be no difficulty in tying stars of magnitude 18--20 into our ground- based photometry. However for the crowded NGC 121 field we will take some short exposures in order to facilitate a tie-in with the ground- based observations. We will take exposures of 6*30s in V and 4*20s in I, which will saturate at about V ~ 15.2. We will avoid a moderately bright (14 mag.) star near NGC 121. We (AW,NS,RS,EO) have been granted 4 nights on the CTIO 4-m and 4 nights on the CTIO 1.5-m with SITe 2048 direct CCD during November and December 1995 for a program entitled ``Photometric Investigation of Magellanic Cloud Field Populations''. This program is aimed at producing accurate photometry in 30x30'\\ square fields in positions containing the proposed HST fields. The areas are extensive enough to accumulate significant samples even for stars in relatively short-lived evolutionary phases, and will complement the deeper, but much smaller field, HST images. We will observe using CTIO's copies of the HST F555W and F814W filters, plus a DDO51 to identify foreground G--K dwarfs. This work will also provide local standards so that the HST and ground-based photometry can be correctly tied together. We (EO, RS, NS) have extensive observations (photometry, abundances, radial velocities) for LMC red giant branch (RGB) stars in seven one -degree fields. The most complete of these is near NGC 2257 (r = 8.4degrees), where in addition proper motions are available (Lin et al 1995). This region also has extensive photometry for the field RR Lyraes (Nemec et al., 1985; Walker 1988). The proposed HST observations will provide deeper and more accurate photometry in the magnitude range V = 22-25, at and below the MS turn-offs for older populations. This accurate photometry is crucial when trying to disentangle mixed populations that have differing ages and metallicities. As described in the scientific justification, we estimate how many stars are near the old-population turn- off (V = 21.75--22.5, B-V = 0.2--0.6) by scaling from CTIO 4m V, B-V photometry by one of us (NS). For a single WFPC2 pointing there are ~ 26 such stars. Real Time Justification ----------------------- Calibration Justification ------------------------- Additional Comments ------------------- ------------------------------------------------------------------------------------ 6604( 8) - 03/20/96 10:57 - [ 3] Data Distribution Media: 8MM Blocking Factor: 10 Ship To: PI_Address Ship Via: UPS Email: , ------------------------------------------------------------------------------------ 6604( 8) - 03/20/96 10:57 - [ 4] TARGET LIST a) Fixed Targets ------------------------------------------------------------------------------------------------------------------------------------ Tar| Target | Target | Target |Coord | Radial | Flux data No | Name | Description | Position |Eqnx | Vel. | ------------------------------------------------------------------------------------------------------------------------------------ 1 LMC-062630- Unidentified, Blank RA=06H 26M 30.0S +/- 0.1S, DEC=- 2000 V = 24 644500 Field 64D 45' 00.0" +/- 0.1", PLATE- B-V = 0.5 ID=00MS 2 LMC-062930- Unidentified, Blank RA=06H 29M 30.0S +/- 0.1S, DEC=- 2000 V = 24 640200 Field 64D 02' 00.0" +/- 0.1", PLATE- B-V = 0.5 ID=00MS 3 LMC-063600- Unidentified, Blank RA=06H 36M 00.0S +/- 0.1S, DEC=- 2000 V = 24 640600 Field 64D 06' 00.0" +/- 0.1", PLATE- B-V = 0.5 ID=00MS 4 LMC-062806- Unidentified, Blank RA=06H 28M 06.0S +/- 0.1S, DEC=- 2000 V = 24 650800 Field 65D 08' 00.0" +/- 0.1", PLATE- B-V = 0.5 ID=00MS 5 SMC-004600- Unidentified, Blank RA=00H 46M 00.0S +/- 0.1S, DEC=- 2000 V = 24 703500 Field 70D 35' 00.0" +/- 0.1", PLATE- B-V = 0.5 ID=015Q 6 SMC-004612- Unidentified, Blank RA=00H 46M 12.0S +/- 0.1S, DEC=- 2000 V = 24 704700 Field 70D 47' 00.0" +/- 0.1", PLATE- B-V = 0.5 ID=015Q 7 SMC-004900- Unidentified, Blank RA=00H 49M 00.0S +/- 0.1S, DEC=- 2000 V = 24 704800 Field 70D 48' 00.0" +/- 0.1", PLATE- B-V = 0.5 ID=015Q 8 SMC-004900- Unidentified, Blank RA=00H 49M 00.0S +/- 0.1S, DEC=- 2000 V = 24 703300 Field 70D 33' 00.0" +/- 0.1", PLATE- B-V = 0.5 ID=015Q 9 NGC121 Ext-Cluster, Globular RA=00H 26M 48.9S +/- 0.1S, DEC=- 2000 V = 24 cluster 71D 32' 12.0" +/- 0.1" B-V = 0.5 Comments: Flux refers to the faintest stars which we wish to accurately measure 6604( 8) - 03/20/96 10:57 - [ 5] Visit: 01 Visit Requirements: 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 LMC-062630- WFPC2 IMAGE PC1-FIX F555W 4 500.00S 644500 ------------------------------------------------------------------------------------------------------------------------------------ 2 LMC-062630- WFPC2 IMAGE PC1-FIX F814W 3 600.00S 644500 ------------------------------------------------------------------------------------------------------------------------------------ 3 LMC-062630- WFPC2 IMAGE PC1-FIX F814W 1 500.00S 644500 ------------------------------------------------------------------------------------------------------------------------------------ 4 LMC-062930- WFPC2 IMAGE PC1-FIX F555W 4 500.00S 640200 ------------------------------------------------------------------------------------------------------------------------------------ 5 LMC-062930- WFPC2 IMAGE PC1-FIX F814W 3 600.00S 640200 ------------------------------------------------------------------------------------------------------------------------------------ 6 LMC-062930- WFPC2 IMAGE PC1-FIX F814W 1 500.00S 640200 ------------------------------------------------------------------------------------------------------------------------------------ 6604( 8) - 03/20/96 10:57 - [ 6] Visit: 02 Visit Requirements: 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 LMC-063600- WFPC2 IMAGE PC1-FIX F555W 4 500.00S 640600 ------------------------------------------------------------------------------------------------------------------------------------ 2 LMC-063600- WFPC2 IMAGE PC1-FIX F814W 3 600.00S 640600 ------------------------------------------------------------------------------------------------------------------------------------ 3 LMC-063600- WFPC2 IMAGE PC1-FIX F814W 1 500.00S 640600 ------------------------------------------------------------------------------------------------------------------------------------ 6604( 8) - 03/20/96 10:57 - [ 7] Visit: 03 Visit Requirements: 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 LMC-062806- WFPC2 IMAGE PC1-FIX F555W 4 500.00S 650800 ------------------------------------------------------------------------------------------------------------------------------------ 2 LMC-062806- WFPC2 IMAGE PC1-FIX F814W 4 600.00S 650800 ------------------------------------------------------------------------------------------------------------------------------------ 6604( 8) - 03/20/96 10:57 - [ 8] Visit: 04 Visit Requirements: 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 SMC-004600- WFPC2 IMAGE PC1-FIX F555W 4 600.00S 703500 ------------------------------------------------------------------------------------------------------------------------------------ 2 SMC-004600- WFPC2 IMAGE PC1-FIX F814W 4 600.00S 703500 ------------------------------------------------------------------------------------------------------------------------------------ 3 SMC-004612- WFPC2 IMAGE PC1-FIX F555W 4 600.00S 704700 ------------------------------------------------------------------------------------------------------------------------------------ 4 SMC-004612- WFPC2 IMAGE PC1-FIX F814W 4 600.00S 704700 ------------------------------------------------------------------------------------------------------------------------------------ 5 SMC-004900- WFPC2 IMAGE PC1-FIX F555W 4 600.00S 704800 ------------------------------------------------------------------------------------------------------------------------------------ 6 SMC-004900- WFPC2 IMAGE PC1-FIX F814W 4 600.00S 704800 ------------------------------------------------------------------------------------------------------------------------------------ 7 SMC-004900- WFPC2 IMAGE PC1-FIX F555W 4 600.00S 703300 ------------------------------------------------------------------------------------------------------------------------------------ 8 SMC-004900- WFPC2 IMAGE PC1-FIX F814W 4 600.00S 703300 ------------------------------------------------------------------------------------------------------------------------------------ 6604( 8) - 03/20/96 10:57 - [ 9] Visit: 05 Visit Requirements: ORIENT 220D TO 330D 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 NGC121 WFPC2 IMAGE PC1-FIX F555W 6 40.00S ------------------------------------------------------------------------------------------------------------------------------------ 2 NGC121 WFPC2 IMAGE PC1-FIX F555W 4 400.00S ------------------------------------------------------------------------------------------------------------------------------------ 3 NGC121 WFPC2 IMAGE PC1-FIX F814W 4 20.00S ------------------------------------------------------------------------------------------------------------------------------------ 4 NGC121 WFPC2 IMAGE PC1-FIX F814W 4 500.00S ------------------------------------------------------------------------------------------------------------------------------------ 6604( 8) - 03/20/96 10:57 - [ 10] Summary Form for Proposal 6604 Item Used in this proposal ------------------------------------------------------------------------------------------------------------------------------------ Configurations WFPC2 ------------------------------------------------------------------------------------------------------------------------------------ Opmodes IMAGE ------------------------------------------------------------------------------------------------------------------------------------ Proposal Category GO ------------------------------------------------------------------------------------------------------------------------------------ Scientific Category STELLAR POPULATIONS ------------------------------------------------------------------------------------------------------------------------------------ Special Requirements ORIENT 220D TO 330D ------------------------------------------------------------------------------------------------------------------------------------ Spectral Elements F555W F814W ------------------------------------------------------------------------------------------------------------------------------------ Target Names LMC-062630-644500 LMC-062930-640200 LMC-063600-640600 LMC-062806-650800 SMC-004600-703500 SMC-004612-704700 SMC-004900-704800 SMC-004900-703300 NGC121 ------------------------------------------------------------------------------------------------------------------------------------