5916( 7) - 11/27/95 12:02 - [ 1] PROPOSAL FOR HUBBLE SPACE TELESCOPE OBSERVATIONS ST ScI Use Only ID: 5916 Version: 7 Check-in Date: ********** 1.Proposal Title: The Oldest Globular Clusters in the Large Magellanic Cloud ------------------------------------------------------------------------------------ 2. Scientific Category 3. Proposal For 4. Cycle Stellar Populations GO 5 ------------------------------------------------------------------------------------ 5. Investigators Contact? PI: Nicholas B. Suntzeff Cerro Tololo Inter-American Observatory CoI: Paul W. Hodge Department of Astronomy, FM-20,University N of Washington,Seattle, WA 98195 CoI: Mario L. Mateo Department of Astronomy,834 N Dennnison,University of Michigan,Ann CoI: Edward W. Olszewski Steward Observatory,University of N Arizona,Tucson, AZ 85721 CoI: Robert A. Schommer Cerro Tololo Inter-American Observatory- N NOAO,P.O. Box 26732,Tucson AZ 85726 CoI: Alistair R. Walker Cerro Tololo Inter-American Observatory- N NOAO,P.O. Box 26732,Tucson AZ 85726 ------------------------------------------------------------------------------------ 6. Abstract We propose to obtain deep V(F555W) and I(F814W) images of a sample of the metal-poor globular clusters in the Large Magellanic Cloud (LMC). With these observations we will be able to measure the ages of the clusters, which will allow us to place the formation epoch of the initial population of stars in the LMC in the general context of the formation of the Milky Way Galaxy. Specifically, there is mounting data that the Milky Way Galaxy contains two distinct populations of old metal-poor stars: the initial population is a flattened disk with a small amount of rotation that dominates the Population II in the solar neighborhood; and a second population of the same metallicity, which forms a non-rotating spherical halo with an R**-3.5 density law, and is apparently a few Gyrs younger than the first population. The former population is perhaps associated with a spherical collapse model (Eggen, Lynden-Bell, & Sandage 1962) and the latter with a slower cellular accretion model of Searle & Zinn (1978). If the LMC did not form via an ELS process at the same epoch as our Galaxy, then we might expect that the oldest LMC clusters are younger than the oldest components of our Galaxy. There is some evidence for this derived from ground-based photometry for a few of the LMC clusters. We propose to test whether formation of Pop II LMC clusters was indeed delayed by a few Gyr, and in particular to test whether there is coevality between the (mean) ages of the LMC Pop II clusters and clusters in the outer halo of our Galaxy. ------------------------------------------------------------------------------------ 5916( 7) - 11/27/95 12:02 - [ 2] Observations Description ------------------------ We propose to observe 6 old LMC clusters with the WFPC2 in the HST F555W and F814W (VI equivalent) filters. We request CVZ observations, since a complete set of observations of a single cluster can be fit into the 96 minute viewing window. By squeezing in one cluster per orbit with only a single filter change during an orbit, we can use HST at a very high efficiency. We will observe each cluster as: 3x500s + 2x30s (V) and 3x600s + 3x20s(I). The observations will be taken with the clusters centered on the PC. The extreme crowding in the cluster+field requires images which are not undersampled. We have calculated the estimated exposure times as follows: The average magnitude for the LMC horizontal branch is ~19.3 (Suntzeff, etal 1992, Table 6). The difference in magnitude between the horizontal branch and the main-sequence turnoff for the average Galactic globular cluster is Delta V^TO_HB=3.55+/-0.09 (Buonanno etal 1989). We estimate that we must reach 2.5 magnitudes below the turnoff with ~0.1 mag accuracy in (V-I) to measure a reliable age, implying an error of about 0.07 in each color at V=25.3. Using the data in the WFPC2 Handbook (version 2.0 1994), and assuming F0 spectral type, V_sky=22.0, read noise of 5.2e-, dark of 0.016e- s^-1, quantization noise equivalent of 3.3e-, and and average sharpness factor of 0.05 for the PC (Table 6.4), we find we will reach S/N=6 in 600s in I and and S/N=8 in 500s in V. With 3 exposures per color, we will reach an error of 0.12 mag in (V-I), and 0.07 in V. According to Table 2.2, with 7e- DN^-1, a 500s exposure with the PC will saturate at V~18.4. To allow a tie-in with ground-based observations, we will also obtain short exposures of 2x30s in V and 3x20s in I which will saturate at about V~15.5. The shorter exposures will allow us to construct a cmd of the whole red giant branch region, and give us candidate stars for future spectroscopic work. (Even the brightest stars in these crowded and compact clusters cannot be measure reliably from ground based telescopes.) We have used the WFPC2 Handbook to calculate read times and exposures. If we instead use the information in the Phase I proposal instructions, the total time per cluster is slightly greater than one orbit. If this is the actual case, we would reduce the number of shorter exposures to fit a cluster into a single orbit. The main uncertainty in this calculation is what we should use for the effective sky brightness in the bar region of the LMC. From Bothun & Thompson (1988), the central surface brightness of the LMC is sigma_B = 21.2 with color B-V = 0.52. The implied surface brightness of sigma_V = 20.7 must be a significant overestimate for our purposes, because it is only the faint stellar component below V~25 which will appear as a ``diffuse'' sky. We can roughly estimate the effective surface brightness of the stars fainter than ~25 by merging the deep luminosity function for LMC field stars (at 4 degrees NW of the LMC center; Butcher 1977) with the shallower luminosity function of the LMC bar (1.4 degrees from the center ; Hardy etal 1984). We extrapolate to V=25.5 and then calculate the fraction of the surface brightness in the bar regions which are due to stars fainter than this limit. Our worst case estimate for the surface brightness for the faint stars is sigma_V~21.8 in the central regions measured by Bothun & Thompson, and at worse sigma_V~22.3 in the regions at 0.5-1 degrees, which are the typical positions of the inner LMC clusters under consideration here. The natural sky background is sigma_V~23 (Table 6.3 in the WFPC2 Handbook). If we combine both the natural and LMC faint star background, we end up with sigma_V~22. Real Time Justification ----------------------- The CVZ requirement is justified in the previous section. We also request that the telescope be ``jogged'' by a small amount between all exposures, to reduce the photometric errors in the WF images where there can be real sensitivity variations across a pixel. We understand that this is possible with no additional loss in observing time. ARW and NBS have been granted 4 nights of observing time with the CTIO 1.5m telescope and imaging CCD in Jan 1995 to being a search for variable stars in the clusters SL244, N1754, N1865, N1898, N1916, N1928, N1939, N2005, and N2019. We had planned to use 4min V and 6min B (Johnson system) exposures on each cluster, cycling through all the clusters for 4 nights (plus photometric standards, and a few Galactic globular clusters for tie-in). However, if we are awarded HST time, we will instead incorporate the CTIO set of F555W (HST V equivalent) and F814W (HST I equivalent) filters into our scheduled ground-based observations and observe in B(Johnson)V(F555W)I(F814W). We have bought the FW555 and FW814 filters for CTIO as part of our HST project 5423 (The Tucana Dwarf Spheroidal Galaxy). We would use the 1.5m images as direct calibrations for the LMC cluster images, and as a direct tie-in with Galactic globular clusters measured with our HST equivalent filters. It is our goal to observe ALL the old clusters in the LMC to measure ages. The outer clusters (Hodge 11, NGC 1466, NGC 1841, NGC 2210, NGC 2257, and Reticulum) are in uncrowded regions of the LMC, and ground- based photometry in good seeing can be used to reach V~25. NGC 2257 has been observed on the ESO NNT telescope by Ferraro, etal (1994, preprint) in BV colors, and they find an age several Gyr younger for the cluster than a typical Galactic globular. The VI observations will be made using the CTIO HST filter (F555W and F814W) to provide a consistent system of colors for all the old LMC colors. Calibration Justification ------------------------- Additional Comments ------------------- ------------------------------------------------------------------------------------ 5916( 7) - 11/27/95 12:02 - [ 3] Data Distribution Media: 8MM Blocking Factor: 10 Ship To: Dr. Nicholas B. Suntzeff*NOAO*950 North Cherry Avenue*Tucson, AZ 85719 Ship Via: UPS Email: nsuntzeff@noao.edu , ------------------------------------------------------------------------------------ 5916( 7) - 11/27/95 12:02 - [ 4] TARGET LIST a) Fixed Targets ------------------------------------------------------------------------------------------------------------------------------------ Tar| Target | Target | Target |Coord | Radial | Flux data No | Name | Description | Position |Eqnx | Vel. | ------------------------------------------------------------------------------------------------------------------------------------ 1 NGC1754 EXT-CLUSTER, GLOBULAR RA=4H 54M 18.6S +/- 1.5S, DEC=- J2000 V=+236 V=15.5+/-0.5 B-V=1.5+/-0.5 E(B-V)=0.08+/ CLUSTER 70D 26' 32.1" +/- 8" -0.03 SURF-BKG(V)=22+/-1 Comments: BV MAGNITUDES ARE ESTIMATED FOR BRIGHTEST CLUSTER MEMBERS. LMC FIELD STARS MAY BE AS BRIGHT AS V=12.5. BRIGHT STAR WITH V=11.0 and V-I=0.9 AT (26"S,19"E) OR PA= 144D (N INTO E) AT R= 32.2" WITH RESPECT TO CLUSTER CENTER. THESE ARE APPROXIMATE COORDINATES, AND NOT FINAL GSSS COORDS. 2 NGC1835 EXT-CLUSTER, GLOBULAR RA=5H 05M 06.4S +/- 1.5S, DEC=- J2000 V=+188 V=15.5+/-0.5 B-V=1.5+/-0.5 E(B-V)=0.13+/ CLUSTER 69D 24' 16.5" +/- 8" -0.03 SURF-BKG(V)=22+/-1 Comments: BV MAGNITUDES ARE ESTIMATED FOR BRIGHTEST CLUSTER MEMBERS. LMC BAR FIELD STARS MAY BE AS BRIGHT AS V=12.5. ANY ORIENTATION IS OKAY. THESE ARE APPROXIMATE COORDINATES, AND NOT FINAL GSSS COORDS. 3 NGC1898 EXT-CLUSTER, GLOBULAR RA=5H 16M 42.5S +/- 1.5S, DEC= J2000 V=+210 V=15.5+/-0.5 B-V=1.5+/-0.5 E(B-V)=0.06+/ CLUSTER -69D 39' 22.5" +/- 8" -0.03 SURF-BKG(V)=22+/-1 Comments: BV MAGNITUDES ARE ESTIMATED FOR BRIGHTEST CLUSTER MEMBERS. LMC BAR FIELD STARS MAY BE AS BRIGHT AS V=12.5. MANY BRIGHT STARS IN FIELD. ORIENT SO THAT CENTER OF PC1 TO CORNER OF WF3 IS POINTING N. THESE ARE APPROXIMATE COORDINATES, AND NOT FINAL GSSS COORDS. 4 NGC1916 EXT-CLUSTER, GLOBULAR RA=5H 18M 37.66S +/- 1.5S, J2000 V=+278 V=15.5+/-0.5 B-V=1.5+/-0.5 E(B-V)=0.18+/ CLUSTER DEC=-69D 24' 25.6" +/- 8" -0.03 SURF-BKG(V)=22+/-1 Comments: BV MAGNITUDES ARE ESTIMATED FOR BRIGHTEST CLUSTER MEMBERS. LMC BAR FIELD STARS MAY BE AS BRIGHT AS V=12.5. BRIGHT STAR AT 135"W OF CLUSTER CENTER WITH V=14. ANY ORIENTATION IS OKAY. THESE ARE APPROXIMATE COORDINATES, AND NOT FINAL GSSS COORDS. 5 NGC2005 EXT-CLUSTER, GLOBULAR RA=5H 30M 10.4S +/- 1.5S, DEC=- J2000 V=+270 V=15.5+/-0.5 B-V=1.5+/-0.5 E(B- CLUSTER 69D 45' 10.1" +/- 8" V)=0.012+/-0.03 SURF-BKG(V)=22+/-1 Comments: BV MAGNITUDES ARE ESTIMATED FOR BRIGHTEST CLUSTER MEMBERS. LMC BAR FIELD STARS MAY BE AS BRIGHT AS V=12.5. VERY BRIGHT STAR AT (120"S,20"W) OR PA=189D AND R=121". THESE ARE APPROXIMATE COORDINATES, AND NOT FINAL GSSS COORDS. 6 NGC2019 EXT-CLUSTER, GLOBULAR RA=5H 31M 56.8S +/- 1.5S, DEC= J2000 V=+269 V=15.5+/-0.5 B-V=1.5+/-0.5 E(B-V)=0.12+/ CLUSTER -70D 09' 34.0" +/- 8" -0.03 SURF-BKG(V)=22+/-1 Comments: BV MAGNITUDES ARE ESTIMATED FOR BRIGHTEST CLUSTER MEMBERS. LMC BAR FIELD STARS MAY BE AS BRIGHT AS V=12.5. ANY ORIENTATION IS OKAY. THESE ARE APPROXIMATE COORDINATES, AND NOT FINAL GSSS COORDS. 5916( 7) - 11/27/95 12:02 - [ 5] Visit: 01 Visit Requirements: CVZ On Hold Comments: Additional Comments: WOULD LIKE TO ORIENT 109D TO 179D, BUT NO CVZ ZONE THEN. Exposures ------------------------------------------------------------------------------------------------------------------------------------ Exposure| Target |Instr | Oper. | Aper |Spectral|Central| Optional |Num| Time | Special Number | Name |Config| Mode |or FOV |Element |Waveln.| Parameters |Exp| | Requirements ------------------------------------------------------------------------------------------------------------------------------------ 101 NGC1754 WFPC2 IMAGE PC1-FIX F555W CLOCKS=NO 2 20S ------------------------------------------------------------------------------------------------------------------------------------ 102 NGC1754 WFPC2 IMAGE PC1-FIX F555W CLOCKS=NO 3 500S ------------------------------------------------------------------------------------------------------------------------------------ 103 NGC1754 WFPC2 IMAGE PC1-FIX F814W CLOCKS=NO 3 20S ------------------------------------------------------------------------------------------------------------------------------------ 104 NGC1754 WFPC2 IMAGE PC1-FIX F814W CLOCKS=NO 3 600S ------------------------------------------------------------------------------------------------------------------------------------ 5916( 7) - 11/27/95 12:02 - [ 6] Visit: 02 Visit Requirements: CVZ On Hold Comments: Additional Comments: ANY ORIENTATION IS OKAY Exposures ------------------------------------------------------------------------------------------------------------------------------------ Exposure| Target |Instr | Oper. | Aper |Spectral|Central| Optional |Num| Time | Special Number | Name |Config| Mode |or FOV |Element |Waveln.| Parameters |Exp| | Requirements ------------------------------------------------------------------------------------------------------------------------------------ 201 NGC1835 WFPC2 IMAGE PC1-FIX F555W CLOCKS=NO 2 20S ------------------------------------------------------------------------------------------------------------------------------------ 202 NGC1835 WFPC2 IMAGE PC1-FIX F555W CLOCKS=NO 3 500S ------------------------------------------------------------------------------------------------------------------------------------ 203 NGC1835 WFPC2 IMAGE PC1-FIX F814W CLOCKS=NO 3 20S ------------------------------------------------------------------------------------------------------------------------------------ 204 NGC1835 WFPC2 IMAGE PC1-FIX F814W CLOCKS=NO 3 600S ------------------------------------------------------------------------------------------------------------------------------------ 5916( 7) - 11/27/95 12:02 - [ 7] Visit: 03 Visit Requirements: CVZ ORIENTation 320D to 40D On Hold Comments: Additional Comments: ORIENT -40D to +40D Exposures ------------------------------------------------------------------------------------------------------------------------------------ Exposure| Target |Instr | Oper. | Aper |Spectral|Central| Optional |Num| Time | Special Number | Name |Config| Mode |or FOV |Element |Waveln.| Parameters |Exp| | Requirements ------------------------------------------------------------------------------------------------------------------------------------ 301 NGC1898 WFPC2 IMAGE PC1-FIX F555W CLOCKS=NO 2 20S ------------------------------------------------------------------------------------------------------------------------------------ 302 NGC1898 WFPC2 IMAGE PC1-FIX F555W CLOCKS=NO 3 500S ------------------------------------------------------------------------------------------------------------------------------------ 303 NGC1898 WFPC2 IMAGE PC1-FIX F814W CLOCKS=NO 3 20S ------------------------------------------------------------------------------------------------------------------------------------ 304 NGC1898 WFPC2 IMAGE PC1-FIX F814W CLOCKS=NO 3 600S ------------------------------------------------------------------------------------------------------------------------------------ 5916( 7) - 11/27/95 12:02 - [ 8] Visit: 04 Visit Requirements: CVZ On Hold Comments: Additional Comments: ANY ORIENTATION OKAY Exposures ------------------------------------------------------------------------------------------------------------------------------------ Exposure| Target |Instr | Oper. | Aper |Spectral|Central| Optional |Num| Time | Special Number | Name |Config| Mode |or FOV |Element |Waveln.| Parameters |Exp| | Requirements ------------------------------------------------------------------------------------------------------------------------------------ 401 NGC1916 WFPC2 IMAGE PC1-FIX F555W CLOCKS=NO 2 20S ------------------------------------------------------------------------------------------------------------------------------------ 402 NGC1916 WFPC2 IMAGE PC1-FIX F555W CLOCKS=NO 3 500S ------------------------------------------------------------------------------------------------------------------------------------ 403 NGC1916 WFPC2 IMAGE PC1-FIX F814W CLOCKS=NO 3 20S ------------------------------------------------------------------------------------------------------------------------------------ 404 NGC1916 WFPC2 IMAGE PC1-FIX F814W CLOCKS=NO 3 600S ------------------------------------------------------------------------------------------------------------------------------------ 5916( 7) - 11/27/95 12:02 - [ 9] Visit: 05 Visit Requirements: CVZ On Hold Comments: Additional Comments: WOULD LIKE TO ORIENT 149D TO 229D, BUT NO CVZ ZONE THEN Exposures ------------------------------------------------------------------------------------------------------------------------------------ Exposure| Target |Instr | Oper. | Aper |Spectral|Central| Optional |Num| Time | Special Number | Name |Config| Mode |or FOV |Element |Waveln.| Parameters |Exp| | Requirements ------------------------------------------------------------------------------------------------------------------------------------ 501 NGC2005 WFPC2 IMAGE PC1-FIX F555W CLOCKS=NO 2 20S ------------------------------------------------------------------------------------------------------------------------------------ 502 NGC2005 WFPC2 IMAGE PC1-FIX F555W CLOCKS=NO 3 500S ------------------------------------------------------------------------------------------------------------------------------------ 503 NGC2005 WFPC2 IMAGE PC1-FIX F814W CLOCKS=NO 3 20S ------------------------------------------------------------------------------------------------------------------------------------ 504 NGC2005 WFPC2 IMAGE PC1-FIX F814W CLOCKS=NO 3 600S ------------------------------------------------------------------------------------------------------------------------------------ 5916( 7) - 11/27/95 12:02 - [ 10] Visit: 06 Visit Requirements: CVZ On Hold Comments: Additional Comments: ANY ORIENTATION OKAY Exposures ------------------------------------------------------------------------------------------------------------------------------------ Exposure| Target |Instr | Oper. | Aper |Spectral|Central| Optional |Num| Time | Special Number | Name |Config| Mode |or FOV |Element |Waveln.| Parameters |Exp| | Requirements ------------------------------------------------------------------------------------------------------------------------------------ 601 NGC2019 WFPC2 IMAGE PC1-FIX F555W CLOCKS=NO 2 20S ------------------------------------------------------------------------------------------------------------------------------------ 602 NGC2019 WFPC2 IMAGE PC1-FIX F555W CLOCKS=NO 3 500S ------------------------------------------------------------------------------------------------------------------------------------ 603 NGC2019 WFPC2 IMAGE PC1-FIX F814W CLOCKS=NO 3 20S ------------------------------------------------------------------------------------------------------------------------------------ 604 NGC2019 WFPC2 IMAGE PC1-FIX F814W CLOCKS=NO 3 600S ------------------------------------------------------------------------------------------------------------------------------------ 5916( 7) - 11/27/95 12:02 - [ 11] Summary Form for Proposal 5916 Item Used in this proposal ------------------------------------------------------------------------------------------------------------------------------------ Configurations WFPC2 ------------------------------------------------------------------------------------------------------------------------------------ Opmodes IMAGE ------------------------------------------------------------------------------------------------------------------------------------ Optional Parameters CLOCKS=NO ------------------------------------------------------------------------------------------------------------------------------------ Proposal Category GO ------------------------------------------------------------------------------------------------------------------------------------ Scientific Category Stellar Populations ------------------------------------------------------------------------------------------------------------------------------------ Special Requirements CVZ CVZ ORIENTation 320D to 40D ------------------------------------------------------------------------------------------------------------------------------------ Spectral Elements F555W F814W ------------------------------------------------------------------------------------------------------------------------------------ Target Names NGC1754 NGC1835 NGC1898 NGC1916 NGC2005 NGC2019 ------------------------------------------------------------------------------------------------------------------------------------