5906( 6) - 11/21/95 09:03 - [ 1] PROPOSAL FOR HUBBLE SPACE TELESCOPE OBSERVATIONS ST ScI Use Only ID: 5906 Version: 6 Check-in Date: ********** 1.Proposal Title: Star Counts and Color--Magnitude Diagrams of Four Bright Globular Clusters in M31 ------------------------------------------------------------------------------------ 2. Scientific Category 3. Proposal For 4. Cycle Stellar Populations GO 5 ------------------------------------------------------------------------------------ 5. Investigators Contact? PI: Stephen Holland University of British Columbia CoI: Dr. Greg Fahlman University of British Columbia N CoI: Dr. Harvey Richer University of British Columbia N ------------------------------------------------------------------------------------ 6. Abstract The combination of the large field of view and the high resolution imaging capabilities of the HST Wide Field Camera makes it an ideal instrument for studying the stellar populations of globular star clusters in M31. We propose to obtain deep (V ~= 26) V- and I-band images of four M31 globular clusters to study their structural parameters and stellar populations. We will construct surface density profiles, search for evidence of collapsed cores and extended halos of escaped stars, and investigate the ellipticity and orientation of each cluster. The HST data will be combined with existing ground-based data from the same clusters to search for systematic biases in deriving structural parameters of semi-resolved objects. The stellar surface density profiles for each cluster will be compared to the surface brightness profiles determined from our ground-based observations. Color--magnitude diagrams (CMDs) will be built for each cluster to determine cluster membership, investigate horizontal branch morphology, and to obtain CMD-based metallicity estimates that can be compared with existing spectral estimates. Star counts and CMDs will be made for the field around each cluster to study population gradients in the halo of M31. ------------------------------------------------------------------------------------ 5906( 6) - 11/21/95 09:03 - [ 2] Observations Description ------------------------ We request that WFPC2 be used to image the M31 globular clusters G2, G302, G312 and G352. We request that two exposure of 1000 seconds each be made using the F555W filter and two exposures of 900 seconds each be made using the F814W filter for each target. The exposure times were selected to obtain a photometric uncertainty in (V - I) of +/- 0.07 magnitudes at V = 25, the level of the horizontal branch. The WFC CCDs have linear dimensions of 1 by 1 arcminutes each whereas the limiting radii of the globular clusters are approximately 10 to 20 arcseconds. We request that the targets be centered on the WF3 CCD so that an entire cluster can be imaged on a single CCD. This will simplify the reductions since the entire cluster will be subjected to uniform read-out conditions and preprocessing. In addition, having the entire cluster on a single CCD will reduce the geometric corrections that will need to be applied to the observed stellar positions. Precise astrometry is essential when determine ellipticities and orientations from star count data. The images from the WF1 and WF2 CCDs will be used to determine the background stellar density corrections to be applied to the globular cluster data. These background images will also be used to study stellar populations in the M31 halo. In order to minimize the overhead times needed we have selected exposure times that allow each orbit to be dedicated to exposures of a single GC using a single filter. We request that each 1000 second exposure be CR-split into two 500 second exposures and each 900 second exposure be CR-split into two 450 second exposures. This will give us four images per filter per object, sufficient to remove cosmic ray events via image combination. Should the degree of cosmic ray activity warrant it all eight exposures of each object may be used to identify cosmic rays. The field of view of each WFC CCD is sufficient that there will be no difficulty in identifying sufficient psf-stars on each CCD to perform profile-fitting photometry and the artificial star tests necessary to obtain the completeness of the star count data. Real Time Justification ----------------------- None. Calibration Justification ------------------------- None. Additional Comments ------------------- We have deep, photometrically calibrated V- and I-band images of two dozen M31 globular clusters obtained using the HRCam at CFHT. This data set includes sub -arcsecond images of G2, G302, G312 and G353 with limiting magnitudes of V ~= 24. This data is currently being analysed to derive surface brightness profiles, ellipticities, and orientations for these objects. Preliminary results for G302 were presented at the 182nd meeting of the AAS (Holland et al. 1993, BAAS, 25, 885). We have attempted to fit seeing-convolved Michie--King models to these globular clusters. Unfortunately Monte-Carlo simulations (Holland et al., in progress) show that seeing has a significant affect on the observed structure of an M31 globular cluster over the entire cluster. We have found that one-dimensional seeing convolution systematically overestimates the core radius of a GC and that seeing effects make a significant contribution to the observed ellipticity out to approximately 8 times the seeing FWHM. To overcome this problem we have developed techniques for modelling semi-resolved clusters that account for seeing. HST observations of G2, G302, G312, and G353 will provide a check on these methods. Our techniques can be used to study any unresolved globular cluster cores in the WFC images as well as other semi- resolved objects such as globular clusters in the Virgo cluster or dwarf galaxies in the Coma cluster. All of the images described above have fields of view of 2 by 2 arcminutes and are located along the minor axis of M31. In addition we have deep images of five fields located along the major axis of M31. We have performed photometry of the M31 halo on these images and will combine this data with the HST data. ------------------------------------------------------------------------------------ 5906( 6) - 11/21/95 09:03 - [ 3] Data Distribution Media: 8MM Blocking Factor: 10 Ship To: PI_Address Ship Via: UPS Email: , ------------------------------------------------------------------------------------ 5906( 6) - 11/21/95 09:03 - [ 4] TARGET LIST a) Fixed Targets ------------------------------------------------------------------------------------------------------------------------------------ Tar| Target | Target | Target |Coord | Radial | Flux data No | Name | Description | Position |Eqnx | Vel. | ------------------------------------------------------------------------------------------------------------------------------------ 1 STAR-CLUS- EXT-CLUSTER, Globular RA = 0H 45M 25.21S +/- 1S, DEC = 2000 V = -8 SURF(V) = 15.5 +/- 0.2 004525+41053 Cluster, +41D 06' 22.8" +/- 3" SURF(I) = 14.6 +/- 0.2 0 E(B-V) = 0.08 +/- 0.01 E(V-I) = 0.11 +/- 0.01 2 STAR-CLUS- EXT-CLUSTER, Globular RA = 0H 45M 58.8S +/- 1S, DEC = 2000 V = -352 SURF(V) = 15.9 +/- 0.2 004559+40423 Cluster, +40D 42' 32.0" +/- 3" SURF(I) = 14.8 +/- 0.2 2 E(B-V) = 0.08 +/- 0.01 E(V-I) = 0.11 +/- 0.01 5906( 6) - 11/21/95 09:03 - [ 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 ------------------------------------------------------------------------------------------------------------------------------------ 11 STAR-CLUS- WFPC2 IMAGE WF3-FIX F814W ATD-GAIN = 7 CR 3 500S 004525+4105 -SPLIT = NO 30 ------------------------------------------------------------------------------------------------------------------------------------ 12 STAR-CLUS- WFPC2 IMAGE WF3-FIX F814W ATD-GAIN = 7 CR 1 400S 004525+4105 -SPLIT = NO 30 ------------------------------------------------------------------------------------------------------------------------------------ 13 STAR-CLUS- WFPC2 IMAGE WF3-FIX F814W ATD-GAIN = 7 CR 4 500S 004525+4105 -SPLIT = NO 30 ------------------------------------------------------------------------------------------------------------------------------------ 14 STAR-CLUS- WFPC2 IMAGE WF3-FIX F814W ATD-GAIN = 7 CR 1 160S 004525+4105 -SPLIT = NO 30 ------------------------------------------------------------------------------------------------------------------------------------ 15 STAR-CLUS- WFPC2 IMAGE WF3-FIX F555W ATD-GAIN = 7 CR 4 500S 004525+4105 -SPLIT = NO 30 ------------------------------------------------------------------------------------------------------------------------------------ 16 STAR-CLUS- WFPC2 IMAGE WF3-FIX F555W ATD-GAIN = 7 CR 1 160S 004525+4105 -SPLIT = NO 30 ------------------------------------------------------------------------------------------------------------------------------------ 17 STAR-CLUS- WFPC2 IMAGE WF3-FIX F555W ATD-GAIN = 7 CR 4 500S 004525+4105 -SPLIT = NO 30 ------------------------------------------------------------------------------------------------------------------------------------ 18 STAR-CLUS- WFPC2 IMAGE WF3-FIX F555W ATD-GAIN = 7 CR 1 160S 004525+4105 -SPLIT = NO 30 ------------------------------------------------------------------------------------------------------------------------------------ 5906( 6) - 11/21/95 09:03 - [ 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 ------------------------------------------------------------------------------------------------------------------------------------ 21 STAR-CLUS- WFPC2 IMAGE WF3-FIX F814W ATD-GAIN = 7 CR 3 500S 004559+4042 -SPLIT = NO 32 ------------------------------------------------------------------------------------------------------------------------------------ 22 STAR-CLUS- WFPC2 IMAGE WF3-FIX F814W ATD-GAIN = 7 CR 1 400S 004559+4042 -SPLIT = NO 32 ------------------------------------------------------------------------------------------------------------------------------------ 23 STAR-CLUS- WFPC2 IMAGE WF3-FIX F814W ATD-GAIN = 7 CR 4 500S 004559+4042 -SPLIT = NO 32 ------------------------------------------------------------------------------------------------------------------------------------ 24 STAR-CLUS- WFPC2 IMAGE WF3-FIX F814W ATD-GAIN = 7 CR 1 160S 004559+4042 -SPLIT = NO 32 ------------------------------------------------------------------------------------------------------------------------------------ 25 STAR-CLUS- WFPC2 IMAGE WF3-FIX F555W ATD-GAIN = 7 CR 4 500S 004559+4042 -SPLIT = NO 32 ------------------------------------------------------------------------------------------------------------------------------------ 26 STAR-CLUS- WFPC2 IMAGE WF3-FIX F555W ATD-GAIN = 7 CR 1 160S 004559+4042 -SPLIT = NO 32 ------------------------------------------------------------------------------------------------------------------------------------ 27 STAR-CLUS- WFPC2 IMAGE WF3-FIX F555W ATD-GAIN = 7 CR 4 500S 004559+4042 -SPLIT = NO 32 ------------------------------------------------------------------------------------------------------------------------------------ 28 STAR-CLUS- WFPC2 IMAGE WF3-FIX F555W ATD-GAIN = 7 CR 1 160S 004559+4042 -SPLIT = NO 32 ------------------------------------------------------------------------------------------------------------------------------------ 5906( 6) - 11/21/95 09:03 - [ 7] Summary Form for Proposal 5906 Item Used in this proposal ------------------------------------------------------------------------------------------------------------------------------------ Configurations WFPC2 ------------------------------------------------------------------------------------------------------------------------------------ Opmodes IMAGE ------------------------------------------------------------------------------------------------------------------------------------ Optional Parameters ATD-GAIN = 7 CR-SPLIT = NO ------------------------------------------------------------------------------------------------------------------------------------ Proposal Category GO ------------------------------------------------------------------------------------------------------------------------------------ Scientific Category Stellar Populations ------------------------------------------------------------------------------------------------------------------------------------ Spectral Elements F814W F555W ------------------------------------------------------------------------------------------------------------------------------------ Target Names STAR-CLUS-004525+410530 STAR-CLUS-004559+404232 ------------------------------------------------------------------------------------------------------------------------------------