8696( 2) - 01/28/00 09:26 - [ 1] PROPOSAL FOR HUBBLE SPACE TELESCOPE OBSERVATIONS ST ScI Use Only ID: 8696 Version: 2 Check-in Date: 28-Jan-2000 09:25:56 1.Proposal Title: Disentangling the Bulge and NGC 6528 - a proper motion study ------------------------------------------------------------------------------------ 2. Proposal For 3. Cycle GO 9 ------------------------------------------------------------------------------------ 4. Investigators Contact? PI: Sofia Feltzing Lund University CoI: Dr Rachel Johnson Institute of Astronomy N ------------------------------------------------------------------------------------ 5. Abstract We propose to obtain second epoch WFPC2 observations of the Galactic globular cluster NGC 6528. These will be used to obtain proper motions of the cluster relative to the background bulge field and thus provide, for the first time, a colour-magnitude diagram of NGC 6528 free from background stars. NGC 6528 is perhaps the most metal-rich globular cluster known and as such serves as an invaluable template for calibrating and understanding metal-rich stellar populations in extra- galactic bulges and disks, as well as serving as a template in studies of our own Galactic Bulge. Observations of this cluster are, however, severely hampered by it being superimposed on and physically close to the Galactic Bulge itself, resulting in a colour-magnitude diagram made up of Bulge and cluster stars. Thus the images currently available in the HST-archive are of limited, if any, use as a template for metal-rich stellar populations. Proper-motion studies of globular clusters, eg. NGC 6379, have proved extremely efficient in cleaning the CMD from field stars. ------------------------------------------------------------------------------------ 8696( 2) - 01/28/00 09:26 - [ 2] Observations Description ------------------------ The proposed observations will provide a second epoch of observations of NGC 6528 from which proper motions will be derived. It is therefore extremely important that both position and roll angle are identical to those of the old archive observations, GO 5436 (PI Ortolani). GO 5436 integrated for 2*50 seconds in F814W, 2*100 seconds in F555W, and a total 1600 seconds in F336. Single short exposures in F555W and F814W were also obtained. These integrations times resulted in a nominal S/N ratio of ~ 10 for the faintest stars (V~ 24). In order to achieve the greatest possible scientific output from our proposed observations we ask for dithered observations in F555W to increase the spatial sampling to reduce the error in the derived positions in the second epoch to a minimum. Leaving the first epoch observations to provide the dominating errors in the positioning. The best way for such a dither is a boxed patter. With CR-split exposures of 200s length at each corner this results in a total exposure time, including overheads, of 29 minutes. In addition we ask for a 100s CR-split observation in F814W to improve on the photometry in F814W and furthermore a short expousre in F555W to obtain proper motions for the brightest stars. This gives a grand total of 53-54 minutes which just fits in to one orbit. For the astrometry we intend to use the same strategy as King et al. (1998). The aim here is to disentangle two populations of stars with different kinematics. The method of King et al. may be summarized as follows. For each star measured in the first epoch one use the positions of the nearest ~10 stars that are suspected cluster members are used to provide the reference frame. Then the second frame is searched for the star. If it is a cluster member it will appear where expected, if not it will be displaced. In this way the cluster proper motions will be centered on 0, 0 and the Bulge stars displaced. The procedure is continued and iterated. Real Time Justification ----------------------- None Calibration Justification ------------------------- None Additional Comments ------------------- ------------------------------------------------------------------------------------ 8696( 2) - 01/28/00 09:26 - [ 3] TARGET LIST Fixed Targets ------------------------------------------------------------------------------------------------------------------------------------ Tar| Target | Target | Target |Coord | Radial | Flux data No | Name | Description | Position |Eqnx | Vel. | ------------------------------------------------------------------------------------------------------------------------------------ 1 NGC6528 STELLAR CLUSTER, RA=18H 4M 49.59S +/- 10S, DEC=- J2000 TYPE=G3 GLOBULAR CLUSTER 30D 3' 17.62" +/- 10", PLATE- E(B-V) = 0.62 ID=067O B-V = 1.53 V = 16.13 8696( 2) - 01/28/00 09:26 - [ 4] Patterns ------------------------------------------------------------------------------------------------------------------------------------ Pattern_Number: 1 Primary_Pattern Secondary_Pattern Pattern_Type WFPC2-BOX Pattern_Purpose DITHER Number_Of_Points 4 Point_Spacing 0.559017 Coordinate_Frame POS-TARG Pattern_Orient 26.565051 Center_Pattern NO ------------------------------------------------------------------------------------------------------------------------------------ 8696( 2) - 01/28/00 09:26 - [ 5] Visit: 01 Visit Priority: Visit Requirements: ORIENT 260D TO 271D On Hold Comments: Additional Comments: We have specified the roll angle which will maximize the scientific output of the observations. It is possible to observe also at 350 to 1 degrees or 170 to 181 degrees however the scientific output would be much diminished. However, if the observations are impossible to schedule in the window defined by 260 to 271 degrees roll angle we can also accept either of two alteratives defined above. However, 260 to 271 degrees are strongly preferred. Exposures ------------------------------------------------------------------------------------------------------------------------------------ Exposure| Target |Instr | Oper. | Aper |Spectral|Central| Optional |Num| Time | Special Number | Name |Config| Mode |or FOV |Element |Waveln.| Parameters |Exp| | Requirements ------------------------------------------------------------------------------------------------------------------------------------ 1 NGC6528 WFPC2 IMAGE WFALL F555W CR-SPLIT=0.5 1 200 S PATTERN 1 1 ------------------------------------------------------------------------------------------------------------------------------------ 2 NGC6528 WFPC2 IMAGE WFALL F555W 1 15 S ------------------------------------------------------------------------------------------------------------------------------------ 4 NGC6528 WFPC2 IMAGE WFALL F814W CR-SPLIT=0.5 1 100 S ------------------------------------------------------------------------------------------------------------------------------------ 8696( 2) - 01/28/00 09:26 - [ 6] Summary Form for Proposal 8696 Item Used in this proposal ------------------------------------------------------------------------------------------------------------------------------------ Apertures WFALL ------------------------------------------------------------------------------------------------------------------------------------ Configurations WFPC2 ------------------------------------------------------------------------------------------------------------------------------------ Opmodes IMAGE ------------------------------------------------------------------------------------------------------------------------------------ Optional Parameters CR-SPLIT=0.5 ------------------------------------------------------------------------------------------------------------------------------------ Proposal Category GO ------------------------------------------------------------------------------------------------------------------------------------ Special Requirements ORIENT 260D TO 271D PATTERN 1 1 ------------------------------------------------------------------------------------------------------------------------------------ Spectral Elements F555W F814W ------------------------------------------------------------------------------------------------------------------------------------ Target Names NGC6528 ------------------------------------------------------------------------------------------------------------------------------------