9436( 4) - 03/18/02 14:55 - [ 1] PROPOSAL FOR HUBBLE SPACE TELESCOPE OBSERVATIONS ST ScI Use Only ID: 9436 Version: 4 Check-in Date: 18-Mar-2002 14:55:00 1.Proposal Title: Proper Motions of Bulge Stars at b=-6: The Shape of the Potential in the Central kpc of the Galaxy ------------------------------------------------------------------------------------ 2. Proposal For 3. Cycle GO 11 ------------------------------------------------------------------------------------ 4. Investigators Contact? PI: Konrad Kuijken Kapteyn Astronomical Institute CoI: Prof. R. Michael Rich University of California at Los Angeles N ------------------------------------------------------------------------------------ 5. Abstract We propose to measure proper motions in a bulge field at l, b=0,-6, near the globular cluster NGC 6558. Suitable first- epoch data exist (from 1997). These data will allow us to calculate the vertical potential gradient in the bulge region, and hence the flattening of the potential in the central regions of the potential - a crucial piece of information for deciding measuring the central density of the dark halo of the Galaxy. Central halo densities currently constitute one of the most discrepant predictions of structure formation theories. In addition, the proper motions will allow us to make a kinematic separation between bulge, disk and globular cluster stars. The resulting cleaned colour-magnitude diagrams allow the bulge main sequence turnoff to be viewed cleanly, and compared with lower-latitude regions. Thus age and metallicity gradients in the bulge can be measured. ------------------------------------------------------------------------------------ 9436( 4) - 03/18/02 14:55 - [ 2] Observations Description ------------------------ Astrometry of undersampled, dense starfield images is complicated, but it can be done. An extensive description is given by Anderson and King (2000), and we have been basically following their methodology. We will reobserve the NGC6558 field of proposal 6820, taken (on this PIs birthday) in September 1997. We will once again use WFPC2, in order to avoid any possible systematic differences between the ACS and WFPC2 astrometry. This field lies some 9' from the center of this globular cluster, and is well-suited for study of the `contaminating' bulge population. Observations were taken in F606W and F814W (10.8 ksec in each filter), and were well- dithered. The only concern is the fact that the shortest exposures were 1000 sec, so the brightest stars will be saturated - this will affect stars brighter than I~eq18 (near the bulge turnoff). For somewhat brighter stars, only those falling on the PC will be useable. For our science goal this is not crucial - our main tracers are main sequence stars well below the turnoff (see fig reffig:bwpmlgrad). For proper motions, we need only reobserve the field in one passband. We will follow the strategy adopted for our succesful proposal (8250), namely a four-step dither of the field, in the I band. The I band is preferred as the PSF is best resolved, and hence centroiding accuracy highest; nevertheless dithering is essential. We will integrate somewhat deeper than we did for our cycle 9 fields, in order to match the superior first-epoch data and to take advantage of the lower star density than in our lower-latitude fields. Our total request is thus for 3 orbits. At each of the four dither positions we require four 400-sec exposures (6400sec total). This allows a 5-sigma detection for stars as faint as I=23 in each exposure, and the combined detection in all 16 images is therefore 20-sigma. Using our cycle-9 data we have found that we can centroid to an accuracy (see fig reffig:pmerror) better than 1/70th of a pixel, or 1.5mas, for stars as faint as I=22. Over an epoch separation of 5 years, this gives us better than 0.4mas/yr accuracy. Real Time Justification ----------------------- The main requirement for this work is that the pointing match the first-epoch field, both in center and orientation, closely. Calibration Justification ------------------------- Additional Comments ------------------- ------------------------------------------------------------------------------------ 9436( 4) - 03/18/02 14:55 - [ 3] TARGET LIST Fixed Targets ------------------------------------------------------------------------------------------------------------------------------------ Tar| Target | Target | Target |Coord | Radial | Flux data No | Name | Description | Position |Eqnx | Vel. | ------------------------------------------------------------------------------------------------------------------------------------ 1 NGC6558-FLD1 STELLAR CLUSTER, RA=18H 11M 5S +/- .5", DEC=-31D J2000 V = 28+/-1 GLOBULAR CLUSTER, 45' 49" +/- .5" GALAXY, BULGE Coordinate Source: HST_IMAGE Comments: Galactic Bulge field 9436( 4) - 03/18/02 14:55 - [ 4] Visit: 01 Visit Priority: Visit Requirements: ORIENT 87.6D TO 87.6D On Hold Comments: Additional Comments: All exposures are meant to repeat as closely as possible the pointing of the WFPC2 exposures of proposal 6820 (proper motions). A 4-point dither of 2.5 wf pixels offset is intended. Exposures ------------------------------------------------------------------------------------------------------------------------------------ Exposure| Target |Instr | Oper. | Aper |Spectral|Central| Optional |Num| Time | Special Number | Name |Config| Mode |or FOV |Element |Waveln.| Parameters |Exp| | Requirements ------------------------------------------------------------------------------------------------------------------------------------ 10 NGC6558-FLD WFPC2 IMAGE PC1-FIX F814W 2 500 S POS TARG -0.1,-0.1 GSPAIR 1 0739202181F20739201659F1 ------------------------------------------------------------------------------------------------------------------------------------ 15 NGC6558-FLD WFPC2 IMAGE PC1-FIX F814W 1 400 S POS TARG -.1,-.1 1 ------------------------------------------------------------------------------------------------------------------------------------ 20 NGC6558-FLD WFPC2 IMAGE PC1-FIX F814W 2 500 S POS TARG .15,-.1 1 ------------------------------------------------------------------------------------------------------------------------------------ 25 NGC6558-FLD WFPC2 IMAGE PC1-FIX F814W 1 400 S POS TARG .15,-.1 1 ------------------------------------------------------------------------------------------------------------------------------------ 30 NGC6558-FLD WFPC2 IMAGE PC1-FIX F814W 2 500 S POS TARG .15,.15 1 ------------------------------------------------------------------------------------------------------------------------------------ 35 NGC6558-FLD WFPC2 IMAGE PC1-FIX F814W 1 400 S POS TARG .15,.15 1 ------------------------------------------------------------------------------------------------------------------------------------ 40 NGC6558-FLD WFPC2 IMAGE PC1-FIX F814W 3 500 S POS TARG -.1,.15 1 ------------------------------------------------------------------------------------------------------------------------------------ 9436( 4) - 03/18/02 14:55 - [ 5] Summary Form for Proposal 9436 Item Used in this proposal ------------------------------------------------------------------------------------------------------------------------------------ Apertures PC1-FIX ------------------------------------------------------------------------------------------------------------------------------------ Configurations WFPC2 ------------------------------------------------------------------------------------------------------------------------------------ Opmodes IMAGE ------------------------------------------------------------------------------------------------------------------------------------ Proposal Category GO ------------------------------------------------------------------------------------------------------------------------------------ Special Requirements ORIENT 87.6D TO 87.6D POS TARG -0.1,-0.1 GSPAIR 0739202181F20739201659F1 POS TARG -.1,-.1 POS TARG .15,-.1 POS TARG .15,.15 POS TARG -.1,.15 ------------------------------------------------------------------------------------------------------------------------------------ Spectral Elements F814W ------------------------------------------------------------------------------------------------------------------------------------ Target Names NGC6558-FLD1 ------------------------------------------------------------------------------------------------------------------------------------