We request second-epoch ACS observations of four star fields in the Galactic bar. These will allow us to measure proper motions for tens of thousands of stars well below the turnoff, to construct a dynamical model for the bulge/bar (in combination with data already in hand from other HST fields, and from VLT spectroscopy), and hence to take a unique look at the internal dynamical structure of the central regions of our Galaxy. By relating the kinematics with stellar population we can elucidate the formation history of the bulge and bar, and their relation to the surrounding Galactic disk. This is a resubmission of an approved Cycle 15 proposal that was hit by the ACS malfunction.

We propose to reobserve four fields in the far side (negative longitude) of the Galactic bar. We obtained first-epoch F814W images for these fields in 2003 as part of our cycle 12 proposal (in which we also completed the 2nd epoch observations for our fields in the near side of the bar). With the data requested here we will complete the HST imaging portion of the project, and will have proper motion data in hand for 11 bulge/bar fields, covering the full extent of the bar. We will have at least 10,000 stars per field. Astrometric accuracy is crucial, and we will follow the methodology of our earlier work (Kuijken \& Rich 2002, based on Anderson \& King 2000): a mix of short and long, dithered exposures such that we can correct accurately for sub-pixel position uncertainties. Per field we require one orbit for five (3$\times$7.5 minutes and 2$\times$1 minute) F814W exposures. A second orbit for each field is required for similar exposures in F555W: these data will not be used for proper motion measurements (there being no equivalent first epoch) but the resulting colour-magnitude diagram will be used for photometric parallax measurements so that the kinematics can be related to distance down the line of sight (Kuijken \& Rich 2002). The proper motion accuracy we have been able to achieve with WFPC2 images over a 6-year baseline is better than 1mas/yr (equivalent to 30 km/s at the distance to the bulge), in spite of the poor sampling offered by the WF chips. The key to achieving this accuracy was to observe in F814W, which has the widest PSF of the broad bands, and to dither extensively. We request a final orbit for an F814W-only observation of Baade's window. This field has been observed roughly every three years since 1994, and this final epoch will further improve the astrometric accuracy there. More importantly, it will enable us to cross check in detail the accuracies of proper motions derived from WFPC2/WFPC2, WFPC2/ACS and ACS/ACS combinations. Our total request is thus for nine orbits, covering 5 fields. NOTE on ORIENT values chosen: In order to optimize the overlap with the first-epoch data, we have specified four possible ORIENT ranges for each field (multiples of 90 degrees, $\pm2$, from the first-epoch observations). The preferred value, which corresponds to the first epoch observations, is given in the comments field for each visit. All observations are schedulable with full visibility with the ORIENTS chosen. RESOLVED STELLAR POPULATIONS 12 false 0.0 0.0 false false false false false SUPPORTED UNIDENTIFIED BULGE J2000 17 16 34.0000 -29 34 27.00 J2000 UNIDENTIFIED BULGE J2000 17 07 15.0000 -33 02 43.00 J2000 UNIDENTIFIED BULGE J2000 18 05 6.0000 -35 54 10.00 J2000 UNIDENTIFIED BULGE J2000 17 56 23.0000 -38 56 13.00 J2000 UNIDENTIFIED BULGE J2000 18 03 9.3723 -29 51 42.35 J2000 ACS/WFC Imaging No No F814W ACS/WFC Imaging No No F555W ACS/WFC Imaging No No F814W ACS/WFC Imaging No No F555W ACS/WFC Imaging No No F814W ACS/WFC Imaging No No F555W ACS/WFC Imaging No No F814W ACS/WFC Imaging No No F555W ACS/WFC Imaging No No F814W POS-TARG DITHER 0.265 LINE 0.187 false false 69.05 4 20.67 ACS-WFC-DITHER-BOX 1 false POS-TARG DITHER .1825 LINE false false 2 85.28 ACS-WFC-DITHER-LINE 2 false The scheduling data was generated by CASM 17.2. CASM was run in Phase II mode. field 1716 Preferred ORIENT is 77.5 Alternate ORIENTs may be added only if preferred ORIENT is not possible: 75-80, 165-170, 255-260, 345-350. (Tic-Visit :proposal-id 11655 :visit-id "01" :proposal-version -1 :trans-version "20080624" :date "03-Jul-2008 10:10:48.0" :trans-errors '(:WARNINGS 0 :ERRORS 0 :HEALTH-AND-SAFETY 0 :FATAL-ERRORS 0 :CONTINUABLE-FATALS 0) :number-of-orbits 2 :saa-model "27" :si-bright-earth-limb-angle 20.0 :data-volume 3410688 :tdrss-contact-duration 0 :coron NIL ) (Tic-Obset :proposal-id 11655 :visit-id "01" :obset-id "01" :pcs-mode "FGS" ) (Tic-Orbit :proposal-id 11655 :visit-id "01" :orbit-number 1 :pcs-time 333 :si-vis-utilization 2931 :al-overhead-at-end 0 :internal-dur-at-end 349 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3267 :pcs-gap 0 ) (Tic-Orbit :proposal-id 11655 :visit-id "01" :orbit-number 2 :pcs-time 195 :si-vis-utilization 3069 :al-overhead-at-end 0 :internal-dur-at-end 349 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3267 :pcs-gap 0 ) false 2 0 The scheduling data was generated by CASM 17.2. CASM was run in Phase II mode. field 1707 Preferred ORIENT is 64.8 Alternate ORIENTs may be added only if preferred ORIENT is not possible: 63-67, 153-157, 243-247, 333-337. (Tic-Visit :proposal-id 11655 :visit-id "02" :proposal-version -1 :trans-version "20080624" :date "03-Jul-2008 10:15:45.0" :trans-errors '(:WARNINGS 0 :ERRORS 0 :HEALTH-AND-SAFETY 0 :FATAL-ERRORS 0 :CONTINUABLE-FATALS 0) :number-of-orbits 2 :saa-model "27" :si-bright-earth-limb-angle 20.0 :data-volume 3410688 :tdrss-contact-duration 0 :coron NIL ) (Tic-Obset :proposal-id 11655 :visit-id "02" :obset-id "02" :pcs-mode "FGS" ) (Tic-Orbit :proposal-id 11655 :visit-id "02" :orbit-number 1 :pcs-time 333 :si-vis-utilization 2947 :al-overhead-at-end 0 :internal-dur-at-end 349 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3283 :pcs-gap 0 ) (Tic-Orbit :proposal-id 11655 :visit-id "02" :orbit-number 2 :pcs-time 195 :si-vis-utilization 3085 :al-overhead-at-end 0 :internal-dur-at-end 349 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3283 :pcs-gap 0 ) false 2 0 The scheduling data was generated by CASM 17.2. CASM was run in Phase II mode. field 1805 Preferred ORIENT is 81.5 Alternate ORIENTs may be added only if preferred ORIENT is not possible: 79-84, 169-174, 259-264, 349-354. (Tic-Visit :proposal-id 11655 :visit-id "03" :proposal-version -1 :trans-version "20080624" :date "03-Jul-2008 11:27:59.0" :trans-errors '(:WARNINGS 0 :ERRORS 0 :HEALTH-AND-SAFETY 0 :FATAL-ERRORS 0 :CONTINUABLE-FATALS 0) :number-of-orbits 2 :saa-model "27" :si-bright-earth-limb-angle 20.0 :data-volume 3410688 :tdrss-contact-duration 0 :coron NIL ) (Tic-Obset :proposal-id 11655 :visit-id "03" :obset-id "03" :pcs-mode "FGS" ) (Tic-Orbit :proposal-id 11655 :visit-id "03" :orbit-number 1 :pcs-time 333 :si-vis-utilization 2975 :al-overhead-at-end 0 :internal-dur-at-end 349 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3310 :pcs-gap 0 ) (Tic-Orbit :proposal-id 11655 :visit-id "03" :orbit-number 2 :pcs-time 195 :si-vis-utilization 3113 :al-overhead-at-end 0 :internal-dur-at-end 349 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3310 :pcs-gap 0 ) false 2 0 The scheduling data was generated by CASM 17.2. CASM was run in Phase II mode. field 1756 Preferred ORIENT is 22 Alternate ORIENTs may be added only if preferred ORIENT is not possible: 20-24, 110-114, 200-204, 290-294. (Tic-Visit :proposal-id 11655 :visit-id "04" :proposal-version -1 :trans-version "20080624" :date "03-Jul-2008 10:15:51.0" :trans-errors '(:WARNINGS 0 :ERRORS 0 :HEALTH-AND-SAFETY 0 :FATAL-ERRORS 0 :CONTINUABLE-FATALS 0) :number-of-orbits 2 :saa-model "27" :si-bright-earth-limb-angle 20.0 :data-volume 3410688 :tdrss-contact-duration 0 :coron NIL ) (Tic-Obset :proposal-id 11655 :visit-id "04" :obset-id "04" :pcs-mode "FGS" ) (Tic-Orbit :proposal-id 11655 :visit-id "04" :orbit-number 1 :pcs-time 333 :si-vis-utilization 2975 :al-overhead-at-end 0 :internal-dur-at-end 349 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3310 :pcs-gap 0 ) (Tic-Orbit :proposal-id 11655 :visit-id "04" :orbit-number 2 :pcs-time 195 :si-vis-utilization 3113 :al-overhead-at-end 0 :internal-dur-at-end 349 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3310 :pcs-gap 0 ) false 2 0 The scheduling data was generated by CASM 17.2. CASM was run in Phase II mode. Baade Window Preferred ORIENT is 40 Alternate ORIENTs may be added only if preferred ORIENT is not possible: 38-42, 128-132, 218-222, 308-312. (Tic-Visit :proposal-id 11655 :visit-id "05" :proposal-version -1 :trans-version "20080624" :date "03-Jul-2008 10:15:53.0" :trans-errors '(:WARNINGS 0 :ERRORS 0 :HEALTH-AND-SAFETY 0 :FATAL-ERRORS 0 :CONTINUABLE-FATALS 0) :number-of-orbits 1 :saa-model "27" :si-bright-earth-limb-angle 20.0 :data-volume 1705344 :tdrss-contact-duration 0 :coron NIL ) (Tic-Obset :proposal-id 11655 :visit-id "05" :obset-id "05" :pcs-mode "FGS" ) (Tic-Orbit :proposal-id 11655 :visit-id "05" :orbit-number 1 :pcs-time 333 :si-vis-utilization 2931 :al-overhead-at-end 0 :internal-dur-at-end 349 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3267 :pcs-gap 0 ) false 1 0