We propose to obtain high-resolution images of five hypervelocity stars in the Galactic halo in order to establish the first-epoch astrometric frame for them, as a part of a long-term program to measure precise proper motions. The origin of these recently discovered stars, all with positive radial velocities above 540 km/s, is consistent only with being ejected from the deep potential well of the massive black hole at the Galactic center. The deviations of their space motions from purely radial trajectories probe the departures from spherical symmetry of the Galactic potential, mainly due to the triaxiality of the dark-matter halo. Reconstructing the full three-dimensional space motion of the hypervelocity stars, through astrometric proper motions, provides a unique opportunity to measure the shape and orientation of the dark halo. The hypervelocity stars allow measurement of the potential up to 75 kpc from the center, independently of and at larger distances than are afforded by tidal streams of satellite galaxies such as the Sagittarius dSph galaxy. HVS3 may be associated with the LMC, rather then the Galactic center, and would therefore present a case for a supermassive black hole at the center of the LMC. We request one orbit with ACS/WFC for each of the five hypervelocity stars to establish their current positions relative to background galaxies. We will request a repeated observation of these stars in Cycle 17, which will conclusively measure the astrometric proper motions.

In this program we are obtaining second-epoch images of 5 hypervelocity stars. The aim is to measure their proper motions against the reference frame of background galaxies. Each target will be centered either in the middle of WFC1, or for the brightest targets we will reduce overheads by using the subraster WFC1-2K. The targets will be observed either in F814W or F850LP, which appear to be better filters than the F606W mentioned in the original proposal. The reason is that the crucial background galaxies will give a better S/N at longer wavelengths, according to the ACS ETC. For each target we will take the maximum number of exposures possible in one orbit, with appropriate dithering. Exposures are chosen for each target to be no more than 70% of saturation on the target star. For these second-epoch observations, we are specifying the exact same ORIENT's that were used for the first-epoch observations in Cycle 15. RESOLVED STELLAR POPULATIONS 0 false 0.0 0.0 false false false false false false SUPPORTED STAR BP HALO HVS1 J2000 09 07 45.0000 +02 45 7.00 J2000 STAR BP HALO HVS2 US708 J2000 09 33 20.8600 +44 17 5.40 J2000 STAR BP HALO HVS3 J2000 04 38 12.8000 -54 33 12.00 J2000 STAR BP HALO HVS4 J2000 09 13 1.0110 +30 51 19.83 J2000 STAR BP HALO HVS5 J2000 09 17 59.4750 +67 22 38.35 J2000 ACS/WFC Imaging No No F606W ACS/WFC Imaging No No F606W ACS/WFC Imaging No No F606W ACS/WFC Imaging No No F606W ACS/WFC Imaging No No F606W The scheduling data was generated by CASM 17.2. CASM was run in Phase II mode. The ORIENT is chosen to exactly duplicate that used in the first-epoch observations made in Cycle 15. 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(Tic-Visit :proposal-id 11782 :visit-id "03" :proposal-version -1 :trans-version "20080624" :date "18-Jul-2008 18:04:17.0" :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 421472 :tdrss-contact-duration 0 :coron NIL ) (Tic-Obset :proposal-id 11782 :visit-id "03" :obset-id "03" :pcs-mode "FGS" ) (Tic-Orbit :proposal-id 11782 :visit-id "03" :orbit-number 1 :pcs-time 333 :si-vis-utilization 3110 :al-overhead-at-end 0 :internal-dur-at-end 269 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3453 :pcs-gap 0 ) false 1 0 The scheduling data was generated by CASM 17.2. CASM was run in Phase II mode. The ORIENT is chosen to exactly duplicate that used in the first-epoch observations made in Cycle 15. (Tic-Visit :proposal-id 11782 :visit-id "04" :proposal-version -1 :trans-version "20080624" :date "17-Jul-2008 15:08:52.0" :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 1421120 :tdrss-contact-duration 0 :coron NIL ) (Tic-Obset :proposal-id 11782 :visit-id "04" :obset-id "04" :pcs-mode "FGS" ) (Tic-Orbit :proposal-id 11782 :visit-id "04" :orbit-number 1 :pcs-time 333 :si-vis-utilization 2902 :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 1 0 The scheduling data was generated by CASM 17.2. CASM was run in Phase II mode. 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