Blue hook stars are a class of hot (~35,000 K) subluminous horizontal branch stars that have been recently discovered using HST ultraviolet images of the globular clusters omega Cen and NGC 2808. These stars occupy a region of the HR diagram that is unexplained by canonical stellar evolution theory. Using new theoretical evolutionary and atmospheric models, we have shown that the blue hook stars are very likely the progeny of stars that undergo extensive internal mixing during a late helium core flash on the white dwarf cooling curve. This "flash mixing" produces an enormous enhancement of the surface helium and carbon abundances, which suppresses the flux in the far ultraviolet. Although flash mixing is more likely to occur in stars that are born with high helium abundances, a high helium abundance, by itself, does not explain the presence of a blue hook population - flash mixing of the envelope is required. We propose ACS ultraviolet (SBC/F150LP and HRC/F250W) observations of the five additional globular clusters for which the presence of blue hook stars is suspected from longer wavelength observations. Like omega Cen and NGC 2808, these five targets are also among the most massive globular clusters, because less massive clusters show no evidence for blue hook stars. Because our targets span 1.5 dex in metallicity, we will be able to test our prediction that flash-mixing should be less drastic in metal-rich blue hook stars. In addition, our observations will test the hypothesis that blue hook stars only form in globular clusters massive enough to retain the helium-enriched ejecta from the first stellar generation. If this hypothesis is correct, then our observations will yield important constraints on the chemical evolution and early formation history in globular clusters, as well as the role of helium self-enrichment in producing blue horizontal branch morphologies and multiple main sequence turnoffs. Finally, our observations will provide new insight into the formation of the hottest horizontal branch stars, with implications for the origin of the hot helium-rich subdwarfs in the Galactic field.

Our observations are extremely straight-forward. Blue hook stars may extend up to 1 mag below the canonical ZAHB, at effective temperatures near 30,000 K. To obtain accurate colors and luminosities for these stars, we require photometry with a signal-to-noise ratio of 20 in each bandpass (ACS/HRC/F250W = NUV, ACS/SBC/F150LP = FUV). Note that cooler stars (down to T_eff ~ 10,000 K) at the same luminosity or brighter (such as those on the canonical HB) will have higher signal-to-noise ratio photometry. We have calculated the required exposure times using our own models of such stars, the ACS throughputs, and estimates of the average sky and dark rates. These calculations have been verified using the ACS ETC. As done for the earlier STIS FUV/NUV images of NGC2808, we will sample the core of the four nearby clusters with a small 2x2 tile mosaic pattern. Because all of the tiling can be achieved within a single small visit, we have no orientation constraints. One cluster, NGC2419, is distant enough to cover the entire core with a single tile, although the exposure times will be longer to achieve similar data quality. Although the cores of these clusters appear very crowded in optical bandpasses, in the FUV and NUV, they are much more sparse, so accurate photometry will not be a problem (e.g., see Brown et al. 2001 for the UV image of NGC2808, with very similar sampling and coverage of the cluster core). The observations present no safety concerns for the SBC, because the 3 relatively nearby clusters have extant UV images showing no UV-bright stars, while the other two clusters are significantly distant and reddened. RESOLVED STELLAR POPULATIONS 12 false 0 0.0 0 false false false false false false SUPPORTED STELLAR CLUSTER GLOBULAR CLUSTER J2000 07 38 7.9000 +38 52 48.00 J2000 STELLAR CLUSTER GLOBULAR CLUSTER J2000 17 02 37.7000 -26 16 5.00 J2000 STELLAR CLUSTER GLOBULAR CLUSTER J2000 18 55 3.3000 -30 28 43.00 J2000 STELLAR CLUSTER GLOBULAR CLUSTER J2000 17 36 17.4000 -44 44 8.00 J2000 STELLAR CLUSTER GLOBULAR CLUSTER J2000 17 50 12.8000 -37 03 4.00 J2000 ACS/HRC Imaging No No F250W ACS/SBC Imaging No No F150LP ACS/HRC Imaging No No F250W ACS/SBC Imaging No No F150LP ACS/HRC Imaging No No F250W ACS/SBC Imaging No No F150LP ACS/HRC Imaging No No F250W ACS/SBC Imaging No No F150LP ACS/HRC Imaging No No F250W ACS/SBC Imaging No No F150LP (Tic-Visit :proposal-id 10815 :visit-id "01" :proposal-version -1 :trans-version "20061109" :date "21-Feb-2007 12:02:52.0" :trans-errors '(:WARNINGS 0 :ERRORS 0 :HEALTH-AND-SAFETY 0 :FATAL-ERRORS 0 :CONTINUABLE-FATALS 0) :number-of-orbits 4 :saa-model "27" :si-bright-earth-limb-angle 20.0 :data-volume 1219152 :tdrss-contact-duration 0 :coron NIL ) (Tic-Obset :proposal-id 10815 :visit-id "01" :obset-id "01" :pcs-mode "FGS" ) (Tic-Obset :proposal-id 10815 :visit-id "01" :obset-id "A1" :pcs-mode "FGS" ) (Tic-Orbit :proposal-id 10815 :visit-id "01" :orbit-number 1 :pcs-time 355 :si-vis-utilization 2716 :al-overhead-at-end 0 :internal-dur-at-end 0 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3159 :pcs-gap 0 ) (Tic-Orbit :proposal-id 10815 :visit-id "01" :orbit-number 2 :pcs-time 355 :si-vis-utilization 2718 :al-overhead-at-end 0 :internal-dur-at-end 967 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3159 :pcs-gap 0 ) (Tic-Orbit :proposal-id 10815 :visit-id "01" :orbit-number 3 :pcs-time 355 :si-vis-utilization 2721 :al-overhead-at-end 0 :internal-dur-at-end 0 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3159 :pcs-gap 0 ) (Tic-Orbit :proposal-id 10815 :visit-id "01" :orbit-number 4 :pcs-time 355 :si-vis-utilization 2720 :al-overhead-at-end 0 :internal-dur-at-end 119 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3159 :pcs-gap 0 ) true 4 0 Before ACS Side-2 failure this visit contained HRC & SBC observations of NGC6273. The HRC has been changed to WFPC2, and the SBC has been moved to Visit 52. (Tic-Visit :proposal-id 10815 :visit-id "02" :proposal-version -1 :trans-version "20061109" :date "21-Feb-2007 12:03:05.0" :trans-errors '(:WARNINGS 0 :ERRORS 0 :HEALTH-AND-SAFETY 0 :FATAL-ERRORS 0 :CONTINUABLE-FATALS 0) :number-of-orbits 5 :saa-model "26" :si-bright-earth-limb-angle 20.0 :data-volume 448160 :tdrss-contact-duration 0 :coron NIL ) (Tic-Obset :proposal-id 10815 :visit-id "02" :obset-id "02" :pcs-mode "FGS" ) (Tic-Orbit :proposal-id 10815 :visit-id "02" :orbit-number 1 :pcs-time 355 :si-vis-utilization 2720 :al-overhead-at-end 160 :internal-dur-at-end 0 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3149 :pcs-gap 0 ) (Tic-Orbit :proposal-id 10815 :visit-id "02" :orbit-number 2 :pcs-time 355 :si-vis-utilization 2723 :al-overhead-at-end 157 :internal-dur-at-end 0 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3149 :pcs-gap 0 ) (Tic-Orbit :proposal-id 10815 :visit-id "02" :orbit-number 3 :pcs-time 355 :si-vis-utilization 2720 :al-overhead-at-end 160 :internal-dur-at-end 0 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3149 :pcs-gap 0 ) (Tic-Orbit :proposal-id 10815 :visit-id "02" :orbit-number 4 :pcs-time 355 :si-vis-utilization 2721 :al-overhead-at-end 159 :internal-dur-at-end 0 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3149 :pcs-gap 0 ) (Tic-Orbit :proposal-id 10815 :visit-id "02" :orbit-number 5 :pcs-time 355 :si-vis-utilization 2700 :al-overhead-at-end 180 :internal-dur-at-end 0 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3149 :pcs-gap 0 ) true 5 0 Before ACS Side-2 failure, this visit contained HRC observations of NGC6715. The HRC has now been switched to WFPC2. We will inspect these data as soon as possible after the observations have completed (in conjunction with ground-based wide-field U-band images), and if they show no excessively UV-bright star in the field, we will allow Visit 53 to execute. (Tic-Visit :proposal-id 10815 :visit-id "03" :proposal-version -1 :trans-version "20061109" :date "21-Feb-2007 12:03:13.0" :trans-errors '(:WARNINGS 0 :ERRORS 0 :HEALTH-AND-SAFETY 0 :FATAL-ERRORS 0 :CONTINUABLE-FATALS 0) :number-of-orbits 5 :saa-model "26" :si-bright-earth-limb-angle 20.0 :data-volume 448160 :tdrss-contact-duration 0 :coron NIL ) (Tic-Obset :proposal-id 10815 :visit-id "03" :obset-id "03" :pcs-mode "FGS" ) (Tic-Orbit :proposal-id 10815 :visit-id "03" :orbit-number 1 :pcs-time 355 :si-vis-utilization 2720 :al-overhead-at-end 160 :internal-dur-at-end 0 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3152 :pcs-gap 0 ) (Tic-Orbit :proposal-id 10815 :visit-id "03" :orbit-number 2 :pcs-time 355 :si-vis-utilization 2723 :al-overhead-at-end 157 :internal-dur-at-end 0 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3152 :pcs-gap 0 ) (Tic-Orbit :proposal-id 10815 :visit-id "03" :orbit-number 3 :pcs-time 355 :si-vis-utilization 2720 :al-overhead-at-end 160 :internal-dur-at-end 0 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3152 :pcs-gap 0 ) (Tic-Orbit :proposal-id 10815 :visit-id "03" :orbit-number 4 :pcs-time 355 :si-vis-utilization 2721 :al-overhead-at-end 159 :internal-dur-at-end 0 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3152 :pcs-gap 0 ) (Tic-Orbit :proposal-id 10815 :visit-id "03" :orbit-number 5 :pcs-time 355 :si-vis-utilization 2700 :al-overhead-at-end 180 :internal-dur-at-end 0 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3152 :pcs-gap 0 ) false 5 0 Before ACS Side-2 failure this visit contained HRC & SBC observations of NGC6388. The HRC has been changed to WFPC2, and the SBC has been moved to Visit 54. (Tic-Visit :proposal-id 10815 :visit-id "04" :proposal-version -1 :trans-version "20061109" :date "21-Feb-2007 12:03:21.0" :trans-errors '(:WARNINGS 0 :ERRORS 0 :HEALTH-AND-SAFETY 0 :FATAL-ERRORS 0 :CONTINUABLE-FATALS 0) :number-of-orbits 5 :saa-model "26" :si-bright-earth-limb-angle 20.0 :data-volume 448160 :tdrss-contact-duration 0 :coron NIL ) (Tic-Obset :proposal-id 10815 :visit-id "04" :obset-id "04" :pcs-mode "FGS" ) (Tic-Orbit :proposal-id 10815 :visit-id "04" :orbit-number 1 :pcs-time 355 :si-vis-utilization 2720 :al-overhead-at-end 160 :internal-dur-at-end 0 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3169 :pcs-gap 0 ) (Tic-Orbit :proposal-id 10815 :visit-id "04" :orbit-number 2 :pcs-time 355 :si-vis-utilization 2723 :al-overhead-at-end 157 :internal-dur-at-end 0 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3169 :pcs-gap 0 ) (Tic-Orbit :proposal-id 10815 :visit-id "04" :orbit-number 3 :pcs-time 355 :si-vis-utilization 2720 :al-overhead-at-end 160 :internal-dur-at-end 0 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3169 :pcs-gap 0 ) (Tic-Orbit :proposal-id 10815 :visit-id "04" :orbit-number 4 :pcs-time 355 :si-vis-utilization 2721 :al-overhead-at-end 159 :internal-dur-at-end 0 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3169 :pcs-gap 0 ) (Tic-Orbit :proposal-id 10815 :visit-id "04" :orbit-number 5 :pcs-time 355 :si-vis-utilization 2700 :al-overhead-at-end 180 :internal-dur-at-end 0 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3169 :pcs-gap 0 ) true 5 0 Before ACS Side-2 failure this visit contained HRC & SBC observations of NGC6441. The HRC has been changed to WFPC2, and the SBC has been moved to Visit 55. (Tic-Visit :proposal-id 10815 :visit-id "05" :proposal-version -1 :trans-version "20061109" :date "21-Feb-2007 12:03:29.0" :trans-errors '(:WARNINGS 0 :ERRORS 0 :HEALTH-AND-SAFETY 0 :FATAL-ERRORS 0 :CONTINUABLE-FATALS 0) :number-of-orbits 5 :saa-model "26" :si-bright-earth-limb-angle 20.0 :data-volume 448160 :tdrss-contact-duration 0 :coron NIL ) (Tic-Obset :proposal-id 10815 :visit-id "05" :obset-id "05" :pcs-mode "FGS" ) (Tic-Orbit :proposal-id 10815 :visit-id "05" :orbit-number 1 :pcs-time 355 :si-vis-utilization 2720 :al-overhead-at-end 160 :internal-dur-at-end 0 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3159 :pcs-gap 0 ) (Tic-Orbit :proposal-id 10815 :visit-id "05" :orbit-number 2 :pcs-time 355 :si-vis-utilization 2723 :al-overhead-at-end 157 :internal-dur-at-end 0 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3159 :pcs-gap 0 ) (Tic-Orbit :proposal-id 10815 :visit-id "05" :orbit-number 3 :pcs-time 355 :si-vis-utilization 2720 :al-overhead-at-end 160 :internal-dur-at-end 0 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3159 :pcs-gap 0 ) (Tic-Orbit :proposal-id 10815 :visit-id "05" :orbit-number 4 :pcs-time 355 :si-vis-utilization 2721 :al-overhead-at-end 159 :internal-dur-at-end 0 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3159 :pcs-gap 0 ) (Tic-Orbit :proposal-id 10815 :visit-id "05" :orbit-number 5 :pcs-time 355 :si-vis-utilization 2700 :al-overhead-at-end 180 :internal-dur-at-end 0 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3159 :pcs-gap 0 ) true 5 0 Before ACS Side-2 failure this visit contained HRC & SBC observations of NGC6441. The HRC has been changed to WFPC2, and the SBC has been moved to Visit 55. (Tic-Visit :proposal-id 10815 :visit-id "05" :proposal-version -1 :trans-version "20061109" :date "21-Feb-2007 12:03:29.0" :trans-errors '(:WARNINGS 0 :ERRORS 0 :HEALTH-AND-SAFETY 0 :FATAL-ERRORS 0 :CONTINUABLE-FATALS 0) :number-of-orbits 5 :saa-model "26" :si-bright-earth-limb-angle 20.0 :data-volume 448160 :tdrss-contact-duration 0 :coron NIL ) (Tic-Obset :proposal-id 10815 :visit-id "05" :obset-id "05" :pcs-mode "FGS" ) (Tic-Orbit :proposal-id 10815 :visit-id "05" :orbit-number 1 :pcs-time 355 :si-vis-utilization 2720 :al-overhead-at-end 160 :internal-dur-at-end 0 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3159 :pcs-gap 0 ) (Tic-Orbit :proposal-id 10815 :visit-id "05" :orbit-number 2 :pcs-time 355 :si-vis-utilization 2723 :al-overhead-at-end 157 :internal-dur-at-end 0 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3159 :pcs-gap 0 ) (Tic-Orbit :proposal-id 10815 :visit-id "05" :orbit-number 3 :pcs-time 355 :si-vis-utilization 2720 :al-overhead-at-end 160 :internal-dur-at-end 0 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3159 :pcs-gap 0 ) (Tic-Orbit :proposal-id 10815 :visit-id "05" :orbit-number 4 :pcs-time 355 :si-vis-utilization 2721 :al-overhead-at-end 159 :internal-dur-at-end 0 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3159 :pcs-gap 0 ) (Tic-Orbit :proposal-id 10815 :visit-id "05" :orbit-number 5 :pcs-time 355 :si-vis-utilization 2700 :al-overhead-at-end 180 :internal-dur-at-end 0 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3159 :pcs-gap 0 ) false 5 0 Before ACS Side-2 failure, this visit repeated the HRC and failed SBC observations in Visit 1 to ensure they were at the same position and orientation. Now it is only a repeat of the SBC observations in Visit 1. This visit should execute at the same orientation and position as the HRC observations of Visit 1. (Tic-Visit :proposal-id 10815 :visit-id "51" :proposal-version -1 :trans-version "20061109" :date "21-Feb-2007 12:03:32.0" :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 74064 :tdrss-contact-duration 0 :coron NIL ) (Tic-Obset :proposal-id 10815 :visit-id "51" :obset-id "51" :pcs-mode "FGS" ) (Tic-Orbit :proposal-id 10815 :visit-id "51" :orbit-number 1 :pcs-time 355 :si-vis-utilization 2721 :al-overhead-at-end 0 :internal-dur-at-end 0 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3159 :pcs-gap 0 ) (Tic-Orbit :proposal-id 10815 :visit-id "51" :orbit-number 2 :pcs-time 355 :si-vis-utilization 2720 :al-overhead-at-end 0 :internal-dur-at-end 119 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3159 :pcs-gap 0 ) true 2 0 Before ACS Side-2 failure, these observations were in Visit 2. Wide orient ranges are used here to avoid a bright foreground star. (Tic-Visit :proposal-id 10815 :visit-id "52" :proposal-version -1 :trans-version "20061109" :date "21-Feb-2007 12:03:38.0" :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 222160 :tdrss-contact-duration 0 :coron NIL ) (Tic-Obset :proposal-id 10815 :visit-id "52" :obset-id "52" :pcs-mode "FGS" ) (Tic-Orbit :proposal-id 10815 :visit-id "52" :orbit-number 1 :pcs-time 355 :si-vis-utilization 2772 :al-overhead-at-end 0 :internal-dur-at-end 281 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3149 :pcs-gap 0 ) true 1 0 Before ACS Side-2 failure, these observations were in Visit 6. The exposures in Visit 53 should follow Visit 3. We have introduced a delay between the WFPC2 exposures of NGC6715 (target 3) and the SBC exposures of NGC6715, to allow us to inspect the WFPC2 exposures and ensure the field is safe for the SBC. With the data in hand we are unable to rule out a hot foreground star along the sightline to this cluster. Visit 53 should not execute until we have inspected the data from Visit 3. Note that the SBC exposures in Visit 53 fall completely within the area mapped by the Visit 3 WFPC2 exposures and also ground-based U-band images over a much wider area. Visit 53 does not need to be at the same orient as Visit 3, and it does not need to use the same guide stars. (Tic-Visit :proposal-id 10815 :visit-id "53" :proposal-version -1 :trans-version "20061109" :date "21-Feb-2007 12:03:45.0" :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 222160 :tdrss-contact-duration 0 :coron NIL ) (Tic-Obset :proposal-id 10815 :visit-id "53" :obset-id "53" :pcs-mode "FGS" ) (Tic-Orbit :proposal-id 10815 :visit-id "53" :orbit-number 1 :pcs-time 355 :si-vis-utilization 2772 :al-overhead-at-end 0 :internal-dur-at-end 281 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3152 :pcs-gap 0 ) false 1 0 Before ACS Side-2 failure, these observations were in Visit 4. (Tic-Visit :proposal-id 10815 :visit-id "54" :proposal-version -1 :trans-version "20061109" :date "21-Feb-2007 12:03:51.0" :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 222160 :tdrss-contact-duration 0 :coron NIL ) (Tic-Obset :proposal-id 10815 :visit-id "54" :obset-id "54" :pcs-mode "FGS" ) (Tic-Orbit :proposal-id 10815 :visit-id "54" :orbit-number 1 :pcs-time 355 :si-vis-utilization 2772 :al-overhead-at-end 0 :internal-dur-at-end 281 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3169 :pcs-gap 0 ) true 1 0 Before ACS Side-2 failure, these observations were in Visit 5. (Tic-Visit :proposal-id 10815 :visit-id "55" :proposal-version -1 :trans-version "20061109" :date "21-Feb-2007 12:03:57.0" :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 222160 :tdrss-contact-duration 0 :coron NIL ) (Tic-Obset :proposal-id 10815 :visit-id "55" :obset-id "55" :pcs-mode "FGS" ) (Tic-Orbit :proposal-id 10815 :visit-id "55" :orbit-number 1 :pcs-time 355 :si-vis-utilization 2775 :al-overhead-at-end 0 :internal-dur-at-end 0 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3159 :pcs-gap 0 ) (Tic-Orbit :proposal-id 10815 :visit-id "55" :orbit-number 2 :pcs-time 355 :si-vis-utilization 2697 :al-overhead-at-end 0 :internal-dur-at-end 281 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3159 :pcs-gap 0 ) true 2 0