0 Assigned ID: 1581 ----- Attempting Submission 1 (Sun Oct 01 01:30:23 GMT 2017) ----- HST Phase I Proposal 1581 (apt_ucdpos1715.apt) successfully submitted. Receipt: # 1581-1 0 ----- Attempting Submission 1 (Tue Dec 12 15:45:36 GMT 2017) ----- We request HST/WFC3 imaging to obtain a deep color-magnitude diagram of CenA-MM-UCD1: this peculiar ultra-compact dwarf (UCD) is the first reported UCD with a partially resolved extended halo, and it was serendipitously discovered within our Magellan/Megacam survey of the closest elliptical CenA. The origin of UCD is still uncertain, and mounting observational evidence suggests that these objects could be tidally stripped nuclei of more massive galaxies. CenA-MM-UCD1 represents the missing link in the evolutionary sequence between a nuclear star cluster (pre-stripping phase) and a ``normal'' UCD (post-stripping phase): characterizing the nature of CenA-MM-UCD1 thus represents a milestone for the understanding of the origin of UCDs. The exquisite resolution of the proposed follow-up HST imaging is essential to derive an accurate stellar mass profile for CenA-MM-UCD1, investigate age/metallicity spreads in its populations with a significantly improved accuracy with respect to the ground-based discovery data, and search for possible faint signs of ongoing tidal disruption. The observations are going to be performed with WFC3/UVIS (primary) and ACS/WFC (parallel). We request imaging in the F606W and F814W filters (broad V and I) for both instruments to observe our peculiar UCD target: these observations will enable an accurate characterization of its stellar content (central surface brightness profile, TRGB distance, photometric metallicities, star formation history, structural parameters) and will resolve a significantly larger number of stars with respect to the ground-based discovery data, while optimizing the exposure times. The parallel pointing will allow us to observe a field region next to our target: we will thus be able to estimate more precisely the number of background/foreground contaminants (both Milky Way populations and Centaurus A halo stars) present in our WFC3 field. Our goal is to obtain a SNR of ~10 at ~1.5 mag below the TRGB, i.e., at F814W~25.5 and F606W~26.5, in order to derive a precise distance measurement, star formation history, color variations, structural properties and investigate possible spatial asymmetries for our target. With the requested SNR, we will obtain a photometric uncertainty as low as ~0.1 mag at the TRGB magnitude. Our goal requires ~2400 sec in the F606W filter and ~2300 sec in the F814W filter, which nicely fit into one orbit per filter, after taking into account overheads and subdividing the exposures with a standard three-dither line pattern to remove detector defects and improve the PSF sampling; we additionally insert a 50 sec exposure in each filter to avoid saturation at the center of our target. We combine the two orbits into one visit in order to maintain the same orientation and thus target the same stars in both filters; our loose ORIENT constraints are necessary to avoid ghosts/spikes at the target's position caused by bright stars within our WFC3 field-of-view. Stellar Populations Galaxies and the IGM 3 false false false false false false SUPPORTED UNIDENTIFIED NUCLEUS UNDESIGNATED NO Total apparent magnitude J2000 J2000 ACS/WFC Imaging No No F606W ACS/WFC Imaging No No F814W WFC3/UVIS Imaging No No F606W WFC3/UVIS Imaging No No F814W POS-TARG DITHER 0.135 LINE false false 3 46.84 WFC3-UVIS-DITHER-LINE-3PT 1 false 1513092876730 /Users/denija/OnTravel/HST_PhaseII/PhaseII/OP-cache/apt_ucdpos1715_mod2-v01.xml The scheduling data was generated by CASM 25.2. CASM was run in Phase II mode. (Tic-Visit :proposal-id 15426 :visit-id "01" :proposal-version -1 :trans-version "20171009" :date "12-Dec-2017 15:34:36.0" :trans-errors '(:WARNINGS 0 :ERRORS 0 :HEALTH-AND-SAFETY 0 :FATAL-ERRORS 0 :CONTINUABLE-FATALS 0) :number-of-orbits 2 :saa-model "29" :si-bright-earth-limb-angle 20.0 :data-volume 4015328 :tdrss-contact-duration 0 :coron NIL ) (Tic-Obset :proposal-id 15426 :visit-id "01" :obset-id "01" :pcs-mode "FGS" ) (Tic-Orbit :proposal-id 15426 :visit-id "01" :orbit-number 1 :pcs-time 333 :si-vis-utilization 2965 :al-overhead-at-end 116 :internal-dur-at-end 347 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3298 :pcs-gap 0 ) (Tic-Orbit :proposal-id 15426 :visit-id "01" :orbit-number 2 :pcs-time 222 :si-vis-utilization 3076 :al-overhead-at-end 116 :internal-dur-at-end 1015 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3298 :pcs-gap 0 ) true 2 0