We are requesting 32 HST orbits to help ascertain the nature of the
population that gives rise to the observed set of microlensing events
towards the LMC. The SuperMACHO project is an ongoing ground-based
survey on the CTIO 4m that has demonstrated the ability to detect LMC
microlensing events in real-time via frame subtraction. The improvement
in angular resolution and photometric accuracy available from HST will
allow us to 1} confirm that the detected flux excursions arise from LMC
source stars rather than extended objects {such as for background
supernovae or AGN}, and 2} obtain reliable baseline flux measurements
for the objects in their unlensed state. The latter measurement is
important to resolve degeneracies between the event timescale and
baseline flux, which will yield a tighter constraint on the microlensing
optical depth.

We propose to measure the radius of the recently-discovered transiting
extrasolar planet XO-1b. XO-1b's nominal radius is 1.30 times the radius
of Jupiter, which is nearly as large as HD 209458b {1.32 R_J}. We will
use two independent methods to measure XO-1b's radius: 1} precision
light curve analysis, and 2} measurement of its trigonometric parallax
combined with its spectroscopically-determined effective temperature and
its apparent magnitude.

A new procedure proposed to alleviate the CR-persistence problem of
NICMOS. Dark frames will be obtained immediately upon exiting the SAA
contour 23, and every time a NICMOS exposure is scheduled within 50
minutes of coming out of the SAA. The darks will be obtained in parallel
in all three NICMOS Cameras. The POST-SAA darks will be non-standard
reference files available to users with a USEAFTER date/time mark. The
keyword 'USEAFTER=date/time' will also be added to the header of each
POST-SAA DARK frame. The keyword must be populated with the time, in
addition to the date, because HST crosses the SAA ~8 times per day so
each POST-SAA DARK will need to have the appropriate time specified, for
users to identify the ones they need. Both the raw and processed images
will be archived as POST-SAA DARKSs. Generally we expect that all NICMOS
science/calibration observations started within 50 minutes of leaving an
SAA will need such maps to remove the CR persistence from the science i
mages. Each observation will need its own CRMAP, as different SAA
passages leave different imprints on the NICMOS detectors.

In the fall of 2005, a serious anomaly was found in images from the WF4
CCD in WFPC2. The WF4 CCD bias level appeared to have become unstable,
resulting in sporadic images with either low or zero bias level. The
severity and frequency of the problem was rapidly increasing, making it
possible that WF4 would soon become unusable if no work-around were
found. Examination of bias levels during periods with frequent WFPC2
images showed low and zero bias episodes every 4 to 6 hours. This
periodicity is driven by cycling of the WFPC2 Replacement Heater, with
the bias anomalies occurring at the temperature peaks. The other three
CCDs {PC1, WF2, and WF3} appear to be unaffected and continue to operate
properly. Lowering the Replacement Heater temperature set points by a
few degrees C effectively eliminates the WF4 anomaly. On 9 January 2006,
the upper set point of the WFPC2 Replacement Heater was reduced from
14.9C to 12.2C. On 20 February 2006, the upper set point was reduced
from 12.2C to 11.3C, and the lower set point was reduced from 10.9C to
10.0C. These changes restored the WF4 CCD bias level; however, the bias
level has begun to trend downwards again, mimicking its behavior in late
2004 and early 2005. A third temperature reduction is planned for March
2007. We will reduce the upper set point of the heater from 11.3C to
10.4C and the lower set point from 10.0C to 9.1C. The observations
described in this proposal will test the performance of WFPC2 before and
after this temperature reduction. Additional temperature reductions may
be needed in the future, depending on the performance of WF4. Orbits:
internal 26, external 1

A surprising result has emerged from the ACS Virgo Cluster Survey
{ACSVCS}, a program to obtain ACS/WFC gz imaging for a large, unbiased
sample of 100 early-type galaxies in the Virgo Cluster. On subarcsecond
scales {i.e., <0.1"-1"}, the HST brightness profiles vary systematically
from the brightest giants {which have nearly constant surface brightness
cores} to the faintest dwarfs {which have compact stellar nuclei}.
Remarkably, the fraction of galaxy mass contributed by the nuclei in the
faint galaxies is identical to that contributed by supermassive black
holes in the bright galaxies {0.2%}. These findings strongly suggest
that a single mechanism is responsible for both types of Central Massive
Object: most likely internally or externally modulated gas inflows that
feed central black holes or lead to the formation of "nuclear star
clusters". Understanding the history of gas accretion, star formation
and chemical enrichment on subarcsecond scales has thus emerged as the
single most pressing question in the study of nearby galactic nuclei,
either active or quiescent. We propose an ambitious HST program {199
orbits} that constitutes the next, obvious step forward:
high-resolution, ultraviolet {WFPC2/F255W} and infrared {NIC1/F160W}
imaging for the complete ACSVCS sample. By capitalizing on HST's unique
ability to provide high-resolution images with a sharp and stable PSF at
UV and IR wavelengths, we will leverage the existing optical HST data to
obtain the most complete picture currently possible for the history of
star formation and chemical enrichment on these small scales. Equally
important, this program will lead to a significant improvement in the
measured structural parameters and density distributions for the stellar
nuclei and the underlying galaxies, and provide a sensitive measure of
"frosting" by young stars in the galaxy cores. By virtue of its superb
image quality and stable PSF, NICMOS is the sole instrument capable of
the IR observations proposed here. In the case of the WFPC2
observations, high-resolution UV imaging {< 0.1"} is a capability unique
to HST, yet one that could be lost at any time.

Recent systematic surveys of strong galaxy-galaxy lenses {CLASS, SLACS,
GOODS, etc.} are producing spectacular results for galaxy masses roughly
below a transition mass M~10^13 Mo. The observed lens properties and
their evolution up to z~0.2, consistent with numerical simulations, can
be described by isothermal elliptical potentials. In contrast, modeling
of giant arcs in X-ray luminous clusters {halo masses M >~10^13 Mo}
favors NFW mass profiles, suggesting that dark matter halos are not
significantly affected by baryon cooling. Until recently, lensing
surveys were neither deep nor extended enough to probe the intermediate
mass density regime, which is fundamental for understanding the assembly
of structures. The CFHT Legacy Survey now covers 125 square degrees, and
thus offers a large reservoir of strong lenses probing a large range of
mass densities up to z~1. We have extracted a list of 150 strong lenses
using the most recent CFHTLS data release via automated procedures.
Following our first SNAPSHOT proposal in cycle 15, we propose to
continue the Hubble follow-up targeting a larger list of 130 lensing
candidates. These are intermediate mass range candidates {between
galaxies and clusters} that are selected in the redshift range of 0.2-1
with no a priori X-ray selection. The HST resolution is necessary for
confirming the lensing candidates, accurate modeling of the lenses, and
probing the total mass concentration in galaxy groups up to z~1 with the
largest unbiased sample available to date.