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21 April 2003

HUBBLE SPACE TELESCOPE

DAILY REPORT     #3345

PERIOD COVERED:  DOY 108-110

Part 2 of 5

ACS 9454

The Nature of the UV Continuum in LINERs: A Variability Test

LINERs may be the most common AGNs, and the signposts of accretion
onto the massive black holes present in most galaxies. However, the
LINER spectrum is the result of UV excitation, and, in at least some
LINERs, a nuclear cluster of hot stars, rather than an AGN, dominates
the energetics in the UV. Thus, it is still unknown if the UV
continuum, or the optical emission lines it excites, have anything to
do with an AGN. The demographics and accretion physics of
low-luminosity  AGNs hinge on this question. We propose to search for
variability in a sample of 17 LINERs with compact UV nuclei.
Variability can reveal an AGN component in the UV continuum, even
when its light is not dominant. We will test systematically the
handful of non-definitive reports of UV variability, and potentially
quantify the AGN contribution to the UV emission. Variability in all
or most objects will be strong evidence that LINERs mark dormant AGNs
in most galaxies. Alternatively, a general null detection of
variability will suggest that, even in LINERs with additional AGN
signatures, the UV continuum is stellar in origin.  Contemporaneous
monitoring with the VLA/VLBA of 11 objects which have radio cores
{five of which we already know are radio-variable} will reveal the
relations between UV and radio variations. The UV-variable objects
will be targeted for future, better-sampled, monitoring.

ACS/HRC 9391

High-Resolution Imaging of Pluto's Surface

We will collect a series of observations with the ACS/HRC from which
we will derive a two-color global map of Pluto's surface. We will
image Pluto at F435W and F555W, wavelengths that have been
extensively studied from the ground over the past 50 years. The maps
will provide albedos with accurate error determinations down to 52
degrees South latitude. These observations will provide a second
epoch of HST mapping of the active surface of Pluto as it continues
to recede from the Sun and will provide an important context for
other detailed studies of Pluto.

ACS/WFC/HRC 9445

Gravitational Microlensing in the NGC 3314A-B Galaxy Pair.

The Advanced Camera for Surveys (WFC and HRC) was used to determine
the composition of the dark matter that dominates the masses of
galaxies which is an important unsolved problem.

ACS/WFPC2 9488

Cosmic Shear - with ACS Pure Parallel Observations

The ACS, with greater sensitivity and sky coverage, will extend our
ability to measure the weak gravitational lensing of galaxy images
caused by the large scale distribution of dark matter. We propose to
use the ACS in pure parallel {non- proprietary} mode, following the
guidelines of the ACS Default Pure Parallel Program. Using the HST
Medium Deep Survey WFPC2 database we have measured cosmic shear at
arc-min angular scales. The MDS image parameters, in particular the
galaxy orientations and axis ratios, are such that any residual
corrections due to errors in the PSF or jitter are much smaller than
the measured signal. This situation is in stark contrast with
ground-based observations. We have also developed a statistical
analysis procedure to derive unbiased estimates of cosmic shear from
a large number of fields, each of which has a very small number of
galaxies. We have therefore set the stage for measurements with the
ACS at fainter apparent magnitudes and smaller, 10 arc-second scales
corresponding to larger cosmological distances. We will adapt
existing MDS WFPC2 maximum likelihood galaxy image analysis
algorithms to work with the ACS. The analysis would also yield an
online database similar to that in archive.stsci.edu/mds/

FGS/CAL 9602

Long Term Monitoring of FGS1r in Position Mode

It is known from our experience with FGS3, and later with FGS1r, that
an FGS on orbit experiences long term evolution, presumably due to
disorption of water from the instrument's graphite epoxy composites.
This manifests principally as a change in the plate scale and
secondarily as a change in the geometric distortions. These effects
are well modeled by adjustments to the rhoA and kA parameters which
are used to transform the star selector servo angles into FGS {x, y}
detector space coordinates. By observing the relative positions of
selected stars in a standard cluster at a fixed telescope pointing
and orientation, the evolution of rhoA and kA can be monitored and
calibrated to preserve the astrometric performance of FGS1r.

FGS1R 9408

Calibrating the Mass-Luminosity Relation at the End of the Main
Sequence 

We propose to use HST-FGS1R to calibrate the mass-luminosity
relation {MLR} for stars less massive than 0.2 Msun, with special
emphasis on objects near the stellar/brown dwarf border. Our goals
are to determine M_V values to 0.05 magnitude, masses to 5 than
double the number of objects with masses determined to be less than 0.
20 Msun. This program uses the combination of HST-FGS3/FGS1R at
optical wavelengths and ground-based infrared interferometry to
examine nearby, subarcsecond binary systems.  The high precision
measurements with HST-FGS3/FGS1R {to 1 mas in the separations} for
these faint targets {V = 10--15} simply cannot be equaled by any
ground based technique. As a result of these measurements, we are
deriving high quality luminosities and masses for the components in
the observed systems, and characterizing their spectral energy
distributions from 0.5 to 2.2 Mum. Several of the objects included
have M < 0.1 Msun, placing them at the very end of the stellar main
sequence. Three of the targets are brown dwarf candidates, including
the current low mass record holder, GJ 1245C, with a mass of
0.062 +/- 0.004 Msun. The payoff of this proposal is high because all
10 of the systems selected have already been resolved with
HST- FGS3/FGS1R during Cycles 5--10 and contain most of the reddest
objects for which masses can be determined. 

 - Continued -

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