Status Report

NASA Hubble Space Telescope Daily Report # 3676

By SpaceRef Editor
August 18, 2004
Filed under , ,

HUBBLE SPACE TELESCOPE – Continuing to collect World Class Science




ACS/HRC 10182

Towards a Comprehensive Understanding of Type Ia Supernovae: The
Necessity of UV Observations

Type Ia supernovae {SNe Ia} are very important to many diverse areas
of astrophysics, from the chemical evolution of galaxies to
observational cosmology which led to the discovery of dark energy and
the accelerating Universe. However, the utility of SNe Ia as
cosmological probes depends on the degree of our understanding of SN
Ia physics, and various systematic effects such as cosmic chemical
evolution. At present, the progenitors of SNe Ia and the exact
explosion mechanisms are still poorly understood, as are evolutionary
effects on SN Ia peak luminosities. Since early-time UV spectra and
light curves of nearby SNe Ia can directly address these questions, we
propose an approach consisting of two observational components: {1}
Detailed studies of two very bright, young, nearby SNe Ia with HST UV
spectroscopy at 13 epochs within the first 1.5 months after discovery;
and {2} studies of correlations with luminosity for five somewhat more
distant Hubble-flow SNe Ia, for which relative luminosities can be
determined with precision, using 8 epochs of HST UV spectroscopy
and/or broad-band imaging. The HST data, along with extensive
ground-based optical to near-IR observations, will be analyzed with
state-of-the-art models to probe SN Ia explosion physics and constrain
the nature of the progenitors. The results will form the basis for the
next phase of precision cosmology measurements using SNe Ia, allowing
us to more fully capitalize on the substantial past {and future}
investments of time made with HST in observations of high-redshift SNe

ACS/HRC 10185

When does Bipolarity Impose itself on the Extreme Mass Outflows from
AGB Stars? An ACS SNAPshot Survey

Essentially all well-characterized preplanetary nebulae {PPNe} —
objects in transition between the AGB and planetary nebula
evolutionary phases – are bipolar, whereas the mass-loss envelopes of
AGB stars are strikingly spherical. In order to understand the
processes leading to bipolar mass-ejection, we need to know at what
stage of stellar evolution does bipolarity in the mass-loss first
manifest itself? Our previous SNAPshot surveys of a PPNe sample {with
ACS & NICMOS} show that roughly half our targets observed are
resolved, with well-defined bipolar or multipolar morphologies.
Spectroscopic surveys of our sample confirm that these objects have
not yet evolved into planetary nebulae. Thus, the transformation from
spherical to aspherical geometries has already fully developed by the
time these dying stars have become preplanetary nebulae. From this
new and surprising result, we hypothesize that the transformation to
bipolarity begins during the very late AGB phase, and happens very
quickly, just before, or as the stars are evolving off the AGB. We
propose to test this hypothesis quantitatively, through a SNAPshot
imaging survey of very evolved AGB stars which we believe are nascent
preplanetary nebulae; with our target list being drawn from published
lists of AGB stars with detected heavy mass-loss {from millimeter-wave
observations}. This survey is crucial for determining how and when the
bipolar geometry asserts itself. Supporting kinematic observations
using long-slit optical spectroscopy {with the Keck}, millimeter and
radio interferometric observations {with OVRO, VLA & VLBA} are being
undertaken. The results from this survey {together with our previous
work} will allow us to draw general conclusions about the onset of
bipolar mass-ejection during late stellar evolution, and will provide
crucial input for theories of post-AGB stellar evolution. Our survey
will produce an archival legacy of long-standing value for future
studies of dying stars.

ACS/HRC 10198

Probing the Dynamics of the Galactic Bar through the Kinematics of
Microlensed Stars

The observed optical depths to microlensing of stars in the Galactic
bulge are difficult to reconcile with our present understanding of
Galactic dynamics. The main source of uncertainty in those comparisons
is now shifting from microlensing measurements to the dynamical models
of the Galactic bar. We propose to constrain the Galactic bar models
with proper motion observations of Bulge stars that underwent
microlensing by determining both the kinematic identity of the
microlensed sources and the importance of streaming motions. The
lensed stars are typically farther than randomly selected stars.
Therefore, our proper motion determinations for 36 targeted MACHO
events will provide valuable constraints on the dynamics of bulge
stars as a function of distance. The first epoch data for our proposed
events is already available in the HST archive so the project can be
completed within a single HST cycle. The exceptional spatial
resolution of HST is essential for completion of the project.
Constraints on the total mass in the bulge will ultimately lead to the
determination of the amount of dark matter in inner Galaxy.

ACS/HRC 10255

A Never Before Explored Phase Space: Resolving Close White Dwarf / Red
Dwarf Binaries

We propose an ACS Snapshot imaging survey to resolve a well-defined
sample of highly probable white dwarf plus red dwarf close binaries.
These candidates were selected from a search for white dwarfs with
infrared excess from the 2MASS database. They represent unresolved
systems {separations less than approximately 2" in the 2MASS images}
and are distributed over the whole sky. Our HST+ACS observations will
be sensitive to a separation range {1-20 AU} never before probed by
any means. The proposed study will be the first empirical test of
binary star parameters in the post-AGB phase, and cannot be
accomplished from the ground. By resolving as few as 20 of our ~100
targets with HST, we will be able to characterize the distribution of
orbital semi-major axes and secondary star masses.

ACS/HRC 10262

The 3D Morphology of the Extreme Red Supergiant VY Cma

The extreme RSG and powerful OH/IR source VY CMa is surrounded by an
asymmetric reflection nebula dominated by a prominent nebulous arc,
bright filamentary arcs, and several clumps of dusty knots that are
evidence for multiple and asymmetric mass loss events. Our groundbased
velocities show that these structures are kinematically distinct from
the general flow of the diffuse gas and may be directional. We have
speculated that these arcs and knots may be caused by localized
activity on the star involving convection and possibly magnetic fields
analogous to lower mass stars. If correct this would have important
implications for the causes of high mass loss events in evolved
massive stars. Fortunately, VY CMa provides us with an opportunity to
learn more about its possible mass loss mechanisms and history from
the morphology of its ejecta. We propose to use polarimetry and second
epoch images combined with our radial velocities to map the morphology
of the nebula and the discrete structures embedded in it.


CCD Daily Monitor

This program consists of basic tests to monitor, the read noise, the
development of hot pixels and test for any source of noise in ACS CCD
detectors. This programme will be executed once a day for the entire
lifetime of ACS.

ACS/WFC 10098

Probing the nature of Type Ia SNe through HST astrometry

Type Ia supernovae are of key importance in cosmology. Empirical
relations allow their use as cosmological standard candles. The
generally accepted picture is that the exploding star is a C+O white
dwarf which accretes matter from a companion in a binary system.
However, the nature of the companion is still unknown. It could either
be another WD, or be a giant, subgiant, or main-sequence star.
Calculations have shown that it is possible to distinguish among those
possibilities by the effect that the supernova explosion has on the
companion star. We propose to identify the companion star of the two
historical well-known SNeIa through ACS imaging of the targets
complemented by WFPC2 observations. A radial-velocity study of the
stars in those two Galactic SNeIa has been done from ground-based
facilities. To obtain the full motion vector of those stars, we plan
to use ACS for high-resolution astrometry in two different epochs.
That should allow to detect motion imparted during the explosion in
the direction perpendicular to the line of sight, down to a level of a
few milliarcsecs/yr.


Searching for the Bottom of the Initial Mass Function

The minimum mass of the Initial Mass Function {IMF} should be a direct
reflection of the physical processes that dominate in the formation of
stars and brown dwarfs. To date, the IMF has been measured down to 10
M_Jup in a few young clusters; there is no sign of a low-mass cutoff
in the data for these clusters. We propose to obtain deep images in
the SDSS i and z filters {i=26, z=25} with the ACS/WFC on HST for a
800"x1000" field in the Chamaeleon I star-forming region {2 Myr, 160
pc}. By combining these HST data {0.8, 0.9 um} with comparably deep
broad-band photometry from ground-based telescopes {1.2, 1.6, 2.2 um}
and SIRTF {3.6, 4.5, 5.8, 8.0 um}, we will measure the mass function
of brown dwarfs down to the mass of Jupiter and thus determine the
lowest mass at which objects can form in isolation in a typical star
forming cluster.


Accurately Mapping M31’s Microlensing Population

We propose to augment an existing microlensing survey of M31 with
source identifications provided by a modest amount of ACS {and WFPC2
parallel} observations to yield an accurate measurement of the masses
responsible for microlensing in M31, and presumably much of its dark
matter. The main benefit of these data is the determination of the
physical {or "Einstein"} timescale of each microlensing event, rather
than an effective {"FWHM"} timescale, allowing masses to be determined
more than twice as accurately as without HST data. The Einstein
timescale is the ratio of the lensing cross-sectional radius and
relative velocities. Velocities are known from kinematics, and the
cross-section is directly proportional to the {unknown} lensing mass.
We cannot easily measure these quantities without knowing the
amplification, hence the baseline magnitude, which requires the
resolution of HST to find the source star. This makes a crucial
difference because M31 lens mass determinations can be more accurate
than those towards the Magellanic Clouds through our Galaxy’s halo
{for the same number of microlensing events} due to the better
constrained geometry in the M31 microlensing situation. Furthermore,
our larger survey, just completed, should yield at least 100 M31
microlensing events, more than any Magellanic survey. A small amount
of ACS+WFPC2 imaging will deliver the potential of this large database
{about 350 nights}. For the whole survey {and a delta-function mass
distribution} the mass error should approach only about 15%, or about
6% error in slope for a power-law distribution. These results will
better allow us to pinpoint the lens halo fraction, and the shape of
the halo lens spatial distribution, and allow
generalization/comparison of the nature of halo dark matter in spiral
galaxies. In addition, we will be able to establish the baseline
magnitude for about 50, 000 variable stars, as well as measure an
unprecedentedly detailed color-magnitude diagram and luminosity
function over much of M31.

FGS 10010

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.

FGS 10202

Resolving OB Binaries in the Carina Nebula, Resuming the Survey

In March 2002 we carried out a small, high-angular resolution survey
of some of the brightest OB stars in the Carina Nebula with FGS1r in
an attempt to resolve binary systems which had thus far evaded
detection by other techniques. Of 23 stars observed, 5 new OB binaries
were discovered with component separations ranging from 0.015"
to0.325". This yield over the spatial domain of FGS1r’s angular
resolution, coupled with published statistics of the incidence of OB
stars in short-period spectroscopic, and long-period visual binaries
suggests that the fraction of binarity or multiplicity among OB stars
is near unity. Our unexpected resolution of the prototype O2 If* star
HD 93129A as a 55 milli-arcsecond double is a case in point that great
care must be exercised when one attempts to establish the IMF and
upper-mass cuttoff at the high-mass end of the HR diagram. We propose
to resume the survey to observe a larger, statistically meaningful
sample of OB stars to establish a firm assessment of multiplicity at
the high-mass end of the IMF in these clusters. We will also
investigate the single-star/binary-star status of several
astrophysically important, individual stars in order to enable a
better understanding of the evolution of high-mass stars.

NIC1/NIC2/NIC3 8793

NICMOS Post-SAA calibration – CR Persistence Part 4

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 images. Each observation will need its
own CRMAP, as different SAA passages leave different imprints on the
NICMOS detectors.

NIC2 10176

Coronagraphic Survey for Giant Planets Around Nearby Young Stars

A systematic imaging search for extra-solar Jovian planets is now
possible thanks to recent progress in identifying "young stars near
Earth". For most of the proposed young {<~ 30 Myrs} and nearby {<~ 60
pc} targets, we can detect a few Jupiter-mass planets as close as a
few tens of AUs from the primary stars. This represents the first time
that potential analogs of our solar system – that is planetary systems
with giant planets having semi-major axes comparable to those of the
four giant planets of the Solar System – come within the grasp of
existing instrumentation. Our proposed targets have not been observed
for planets with the Hubble Space Telescope previously. Considering
the very successful earlier NICMOS observations of low mass brown
dwarfs and planetary disks among members of the TW Hydrae Association,
a fair fraction of our targets should also turn out to posses low mass
brown dwarfs, giant planets, or dusty planetary disks because our
targets are similar to {or even better than} the TW Hydrae stars in
terms of youth and proximity to Earth. Should HST time be awarded and
planetary mass candidates be found, proper motion follow-up of
candidate planets will be done with ground-based AOs.

NIC2 10177

Solar Systems In Formation: A NICMOS Coronagraphic Survey of
Protoplanetary and Debris Disks

Until recently, despite decades of concerted effort applied to
understanding the formation processes that gave birth to our solar
system, the detailed morphology of circumstellar material that must
eventually form planets has been virtually impossible to discern. The
advent of high contrast, coronagraphic imaging as implemented with the
instruments aboard HST has dramatically enhanced our understanding of
natal planetary system formation. Even so, only a handful of evolved
disks {~ 1 Myr and older} have been imaged and spatially resolved in
light scattered from their constituent grains. To elucidate the
physical processes and properties in potentially planet-forming
circumstellar disks, and to understand the nature and evolution of
their grains, a larger spatially resolved and photometrically reliable
sample of such systems must be observed. Thus, we propose a highly
sensitive circumstellar disk imaging survey of a well-defined and
carefully selected sample of YSOs {1-10 Myr T Tau and HAeBe stars} and
{> app 10 Myr} main sequence stars, to probe the posited epoch of
planetary system formation, and to provide this critically needed
imagery. Our resolved images will shed light on the spatial
distributions of the dust in these thermally emissive disks. In
combination with their long wavelength SEDs the physical properties of
the grains will be discerned, or constrained by our photometrically
accurate surface brightness sensitivity limits for faint disks which
elude detection. Our sample builds on the success of the exploratory
GTO 7233 program, using two-roll per orbit PSF-subtracted NICMOS
coronagraphy to provide the highest detection sensitivity to the
smallest disks around bright stars which can be imaged with HST. Our
sample will discriminate between proposed evolutionary scenarios while
providing a legacy of cataloged morphologies for interpreting mid- and
far-IR SEDs that the recently launched Spitzer Space Telescope will

WFPC2 10071

WFPC2 CYCLE 12 Supplemental Darks Part 3/3

This dark calibration program obtains 3 dark frames every day to
provide data for monitoring and characterizing the evolution of hot


Significant Spacecraft Anomalies: (The following are preliminary
reports of potential non-nominal performance that will be
investigated.) None



                           SCHEDULED     SUCCESSFUL    FAILURE TIMES
FGS GSacq               17                        17
FGS REacq               05                         05
FHST Update             15                         15


SMS SA229O01 is being rerun to accommodate a Target-of-Opportunity.
Products will be available Day 232 around noon, planned intercept time
is 233/03:45Z with first uplink opportunity @ 232/20:25Z

SpaceRef staff editor.