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Daily #4025



 
 
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Old January 11th 06, 02:51 PM posted to sci.astro.hubble
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Default Daily #4025

HUBBLE SPACE TELESCOPE - Continuing to collect World Class Science

DAILY REPORT #4025

PERIOD COVERED: UT January 10, 2006 (DOY 010)

OBSERVATIONS SCHEDULED

ACS/HRC 10476

Accurate Mass Determination of the Ancient White Dwarf ER 8 Through
Astrometric Microlensing

We propose to determine the mass of the very cool white dwarf ER 8
through astrometric microlensing. We have predicted that ER 8 will
pass very close to a 15th-mag background star in January 2006, with an
impact parameter of less than 0.05 arcsec. As it passes in front, it
will cause a deflection of the background star's image by 8
milliarcsec, an amount easily detectable with HST/FGS. The
gravitational deflection angle depends only on the distances and
relative positions of the stars, and on the mass of the white dwarf.
Since the distances and positions can be determined precisely before
the event, the astrometric measurement offers a unique and direct
method to measure the mass of the white dwarf to high accuracy {5%}.
Unlike all other stellar mass determinations, this technique works for
single stars {but only if they are nearby and of sufficient mass}. The
mass of ER 8 is of special interest because it is a member of the
Galactic halo, and appears to be the oldest known field white dwarf.
This object can thus set a lower limit on the age of the Galactic
halo, but since white-dwarf cooling rates depend on their masses, the
mass is a necessary ingredient in the age determination. As a
byproduct, we will obtain an accurate parallax for ER 8, and thus its
luminosity and {from its effective temperature} its radius. Such
quantities are at present rather poorly known for the coolest white
dwarfs, and will provide strong constraints on white-dwarf physics.

ACS/WFC 10496

Decelerating and Dustfree: Efficient Dark Energy Studies with
Supernovae and Clusters

We propose a novel HST approach to obtain a dramatically more useful
"dust free" Type Ia supernovae {SNe Ia} dataset than available with
the previous GOODS searches. Moreover, this approach provides a
strikingly more efficient search-and-follow-up that is primarily pre-
scheduled. The resulting dark energy measurements do not share the
major systematic uncertainty at these redshifts, that of the
extinction correction with a prior. By targeting massive galaxy
clusters at z 1 we obtain a five-times higher efficiency in
detection of Type Ia supernovae in ellipticals, providing a
well-understood host galaxy environment. These same deep cluster
images then also yield fundamental calibrations required for future
weak lensing and Sunyaev-Zel'dovich measurements of dark energy, as
well as an entire program of cluster studies. The data will make
possible a factor of two improvement on supernova constraints on dark
energy time variation, and much larger improvement in systematic
uncertainty. They will provide both a cluster dataset and a SN Ia
dataset that will be a longstanding scientific resource.

ACS/WFC 10543

Microlensing in M87 and the Virgo Cluster

Resolving the nature of dark matter is an urgent problem. The results
of the MACHO survey of the Milky Way dark halo toward the LMC indicate
that a significant fraction of the halo consists of stellar mass
objects. The VATT/Columbia survey of M31 finds a similar lens fraction
in the M31 dark halo. We propose a series of observations with ACS
that will provide the most thorough search for microlensing toward
M87, the central elliptical galaxy of the Virgo cluster. This program
is optimized for lenses in the mass range from 0.01 to 1.0 solar
masses. By comparing with archival data, we can detect lenses as
massive as 100 solar masses, such as the remnants of the first stars.
These observations will have at least 15 times more sensitivity to
microlensing than any previous survey, e.g. using WFPC2. This is due
to the factor of 2 larger area, factor of more than 4 more sensitivity
in the I-band, superior pixel scale and longer baseline of
observations. Based on the halo microlensing results in the Milky Way
and M31, we might expect that galaxy collisions and stripping would
populate the overall cluster halo with a large number of stellar mass
objects. This program would determine definitively if such objects
compose the cluster dark matter at the level seen in the Milky Way. A
negative result would indicate that such objects do not populate the
intracluster medium, and may indicate that galaxy harassment is not as
vigorous as expected. We can measure the level of events due to the
M87 halo: this would be the best exploration to date of such a lens
population in an elliptical galaxy. Star-star lensing should also be
detectable. About 20 erupting classical novae will be seen, allowing
to determine the definitive nova rate for this giant elliptical
galaxy. We will determine if our recent HST detection of an M87
globular cluster nova was a fluke, or indicative of a 100x higher rate
of incidence of cataclysmic variables and nova eruptions in globulars
than previously believed. We will examine the populations of variable
stars, and will be able to cleanly separate them from microlensing.

FGS 10611

Precise Distances to Nearby Planetary Nebulae

We propose to carry out astrometry with the FGS to obtain accurate and
precise distances to four nearby planetary nebulae. In 1992, Cahn et
al. noted that ``The distances to Galactic planetary nebulae remain a
serious, if not THE most serious, problem in the field, despite
decades of study.'' Twelve years later, the same statement still
applies. Because the distances to planetary nebulae are so uncertain,
our understanding of their masses, luminosities, scale height, birth
rate, and evolutionary state is severely limited. To help remedy this
problem, HST astrometry can guarantee parallaxes with half the error
of any other available approach. These data, when combined with
parallax measurements from the USNO, will improve distance
measurements by more than a factor of two, producing more accurate
distances with uncertainties that are of the order of ~6%. Lastly,
most planetary nebula distance scales in the literature are
statistical. They require several anchor points of known distance in
order to calibrate their zero point. Our program will provide "gold
standard" anchor points by the end of 2006, a decade before any
anticipated results from future space astrometry missions.

WFPC2 10777

WFPC2 WF4 Bay 1 Temperature Reduction Test #1

A anomaly has been found in images from the WF4 CCD in WFPC2. The WF4
CCD bias level appears to have become unstable, resulting in sporadic
images with either low or zero bias level. The severity and frequency
of the problem is rapidly increasing, and it is possible that WF4 will
soon become unusable if no work-around is found. The other three CCDs
{PC1, WF2, and WF3} appear to be unaffected and continue to operate
properly. This proposal tests methods to fix the problem by adjusting
some temperatures inside WFPC2. 1 external and 24 internal orbits.

FLIGHT OPERATIONS SUMMARY:

Significant Spacecraft Anomalies: (The following are preliminary
reports of potential non-nominal performance that will be
investigated.)

HSTARS: (None)

COMPLETED OPS REQUEST: (None)

COMPLETED OPS NOTES: (None)

SCHEDULED SUCCESSFUL
FGS GSacq 08 08
FGS REacq 06 06
OBAD with Maneuver 27 27

SIGNIFICANT EVENTS: (None)


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