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3\11 HST Daily Rpt No 3317
Part 1 of 2

 HUBBLE SPACE TELESCOPE
  DAILY REPORT # 3317

PERIOD COVERED: DOY 70

OBSERVATIONS SCHEDULED

NICMOS 8791

NICMOS Post-SAA calibration - CR Persistence Part 2

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.

ACS 9352

The Deceleration Test from Treasury Type Ia Supernovae at Redshifts 
1.2 to 1.6

Type Ia supernovae {SNe Ia} provide the only direct evidence for an 
accelerating universe, an extraordinary result that needs a rigorous 
test. The case for cosmic acceleration rests on the observation that 
SNe Ia at z ~ 0.5 are ~ 0.25 mag fainter than they would be in a 
universe without acceleration. A powerful and straightforward way to 
assess the reliability of the SN Ia measurement and the conceptual 
framework of its interpretation is to look for cosmic deceleration at 
z >= 1. This would be a clear signature of a mixed dark-matter and 
dark-energy universe. Systematic errors in the SN Ia result attributed 
to grey dust or cosmic evolution of the SN Ia peak luminosity would 
not show this change of sign. We have demonstrated proof of this 
concept with a single SN Ia, SN 1997ff at z = 1.7, found and followed 
by HST. The results suggest an early epoch of deceleration, but this 
is too important a conclusion to rest on just one object. Here we 
propose to use HST for observations of six SNe Ia in the range 1.2 <= 
z <= 1.6, that will be discovered as a byproduct from proposed 
Treasury programs for high-latitude ACS surveys. Six objects will 
provide a much firmer foundation for a conclusion that touches on 
important questions of fundamental physics.

NICMOS 9360

Paschen-alpha Imaging of a SIRTF-Selected Nearby Galaxy Sample

We propose to carry out a NICMOS snapshot survey in the Paschen-alpha 
{PAlpha} emission line and H-band of the sample of galaxies being 
observed at 3.5 -- 160 microns as part of SIRTF Nearby Galaxies Survey 
{SINGS} and a related guaranteed time survey of starburst galaxies. 
The PAlpha images, accessible only from HST, will be combined with 
groundbased HAlpha imaging to measure the extinction in the 
star-forming centers of these galaxies, and obtain robust, extinction-
corrected maps of the massive star formation rate {SFR}. The PAlpha 
data by themselves will provide reliable `extinction- free' SFRs, and 
a cross-calibration of the {dust--affected} HAlpha-- and UV--based 
SFRs. The PAlpha--based SFR measurements will extend the SFR-vs.-gas 
density law {Schmidt--law} to surface densities at least 30 times 
higher than what is accessible using HAlpha--based SFR measurements 
alone, bridging the gap between normal galaxies and IR--luminous 
starbursts. Furthermore, the combination of the HST PAlpha images with 
the SIRTF images and spectra, as well as ancillary ground--based 
UBVRIJHK images and GALEX UV images being obtained as part of the
SINGS project, will provide a definitive study of the radiative 
transfer of starlight and dust heating in star--forming galaxies. The 
processed NICMOS images will be incorporated into the public SINGS 
Legacy Data Archive, to enable scores of follow-up studies by the 
astronomical community at large.

ACS 9450

The lensing galaxy of JVAS B0218+357: determination of H_0

Much effort has been devoted to estimating Hubble's constant H_0 using
observations of very nearby objects. Gravitational lensing time delays 
offer potentially the most accurate method for determining H_0 using 
observations on cosmological scales; it is a very clean method in that 
little complicated astrophysics is involved, and it is a single--step 
method compared to the traditional multi--step distance ladder. The 
major problem with most such determinations in the past has been 
systematic errors due to uncertainties in the lens mass model, leading 
to 20 Einstein-ring lens system, is the one system for which these 
systematic uncertainties can be reduced very substantially, and in 
particular is unique in that the modeling systematics can be reduced 
to the level of the uncertainties in the measurement of the time 
delay. The only requirement left is to be able accurately to locate 
the center of the lensing galaxy. We propose an extremely deep ACS 
image in I-band of this system for this purpose; the prize is a robust 
5 {lens mass model}. We have conducted simulations to estimate the 
necessary S:N ratio in an ACS observation in order to be able to 
achieve a successful deconvolution of the lens galaxy and lensed 
images with the required accuracy.

ACS 9463

Are OH/IR stars the youngest post-AGB stars? An ACS SNAPshot imaging 
survey

Essentially all well-characterized preplanetary nebulae {PPNs}-- 
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? We have recently hypothesized that most OH/IR stars 
{evolved mass- losing stars with OH maser emission} are very young 
PPNe. We propose an ACS/SNAPshot imaging survey of a large, 
morphologically unbiased sample of these objects, selected using their 
IRAS 12-to-25micron colors. Our ground-based imaging study of OH/IR 
stars has revealed a few compact bipolar objects, supporting our 
hypothesis. However since most objects remain unresolved, HST 
observations are needed to determine how and when the bipolar geometry 
asserts itself. Our complementary program of interferometric mapping 
of the OH maser emission in our sources is yielding kinematic 
information with spatial resolution comparable to that in the HST 
images. The HST/radio data will provide crucial input for theories of 
post-AGB stellar evolution. In addition, these data will also indicate 
whether the multiple concentric rings, ``searchlight beams'', and 
truncated equatorial disks recently discovered with HST in a few PPNs, 
are common or rare phenomena.

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