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3\10 HST Daily Rpt No 3316
Part 2 of 3

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.

ACS 9480

Cosmic Shear With ACS Pure Parallels

Small distortions in the shapes of background galaxies by foreground 
mass provide a powerful method of directly measuring the amount and 
distribution of dark matter. Several groups have recently detected 
this weak lensing by large-scale structure, also called cosmic shear. 
The high resolution and sensitivity of HST/ACS provide a unique 
opportunity to measure cosmic shear accurately on small scales. Using 
260 parallel orbits in Sloan textiti {F775W} we will measure for the 
first time: beginlistosetlength sep0cm setlengthemsep0cm setlength 
opsep0cm em the cosmic shear variance on scales <0.7 arcmin, em the
skewness of the shear distribution, and em the magnification effect. 
endlist Our measurements will determine the amplitude of the mass 
power spectrum sigma_8Omega_m^0.5, with signal-to-noise {s/n} ~ 20, 
and the mass density Omega_m with s/n=4. They will be done at small 
angular scales where non-linear effects dominate the power spectrum, 
providing a test of the gravitational instability paradigm for 
structure formation. Measurements on these scales are not possible 
from the ground, because of the systematic effects induced by PSF
smearing from seeing. Having many independent lines of sight reduces 
the uncertainty due to cosmic variance, making parallel observations 
ideal.

ACS/WFPC2 9481

Pure Parallel Near-UV Observations with WFPC2 within High-Latitude ACS 
Survey Fields

In anticipation of the allocation of ACS high-latitude imaging 
survey{s}, we request a modification of the default pure parallel 
program for those WFPC2 parallels that fall within the ACS survey 
field. Rather than duplicate the red bands which will be done much 
better with ACS, we propose to observe in the near-ultraviolet F300W 
filter. These data will enable study of the rest-frame ultraviolet 
morphology of galaxies at 0 * Origin: SpaceBase[tm] Vancouver Canada [3 Lines] 604-473-9357 (1:153/719)

SEEN-BY: 633/267 270

@PATH: 153/719 715 7715 140/1 106/2000 633/267