HUBBLE SPACE TELESCOPE - Continuing to collect World Class Science

DAILY REPORT      #4837

PERIOD COVERED: 5am April 20 - 5am April 21, 2009 (DOY

                           110/0900z-111/0900z)

OBSERVATIONS SCHEDULED

ACS/SBC 11982

Spanning the Reionization History of IGM Helium: a Large and Efficient

HST Spectral Survey of Far-UV-Bright Quasars

The reionization of IGM helium is thought to have occurred at redshifts

of z=3 to 4. Detailed studies of HeII Lyman-alpha absorption toward a

handful of QSOs at 2.7<z<3.3 demonstrated the high potential of such IGM

probes, but the small sample size and redshift range limit confidence in

cosmological inferences. The requisite unobscured sightlines to high-z

are extremely rare, but we've cross-correlated 10, 000 z>2.8 SDSS DR7

(and other) quasars with GALEX GR4 UV sources to obtain 550 new, high

confidence, sightlines potentially useful for HST HeII studies; and in

cycle 15-16 trials we demonstrated the efficacy of our SDSS/GALEX

selection approach identifying 9 new HeII quasars at unprecedented 67%

efficiency. We propose the first far-UV-bright HeII quasar survey that

is both large in scale and also efficient, via 2-orbit reconnaissance

ACS/SBC prism spectra toward a highly select subset of 40 new SDSS/GALEX

quasars at 3.1<z<5.1. These will provide a community resource list that

includes 5 far-UV-bright (restframe) HeII sightlines in each of 8

redshift bins spanning 3.1<z<3.9 (and perhaps several objects at z>4),

enabling superb post-SM4 follow-up spectra with COS or STIS. But

simultaneously and independent of any SM4 uncertainties, we will hereby

directly obtain 10-orbit UV spectral stacks from the 5 HeII quasars in

each of the 8 redshift bins to trace the reionization history of IGM

helium over at least 3.1<z<3.9. These spectral stacks will average over

cosmic variance and individual object pathology. Our new high-yield HeII

sightline sample and spectral stacks, covering a large redshift range,

will allow confident conclusions about the spectrum and evolution of the

ionizing background, the evolution of HeII opacity, the density of IGM

baryons, and the epoch of helium reionization.

WFPC2 11302

WFPC2 CYCLE 16 Standard Darks - Part III

This dark calibration program obtains dark frames every week in order to

provide data for the ongoing calibration of the CCD dark current rate,

and to monitor and characterize the evolution of hot pixels. Over an

extended period these data will also provide a monitor of radiation

damage to the CCDs.

WFPC2 11793

WFPC2 Cycle 16 Internal Monitor

This calibration proposal is the Cycle 15 routine internal monitor for

WFPC2, to be run weekly to monitor the health of the cameras. A variety

of internal exposures are obtained in order to provide a monitor of the

integrity of the CCD camera electronics in both bays (both gain 7 and

gain 15 -- to test stability of gains and bias levels), a test for

quantum efficiency in the CCDs, and a monitor for possible buildup of

contaminants on the CCD windows. These also provide raw data for

generating annual super-bias reference files for the calibration

pipeline.

WFPC2 11975

UV Light from Old Stellar Populations: a Census of UV Sources in

Galactic Globular Clusters

In spite of the fact that HST has been the only operative

high-resolution eye in the UV-window over the last 18 years, no

homogeneous UV survey of Galactic globular clusters (GGCs) has been

performed to date. In order to fill this gap in the stellar population

studies, we propose a program that exploits the unique capability of the

WFPC2 and the SBC in the far-/mid- UV for securing deep UV imaging of 46

GGCs. The proposed observations will allow to study with unprecedented

accuracy the hottest GGC stars, comprising the extreme horizontal branch

(HB) stars and their progeny (the so-called AGB-manque', and Post-early

AGB stars), and "exotic stellar populations" like the blue straggler

stars and the interacting binaries. The targets have been selected to

properly sample the GGC metallicity/structural parameter space, thus to

unveil any possible correlation between the properties of the hot

stellar populations and the cluster characteristics. In addition, most

of the targets have extended HB "blue tails", that can be properly

studied only by means of deep UV observations, especially in the far-UV

filters like the F160BW, that is not foreseen on the WFC3. This data

base is complemented with GALEX observations in the cluster outermost

regions, thus allowing to investigate any possible trend of the

UV-bright stellar types over the entire radial extension of the

clusters. Although the hottest GGC stars are just a small class of

"special" objects, their study has a broad relevance in the context of

structure formation and chemical evolution in the early Universe,

bringing precious information on the basic star formation processes and

the origin of blue light from galaxies. Indeed, the proposed

observations will provide the community with an unprecedented data set

suitable for addressing a number of still open astrophysical questions,

ranging from the main drivers of the HB morphology and the mass loss

processes, to the origin of the UV upturn in elliptical galaxies, the

dating of distant systems from integrated light, and the complex

interplay between stellar evolution and dynamics in dense stellar

aggregates. In the spirit of constructing a community resource, we

entirely waive the proprietary period for these observations.

WFPC2 11981

FUV Imaging Survey of Galactic Open Clusters

We propose a WFPC2 FUV imaging survey of 6 Galactic open clusters with

ages ranging from 1 Myr to 300 Myr complemented with NUV/optical imaging

of the same fields. No such survey has ever been attempted before in the

FUV at the resolution of WFPC2 (indeed, no WFPC2 FUV images of any

Galactic open cluster exist in the HST archive) and, since WFPC2 will be

retired in SM4 and none of the other HST instruments can do FUV imaging

of bright objects, this is the last chance to do such a survey before

another UV telescope is launched. This survey will provide a new

perspective on young/intermediate age Galactic clusters and a key

template for the study of star formation at high redshift, where the

intensity peak we observe in the optical/NIR from Earth is located in

the FUV in its rest frame. For clusters still associated with an H II

region, UV imaging maps the continuum emission of the ionized gas and

the radiation scattered by background dust and, combined with optical

nebular images, can be used to determine the 3-D structure of the H II

region. For all young clusters, FUV+NUV+optical photometry can be used

to study the UV excesses of T-Tauri stars. For clusters older than ~40

Myr, the same photometric combination is the easiest method to detect

companion white dwarfs which are invisible using only the optical and

NIR. WFPC2 is also an excellent instrument to discover close companions

around bright stars and improve our knowledge of their multiplicity

fraction. Finally, for all clusters, the combination of

high-spatial-resolution UV and optical photometry can be used to

simultaneously measure the temperature, extinction, extinction law,

distance, and existence of companions (resolved and unresolved) and,

thus, produce clean HR diagrams with resolved cluster membership and

much-reduced systematic uncertainties.

FLIGHT OPERATIONS SUMMARY:

Significant Spacecraft Anomalies: (The following are preliminary reports

of potential non-nominal performance that will be investigated.)

HSTARS:

11775 - REAcq(2,1,1) scheduled at 110/09:28:24 failed due to receiving

           QF1STOPF,QSTOP,QF1SSDIF,and QF2SSDIF on FGS2.

           Observations affected: WFPC 42 - 44, Proposal ID# 11981

COMPLETED OPS REQUEST: (None)

COMPLETED OPS NOTES: (None)

                      SCHEDULED      SUCCESSFUL

FGS GSAcq               04                 04

FGS REAcq               09                 08                    

OBAD with Maneuver 26                 26

SIGNIFICANT EVENTS: (None)