HUBBLE SPACE TELESCOPE - Continuing to collect World Class Science

DAILY REPORT       #4807

PERIOD COVERED: 5am March 9 - 5am March 10, 2009 (DOY

                           068/1000z-069/1000z)

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.

FGS 11704

The Ages of Globular Clusters and the Population II Distance Scale

Globular clusters are the oldest objects in the universe whose age can

be accurately determined. The dominant error in globular cluster age

determinations is the uncertain Population II distance scale. We propose

to use FGS 1r to obtain parallaxes with an accuracy of 0.2

milliarcsecond for 9 main sequence stars with [Fe/H] < -1.5. This will

determine the absolute magnitude of these stars with accuracies of 0.04

to 0.06mag. This data will be used to determine the distance to 24

metal-poor globular clusters using main sequence fitting. These

distances (with errors of 0.05 mag) will be used to determine the ages

of globular clusters using the luminosity of the subgiant branch as an

age indicator. This will yield absolute ages with an accuracy 5%, about

a factor of two improvement over current estimates. Coupled with

existing parallaxes for more metal-rich stars, we will be able to

accurately determine the age for globular clusters over a wide range of

metallicities in order to study the early formation history of the Milky

Way and provide an independent estimate of the age of the universe.

The Hipparcos database contains only 1 star with [Fe/H] < -1.4 and an

absolute magnitude error less than 0.18 mag which is suitable for use in

main sequence fitting. Previous attempts at main sequence fitting to

metal-poor globular clusters have had to rely on theoretical

calibrations of the color of the main sequence. Our HST parallax program

will remove this source of possible systematic error and yield distances

to metal-poor globular clusters which are significantly more accurate

than

possible with the current parallax data. The HST parallax data will have

errors which are 10 times smaller than the current parallax data. Using

the HST parallaxes, we will obtain main sequence fitting distances to 11

globular clusters which contain over 500 RR Lyrae stars. This will allow

us to calibrate the absolute magnitude of RR Lyrae stars, a commonly

used Population II distance indicator.

WFPC2 11944

Binaries at the Extremes of the H-R Diagram

We propose to use HST/Fine Guidance Sensor 1r to survey for binaries

among some of the most massive, least massive, and oldest stars in our

part of the Galaxy. FGS allows us to spatially resolve binary systems

that are too faint to observe using ground-based, speckle or optical

long baseline interferometry, and too close to resolve with AO. We

propose a SNAP-style program of single orbit FGS TRANS mode observations

of very massive stars in the cluster NGC 3603, luminous blue variables,

nearby low mass main sequence stars, cool subdwarf stars, and white

dwarfs. These observations will help us to (1) identify systems suitable

for follow up studies for mass determination, (2) study the role of

binaries in stellar birth and in advanced evolutionary states, (3)

explore the fundamental properties of stars near the main sequence-brown

dwarf boundary, (4) understand the role of binaries for X-ray bright

systems, (5) find binaries among ancient and nearby subdwarf stars, and

(6) help calibrate the white dwarf mass - radius relation.

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, expecially 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 11987

The Recent Star Formation History of SINGS Galaxies

The Spitzer Legacy project SINGS provided a unique view of the current

state of star formation and dust in a sample of galaxies of all Hubble

types. This multi-wavelength view allowed the team to create current

star formation diagnostics that are independent of the dust content and

increased our understanding of the dust in galaxies. Even so, using the

SINGS data alone we can only make rough estimates of the recent star

formation history of these galaxies. The lack of U-band observations

means that it is impossible to estimate the ages of young clusters. In

addition, the low resolution of the Spitzer and ground-based

observations means that what appear to be individual Spitzer sources can

actually be composed of many individual clusters with varying ages. In

this proposal we plan to address this missing area in SINGS by obtaining

high-resolution WFPC2 UBVI observations to accurately find and determine

the ages of the young stellar clusters in a subset of the SINGS

galaxies. These observations will greatly enhance the legacy value of

the SINGS observations while also directly answering questions

pertaining to star formation in galaxies.

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: 

18406-2 - Modify OBAD Angular Separation Limit @ 068/1512z

COMPLETED OPS NOTES: (None)

                      SCHEDULED      SUCCESSFUL 

FGS GSAcq               07                 07                

FGS REAcq               06                 06          

OBAD with Maneuver 22                 22

SIGNIFICANT EVENTS:

OBAD Angular Separation Limit Modification:

At 068/1512z Ops Request 18406 was successfully executed to modify the

OBAD angular separation limit from 18 to 75 arc-seconds. The increase in

the angular separation limit was desired to prevent duplicate stars from

being used in the attitude orientation check process (used for finding

star pairs).