NASA Hubble Space Telescope Daily Status Report # 4406

Notice: Due to the conversion of some ACS WFC or HRC observations into WFPC2, or NICMOS observations after the loss of ACS CCD science capability in January, there may be an occasional discrepancy between a proposal’s listed (and correct) instrument usage and the abstract that follows it.

HUBBLE SPACE TELESCOPE DAILY REPORT # 4406

– Continuing to collect World Class Science

PERIOD COVERED: UT July 17, 2007 (DOY 198)

OBSERVATIONS SCHEDULED

ACS/WFC 10787

Modes of Star Formation and Nuclear Activity in an Early Universe Laboratory

Nearby compact galaxy groups are uniquely suited to exploring the mechanisms of star formation amid repeated and ongoing gravitational encounters, conditions similar to those of the high redshift universe. These dense groups host a variety of modes of star formation, and they enable fresh insights into the role of gas in galaxy evolution. With Spitzer mid-IR observations in hand, we have begun to obtain high quality, multi-wavelength data for a well-defined sample of 12 nearby {<4500km/s} compact groups covering the full range of evolutionary stages. Here we propose to obtain sensitive BVI images with the ACS/WFC, deep enough to reach the turnover of the globular cluster luminosity function, and WFPC2 U-band and ACS H-alpha images of Spitzer- identified regions hosting the most recent star formation. In total, we expect to detect over 1000 young star clusters forming inside and outside galaxies, more than 4000 old globular clusters in

>40 giant galaxies {including 16 early-type galaxies}, over 20 tidal

features, approximately 15 AGNs, and intragroup gas in most of the 12 groups. Combining the proposed ACS images with Chandra observations, UV GALEX observations, ground-based H-alpha imaging, and HI data, we will conduct a detailed study of stellar nurseries, dust, gas kinematics, and AGN. ACS/WFC/WFPC2 10904

Star formation in extended UV disk {XUV-disk} galaxies

The Galaxy Evolution Explorer {GALEX} has discovered the existence of extended UV-disk {XUV-disk} galaxies. This class of intriguing spiral galaxies is distinguished by UV-bright regions of star formation located at extreme galactocentric radii, commonly reaching many times the optical extent of each target. XUV-disks represent a population of late-type galaxies still actively building, or significantly augmenting, their stellar disk in the outer, low-density environment. Prior to GALEX, such regions were considered to be far more stable against star formation than now realized. Our work on these targets has led to the recognition of the XUV phenomenon as probing a diverse population of galaxies which, although having certain commonality in terms of their present XUV star formation, have apparently experienced different star formation histories {as judged by their outer disk UV-optical colors and morphology}. In ordinary spirals, disk formation occurred at a much earlier epoch, making today’s XUV-disks useful templates for commonplace, high z galaxies. The diverse XUV-disks in our sample may represent snapshots of different phases in the disk building process. We seek to characterize the demographics of star forming regions occupying this environmental range, especially in contrast to their inner disk counterparts. HST imaging is needed to accurately characterize the massive stars and clusters which have, in fact, managed to form. The GALEX observations are limited by 5″ resolution. Deep ACS FUV, B, V, I, and H-alpha imaging {along with parallel WFPC2 data} will allow: {1} photometric classification of the OB star population, {2} constraint on the cluster mass function and age distribution, {3} critical accounting for possible leakage of Lyman continuum photons in a porous ISM or an IMF change, and {4} population synthesis modeling of the field SFH on Gyr timescales. We benefit from extensive archival HST observations of our target galaxies, although the outer disk has yet to be probed.

WFPC2 11105

The LBV progenitor of SN 2005gl – a new key to massive star evolution puzzles

The currently accepted theory regarding the last stages of massive star evolution maintains that the evolution of the envelope is coupled to that of the stellar core. For this reason, very massive stars are expected to shed their outer hydrogen envelopes before they develop large iron cores, and ultimately, explode as core-collapse supernovae {SNe}. It is therefore a strict prediction of current models that massive stars {certainly those above ~40 solar mass} will explode as hydrogen-poor SNe, i.e., of Types Ib and Ic. In particular, the class of luminous blue variables {LBVs} such as eta-Carina, which are known to be very massive {up to 100 solar masses and above} are expected to lose their entire hydrogen envelopes prior to their ultimate explosions as SNe. However, using pre-explosion HST/WFPC2 imaging of the location of the recent hydrogen- rich type IIn SN 2005gl, we have identified {Gal-Yam et al. 2007} its putative progenitor as a very luminous point source {with absolute V magnitude of -10.2}. If this is a single star, it must be an LBV from luminosity considerations {no other stars are as luminous}. If our progenitor identification is correct, at least in some cases, massive stars explode before losing most of their hydrogen envelope, indicating the core and envelope are decoupled, and requiring revision of stellar evolution theory. Here, we propose a single-orbit HST observation of the location of SN 2005gl designed to test whether the point source we identified as its LBV progenitor has indeed disappeared {as expected from a single star} or remained unchanged {as expected, e.g., if it is a compact star cluster}. These data are the last observational ingredient required to firmly establish {or refute} the explosion of an LBV as a type IIn SN, with fundamental implications for the theory of massive star evolution. Since the new data will be compared to pre-explosion WFPC2 images, this program is perfectly suited to be carried out with the WFPC2 camera.

WFPC2 11178

Probing Solar System History with Orbits, Masses, and Colors of Transneptunian Binaries

The recent discovery of numerous transneptunian binaries {TNBs} opens a window into dynamical conditions in the protoplanetary disk where they formed as well as the history of subsequent events which sculpted the outer Solar System and emplaced them onto their present day heliocentric orbits. To date, at least 47 TNBs have been discovered, but only about a dozen have had their mutual orbits and separate colors determined, frustrating their use to investigate numerous important scientific questions. The current shortage of data especially cripples scientific investigations requiring statistical comparisons among the ensemble characteristics. We propose to obtain sufficient astrometry and photometry of 23 TNBs to compute their mutual orbits and system masses and to determine separate primary and secondary colors, roughly tripling the sample for which this information is known, as well as extending it to include systems of two near-equal size bodies. To make the most efficient possible use of HST, we will use a Monte Carlo technique to optimally schedule our observations.

WFPC2 11218

Snapshot Survey for Planetary Nebulae in Globular Clusters of the Local Group

Planetary nebulae {PNe} in globular clusters {GCs} raise a number of interesting issues related to stellar and galactic evolution. The number of PNe known in Milky Way GCs, 4, is surprisingly low if one assumes that all stars pass through a PN stage. However, it is likely that the remnants of stars now evolving in Galactic GCs leave the AGB so slowly that any ejected nebula dissipates long before the star becomes hot enough to ionize it. Thus there should not be ANY PNe in Milky Way GCs–but there are four! It has been suggested that these PNe are the result of mergers of binary stars within GCs, i.e., that they are descendants of blue stragglers. The frequency of occurrence of PNe in external galaxies poses more questions, because it shows a range of almost an order of magnitude. I propose a Snapshot survey aimed at discovering PNe in the GC systems of Local Group galaxies more distant than the Magellanic Clouds. These clusters, some of which may be much younger than their counterparts in the Milky Way, might contain many more PNe than those of our own galaxy. I will use the standard technique of emission-line and continuum imaging, which easily discloses PNe.

WFPC2 11223

The Key to Understanding RR Lyr Stars: WFPC2 Observations of a Unique LMC EB with a RR Lyr Component

We are proposing HST/WFPC2 2550-10420A multi-band photometry of an important “unique” LMC eclipsing binary with an RR Lyr component. This binary is the only bona fide eclipsing binary {EB} with an RR Lyr component. Because of their constant mean luminosities {L ~ 45 Lsun; ~ +0.5 mag} and easily recognizable light curves, RR Lyr variables have long served as the “cornerstone” of the Pop II distance scale in our galaxy and for Local Group galaxies. However, in spite of their critical importance to astronomy, there is a paucity of fundamental data available for RR Lyr stars. In fact, there are no direct measures of their most fundamental properties – such as Mass, Radius and Luminosity. The astrophysical and cosmological consequences of finding an RR Lyr star in an EB are considerable, because the masses and absolute radii of the stars of eclipsing binaries can be determined to within a few percent from time-tested analyses of their light and radial velocity curves. With accurate temperatures and ISM absorption values, determined from the proposed WFPC2 observations, it is possible to determine reliable stellar luminosities and distances. It is for these reasons that we propose WFPC2 observations of the recently discovered detached LMC eclipsing binary OGLE J052218.07-692827.4 { ~18.6- mag; 0 ~+0.27; Porb = 8.9231-d}; the RR Lyr primary component has a pulsation period of P{RR} = 0.564876-d. This important binary star is an integral part of our on-going multi- wavelength study of selected eclipsing binaries in nearby galaxies. Three HST/WFPC2 orbits are requested to determine complementary accurate Teff, log g and ISM absorption {A-lambda} for the component stars. These quantities will be combined with the fundamental stellar data being determined from our ground-based radial velocity and photometric observations. The combined observations will yield accurate stellar masses, radii, temperatures and luminosities, as well as a direct distance to the binary and LMC-Bar. This RR Lyr/EB thus offers the unprecedented opportunity to: {1} determine directly {and for the first time} the fundamental physical properties {M, R, L} of an RR Lyr star, {2} directly calibrate “in situ” the zero-point of the LMC RR Lyr – P – Mv – Z relation and {3} to derive an additional accurate distance to the Bar region of the LMC.

WFPC2 11293

The Dynamical Mass of the Bright Cepheid Polaris

Cepheid variables are of central importance in Galactic and extragalactic astronomy. They are the primary standard candles for measuring extragalactic distances, and they provide critical tests of stellar-evolution theory. Surprisingly, however, until now there was not a single Cepheid with a purely dynamical measurement of its mass. Polaris {alpha UMi} is the nearest and brightest of all Cepheids. It offers the unique opportunity to measure the dynamical mass of a Cepheid, because it is in a binary system for which a single-lined spectroscopic orbit is already available. In Cycle 14, we resolved the system in the UV using ACS/HRC, thus providing the first direct detection of the companion, as well as a first approximation to the dynamical mass. In the present proposal we request one HST orbit per year for the next 3 Cycles, in order to refine the visual orbit. Combined with the HST/FGS parallax {see below}, this program will provide an accurate mass for the Cepheid {the error should be about 0.5 Msun by Cycle 17}, and the only one based purely on dynamical information. Only HST’s combination of high spatial resolution and UV sensitivity can achieve this result. The parallax is a key ingredient in the mass determination. In an ongoing multi-year program {GO-9888, GO-10113, GO-10482}, we are using the FGS to improve significantly upon the Hipparcos parallax of Polaris. The WFPC2 imaging proposed here is a continuation of our program initiated with the ACS. These observations will provide extremely valuable astrophysical information from a very modest additional investment of observing time.

FLIGHT OPERATIONS SUMMARY:

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

HSTARS:

#10898 – GSAcq (1,2,1) failed to RGA hold (Gyro Control).

Upon acquisition of signal (AOS) at 198/17:38:14, the GSAcq(1,2,1) scheduled at 198/17:27:23 – 17:35:14 had failed to RGA Hold due to (QF1STOPF) stop flag indication on FGS-1. Pre-acquisition OBADs attitude correction values not available due to LOS. Post-acq OBAD/MAP had (RSS) value of 18.16 arcseconds.

#10899 – REacq(2,1,2) failed, Search Radius Limit exceeded on FGS 2.

Upon acquisition of signal at 198/22:41:27 vehicle was in gyro control, REacq(2,1,2) at 22:14:25 failed with search radius limit exceeded on FGS 2. OBAD prior to GSACQ at 22:10:05 had RSS error of 18.01 arcseconds.

COMPLETED OPS REQUEST: (None)

                                SCHEDULED SUCCESSFUL 
FGS GSacq                       06             05 
FGS REacq                       06             05 
OBAD with Maneuver              24              24 

COMPLETED OPS NOTES: (None)

SIGNIFICANT EVENTS: (None)