This week on HST

HST Programs: August 3 - August 9, 2009

SMOV still under way, but science observations being made.

Selected highlights

GO 11564: Optical and Ultraviolet Photometry of Isolated Neutron Stars

Artist's impression of a neutron star

Neutron stars are extremely compact (~10 km diameter) massive (~1.4 to 2 solar mass) remnants of high mass (> 7 solar mass) stars. Formed during the gravitational core-collapse of Type Ib, Ic and II supernovae, they have initial temperatures exceeding 1011 Kelvon, but cool very rapidly with time. Their existence was originally proposed by Zwicky and Baade in the 1930s, but they remained unobserved until the late-1960s, when Hewish and Bell identified a pulsing radio source in the Crab nebula as a neutron star. (Hewish and Okoye had previously identified "an unusual source of high radio brightness temperature" within the crab.) Neutron stars have strong magnetic fields and rapid rotation, leading to radiation beaming along the magnetic poles; the observed pulses result from the beams sweeping across the terrestrial line of sight. Numerous pulsars are currently known, but direct observations of the neutron star's "photosphere" are much rarer. The present observations will used WFPC2 to image five previously-detected neutron stars in the U and B bands, mapping the blue/ultraviolet flux distribution and enabling the potential measurement of proper motions.

GO 11570: Narrowing in on the Hubble Constant and Dark Energy

A GALEX image of Messier 106 (NGC 4258), one of the galaxies targeted in this program

The Hubble constant remains a key parameter in understanding cosmology and the evolution of the Universe. Refining measurements of H0 therefore still represents a vital means of probing the nature of dark energy. The present program aims to tackle this question by laying a firmer foundation to the SNe Ia distance scale. The WFC3 IR camera will be used to identify and characterise Cepheid variables in eight relatively nearby galaxies that have hosted Type Ia SNe. Cepheids have signficantly lower amplitude at near-infrared wavelengths, and the measured magnitudes are less subject to uncertainties due to foreground reddening and variations in metallicity. As a consequence, determining the mean apparent magnitude, and hence the period/apparent magnitude relation, is substantially more straightforward than at optical wavelengths. Matching the observed relation against reference stars from the LMC allows a more reliable determination of the distance to the parent galaxy, and hence a firmer zeropoint for the SNe Ia distance scale. The aim is to reduce the level of systematics in determinations oif H0 to the 3 percent level.

GO 11680: The Main Sequence Luminosity Function of Low-Mass Globular Clusters

Tidals tails due to stars stripped from the loose globular, Palomar 5

Globular clusters are members of the Galactic halo population that formed during the first extensive period of star formation in the Milky Way. As such, the properties of the stellar constituents within those systems can provide crucial insight into the earliest stages of galaxy formation. Most globulars are prominent concentrations of 106 to several x 10,sup>7 stars, easily distinguishable from background (and foreground) stars in the Galactic halo, disk and bulge. A handful of clusters, however, are more diffuse, with lower total mass and low central concentrations to the extent that their existence is barely discernible. Such systems are generally believed to be the product of many Gyrs of dynamical evolution, as gravitational encounters lead to stellar evaporation and tidal stripping. Previous WFPC2 observations of one such cluster, Palomar 5, revealed a strong deficiency of low-mass stars as compared with "normal" globulars. This is consistent with the hypothesis of dynamical evolution, since mass segregation due to dynamical relaxation within the cluster re-distributes low-mass stars to larger radii, and therefore renders them more liable to tidal stripping. The present program will use WFC3 to observe two other low-concentration clusters, AM-4 and Palomar 13, and determine whether this deficiency of low-mass stars is a common feature of these systems.

GO 11734: The hosts of high redshift gamma-ray bursts

Artist's impression of a gamma-ray burst

Gamma ray bursts are events that tap extraordinary energies (1045 to 1047 joules) in remarkably short periods of time. Several thousands bursts have been detected over the last 30+ years, and analyses indicate that they can be divided into two classes with durations longer or shorter than 2 seconds. The short bursts appear to release more high energy radiation, so the two subsets are known as long/soft and short/hard bursts.The short/hard bursts appear to arise from coalescing binary systems (probably pairs of neutron stars or black holes), but the long/soft bursts appear to originate in the collapse of very massive stars. The latter sources are therefore almost certainly associated with star formation, so they act as signposts to active star-forming regions in the high redshift universe. Many of these bursts are sufficiently bright that ground-based spectroscopic observations allow reliable measurement of the redshift. In many cases, ground-based observations at later epochs are insufficient to detect the underlying host galaxy, and characterise its properties. This program targets such systems at redshifts z>3, and aims to use WFC3 and ACS observations to reveal the host galaxy.