This week on HST

HST Programs: April 26, 2010 - May 2, 2010

Selected highlights

GO 11578: The Extremely Metal-Poor BCD Galaxy DDO 68: a Young Galaxy in the Local Universe

Ground-based imaging of the dwarf galaxy, DDO 68; the location of a luminous blue variable is marked

DDO 68 (or UGC 5340) is a star-forming, dwarf galaxy lying at a distance of rouhgly 10 Mpc from the Milky Way. Observations indicate that it is metal-poor, with an oxygen abundance less than 1/30th that of the Sun, comparable with the much better known system, I Zw 18. As such, it, like I Zw 18, is a candidate for a galaxy undergoing its first extensive period of star formation, a local analogue to "primordial" (or maybe second generation) galaxies that are observed at higher redshifts. It remains unclear, however, whether the star formation that we currently observe in these systems really is the first burst, or whether it sits upon an older underlying population, whose main sequence has evolved to the point where it is difficult to detect. The key discriminant in resolving such issues is the morphology of thered giant branch. The present proposal tackles that question for DDO 68 by using ACS and WFC3 (in parallel) to construct deep colour-magnitude diagrams to probe the evolved population in that system.

GO 11650: Orbits, Masses, and Densities of Three Cold Classical Transneptunian Binaries

Preliminary orbital determination for the KBO WW31, based on C. Veillet's analysis of CFHT observations; the linked image shows the improved orbital derivation, following the addition of HST imaging

The Kuiper Belt consists of icy planetoids that orbit the Sun within a broad band stretching from Neptune's orbit (~30 AU) to distance sof ~50 AU from the Sun (see David Jewitt's Kuiper Belt page for details). Over 500 KBOs (or trans-Neptunian objects, TNOs) are currently known out of a population of perhaps 70,000 objects with diameters exceeding 100 km. Approximately 2% of the known KBOs are binary (including Pluto, one of the largest known KBOs, regardless of whether one considers it a planet or not). This is a surprisingly high fraction, given the difficulties involved in forming such systems and the relative ease with which they can be disrupted. It remains unclear whether these systems formed from single KBOs (through collisions or 3-body interactions) as the Kuiper Belt and the Solar System have evolved, or whether they represent the final tail of an initial (much larger) population of primordial binaries. These issues can be addressed, at least in part, through deriving a better understanding of the composition of KBOs - and those properties can be deduced by measuring the orbital parameters for binary systems. The present proposal aims to use HST WFC3 observations to map the orbits of three binary systems. Those observations will be ued to determine the orbital period and semi-major axis and the total system mass, while the mid-infrared properties (measured by Spitzer) allow an assessment of the surface area/diameters; combining these measurements gives an estimate of the mean density.

GO 11669: The Origins of Short Gamma-Ray Bursts

An 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 long/soft bursts appear to originate in the collapse of very massive stars, while the short/hard bursts are coalescing binary systems (probably pairs of netron stars or black holes). The first optical counterpart to a gamma ray burst was identified in 1998, allowing confirmation of their extragalactic nature, and, since then, more than 60 bursts have been detected at X-ray wavelengths, and half that number detected at either optical or radio wavelengths; all of these detections are long/soft bursts. The present program will use a two-pronged approach to probe the nature of soft GRB progenitors by gaining a better understanding of the nature of the surrounding stellar population. One approach is statistical, with high angular resolution WFC3 imaging used to examine the colours of stellar populations in a sample of galaxies that have hosted past GRBs - are they characteristic of young, star-forming regions, or of older, more mature environments? The drawback with this study lies in the modest accuracies associated with the GRB positions in those galaxies. Complementing that analysis, the program will trigger ToO observations of two yet-to-be discovered GRBs, one in a star-forming galaxy and the other in an elliptical. The HST imaging of those sources should lead to more precise positoins, and hence a clearer idea of the local stellar environment.

GO 11675: Stellar Forensics: A post-explosion view of the progenitors of core-collapse supernovae

Chandra X-ray image of G292.0+1.9, a ~3000-year old supernova remnant

Supernovae are generally believed to originate through two mechanisms: accretion onto a white dwarf in a close binary system, driving the white dwarf above the Chandrasekhar limit; and the implosion of the core of very massive (> 7 solar masses) stars. Both processes result in explosive nucleosythesis that enriches the interstellar medium, with the ejecta forming a rapidly expanding shell. Supernovae are intrinsically rare: Tycho's star (1604) was the last Galactic supernova identified by contemporary astronomers, although the radio remnant Cas A (identiied as 3C 461 in the 1959 Third Cambridge Catalogue of Radio Stars) may have been recorded, if not recognised as unusual, by Flamsteed in 1680. Understanding their progenitors therefore demands that we expand observations to external galaxies. The present program builds on several from previous cycles, and uses HST to obtain high-resolution imaging of the sites of several Type II supernovae in nearby galaxies. By now, the supernova themselves have faded from view, but the HST data can reveal the stellar population in the immediate environs. detailed analysis of the colour magnitude diagrams can probe the likely age of the star forming regions, and hence set limits of the progenitor mass.