This week on HST

HST Programs: May 4 - May 10, 2015

Selected highlights

GO 13297: The HST Legacy Survey of Galactic Globular Clusters: Shedding UV Light on Their Populations and Formation

Hubble image of the metal-poor globular cluster, M15

Globular clusters are members of the Galactic halo population, representing remnants from the first extensive period of star formation in the Milky Way. As such, the properties of the 106 to 107 stellar constituents can provide crucial insight into the earliest stages of galaxy formation. Until recently, conventional wisdom was that these are simple systems, where all the stars formed in a single starburst and, as a consequence, have the same age and metallicity. One of the most surprising disoveries in recent years is the realisation that this simple picture no longer holds. Up until about 5 years ago, the only known counter-example to convention was the cluster Omega Centauri, which is significantly more massive than most clusters and has both a complex main sequence structure and a range of metallicities among the evolved stars. High precision photometric observations with HST has demonstrated that Omega Cen is far from unique, with multiple populations evident in numerous other clusters, including NGC 2808, NGC 1851, 47 Tuc and NGC 6752. Multiple populations have also been discerned in a number of clusters in the Magellanic clouds. Sustaining multiple bursts of star formation within these systems demands that they retain gas beyond the first star forming event, which appears to set a requirement that these clusters were significantly more massive during their epoch of formation; put another way, the current globulars may represent the remnant cores of dwarf galaxy-like systems. That, in turn, implies that the stars ejected from those systems make a significant contribution to the current galactic halo. One of the most effective means of identifying and studying multi-population clusters is combining high-precision photometry over a wide wavelength range, particularly extending to UV wavelengths. Sixty-five globular clusters already have R/I (F606W, F814W) Hubble imaging and photometry thanks to the Cycle 14 program, An ACS Survey of Galactic Globular Clusters (GO 10775). The present program aims to build on those data by adding UV/blue observations using the F275W, F336W and F438W filters on the WFC3-UVIS camera. The colorus derived from these filters enable characterisation of the C, N and O abundances of the component stellar populations in these systems.

GO 13665: Exploring the Diversity of Exoplanet Atmospheres in the Super-Earth Regime

Artist's impression of the GJ 1214 system

The first exoplanet, 51 Peg b, was discovered through radial velocity measurements in 1995. 51 Pegb was followed by a trickle, and then a flood of other discoveries, as astronomers realised that there were other solar systems radically different from our own, where "hot jupiters" led to short-period, high-amplitude velocity variations. Then, in 1999, came the inevitable discovery that one of those hot jupiters. HD 209458b, was in an orbit aligned with our line of sight to the star, resulting in transits. Since that date, the number of known transiting exoplanet systems has grown to more than 100 from ground-based observations, most detected through wide-field photometric surveys, while the high-sensitivity data provided by Kepler has added a further 1000+ confirmed systems and ~2000 additional candidates. With the added numbers, observations have pushed detections to lower and lower masses, and it is now clear that the most common type of planet is the "super-Earth" - planets with masses that are several (3-6) times that of Earth and radii 2-4 times larger than Earth. One of the earliest examples is the planet circling the M dwarf, GJ 1214. Such planets have no obvious analogue in the Solar System, and the measured masses and diameters might reflect a range of interior structurees: large rocky bodies with relatively thin atmospheres; dense cores surrounded by a steam atmosphere; or "mini-Neptunes", with rock or ice cores surrounded by extended hydrogen or helium atmospheres. The present program aims to probe the diversity of these systems by using the G141 grism on Wide-Field Camera 3 to obtain time-resolved scanning observations of five transiting systems. The goal is to obtain data that will clearky distinguish between large scaleheight, hydrogen-dominated atmospheres and a more compact, steam-dominated systems.

GO 13704: Testing the single degenerate channel for Supernova Ia

Artist's impression of the evolution of a Type Ia progenitor

Supernovae are the most spectacular form of stellar obituary. Since B2FH, the physical processes underlying their eruptive deaths have been known to play a key role in populating the ISM with metals beyond the iron peak. More recently, these celestial explosions have acquired even greater significance through the use of Type Ia supernovae as distance indicators in mapping the `dark energy' acceleration term of cosmic expansion. However, while there are well-established models for the two main types of supernovae (runaway fusion on the surface of a white dwarf in a binary system for Type Ia, or detonation of the core in Type II), some significant uncertainties remain concerning the physical details of the disruption, and, potentially, the overall uniformity of these events. In particular, questions remain over how much material the white dwarf accretes before it becomes unstable, hence setting limits of the total mass at the time of the supernova explosion. This accretion phase is extremely short-lived and, as a result, a bare handful of such systems are known within the Milky Way, none accessible tod etailed study. An alternative approach is to study these systems at an earlier phase in their lifeitme, when the white dwarf is companion to an FGK dwarf. Those systems are more numerous, but also extremely difficult to locate since the white dwarf is well outshone by its companion. The present program aims to expand the sample of known systems by using COS to obtain follow-up observations of 5 candidate FGK binaries, systems that show both radial velocity variations and UV excess, strongly suggesting the presence of a hot white dwarf companion.

GO 14053: Astrometric Follow-Up of Kuiper Belt Objects for the New Horizons Mission

Hubble Space Telescope images of the Pluto system, including the recently discovered moons, P4 and P5

The Kuiper Belt lies beyond the orbit of Neptune, extending from ~30 AU to ~50 AU from the Sun, and includes at least 70,000 objects with diameters exceeding 100 km. Setting aside Pluto, the first trans-Neptunian objects were discovered in the early 1990s. Most are relatively modest in size, with diameters of a few hundred km and photometric properties that suggest an icy composition, similar to Pluto and its main satellite, Charon. In recent years, a handful of substantially larger bodies have been discovered, with diameters of more than 1000 km; indeed, one object, Eris (2003 UB13), is slightly larger than Pluto (2320 km) and 25% more massive. We know the mass for Eris because it has a much lower mass companion, Dysnomia, which orbits Eris with a period of 16 days (see this recent press release ). Pluto itself has at least 5 companions: Charon, which is about 1/7th the mass of Pluto, and the much smaller bodies, Hydra, Nix, P4 and P5 discovered through HST observations within the last few years. The New Horizons Mission was launched on January 19th 2006 with the prime purpose of providing the first detailed examination of Pluto. The Pluto encounter represents the first phase of the originally-proposed mission. Following the fly-by, set for Bastille day in 2015, the aim is to re-direct New Horizons towards one or more smaller members of the Kuiper Belt, with the goal of providing a closer look at these icy bodies. However, New Horizons needs to identify an appropriate target - a KBO with orbital parameters such that New Horizons can use its modest complement of remaining fuel to reach the target. Adding a further complication, Pluto happens to lie within 5 degrees of the Galactic Plane and the consequent high star density has proven a barrier to deep ground-based searches. As a consequence, the New Horizons team was awarded Hubble time to search an area roughly the size of the full moon for a suitable target. Those observations have succeeded in detecting at least two KBOs that should be accessible to New Horizons. The present observations aim to refine the orbital parameters and define the final choice of a target for a potential extended mission.