This week on HST

HST Programs: August 26 - September 1, 2013

Selected highlights

GO 12468: How Fast Did Neptune Migrate? A Search for Cold Red Resonant 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 TNOs are binary (including Pluto, one of the largest known TNOs, regardless of whether one considers it a planet or not). TNOs are grouped within three broad classes: resonant objects, whose orbits are in mean motion resonance with Neptune, indicating capture; scattered objects, whose current orbits have evolved through gravitational interactions with Neptune or other giant planets; and classical TNOs, which are on low eccentricity orbits beyond Neptune, with no orbital resonance with any giant planet. The latter class are further sub-divided into "hot" and "cold" objects, depending on whether the orbits have high or low inclinations with respect to the ecliptic. Cold, classical TNOs show relatively uniform characteristics, including red colours, high albedos and an extremely high binary fraction (>30%). They are believed to have formed in situ, and were therefore in place to experience the range of gravitational interactions as the giant planets migrated to their present location. As that migration occurred, subsets are expected to have been trapped in transitory resonance orbits. The present proposal aims to use HST to complete a photometric survey of all known resonant TNOs, with the goal of identifying the proportion of cold classical TNOs that have been captured. The relative number of such objects can be used to constrain models for Neptune's orbital migration in the early Solar System.

GO 13020: A Comprehensive COS Study of the Magnetic Dynamos, Rotations, UV Irradiances and Habitability of dM Stars with a Broad Span of Ages

An artist's impression of a giant flare on an M dwarf star

M dwarfs constitute the overwhelming majority of stars in the Milky Way, contributing more than 70% of the stellar constituents in the Solar neighbourhood. These stars are well known for possessing active chromospheres and coronae that can lead to significant photometric variations, particularly for the subset known as flare stars. Their discovery as highly variable objects happened largely by chance. Willem Luyten had noticed in 1924 that certain M dwarfs showed spectroscopic variability, with the occasional appearance of emission lines, while in the early 1940s van Maanen commented that two late-type dwarfs, Gl 412B (WX UMa) and Gl 285 (YZ CMi), had brightened by over a magnitude on a handful of parallax plates. The crucial observations came in 1948, when E.F Carpenter noticed that the fainter component of a wide binary system had brightened by more than 3 magnitudes in a matter of minutes. In the succeeding decades, these stars have been subjected to extensive observations, particularly at optical and X-ray wavelengths, and the underlying physical processes are relatively well understood. However, most attention has focused on the more active flare stars, and we still have a relatively uncertain grasp on the flare frequency among less active stars. This issue has acquired increased importance as more attention has been devoted to the potential of M dwarfs as planetary hosts. The habitable zones lie much closer to the parent star, and planets are correspondingly vulnerable to detrimental effects from enhanced UV radiation, particularly short-wavelength UV-C. This proposal aims to use the Cosmic Origins Spectrograph to obtain low-resolution far-UV spectra for a sample of nine nearby M dwarfs, complementing X-ray observations currently being undertaken by Chandra. All of these stars are in wide binary systems with white dwarf companions, and the properties of the degenerate companion have been used to estimate the age of each system. The observations therefore offer the prospect of mapping the time evolution of coronal and chromospheric activity, probing tbe likely long-term impact of stellar activity on hypothetical planets in the habitable zone.

GO 13046: RAISIN: Tracers of cosmic expansion with SN IA in the IR

The first supernova discoevered by the Pan-STARRs survey

Supernovae are the most spectacular form of stellar obituary. In recent years, these celestial explosions have acquired even more 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 uncertainties remain as to the uniformity of the events. Moreover, as the sample of known supernova has grown, so has the range of photometric systems and the methods used to fit the light curve and account for the ever-present uncertainites inroduced by dust absorption. Consequently, the potential remains for systematic bias in distance estimates due both to intrinsic differences and to measurement errors. The persent program aims to minimise these systematics by compiling standard sequences of observations, primarily in the Y, J, and H filters, of supernovae at redshifts between z~0.3 and 0.5. Focusing on those wavelengths minises the effects, and hence the uncertainties, due to dust absorption. The supernovae themselves are drawn from the Pan-STARRS survey, with the WFC3-IR camera on HST employed to obtain the photometry.

GO 13295: Do the globular clusters in the Fornax dSph have multiple stellar populations?

The Fornax dwarf spheroidal galaxy

Globular clusters are remnants of the first substantial burst of star formation in the Milky Way. With typical masses of a few x 105 solar masses, distributed among several x 106 stars, the standard picture holds 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. Until recently, the only known exception 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 cotnribution to the current galactic halo. The present program aims to expand observations to the globular clusters within the nearby Fornax dwarf spheroidal system. The goal is to measure the width of the giant branch to test for the presence of substantial metallicity variations. These clusters are very metal-poor, [Fe/H]<-2; however, there does not appear to be a significant component of metal-poor field stars within Fornax, which one would expect if the present-day clusters were remnants of much more massive systems. Thus, the detection of multiple stellar populations in these systems would require an alternatvie formation mechanism.