This week on HST

HST Programs: March 10 - March 16, 2014

Selected highlights

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 13330: Mapping the AGN Broad Line Region by Reverberation

Simulations of the appearance and velocity structure within an AGN disk (see

Active galaxies (AGNs) are generally luminous systems, characterised by the presence of strong nuclear emission lines of numerous species including H, He I, He II, and Fe, Ca, O, C and S over a range of ionisations. These features originate from gas clouds in the nuclear regions, with the energy supplied through accretion onto a central massive black hole. The high-temperature, rapidly-rotating gas clouds nearest the central engine are responsible for producing broad emission lines (hence, the "Broad Line Region"). The structure of the BLR can be discerned using a technique known as reverberation mapping: variations in the accretion rate lead to fluctuations in luminosity; those variations lead, in turn, to variations in the photoionisation of the BLR, and corresponding changes in spectral line strengths and velocities; monitoring those changes, and correlating them with the photometric variability of the central source, measures the light travel time from nucleus to BLR gas, and hence maps the size of the BLR. The present prorgam will use the Cosmic Origins Spectrograph to undertake systematic monitoring of the nuclear regions of the Seyfert I galaxy, NGC 5548. The observatons are spread over the next 6 months, with one orbit per day for 179 days.

GO 13453: The Elemental Compositions of Extrasolar Minor Planets

Artist's impression of a comet spiralling in to the white dwarf variable, G29-38

During the 1980s, one of the techniques used to search for brown dwarfs was to obtain near-infrared photometry of white dwarf stars. Pioneered by Ron Probst (KPNO), the idea rests on the fact that while white dwarfs are hot (5,000 to 15,000K for the typcail targets0, they are also small (Earth-sized), so they have low luminosities; consequently, a low-mass companion should be detected as excess flux at near- and mid-infrared wavelengths. In 1988, Ben Zuckerman and Eric Becklin detected just this kind of excess around G29-38, a relatively hot DA white dwarf that also happens to lie on the WD instability strip. However, follow-up observations showed that the excess peaked at longer wavelengths than would be expected for a white dwarf; rather, G 29-38 is surrounded by a dusty disk. Given the orbital lifetimes, those dust particles must be regularly replenished, presumably from rocky remnants of a solar system. G 29-38 stood as a lone prototype for almost 2 decades, until a handful of other dusty white dwarfs were identified from Spitzer observations within the last couple of years.In subsequent years, a significant number of DA white dwarfs have been found to exhibit narrow metallic absorption lines in their spectra. Those lines are generally attributed to "pollution" of the white dwarf atmospheres. Given that the diffusion time for metals within the atmospheres is short (tens to hundreds of years), the only reasonable means of maintaining such lines in ~20% of the DA population is to envisage continuous accretion from a surrounding debris disk, probably generated by collisions within a population of asteroids. The present program aims to use these features to probe the composition of the parent asteroids through near- and far-UV spectroscopic observations of five white dwarfs, GD 408, GD 303, WD1425+540, GD 205 and WD2253+803. Given the presence of Ca II and Mg II lines in the optical spectra of these degenerates, the UV observations, with the Cosmic Origins Spectrograph, are expected to reveal absorpton lines due to neutral oxygen and ionised silicon and carbon, probing the relative abundances of these various elements within the accreted material.

GO 13610: Imaging Comet C/2013 A1 (Siding Spring) to Support Risk Assessment for Mars Orbiters during the Close Mars Encounter

The projected orbit of Comet C/2013 A1

2013 has been an interesting year for Hubble and comets: Comet ISON is being tracked as a potential great comet, passing through perihelion in late November; PanSTARRS and other surveys are turning up more evidence of residual cometary activity within objects in the asteroid belt, main belt comets; and an Oort cloud comet discovered in early January is slated to undergo a close encounter with Mars in October next year. The last-named object is Comet 2013/A1 (Siding Spring), originally found by Robert McNaught in the course of the ongoing Siging Spring Survey for near-Earth asteroids and comets. Pre-discovery images have been located among observations by the Pan-STARRS survey and the Catalina Sky survey, and it quickly became apparent that the orbit will take the comet extremely close to Mars in late 2014. More details calculations show that the comet will pass within 134,000 km of the planet. As an Oort comet, potentially on its first pass through the inner Solar System, Comet 2013/A1 is likely to have a substantial component of icy, volatile substances, generating an extensive coma and tail of gas and dust that will envelope the planet and its surrounding. Those dusty materials present a potential hazard for the planetary probes that are either currently in orbit around Mars or (in the case of Maven) slated to arrive there shrtly before the comet. The present program will use HST to obtain imaging observations at there epochs with the aim of characterising the evolution of surface activity and the consequent likelihood of adverse effects on the Martian probes.