This week on HST

HST Programs: May 30 - June 5, 2016

Selected highlights

GO 14076: An HST legacy ultraviolet spectroscopic survey of the 13pc white dwarf sample

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 typical targets), 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. The Cosmic Origins Spectrograph (COS) is an ideal instrument for probing the abundance of trace elements in white dwarfs atmospheres: more than 70 systems have been observed, with detection rates running at around 50%. The present program is using COS to refine the statistics by targeting a volume-limited sample of 37 white dwarfs within 13 parsecs of the Sun. This sample is sufficient to provide an estimate of the overall occurence of accreting systems.

GO 14182: The Coma Cluster Core Project

Hubble image covering part of the central regions of the Coma cluster

The Coma cluster is the nearest rich galaxy cluster, lying at a distance of ~100 Mpc from the Milky Way. The cluster includes well over 1000 major galaxies, centred on two giant ellipticals, NGC 4874 and NGC 4889. Chandra observations show that the galaxies are embedded in very hot intracluster gas (see this site ). Cluster galaxies have also been surveyed at mid-infrared wavelengths by Spitzer, and in the ultraviolet by GALEX. Individual galaxies have been studied in the past using HST. In Cycle 15 the Advanced Camera for Surveys started a systematic imaging with ACS (with parallel observations with NICMOS) of the cluster core ( a 7x6 mosaic, covering approximately 400 sq. arcmin.), together with 40 fields at larger radii.That program was curtailed by the failure of ACS in January 27th. The present program builds on those observations, using Wide-Field Camera 3 to obtain complementary UV and IR data for the existing ACS images. ACS itself is being used in parallel to cover other regions with the cluster. Those observations will be capable of detecting dwarf galaxies with absolute magnitudes as faint as MB ~ -8, fainter than most of the Milky Way's dwarf spheroidal companions. Besides completing a detailed census of the low luminosity tail of the galaxian mass function, the observations will be used to probe colour gradients and internal chemical evolution.

GO 14216: RAISIN2: Tracers of cosmic expansion with SN IA in the IR

The first supernova discovered 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 present program builds on a Cycle 21 program, and 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 14241: Cloud Atlas: Vertical Cloud Structure and Gravity in Exoplanet and Brown Dwarf Atmospheres

Ground-based imaging the the very low-mass brown dwarf binary, 2MASS1207

Recent years have seen the discovery of numerous extrasolar planets. Initially, most were discovered through radial velocity monitoring; more recently the superb photometric properties of the Kepler mission have led to the discovery of numerous transiting systems. Lagging behind in numbers, but offering the greatest prospect for probing physical conditions, is direct imaging. A handful of systems with resolved planetary companions have been discovered, most notably the multi-planet system around the nearby A-type star, but they are much more massive and therefore have higher gravities. The present program aims to gain insight into their structure by comparing resolved exoplanets and higher mass, but similar temperature, brown dwarfs. Observations of isolated brown dwarfs have shown evidence for systematic variations in brightness, possibly due to dust within the atmosphere forming clouds, perhaps giving the dwarf's surface a banded appearance, similar to Jupiter. The clouds themselves may appear and disappear over relatively short timescales, leading to photometric variations at particular wavelengths. Past programs have used both Spitzer and HST to monitor spectral variability in a number of systems. The present program will use Wide-field Camera 3 to obtain time-series near-infrared grism spectra of ten brown dwarfs and high-mass exoplanets. Variations in those spectra map the atmospheric cloud structure in each system, and those variations can be examined for correlations with the mass/gravity of the parent object.