This week on HST

HST Programs: June 4 - June 10, 2012

Selected highlights

GO 12474: The frequency and chemical composition of rocky planetary debris around young white dwarfs

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. The present program aims to address this question by using COS to obtain UV observations of young white dwarfs, probing correlations with progenitor mass and examining the detailed composition of the accreted materials.

GO 12493: A Candidate Lensed Quasar at z=6.25

An image of SDSSJ1226-0006, a z=1.12 quasar lensed by a galaxy at z=0.57

Gravitational lensing is a consequence of general relativity. Its importance as an astrophysical tool first became apparent with the realisation (in 1979) that the quasar pair Q0957+561 actually comprised two lensed images of the same background quasar. In the succeeding years, lensing has been used primarily to probe the mass distribution of galaxy clusters, using theoretical models to analyse the arcs and arclets that are produced by strong lensing of background galaxies, and the large-scale mass distribution, through analysis of weak lensing effects on galaxy morphologies. Gravitational lensing also increases the apparent brightness of the background sources. This effect can be used to our advantage, enabling detailed observations of high-redshift sources that be too faint to observe under normal circumstances, but it can also lead to statistical biases in parameters such as luminosity functions. These effects are likely to be of most importance for higher redshift sources, where the longer pathlength leads to a higher probability of the light encountering a foreground lens. The present HST program builds on results from a Cycle 18 SNAP program that targetted z>6 QSOs drawn from the Sloan Digital Sky Survey. The present program targets CFHQS-J0050+3445, a z=6.25 QSO that shows multiple images suggstive of lensing. The multicolour data taken with ACS/WFC and WFC3-IR will better characterise this system.

GO 12533: Escape of lyman-Alpha Photons from Dusty Starbursts

A NICMOS image of the interacting Luminous IR Galaxy, NGC 6090

Ultraluminous infrared galaxies (LIRGs) are systems that are characterised as having luminosities that exceed 1012 LSun, with most of the energy emitted at wavelengths longward of 10 microns. Many (perhaps most) of these galaxies are interacting or merging disk galaxies, with the excess infrared luminosity generated by warm dust associated with the extensive star formation regions. Many systems also exhibit an active nucleus, and may be in the process of evolving towards an S0 or elliptical merger remnant. One of the surprising discoveries in recent years has been the extent to which Lyman-alpha ionising emission can be detected escaping from these dusty systems. The present program looks to quantify the distribution of these properties through COS observations of sixteen ULIRGs in the local universe (z~0.1). These relatively nearby systems can provide insight into the structure of these systems, and give clues to the likely behaviour at higher redshifts.

GO 12537: Venus observed as an extrasolar planet

An image of the 2004 Venus transit - with a terrestrial interloper (taken by Jerry Zhu

Extrasolar planets that transit their parent star from the vantage point of the Earth are much sought after in exoplanet research. Not only do those systems eliminate the sin(i) uncertainty in estimating planetary masses,they also provide an opportunity to probe atmospheric properties through comparative spectroscopy of the host star during and outside primary transit - differencing the spectra can reveal absorption features introduced as the starlight passes through the exoplanet atmosphere. Earth's inhabitants are also occasionally in a position to view Solar System transits, as bodies interior to the earth's orbit, notably Mercury and Venus, pass across the face of the Sun. The planetary transits occur when inferior conjunction (the planet lies directly between Earth and the Sun) occurs close to the orbital node (where the planetary orbit crosses the ecliptic. In the case of Mercury, this happens relatively frequently, with transits in May or November and occuring at intervals of 7, 13 or 33 years - the most recent events were November 15 1999, May 7 2003 and November 8 2006. Venusian transits occur in June or December, but at a much lower frequency. Transits occur in pairs, 8 years apart, separated by gaps of 121.5 and 105.5 years, with the cycle recurring every 243 years. Thus, the June transits of 1761 and 1769 were followed by December transits in 1874 and 1882, and repeated as June transits in 2004 and 2012. The 18th and 19th century transits were the targets of numerous scientific expeditions, including James Cook's expedition to Tahiti in 1769, since accurate timing could be applied to measuring the astronomical unit, and scaling the solar system. Nowadays, there are much more effective ways of measuring that parameter (that don't have to deal with the uncertainties introduced by the 'black drop"). However, the upcoming transit can be turned to advantage in probing our sensitivity to detecting the atmospheres of terrestrial exoplanets. Direct observations of the Sun are out of the question with HST - indeed, Hubble can only observe Venus itself when the planet is at its furthest elongation from the Sun (see HST's observations in support of Venus express - GO 12433 ). However, HST can observe reflected sunlight by observing the Moon. The present program uses the Space Telescope Imaging Spectrograph to observe two regions of the lunar surface, near the craters Hipparchus and Dollond, during the Venusian transit on June 5, 2012. Those observations will be differenced against reference lunar spectra, with an initial set being taken this week.