This week on HST

HST Programs: August 13 - August 19, 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 12506:A Precise Mass-Luminosity-Temperature Relation for Young Stars

Artist's impression of the young, low-mass binary system, Coku Tau4

Mass, luminosity and temperature are the three of the five fundamental quantities that we'd like to know for every star (the other two are chemical composition and age, with age being the least accessible to direct measurement). Binary systems offer one of the most effective means of determining the former three parameters, where measurements of the orbital period and velocity variation permit direct determination of the system mass and, by scaling against angular measurements for visual binaries. the distance. Given the luminosity and temperature, the individual stellar radii can also be derived. These quantities are particularly useful in constraining models of young stars. The present program focuses on observations 16 low-mass systems in nearby star-forming regions. All 16 have orbital determinations, derived from AO-assisted K-band ground-based imaging. the WFC3-UVIS camera will be used to obtain multi-colour imaging, providing measurements of the spectral energy distributions of the individual components in each system, and hence estimates of the surface temperatures.

GO 12534: The Panchromatic Hubble Ultra Deep Field: Ultraviolet Coverage

The ACS optical/far-red image of the Hubble Ultra Deep Field

Galaxy evolution in the early Universe is a discipline of astronomy that has been transformed by observations with the Hubble Space Telescope. The original Hubble Deep Field, the product of 10 days observation in December 1995 of a single pointing of Wide Field Planetary Camera 2, demonstrated conclusively that galaxy formation was a far from passive process. The images revealed numerous blue disturbed and irregular systems, characteristic of star formation in galaxy collisions and mergers. Building on this initial progam, the Hubble Deep Field South (HDFS) provided matching data for a second southern field, allowing a first assessment of likely effects due to field to field cosmic variance, and the Hubble Ultra-Deep Field (UDF) probed to even fainter magitude with the Advanced Camera for Surveys (ACS). The original UDF program comprised 412 orbits directed at a single ACS field within the Chandra Deep Field-South (CDF-S) GOODS area. Those 412 orbits were divided among four filters - F435W (broad B-band: 56 orbits), F606W (broad V/R-band: 56 orbits), F775W (I-band: 150 orbits), and F850LP (z-band: 150 orbits). A further 144 orbits were devoted to a 3x3 grid of F110W (J) and F160W (H) NICMOS images covering the same field.(GO program 9803). Immediately following Servicing Mission 4, the Wide-Field Camera 3 infrared camera was used to obtain obtain deep F850LP (Y), F105W (J) and F160W (H) images centred on the UDF and two flanking fields (GO 11563), resulting in the identification of significant numbers of galaxies at redshifts z~7 and 8, and even the identification of a z~10 galaxy candidate. The present program aims to fill in details at lower redshifts. Deep images will be obtained with the WFC3-UVIS camera at near-UV waveelngths using the F225W, F275W and F336W filters. Those data will extendn to 29th magnitude (AB), and provide invaluable information on the level of star forming activity in galaxies at redshifts in the range 1 < z < 2.5,

GO 12957: The Proper Motion of Palomar 5 and Tidal Tails

The globular cluster Palomar 5's tidal tails, from analysis of SDSS data

Palomar 5 is one of the more distant members of the Milky Way globular cluster system, lying at a Galactocentric radius of 18.6 kiloparsecs and a distance of ~20 kpc from the Sun. It was discovered in 1950 by Walter Baade, and independently rediscovered from Palomar Schmidt plates by A. G. Wilson in 1955. Wilson originally classed it a a dwarf galaxy, but follow-up 200-inch plates taken by Allan Sandage confirmed the system as a true globular cluster, and it was listed as number 5 in George Abell's catalogue of newly-discovered POSS clusters. Pal 5 is a relatively sparse system, with a present-day mass of only ~30,000 solar masses. The original cluster may have been much more massive, however. In 2002, analyses of wide-field imaging data from the Sloan Digital Sky Survey (Odenkirchen et al) revealed the presence of an extensive tidal tail system centred on the cluster. The tails extend for ~10 degrees on the sky, and are the product of dynamical evolution due to gravitational interactions as the cluster plunges through the galactic disk. Modeling these interactions in detail demands knowledge of Palomar 5's orbit. Measuring the line-of-sight radial velocity component is straightforward but, at ~20 kpc, determining the tangential proper motion is mucht rickier and multiple ground-based observations have not converged on an unambiguous value. The present proposal aims to bring HST's astrometric precision to ebar on the problem, with a first set of observations taken this cycle and a second set of images taken in Cycle 22.