This week on HST

HST Programs: August 29, 2011 - September 4, 2011

Selected highlights

GO 12016: A Panchromatic Hubble Andromeda Treasury

M31: the Andromeda spiral galaxy

M31, the Andromeda galaxy, is the nearest large spiral system to the Milky Way (d ~ 700 kpc), and, with the Milky Way, dominates the Local Group. The two galaxies are relatively similar, with M31 likely the larger system; thus, Andromeda provides the best opportunity for a comparative assessment of the structural properties of the Milky Way. Moreover, while M31 is (obviously) more distant, our external vantage point can provide crucial global information that complements the detailed data that we can acquire on individual members of the stellar populations of the Milky Way. With the advent on the ACS and, within the last 2 years, WFC3 on HST, it has become possible to resolve main sequence late-F and G dwarfs, permitting observations that extend to sub-solar masses in M31's halo and disk. Initially, most attention focused on the extended halo of M31 (eg the Cycle 15 program GO 10816 ), with deep imaging within a limited number of fields revealing the complex metallicity structure within that population. With the initiation of the present Multi-Cycle Treasury program, attention switches to the M31 disk. "PHAT" will conduct a multi-waveband survey of approximately one third of disk and bulge, focusing on the north-east quadrant. Observations will extend over the next three cycles, and will provide a thorough census of upper main-sequence stars and star forming regions, matching the stellar distribution against the dust and gas distribution.

Hubble ACS image of Uranus from 2004

Uranus is unique among the major bodies in the Solar System in that it rotates on its side. Unlike the other planets, the polar obliquity is 98o degrees, so the equator is alomost perpendicular to the ecliptic plane. Consequently, as Uranus circles the Sun, each pole spends almost half of a Uranian "year" (or 42 terrestrial years) hidden from the Sun in total darkness. Perhaps as a result, the Uranian upper atmosphere is significantly different than the atmospheres of the other solar system gas giants. Observations by Voyager 2 during its 1986 flyby showed that Uranus has a hot thermosphere and extensive corona, but a very quiescent stratosphere. At that time, the polar axis was almost directly pointed towards the Sun. On December 7th 2007, Uranus passed through its equinox, and the Sun now passes directly overhead at the Uranian equator. The present program will use STIS to obtain far-UV spectra of Uranian airglow within the upper atmosphere. Those observations will be compared with similar data from Voyar=ger 2 to determine the extent of seasonal variations in the structure of the upper atmosphere.

GO 12254: Helium-core White Dwarfs and Cataclysmic Variables in NGC 6752: New Clues to the Dynamical Evolution of Globular Clusters

The intermediate-metallicity cluster, NGC 6752

Lying at a distance of ~4 kpc in the southern constellation of Pavo, NGC 6752 is the third brightest globular cluster and one of the closest to the Sun. It is moderately metal-poor, with a metallicity [Fe/H] ~ -1.5, or ~3% that of the Sun. Given its proximity and the relatively low reddening due to foreground dust, NGC 6752 has received substantial observational attention since its discovery in the mid 1820s by the Scots astronomer, James Dunlop. [At the time, Dunlop was working for Sir Thomas Brisbane at Paramatta Observatory, NSW.] Ground-based and HST observations reveal a colour-magnitude diagram characterised by an extended horizontal branch, a main sequence of some modest width, diverse abundances on the giant branch and a number of white dwarfs outlining a degenerate sequence. Recent detailed observations suggest that this cluster harbours multiple stellar populations. The present program focuses on the white dwarfs, using a combination of blue and red, wide- and narrow-band imaging with the Wide-Field camera on ACS in an attempt to identify candidate binary systems, both double-degenerates and cataclysmic variables.

GO 12298: Towards a Physical Understanding of the Diversity of Type Ia Supernovae

The discovery image of the August 2011 SNe in M101

Supernovae are the most spectacular form of stellar obituary. Since B2FH, the physical processes underlying their eruptive deaths have been known to play a key role in populating the ISM with metals beyond the iron peak. More recently, these celestial explosions have acquired even greater 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 significant uncertainties remain concerning the physical details of the disruption, and, potentially, the overall uniformity of these events. Consequently, there is potential for systematic bias in the distance estimates. The present program aims to address this issue through detailed observations of a small number of relatively nearby Type Ia Sne. The program aims to pick up the supernovae well before maximum, permitting detailed UV spectroscopy on the rising side of the light curve, and following the spectral evolution to well past maximum. The SNe targets are selected from observations made by the Palomar Transit Factory, an on-going monitoring prorgam using the 60-inch telescope on Palomar mountain. The present observations target the type Ia supernova designated as PTF11kly in the nearby galaxy, M101, discovered on August 25th by the Palomar Transit Factory. This object is of particular interest, since archival HST images allow a search for the possible progenitor.