This week on HST

HST Programs: August 22, 2011 - August 28, 2011

Selected highlights

GO 12192: A SNAPSHOT Survey of Interstellar Absorption Lines

A map of the Local Stellar Neighbourhood

Understanding the nature and structure of gas within the interstellar medium is a key step towards understanding how material is recycled and how energetic processes, such as stellar winds and outflows, feed energy into the overall system. UV spectroscopy plays a key role in probing these effects: hot, background objects that produce relatively few intrinsic absorption features serve to map the the velocities and temperatures within the intervening gas along the line of sight. Observations of quasars are used to probe galaxy halos at moderate and high redshift; observations of hot stars provide similar information for gas in the Milky Way. The present program is using high-resolution, echelle observations with STIS to target O and B stars with a few kpc of the Sun, probing the interstellar medium along the line of sight. All of these stars have prior low-resolution observations at far-UV wavelengths with FUSE, providing a reliable guide to the flux levels. The overall goal is to determine the density, temperature and abundance distributions within the ISM along a wide variety of sight-lines throught the Galacic disk and inner halo.

GO 12273: Mass of the Local Group from Proper Motions of Distant Dwarf Galaxie

The dwarf galaxy, Leo A, as imaged by the Subaru telescope

M31 and the Milky Way are the two largest members of the Local Group, with masses of ~4 x 1011 and ~1011 MSun, respectively. As such, they dominate the system dynamics; M33 and the LMC are the next largest systems, with masses lower by a factor of 10. Radial velocity measurement show that M31 and the Milky Way are converging at a velocity of ~125 km/sec; however, interpreting that result in cosmological terms requires a better understanding of the total mass of the Local Group. Using a variety of techniques, current estimates range over a factor of 5, from ~1.3 x 1012 MSun to ~5.6 x 1012 MSun. T%he present program aims to apply stronger constraints to this fundamental value by measuring proper motions for four dwarf galaxies that lie towards the edge of the local group: Cetus, Leo A, Tucana and the Sagittarius Dwarf Irregular. First epoch observations with the ACS/WFC are already available in the archive for these four systems. The present program will build on those, obtaining new I-band (F814W) observations with the ACS/WFC, while simulateously using the WFC3-UVIS camera in parallel to obtain deep B (F475W) and I (F814W) colour-magnitude data for these low-mass systems.

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

Recent supernova in M100

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 SN2011EK in NGC 918, discovered on August 4th by Koichi Itagaki.

GO 12576: Orbit determination for Fomalhaut b and the origin of the debris belt halo

HST-ACS image of the planetary-mass companion of Fomalhaut

Fomalhaut, or alpha Piscis Austrini, is one of the Sun's closest neighbours, an A-type star with a mass approximately twice that of the Sun and an age between 100 and 300 million years, lying at a distance of only ~7.7 parsecs. Observations with the IRAS satellite in the early 1980s revealed the presence of significant excess radiation at mid-infrared wavelengths, indicating the presence of substantial dust within a disk that is being irradiated by the luminous central star. Since then, observations of Fomalhaut and nearby stars of that ilk have led to a much more detailed characterisation of the debris disk phase. In particular, Spitzer has mapped warm dust in these systems, while HST imaging has provided exquisite resolution in reflected light. It is now recogised that debris disks are the evolutionary stage where planet formation has likely run to completion, the gas has fully dissipated but the disk remains well populated with dusty material spanning a wide range of sizes. Indeed, it is likely that this phase coincides with the heavy bombardment epoch within the Solar System. ACS imaging of Fomalhaut reveals extensive structure in the disk, notably a sharply-defined, eccentric inner edge to the disk, which led to the prediction of a ~Saturn-mass planet at that location. Subsequent ACS/HRC observations led to the identification of that planet, the first direct imaging of a "conventional" exoplanet. The detection was confirmed in 2010 by HST observations with STIS. The exoplanet is variable at optical wavelengths, suggesting that the observed flux is not only contributed by reflected light from the planetary "surface". The current program combines further STIS observations, which are designed to detect the planet at the current epoch and hence map the orbit, with observations with the UVIS channel on WFC3, which are designed to probe the nature of the dusty debris disk.