This week on HST

HST Programs: August 1, 2011 - August 7, 2011

Selected highlights

GO 11670: The Host Environments of Type Ia Supernovae in the SDSS Survey

SN 2007uy and 2008D in NGC 2770

Supernovae have long attracted the attention of both amateur and professional astronomers as a means of studying the violent eruption and death of massive stars and degenerates. However, in the last decade they have also acquired considerable importance as distance indicators, tracing the expansion of the universe to redshifts well beyond the reach of more conventional yardsticks, such as cepheids, and providing a key underpinning for the hypothesised existcen of dark energy. Understanding the supernovae themselves, and, in particular, their progenitors, is key to accurately interpreting their luminosities and distances. The present program aims to tackle that aspect of the problem by using ACS to obtain deep, high resolution images of galaxies that have harboured recent type Ia supernovae. The targets are all drawn from the Sloan Digital Sky Survey, which has uncovered more than than 500 type Ia supernovae,. The supernovae themselves are long gone from view, but the ACS data will be used to probe the stellar populations in the immediate vicinity of the explosion, and hence gain a better understanding of the likely progenitor.

GO 12059: 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.

Temporal variation in the intensities of the lensed components of the Einstein Cross

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, in 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 program aims to address this issue for by using WFC3 to obtain high resolution images F105W ("Y"-band) of a subset of 54 QSOs with redshifts in the range 5.7 < z < 6.4.

GO 12215: Searching for Missing Low-Mass Companions of massive Stars

Artist's impression of an O star binary system

Many stars reside in binary systems; indeed, it used to be stated as fact that most stars resided in binary systems. As Charles Lada has pointed, we now know that the latter staement is no valid, since the binary frequency among M dwafs is less than 30%, and most stars are M dwarfs. Nonetheless, understanding how stellar multiplicity and component mass ratios vary with the mass of the primary in multiple systems is likely to provide significant insight into the overall star formation process. The present program focuses on high mass stars. Detecting binary companions clearly depends on the flux ratio (for direct imaging) and the radial velocity amplitude (for spectroscopic surveys); in both cases, higher mass ratio systems are more difficult to dtect, since the lower mass companion is generally both fainter and produces a smaller reflex motion on the part of the primary. The situation is particularly difficult for high mass O stars, where even solar-type companions can fall below the threshold for detection. The present proposal aims to use HST to probe this issue, by using WFC3 to image a sample of Cepheids (which are descended from more luminous B stars), and search for M-type companions.