This week on HST

HST Programs: May 1 - May 7, 2006

Some selected highlights

GO 10482: Trigonometric Calibration of the Period- Luminosity Relations for Fundamental and First-Overtone Galactic Cepheids

Cepheids in the LMC: the original PLC

Cepheids are the original distance indicator, and remain the primary calibrator for the extragalactic distance scale. Most investigations tie the zeropoint for the latter scale to the Large Magellanic Cloud, which has a large population of Cepheid variables that provide a well-populated period-luminosity-colour (PLC) relation. In the case of the Hubble H0 Key Project, the LMC distance modulus is set at 18.5, corresponding to a distance of ~50 kpc. However, a variety of studies using a variety of distance estimators give LMC distance molduli that range from 18.1 to 18.8 (~42 to ~58 kpc). Moreover, the average metallicity of the LMC Cepheids is somewhat lower than the average for the tyypical spiral galaxies that serve as the key links in the H0 chain. It is therefore important to verify the LMC calibration using Galactic Cepheids. Unfortunately, there are few such variables within 500 parsecs (Polaris is the nearest, at ~130 parsecs), and, as a result, most have trigonometric parallaxes, even with Hipparcos, that are accurate to no better than 20%. Eventually, astrometric missions such as SIM and Gaia will nail down these distances and the Galactic PLC calibration. In the meantime, the Fine Guidance Sensors on HST are capable of an astrometric accuracy of ~0.2 milliarcseconds. This program targets nine Cepheids, spanning a range of period, and will provide a local calibration that will test the LMC zeropoint at the 0.05 magnitude level.

GO 10525: Characterizing the Near-UV Environment of M Dwarfs: Implications for Extrasolar Planetary Searches and Astrobiology

SOHO image of an extremely strong solar flare

M dwarfs - at least, the subset of M dwarfs known as flare stars - are renowned for possessing extremely active chromospheres and coronae. Their discovery as highly variable objects happened largely by chance. Willem Luyten had noticed in 1924 that certain M dwarfs showed spectroscopic variability, with the occasional appearance of emission lines, while in the early 1940s van Maanen commented that two late-type dwarfs, Gl 412B (WX UMa) and Gl 285 (YZ CMi), had brightened by over a magnitude on a handful of parallax plates. The crucial observations came in 1948, when E.F Carpenter noticed that the fainter component of a wide binary system had brightened by more than 3 magnitudes in a matter of minutes. In the succeeding 50 years, these stars have been subjected to extensive observations, particularly at optical and X-ray wavelengths, and the underlying physical processes are relatively well understood. However, most attention has focused on the more active flare stars, and we still have a relatively uncertain grasp on the flare frequency among less active stars. This issue has acquired increased importance as more attention has been devoted to the potential of M dwarfs as planetary hosts. The habitable zones lie much closer to the parent star, and planets are correspondingly vulnerable to detrimental effects from enhanced UV radiation, particularly short-wavelength UV-C. This proposal uses ACS HRC prism to obtain low-resolution near-UV spectra of nearby M dwarfs, providing a broad sampling of the range of activity levels among these low mass dwarfs.

GO 10587: Measuring the Mass Dependence of Early-Type Galaxy Structure

Galaxy-galaxy lensing

Gravitational lensing amplifies and distorts the light from background sources. Most recent studies have focused either on micolensing, where the lensed sources are individual stars in either the Milky Way or its immediate neighbours, or on lensing of background galaxies by clusters of galaxies. In the former case, the main goal has been searching for possible contributors to the dark matter halo of our Galaxy (and M31); in the latter, the extended background sources are distorted into arcs and arclets whose morphjology allows an estimate of the mass distribution within the foreground cluster. Individual galaxies can also act as gravitational lenses, in a manner analogous to galaxy clusters. In that case, the morphology of the lensed source probes the mass distribution of the foreground galaxy. This proposal capitalises on these circumstances by using ACS to target a sample of 118 early-type lens candidates in a Snapshot survey. These systems were identified from Sloan Digital SKy Survey (SDSS) data, have known redshifts and their internal dynamics are currently the subject of ground-based spectrscopic study. Combining those data with the detailed photometric profiles obtained by ACS will constrain the mass profiles of the luminous and dark matter components in a range of early-type galaxies.

GO 10630: The Fine Structure of Elliptical Galaxies in Voids

The elliptical galaxy, NGC 1316

"The sequence of elliptical nebulae appears to offer a simpler problem, for the individual objects at any given stage in this sequence are curiously similar. There are, in general, no structural details whatsoever beyond the general appearance of rotational symmetry." Edwin Hubble's 1930 description of elliptical galaxies remained the paradigm for over 50 years. Hubble added a qualifying footnote ("Occasionally, small dark markings, generally arcs or crescents, are found near the nuclei..."), and as higher sensitivity observations were acquired in the 1980s and 90s, it became clear that ellipticals harboured a wealth of detailed structure, including dust lanes, shells, tidal tails and, in some cases, residual star forming regions. Many of these features are likely related to galaxy-galaxy interactions, including full-scale mergers. Taken at face value, one would expect these characteristics to depend on the environment, particularly the galaxy density and the likelihood of close encounters. This proposal tests this hypothesis by using ACS to image isolated elliptical galaxies and search for evidence of detailed structure within those systems.