This week on HST

HST Programs: November 28, 2011 - December 4, 2011

Selected highlights

GO 11704: The Ages of Globular Clusters and the Population II Distance Scale

Hubble Heritage image of the globular cluster, M15

Globular clusters are the oldest structures within the Milky Way that are directly accessible to observation. They are relatively simple systems, with relatively simple colour-magnitude diagrams (albeit with some complexities adduced from recent HST observations, see GO 11233 ). Matching those CMDs against theoretical models allows us to set constraints on the age of the oldest stars in the Galaxy, and hence on the age of the Milky Way and the epoch of galaxy formation. However, the accuracy of those age determinations rest crucially on the accuracy of the cluster distance determinations. The clusters themselves lie at distances of several kpc at best, and tens of kpc at worst; thus, direct trigonometric parallax measurements must await microacrsecond astrometric missions. The classical method of deriving distances is main sequence fitting - using nearby stars, with similar chemical abundances and accurate parallax measurements, to map out the main sequence in absolute units, and then scaling the cluster data to fit. The problem with this method is that metal-poor subdwarfs are rare, so even Hipparcos was only able to obtain accurate distances to a handful of stars. The present program aims to improve the distance measurements by using the Fine Guidance Sensors on HST to determine sub-millarcsecond trigonometric parallaxes to 9 subdwarfs, almost doubling the sample available for MS fitting.

GO 12246: Weak Lensing Mass Calibration of SZ-Selected Clusters

The ACS optical/far-red image of the galaxy cluster, Abell 2218, revealing an extensive number of lensed arcs

The overwhelming majority of galaxies in the universe are found in clusters. As such, these systems offer an important means of tracing the development of large-scale structure through the history of the universe. Galaxy clusters can be identified at moderate redshifts by searching for signatures of the Sunyaev-Zeldovich effect: high energy electrons in the hot intercluster medium interact with radiation from the cosmic microwave background to distort the microwave spectrum. The South Pole Telescope is a 10-metre microwave/millimetre telescope located at Amundsen-Scott South Pole Station on the Antarctiva high plateau, close to the geographic South Pole. That telescope has been used to search for galaxy clusters. As intense mass concentrations, these systems are highly efficient gravitational lenses, capable of concentrating and magnifying light from background high redshift galaxies to allow detailed spectropic investigations of star formation in the early universe. Hubble imaging has already revealed lensed arcs and detailed sub-structure within a handful of rich clusters. At the same time, the lensing characteristics provide information on the mass distribution within the lensing cluster. The present program aims to use the Wide Field Camera on the Advanced Camera for Surveys to obtain 2x2 mosaic images of a unfiromly selected sample of high mas clusters in the redshift range 0.58 < z < 0.88. The HST data will be analysed for weak lensing signatures that can be combined with X-ray, IR and optical observations to constrain the mass estimates for these clusters.

GO 12378: The different environments of dark gamma-ray bursts

The life-cycle of agamma-ray burst

Gamma ray bursts are events that tap extraordinary energies (1045 to 1047 joules) in remarkably short periods of time. Several thousands bursts have been detected over the last 30+ years, and analyses indicate that they can be divided into two classes with durations longer or shorter than 2 seconds. The short bursts appear to release more high energy radiation, so the two subsets are known as long/soft and short/hard bursts. The long/soft bursts appear to originate in the collapse of very massive stars, while the short/hard bursts are coalescing binary systems (probably pairs of neutron stars or black holes). The first optical counterpart to a gamma ray burst was identified in 1998, allowing confirmation of their extragalactic nature, and, since then, more than 60 bursts have been detected at X-ray wavelengths, and half that number detected at either optical or radio wavelengths; all of these detections are long/soft bursts. In the past decade, optical counterparts have been detected for many of these sources, allowing not only direct study of their characteristics, but also investigations of the properties of the underlying host galaxies. Some sources, however, do not appear to generate optical counetrparts, perhaps because the optical emission is suppressed due to substantial local dust obscuration. The absence of those data has hampered previous attempts to match those sources, and their environments, against GRBs with optical counetrparts. The present program aims to circumvent this issue by using Chandra observations to obtain accurate positions for the X-ray counterparts of such sources, and then match that astrometry against deep visual (F606W) and near-infrared (F160W) HST WFC3 images.

GO 12475: Cool Star Winds and the Evolution of Exoplanetary Atmospheres

The solar corona

All stars lose mass although, in the best 1984 tradition, some lose mass more equally than others. With growing evidence for the near-ubiquity of planetary systems, increasing attention is being devoted to how mass loss varies from star to star, and how varying mass loss rates might affect the conditions on planets orbiting within the habitable zone of that system. On Earth, we have direct experience of the impact of mild solar variability, where coronal mass ejections can lead to spectracular auroral displays and satellite disruption; more energetic outbursts on other stars could lead to significant erosion of the outer atmosphere, and perhaps radiation-induced mutation of hypothetical life forms. The present proposal aims to probe one aspect of this subject by using the Space Telescope Imaging Spectrograph (STIS) to probe mass loss rates in a sample of four late-type dwarfs thata re known to ahve planetary companions. All four of these stars are known to haver ealtively low mass-loss rates. High resolution ultraviolet spectra will be used to search for emission due to Mg II and Fe II, characteristic of stellar winds; in addition, lower resolution spectra will be used to search for Lyman-alpha absorption due to hydrogen within the putative stellar winds.