This week on HST

HST Programs: April 12, 2010 - April 18, 2010

Selected highlights

GO 11594: A WFC3 Grism Survey for Lyman limit absorption at z=2

HRC grism image of SN 1987A, as displayed in the Hubble Legacy Archive

Grisms are optical components with finely ruled gratings that canbe introduced into the beam of an imaging camera to produce low-resolution spectra of objects within the field of view. Wide Field Camera 3 on HST has three such components: an ultraviolet grism, G280, providing spectroscopy from ~2100 to 3900 Angstroms; and two grisms, G102 and G141, for use at near-infrared wavelengths. The present program aims to use the UV grism to search for galaxies at redshifts in the range 1.8 < z < 2.5. The observations target 64 quasars, drawn from the Sloan Digital Sky Survey, that have absorption features that are characteristic of that redshift range. Those features are likely due to foreground galaxies, whose extended gaseous halos absorb light from the QSO. The grism data will used to search for sources with emission lines that indciate that they are at the appropriate redshift.

GO 11666: Chilly Pairs: A Search for the Latest-type Brown Dwarf Binaries and the Prototype Y Dwarf

NICMOS images of the ultracool L/T binary, 2MASS J22521073-1730134; the northern component, notably fainter at F160W, is the T dwarf.

Ultracool dwarfs are defined as having spectral types later than M7, and therefore include the recently discovered L and T dwarfs. They encompass the lowest mass stars (masses < ~0.1 MSub) and sub-stellar mass brown dwarfs, with surface temperatures ranging from ~2500K (~M7) to <700K (late-type T dwarfs). Following their discovery over a decade ago, considerable theoretical attention has focused on the evolution of the intrinsic properties, particularly the details of the atmospheric changes in the evolution from type L to type T. This point marks the emergence of methane as a dominant absorber at near-infrared wavelengths. Current models suggest the transition occurs at ~1400-1200K, and that the spectral changes are at least correlated with, and perhaps driven by, the distribution and properties of dust layers ("clouds") within the atmosphere. The overall timescales associated with the process remain unclear. The present proposal aims to tackle this issue through identifying, and characterising, ultracool binary systems with extremely cool components. Since these systems are almost certainly coeval, the relative spectral energy distributions of the two components can be used to set constraints on evolutionary models. More than 80 ultracool binary systems are currently known; almost all have relatively small linear separations (<15 AU), and components with mass ratios close to one. The present program targets 27 ultracool dwarfs with spectral types in the range T5 to T9, and will use WFC3 IR observations to search for previously unrecognised close, faint companions.

GO 11712: Calibration of Surface Brightness Fluctuations for WFC3/IR

Simulations of a nearby dwarf galaxy, a nearby giant galaxy and a distant giant galaxy; note that the last is similar in angular size to the dwarf, but has a much smoother brightness distribution (simulations from Ned Wright's ABC of distances

The determination of the Cosmic Distance Scale remains one of the major goals of cosmological programs in the early 21st century. Achieving this goal requires a reliable distance indicator. While observing programs continue to pursue conventional primary distance indicators (such as RR Lyraes and Cepheids) and secondary distance indicators (such as the RGB tip and the Tully-Fisher relation), attention is also being given to the method of surface brightness fluctuations. This method rests primarily on the hypothesis that the stellar populations in most galaxies have similar colour-magnitude diagrams. Thus, the total luminosity of the galaxy is generated by similar stars - mainly red giants. In a nearby low-luminosity galaxy, most of the light comes from a relatively small numebr of giant branch stars; consequently, that galaxy has a "grainier" appearance than a distant high-luminosity galaxy of the same apparent magnitude (see figure). The degree of granularity can therefore serve as a distance indicator. The present program will use the IR channel of Wide-Field Camera 3 (F110W and F160W filters) to observe seventeen galaxies in the Fornax and Virgo clusters to provide a reliable calibration of this technique.

GO 11792: Extrasolar Planet XO-2b

Artist's impression of a planetary transit against an active solar-type star

Transiting extrasolar planets offer the opportunity to gain valuable insight into the interior structure and atmospheres of gas giants beyond the Solar System. Besides providing direct measures of mass (with no complications for v sin(i)) and radius (from accurate time-series photometry), spectroscopic observations obtained during either transit or planetary eclipse can probe the atmospheric structure and chemical composition. The present proposal targets the transiting system designated XO-2b, discovered in 2007 by an international team of professional and amateur astronomers using a fleet of telescopes with very modest apertures. As an example, the primary survey telescope, the XO telescope, is a pair of 200-mm telephoto lenses with a very wide field of view. These small telescopes are used to survey large areas of the celestial sphere, searching for photometric variations characteristic of planetary transits (i.e. periodic dips in brightness of 1-2%); transit candidates are then verified using higher accuracy photometric observations with larger telescopes, and finally radial velocity measurements to confirm the companion mass. XO-2b, the second system discovered in the course of this program, is a ~0.6 MJ object in a 2.6-day period around a 10th magnitude K dwarf. Photometric and spectroscopic parallaxes place the star at a distance of around 150 parsecs, with an uncertainty of 10%. The present program will use the HST Fine Guidance Sensors to measure a trigonometric parallax accurate to 0.2 milliarcseconds, corresponding to uncertainties better than 5% in distance.