This week on HST

HST Programs: May 5 - May 11, 2014

Selected highlights

GO 12893: Study of Small and Cool Kepler Planet Candidates with High Resolution Imaging

The Kepler satellite

Kepler is a NASA Discovery-class mission, designed to search for extrasolar planets by using high-precision photometric observations to detect transits. Launched on 7 March 2009, Kepler continuously monitored ~100,000 (mainly) solar-type stare within a ~100 square degree region in Cygnus for more than 4 years. Routine observations ceased on May 11 2013 when a second reaction wheel failed; efforts are currently under way to examine the options for restoring observations. Regardless, the mission has been an astounding success. Ground-based observations have successfully detected a couple of dozen transiting planets (e.g. HD 209458); almost all are "hot jupiters", gas giants on short-period orbits which produce a photometric dip of ~10-2 with a period of a few days, with a smattering of neptune-sized "super-Earths". Kepler, in contrast, has identified more than 2,700 exoplanet candidates around over 2,000 candidate host stars. More significantly, the exquisite precision of Kepler's photometric observations enables it to detect the 0.01% transit signature of earth analogues in these systems. A subset of stellar binaries provide one of the main sources of confusion in searching for planetary transits, since "grazing" transits can mimic the planetary signature. This is particularly an issue with Kepler, since the optical system is designed to provide a broad psf, spreading the stellar flux over a large area on the detector to allow high photometric accuracy. As a result, faint eclipsing stellar binaries will contribute to the source counts. Moreover, since the target field is (intentionally) within the Milky Way, there is a significant potential for unresolved stars within the (relatively broad) Kepler psf to increase the total signal, and hence dilute the depth of transits, giving the appearance of a smaller diameter exoplanet. This program is using the high spatial resolution imaging provided by HST to study a subset of the Kepler Earth-like candidates to assess the potential of this effect.

GO 13046: RAISIN: Tracers of cosmic expansion with SN IA in the IR

The first supernova discoevered by the Pan-STARRs survey

Supernovae are the most spectacular form of stellar obituary. In recent years, these celestial explosions have acquired even more 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 uncertainties remain as to the uniformity of the events. Moreover, as the sample of known supernova has grown, so has the range of photometric systems and the methods used to fit the light curve and account for the ever-present uncertainites inroduced by dust absorption. Consequently, the potential remains for systematic bias in distance estimates due both to intrinsic differences and to measurement errors. The persent program aims to minimise these systematics by compiling standard sequences of observations, primarily in the Y, J, and H filters, of supernovae at redshifts between z~0.3 and 0.5. Focusing on those wavelengths minises the effects, and hence the uncertainties, due to dust absorption. The supernovae themselves are drawn from the Pan-STARRS survey, with the WFC3-IR camera on HST employed to obtain the photometry.

GO 13280: Evolution of heterogeneous cloud structure through the T dwarf sequence

An artist's rendition of brown dwarf atmospheric structure

Brown dwarfs are failed stars - objects that form like stars, by gravitational collapse within giant molecular clouds, but which have insufficient mass to raise the central temperature above 107 K, and which therefore are unable to ignite hydrogran fusion and maintain a long-lived central energy source. As such, these objects reach a maximum surface temperature of perhaps 3,000K some tens of millions of years after their formation, and subsequently cool and fade into oblivion. As they cool, they move through spectral types M, L and T, with the oldest brown dwarfs now likely to have temperatures close to 300K and emergent spectra characterised by water and ammonia bands, the putative signatures of the spectral class Y. As these dwarfs cool from L to T (~1500 to ~1200K), the atmospheres undergo significant changes, with heavier elements condensing to form dust. That dust can form clouds, perhaps giving the dwarf's surface a banded appearance, similar to Jupiter. The clouds themselves may appear and disappear over relatively short timescales, leading to photometric variations at particular wavelengths. Past programs have used both Spitzer and HST to monitor spectral variability in a number of systems, primarily objects with spectral types in the L/T transition zone. The present program focues on coole dwarfs, targeting three mid-type T dwarfs identified as variable objects from the Cycle 19 HST SNAP program 12550 (PI: D. Apai). The WFC3 grism G141 will be used to obtain time-resolved spectroscopy of the three objects, 2MASSJ559-0441 (T5), 2MASSJ1624+0029 (T6) and 2MASSJ1049-5319 (Luhman 16).

GO 13293: Green Pea Galaxies: Extreme, Optically-Thin Starbursts?

A montage of green pea galaxies discovered by the Galaxy Zoo project

Understanding the galaxy formation and galaxy evolution has been a strong focus of astronomical research since thne 1970s and remains a key issue for 21st century astrophysics. Since we cannot follow an individual galaxy through time, tackling these questions requires a statistical approach, and, as a result, large-scale surveys have played a crucial role in the field. In particular, the Sloan Digital Sky Survey, conducted from Apache point Observatory from the 1990s through the early years of this century, has provided a treasure trove of information of galaxies at low and moderate redshifts. One of the interesting discoveries that originated from SDSS was the discovery of so-called "green pea" galaxies - compact objects whose combined images have a greenish hue, originally uncovered by citizen scientists as part of the Galaxy Zoo project. Closer inspection shows that these are gas-rich galaxies lying at relatiVEly low redshifts, 0.1 < z < 0.36, with the green-tinged hue partly stemming from the presence of strong oxygen ([O III]) emission. These characteristics indicate that the galaxies are undergoing strong star-forming episodes. Teh present program aims to take advantage of HST's unparalleled resolution and extraordinary sensitivity at ultraviolet wavelengths, and will use the Cosmic Origins Spectrograph to obtain spectra are far-UV wavelengths and probe the physical structure of the underlying star forming regions.