This week on HST

HST Programs: June 3 - June 9, 2013

Selected highlights

GO 12881: Spanning the chasms: re-observing the transiting exoplanet HD 189733b

Key events in a planetary transit

HD 198733 is a 7th magnitude G5 dwarf that lies at a distance of ~20 parsecs from the Sun in the constellation of Vulpecula. Like many other nearby solar-type stars, HD 189733 has an associated planetary system, including a hot Jupiter, a ~1.15 MJ gas giant with an orbital period of 2.12 days. Most significantly, that inner planet transits the central star, making HD 189733 the closest transiting system found so far. Transiting systems offer a potential gold-mine for extrasolar planetary studies, since not only is the orbital inclination well defined, but the diameter (and hence the average density) is directly measureable form the eclipse depth, while the atmospheric composition can be probed through line absorption or re-radiated thermal flux. The results from these measurments can be used to test, and improve, theoretical models of extrasolar planets. These observations are best done from space: indeed, the only successful atmospheric observations to date have been with HST and Spitzer. HD 189733 has been one of the kost popular targets, with previous observations used to measure the diameter (GO 10923: ACS/HRC in Cycle 15), search for atmospheric water (GO 11117: NICMOS, Cycle 16; GO 11740, STIS & WFC3/IR, Cycle 17) and sodium (GO 11572: STIS, Cycle 17), and probe the outer atmospheric dynamics (GO 11572: STIS, Cycle 17). The results are somewhat controversial, with alternative ijnterpretation of the NICMOS and (shorter-wavelength) WFC3-IR data. The present prorgam aims to resolve those controversies by obtaing new WFC3-IR G141 data that span the gap between the older WFC3-IR and NICMOS observations.

GO 12902: WISP - A Survey of Star Formation Across Cosmic Time

A region of massive star formation

Star formation is the key astrophysical process in determining the overall evolution of galactic systems, the generation of heavy elements, and the overall enrichment of interstellar and intergalactic material. Tracing the overall evolution through a wide redshift range is crucial to understanding how gas and stars evolved to form the galaxies that we see around us now. The present program builds on the ability of HST to carry out parallel observations, using more than one instrument. While the Cosmic Origins Spectrograph is focused on obtaining ultraviolet spectra of unparalleled signal-to-noise, this program uses the near-infrared grisms mounted on the Wide-Field Camera 3 infrared channel to obtain low resolution spectra between 1 and 1.6 microns of randomly-selected nearby fields. The goal is to search for emission lines characteristic of star-forming regions. In particular, these observations are capable of detecting Lyman-alpha emission generated by star formation at redshifts z > 5.6. A total of up to 40 "deep" (4-5 orbit) and 20 "shallow" (2-3 orbit) fields will be targeted in the course of this observing campaign.

GO 12972: In Search of the Coldest Atmospheres: Identifying Companions to the Latest WISE Brown Dwarfs

The stellar menagerie: Sun to Jupiter, via brown dwarfs

Brown dwarfs are objects that form in the same manner as stars, by gravitational collapse within molecular clouds, but which do not accrete sufficient mass to raise the central temperature above ~2 million Kelvin and ignite hydrogen fusion. As a result, these objects, which have masses less than 0.075 MSun or ~75 M<\sub>Jup, lack a sustained source of energy, and they fade and cool on relatively short astronomical (albeit, long anthropological) timescales. Following their discovery over a decade ago, considerable observational and theoretical attention has focused on the evolution of their intrinsic properties, particularly the details of the atmospheric changes. At their formation, most brown dwarfs have temperatures of ~3,000 to 3,500K, comparable with early-type M dwarfs, but they rapidly cool, with the rate of cooling increasing with decreasing mass. As temperatures drop below ~2,000K, dust condenses within the atmosphere, molecular bands of titanium oxide and vanadium oxide disappear from the spectrum to be replaced by metal hydrides, and the objects are characterised as spectral type L. Below 1,300K, strong methane bands appear in the near-infrared, characteristics of spectral type T. At present, the coolest T dwarfs known have temperatures of ~650 to 700K. At lower temperatures, other species, notably ammonia, are expected to become prominent, and a number of efforts have been undertaken recently to find examples of these "Y" dwarfs. The search is complicated by the fact that such objects are extremely faint instrinsically, so only the nearest will be detectable.Wide-field surveys have been undertaken at infrared wavelengths with both ground-based telescopes (eg UKIDDS) and satellite observatories. Most recently, the Wide-field Infrared Survey Exoplorer, WISE satellite mission, completed an all-sky survey and succeeded in identifying several tens of late-T and Y dwarfs. As a complement tot he wide-field approach, one can "look under the lamp-post": both stars and brown dwarfs are often found as binary or multiple systems, so one can take a sample of low-mass obejcts known to be within the Solar Neighbourhood, and look for even lower luminosity companions. That technique served in the past to identify van Biesbroeck 10, the first ultracool dwarf; GD 165B, the first L dwarf; and Gl 229B, the first T dwarf. The present program is applying the latter technique to the results of the WISE survey: thirteen brown dwarfs with spectral types T8 or later are being targeted for observation with WFC3-IR (J and H bands), with the aim of detecting even lower luminosity (and lower mass) companions.

GO 13033: COS-Halos: New FUV Measurements of Baryons and Metals in the Inner Circumgalactic Medium

A computer simulation of galactic gas accretion and outflow

Galaxy formation, and the overall history of star formation within a galaxy, clearly demands the presence of gas. The detailed evolution therefore depends on how gas is accreted, recycled, circulated through the halo and, perhaps, ejected back into the intergalactic medium. Tracing that evolutionary history is difficult, since gas passes through many different phases, some of which are easier to detect than others. During accretion and, probably, subsequent recycling, the gas is expected to be reside predominantly at high temperatures. The most effective means of detecting such gas is through ultraviolet spectroscopy, where gas within nearby systems can be detected as absorption lines superimposed on the spectra of more distant objects, usually quasars. The present program builds on a Cycle 17 program (GO 11598) that used the Cosmic Origins Spectrograph to observe z>1 QSOs that lie at small angular separations from SDSS galaxies at redshifts between z=0.15 and 0.35. The sightlines run through the halos of the galaxies, and the QSOs therefore provide a pencilbeam backlight that probes hot gas in the foreground systems. The original observations covered the spectral region 1150-1750 Angstroms; the present program targets 14 quasars for observations at shorter wavelengths, adding information on the neutral hydrogen column density by measuring absorption at the Lyman limit in these systems.