This week on HST

HST Programs: May 6 - May 12, 2013

Selected highlights

GO 12903: 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 12920: Testing the paradigm of X-ray driven exoplanet evaporation with XMM+HST

Artist's conception of atmospheric ablation on a hot jupiter

HD 189733 is an early K-type dwarf lying at a distance of ~19.3 parsecs in the constellation of Vulpecula. It has an M dwarf companion, HD 189733B, at a separation of ~220 AU, and also harbours a planetary system. HD 209458 is G0, near-solar metallicity main-sequence dwarf lying 50 parsecs from the Sun. Both of these stars have "hot Jupiters" - gas giants that occupy orbits whose semi-major axis is smaller than that of mercury in the Solar System. HD 1897733b orbits the parent star at a distance of 0.031 AU in a period of 2.219 days; HD 209458b lies at a distance of 0.045 AU, has a period of 3.52474859 day; both also happen to transit the parent star. At these distances, the planetary atmosphere is heated to temperatures exceeding 1000K, leading to extensive evaporation.Past HST observations have resulted in spectroscopic detections of the evaporating gas. The present program aims to determine the role that X-ray irradiation plays in driving the evaporation by coupling XMM oobservations with near-simultaneous HST STIS spectroscopy of HD 189733.

GO 12970: Completing the Census of Ultracool Brown Dwarfs in the Solar Neighborhood using HST/WFC3

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. Identifying such ultra-ultracool dwarfs was a goal of the WISE satellite mission, which recently completed its all-sky survey. WISE has succeeded in identifying a number of extremely interesting sources, including at least 4 objects that have been confirmed as dwarfs with temperatures lower than 350K. These are among the first examples of Y dwarfs, and all are too faint to be characterised with any degree of certainty using ground-based observations. The current program will use WFC3 G102 grism spectroscopy to verify the nature of a further 20 candidates.

GO 13229: Hubble Heritage imaging of Comet ISON

HST image of Comet ISON, late April 2013

At least once every decade, the astronomical community gets excited by the discovery of a comet that offers the prospect of becoming a "comet of the century", developing a spectacular tail speading across the sky at sunset or sunrise. Unfortunately, while Comet McNaught put on a spectaculal shows for the southern hemisphere in 2007, most of the predicted rivals of the Great Comets of 1680/1811/1882 have fizzled. The latest in line is Comet ISON, currently lying somewhat beyond jupiter's orbit, but scheduled to pass within 700,000 miles of the Sun in late November. Despite the distance, the comet already has a signfiicant coma and tail, leading to hopes that it might be highly volatile rich. As a sun-grazer, there is a significant chance that the comet might break up close to peihelion, potentially offering an even more spectacular show and more volatiles are exposed to solar radiation. In the meantime, Hubble will continue to monitor its progression - at least until it entes the solar avoidance zone, withi 50 degrees of the Sun.