This week on HST

HST Programs: October 28 - November 3, 2013

Selected highlights

GO 13299: Silver Linings: Using Cloud Maps to Understand the L/T Spectral Transition

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 further broaden the sample by monitoring three brown dwarfs for variability using time-resolved WFC3 grism G141 spectroscopy.

GO 13003: Resolving the Star Formation in Distant Galaxies

ACS images of galaxy-galaxy Einstein ring lenses from the Sloan survey

Gravitational lensing is a consequence the theory of general relativity. Its importance as an astrophysical tool first became apparent with the realisation (in 1979) that the quasar pair Q0957+561 actually comprised two lensed images of the same background quasar. In the succeeding years, lensing has been used primarily to probe the mass distribution of galaxy clusters, using theoretical models to analyse the arcs and arclets that are produced by strong lensing of background galaxies, and the large-scale mass distribution, through analysis of weak lensing effects on galaxy morphologies. Gravitational lensing can also be used both to investigate the mass distribution of individual foreground galaxies and to probe the properties of more distant systems. Until recently, the most common background sources were quasars. Galaxy-galaxy lenses, however, offer a distinct advantage, since the background source is extended, and therefore imposes a stronger constraints on the mass distribution of the lensing galaxy than a point-source QSO. Moreover, the lensed image of the background galaxy is not only amplified, but expanded in angular size, enabling much more detailed investigation of the properties of such systems than in un-lensed field galaxies at the same redshift. The present program is capitalising on galaxy-galaxy lenses as natural telescopes to probe star formation at redshifts z>1. WFC3 is being used to observe 73 galaxy-galaxy lensed systems identified from the Sloan Digital Sky Survey. The systems are imaged in both the UVIS and IR channels, mapping the unrerlying spectral energy distribution, and hence the star formation rate, as a function of location in the background galaxies.

GO 13348: Imaging the Crab nebula while it is flaring in gamma-rays

The Crab Nebula

Messier 1, the Crab Nebula, presents one of astronomy's iconic images. The remnant of a bright supernova observed in 1054 by Arabian and Chinese astronomers, the Crab was first recorded in 1731 by the English astronomer, John Bevis, 37 years before Messier compiled his catalogue of non-comets. The energy source for the gaseous emission is the neutron star that lies in the centre of nebulosity, and was one of the first pulsars to be identified. The Crab is also a source of high energy emission, including gamma rays. In late September 2010, the gamma ray flux was observed to double in strength by both the Fermi and Agile spacecrafts, while for a few days in April 2011, the Crab was tghe brightest obejct in the gamma-ray sky. Follow-up high-energy observations were obtained with the Integral, Swift and Chandra observatories. and HST observations were used to study the detailed structure of the complex knots and filaments, to search for further insight into the nature of this event. The Crab has undergone another outburst, and the present observations have been triggered to obtain further supplementary optical imaging with HST.

GO 13610: Imaging Comet C/2013 A1 (Siding Spring) to Support Risk Assessment for Mars Orbiters during the Close Mars Encounter

The projected orbit of Comet C/2013 A1

2013 has been an interesting year for Hubble and comets: Comet ISON is being tracked as a potential great comet, passing through perihelion in late November; PanSTARRS and other surveys are turning up more evidence of residual cometary activity within objects in the asteroid belt, main belt comets; and an Oort cloud comet discovered in early January is slated to undergo a close encounter with Mars in October next year. The last-named object is Comet 2013/A1 (Siding Spring), originally found by Robert McNaught in the course of the ongoing Siging Spring Survey for near-Earth asteroids and comets. Pre-discovery images have been located among observations by the Pan-STARRS survey and the Catalina Sky survey, and it quickly became apparent that the orbit will take the comet extremely close to Mars in late 2014. More details calculations show that the comet will pass within 134,000 km of the planet. As an Oort comet, potentially on its first pass through the inner Solar System, Comet 2013/A1 is likely to have a substantial component of icy, volatile substances, generating an extensive coma and tail of gas and dust that will envelope the planet and its surrounding. Those dusty materials present a potential hazard for the planetary probes that are either currently in orbit around Mars or (in the case of Maven) slated to arrive there shrtly before the comet. The present program will use HST to obtain imaging observations at there epochs with the aim of characterising the evolution of surface activity and the consequent likelihood of adverse effects on the Martian probes.