This week on HST

HST Programs: January 3, 2011 - January 9, 2011

Selected highlights

GO 11616: The Disks, Accretion, and Outflows (DAO) of T Tau stars

Wide-field image, from NOAO, of T Tauri and its immediate environs

The T Tauri stage of evolution occurs early in a star's lifetime, within ~10 Myrs of its birth, when it still retains a dense, dust and gas-rich circumstellar disk. During this phase, there is substantial accretion of material onto the central star. This leads to heating of the inner regions of the accretion disk, and significant emission at ultraviolet and X-ray wavelengths. Previous HST programs (e.g. GO 10840 ) have used the STIS and the ACS/SBC to investigate these processes at FUV wavelengths. The present program will extend those investigations using COS, which provides more than an order of magnitude more sensitivity and resolution. The survey will target 32 T Tauri stars, including 26 "classical" T Tauris and 6 "weak-lined" T Tauris (the latter are surrounded by less disk material, and are generally believed to be at a later stage of evolution than the CTTs). COS will be used to measure the emission profiles of an extensive number of lines, probing opacities, temperatures and densities in the disk and outflow regions.

GO 12064: Cosmic Assembly Near-IR Deep Extragalactic Legacy Survey -- the UDS fields

Part of the GOODS/Chandra Deep Field South field, as imaged by HST

CANDELS is one of three Multi-Cycle Treasury Program, whose observations will be executed over the next three HST Cycles. It builds on past investment of both space- and ground-based observational resources. In particular, it includes coverage of the two fields of the Great Observatory Origins Deep Survey (GOODS), centred on the northern Hubble Deep Field (HDF) in Ursa Major and the Chandra Deep Field-South in Fornax. In addition to deep HST data at optical and near-infrared wavelengths, the fields have been covered at X-ray wavelengths by Chandra (obviously) and XMM-Newton; at mid-infrared wavelengths with Spitzer; and ground-based imaging and spectroscopy using numerous telescopes, including the Kecks, Surbaru and the ESO VLT. This represents an accumulation of almost 1,000 orbits of HST time, and comparable scale allocations on Chandra, Spitzer and ground-based facilities. The CANDELS program is capitalising on this large investment, with new observations with WFC3 and ACS on both GOODS fields, and on three other fields within the COSMOS, EGS and UDS survey areas (see this link for more details). The prime aims of the program are twofold: reconstructing the history of galaxy formation, star formation and nuclear galactic activity at redshifts between z=8 and z=1.5; and searching for high-redshift supernovae to measure their properties at redshifts between z~1 and z~2. The program incorporates a tiered set of observations that complement, in areal coverage and depth, the deep UDF observations, while the timing of individual observations will be set to permit detection of high redshift SNe candidates, for subsequent separate follow-up.

GO 12241: SAINTS - Supernova 1987A INTensive Survey

November 2003 HST image of the SN1987A gaseous ring

SN1987A, in the Large Magellanic Cloud, is (as far as we know) the nearest supernova to the Sun since Kepler's supernova of 1604. While its eruption, in January 1987, predated HST's launch by over 3 years, the remnant has been a regular observational target since the installation of COSTAR at the first servicing mission. Those high resolution observations have revealed the development, and evolution, of extensive, intricate structures as the blast wave from SN1987A encounters the surrounding interstellar medium. In particular, a striking circum-remnant ring has developed, with numerous hot spots stimulated by the fastest moving debris. The present HST program both the UVIS and IR channels on WFC3 to continue monitoring the development of those features, using a series of observations that are co-ordinated with the Chandra X-ray Observatory. In addition, COS will be used to obtain UV spectra of a number of specific features. Over the past few cycles, the hotspots are fusing as the shock fully enters the ring, and photons from these regions are exciting previously hidden gas outside the ring, illuminating mass lost from the progenitor before the explosion. The inner debris are now well resolved, and clearly aspherical. Overall, these observations provide crucial insight into the earliest stages of formation of a supernova remnant.

GO 12435: Investigating the Outburst of Asteroid 596 Scheila: Main Belt Comet vs Collisional Origin

Asteroid 596-Scheila, imaged by Peter Lake in December, 2010

Most asteroids within the inner solar system lie between Mars and Jupiter in the Main Belt, with relatively circular orbits having typical semi-major axes of 2-3 AU. The original members, Ceres, Pallas and Vesta, have diameters between 500 and 1000 kilometers and were dubbed "minor planets" at the time of their discovery in the early 19th century, but the vast majority currently known have dimensions of tens of kilometers. The asteroid 596-Scheila is among the latter. Discovered by the Heidelberg astronomer August Kopff in 1906, and named after an English student with whom he was acquainted, this is one of the larger known asteroids, with a diameter estimated as ~110 km. For over a century, its diameter was the only claim to fame, but in early December 2010, Steve Larson (of Arizonaâ^À^Ùs Lunar and Planetary Laboratory) noted that Scheila had sprouted a coma halo in observations taken by the Catalina Sky Survey. An examination of archival images revealed no evidence for activity throughout October and November, but a possible onset on December 3rd. Scheila is not the only main belt asteroid to have exhibited such behavior: the asteroid 1979 OW7/1996 N2 developed a comet-like appearance in 1996 and again in 2002; the initial outburst was ascribed to a collision, but the second event suggested that the activity is intrinsic to the asteroid; subsequently, a handful of other examples have been discovered, leading to the introduction of an entirely new class of objects, main belt comets. Scheila is likely an addition to that catalogue. HST will obtain WFC3 imaging to better characterize the current outburst.