This week on HST

HST Programs: April 17 - April 23, 2006

Some selected highlights

GO 10114: Lyman-alpha far-UV observations of the Sun in time, and effects on planetary atmospheres

Lyman-alpha image of a solar flare (from the TRACE satellite)

The `Sun in time' program is designed to combine solar astronomy and archaeoastronomy, tracing how changes in solar properties over the last 4.5 billion years may have affected life on Earth, and looking forward to future (long-term) developments. Ultraviolet radiation plays an important role in these changes, photoionising and eroding the upper atmosphere, and providing a means of photochemical evolution. The Lyman alpha emission line (at 1215 Angstroms) is the strongest source of UV radiations, contributing 80-90% of far UV radiation, and 30-60% of the flux between 1 and 1500 Angstroms. This proposal aims to supply data on the likely evolution of this emission line over the Sun's history. The targets are a sample of G0-G5 dwarfs with ages between 130 Myrs and 6.7 Gyrs; these dwarfs serve as proxies for the Sun at different stages of its main sequence evolution. Observations are made with the solar-blind channel (SBC) of the Advance Camera for Surveys (ACS), using both narrowband imaging and low-resolution grism spectroscopy. The resulting fluxes will also be useful in assessing how UV radiation might affect the atmospheres of known extrasolar planets, particularly the `hot Jupiters' that orbit their parent star at separations of only a few stellar radii.

GO 10173/10606: Multicolour snapshot observations of 3C Radio Galaxies

Hubble WFPC2 planetary camera images of the central regions of several 3C radio galaxies

The Third Cambridge Catalogue of Radio Sources was compiled originally in 1959 from observations at 159 MHz made by the Cambridge Radio Astronomy Group using the Cambridge interferometer (Edge et al, Mem RAS, 68, 37). The addition of observations at 178 MHz led to the revised catalogue, the 3CR (Bennett, 1962, Mem RAS, 68, 137). This catalogue was the main basis for the Fanaroff-Riley classification scheme (FR-Is are double-lobed sources with small separation between the peaks; FR-IIs have a separation that exceeds half the largest size of the source), and includes many of the brightest known radio galaxies. As such, the 3CR sources have remained extremely important observational targets for understanding the nature and structure these highly energetic sources. Over the past several cycles, many 3CR galaxies have been observed at a variety of wavelengths by HST. Those observations have revealed new optical jets, dust lanes, face-on disks with optical jets, besides point-like nuclei whose properties support FR-I/BL Lac unified schemes. The present programs aim to use NICMOS and the ACS/HRC to extend coverage to near-infrared and ultraviolet wavelengths, searching for dust and regions of star formation in the central regions.

GO 10524: Blue Stragglers: a key stellar population to probe internal cluster dynamics

Blue stragglers in NGC 6397 (Hubble heritage team)

Blue stragglers are found in old open clusters and globular cluster systems. Most appear to lie on the main sequence, but in a location `up and to the right' of the main-sequence turnoff; that is, these stars have higher mass than the turnoff stars, and, if coeval, ought to have evolved through the giant branches to become white dwarfs. The resolution of this paradox appears to lie in stellar mergers; blue stragglers represent the remnants of violent dynamical processes that can only occur within high density star clusters. Most recent models favour stellar collisions as the likely formation mmode for these stars. However, previous observations of M3, 47 Tuc and NGC 6752 suggest that the blue stragglers folla a bimodal radial distribution, a result that this team interprets in favour of formation by mass transfer in close binary systems, rather than by direct stellar collisions The present proposal aims to combine HST ACS observations with ground-based CCD imaging to trace the radial distribution of these stars in eight globular clusters, including M15, NGC 2808, NGC 5466 and M71.

GO 10556: Neutral gas at redshift z=0.5

Hubble WFPC2 images of a faint galaxy near a z=2.81 QSO; the galaxy, lying ~1" from the QSO, is only visible after psf subtraction (lower panel), but produces damped Ly-alpha absorption in the QSO spectrum.

Bahcall & Salpeter originally suggested in 1965 that there might be a population of intergalactic hydrogen clouds lying along the line of sight to quasars (or high redshift galaxies) that could be detected through absorption in the QSO spectrum. That prediction was soon confirmed, with the detection of numerous absorption lines blueward of the QSO Lyman alpha emission line - the Lyman alpha forest. Most of those lines are extremely narrow, but a subset are suficiently strong that they are completely opaque at the base. These damped Lyman-alpha absorption lines must arise in systems that have a hydrogen column density exceeding 12019 cm-2, indicating a substantial mass of neutral hydrogen gas. A subset of DLA lines have been identified with particular galaxies (through redshift measurements), many lying far from the QSO line of sight. It is possible that both those systems and DLA systems that lack an optical counterpart are progenitors of spiral or irrgeular galaxies. Ground-based observations of DLA systems are restricted to redshifts z>1.65 by the atmospheric cutoff at 3100 Angstroms, but HST allows observations to extend to lower redshifts. The present proposal is using the PR200L prism on the ACS/HRC to survey 109 targets. These observations will extend the DLA census to redshifts between ~0.37 and ~0.7, and will therefore provide valuable information on the evolution of these systems at intermediate redshifts.

GO 10625/10992: Investigating the disintegration of Comet 73P/Schwassmann-Wachmann 3

Comet 73P/Schwassmann-Wachmann 3, 14 Dec 1995

Comet Schwassmann-Wachmann 3 was discovered by Arnold Schwassmann and Arno Arthur Wachmann on 2 May 1930 on photographcs taken as part of a minor planet survey being carried out by Hamberg observatory (see cometography ). During that initial apparition, the comet passed within 6 million miles of the Earth, and almost achieved naked eye brightness. The orbit was determined with sufficient precision to determine that this is a short-period comet, with period between 5.43 and 5.46 years, but the comet was not recovered again until 1979. The comet was missed in 1986, but recovered in 1990, when it passed within 0.35 AU of Earth and reached 9th magnitude. During the 1995 apparition, the comet underwent a series of outbursts, brightening from 14th to 8th magnitude, and it became clear that the nucleus had split into three or four components (A, B, C and D), at least two of which (components B and C) were still visible in 2001. (Unlike Comet Shoemaker-Levy, this disruption almost certainly resulted from internal forces, rather tidal disruption by an eternal body.) Comet Schwassmann-Wachmann 3 is back again this year. Working in conjunction with ground-based observatories, HST has been obtaining high-resolution ACS images at optical wavelengths, while Spitzer is conducting mid-infrared observations. Components B and C are still discernible, so are many others (up to S at the last count). All three of the brightest fragments show evidence for continued disintegration; thus, we may have a ringside seat at the disintegration of a cometary nucleus.