This week on HST

HST Programs: October 29 - November 4, 2007

Some selected highlights

GO 10840: The FUV fluxes of Tauri stars in the Taurus molecular cloud

Schematic view of a T Tauri star

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 significant 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. The aim of this proposal is to use combine observations with WFPC2 and with the Solar Blind Channel (SBC) on the Advanced Camera for Surveys (ACS) to study the ultraviolet energy distribution of a number of T Tauris spanning a range of ages. The WFPC2 will be used to obtain images at near-UV wavelengths (2000 - 3000 Angstroms), while the SBC will provide comparable data at Far-UV wavelengths. The present observations target FM Tauri, a ~2 Myr-old classical T Tauri star in the Taurus molecular cloud complex.

GO 10907: New Sightlines for the Study of Intergalactic Helium: A Dozen High-Confidence, UV- Bright Quasars from SDSS/GALEX

Probing the intergalactic medium via QSO absorption lines

Besides being interesting in their own right as the sites of super-mass black holes, QSOs serve an important role as cosmic searchlights, probing the composition of the intergalactic medium. Numerous galaxies and gas clouds lie along the line of sight to these distant objects, and each intervening cloud absorbs radiation, introducing absorption lines at the appropriate redshift and mapping the large-scale structure of the cosmic web. Absorption line spectroscopy, however, demands high signal-to-noise spectra which, in turn, demands bright QSOs. The present program aims to study the reionisation of He II, which is predicted to occur between redshift 3 and 4. The program has used SDSS data to identify QSOs with the appropriate redshift, and cross-referenced that dataset against GALEX data to select sources that are sufficiently bright for ACS/SBC observations. The brighter sources in this sample are also likely to be targets in future spectroscopic programs with either STIS or COS.

GO 11079: Treasury Imaging of Star Forming Regions in the Local Group: Complementing the GALEX and NOAO Surveys

The star forming region, N11, in the Small Magellanic Cloud

Star formation is a complex process that takes place on many scales, from diffuse, low-level activity within dispersed clouds like the nearby Taurus association, through massive star forming regions, like Orion and 30 Doradus, to intense starbursts within galactic nucle and mergers. The aim of the present proposal is to use WFPC2 to survey a representative sample of active star-forming regions in local Group galaxies. In particular, the observations will cover a wide range of OB associations in the two nearest large spirals, the Andromeda galaxy, M31, and the Triangulum system, M33. Multicolour, high-resolution images, taken at passbands from the ultraviolet (F170W) to far-red (F814W) will allow H-R diagrams to be constructed, probing the distribution of ages and metallicities, and setting constraints on variations in the underlying initial mass function(s). In the coming week, observations are scheduled of OB associations in the M33.

GO 11418: Investigating the Spectacular Outburst of Comet 17P/Holmes

Comet Holmes with SPICAM from Apache Point Observatory (taken Jack Dembicky)

Comet P/17 Holmes was discovered almost exactly 116 years ago, on Nov 6 1892, by the English gentleman astronomer, Edwin Holmes. At its discovery, the comet was detectbale by the naked eye, but within 2 months it had faded by 7 or 8 magnitudes. The following year, the comet again flared, brightening by ~6 magnitudes in January and gradually fading to ~15th magnitude by mid-March 1893. The comet has a period of 7 years, and was recovered in 1899 and 1906, but lost until its 1964 apparition. Since then, it has been recovered every cycle, but has remained quiescent - until October 24th this year, when Juan Antonia Henr'quez Santana (spain) reported significant brightening. Within a matter of hours, the comet had brightened from 15th to 3rd magnitude and is now the focus of much attention (see, for example, Frank Roylance's blog ). Comet P/17 Holmes has an orbit with a period of 6.88 years and an eccentricity of 0.40 (the orbit is subject to extensive perturbations by Jupiter, and the orbital parameters have shown notable changes over the last century). With a perihelion distance of 2.05 AU (which it passed in May this year), it does not get particularly close to the Sun, and, as a result, it is regarded as extremely unlikely that these flares could be produced by outgassing due to solar radiation. Fred Whipple (SAO) studied the 1892/3 episodes, and suggested that they might be due to collisions between the main cometary nucleus and a large satellite - a model that Whipple himself admitted was somehting of a "Just so" story. With the new outburst, the present proposal will use the WFPC2 camera on HST to image the nucleus of the comet and search for potential satellite nuclei.