This week on HST

HST Programs: January 25, 2010 - January 31, 2010

Selected highlights

GO 11565: A search for astrometric companions to very low-mass, Population II stars

HST image of Gliese 623AB, a nearby disk-dwarf binary system

Binary stars provide an important avenue for setting constraints on stellar evolution, particularly binary stars that consist of components with significantly different masses. The simplest means of producing stars in a dynamically bound system is during the formation process; consequently, it is reasonable to assume that the the stars are coeval and a comparison between the observed properties and predictions can constrain stellar models. Binary systems are common in stars in the Galactic disk, although the overall frequency does decrease with stellar mass, ranging from ~60-70% for G dwarfs to closer to 20% for late-type M dwarfs and ultracool dwarfs. Binaries are much rarer among the metal-poor members of the Galactic halo, with only a handful known among cool, late-type subdwarfs. The present program aims to expand the sample by using WFC3 on HST to image low-mass, metal-poor subdwarfs within 120 parsecs of the Sun, searching for close, faint companions that can be monitored for astrometric orbit determinations.

GO 11577: Opening New Windows on the Antennae with WFC3

HST ACS image of NGC 4038/4039, the Antennae

Galaxy merging has been recognised in recent years as a major process that influences galaxy evolution. One of the key systems that has contributed to that understanding is the Antennae galaxy, also known as NGC 4038/4039 or Arp 244. Originally identified by William Herschel in the course of his star sweeps, the Antennae is a nearby pair of galaxies (a spiral and a barred spiral) undergoing a major merger that started approximately 1 billion years ago. The interactions have led to etensive gravitational distortions, including two prominent tidal tails, and extensive star formation. HST observations over the last 15 years have revealed more and more details of the star formation\ triggered by the merger, with first WFPC2, then NICMOS and ACS imaging of key regions. The present program aims to build on those previous investigations by using ultraviolet imaging with WFC3 to probe young star clusters within the colliding disks.

GO 11653: 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 continues to monitor the development of those features, using a series of observations with WFC3/UVIS that are co-ordinated with the Chandra X-ray Observatory. 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 11732: The Temperature Profiles of Quasar Accretion Disks

The first Einstein cross, the gravitational lensed QSO, G2237+0305

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 to probe the mass distributions on a variety of scales: of galaxies (primarily via multiply-imaged quasars); of galaxy clusters (arcs and arclets); and at the largest scales (weak lensing). However, lensing can also provide insight on the small-scale properties of the object being lensed. In a lensed QSO, the light from the QSO follows different paths to produce the separate images; each of those paths has a different length; consequently, flux variations in the source show up at different times in the separate images. The present program aims to take advantage of this property to probe the structure of the accretion disks surrounding the central black hole in a number of lensed QSOs. The program will combine ultraviolet observations with the WFC3/UVIS camera on HST with GALEX UV data for 5 lenses spanning as broad range of black hole masses. Studying the variation as a function of wavelength should probe the accretion disk structure, since light from the inner regions are expected to dominate at shorter wavelengths, while the outer regions dominate at longer wavelengths.