This week on HST

HST Programs: March 23 - March 29, 2015

Selected highlights

GO 13648: Uncovering the Putative B-Star Binary Companion of the SN 1993J Progenitor

SN 1993J in M81; the supernova is the bright star at ~2 o'clock from the galaxy centre

Supernovae come in several different flavours. In recent years, attention has focused primarily on Type Ia systems, generally believed to stem from thermonuclear runaway reactions on white dwarf stars in binary systems. Type Ia supernovae serve as extremely luminous standard candles, and have therefore come to play a significant role in mapping the expansion history of the universe. Type II supernovae, on the other hand, are generally ascribed to core collapse in extremely massive stars, as the rate of nuclear energy generation falls below the level required to provide support against gravitational collapse. There are several categories of Type II supernovae, notably Type IIL, which show a linear decline in brightness post-maximum, and Type IIP, which exhibit a plateau in brightness before entering the decline. SN 1993J, one of the nearest and brightest Type II supernovae of the last 50 yhears, falls into neither of those classes. Discovered by the amateur astronomer Francisco Garcia, the initial spectra revealed strong hydrogen emission, suggestive of a Type II system. The light curve, however, was more characteristic of a Type Ib supernova - also identified as dur to core collapse in massive stars, but in systems that have lost the hydrogen envelope and are therefore characterised by helium emission. Indeed, during the later phases of evolution, SN 1993J exhibited strong helium emission. The supernova is therefore one of the few supernovae classified as a Type IIb system - a massive star that has lost some, but not all, of its hydrogen envelope, and therefore lies between the pure Type II and Type Ib systems. Stripping the outer layers of a star generally requires prolonged gravitational interaction with a companion star - and the companion star, which must eb relatively massive itself, is expected to survive the supernova explosion. However, to date there is no observational confirmation of a companion in any Type IIb system. As the nearest such system, SN 1993J is the best candidate for resolving this issue, and the present program aims to use WFC3 to search for SN1993J's companion.

GO 13667: Observations of the Pluto System during the New Horizons Encounter

The first New Horizon image Pluto and Charon, from a distance of ~200 million km

Pluto, one of the largest members of the Kuiper Belt and, until recently, the outermost planet in the solar system, is the primary target of NASA's New Horizons Mission and has been the subject of a range of supporting HST programs over the past few years. Those observing programs have added four moons to the Pluto system. James Christy originally identified the largest moon, Charon, in 1978 from photographic plates Charon has a diameter of ~1200 km, or almost half that of Pluto itself. In 2005, Hubble added two small moons, christened Nix and Hydra, approximately 5,000 times fainter than Pluto itself, implying diameters as small as ~30-50 km if the surface composition is similar. More recent observations, in support of New Horizons mission, aimed to use WFC3 to search for faint rings due to dust particles that might jeopardise the space craft and require a course correction. While no rings were detected unequivocally, two small satellite, christened "P4" and "P5", have been discovered. Both are significantly fainter than Nix and Hydra, and may well be as small as 10-13 km in size. There is also some evidence that might point to the presence of a debris ring within Charon's orbit. Most recently, Hubble carried out an extensive imaging survey to identify KBOs lying beyond pluto that might seve as targets for an extended new Horizons mission. Two such candidates were identified, and hubble will be conducting follow up observations to further constrain their orbits. The present observations are in direct support of the primary New Horizons mission. WFC3 will be used to provide a comprehensive set of direct imaging and grism observations that will provide reference data to the broad context for interpreting New Horizons data compiled during the July 14th system fly-by.

GO 13686: The Longest Period Cepheids, a bridge to the Hubble Constant

ESO image of the material surrounding the long-period Cepheid, RS Puppis

Cepheid variable stars have been the prime extragalactic distance indicator since Henrietta Leavitt's discovery of the period-luminosity relation outlined by variables in the Small Magellanic Cloud. It was Hubble's identification of Cepheids in NGC 6822 that finally established that at least some nebulae were island universes. Cepheids and the extragalactic distance scale figure largely in HST's history, notably through the Hubble Constant Program, one of the initial Key Projects. HST has since observed Cepheids in more than 30 galaxies. Establishing a Galactic sample with reliable distance determinations is obviously crucial to this process. Long period Cepheids, with pulsation periods in excess of 25 days, play a key role, since they are more luminous, easier to detect and can be observed with higher photometric accuracy in distant galaxies. The present program aims to add to the sample of well-observed Galactic Cepheids by using spatial scanning on WFC3 to determine accurate parallaxes for nine Cepheids at distances up to 4 kpc from the Sun. Spatial scanning enables astrometry to an acuracy of ~40 microarcseconds, offering the prospect of distances accurate to 4% for individual Cepheids, and an overall distance scal calibration accurate to ~1%.

GO 13871: A Spectroscopic Redshift for the Most Luminous Galaxy Candidate at z~10

ACS images of a section of the GOODS fields

The Great Observatories Origins Deep Survey, GOODS, originated as a Spitzer Legacy program coupled with a Cycle 12 HST Treasury program. The program was designed to probe galaxy formation and evolution at redshifts from z~1 to z~6. GOODS covers two ~150 sq. arcminute fields, one centred on the Hubble Deep Field in Ursa Major and the Chandra Deep Field-South in Fornax. Initially, the program combined deep optical/far-red imaging (F435W, F606W, F775W and F850LP filters) using ACS on HST with deep IRAC (3.6 to 8 micron) and MIPS (25 micron) imaging with Spitzer. These two fields have become among the most studied celestial regions. In addition to deep HST data at optical and near-infrared wavelengths (both fields have been covered by NICMOS), the fields have been covered at X-ray wavelengths by Chandra (obviously) and XMM-Newton, and ground-based imaging and spectroscopy using numerous telescopes, including the Kecks, Gemini, Surbaru and the ESO VLT. Part of the GOODS South field was covered by the WFC3 Early Release Science observations (see WFC3 ERS ), and both fields are also covered partially by one of the three Multi-Cycle Treasury programs allocated time in Cycle 18-20. Combining all these imaging data, Oesch et al have recently identified four surprisingly bright galaxies with photometric redshifts exceeding z=9. One of these objects, GN-z10-1, is placed at erdshift z=103., less than 500 Myrs after the Big Bang. The detection of such a luminous and (presumably) massive object at such a young epoch is surprising. The present program aims to solidify the observational result by using the WFC3-IR G141 grism to obtain a spectrum and determine a spectroscopic redshift for the object.