This week on HST

HST Programs: January 23 - January 29, 2012

Selected highlights

GO 12181: The Atmospheric Structure of Giant Hot Exoplanets

Probing the atmosphere of a transiting exoplanet

The first exoplanet, 51 Peg b, was discovered through radial velocity measurements in 1995. 51 Pegb was followed by a trickle, and then a flood of other discoveries, as astronomers realised that there were other solar systems radically different from our own, where "hot jupiters" led to short-period, high-amplitude velocity variations. Then, in 1999, came the inevitable discovery that one of those hot jupiters. HD 209458b, was in an orbit aligned with our line of sight to the star, resulting in transits. Since that date, the number of known transiting exoplanet systems has grown to more than 100, most detected through wide-field photometric surveys with the Kepler satellite providing the highest sensitivity dataset. These transiting systems are invaluable, since they not only provide unambiguous measurements of mass and diameter, but they also provide an opportunity to probe the atmospheric structure by differencing spectra taken during and between primary secondary transit. Such observations are best done from space: indeed, the only successful atmospheric observations to date have been with HST and Spitzer. The present program aims to set these measurements on a systematic basis by targeting 13 transiting exoplanets. The WFC3-IR G141 grism will be used to search for characteristic near-infrared spectral features in those systems.

GO 12283/12568: WISP - A Survey of Star Formation Across Cosmic Time

A region of massive star formation

Star formation is the key astrophysical process in determining the overall evolution of galactic systems, the generation of heavy elements, and the overall enrichment of interstellar and intergalactic material. Tracing the overall evolution through a wide redshift range is crucial to understanding how gas and stars evolved to form the galaxies that we see around us now. The present program builds on the ability of HST to carry out parallel observations, using more than one instrument. While the Cosmic Origins Spectrograph is focused on obtaining ultraviolet spectra of unparalleled signal-to-noise, this program uses the near-infrared grisms mounted on the Wide-Field Camera 3 infrared channel to obtain low resolution spectra between 1 and 1.6 microns of randomly-selected nearby fields. The goal is to search for emission lines characteristic of star-forming regions. In particular, these observations are capable of detecting Lyman-alpha emission generated by star formation at redshifts z > 5.6. A total of up to 40 "deep" (4-5 orbit) and 20 "shallow" (2-3 orbit) fields will be targeted in the course of this observing campaign.

GO 12476: Measuring the Hubble Flow Constant

NGC 4911, from DSS scans of POSS II IIIaJ plate material

Cepheid variable stars have been the prime extragalactic distance indicator since Henrietta Leavitt's discovery of the period-luminosity relation described by Cepheids 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. Hubble has accumulated WFPC2 and NICMOS observations of Cepheids in 31 galaxies. All of those galaxies lie within 25 Mpc; thus, both the Key Project's derivation of H0 = 72 +/- 8 km/sec/Mpc and the competing value, H0 = 56 +/- 7 km/sec/Mpc, (an offset of 1.5 sigma), rely on secondary indicators to take measurements to the far-field Hubble flow. The aim of the present project is to extend covrage to Cepheids within the Coma cluster. The present program has its genesis in a Cycle 15 Treasury program designed to use the high sensitivity and high resolution of the Adnaced Camera for Surveys to search for Cepheids in two spiral galaxies in the cluster, NGC 4911 and NGC 4921. If Coma lies at a distance of 100 Mpc ( (m-M)=35.0), then long-period Cepheids (P~50 days) have mean apparent magnitudes of V~29 - challenging observations even for ACS. Unfortunately, the ACS failure in January 2007 left the original program barely 40% complete. However, the initial dataset was sufficient to identify 50 long-period Cepheids candidates in NGC 8921. The present program aims to build on those incomplete results by using the UVIS channel on Wide-field Camera 3 to obtain cadenced observations in the F350LP and F606W filters, and determine periods for verified variable stars..

GO 12528: Probing the nature of LBVs in M31 and M33 - Blasts from the Past

GR290, Romano's star, a luminous blue varibale in M33

Luminous Blue Variables (LBVs) are just what they sound like - bright blue stars that show low-level (few tenths of a magnitude), irregular changes in brightness over timescales of years, punctuated by substantial outbursts, when the star brightens by several magnitudes. The prototype is S Doradus, the brightest star in the Large Magellanic Cloud (MV < -10, so brighter than many dwarf galaxies), and there are only around 20 other such stars known, including P Cygni and eta Carinae. They are massive hypergiants, with radii over 100 times that of the Sun and system masses that may exceed 100 MSun. At least some of these systems are binary stars. The outbursts are very likely caused by substantial mass loss events, perhaps stimulated by the binary companion, and can lead to the star's brightness increasing over a hundredfold over a period of 5-10 years. The classic example is Eta Carinae, which was originally catalogued by Edmond Halley as 4th magnitude in 1677, but had brightened by 3-4 magnitudes by 1730, returning to its previous brightness by 1782. There have been a number of subsequent outbursts, and detailed, high-resoution imaging ahs revealed a complex series of nebular shells surrounding the stellar system, indicative of substantial mass-loss events. The present HST program aims to probe the LBV phenomenon through high spatial-resolution spectroscopy of the 6 known LBVs in the nearby spirals, M31 and M33. The goal is to use those spectral data to probe the outburst history of those stars, as traced by the surrounding circumstellar material, and hence shed light on the long term evolution of these systems.