This week on HST

HST Programs: September 30 - October 6, 2013

Program Number Principal Investigator Program Title
12817 Richard J. Massey, University of Durham Longevity of dark matter substructure in Abell 3827
12880 Adam Riess, The Johns Hopkins University The Hubble Constant: Completing HST's Legacy with WFC3
12884 Harald Ebeling, University of Hawaii A Snapshot Survey of The Most Massive Clusters of Galaxies
12893 Ronald L Gilliland, The Pennsylvania State University Study of Small and Cool Kepler Planet Candidates with High Resolution Imaging
12937 Dennis Zaritsky, University of Arizona Direct Confirmation of Intracluster Stars as SN Ia Progenitors
12961 Misty C. Bentz, Georgia State University Research Foundation A Cepheid Distance to NGC6814
13014 Michael A. Strauss, Princeton University The Host Galaxies of High-Luminosity Obscured Quasars at z~2.5
13024 John S. Mulchaey, Carnegie Institution of Washington A Public Snapshot Survey of Galaxies Associated with O VI and Ne VIII Absorbers
13028 Evan D. Skillman, University of Minnesota - Twin Cities Is the First Epoch of Star Formation in Satellite Galaxies Universal?: M31 vs. Milky Way dSphs
13031 William M. Grundy, Lowell Observatory Testing Collisional Grinding in the Kuiper Belt
13046 Robert P. Kirshner, Harvard University RAISIN: Tracers of cosmic expansion with SN IA in the IR
13109 Martin C. Weisskopf, NASA Marshall Space Flight Center Joint Chandra and HST Monitoring and Studies of the Crab Nebula
13302 J. Michael Shull, University of Colorado at Boulder COS Spectra of High-Redshift AGN: Probing Deep into the Rest-Frame Ionizing Continuum and Broad Emission Lines
13344 Adam Riess, The Johns Hopkins University A 1% Measurement of the Distance Scale with Perpendicular Spatial Scanning
13346 Thomas R. Ayres, University of Colorado at Boulder Advanced Spectral Library II: Hot Stars
13368 Nancy R. Evans, Smithsonian Institution Astrophysical Observatory A Precision Measurement of the Mass of the Cepheid V350 Sgr
13400 Arlin Crotts, Columbia University in the City of New York The Surprising Ejecta Geometry of Recurrent Nova T Pyx
13445 Joshua S. Bloom, University of California - Berkeley Absolute Calibration of the Extragalactic Mira Period-Luminosity Relation
13451 Frederick Hamann, University of Florida A Study of PG Quasar-Driven Outflows with COS
13457 Kailash C. Sahu, Space Telescope Science Institute Accurate Mass Determination of the Nearby Old White Dwarf Stein 2051B through Astrometric Microlensing
13476 Nitya Kallivayalil, The University of Virginia Proper Motion and Internal Kinematics of the SMC: are the Magellanic Clouds bound to one another?

Selected highlights

GO 12893: Study of Small and Cool Kepler Planet Candidates with High Resolution Imaging

The Kepler satellite
Kepler is a NASA Discovery-class mission, designed to search for extrasolar planets by using high-precision photometric observations to detect transits. Launched on 7 March 2009, Kepler continuously monitored ~100,000 (mainly) solar-type stare within a ~100 square degree region in Cygnus for more than 4 years. Routine observations ceased on May 11 2013 when a second reaction wheel failed; efforts are currently under way to examine the options for restoring observations. Regardless, the mission has been an astounding success. Ground-based observations have successfully detected a couple of dozen transiting planets (e.g. HD 209458); almost all are "hot jupiters", gas giants on short-period orbits which produce a photometric dip of ~10-2 with a period of a few days, with a smattering of neptune-sized "super-Earths". Kepler, in contrast, has identified more than 2,700 exoplanet candidates around over 2,000 candidate host stars. More significantly, the exquisite precision of Kepler's photometric observations enables it to detect the 0.01% transit signature of earth analogues in these systems. A subset of stellar binaries provide one of the main sources of confusion in searching for planetary transits, since "grazing" transits can mimic the planetary signature. This is particularly an issue with Kepler, since the optical system is designed to provide a broad psf, spreading the stellar flux over a large area on the detector to allow high photometric accuracy. As a result, faint eclipsing stellar binaries will contribute to the source counts. Moreover, since the target field is (intentionally) within the Milky Way, there is a significant potential for unresolved stars within the (relatively broad) Kepler psf to increase the total signal, and hence dilute the depth of transits, giving the appearance of a smaller diameter exoplanet. This program is using the high spatial resolution imaging provided by HST to study a subset of the Kepler Earth-like candidates to assess the potential of this effect.

GO 13031: Testing Collisional Grinding in the Kuiper Belt

Preliminary orbital determination for the KBO WW31, based on C. Veillet's analysis of CFHT observations; the linked image shows the improved orbital derivation, following the addition of HST imaging
The Kuiper Belt consists of icy planetoids that orbit the Sun within a broad band stretching from Neptune's orbit (~30 AU) to distance sof ~50 AU from the Sun (see David Jewitt's Kuiper Belt page for details). Over 500 KBOs (or trans-Neptunian objects, TNOs) are currently known out of a population of perhaps 70,000 objects with diameters exceeding 100 km. Approximately 2% of the known KBOs are binary (including Pluto, one of the largest known KBOs, regardless of whether one considers it a planet or not).TNOs are grouped within three broad classes: resonant objects, whose orbits are in mean motion resonance with Neptune, indicating capture; scattered objects, whose current orbits have evolved through gravitational interactions with Neptune or other giant planets; and classical TNOs, which are on low eccentricity orbits beyond Neptune, with no orbital resonance with any giant planet. The latter class are further sub-divided into "hot" and "cold" objects, depending on whether the orbits have high or low inclinations with respect to the ecliptic. Cold, classical TNOs show relatively uniform characteristics, including red colours, high albedos and an extremely high binary fraction (>30%). They are believed to have formed in situ, and were therefore in place to experience the range of gravitational interactions as the giant planets migrated to their present location. As that migration occurred, subsets are expected to have been trapped in transitory resonance orbits. The present SNAP program aims to use HST to survey up to 56 cold, classical TNOs, aiming to deermine both the binary frequency and the colour distribution of the sample. Collisional grinding models have been invoked to explain the number-magnitude distribution of these obejcts; if those models are valid, then the expectation is that small binaries should also have been disrupted, and the surface of these eroded by collisions to expose the different-composition (colour) interior.

GO 13109: Joint Chandra and HST Monitoring of the Crab Nebula

The Crab Nebula
Messier 1, the Crab Nebula, provides astronomy with one of its iconic images. The remnant of a bright supernova observed in 1054 by Arabian and Chinese astronomers, the Crab was first recorded in 1731 by the English astronomer, John Bevis, thirt-seven years before Messier compiled his catalogue of non-comets. The energy source for the gaseous emission is the neutron star that lies in the centre of nebulosity, and was one of the first pulsars to be identified. The Crab is also a source of high energy emission, including radiation at X-ray and gamma ray wavelengths. Overall, this system plays a crucial role in aiding our understanding of post-supernova evolutionary processes. However, there are still some notable undertainties in the detailed processes within even this system. In particular, in September of 2010 the Crab surprised the astronomial community by producing a powerful flare at gamma-ray wavelengths that persisted for 4 days (see GO 12381 ).A second flare of similar magnitude occurred in May, 2011. Observations taken during the 2010 flare by HST and Chandra provided some insight into the effects of the flare, but analysis was hampered by the absence of a comparison set of pre-outburst images of comparable resolution and depth. The present program aims to address that issue through coordinated monitoring of the Crab at X-ray and optical wavelengths. Tne Advanced Camera for Surveys on HST has been used to take images in the F550M filter at 6 epochs in 2012, with the observations timed to be within 10 days of X-ray images taken by Chandra using the AXAF CCD Imaging Spectrometer; the present program will obtain a further series of observations through 2013. These data will establish a reference set should a further flare occur.

GO 13445: Absolute Calibration of the Extragalactic Mira Period-Luminosity Relation

The spiral galaxy, NGC 4258, as imaged by GALEX
Defining a reliable cosmic distance scale remains one of the most important tasks facing modern cosmologists. 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. Cepheids are not the only pulsating variables that can serve as distance indicators, however: RR Lyraes serve as distance indicators for old populations; and pulsating red giant variables offer an alternatiev in intermediate-age populations. Most long period variable stars (LPVs) are red giants on the asymptotic giant branch (AGB). These are intermediate mass stars, between ~1.5 and ~7 solar masses, that are powered by hydrogen and helium shell-burning. The interactions between the two energy sources lead to instabilities that can generate substantial pulsations, with periods from ~50 to ~500 days. Mira, or omicron Ceti, is the Galactic prototype for this type of variable, and numerous LPVs have been identified throughout the Milky Way and in the neighbouring Small and Large Magellanic Clouds. The LMC and SMC variables show a clear period-luminosity relation, particularly at near-infrared wavelengths, with longer period stars having higher intrinsic luminosities. This P-L relation, while not as well established for classical cepheids, allows LPVs to contribute to measurements of the extragalactic distance scale, particularly since miras are more luminous than Cepheids at inreared wavelengths. The present program aims to capitalise on these characteristics by using the WFC3 IR camera to search for mira variables in NGC 4258, a spiral galaxy that hosts several megamaser sources that enable accuate distance measurements. The aim is to use these miras to define a period-luminosity relation that can serve as a reference point for measuring distances to more distant galaxies.

Past weeks:
page by Neill Reid, updated 14/10/2012
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