We obtained Spitzer Space Telescope Multiband Imaging Photometer for Spitzer (MIPS) 24 mu m and 70 mu m observations of 182 nearby, Hipparcos F-and G-type common proper motion single and binary systems in the nearest OB association, Scorpius-Centaurus. We also obtained Magellan/MIKE R similar to 50,000 visual spectra at 3500-10500 angstrom for 181 candidate ScoCen stars in single and binary systems. Combining our MIPS observations with those of other ScoCen stars in the literature, we estimate 24 mu mF+G-type disk fractions of 9/27 (33% +/- 11%), 21/67 (31% +/- 7%), and 25/71 (35% +/- 7%) for Upper Scorpius (similar to 10 Myr), Upper Centaurus Lupus (similar to 15 Myr), and Lower Centaurus Crux (similar to 17 Myr), respectively. We confirm previous IRAS and MIPS excess detections and present new discoveries of 41 protoplanetary and debris disk systems, with fractional infrared luminosities ranging from L-IR/L-* = 10(-5) to 10(-2) and grain temperatures ranging from T-gr = 40-300 K. We searched for an increase in 24 mu m excess at an age of 15-20 Myr, consistent with the onset of debris production predicted by coagulation N-body simulations of outer planetary systems. We found such an increase around 1.5M(circle dot) stars but discovered a decrease in the 24 mu m excess around 1.0 M-circle dot stars. We additionally discovered that the 24 mu m excess around 1.0 M-circle dot stars is larger than predicted by self-stirred models. Finally, we found a weak anti-correlation between fractional infrared luminosity (L-IR/L-*) and chromospheric activity (R'(HK)), that may be the result of differences in stellar properties, such as mass, luminosity, and/or winds.

We have obtained near-infrared adaptive optics imaging and collected additional radial velocity observations to search for a third component in the extremely dusty short-period binary system BD +20 degrees 307. Our image shows no evidence for a third component at separations greater than 19AU. Our four seasons of radial velocities have a constant center-of-mass velocity and are consistent with the systemic velocities determined at two earlier epochs. Thus, the radial velocities also provide no support for a third component. Unfortunately, the separation domains covered by our imaging and radial velocity results do not overlap. Thus, we examined the parameters for possible orbits of a third component that could have been missed by our current observations. With our velocities we determined improved circular orbital elements for the 3.4 day double-lined binary. We also performed a spectroscopic abundance analysis of the short-period binary components and conclude that the stars are a mid-and a late-F dwarf. We find that the iron abundances of both components, [ Fe/H] = 0.15, are somewhat greater than the solar value and comparable to that of stars in the Hyades. Despite the similarity of the binary components, the lithium abundances of the two stars are very unequal. The primary has log epsilon (Li) = 2.72, while in the secondary log epsilon (Li) <= 1.46, which corresponds to a difference of at least a factor of 18. The very disparate lithium abundances in very similar stars make it impossible to ascribe a single age to them. While the system is likely at least 1 Gyr old, it may well be as old as the Sun.

Single metal-polluted white dwarfs with no dusty disks are believed to be actively accreting metals from a circumstellar disk of gas caused by the destruction of asteroids perturbed by planetary systems. We report, for the first time, the detection of circumstellar Ca II gas in absorption around the DAZ WD 1124-293, which lacks an infrared excess. We constrain the gas to >7 R-WD and <32000 AU, and estimate it to be at similar to 54 R-WD, well within WD 1124-293's tidal disruption radius. This detection is based on several epochs of spectroscopy around the Ca II H and K lines (lambda = 3968 angstrom, 3933 angstrom) with the MIKE spectrograph on the Magellan/Clay Telescope at Las Campanas Observatory. We confirm the circumstellar nature of the gas by observing nearby sightlines and finding no evidence for gas from the local interstellar medium. Through archival data we have measured the equivalent width of the two photospheric Ca lines over a period of 11 years. We see <5%-7% epoch-to-epoch variation in equivalent widths over this time period, and no evidence for long term trends. The presence of a circumstellar gas implies a near edge-on inclination to the system, thus we place limits to short period transiting planetary companions with R > R-circle plus using theWide Angle Search for Planets survey. The presence of gas in orbit around WD 1124-293 implies that most DAZs could harbor planetary systems. Since 25%-30% of white dwarfs show metal line absorption, the dynamical process for perturbing small bodies must be robust.

Debris dust in the habitable zones of stars-otherwise known as exozodiacal dust-comes from extrasolar asteroids and comets and is thus an expected part of a planetary system. Background flux from the solar system's zodiacal dust and the exozodiacal dust in the target system is likely to be the largest source of astrophysical noise in direct observations of terrestrial planets in the habitable zones of nearby stars. Furthermore, dust structures like clumps, thought to be produced by dynamical interactions with exoplanets, are a possible source of confusion. In this article, we qualitatively assess the primary impact of exozodiacal dust on high-contrast direct imaging at optical wavelengths, such as would be performed with a coronagraph. Then we present the sensitivity of previous, current, and near-term facilities to thermal emission from debris dust at all distances from nearby solar-type stars, as well as our current knowledge of dust levels from recent surveys. Finally, we address the other method of detecting debris dust, through high-contrast imaging in scattered light. This method is currently far less sensitive than thermal emission observations, but provides high spatial resolution for studying dust structures. This article represents the first report of NASA's Exoplanet Exploration Program Analysis Group (ExoPAG).

We present a 0.5-2.2 mu m scattered light spectrum of the circumstellar disk around TW Hya from a combination of spatially resolved Hubble Space Telescope STIS spectroscopy and NICMOS coronagraphic images of the disk. We investigate the morphology of the disk at distances >40 AU over this wide range of wavelengths, and identify the presence of a depression in surface brightness at similar to 80 AU that could be caused by a gap in the disk. Additionally, we quantify the surface brightness, azimuthal symmetry, and spectral character of the disk as a function of radius. Our analysis shows that the scattering efficiency of the dust is largely neutral to blue over the observed wavelengths. We model the disk as a steady alpha-disk with an ad hoc gap structure. The thermal properties of the disk are self-consistently calculated using a three-dimensional radiative transfer code that uses ray tracing to model the heating of the disk interior and scattered light images. We find a good fit to the data over a wide range of distances from the star if we use a model disk with a partially filled gap of 30% depth at 80 AU and with a self-similar truncation knee at 100 AU. The origin of the gap is unclear, but it could arise from a transition in the nature of the disk's dust composition or the presence of a planetary companion. Based on scalings to previous hydrodynamic simulations of gap-opening criteria for embedded proto-planets, we estimate that a planetary companion forming the gap could have a mass between 6 and 28M(circle plus).

We report the discovery of a planetary-mass companion, HD 106906 b, with the new Magellan Adaptive Optics (MagAO) + Clio2 system. The companion is detected with Clio2 in three bands: J, K-S, and L', and lies at a projected separation of 7.'' 1 (650AU). It is confirmed to be comoving with its 13 +/- 2 Myr F5 host using Hubble Space Telescope Advanced Camera for Surveys astrometry over a time baseline of 8.3 yr. DUSTY and COND evolutionary models predict that the companion's luminosity corresponds to a mass of 11 +/- 2M(Jup), making it one of the most widely separated planetary-mass companions known. We classify its Magellan/Folded-Port InfraRed Echellette J/H/K spectrum as L2.5 +/- 1; the triangular H-band morphology suggests an intermediate surface gravity. HD 106906 A, a pre-main-sequence Lower Centaurus Crux member, was initially targeted because it hosts a massive debris disk detected via infrared excess emission in unresolved Spitzer imaging and spectroscopy. The disk emission is best fit by a single component at 95 K, corresponding to an inner edge of 15-20 AU and an outer edge of up to 120 AU. If the companion is on an eccentric (e > 0.65) orbit, it could be interacting with the outer edge of the disk. Close-in, planet-like formation followed by scattering to the current location would likely disrupt the disk and is disfavored. Furthermore, we find no additional companions, though we could detect similar-mass objects at projected separations > 35 AU. In situ formation in a binary-star-like process is more probable, although the companion-to-primary mass ratio, at <1%, is unusually small.

The Hunt for Observable Signatures of Terrestrial planetary Systems (HOSTS) program on the Large Binocular Telescope Interferometer (LBTI) will survey nearby stars for faint exozodiacal dust (exozodi). This warm circumstellar dust, analogous to the interplanetary dust found in the vicinity of the Earth in our own system, is produced in comet breakups and asteroid collisions. Emission and/or scattered light from the exozodi will be the major source of astrophysical noise for a future space telescope aimed at direct imaging and spectroscopy of terrestrial planets (exo-Earths) around nearby stars. About 20% of nearby field stars have cold dust coming from planetesimals at large distances from the stars (Eiroa et al. 2013, A&A, 555, A11; Siercho et al. 2014, ApJ, 785, 33). Much less is known about exozodi; current detection limits for individual stars are at best similar to 500 times our solar system's level (aka. 500 zodi). LBTI-HOSTS will be the first survey capable of measuring exozodi at the 10 zodi level (3 sigma). Detections of warm dust will also reveal new information about planetary system architectures and evolution. We will describe the motivation for the survey and progress on target selection, not only the actual stars likely to be observed by such a mission but also those whose observation will enable sensible extrapolations for stars that will not be observed with LBTI. We briefly describe the detection of the debris disk around eta Crv, which is the first scientific result from the LBTI coming from the commissioning of the instrument in December 2013, shortly after the first time the fringes were stabilized.

MagAO is the new adaptive optics system with visible-light and infrared science cameras, located on the 6.5-m Magellan "Clay" telescope at Las Campanas Observatory, Chile. The instrument locks on natural guide stars (NGS) from 0th to 16th R-band magnitude, measures turbulence with a modulating pyramid wavefront sensor binnable from 28x28 to 7x7 subapertures, and uses a 585-actuator adaptive secondary mirror (ASM) to provide flat wavefronts to the two science cameras. MagAO is a mutated clone of the similar AO systems at the Large Binocular Telescope (LBT) at Mt. Graham, Arizona. The high-level AO loop controls up to 378 modes and operates at frame rates up to 1000 Hz. The instrument has two science cameras: VisAO operating from 0.5-1 mu m and Clio2 operating from 1-5 mu m. MagAO was installed in 2012 and successfully completed two commissioning runs in 2012-2013. In April 2014 we had our first science run that was open to the general Magellan community. Observers from Arizona, Carnegie, Australia, Harvard, MIT, Michigan, and Chile took observations in collaboration with the MagAO instrument team. Here we describe the MagAO instrument, describe our on-sky performance, and report our status as of summer 2014.

Optical and synchrotron x-ray diffraction diamond anvil cell experiments have been combined with first-principles theoretical structure predictions to investigate mixtures of N-2 and H-2 up to 55 GPa. Our experiments show the formation of structurally complex van der Waals compounds [see also D. K. Spaulding et al., Nat. Commun. 5, 5739 (2014)] above 10 GPa. However, we found that these NxH (0.5 < x < 1.5) compounds transform abruptly to new oligomeric materials through barochemistry above 47 GPa and photochemistry at pressures as low as 10 GPa. These oligomeric compounds can be recovered to ambient pressure at T < 130 K, whereas at room temperature, they can be metastable on pressure release down to 3.5 GPa. Extensive theoretical calculations show that such oligomeric materials become thermodynamically more stable in comparison to mixtures of N-2, H-2, and NH3 above approximately 40 GPa. Our results suggest new pathways for synthesis of environmentally benign high energy-density materials. These materials could also exist as alternative planetary ices. (C) 2015 AIP Publishing LLC.

Pressure dependent angle-dispersive x-ray powder diffraction measurements of alpha-phase aluminum trifluoride (alpha-AlF3) and separately, aluminum triiodide (AlI3) were conducted using a diamond-anvil cell. Results at 295 K extend to 50 GPa. The equations of state of AlF3 and AlI3 were determined through refinements of collected x-ray diffraction patterns. The respective bulk moduli and corresponding pressure derivatives are reported for multiple orders of the Birch-Murnaghan (B-M), finite-strain (F-f), and higher pressure finite-strain (G-g) EOS analysis models. Aluminum trifluoride exhibits an apparent isostructural phase transition at approximately 12 GPa. Aluminum triiodide also undergoes a second-order atomic rearrangement: applied stress transformed a monoclinically distorted face centered cubic (fcc) structure into a standard fcc structural arrangement of iodine atoms. Results from semi-empirical thermochemical computations of energetic materials formulated with fluorine containing reactants were obtained with the aim of predicting the yield of halogenated products. (C) 2015 AIP Publishing LLC.