Exo-Earth/Super-Earth Yield of JWST plus a Starshade External Occulter

We examine the scientific viability of an imaging mission to find exo-Earths combining the James Webb Space Telescope (JWST) with a starshade external occulter under a realistic set of astrophysical assumptions. We define an exo-Earth as a planet of 1 to 10 Earth masses orbiting in the habitable zone (HZ) of a solar-type star. We show that for a survey strategy that relies on a single image to detect an exo-Earth, roughly half of all exo-Earth detections will be false alarms. Here, a false alarm is a mistaken identification of a planet as an exo-Earth. We consider two survey strategies designed to mitigate the false alarm problem. The first is to require that for each potential exo-Earth, a sufficient number of detections are made to measure the orbit. When the orbit is known we can determine if the planet is in the habitable zone. With this strategy, we find that the number of exo-Earths found is on average 0.9, 1.9 and 2.7 for {\eta}_Earth = 0.1, 0.2 and 0.3. Here, {\eta}_Earth is the frequency of exo-Earths orbiting solar-type stars. There is a ~40% probability of finding zero exo-Earths for {\eta}_Earth = 0.1. A second strategy can be employed if a space astrometry mission has identified and measured the orbits and masses of the planets orbiting nearby stars. We find that with prior space-based astrometry from a survey of 60 nearby stars, JWST plus an external occulter can obtain orbital solutions for the majority (70% to 80%) of the exo-Earths orbiting these 60 stars. The exo-Earth yield is approximately five times higher than the yield for the JWST plus occulter mission without prior astrometry. With prior astrometry, the probability that an imaging mission will find zero exo-Earths is reduced to below 1% for the case of {\eta}_Earth = 0.1.Comment: Accepted by PASP. To appear in February 2010 issue. 15 pages, 2 figure

Utilizing Astrometric Orbits to Obtain Coronagraphic Images

We present an approach for utilizing astrometric orbit information to improve the yield of planetary images and spectra from a follow-on direct detection mission. This approach is based on the notion-strictly hypothetical-that if a particular star could be observed continuously, the instrument would in time observe all portions of the habitable zone so that no planet residing therein could be missed. This strategy could not be implemented in any realistic mission scenario. But if an exoplanet's orbit is known from astrometric observation, then it may be possible to plan and schedule a sequence of imaging observations that is the equivalent of continuous observation. A series of images-optimally spaced in time-could be recorded to examine contiguous segments of the orbit. In time, all segments would be examined, leading to the inevitable detection of the planet. In this paper, we show how astrometric orbit information can be used to construct such a sequence. Using stars from astrometric and imaging target lists, we find that the number of observations in this sequence typically ranges from 2 to 7, representing the maximum number of observations required to find the planet. The probable number of observations ranges from 1.5 to 3.1. This is a dramatic improvement in efficiency over previous methods proposed for utilizing astrometric orbits. We examine how the implementation of this approach is complicated and limited by operational constraints. We find that it can be fully implemented for internal coronagraph and visual nuller missions, with a success rate approaching 100%. External occulter missions will also benefit, but to a lesser degree.Comment: 28 pages, 14 figures, submitted to PAS

The Synergy of Direct Imaging and Astrometry for Orbit Determination of exo-Earths

The holy grail of exoplanet searches is an exo-Earth, an Earth mass planet in the habitable zone around a nearby star. Mass is the most important parameter of a planet and can only be measured by observing the motion of the star around the planet-star center of mass. A single image of a planet, however, does not provide evidence that the planet is Earth mass or that it is in a habitable zone orbit. The planet's orbit, however, can be measured either by imaging the planet at multiple epochs or by measuring the position of the star at multiple epochs by space-based astrometry. The measurement of an exo-planet's orbit by direct imaging is complicated by a number of factors: (1) the inner working angle (IWA); (2) the apparent brightness of the planet depending on the orbital phase; (3) confusion arising from the presence of multiple planets; and (4) the planet-star contrast. In this paper we address the question: "Can a prior astrometric mission that can identify which stars have Earthlike planets significantly improve the science yield of a mission to image exo-Earths?" We find that the Occulting Ozone Observatory (a small external occulter mission that cannot measure spectra) could confirm the orbits of ~4 to ~5 times as many exo-Earths if an astrometric mission preceded it to identify which stars had such planets. We find that in the case of an internal coronagraph, a survey of the nearest ~60 stars could be done with a telescope of half the size if an astrometric mission had first identified the presence of Earth-like planets in the habitable zone and measured their orbital parameters.Comment: ApJ, in press; 28 pages, 8 figure

Constructing a "Culture of Life": Legislation, Rhetoric, and Public Discourse

The purpose of the study is to 1) demonstrate through content analysis that the rhetorical, linguistic, and contextual choices employed in legislation can shape public discourse about political issues, 2) establish that the state uses legislation as a method of reinforcing heteronormative values, and 3) affirm that more focus should be afforded to legislation in the field of rhetorical studies. The focus of this study concerns itself specifically with Kansas House Bill 2253, or the “No Taxpayer Funds for Abortion Act.” A close reading of the Democratic and Republican Party’s respective platform statements on reproductive rights in 2012 serves as the basis for the content analysis; key words and themes were identified from each of those statements as aligning with the Pro-Choice and Pro-Life contexts, which were then applied to H.B. 2253. The legislation defines how the public should consider and discuss the issue of abortion through a heteronormative lens, framing it as an undesirable and amoral act with the multiple stipulations and mandates found in the legislation. Given the textual cues and the context within which such politically charged language like “unborn child” is used, the legislation is clearly invoking a conservative audience even though it is a addressing a general and diverse audience of women. The rhetorical effect that this language has on public discourse is affirmed with an analysis of the comments section of a Huffington Post article about H.B. 2253, in which 25% of the commenters consistently model the rhetoric employed in the legislation. This study demonstrates that the heteronormative rhetoric employed in legislation has a real, tangible impact on public discourse

Biomechanical Evaluation of Two Methods of Humeral Shaft Fixation

Biomechanical evaluations of fracture fixation devices attempt to determine implant performance by approximating the in vivo conditions. This performance is affected by many factors and relies on the complex bone-implant interface. Biomechanical tests can be designed in a variety of ways in order to evaluate device performance with respect to any number of these bone-implant interactions. Standardized tests, designed by groups such as the American Society for Testing and Materials (ASTM), are often designed either to determine the performance of a specific type of fixation device or for direct comparison between different devices. Additionally, many biomechanical evaluations are designed for direct comparison between the devices being evaluated. Often times these tests utilize bone analogs in order to eliminate variability. Finally, the method and location of load application greatly influences device performance outcomes. Cyclic tests determine fatigue performance whereas quasi-static tests are used to define device limits (i.e. - Young\u27s modulus, and ultimate/yield properties). Physiologically equivalent loading patterns expose fixation devices to combined loading modalities most closely resembling the in vivo conditions. This paper will explore the variety of ways in which biomechanical testing of fracture fixation devices are performed. Specific focus will be given to the design and application of biomechanical tests which simulate physiologically relevant loading. Physiologically relevant/equivalent loading refers to the simulation of in vivo loads with respect to anatomic alignment. This examination will include details regarding the differences in biomechanical test designs between weight-bearing (i.e. - lower limb) and non-weight-bearing (i.e. - upper limb) fixation devices. These concepts will then be put to use for the purpose of evaluating the biomechanical performance of two methods of humeral shaft fixation. The results of this study have been submitted for publication in the Journal of Surgical Orthopaedic Advances

Astrometric Detection of Terrestrial Planets in the Habitable Zones of Nearby Stars with SIM PlanetQuest

SIM PlanetQuest (Space Interferometry Mission) is a space-borne Michelson interferometer for precision stellar astrometry, with a nine meter baseline, currently slated for launch in 2015. One of the principal science goals is the astrometric detection and orbit characterization of terrestrial planets in the habitable zones of nearby stars. Differential astrometry of the target star against a set of reference stars lying within a degree will allow measurement of the target star's reflex motion with astrometric accuracy of 1 micro-arcsecond in a single measurement. We assess SIM's capability for detection (as opposed to characterization by orbit determination) of terrestrial planets in the habitable zones of nearby solar-type stars. We compare SIM's performance on target lists optimized for the SIM and Terrestrial Planet Finder Coronograph (TPF-C) missions. Performance is quantified by three metrics: minimum detectable planet mass, number and mass distribution of detected planets, and completeness of detections in each mass range. Finally, we discuss the issue of confidence in detections and non-detections, and show how information from SIM's planet survey can enable TPF to increase its yield of terrestrial planets.Comment: Minor corrections to figures and tables. 46 pages, 27 figures. To appear in PASP (Publications of the Astronomical Society of the Pacific), May 200