177 research outputs found
Electron beam characterization of carbon nanostructures
Atom-thick carbon nanostructures represent a class of novel materials that are of interest to those studying carbon\u27s role in fossil fuel, hydrogen storage, scaled-down electronics, and other nanotechnology. Electron microscope images of edge-on graphene sheets show linear image features due to the projected potential of the sheets. Here, intensity profiles along these linear features can measure the curvature of the sheet, as well as the shape of the sheet (i.e. hexagonal, triangular). Also, electron diffraction powder profiles calculated for triangular graphene sheet shapes show a broadening of the low frequency edge of diffraction rings, in comparison to those calculated for hexagonal sheets with a similar number of atoms. Calculated powder profiles further indicate that curvature of a sheet will broaden the tailing edge of the diffraction peaks. These simulation results are applied to the characterization of nanocrystalline carbon cores found in a subset of graphitic presolar stardust. Electron diffraction data from these cores indicates they are comprised primarily of unlayered graphene sheets. Comparison to simulations indicates that these sheets are more triangular than equant, and thus likely the result of some anisotropic growth process. This assertion is separately supported by intensity profiles of linear features in HRTEM images. The density of the cores is further shown to be less than 90% of the density of graphitic rims surrounding these cores. This structural data constrains proposed grain formation mechanisms in AGB atmospheres, and opens up the unexpected possibility that these presolar cores may have been formed by the dendritic crystallization of liquid carbon droplets --Abstract, page iii
Electron beam characterization of carbon nanostructures
Atom-thick carbon nanostructures represent a class of novel materials that are of interest to those studying carbon\u27s role in fossil fuel, hydrogen storage, scaled-down electronics, and other nanotechnology. Electron microscope images of edge-on graphene sheets show linear image features due to the projected potential of the sheets. Here, intensity profiles along these linear features can measure the curvature of the sheet, as well as the shape of the sheet (i.e. hexagonal, triangular). Also, electron diffraction powder profiles calculated for triangular graphene sheet shapes show a broadening of the low frequency edge of diffraction rings, in comparison to those calculated for hexagonal sheets with a similar number of atoms. Calculated powder profiles further indicate that curvature of a sheet will broaden the tailing edge of the diffraction peaks. These simulation results are applied to the characterization of nanocrystalline carbon cores found in a subset of graphitic presolar stardust. Electron diffraction data from these cores indicates they are comprised primarily of unlayered graphene sheets. Comparison to simulations indicates that these sheets are more triangular than equant, and thus likely the result of some anisotropic growth process. This assertion is separately supported by intensity profiles of linear features in HRTEM images. The density of the cores is further shown to be less than 90% of the density of graphitic rims surrounding these cores. This structural data constrains proposed grain formation mechanisms in AGB atmospheres, and opens up the unexpected possibility that these presolar cores may have been formed by the dendritic crystallization of liquid carbon droplets --Abstract, page iii
Large UV/Optical/Infrared Surveyor: Telling the Story of Life in the Universe. Exoplanet Transmission Spectroscopy with LUVOIR
Exoplanet Transmission Spectroscopy with LUVOI
Lithium in the Upper Centaurus Lupus and Lower Centaurus Crux Subgroups of Scorpius-Centaurus
We utilize spectroscopically derived model atmosphere parameters and the
\ion{Li}{1} subordinate line and the doublet to
derive lithium abundances for 12 members of the Upper-Centaurus Lupus (UCL) and
Lower-Centaurus Crux (LCC) subgroups of the Scorpius Centaurus OB Association.
The results indicate any intrinsic Li scatter in our 0.9-1.4 stars
is limited to dex, consistent with the lack of dispersion in
stars in the 100 Myr Pleiades and 30-50 Myr IC 2391 and
2602 clusters. Both ab initio uncertainty estimates and the derived abundances
themselves indicate that the 6104 line yields abundances with
equivalent or less scatter than is found from the 6708 doublet as a
result of lower uncertainties for the subordinate feature, a result of low
sensitivity to broadening in the subordinate feature. Because NLTE corrections
are less susceptible to changes in surface gravity and/or metallicity for the
6104 {\AA} line, the subordinate Li feature is preferred for deriving lithium
abundances in young Li-rich stellar association stars with K.Comment: Accepted for publication in Astronomical Journal (abstract shortened
for astro-ph submission
Commissioning and performance results of the WFIRST/PISCES integral field spectrograph
The Prototype Imaging Spectrograph for Coronagraphic Exoplanet Studies
(PISCES) is a high contrast integral field spectrograph (IFS) whose design was
driven by WFIRST coronagraph instrument requirements. We present commissioning
and operational results using PISCES as a camera on the High Contrast Imaging
Testbed at JPL. PISCES has demonstrated ability to achieve high contrast
spectral retrieval with flight-like data reduction and analysis techniques.Comment: Author's copy - Proceedings of SPIE Volume 10400. Citation to SPIE
proceedings volume will be added when availabl
Simulating the WFIRST coronagraph Integral Field Spectrograph
A primary goal of direct imaging techniques is to spectrally characterize the
atmospheres of planets around other stars at extremely high contrast levels. To
achieve this goal, coronagraphic instruments have favored integral field
spectrographs (IFS) as the science cameras to disperse the entire search area
at once and obtain spectra at each location, since the planet position is not
known a priori. These spectrographs are useful against confusion from speckles
and background objects, and can also help in the speckle subtraction and
wavefront control stages of the coronagraphic observation. We present a
software package, the Coronagraph and Rapid Imaging Spectrograph in Python
(crispy) to simulate the IFS of the WFIRST Coronagraph Instrument (CGI). The
software propagates input science cubes using spatially and spectrally resolved
coronagraphic focal plane cubes, transforms them into IFS detector maps and
ultimately reconstructs the spatio-spectral input scene as a 3D datacube.
Simulated IFS cubes can be used to test data extraction techniques, refine
sensitivity analyses and carry out design trade studies of the flight CGI-IFS
instrument. crispy is a publicly available Python package and can be adapted to
other IFS designs.Comment: 15 page
Engineering Considerations Applied to Starshade Repointing
Engineering analysis has been carried out on orbit dynamics that drive the delta-v budget for repointing a free-flying starshade occulter for viewing exoplanets with a space telescope. This analysis has application to the design of starshade spacecraft and yield calculations of observations of exoplanets using a space telescope and a starshade. Analysis was carried out to determine if there may be some advantage for the global delta-v budget if the telescope performs orbit changing delta-v maneuvers as part of the telescope-starshade alignment for observing exoplanets. Analysis of the orbit environmental forces at play found no significant advantage in having the telescope participate in delta-v maneuvers for exoplanet observation repointing. A separate analysis of starshade delta-v for repointing found that the orbit dynamics of the starshade is driven by multiple simultaneous variables that need to be considered together in order to create an effective estimate of delta-v over an exoplanet observation campaign. These include area of the starshade, dry mass of the starshade spacecraft, and propellant mass of the starshade spacecraft. Solar radiation pressure has the potential to play a dominant role in the orbit dynamics and delta-v budget. SRP effects are driven by the differences in the mass, area, and coefficients of reflectivity of the observing telescope and the starshade. The propellant budget cannot be effectively estimated without a conceptual design of a starshade spacecraft including the propulsion system. The varying propellant mass over the mission is a complexity that makes calculating the propellant budget less straightforward
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