1,162 research outputs found
The search for habitable worlds: 1. The viability of a starshade mission
As part of NASA's mission to explore habitable planets orbiting nearby stars,
this paper explores the detection and characterization capabilities of a 4-m
space telescope plus 50-m starshade located at the Earth-Sun L2 point, a.k.a.
the New Worlds Observer (NWO). Our calculations include the true spectral types
and distribution of stars on the sky, an iterative target selection protocol
designed to maximize efficiency based on prior detections, and realistic
mission constraints. We carry out both analytical calculations and simulated
observing runs for a wide range in exozodiacal background levels ({\epsilon} =
1 - 100 times the local zodi brightness) and overall prevalence of Earth-like
terrestrial planets ({\eta}\oplus = 0.1 - 1). We find that even without any
return visits, the NWO baseline architecture (IWA = 65 mas, limiting FPB =
4\times10-11) can achieve a 95% probability of detecting and spectrally
characterizing at least one habitable Earth-like planet, and an expectation
value of ~3 planets found, within the mission lifetime and {\Delta}V budgets,
even in the worst-case scenario ({\eta}\oplus = 0.1 and {\epsilon} = 100 zodis
for every target). This achievement requires about one year of integration time
spread over the 5 year mission, leaving the remainder of the telescope time for
UV-NIR General Astrophysics. Cost and technical feasibility considerations
point to a "sweet spot" in starshade design near a 50-m starshade effective
diameter, with 12 or 16 petals, at a distance of 70,000-100,000 km from the
telescope.Comment: Refereed and accepted to PASP, scheduled for publication in the May
2012 issue (Vol. 124, No. 915
Correlation Between the Deuteron Characteristics and the Low-energy Triplet np Scattering Parameters
The correlation relationship between the deuteron asymptotic normalization
constant, , and the triplet np scattering length, , is
investigated. It is found that 99.7% of the asymptotic constant is
determined by the scattering length . It is shown that the linear
correlation relationship between the quantities and
provides a good test of correctness of various models of nucleon-nucleon
interaction. It is revealed that, for the normalization constant and
for the root-mean-square deuteron radius , the results obtained with the
experimental value recommended at present for the triplet scattering length
are exaggerated with respect to their experimental counterparts. By
using the latest experimental phase shifts of Arndt et al., we obtain, for the
low-energy scattering parameters (, , ) and for the
deuteron characteristics (, ), results that comply well with
experimental data.Comment: 19 pages, 1 figure, To be published in Physics of Atomic Nucle
Long-term photometry and periods for 261 nearby pulsating M giants
We present the results of a 5.5-year CCD photometric campaign that monitored
261 bright, southern, semi-regular variables with relatively precise Hipparcos
parallaxes. The data are supplemented with independent photoelectric
observations of 34 of the brightest stars, including 11 that were not part of
the CCD survey, and a previously unpublished long time-series of VZ Cam.
Pulsation periods and amplitudes are established for 247 of these stars, the
majority of which have not been determined before. All M giants with sufficient
observations for period determination are found to be variable, with 87% of the
sample (at S/N >= 7.5) exhibiting multi-periodic behaviour. The period ratios
of local SRVs are in excellent agreement with those in the Large Magellanic
Cloud. Apparent K-band magnitudes are extracted from multiple NIR catalogues
and analysed to determine the most reliable values. We review the effects of
interstellar and circumstellar extinction and calculate absolute K-band
magnitudes using revised Hipparcos parallaxes.Comment: 18 pages, 15 figures; accepted for publication in Monthly Notices of
the Royal Astronomical Societ
Probing Stereoselectivity in Ring-Opening Metathesis Polymerization Mediated by Cyclometalated Ruthenium-Based Catalysts: A Combined Experimental and Computational Study
The microstructures of polymers produced by ring-opening metathesis polymerization (ROMP) with cyclometalated Ru-carbene metathesis catalysts were investigated. A strong bias for a cis,syndiotactic microstructure with minimal head-to-tail bias was observed. In instances where trans errors were introduced, it was determined that these regions were also syndiotactic. Furthermore, hypothetical reaction intermediates and transition structures were analyzed computationally. Combined experimental and computational data support a reaction mechanism in which cis,syndio-selectivity is a result of stereogenic metal control, while microstructural errors are predominantly due to alkylidene isomerization via rotation about the Ru═C double bond
Kepler Cycle 1 Observations of Low Mass Stars: New Eclipsing Binaries, Single Star Rotation Rates, and the Nature and Frequency of Starspots
We have analyzed Kepler light curves for 849 stars with T_eff < 5200 K from
our Cycle 1 Guest Observer program. We identify six new eclipsing binaries, one
of which has an orbital period of 29.91 d, and two of which are probably W UMa
variables. In addition, we identify a candidate "warm Jupiter" exoplanet. We
further examine a subset of 670 sources for variability. Of these objects, 265
stars clearly show periodic variability that we assign to rotation of the
low-mass star. At the photometric precision level provided by Kepler, 251 of
our objects showed no evidence for variability. We were unable to determine
periods for 154 variable objects. We find that 79% of stars with T_eff < 5200 K
are variable. The rotation periods we derive for the periodic variables span
the range 0.31 < P_rot < 126.5 d. A considerable number of stars with rotation
periods similar to the solar value show activity levels that are 100 times
higher than the Sun. This is consistent with results for solar-like field
stars. As has been found in previous studies, stars with shorter rotation
periods generally exhibit larger modulations. This trend flattens beyond P_rot
= 25 d, demonstrating that even long period binaries may still have components
with high levels of activity and investigating whether the masses and radii of
the stellar components in these systems are consistent with stellar models
could remain problematic. Surprisingly, our modeling of the light curves
suggests that the active regions on these cool stars are either preferentially
located near the rotational poles, or that there are two spot groups located at
lower latitudes, but in opposing hemispheres.Comment: 48 pages, 11 figure
Neurology
Contains reports on five research projects.United States Navy, Office of Naval Research (Nonr-609(39))United States Public Health Service (B-3055, B-3090)Unites States Air Force (Contract AF33(616)-7282)Unites States Air Force (Contract AF-33(616)-7588, Project: 61(8-7232); Task 71784))United States Army Chemical Corps (DA-18-108-405-Cml-942
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X-231A demonstration of in-situ remediation of DNAPL compounds in low permeability media by soil fracturing with thermally enhanced mass recovery or reactive barrier destruction
The overall goal of the program of activities is to demonstrate robust and cost-effective technologies for in situ remediation of DNAPL compounds in low permeability media (LPM), including adaptations and enhancements of conventional technologies to achieve improved performance for DNAPLs in LPM. The technologies sought should be potential for application at simple, small sites (e.g., gasoline underground storage tanks) as well as at complex, larger sites (e.g., DOE land treatment units). The technologies involved in the X-231A demonstration at Portsmouth Gaseous Diffusion Plant (PORTS) utilized subsurface manipulation of the LPM through soil fracturing with thermally enhanced mass recovery or horizontal barrier in place destruction. To enable field evaluation of these approaches, a set of four test cells was established at the X-231A land treatment unit at the DOE PORTS plant in August 1996 and a series of demonstration field activities occurred through December 1997. The principal objectives of the PORTS X-231A demonstration were to: determine and compare the operational features of hydraulic fractures as an enabling technology for steam and hot air enhanced soil vapor extraction and mass recovery, in situ interception and reductive destruction by zero valent iron, and in situ interception and oxidative destruction by potassium permanganate; determine the interaction of the delivered agents with the LPM matrix adjacent to the fracture and within the fractured zone and assess the beneficial modifications to the transport and/or reaction properties of the LPM deposit; and determine the remediation efficiency achieved by each of the technology strategies
Neurology
Contains reports on eight research projects.United States Air Force (AF33(616)-7588, AF49(638)-1130)National Science Foundation (Grant G-16526)United States Army Chemical Corps (DA-18-108-405-Cml-942)United States Public Health Service (B-3055, B-3090)United States Navy, Office of Naval Research (Contract Nonr-1841(70)
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