15,467 research outputs found
Guadalupe pluton–Mariposa Formation age relationships in the southern Sierran Foothills: Onset of Mesozoic subduction in northern California?
We report a new 153 ± 2 Ma SIMS U-Pb date for zircons from the hypabyssal Guadalupe pluton which crosscuts and contact metamorphoses upper crustal Mariposa slates in the southern Sierra. A ~950 m thick section of dark metashales lies below sandstones from which clastic zircons were analyzed at 152 ± 2 Ma. Assuming a compacted depositional rate of ~120 m/Myr, accumulation of Mariposa volcanogenic sediments, which overlie previously stranded Middle Jurassic and older ophiolite + chert-argillite belts in the Sierran Foothills, began no later than ~160 Ma. Correlative Oxfordian-Kimmeridgian strata of the Galice Formation occupy a similar position in the Klamath Mountains. We speculate that the Late Jurassic was a time of transition from (1) a mid-Paleozoic–Middle Jurassic interval of mainly but not exclusively strike-slip and episodic docking of oceanic terranes; (2) to transpressive plate underflow, producing calcalkaline igneous arc rocks ± outboard blueschists at ~170–150 Ma, whose erosion promoted accumulation of the Mariposa-Galice overlap strata; (3) continued transpressive underflow attending ~200 km left-lateral displacement of the Klamath salient relative to the Sierran arc at ~150–140 Ma and development of the apparent polar wander path cusps for North and South America; and (4) then nearly orthogonal mid and Late Cretaceous convergence commencing at ~125–120 Ma, during reversal in tangential motion of the Pacific plate. After ~120 Ma, nearly head-on subduction involving minor dextral transpression gave rise to voluminous continent-building juvenile and recycled magmas of the Sierran arc, providing the erosional debris to the Great Valley fore arc and Franciscan trench
New Insights on Interstellar Gas-Phase Iron
In this paper, we report on the gas-phase abundance of singly-ionized iron
(Fe II) for 51 lines of sight, using data from the Far Ultraviolet
Spectroscopic Explorer (FUSE). Fe II column densities are derived by measuring
the equivalent widths of several ultraviolet absorption lines and subsequently
fitting those to a curve of growth. Our derivation of Fe II column densities
and abundances creates the largest sample of iron abundances in moderately- to
highly-reddened lines of sight explored with FUSE, lines of sight that are on
average more reddened than lines of sight in previous Copernicus studies. We
present three major results. First, we observe the well-established correlation
between iron depletion and and also find trends between iron depletion
and other line of sight parameters (e.g. f(H_2), E_(B-V), and A_V), and examine
the significance of these trends. Of note, a few of our lines of sight probe
larger densities than previously explored and we do not see significantly
enhanced depletion effects. Second, we present two detections of an extremely
weak Fe II line at 1901.773 A in the archival STIS spectra of two lines of
sight (HD 24534 and HD 93222). We compare these detections to the column
densities derived through FUSE spectra and comment on the line's f-value and
utility for future studies of Fe II. Lastly, we present strong anecdotal
evidence that the Fe II f-values derived empirically through FUSE data are more
accurate than previous values that have been theoretically calculated, with the
probable exception of f_1112.Comment: Accepted for publication in ApJ, 669, 378; see ApJ version for small
updates. 53 total pages (preprint format), 7 tables, 11 figure
Asteroseismic Signatures of Stellar Magnetic Activity Cycles
Observations of stellar activity cycles provide an opportunity to study
magnetic dynamos under many different physical conditions. Space-based
asteroseismology missions will soon yield useful constraints on the interior
conditions that nurture such magnetic cycles, and will be sensitive enough to
detect shifts in the oscillation frequencies due to the magnetic variations. We
derive a method for predicting these shifts from changes in the Mg II activity
index by scaling from solar data. We demonstrate this technique on the
solar-type subgiant beta Hyi, using archival International Ultraviolet Explorer
spectra and two epochs of ground-based asteroseismic observations. We find
qualitative evidence of the expected frequency shifts and predict the optimal
timing for future asteroseismic observations of this star.Comment: 5 pages including 3 figures and 1 table, MNRAS Letters accepte
The Abundance of Interstellar Fluorine and Its Implications
We report results from a survey of neutral fluorine (F I) in the interstellar
medium. Data from the Far Ultraviolet Spectroscopic Explorer (FUSE) were used
to analyze 26 lines of sight lying both in the galactic disk and halo,
including lines to Wolf-Rayet stars and through known supernova remnants. The
equivalent widths of fluorine resonance lines at 951.871 A and 954.827 A were
measured or assigned upper limits and combined with a nitrogen curve of growth
to obtain F I column densities. These column densities were then used to
calculate fluorine depletions. Comparisons are made to the previous study of F
I by Federman et al. (2005) and implications for F I formation and depletion
are discussed.Comment: 32 pages, 10 figures, Accepted to Ap
Is There Enhanced Depletion of Gas-Phase Nitrogen in Moderately Reddened Lines of Sight?
We report on the abundance of interstellar neutral nitrogen (NI) for 30
sightlines, using data from the Far Ultraviolet Spectroscopic Explorer (FUSE)
and the Hubble Space Telescope (HST). NI column densities are derived by
measuring the equivalent widths of several ultraviolet absorption lines and
subsequently fitting those to a curve of growth. We find a mean interstellar
N/H of 51+/-4 ppm. This is below the mean found by Meyer et al. of 62(+4,-3)
ppm (adjusted for a difference in f-values). Our mean N/H is similar, however,
to the (f-value adjusted) mean of 51+/-3 ppm found by Knauth et al. for a
larger sample of sightlines with larger hydrogen column densities comparable to
those in this study. We discuss the question of whether or not nitrogen shows
increased gas-phase depletion in lines of sight with column densities
log(H_tot) >~ 21, as claimed by Knauth et al. The nitrogen abundance in the
line of sight toward HD 152236 is particularly interesting. We derive very
small N/H and N/O ratios for this line of sight that may support a previous
suggestion that members of the Sco OB1 association formed from an N-deficient
region.Comment: Accepted in The Astrophysical Journal, 9/2006 (expected pub. date:
1/2007) 38 pages, 5 figures (4 color
Space-time variation of malaria incidence in Yunnan province, China
BACKGROUND Understanding spatio-temporal variation in malaria incidence provides a basis for effective disease control planning and monitoring. METHODS Monthly surveillance data between 1991 and 2006 for Plasmodium vivax and Plasmodium falciparum malaria across 128 counties were assembled for Yunnan, a province of China with one of the highest burdens of malaria. County-level Bayesian Poisson regression models of incidence were constructed, with effects for rainfall, maximum temperature and temporal trend. The model also allowed for spatial variation in county-level incidence and temporal trend, and dependence between incidence in June-September and the preceding January-February. RESULTS Models revealed strong associations between malaria incidence and both rainfall and maximum temperature. There was a significant association between incidence in June-September and the preceding January-February. Raw standardised morbidity ratios showed a high incidence in some counties bordering Myanmar, Laos and Vietnam, and counties in the Red River valley. Clusters of counties in south-western and northern Yunnan were identified that had high incidence not explained by climate. The overall trend in incidence decreased, but there was significant variation between counties. CONCLUSION Dependence between incidence in summer and the preceding January-February suggests a role of intrinsic host-pathogen dynamics. Incidence during the summer peak might be predictable based on incidence in January-February, facilitating malaria control planning, scaled months in advance to the magnitude of the summer malaria burden. Heterogeneities in county-level temporal trends suggest that reductions in the burden of malaria have been unevenly distributed throughout the province.This project was supported by a University of Queensland New Research Scientist Start-Up Fund grant. RWS is a Wellcome Trust Principal Research Fellow (#079080) and receives additional support from the Wellcome Trust for the Malaria Atlas Project (MAP, http://www.map.ox.ac.uk)
Precision determination of absolute neutron flux
A technique for establishing the total neutron rate of a highly-collimated
monochromatic cold neutron beam was demonstrated using a method of an
alpha-gamma counter. The method involves only the counting of measured rates
and is independent of neutron cross sections, decay chain branching ratios, and
neutron beam energy. For the measurement, a target of 10B-enriched boron
carbide totally absorbed the neutrons in a monochromatic beam, and the rate of
absorbed neutrons was determined by counting 478keV gamma rays from neutron
capture on 10B with calibrated high-purity germanium detectors. A second
measurement based on Bragg diffraction from a perfect silicon crystal was
performed to determine the mean de Broglie wavelength of the beam to a
precision of 0.024 %. With these measurements, the detection efficiency of a
neutron monitor based on neutron absorption on 6Li was determined to an overall
uncertainty of 0.058 %. We discuss the principle of the alpha-gamma method and
present details of how the measurement was performed including the systematic
effects. We also describe how this method may be used for applications in
neutron dosimetry and metrology, fundamental neutron physics, and neutron cross
section measurements.Comment: 44 page
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