2,292 research outputs found
Applications of stable water and carbon isotopes in watershed research: Weathering, carbon cycling, and water balances
Research on rivers has traditionally involved concentration and flux measurements to better understand
weathering, transport and cycling of materials from land to ocean. As a relatively new tool, stable isotope
measurements complement this type of research by providing an extra label to characterize origin of the
transportedmaterial, its transfer mechanisms, and natural versus anthropogenic influences. These new stable
isotope techniques are scalable across a wide range of geographic and temporal scales. This review focuses on
three aspects of hydrological and geochemical river research that are of prime importance to the policy issues
of climate change and include utilization of stable water and carbon isotopes: (i) silicate and carbonate
weathering in river basins, (ii) the riverine carbon and oxygen cycles, and (iii) water balances at the
catchment scale. Most studies at watershed scales currently focus on water and carbon balances but future
applications hold promise to integrate sediment fluxes and turnover, ground and surface water interactions,
as well as the understanding of contaminant sources and their effects in river systems
The competitive edge: competition and biofilm composition, an individual-based modelling approach
The Host Galaxy and Central Engine of the Dwarf AGN POX 52
We present new multi-wavelength observations of the dwarf Seyfert 1 galaxy
POX 52 in order to investigate the properties of the host galaxy and the active
nucleus, and to examine the mass of its black hole, previously estimated to be
~ 10^5 M_sun. Hubble Space Telescope ACS/HRC images show that the host galaxy
has a dwarf elliptical morphology (M_I = -18.4 mag, Sersic index n = 4.3) with
no detected disk component or spiral structure, confirming previous results
from ground-based imaging. X-ray observations from both Chandra and XMM show
strong (factor of 2) variability over timescales as short as 500 s, as well as
a dramatic decrease in the absorbing column density over a 9 month period. We
attribute this change to a partial covering absorber, with a 94% covering
fraction and N_H = 58^{+8.4}_{-9.2} * 10^21 cm^-2, that moved out of the line
of sight in between the XMM and Chandra observations. Combining these data with
observations from the VLA, Spitzer, and archival data from 2MASS and GALEX, we
examine the spectral energy distribution (SED) of the active nucleus. Its shape
is broadly similar to typical radio-quiet quasar SEDs, despite the very low
bolometric luminosity of L_bol = 1.3 * 10^43 ergs/s. Finally, we compare black
hole mass estimators including methods based on X-ray variability, and optical
scaling relations using the broad H-beta line width and AGN continuum
luminosity, finding a range of black hole mass from all methods to be M_bh =
(2.2-4.2) * 10^5 M_sun, with an Eddington ratio of L_bol/L_edd = 0.2-0.5.Comment: 19 pages, 16 figures, accepted for publication in Ap
Polarized Narrow-Line Emission from the Nucleus of NGC 4258
The detection of polarized continuum and line emission from the nucleus of
NGC 4258 by Wilkes et al. (1995) provides an intriguing application of the
unified model of Seyfert nuclei to a galaxy in which there is known to be an
edge-on, rotating disk of molecular gas surrounding the nucleus. Unlike most
Seyfert nuclei, however, NGC 4258 has strongly polarized narrow emission lines.
To further investigate the origin of the polarized emission, we have obtained
spectropolarimetric observations of the NGC 4258 nucleus at the Keck-II
telescope. The narrow-line polarizations range from 1.0% for [S II] 6716 to
13.9% for the [O II] 7319,7331 blend, and the position angle of polarization is
oriented nearly parallel to the projected plane of the masing disk. A
correlation between critical density and degree of polarization is detected for
the forbidden lines, indicating that the polarized emission arises from
relatively dense (n_e > 10^4 cm^-3) gas. An archival Hubble Space Telescope
narrow-band [O III] image shows that the narrow-line region has a compact,
nearly unresolved core, implying a FWHM size of <2.5 pc. We discuss the
possibility that the polarized emission might arise from the accretion disk
itself and become polarized by scattering within the disk atmosphere. A more
likely scenario is an obscuring torus or strongly warped disk surrounding the
inner portion of a narrow-line region which is strongly stratified in density.
The compact size of the narrow-line region implies that the obscuring structure
must be smaller than ~2.5 pc in diameter.Comment: To appear in the Astronomical Journal. 13 pages, including 1 table
and 4 figures. Uses emulateapj.st
Hubble Space Telescope WFPC2 Imaging of SN 1979C and Its Environment
The locations of supernovae in the local stellar and gaseous environment in
galaxies contain important clues to their progenitor stars. As part of a
program to study the environments of supernovae using Hubble Space Telescope
(HST) imaging data, we have examined the environment of the Type II-L SN 1979C
in NGC 4321 (M100). We place more rigorous constraints on the mass of the SN
progenitor, which may have had a mass M \approx 17--18 M_sun. Moreover, we have
recovered and measured the brightness of SN 1979C, m=23.37 in F439W (~B;
m_B(max) = 11.6), 17 years after explosion. .Comment: 18 pages, 8 figures, submitted to PAS
Aortic cusp extension valvuloplasty with or without tricuspidization in children and adolescents: Long-term results and freedom from aortic valve replacement
ObjectiveAortic cusp extension valvuloplasty is increasingly used in the management of children and adolescents with aortic stenosis or regurgitation. The durability of this approach and the freedom from valve replacement are not well defined. A study was undertaken to investigate outcomes.MethodsFrom July 1987 to November 2008, 142 patients aged less than 19 years underwent aortic cusp extension valvuloplasty in the form of pericardial cusp extension and tricuspidization (when needed). Three patients with truncus arteriosus and severe truncal valve insufficiency were excluded. From the available follow-up data of 139 patients, 50 had bicuspid aortic valves, 40 had congenital aortic valve stenosis, 41 had combined congenital aortic valve stenosis/insufficiency, and 8 had other diagnoses. Median follow-up was 14.4 years (0.1–21.4). Long-term mortality and freedom from aortic valve replacement were studied.ResultsThere were no early, intermediate, or late deaths. Z-values of left ventricular end-diastolic dimension, aortic annulus, aortic sinus diameter, and sinotubular junction diameter before aortic valve replacement were 4.2 ± 3.11, 2.3 ± 1.25, 4.4 ± 1.23, and 1.84 ± 1.28, respectively. During the follow-up period, 64 patients underwent aortic valve reinterventions. The Ross procedure was performed in 32 of 139 patients (23%) undergoing aortic cusp extension valvuloplasty. Other aortic valve replacements were undertaken after 16 aortic cusp extension valvuloplasties (11.5%). Freedom from a second aortic cusp extension valvuloplasty or aortic valve replacement at 18 years was 82.1% ± 4.2% and 60.0% ± 7.2%, respectively.ConclusionAortic cusp extension valvuloplasty is a safe and effective surgical option with excellent survival and good long-term outcomes in children and adolescents. The procedure provides acceptable durability and satisfactory freedom from aortic valve replacement
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