726 research outputs found
Instabilities in Multi-Planet Circumbinary Systems
The majority of the discovered transiting circumbinary planets are located
very near the innermost stable orbits permitted, raising questions about the
origins of planets in such perturbed environments. Most favored formation
scenarios invoke formation at larger distances and subsequent migration to
their current locations. Disk-driven planet migration in multi-planet systems
is likely to trap planets in mean motion resonances and drive planets inward
into regions of larger dynamical perturbations from the binary. We demonstrate
how planet-planet resonances can interact with the binary through secular
forcing and mean-motion resonances, driving chaos in the system. We show how
this chaos will shape the architecture of circumbinary systems, with specific
applications to Kepler 47 and the Pluto-Charon system, limiting maximum
possible stable eccentricities and indicating what resonances are likely to
exist. We are also able to constrain the minimum migration rates of resonant
circumbinary planets.Comment: Accepted for publication in MNRA
Characterizing octagonal and rectangular fibers for MAROON-X
We report on the scrambling performance and focal-ratio-degradation (FRD) of
various octagonal and rectangular fibers considered for MAROON-X. Our
measurements demonstrate the detrimental effect of thin claddings on the FRD of
octagonal and rectangular fibers and that stress induced at the connectors can
further increase the FRD. We find that fibers with a thick, round cladding show
low FRD. We further demonstrate that the scrambling behavior of non-circular
fibers is often complex and introduce a new metric to fully capture non-linear
scrambling performance, leading to much lower scrambling gain values than are
typically reported in the literature (<1000 compared to 10,000 or more). We
find that scrambling gain measurements for small-core, non-circular fibers are
often speckle dominated if the fiber is not agitated.Comment: 10 pages, 8 figures, submitted to SPIE Advances in Optical and
Mechanical Technologies for Telescopes and Instrumentation 2016 (9912-185
Creek College
Creek College is a project bridging art and environmental conservation. We offer a range of art classes and experiences in exchange for activities that aid in the restoration of watersheds suffering from environmental degradation.https://pdxscholar.library.pdx.edu/creek_college/1000/thumbnail.jp
A physically-based model of the ionizing radiation from active galaxies for photoionization modeling
We present a simplified model of Active Galactic Nucleus (AGN) continuum
emission designed for photoionization modeling. The new model {\sc oxaf}
reproduces the diversity of spectral shapes that arise in physically-based
models. We identify and explain degeneracies in the effects of AGN parameters
on model spectral shapes, with a focus on the complete degeneracy between the
black hole mass and AGN luminosity. Our re-parametrized model {\sc oxaf}
removes these degeneracies and accepts three parameters which directly describe
the output spectral shape: the energy of the peak of the accretion disk
emission , the photon power-law index of the non-thermal
emission , and the proportion of the total flux which is emitted in the
non-thermal component . The parameter is
presented as a function of the black hole mass, AGN luminosity, and `coronal
radius' of the {\sc optxagnf} model upon which {\sc oxaf} is based. We show
that the soft X-ray excess does not significantly affect photoionization
modeling predictions of strong emission lines in Seyfert narrow-line regions.
Despite its simplicity, {\sc oxaf} accounts for opacity effects where the
accretion disk is ionized because it inherits the `color correction' of {\sc
optxagnf}. We use a grid of {\sc mappings} photoionization models with {\sc
oxaf} ionizing spectra to demonstrate how predicted emission-line ratios on
standard optical diagnostic diagrams are sensitive to each of the three {\sc
oxaf} parameters. The {\sc oxaf} code is publicly available in the Astrophysics
Source Code Library.Comment: 14 pages, 9 figures, 1 table. Accepted for publication in Ap
The mass-metallicity relation of local active galaxies
We systematically measure the gas-phase metallicities and the
mass-metallicity relation of a large sample of local active galaxies for the
first time. Observed emission-line fluxes from the Sloan Digital Sky Survey
(SDSS) are compared to a four-dimensional grid of photoionization models using
the Bayesian parameter estimation code NebulaBayes. For the first time we take
into account arbitrary mixing between HII region and narrow-line region (NLR)
emission, and the models are also varied with metallicity, ionization parameter
in the NLR, and the gas pressure. The active galactic nucleus (AGN) oxygen
abundance is found to increase by dex as a function
of host galaxy stellar mass over the range .
We also measure the metallicity and ionization parameter of 231000 star-forming
galaxies for comparison with the sample of 7670 Seyfert 2 galaxies. A
systematic offset in oxygen abundance of 0.09 dex is observed between the
mass-metallicity relations of the star-forming and active galaxies. We
investigate potential causes of the offset, including sample selection and the
treatment in the models of diffuse ionized gas, pressure, and ionization
parameter. We cannot identify the major cause(s), but suspect contributions due
to deficiencies in modeling the ionizing spectra and the treatment of dust
physics. Optical diagnostic diagrams are presented with the star-forming and
Seyfert data colored by the inferred oxygen abundance, ionization parameter and
gas pressure, clearly illustrating the trends in these quantities.Comment: 12 pages, 4 figures and 1 table; accepted for publication in Ap
The Apple store
An exhibit within an exhibition at Hauser and Wirth Somerset entitled 'The land we live in - The land we left behind'
A collaborative work with Artist Marcus Coates and Adam Sutherland (curator)
An installation constructed for performance artist Marcus Coates to play out his persona as a an apple store guru advisng participants on his knowledge and experience of apples
Optimal non-circular fiber geometries for image scrambling in high-resolution spectrographs
Optical fibers are a key component for high-resolution spectrographs to
attain high precision in radial velocity measurements. We present a custom
fiber with a novel core geometry - a 'D'-shape. From a theoretical standpoint,
such a fiber should provide superior scrambling and modal noise mitigation,
since unlike the commonly used circular and polygonal fiber cross sections, it
shows chaotic scrambling. We report on the fabrication process of a test fiber
and compare the optical properties, scrambling performance, and modal noise
behavior of the D-fiber with those of common polygonal fibers.Comment: 7 pages, 6 figures, submitted to SPIE Astronomical Telescopes &
Instrumentation 2016 (9912-192
Interrogating Seyferts with NebulaBayes: Spatially probing the narrow-line region radiation fields and chemical abundances
NebulaBayes is a new Bayesian code that implements a general method of
comparing observed emission-line fluxes to photoionization model grids. The
code enables us to extract robust, spatially resolved measurements of
abundances in the extended narrow line regions (ENLRs) produced by Active
Galactic Nuclei (AGN). We observe near-constant ionization parameters but
steeply radially-declining pressures, which together imply that radiation
pressure regulates the ENLR density structure on large scales. Our sample
includes four `pure Seyfert' galaxies from the S7 survey that have extensive
ENLRs. NGC2992 shows steep metallicity gradients from the nucleus into the
ionization cones. An {\it inverse} metallicity gradient is observed in
ESO138-G01, which we attribute to a recent gas inflow or minor merger. A
uniformly high metallicity and hard ionizing continuum are inferred across the
ENLR of Mrk573. Our analysis of IC5063 is likely affected by contamination from
shock excitation, which appears to soften the inferred ionizing spectrum. The
peak of the ionizing continuum E_peak is determined by the nuclear spectrum and
the absorbing column between the nucleus and the ionized nebula. We cannot
separate variation in this intrinsic E_peak from the effects of shock or HII
region contamination, but E_peak measurements nevertheless give insights into
ENLR excitation. We demonstrate the general applicability of NebulaBayes by
analyzing a nuclear spectrum from the non-active galaxy NGC4691 using a HII
region grid. The NLR and HII region model grids are provided with NebulaBayes
for use by the astronomical community.Comment: Accepted for publication in ApJ; 29 pages with 10 figures and 3
table
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