211 research outputs found
Smooth and Clumpy Dust Distribution in AGN: a Direct Comparison of two Commonly Explored Infrared Emission Models
The geometry of the dust distribution within the inner regions of Active
Galactic Nuclei (AGN) is still a debated issue and relates directly with the
AGN unified scheme. Traditionally, models discussed in the literature assume
one of two distinct dust distributions in what is believed to be a toroidal
region around the Supermassive Black Holes: a continuous distribution,
customarily referred to as smooth, and a concentration of dust in clumps or
clouds, referred to as clumpy.
In this paper we perform a thorough comparison between two of the most
popular models in the literature, namely the smooth models by Fritz. et al.
2006 and the clumpy models by Nenkova et al. 2008a, in their common parameters
space. Particular attention is paid to the silicate features at ~9.7 and ~18
micron, the width of the infrared bump, the near-infrared index and the
luminosity at 12.3 micron, all previously reported as possible diagnostic tools
to distinguish between the two dust distributions. We find that, due to the
different dust chemical compositions used in the two models, the behaviour of
the silicate features at 9.7 and 18 micron is quite distinct between the two
models. The width of the infrared bump and the peak of the infrared emission
can take comparable values, their distributions do, however, vary. The
near-infrared index is also quite different, due partly to the primary sources
adopted by the two models. Models with matched parameters do not produce
similar SEDs and virtually no random parameter combinations can result in
seemingly identical SEDs.Comment: 9 Pages, 6 Figures, 1 Table. Accepted for publication in MNRA
A MASSIF Effort To Determine The Mass-Luminosity Relation for Stars of Various Ages, Metallicities, and Evolution States
The MASSIF (Masses and Stellar Systems with Interferometry) Team will use SIM to investigate the mass content of the Galaxy - from huge stars to barely glimmering brown dwarfs, and from hot white dwarfs to exotic black holes. We will target various samples of the Galactic population to determine and relate the fundamental characteristics of mass, luminosity, age, composition, and multiplicity - attributes that together yield an extensive understanding of the stars. Our samples will include distant clusters that span a factor of 5000 in age, and commonplace stars and substellar objects that lurk near the Sun. The principal goals of the MASSIF Key Project are to (1) define the mass-luminosity relation for main sequence stars in five fundamental clusters so that effects of age and metallicity can be mapped (Trapezium, TW Hydrae, Pleiades, Hyades, and M67), and (2) determine accurate masses for representative examples of nearly every type of star, stellar descendant or brown dwarf in the Galaxy
Logical Step-Indexed Logical Relations
Appel and McAllester's "step-indexed" logical relations have proven to be a
simple and effective technique for reasoning about programs in languages with
semantically interesting types, such as general recursive types and general
reference types. However, proofs using step-indexed models typically involve
tedious, error-prone, and proof-obscuring step-index arithmetic, so it is
important to develop clean, high-level, equational proof principles that avoid
mention of step indices. In this paper, we show how to reason about binary
step-indexed logical relations in an abstract and elegant way. Specifically, we
define a logic LSLR, which is inspired by Plotkin and Abadi's logic for
parametricity, but also supports recursively defined relations by means of the
modal "later" operator from Appel, Melli\`es, Richards, and Vouillon's "very
modal model" paper. We encode in LSLR a logical relation for reasoning
relationally about programs in call-by-value System F extended with general
recursive types. Using this logical relation, we derive a set of useful rules
with which we can prove contextual equivalence and approximation results
without counting steps
The Mass of the Candidate Exoplanet Companion to HD 33636 from Hubble Space Telescope Astrometry and High-Precision Radial Velocities
We have determined a dynamical mass for the companion to HD 33636 which
indicates it is a low-mass star instead of an exoplanet. Our result is based on
an analysis of Hubble Space Telescope (HST) astrometry and ground-based radial
velocity data. We have obtained high-cadence radial velocity measurements
spanning 1.3 years of HD 33636 with the Hobby-Eberly Telescope at McDonald
Observatory. We combined these data with previously published velocities to
create a data set that spans nine years. We used this data set to search for,
and place mass limits on, the existence of additional companions in the HD
33636 system. Our high-precision astrometric observations of the system with
the HST Fine Guidance Sensor 1r span 1.2 years. We simultaneously modeled the
radial velocity and astrometry data to determine the parallax, proper motion,
and perturbation orbit parameters of HD 33636. Our derived parallax, pi = 35.6
+/- 0.2 mas, agrees within the uncertainties with the Hipparcos value. We find
a perturbation period P = 2117.3 +/- 0.8 days, semimajor axis a_A = 14.2 +/-
0.2 mas, and system inclination i = 4.1 +/- 0.1 deg. Assuming the mass of the
primary star M_A = 1.02 +/- 0.03 M_sun, we obtain a companion mass M_B = 142
+/- 11 M_jup = 0.14 +/- 0.01 M_sun. The much larger true mass of the companion
relative to its minimum mass estimated from the spectroscopic orbit parameters
(M sin i = 9.3 M_jup) is due to the near face-on orbit orientation. This result
demonstrates the value of follow-up astrometric observations to determine the
true masses of exoplanet candidates detected with the radial velocity method.Comment: 33 pages, 6 figures, accepted for publication in AJ; added reference
to section
Recommended from our members
Family history of Alzheimer's disease alters cognition and is modified by medical and genetic factors
In humans, a first-degree family history of dementia (FH) is a well-documented risk factor for Alzheimer's disease (AD); however, the influence of FH on cognition across the lifespan is poorly understood. To address this issue, we developed an internet-based paired-associates learning (PAL) task and tested 59,571 participants between the ages of 18-85. FH was associated with lower PAL performance in both sexes under 65 years old. Modifiers of this effect of FH on PAL performance included age, sex, education, and diabetes. The Apolipoprotein E epsilon 4 allele was also associated with lower PAL scores in FH positive individuals. Here we show, FH is associated with reduced PAL performance four decades before the typical onset of AD; additionally, several heritable and non-heritable modifiers of this effect were identified.Mueller Family Charitable Trust; Arizona Department of Health Services; National Institutes of Health [R01-AG041232, R01-AG049465-05]; Flinn FoundationOpen access journalThis item from the UA Faculty Publications collection is made available by the University of Arizona with support from the University of Arizona Libraries. If you have questions, please contact us at [email protected]
The International Deep Planet Survey I. The frequency of wide-orbit massive planets around A-stars
Breakthrough direct detections of planetary companions orbiting A-type stars
confirm the existence of massive planets at relatively large separations, but
dedicated surveys are required to estimate the frequency of similar planetary
systems. To measure the first estimation of the giant exoplanetary systems
frequency at large orbital separation around A-stars, we have conducted a
deep-imaging survey of young (8-400 Myr), nearby (19-84 pc) A- and F-stars to
search for substellar companions in the 10-300 AU range. The sample of 42 stars
combines all A-stars observed in previous AO planet search surveys reported in
the literature with new AO observations from VLT/NaCo and Gemini/NIRI. It
represents an initial subset of the International Deep Planet Survey (IDPS)
sample of stars covering M- to B-stars. The data were obtained with
diffraction-limited observations in H- and Ks-band combined with angular
differential imaging to suppress the speckle noise of the central stars,
resulting in typical 5-sigma detection limits in magnitude difference of 12 mag
at 1", 14 mag at 2" and 16 mag at 5" which is sufficient to detect massive
planets. A detailed statistical analysis of the survey results is performed
using Monte Carlo simulations. Considering the planet detections, we estimate
the fraction of A-stars having at least one massive planet (3-14 MJup) in the
range 5-320 AU to be inside 5.9-18.8% at 68% confidence, assuming a flat
distribution for the mass of the planets. By comparison, the brown dwarf (15-75
MJup) frequency for the sample is 2.0-8.9% at 68% confidence in the range 5-320
AU. Assuming power law distributions for the mass and semimajor axis of the
planet population, the AO data are consistent with a declining number of
massive planets with increasing orbital radius which is distinct from the
rising slope inferred from radial velocity (RV) surveys around evolved A-stars.Comment: 20 pages, 10 figures, 7 tables. Accepted for publication in A&
Quantum phase transitions
In recent years, quantum phase transitions have attracted the interest of
both theorists and experimentalists in condensed matter physics. These
transitions, which are accessed at zero temperature by variation of a
non-thermal control parameter, can influence the behavior of electronic systems
over a wide range of the phase diagram. Quantum phase transitions occur as a
result of competing ground state phases. The cuprate superconductors which can
be tuned from a Mott insulating to a d-wave superconducting phase by carrier
doping are a paradigmatic example. This review introduces important concepts of
phase transitions and discusses the interplay of quantum and classical
fluctuations near criticality. The main part of the article is devoted to bulk
quantum phase transitions in condensed matter systems. Several classes of
transitions will be briefly reviewed, pointing out, e.g., conceptual
differences between ordering transitions in metallic and insulating systems. An
interesting separate class of transitions are boundary phase transitions where
only degrees of freedom of a subsystem become critical; this will be
illustrated in a few examples. The article is aimed on bridging the gap between
high-level theoretical presentations and research papers specialized in certain
classes of materials. It will give an overview over a variety of different
quantum transitions, critically discuss open theoretical questions, and
frequently make contact with recent experiments in condensed matter physics.Comment: 50 pages, 7 figs; (v2) final version as publishe
Chronology of Dune Development in the White River Badlands, Northern Great Plains, USA
Aeolian dune field chronologies provide important information on drought history on the Great Plains. The White River Badlands (WRB) dunes are located approximately 60 km north of the Nebraska Sand Hills (NSH), in the western section of the northern Great Plains. Clifftop dunes, sand sheets, and stabilized northwest-southeast trending parabolic dunes are found on upland mesas and buttes, locally called tables. The result of this study is a dune stabilization history determined from samples collected from stratigraphic exposures and dune crests. Thirty-seven OSL ages, from this and previous investigations, show three periods of dune activity: 1) ∼21,000 years ago to 12,000 years ago (a), 2) ∼9 to 6 ka, and 3) post-700 a. Stratigraphic exposures and low-relief dune forms preserve evidence of late Pleistocene and middle Holocene dune development, while high-relief dune crests preserve evidence of late Holocene dune development. Results of 12 OSL ages from the most recent dune activation event indicate that Medieval Climate Anomaly (MCA) droughts and Little Ice Age (LIA) droughts caused dune reactivation on the tables. Dune reactivation was accompanied by other drought-driven geomorphological responses in the WRB, including fluvial incision of the prairie and formation of sod tables. Regional significance of the MCA and LIA droughts is supported by similarities in the aeolian chronologies of the NSH at 700–600 a and some western Great Plains dune fields at 420–210 a. Aerial photographs of the WRB show little activity during the Dust Bowl droughts of the 1930s
Sulfur isotope measurement of sulfate and sulfide by high-resolution MC-ICP-MS
Author Posting. © Elsevier B.V. , 2008. This is the author's version of the work. It is posted here by permission of Elsevier B.V. for personal use, not for redistribution. The definitive version was published in Chemical Geology 253 (2008): 102-113, doi:10.1016/j.chemgeo.2008.04.017.We have developed a technique for the accurate and precise determination of 34S/32S isotope
ratios (δ34S) in sulfur-bearing minerals using solution and laser ablation multiple-collector
inductively coupled plasma mass spectrometry (MC-ICP-MS). We have examined and
determined rigorous corrections for analytical difficulties such as instrumental mass bias,
unresolved isobaric interferences, blanks, and laser ablation- and matrix-induced isotopic
fractionation. Use of high resolution sector-field mass spectrometry removes major isobaric
interferences from O2+. Standard–sample bracketing is used to correct for the instrumental mass
bias of unknown samples. Blanks on sulfur masses arising from memory effects and residual
oxygen-tailing are typically minor (< 0.2‰, within analytical error), and are mathematically
removed by on-peak zero subtraction and by bracketing of samples with standards determined at
the same signal intensity (within 20%). Matrix effects are significant (up to 0.7‰) for matrix
compositions relevant to many natural sulfur-bearing minerals. For solution analysis, sulfur
isotope compositions are best determined using purified (matrix-clean) sulfur standards and
sample solutions using the chemical purification protocol we present. For in situ analysis, where
the complex matrix cannot be removed prior to analysis, appropriately matrix-matching
standards and samples removes matrix artifacts and yields sulfur isotope ratios consistent with
conventional techniques using matrix-clean analytes. Our method enables solid samples to be
calibrated against aqueous standards; a consideration that is important when certified,
isotopically-homogeneous and appropriately matrix-matched solid standards do not exist.
Further, bulk and in situ analyses can be performed interchangeably in a single analytical session
because the instrumental setup is identical for both. We validated the robustness of our analytical
method through multiple isotope analyses of a range of reference materials and have compared
these with isotope ratios determined using independent techniques. Long-term reproducibility of
S isotope compositions is typically 0.20‰ and 0.45‰ (2σ) for solution and laser analysis,
respectively. Our method affords the opportunity to make accurate and relatively precise S
isotope measurement for a wide range of sulfur-bearing materials, and is particularly appropriate
for geologic samples with complex matrix and for which high-resolution in situ analysis is
critical.Support was provided by National Science Foundations grants OCE-0327448 to P.R.C. and
W.B. and OCE-0622982 to O.J.R. Support for L.A.B. was provided by the Woods Hole
Oceanographic Institution Plasma Facility Development Grant (NSF-EAR/IF-0318137)
- …