17,336 research outputs found
Investigating the Structure of the Windy Torus in Quasars
Thermal mid-infrared emission of quasars requires an obscuring structure that
can be modeled as a magneto-hydrodynamic wind in which radiation pressure on
dust shapes the outflow. We have taken the dusty wind models presented by
Keating and collaborators that generated quasar mid-infrared spectral energy
distributions (SEDs), and explored their properties (such as geometry, opening
angle, and ionic column densities) as a function of Eddington ratio and X-ray
weakness. In addition, we present new models with a range of magnetic field
strengths and column densities of the dust-free shielding gas interior to the
dusty wind. We find this family of models -- with input parameters tuned to
accurately match the observed mid-IR power in quasar SEDs -- provides
reasonable values of the Type 1 fraction of quasars and the column densities of
warm absorber gas, though it does not explain a purely luminosity-dependent
covering fraction for either. Furthermore, we provide predictions of the
cumulative distribution of E(B-V) values of quasars from extinction by the wind
and the shape of the wind as imaged in the mid-infrared. Within the framework
of this model, we predict that the strength of the near-infrared bump from hot
dust emission will be correlated primarily with L/L_Edd rather than luminosity
alone, with scatter induced by the distribution of magnetic field strengths.
The empirical successes and shortcomings of these models warrant further
investigations into the composition and behaviour of dust and the nature of
magnetic fields in the vicinity of actively accreting supermassive black holes.Comment: 11 pages, 6 figures, accepted for publication in MNRA
Responses of Hyalella azteca and Ceridaphnia dubia to reservoir sediments following Chelated Copper Herbicide Applications
In response to nuisance growths of algae and vascular
plants, such as dioecious hydrilla (
Hydrilla verticillata
L.f.
Royle), copper formulations have been applied in lakes and
reservoirs for a number of years. Concerns have arisen regarding
the long-term consequences of copper applications
and those concerns have appropriately focused on sediment
residues. In this study, we evaluated the toxicity of sediments
from treated (for a decade) and untreated areas in Lake
Murray, South Carolina and estimated the capacity of those
sediments to bind additional copper. Two sentinel aquatic invertebrates,
Hyalella azteca
Saussure and
Ceriodaphnia dubia
Richard, were used to measure residual toxicity of treated
and untreated sediments from the field and after laboratory
amendments. (PDF has 5 pages.
Dopamine-D1 and δ-opioid receptors co-exist in rat striatal neurons
Cocaine’s enhancement of dopaminergic neurotransmission in the mesolimbic pathway plays a critical role in the initial reinforcing properties of this drug. However, other neurotransmitter systems are also integral to the addiction process. A large body of data indicates that opioids and dopamine together mediate emotional and reinforced behaviors. In support of this, cocaine-mediated increases in activation of dopamine D1 receptors (D1R) results in a desensitization of δ-opioid receptor (DOR) signaling through adenylyl cyclase (AC) in striatal neurons. To further define cellular mechanisms underlying this effect, the subcellular distribution of DOR and D1R was examined in the rat dorsolateral striatum. Dual immunoperoxidase/gold-silver detection combined with electron microscopy was used to identify DOR and D1R immunoreactivities in the same section of tissue. Semi-quantitative analysis revealed that a subset of dendritic cellular profiles exhibited both DOR and D1R immunoreactivities. Of 165 randomly sampled D1R immunoreactive profiles, 43% contained DOR. Similarly of 198 DOR-labeled cellular profiles, 52% contained D1R. The present data provide ultrastructural evidence for co-existence between DOR and D1R in striatal neurons, suggesting a possible mechanism whereby D1R modulation may alter DOR function
Near-Infrared Variability in the 2MASS Calibration Fields: A Search for Planetary Transit Candidates
The 2MASS photometric calibration observations cover ~6 square degrees on the
sky in 35 "calibration fields" each sampled in nominal photometric conditions
between 562 and 3692 times during the four years of the 2MASS mission. We
compile a catalog of variables from the calibration observations to search for
M dwarfs transited by extra-solar planets. We present our methods for measuring
periodic and non-periodic flux variability. From 7554 sources with apparent Ks
magnitudes between 5.6 and 16.1, we identify 247 variables, including
extragalactic variables and 23 periodic variables. We have discovered three M
dwarf eclipsing systems, including two candidates for transiting extrasolar
planets.Comment: The Astrophysical Journal Supplement, in press; figures compresse
- …