492 research outputs found
Capturing an Evolving Nebular Environment: A Petrographic and Geochemical Study of a Type A, B & C CAI
Calcium, Aluminum-rich Inclusions (CAIs) were the first formed solids in our Solar System, with mineral assemblages reflecting the first phases predicted to condense out of a hot nebular gas of Solar composition. Geochemical, textural and crystallographic information in CAIs can be used to constrain the temperature, pressure, and composition (e.g., oxygen fugacity) of the gaseous reservoir(s) from which they formed, as well as any secondary (nebular and parent body) processes they underwent. Coordinated geochemical and textural analyses provide information on nebular conditions (i.e., astrophysical environments and dynamics of nebular gas reservoirs) in which these CAIs formed. In order to better understand the evolution of nebular reservoirs at the time of CAI formation, we analyzed a Type A, B and C CAI using Electron Probe Micro-Analyzer (EPMA) and Electron BackScatter Diffraction (EBSD) at NASA Johnson Space Center (JSC)
Stochastic processes, galactic star formation, and chemical evolution. Effects of accretion, stripping, and collisions in multiphase multi-zone models
This paper reports simulations allowing for stochastic accretion and mass
loss within closed and open systems modeled using a previously developed
multi-population, multi-zone (halo, thick disk, thin disk) treatment. The star
formation rate is computed as a function of time directly from the model
equations and all chemical evolution is followed without instantaneous
recycling. Several types of simulations are presented here: (1) a closed system
with bursty mass loss from the halo to the thick disk, and from the thick to
the thin disk, in separate events to the thin disk; (2) open systems with
random environmental (extragalactic) accretion, e.g. by infall of high velocity
clouds directly to the thin disk; (3) schematic open system single and multiple
collision events and intracluster stripping. For the open models, the mass of
the Galaxy has been explicitly tracked with time. We present the evolution of
the star formation rate, metallicity histories, and concentrate on the light
elements. We find a wide range of possible outcomes, including an explanation
for variations in the Galactic D/H ratio, and highlight the problems for
uniquely reconstructing star forming histories from contemporary abundance
measurements.Comment: 12 pages, 12 Postscript figures, uses A&A style macros. Accepted for
publication by Astronomy & Astrophysic
Brillouin scattering studies in FeO across the Verwey transition
Brillouin scattering studies have been carried out on high quality single
crystals of FeO with [100] and [110] faces in the temperature range of
300 to 30 K. The room temperature spectrum shows a surface Rayleigh wave (SRW)
mode at 8 GHz and a longitudinal acoustic (LA) mode at 60 GHz. The SRW mode
frequency shows a minimum at the Verwey transition temperature of 123 K.
The softening of the SRW mode frequency from about 250 K to can be
quantitatively understood as a result of a decrease in the shear elastic
constant C, arising from the coupling of shear strain to charge
fluctuations. On the other hand, the LA mode frequency does not show any
significant change around , but shows a large change in its intensity. The
latter shows a maximum at around 120 K in the cooling run and at 165 K in the
heating run, exhibiting a large hysteresis of 45 K. This significant change in
intensity may be related to the presence of stress-induced ordering of
Fe and Fe at the octahedral sites, as well as to stress-induced
domain wall motion.Comment: 14 pages, 3 figures, accepted in Physical Review B 200
Evolution of predator dispersal in relation to spatio-temporal prey dynamics : how not to get stuck in the wrong place!
Peer reviewedPublisher PD
Sloshing in High Speed Galaxy Interactions
Observations of lopsided spiral galaxies motivated us to explore whether the
rapid passage of a companion galaxy could cause them. We examine whether the
center of mass of the visible matter becomes displaced from the center of mass
of the dark halo during the intruder's passage, thereby causing an asymmetric
response and asymmetric structure. Two dimensional -body simulations
indicate that this can happen.
We also explore some consequences of this offset. These include the center of
mass of the visible disk following a decaying orbit around the halo center of
mass and the development of transient one-armed spirals that persist for up to
six rotation periods.
We then study the results of a variety of initial conditions based on such
offsets. We report on the results of several runs in which we initially offset
a disk from its halo's center of mass by an amount typical of the above
interaction. In some runs the halo is free to move, while in others it is held
fixed. We used three different mass distributions for the halo in these runs.
We find that the disk's center of mass spiraled inward creating a variety of
observed or observable phenomena including one-armed spirals, massive clumps of
particles, and counter-rotating waves. The systems settle into relatively
axisymmetric configurations. Whether or not the end states included a bar
depended on a variety of initial conditions.Comment: 20 text pages, 3 tables, 24 figures. A gzipped postscripped version
with higher resolution figures can be downloaded from
http://butch.umephy.maine.edu/kickers/Research/Sloshing/ . Accepted for
publication in The Astrophysical Journa
Quasinormal modes and classical wave propagation in analogue black holes
Many properties of black holes can be studied using acoustic analogues in the
laboratory through the propagation of sound waves. We investigate in detail
sound wave propagation in a rotating acoustic (2+1)-dimensional black hole,
which corresponds to the ``draining bathtub'' fluid flow. We compute the
quasinormal mode frequencies of this system and discuss late-time power-law
tails. Due to the presence of an ergoregion, waves in a rotating acoustic black
hole can be superradiantly amplified. We compute superradiant reflection
coefficients and instability timescales for the acoustic black hole bomb, the
equivalent of the Press-Teukolsky black hole bomb. Finally we discuss
quasinormal modes and late-time tails in a non-rotating canonical acoustic
black hole, corresponding to an incompressible, spherically symmetric
(3+1)-dimensional fluid flow.Comment: 19 pages, 12 figures, ReVTeX4; v2: minor modifications and
correction
Reverberation Mapping Measurements of Black Hole Masses in Six Local Seyfert Galaxies
We present the final results from a high sampling rate, multi-month,
spectrophotometric reverberation mapping campaign undertaken to obtain either
new or improved Hbeta reverberation lag measurements for several relatively
low-luminosity AGNs. We have reliably measured thetime delay between variations
in the continuum and Hbeta emission line in six local Seyfert 1 galaxies. These
measurements are used to calculate the mass of the supermassive black hole at
the center of each of these AGNs. We place our results in context to the most
current calibration of the broad-line region (BLR) R-L relationship, where our
results remove outliers and reduce the scatter at the low-luminosity end of
this relationship. We also present velocity-resolved Hbeta time delay
measurements for our complete sample, though the clearest velocity-resolved
kinematic signatures have already been published.Comment: 52 pages (AASTeX: 29 pages of text, 8 tables, 7 figures), accepted
for publication in the Astrophysical Journa
A Revised Broad-Line Region Radius and Black Hole Mass for the Narrow-Line Seyfert 1 NGC 4051
We present the first results from a high sampling rate, multi-month
reverberation mapping campaign undertaken primarily at MDM Observatory with
supporting observations from telescopes around the world. The primary goal of
this campaign was to obtain either new or improved Hbeta reverberation lag
measurements for several relatively low luminosity AGNs. We feature results for
NGC 4051 here because, until now, this object has been a significant outlier
from AGN scaling relationships, e.g., it was previously a ~2-3sigma outlier on
the relationship between the broad-line region (BLR) radius and the optical
continuum luminosity - the R_BLR-L relationship. Our new measurements of the
lag time between variations in the continuum and Hbeta emission line made from
spectroscopic monitoring of NGC 4051 lead to a measured BLR radius of R_BLR =
1.87 (+0.54 -0.50) light days and black hole mass of M_BH = 1.73 (+0.55 -0.52)
x 10^6 M_sun. This radius is consistent with that expected from the R_BLR-L
relationship, based on the present luminosity of NGC 4051 and the most current
calibration of the relation by Bentz et al. (2009a). We also present a
preliminary look at velocity-resolved Hbeta light curves and time delay
measurements, although we are unable to reconstruct an unambiguous
velocity-resolved reverberation signal.Comment: 38 pages, 7 figures, accepted for publication in ApJ, changes from v1
reflect suggestions from anonymous refere
Ecological Invasion, Roughened Fronts, and a Competitor's Extreme Advance: Integrating Stochastic Spatial-Growth Models
Both community ecology and conservation biology seek further understanding of
factors governing the advance of an invasive species. We model biological
invasion as an individual-based, stochastic process on a two-dimensional
landscape. An ecologically superior invader and a resident species compete for
space preemptively. Our general model includes the basic contact process and a
variant of the Eden model as special cases. We employ the concept of a
"roughened" front to quantify effects of discreteness and stochasticity on
invasion; we emphasize the probability distribution of the front-runner's
relative position. That is, we analyze the location of the most advanced
invader as the extreme deviation about the front's mean position. We find that
a class of models with different assumptions about neighborhood interactions
exhibit universal characteristics. That is, key features of the invasion
dynamics span a class of models, independently of locally detailed demographic
rules. Our results integrate theories of invasive spatial growth and generate
novel hypotheses linking habitat or landscape size (length of the invading
front) to invasion velocity, and to the relative position of the most advanced
invader.Comment: The original publication is available at
www.springerlink.com/content/8528v8563r7u2742
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