440 research outputs found
Valuing Natural Space and Landscape Fragmentation in Richmond, VA
Hedonic pricing methods and GIS (Geographic Information Systems) were used to evaluate relationships between sale price of single family homes and landscape fragmentation and natural land cover. Spatial regression analyses found that sale prices increase as landscapes become less fragmented and the amount of natural land cover around a home increases. The projected growth in population and employment in the Richmond, Virginia region and subsequent increases in land development and landscape fragmentation presents a challenge to sustaining intact healthy ecosystems in the Richmond region. Spatial regression analyses helped illuminate how land cover patterns influence sale prices and landscape patterns that are economically and ecologically advantageous
Debris discs around nearby Solar analogues
An unbiased search for debris discs around nearby Sun-like stars is reported.
Thirteen G-dwarfs at 12-15 parsecs distance were searched at 850 \umum
wavelength, and a disc is confirmed around HD 30495. The estimated dust mass is
0.008 M with a net limit \la 0.0025 M for the average
disc of the other stars. The results suggest there is not a large missed
population of substantial cold discs around Sun-like stars -- HD 30495 is a
bright rather than unusually cool disc, and may belong to a few hundred Myr-old
population of greater dust luminosity. The far-infared and millimetre survey
data for Sun-like stars are well fitted by either steady state or stirred
models, provided that typical comet belts are comparable in size to that in the
Solar System.Comment: published in MNRA
Modeling Collisional Cascades In Debris Disks: Steep Dust-Size Distributions
We explore the evolution of the mass distribution of dust in
collision-dominated debris disks, using the collisional code introduced in our
previous paper. We analyze the equilibrium distribution and its dependence on
model parameters by evolving over 100 models to 10 Gyr. With our numerical
models, we confirm that systems reach collisional equilibrium with a mass
distribution that is steeper than the traditional solution by Dohnanyi (1969).
Our model yields a quasi steady-state slope of n(m) ~ m^{-1.88} [n(a) ~
a^{-3.65}] as a robust solution for a wide range of possible model parameters.
We also show that a simple power-law function can be an appropriate
approximation for the mass distribution of particles in certain regimes. The
steeper solution has observable effects in the submillimeter and millimeter
wavelength regimes of the electromagnetic spectrum. We assemble data for nine
debris disks that have been observed at these wavelengths and, using a
simplified absorption efficiency model, show that the predicted slope of the
particle mass distribution generates SEDs that are in agreement with the
observed ones.Comment: 12 pages, 10 figures, Accepted by ApJ, emulateap
A Comparison of Heart Rate Threshold Points Between High Fit and Recreational Fit Cyclists
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Are debris disks self-stirred?
This paper considers the evidence that debris disks are self-stirred by the
formation of Plutos. A model for the dust produced during self-stirring is
applied to statistics for A stars. As there is no significant difference
between excesses of A-stars <50Myr old, we focus on reproducing the broad
trends, the "rise and fall" of the fraction of stars with excesses. Using a
population model, we find that the statistics and trends can be reproduced with
a self-stirring model of planetesimal belts with radii distributed between
15-120AU. Disks must have this 15AU minimum radius to show a peak in disk
fraction, rather than a monotonic decline. Populations of extended disks with
fixed inner and/or outer radii fail to fit the statistics, due mainly to the
slow 70um evolution as stirring moves further out in the disk. This conclusion,
that debris disks are narrow belts, is independent of the significance of 24um
trends for young A-stars. We show that the statistics can also be reproduced
with a model in which disks are stirred by secular perturbations from a nearby
eccentric planet. Detailed imaging is therefore the best way to characterise
the stirring mechanism. From a more detailed look at beta Pictoris Moving Group
and TW Hydrae Association A-stars we find that the disk around beta Pictoris is
likely the result of secular stirring by the proposed planet at ~10AU; the
structure of the HR 4796A disk also points to sculpting by a planet. The two
other stars with disks, HR 7012 and eta Tel, possess transient hot dust, though
the outer eta Tel disk is consistent with a self-stirred origin. Planet
formation provides a natural explanation for the belt-like nature of debris
disks, with inner regions cleared by planets that may also stir the disk, and
the outer edges set by where planetesimals can form. [abridged]Comment: Accepted to MNRA
Search for Cold Debris Disks around M-dwarfs
Debris disks are believed to be related to planetesimals left over around
stars after planet formation has ceased. The frequency of debris disks around
M-dwarfs which account for 70% of the stars in the Galaxy is unknown while
constrains have already been found for A- to K-type stars. We have searched for
cold debris disks around 32 field M-dwarfs by conducting observations at lambda
= 850 microns with the SCUBA bolometerarray camera at the JCMT and at lambda =
1.2mm with the MAMBO array at the IRAM 30-m telescopes. This is the first
survey of a large sample of M-dwarfs conducted to provide statistical
constraints on debris disks around this type of stars. We have detected a new
debris disk around the M0.5 dwarf GJ842.2 at lambda = 850 microns, providing
evidence for cold dust at large distance from this star (~ 300AU). By combining
the results of our survey with the ones of Liu et al. (2004), we estimate for
the first time the detection rate of cold debris disks around field M-dwarfs
with ages between 20 and 200 Myr. This detection rate is 13^{+6}_{-8} % and is
consistent with the detection rate of cold debris disks (9 - 23 %) around A- to
K-type main sequence stars of the same age. This is an indication that cold
disks may be equally prevalent across stellar spectral types.Comment: A&A accepted on 15 september 200
Modeling Collisional Cascades In Debris Disks: The Numerical Method
We develop a new numerical algorithm to model collisional cascades in debris
disks. Because of the large dynamical range in particle masses, we solve the
integro-differential equations describing erosive and catastrophic collisions
in a particle-in-a-box approach, while treating the orbital dynamics of the
particles in an approximate fashion. We employ a new scheme for describing
erosive (cratering) collisions that yields a continuous set of outcomes as a
function of colliding masses. We demonstrate the stability and convergence
characteristics of our algorithm and compare it with other treatments. We show
that incorporating the effects of erosive collisions results in a decay of the
particle distribution that is significantly faster than with purely
catastrophic collisions.Comment: 24 pages, 20 figues, Published in Ap
Variations on Debris Disks II. Icy Planet Formation as a Function of the Bulk Properties and Initial Sizes of Planetesimals
We describe comprehensive calculations of the formation of icy planets and
debris disks at 30-150 AU around 1-3 solar mass stars. Disks composed of large,
strong planetesimals produce more massive planets than disks composed of small,
weak planetesimals. The maximum radius of icy planets ranges from roughly 1500
km to 11,500 km. The formation rate of 1000 km objects - `Plutos' - is a useful
proxy for the efficiency of icy planet formation. Plutos form more efficiently
in massive disks, in disks with small planetesimals, and in disks with a range
of planetesimal sizes. Although Plutos form throughout massive disks, Pluto
production is usually concentrated in the inner disk. Despite the large number
of Plutos produced in many calculations, icy planet formation is inefficient.
At the end of the main sequence lifetime of the central star, Plutos contain
less than 10% of the initial mass in solid material. This conclusion is
independent of the initial mass in the disk or the properties of planetesimals.
Debris disk formation coincides with the formation of planetary systems
containing Plutos. As Plutos form, they stir leftover planetesimals to large
velocities. A cascade of collisions then grinds the leftovers to dust, forming
an observable debris disk. In disks with small (< 1-10 km) planetesimals,
collisional cascades produce luminous debris disks with maximum luminosity
roughly 0.01 times the stellar luminosity. Disks with larger planetesimals
produce much less luminous debris disks. Observations of debris disks around
A-type and G-type stars strongly favor models with small planetesimals. In
these models, our predictions for the time evolution and detection frequency of
debris disks agree with published observations. We suggest several critical
observations that can test key features of our calculations.Comment: 61 pages of text, 24 tables, and 34 figures; submitted to ApJS;
comments welcome; revised version accepted to ApJS, changed text, modified
tables, added references, no major changes to conclusion
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