1,558 research outputs found
On continuum modeling of sputter erosion under normal incidence: interplay between nonlocality and nonlinearity
Under specific experimental circumstances, sputter erosion on semiconductor
materials exhibits highly ordered hexagonal dot-like nanostructures. In a
recent attempt to theoretically understand this pattern forming process, Facsko
et al. [Phys. Rev. B 69, 153412 (2004)] suggested a nonlocal, damped
Kuramoto-Sivashinsky equation as a potential candidate for an adequate
continuum model of this self-organizing process. In this study we theoretically
investigate this proposal by (i) formally deriving such a nonlocal equation as
minimal model from balance considerations, (ii) showing that it can be exactly
mapped to a local, damped Kuramoto-Sivashinsky equation, and (iii) inspecting
the consequences of the resulting non-stationary erosion dynamics.Comment: 7 pages, 2 Postscript figures, accepted by Phys. Rev. B corrected
typos, few minor change
The Herschel/PACS view of disks around low-mass stars in Chamaleon-I
Circumstellar disks are expected to be the birthplaces of planets. The
potential for forming one or more planets of various masses is essentially
driven by the initial mass of the disks. We present and analyze Herschel/PACS
observations of disk-bearing M-type stars that belong to the young ~2 Myr old
Chamaleon-I star forming region. We used the radiative transfer code RADMC to
successfully model the SED of 17 M-type stars detected at PACS wavelengths. We
first discuss the relatively low detection rates of M5 and later spectral type
stars with respect to the PACS sensitivity, and argue their disks masses, or
flaring indices, are likely to be low. For M0 to M3 stars, we find a relatively
broad range of disk masses, scale heights, and flaring indices. Via a
parametrization of dust stratification, we can reproduce the peak fluxes of the
10 m emission feature observed with Spitzer/IRS, and find that disks
around M-type stars may display signs of dust sedimentation. The Herschel/PACS
observations of low-mass stars in Cha-I provide new constraints on their disk
properties, overall suggesting that disk parameters for early M-type stars are
comparable to those for more massive stars (e.g., comparable scale height and
flaring angles). However, regions of the disks emitting at about 100 m may
still be in the optically thick regime, preventing direct determination of disk
masses. Thus the modeled disk masses should be considered as lower limits.
Still, we are able to extend the wavelength coverage of SED models and start
characterizing effects such as dust sedimentation, an effort leading the way
towards ALMA observations of these low-mass stars
PROGRESS ON MANAGING CATTAIL MARSHES WITH RODEO® HERBICIDE TO DISPERSE ROOSTING BLACKBIRDS
In August and September 1989 and 1990, we aerially sprayed 8 cattail (Typha spp.) marshes with Rodeo® herbicide to begin evaluating its use for fragmenting dense cattail stands used by roosting blackbirds (Icterinae). Treated marshes were effectively eliminated as roost sites for blackbirds. After 2 years, cattail densities in 4 marshes treated with Rodeo® at 5.8 - 7.0 L/ha were 87% lower than pretreatment densities (P = 0.0001). In 1990, we treated 4 marshes with Rodeo® at 4.7 L/ha. One year later, 6% of the cattails survived in the sprayed areas. Of 7 groups of indicator birds, only marsh wren (Cistothorus palustris) and rail (sora, Porzana Carolina and Virginia rail, Rallus limicola) populations were adversely affected by cattail fragmentation. These preliminary results led to an increased research effort to develop marsh management techniques aimed at eliminating blackbird roosts
Influence of the Soret effect on convection of binary fluids
Convection in horizontal layers of binary fluids heated from below and in
particular the influence of the Soret effect on the bifurcation properties of
extended stationary and traveling patterns that occur for negative Soret
coupling is investigated theoretically. The fixed points corresponding to these
two convection structures are determined for realistic boundary conditions with
a many mode Galerkin scheme for temperature and concentration and an accurate
one mode truncation of the velocity field. This solution procedure yields the
stable and unstable solutions for all stationary and traveling patterns so that
complete phase diagrams for the different convection types in typical binary
liquid mixtures can easily be computed. Also the transition from weakly to
strongly nonlinear states can be analyzed in detail. An investigation of the
concentration current and of the relevance of its constituents shows the way
for a simplification of the mode representation of temperature and
concentration field as well as for an analytically manageable few mode
description.Comment: 30 pages, 12 figure
Spring Dispersal Patterns of Red-winged Blackbirds, Agelaius phoeniceus, Staging in Eastern South Dakota
Red-winged Blackbirds (Agelaius phoeniceus) are very abundant summer residents throughout the Prairie Pothole Region of central North America. In late summer they assemble in post-breeding flocks that cause significant amounts of agricultural damage, particularly in sunflower fields near natal sites. In April 2001, we aerially color-marked ~370,000 Red-winged Blackbirds near Badger, South Dakota (44°48'N, 97°21'W), to determine if migrants staging here were summer residents in sunflower production areas ~350 km to the northwest. We measured patterns of migratory dispersal by collecting birds in 54 randomly selected blocks in the northcentral U.S. and the Prairie Provinces of Canada. The marked specimens (n = 33) were categorized into three polygons based on analyses of banding and re-sighting data and proximity to concentrated sunflower production. We estimated that 82% of the migrants that had staged in eastern South Dakota resided within or on the periphery of the sunflower growing area. These birds probably stay near their breeding territories until at least late August and cause early damage to sunflower, which comprises the majority of damage. Resident birds in Alberta and most of Saskatchewan (18%) might arrive too late in the damage season to impact the sunflower crop significantly
Food Preferences of Common Grackles, Red-Winged Blackbirds, and Yellow-Headed Blackbirds Presented with Cracked Corn, Sunflower Meats, and Brown Rice
Blackbirds migrating through North Dakota in late summer damage sunflower crops shortly after the yellow ray petals begin to wither and fall (Besser 1985). DeHaven (1982) reported bird damage losses of $7.4 million on 2.2 million acres in 1980. Various behavioral, chemical, and mechanical strategies have been employed over the past decade to reduce the damage. Although some of these methods have effectively dispersed or reduced blackbird populations, many have proven to be ineffective over time, economically infeasible, or culturally unacceptable (Guarino 1984)
A New Galactic 6cm Formaldehyde Maser
We report the detection of a new H2CO maser in the massive star forming
region G23.71-0.20 (IRAS 18324-0820), i.e., the fifth region in the Galaxy
where H2CO maser emission has been found. The new H2CO maser is located toward
a compact HII region, and is coincident in velocity and position with 6.7 GHz
methanol masers and with an IR source as revealed by Spitzer/IRAC GLIMPSE data.
The coincidence with an IR source and 6.7 GHz methanol masers suggests that the
maser is in close proximity to an embedded massive protostar. Thus, the
detection of H2CO maser emission toward G23.71-0.20 supports the trend that
H2CO 6cm masers trace molecular material very near young massive stellar
objects.Comment: Accepted for publication in The Astrophysical Journal Letter
A Search for H2CO 6cm Emission toward Young Stellar Objects III: VLA Observations
We report the results of our third survey for formaldehyde (H2CO) 6cm maser
emission in the Galaxy. Using the Very Large Array, we detected two new H2CO
maser sources (G23.01-0.41 and G25.83-0.18), thus increasing the sample of
known H2CO maser regions in the Galaxy to seven. We review the characteristics
of the G23.01-0.41 and G25.83-0.18 star forming regions. The H2CO masers in
G23.01-0.41 and G25.83-0.18 share several properties with the other known H2CO
masers, in particular, emission from rich maser environments and close
proximity to very young massive stellar objects.Comment: Accepted for publication in the Astrophysical Journal Supplement
Serie
An H2CO 6cm Maser Pinpointing a Possible Circumstellar Torus in IRAS18566+0408
We report observations of 6cm, 3.6cm, 1.3cm, and 7mm radio continuum,
conducted with the Very Large Array towards IRAS18566+0408, one of the few
sources known to harbor H2CO 6cm maser emission. Our observations reveal that
the emission is dominated by an ionized jet at cm wavelengths. Spitzer/IRAC
images from GLIMPSE support this interpretation, given the presence of 4.5um
excess emission at approximately the same orientation as the cm continuum. The
7mm emission is dominated by thermal dust from a flattened structure almost
perpendicular to the ionized jet, thus, the 7mm emission appears to trace a
torus associated with a young massive stellar object. The H2CO 6cm maser is
coincident with the center of the torus-like structure. Our observations rule
out radiative pumping via radio continuum as the excitation mechanism for the
H2CO 6cm maser in IRAS18566+0408.Comment: 20 pages, 4 figures, ApJ (in press
A multi-wavelength analysis for interferometric (sub-)mm observations of protoplanetary disks: radial constraints on the dust properties and the disk structure
Theoretical models of grain growth predict dust properties to change as a
function of protoplanetary disk radius, mass, age and other physical
conditions. We lay down the methodology for a multi-wavelength analysis of
(sub-)mm and cm continuum interferometric observations to constrain
self-consistently the disk structure and the radial variation of the dust
properties. The computational architecture is massively parallel and highly
modular. The analysis is based on the simultaneous fit in the uv-plane of
observations at several wavelengths with a model for the disk thermal emission
and for the dust opacity. The observed flux density at the different
wavelengths is fitted by posing constraints on the disk structure and on the
radial variation of the grain size distribution. We apply the analysis to
observations of three protoplanetary disks (AS 209, FT Tau, DR Tau) for which a
combination of spatially resolved observations in the range ~0.88mm to ~10mm is
available (from SMA, CARMA, and VLA), finding evidence of a decreasing maximum
dust grain size (a_max) with radius. We derive large a_max values up to 1 cm in
the inner disk between 15 and 30 AU and smaller grains with a_max~1 mm in the
outer disk (R > 80AU). In this paper we develop a multi-wavelength analysis
that will allow this missing quantity to be constrained for statistically
relevant samples of disks and to investigate possible correlations with disk or
stellar parameters.Comment: 19 pages, 15 figures, accepted for publication in A&
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