8,759 research outputs found
Gravitational scattering of stars and clusters and the heating of the Galactic disk
Could the velocity spread, increasing with time, in the Galactic disk be
explained as a result of gravitational interactions of stars with giant
molecular clouds (GMCs) and spiral arms? Do the old open clusters high above
the Galactic plane provide clues to this question? We explore the effects on
stellar orbits of scattering by inhomogeneities in the Galactic potential due
to GMCs, spiral arms and the Galactic bar, and whether high-altitude clusters
could have formed in orbits closer to the Galactic plane and later been
scattered.
Simulations of test-particle motions are performed in a realistic Galactic
potential. The effects of the internal structure of GMCs are explored. The
destruction of clusters in GMC collisions is treated in detail with N-body
simulations of the clusters.
The observed velocity dispersions of stars as a function of time are well
reproduced. The GMC structure is found to be significant, but adequate models
produce considerable scattering effects. The fraction of simulated massive old
open clusters, scattered into orbits with |z| > 400 pc, is typically 0:5%, in
agreement with the observed number of high-altitude clusters and consistent
with the present formation rate of massive open clusters.
The heating of the thin Galactic disk is well explained by gravitational
scattering by GMCs and spiral arms, if the local correlation between the GMC
mass and the corresponding voids in the gas is not very strong. Our results
suggest that the high-altitude metal-rich clusters were formed in orbits close
to the Galactic plane and later scattered to higher orbits. It is possible,
though not very probable, that the Sun formed in such a cluster before
scattering occurred.Comment: 19 pages, 15 figure
Close encounters involving free-floating planets in star clusters
Instabilities in planetary systems can result in the ejection of planets from
their host system, resulting in free-floating planets (FFPs). If this occurs in
a star cluster, the FFP may remain bound to the star cluster for some time and
interact with the other cluster members until it is ejected. Here, we use
-body simulations to characterise close star-planet and planet-planet
encounters and the dynamical fate of the FFP population in star clusters
containing single or binary star members. We find that FFPs ejected
from their planetary system at low velocities typically leave the star cluster
40% earlier than their host stars, and experience tens of close ( AU)
encounters with other stars and planets before they escape. The fraction of
FFPs that experiences a close encounter depends on both the stellar density and
the initial velocity distribution of the FFPs. Approximately half of the close
encounters occur within the first 30 Myr, and only 10% occur after 100 Myr. The
periastron velocity distribution for all encounters is well-described by a
modified Maxwell-Bolzmann distribution, and the periastron distance
distribution is linear over almost the entire range of distances considered,
and flattens off for very close encounters due to strong gravitational
focusing. Close encounters with FFPs can perturb existing planetary systems and
their debris structures, and they can result in re-capture of FFPs. In
addition, these FFP populations may be observed in young star clusters in
imaging surveys; a comparison between observations and dynamical predictions
may provide clues to the early phases of stellar and planetary dynamics in star
clusters.Comment: Accepted for publication in MNRAS; 18 pages, 12 figure
Efficient Ornamentation in Ordovician Anthaspidellid Sponges
Lithistid orchoclad sponges within the family Anthaspidellidae Ulrich in Miller, 1889 include several genera that added ornate features to their outer-wall surfaces during Early Ordovician sponge radiation. Ornamented anthaspidellid sponges commonly constructed annulated or irregularly to regularly spaced transverse ridge-and-trough features on their outer-wall surfaces without proportionately increasing the size of their internal wall or gastral surfaces. This efficient technique of modifying only the sponge’s outer surface without enlarging its entire skeletal frame conserved the sponge’s constructional energy while increasing outer-wall surface-to-fluid exposure for greater intake of nutrient bearing currents. Sponges with widely spaced ridge-and-trough ornament dimensions predominated in high-energy settings. Widely spaced ridges and troughs may have given the sponge hydrodynamic benefits in high wave force settings. Ornamented sponges with narrowly spaced ridge-and-trough dimensions are found in high energy paleoenvironments but also occupied moderate to low-energy settings, where their surface-to-fluid exposure per unit area exceeded that of sponges with widely spaced surface ornamentations
Low Cost Dewatering of Waste Slurries
The U.S. Bureau of Mines has developed a technique for dewatering mineral waste slurries which utilizes polymer and a static screen. A variety of waste slurries from placer gold mines and crushed stone operations have been successfully treated using the system. Depending on the waste, a number of polymers have been used successfully with polymer costs ranging from 0.15 per 1,000 gal treated. The dewatering is accomplished using screens made from either ordinary window screen or wedge wire. The screens used are 8 ft wide and 8 ft long. The capacity of the screens varies from 3 to 7 gpm/sq. ft. The water produced is acceptable for recycling to the plant or for discharge to the environment. For example, a fine grain dolomite waste slurry produced from a crushed stone operation was dewatered from a nominal 2.5 pct solids to greater than 50 pct solids using 0.15 worth of polymer per 1,000 gal of slurry. The resulting waste water had a turbidity of less than 50 NTU and could be discharged or recycled. The paper describes field tests conducted using the polymer-screen dewatering system
Long-term stability of the HR 8799 planetary system without resonant lock
HR 8799 is a star accompanied by four massive planets on wide orbits. The
observed planetary configuration has been shown to be unstable on a timescale
much shorter than the estimated age of the system (~ 30 Myr) unless the planets
are locked into mean motion resonances. This condition is characterised by
small-amplitude libration of one or more resonant angles that stabilise the
system by preventing close encounters. We simulate planetary systems similar to
the HR 8799 planetary system, exploring the parameter space in separation
between the orbits, planetary masses and distance from the Sun to the star. We
find systems that look like HR 8799 and remain stable for longer than the
estimated age of HR 8799. None of our systems are forced into resonances. We
find, with nominal masses and in a narrow range of orbit separations, that 5 of
100 systems match the observations and lifetime. Considering a broad range of
orbit separations, we find 12 of 900 similar systems. The systems survive
significantly longer because of their slightly increased initial orbit
separations compared to assuming circular orbits from the observed positions. A
small increase in separation leads to a significant increase in survival time.
The low eccentricity the orbits develop from gravitational interaction is
enough for the planets to match the observations. With lower masses, but still
comfortably within the estimated planet mass uncertainty, we find 18 of 100
matching and long-lived systems in a narrow orbital separation range. In the
broad separation range, we find 82 of 900 matching systems. Our results imply
that the planets in the HR 8799 system do not have to be in strong mean motion
resonances.Comment: Accepted for publication in A&
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