548 research outputs found
Algae for biofuel:will the evolution of weeds limit the enterprise?
Algae hold promise as a source of biofuel. Yet the manner in which algae are most efficiently propagated and harvested is different from that used in traditional agriculture. In theory, algae can be grown in continuous culture and harvested frequently to maintain high yields with a short turnaround time. However, the maintenance of the population in a state of continuous growth will likely impose selection for fast growth, possibly opposing the maintenance of lipid stores desiriable for fuel. Any harvesting that removes a subset of the population and leaves the survivors to establish the next generation may quickly select traits that escape harvesting. An understanding of these problems should help identify methods for retarding the evolution and enhancing biofuel production
Origin and Dynamical Evolution of Neptune Trojans - II: Long Term Evolution
We present results examining the fate of the Trojan clouds produced in our
previous work. We find that the stability of Neptunian Trojans seems to be
strongly correlated to their initial post-migration orbital elements, with
those objects that survive as Trojans for billions of years displaying
negligible orbital evolution. The great majority of these survivors began the
integrations with small eccentricities (e < 0.2) and small libration amplitudes
(A < 30 - 40{\deg}). The survival rate of "pre-formed" Neptunian Trojans (which
in general survived on dynamically cold orbits (e < 0.1, i < 5 - 10{\deg}))
varied between ~5 and 70%. By contrast, the survival rate of "captured" Trojans
(on final orbits spread across a larger region of e-i element space) were
markedly lower, ranging between 1 and 10% after 4 Gyr. Taken in concert with
our earlier work, we note that planetary formation scenarios which involve the
slow migration (a few tens of millions of years) of Neptune from an initial
planetary architecture that is both resonant and compact (aN < 18 AU) provide
the most promising fit of those we considered to the observed Trojan
population. In such scenarios, we find that the current day Trojan population
would number ~1% of that which was present at the end of the planet's
migration, with the bulk being sourced from captured, rather than pre-formed
objects. We note, however, that even those scenarios still fail to reproduce
the currently observed portion of the Neptune Trojan population moving on
orbits with e 20{\deg}. Dynamical integrations of the currently
observed Trojans show that five out of the seven are dynamically stable on 4
Gyr timescales, while 2001 QR322, exhibits significant dynamical instability.
The seventh Trojan object, 2008 LC18, has such large orbital uncertainties that
only future studies will be able to determine its stability.Comment: 24 pages, 6 figures, accepted for publication in MNRAS (The abstract
was shortened. Original version can be found in the pdf file
The orbit of 2010 TK7. Possible regions of stability for other Earth Trojan asteroids
Recently the first Earth Trojan has been observed (Mainzer et al., ApJ 731)
and found to be on an interesting orbit close to the Lagrange point L4 (Connors
et al., Nature 475). In the present study we therefore perform a detailed
investigation on the stability of its orbit and moreover extend the study to
give an idea of the probability to find additional Earth-Trojans. Our results
are derived using different approaches: a) we derive an analytical mapping in
the spatial elliptic restricted three-body problem to find the phase space
structure of the dynamical problem. We explore the stability of the asteroid in
the context of the phase space geometry, including the indirect influence of
the additional planets of our Solar system. b) We use precise numerical methods
to integrate the orbit forward and backward in time in different dynamical
models. Based on a set of 400 clone orbits we derive the probability of capture
and escape of the Earth Trojan asteroids 2010 TK7. c) To this end we perform an
extensive numerical investigation of the stability region of the Earth's
Lagrangian points. We present a detailed parameter study in the regime of
possible stable tadpole and horseshoe orbits of additional Earth-Trojans, i.e.
with respect to the semi-major axes and inclinations of thousands of fictitious
Trojans. All three approaches underline that the Earth Trojan asteroid 2010 TK7
finds himself in an unstable region on the edge of a stable zone; additional
Earth-Trojan asteroids may be found in this regime of stability.Comment: 11 pages, 16 figure
Origin and Dynamical Evolution of Neptune Trojans - I: Formation and Planetary Migration
We present the results of detailed dynamical simulations of the effect of the
migration of the four giant planets on both the transport of pre-formed Neptune
Trojans, and the capture of new Trojans from a trans-Neptunian disk. We find
that scenarios involving the slow migration of Neptune over a large distance
(50Myr to migrate from 18.1AU to its current location) provide the best match
to the properties of the known Trojans. Scenarios with faster migration (5Myr),
and those in which Neptune migrates from 23.1AU to its current location, fail
to adequately reproduce the current day Trojan population. Scenarios which
avoid disruptive perturbation events between Uranus and Neptune fail to yield
any significant excitation of pre-formed Trojans (transported with efficiencies
between 30 and 98% whilst maintaining the dynamically cold nature of these
objects). Conversely, scenarios with periods of strong Uranus-Neptune
perturbation lead to the almost complete loss of such pre-formed objects. In
these cases, a small fraction (~0.15%) of these escaped objects are later
recaptured as Trojans prior to the end of migration, with a wide range of
eccentricities (<0.35) and inclinations (<40 deg). In all scenarios (including
those with such disruptive interaction between Uranus and Neptune) the capture
of objects from the trans-Neptunian disk (through which Neptune migrates) is
achieved with efficiencies between ~0.1 and ~1%. The captured Trojans display a
wide range of inclinations (<40 deg for slow migration, and <20 deg for rapid
migration) and eccentricities (<0.35), and we conclude that, given the vast
amount of material which undoubtedly formed beyond the orbit of Neptune, such
captured objects may be sufficient to explain the entire Neptune Trojan
population. (Shortened version)Comment: 25 pages, 6 figure
Tidal breakup of binary stars at the Galactic Center. II. Hydrodynamic simulations
In Paper I, we followed the evolution of binary stars as they orbited near
the supermassive black hole (SMBH) at the Galactic center, noting the cases in
which the two stars would come close enough together to collide. In this paper
we replace the point-mass stars by fluid realizations, and use a
smoothed-particle hydrodynamics (SPH) code to follow the close interactions. We
model the binary components as main-sequence stars with initial masses of 1, 3
and 6 Solar masses, and with chemical composition profiles taken from stellar
evolution codes. Outcomes of the close interactions include mergers, collisions
that leave both stars intact, and ejection of one star at high velocity
accompanied by capture of the other star into a tight orbit around the SMBH.
For the first time, we follow the evolution of the collision products for many
() orbits around the SMBH. Stars that are initially too small to
be tidally disrupted by the SMBH can be puffed up by close encounters or
collisions, with the result that tidal stripping occurs in subsequent periapse
passages. In these cases, mass loss occurs episodically, sometimes for hundreds
of orbits before the star is completely disrupted. Repeated tidal flares, of
either increasing or decreasing intensity, are a predicted consequence. In
collisions involving a low-mass and a high-mass star, the merger product
acquires a high core hydrogen abundance from the smaller star, effectively
resetting the nuclear evolution "clock" to a younger age. Elements like Li, Be
and B that can exist only in the outermost envelope of a star are severely
depleted due to envelope ejection during collisions and due to tidal forces
from the SMBH. In the absence of collisions, tidal spin-up of stars is only
important in a narrow range of periapse distances, with the tidal disruption radius.Comment: ApJ accepted, 22 pages, 19 figures. Version with high-resolution
figures, and additional animations, available at this url:
http://astrophysics.rit.edu/fantonini/tbbs2
The apsidal corotation in mean motion resonance: the 55 Cancri as an example
The inner two planets around the 55 Cancri were found to be trapped in the
3:1 mean motion resonance. In this paper, we study the dynamics of this
extra-solar planetary system. Our numerical investigation confirms the
existence of the 3:1 resonance and implies a complex orbital motion. Different
stable motion types, with and without the apsidal corotation, are found. Due to
the high eccentricities in this system, we apply a semi-analytical method based
on a new expansion of the Hamiltonian of the planar three-body problem in the
discussion. We analyse the occurrence of the apsidal corotation in this mean
motion resonance and its influence on the stability of the system.Comment: 8 pages, 9 figures. Revised version for submitting to MNRA
Ectodysplasin signaling in cutaneous appendage development: Dose, duration and diversity
The development of several skin appendages is guided by prenatal Ectodysplasin signalling. Cui et al. (this issue, 2009) report on the dose and duration of Ectodysplasin signalling required for the maintenance and morphogenesis of different appendages. They find that achievement of an intimate arrangement between epithelial and mesenchymal cell populations correlates with the acquisition of autonomy from Ectodysplasin stimulation
Formation of galactic nuclei with multiple supermassive black holes at high redshifts
We examine the formation of groups of multiple supermassive black holes
(SMBHs) in gas-poor galactic nuclei due to the high merger rate of galaxies at
high redshifts. We calculate the relative likelihood of binary, triple, and
quadruple SMBH systems, by considering the timescales for relevant processes
and combining merger trees with N-body simulations for the dynamics of stars
and SMBHs in galactic nuclei. Typical haloes today with mass M have an average mass M at
, while rare haloes with current mass M
have an average mass M at that redshift.
These cluster-size haloes are expected to host single galaxies at . We
expect about 30% galaxies within haloes with present-day mass M to contain more than two SMBHs at redshifts . For larger present-day haloes, with
M, this fraction is almost 60%. The existence of multiple SMBHs at high
redshifts can potentially explain the mass deficiencies observed in the cores
of massive elliptical galaxies, which are up to 5 times the mass of their
central BHs. Multiple SMBHs would also lead to an enhanced rate of tidal
disruption of stars, modified gravitational wave signals compared to isolated
BH binaries, and slingshot ejection of SMBHs from galaxies at high speeds in
excess of 2000 km s.Comment: 18 pages, 14 figures, 2 tables; submitted to MNRAS; comments welcom
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