5,717 research outputs found
Methylobacillus flagellatus KT contains a novel cbo-type cytochrome oxidase
AbstractThe o-type oxidase from the methanol-grown obligate methylotroph Methylobacillus flagellatus KT has been purified to homogeneity. The complex is composed of four subunits (57, 40, 35 and 30 kDa). It contains six haems (4C:1B:1O) and one copper atom per molecule. It is proposed that the haem O-CuB binuclear centre and a low-spin haem B are located in subunit I (57 kDa), two haems C reside in the cytochrome c homodimer (35 kDa), two haems C belong to the dihaem cytochrome c (30 kDa). The presented data provide evidence that cytochrome cbo is a novel representative of the haem–copper oxidase superfamily
The Kambarata 2 blast-fill dam, Kyrgyz Republic: blast event, geophysical monitoring and dam structure modelling
Abstract Background The blast-and earth-fill dam of the Kambarata 2 hydropower station is
situated in the seismically active Central Tien Shan region of the Kyrgyz Republic. More than
70% of the dam volume was produced during a blast event on December 22, 2009. In 2010–
2011, dam construction was completed after earth filling on top of the blasted material and
installing concrete and clay screens together with bentonite grouts. A geophysical survey
had been completed in 2012–2013, mainly to monitor the resistivities inside the dam. ..
On the origin of ionising photons emitted by T Tauri stars
We address the issue of the production of Lyman continuum photons by T Tauri
stars, in an attempt to provide constraints on theoretical models of disc
photoionisation. By treating the accretion shock as a hotspot on the stellar
surface we show that Lyman continuum photons are produced at a rate
approximately three orders of magnitude lower than that produced by a
corresponding black body, and that a strong Lyman continuum is only emitted for
high mass accretion rates. When our models are extended to include a column of
material accreting on to the hotspot we find that the accretion column is
extremely optically thick to Lyman continuum photons. Further, we find that
radiative recombination of hydrogen atoms within the column is not an efficient
means of producing photons with energies greater than 13.6eV, and find that an
accretion column of any conceivable height suppresses the emission of Lyman
continuum photons to a level below or comparable to that expected from the
stellar photosphere. The photospheric Lyman continuum is itself much too weak
to affect disc evolution significantly, and we find that the Lyman continuum
emitted by an accretion shock is similarly unable to influence disc evolution
significantly. This result has important consequences for models which use
photoionisation as a mechanism to drive the dispersal of circumstellar discs,
essentially proving that an additional source of Lyman continuum photons must
exist if disc photoionisation is to be significant.Comment: 6 pages, 4 figures. Accepted for publication in MNRA
Protoplanetary disc evolution and dispersal: the implications of X-ray photoevaportion
(Abridged) We explore the role of X-ray photoevaporation in the evolution and
dispersal of viscously evolving T-Tauri discs. We show that the X-ray
photoevaporation wind rates scale linearly with X-ray luminosity, such that the
observed range of X-ray luminosities for solar-type T-Tauri stars (10e28-10e31
erg\s) gives rise to vigorous disc winds with rates of order 10e-10-10e-7
M_sun/yr. We use the wind solutions from radiation-hydrodynamic models, coupled
to a viscous evolution model to construct a population synthesis model so that
we may study the physical properties of evolving discs and so-called
`transition discs'. Current observations of disc lifetimes and accretion rates
can be matched by our model assuming a viscosity parameter alpha = 2.5e-3. Our
models confirm that X-rays play a dominant role in the evolution and dispersal
of protoplanetary discs giving rise to the observed diverse population of inner
hole `transition' sources which include those with massive outer discs, those
with gas in their inner holes and those with detectable accretion signatures.
To help understand the nature of observed transition discs we present a
diagnostic diagram based on accretion rates versus inner hole sizes that
demonstrate that, contrary to recent claims, many of the observed accreting and
non accreting transition discs can easily be explained by X-ray
photoevaporation. Finally, we confirm the conjecture of Drake et al. (2009),
that accretion is suppressed by the X-rays through `photoevaporation starved
accretion' and predict this effect can give rise to a negative correlation
between X-ray luminosity and accretion rate, as reported in the Orion data.Comment: Figure 12 and 13 have been updated. In the original version the
results from an unused model run were plotted by mistak
From discs to planetesimals I: evolution of gas and dust discs
I review the processes that shape the evolution of protoplanetary discs
around young, solar-mass stars. I first discuss observations of protoplanetary
discs, and note in particular the constraints these observations place on
models of disc evolution. The processes that affect the evolution of gas discs
are then discussed, with the focus in particular on viscous accretion and
photoevaporation, and recent models which combine the two. I then discuss the
dynamics and growth of dust grains in discs, considering models of grain
growth, the gas-grain interaction and planetesimal formation, and review recent
research in this area. Lastly, I consider the so-called "transitional" discs,
which are thought to be observed during disc dispersal. Recent observations and
models of these systems are reviewed, and prospects for using statistical
surveys to distinguish between the various proposed models are discussed.Comment: 28 pages, 9 figures. Refereed review chapter for proceedings of VLTI
summer school on "Circumstellar discs and planets at very high angular
resolution", to appear in New Astronomy Reviews. See http://www.vlti.org/ for
more detail
Constraints on the ionizing flux emitted by T Tauri stars
We present the results of an analysis of ultraviolet observations of T Tauri
Stars (TTS). By analysing emission measures taken from the literature we derive
rates of ionizing photons from the chromospheres of 5 classical TTS in the
range ~10^41-10^44 photons/s, although these values are subject to large
uncertainties. We propose that the HeII/CIV line ratio can be used as a
reddening-independent indicator of the hardness of the ultraviolet spectrum
emitted by TTS. By studying this line ratio in a much larger sample of objects
we find evidence for an ionizing flux which does not decrease, and may even
increase, as TTS evolve. This implies that a significant fraction of the
ionizing flux from TTS is not powered by the accretion of disc material onto
the central object, and we discuss the significance of this result and its
implications for models of disc evolution. The presence of a significant
ionizing flux in the later stages of circumstellar disc evolution provides an
important new constraint on disc photoevaporation models.Comment: 8 pages, 5 figures. Accepted for publication in MNRA
Semisimplicity of the quantum cohomology for smooth Fano toric varieties associated with facet symmetric polytopes
The degree zero part of the quantum cohomology algebra of a smooth Fano toric
symplectic manifold is determined by the superpotential function, W, of its
moment polytope. In particular, this algebra is semisimple, i.e. splits as a
product of fields, if and only if all the critical points of W are
non-degenerate. In this paper we prove that this non-degeneracy holds for all
smooth Fano toric varieties with facet-symmetric duals to moment polytopes.Comment: 16 pages; corrected version, published in Electron. Res. Announc.
Math. Sc
Two populations of transition discs?
We examine the distribution of transition discs as a function of mm flux. We
confirm that as expected in any model in which most primordial discs turn into
transition discs and in which mm flux declines with time, transition discs have
lower mm fluxes on average than primordial discs. However, we find that the
incidence of transition discs does not, as expected, fall monotonically towards
large mm fluxes and we investigate the hypothesis that these mm bright
transition discs may have a distinct physical origin. We find that mm bright
transition discs occupy a separate region of parameter space. Transition discs
in the bright mm sub-sample have systematically higher accretion rates and
inner hole radii than those in the faint mm sub-sample, along with being
systematically weighted to earlier spectral types.Comment: 5 pages, 5 figures, accepted version: mnras letter
Theoretical spectra of photoevaporating protoplanetary discs: An atlas of atomic and low-ionisation emission lines
We present a calculation of the atomic and low-ionisation emission line
spectra of photoevaporating protoplanetary discs. Line luminosities and
profiles are obtained from detailed photoionisation calculations of the disc
and wind structures surrounding young active solar-type stars. The disc and
wind density and velocity fields were obtained from the recently developed
radiation-hydrodynamic models of Owen et al., that include stellar X-ray and
EUV irradiation of protoplanetary discs at various stages of clearing, from
primordial sources to inner hole sources of various hole sizes.
Our models compare favourably with currently available observations, lending
support to an X-ray driven photoevaporation model for disc dispersal. In
particular, we find that X-rays drive a warm, predominantly neutral flow where
the OI 6300A line can be produced by neutral hydrogen collisional excitation.
Our models can, for the first time, provide a very good match to both
luminosities and profiles of the low-velocity component of the OI 6300A line
and other forbidden lines observed by Hartigan et al., which covered a large
sample of T-Tauri stars.
We find that the OI 6300A and the NeII 12.8um lines are predominantly
produced in the X-ray-driven wind and thus appear blue-shifted by a few km/s
for some of the systems when observed at non-edge-on inclinations. We note
however that blue-shifts are only produced under certain conditions: X-ray
luminosity, spectral shape and inner hole size all affect the location of the
emitting region and the physical conditions in the wind. We caution therefore
that while a blueshifted line is a tell-tale sign of an outflow, the lack of a
blueshift should not be necessarily interpreted as a lack of outflow.Comment: 18 pages, 7 figures, accepted to be published in MNRAS - changes in
the revised version: reference list update
The blue stragglers formed via mass transfer in old open clusters
In this paper, we present the simulations for the primordial blue stragglers
in the old open cluster M67 based on detailed modelling of the evolutionary
processes. The principal aim is to discuss the contribution of mass transfer
between the components of close binaries to the blue straggler population in
M67. First, we followed the evolution of a binary of 1.4M+0.9M.
The synthetic evolutionary track of the binary system revealed that a
primordial blue straggler had a long lifetime in the observed blue straggler
region of color-magnitude diagram. Second, a grid of models for close binary
systems experiencing mass exchange were computed from 1Gyr to 6Gyr in order to
account for primordial blue-straggler formation in a time sequence. Based on
such a grid, Monte-Carlo simulations were applied for the old open cluster M67.
Adopting appropriate orbital parameters, 4 primordial blue stragglers were
predicted by our simulations. This was consistent with the observational fact
that only a few blue stragglers in M67 were binaries with short orbital
periods. An upper boundary of the primordial blue stragglers in the
color-magnitude diagram (CMD) was defined and could be used to distinguish blue
stragglers that were not formed via mass exchange. Using the grid of binary
models, the orbital periods of the primordial BSs could be predicted. Compared
with the observations, it is clear that the mechanism discussed in this work
alone cannot fully predict the blue straggler population in M67. There must be
several other processes also involved in the formation of the observed blue
stragglers in M67.Comment: 11 pages, 6 figures, A&A accepte
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