4,488 research outputs found
The composition and size distribution of the dust in the coma of comet Hale-Bopp
We discuss the composition and size distribution of the dust in the coma of
comet Hale-Bopp. We do this by fitting simultaneously the infrared emission
spectrum measured by the infrared space observatory (ISO) and the measured
degree of linear polarization of scattered light at various phase angles and 12
different wavelengths. The effects of particle shape on the modeled optical
properties of the dust grains are taken into account. We constrain our fit by
forcing the abundances of the major rock forming chemical elements to be solar.
The infrared spectrum at long wavelengths reveals that large grains are needed
in order to fit the spectral slope. The size and shape distribution we employ
allows us to estimate the sizes of the crystalline silicates. The ratios of the
strength of various forsterite features show that the crystalline silicate
grains in Hale-Bopp must be submicron sized. We exclude the presence of large
crystalline silicate grains in the coma. Because of this lack of large
crystalline grains combined with the fact that we do need large amorphous
grains to fit the emission spectrum at long wavelengths, we need only
approximately 4% of crystalline silicates by mass. After correcting for
possible hidden crystalline material included in large amorphous grains, our
best estimate of the total mass fraction of crystalline material is
approximately 7.5%, significantly lower than deduced in previous studies in
which the typical derived crystallinity is 20-30%. The implications of this on
the possible origin and evolution of the comet are discussed. The crystallinity
we observe in Hale-Bopp is consistent with the production of crystalline
silicates in the inner solar system by thermal annealing and subsequent radial
mixing to the comet forming region.Comment: Accepted for publication in Icaru
Genetic Locus Required for Antigenic Maturation of \u3cem\u3eRhizobium etli\u3c/em\u3e CE3 Lipopolysaccharide
Rhizobium etli modifies lipopolysaccharide (LPS) structure in response to environmental signals, such as low pH and anthocyanins. These LPS modifications result in the loss of reactivity with certain monoclonal antibodies. The same antibodies fail to recognize previously isolated R. etli mutant strain CE367, even in the absence of such environmental cues. Chemical analysis of the LPS in strain CE367 demonstrated that it lacked the terminal sugar of the wild-type O antigen, 2,3,4-tri-O-methylfucose. A 3-kb stretch of DNA, designated as lpe3, restored wild-type antigenicity when transferred into CE367. From the sequence of this DNA, five open reading frames were postulated. Site-directed mutagenesis and complementation analysis suggested that the genes were organized in at least two transcriptional units, both of which were required for the production of LPS reactive with the diagnostic antibodies. Growth in anthocyanins or at low pH did not alter the specific expression of gusA from the transposon insertion of mutant CE367, nor did the presence of multiple copies of lpe3 situated behind a strong, constitutive promoter prevent epitope changes induced by these environmental cues. Mutations of the lpe genes did not prevent normal nodule development on Phaseolus vulgaris and had very little effect on the occupation of nodules in competition with the wild-type strain
Resolving HD 100546 disc in the mid-infrared: Small inner disc and asymmetry near the gap
A region of roughly half of the solar system scale around the star HD 100546
is largely cleared of gas and dust, in contrast to the bright outer disc.
However, some material is observed in the immediate vicinity of the star. We
investigate how the dust is distributed within and outside the gap, and
constrain the disc geometry with mid-infrared interferometric observations
using VLTI/MIDI. With baseline lengths of 40m, our long baseline observations
are sensitive to the inner few AU from the star, and we combined them with
observations at shorter, 15m baselines, to probe emission beyond the gap at up
to 20AU from the star. We modelled the mid-infrared emission using radial
temperature profiles. Our model is composed of infinitesimal concentric annuli
emitting as black bodies, and it has distinct inner and outer disc components.
We derived an upper limit of 0.7AU for the radial size of the inner disc, from
our longest baseline data. This small dusty disc is separated from the edge of
the outer disc by a large, roughly 10AU wide gap. Our short baseline data place
a bright ring of emission at 11+-1AU, consistent with prior observations of the
transition region between the gap and the outer disc, known as the disc wall.
The inclination and position angle are constrained by our data to i=53+-8deg
and PA=145+-5deg. Compared to the rim and outer disc geometry this suggests
co-planarity. Brightness asymmetry is evident in both short and long baseline
data, and it is unequivocally discernible from any atmospheric or instrumental
effects. The origin of the asymmetry is consistent with the bright disc wall,
which we find to be 1-2AU wide. The gap is cleared of micron-sized dust, but we
cannot rule out the presence of larger particles and/or perturbing bodies.Comment: 12 pages, 9 figures, accepted for publication in A&
Planetary Detection Efficiency of the Magnification 3000 Microlensing Event OGLE-2004-BLG-343
OGLE-2004-BLG-343 was a microlensing event with peak magnification
A_{max}=3000+/-1100, by far the highest-magnification event ever analyzed and
hence potentially extremely sensitive to planets orbiting the lens star. Due to
human error, intensive monitoring did not begin until 43 minutes after peak, at
which point the magnification had fallen to A~1200, still by far the highest
ever observed. As the light curve does not show significant deviations due to a
planet, we place upper limits on the presence of such planets by extending the
method of Yoo et al. (2004b), which combines light-curve analysis with priors
from a Galactic model of the source and lens populations, to take account of
finite-source effects. This is the first event so analyzed for which
finite-source effects are important, and hence we develop two new techniques
for evaluating these effects. Somewhat surprisingly, we find that
OGLE-2004-BLG-343 is no more sensitive to planets than two previously analyzed
events with A_{max}~100, despite the fact that it was observed at ~12 times
higher magnification. However, we show that had the event been observed over
its peak, it would have been sensitive to almost all Neptune-mass planets over
a factor of 5 of projected separation and even would have had some sensitivity
to Earth-mass planets. This shows that some microlensing events being detected
in current experiments are sensitive to very low-mass planets. We also give
suggestions on how extremely high-magnification events can be more promptly
monitored in the future.Comment: 50 pages, 13 figures, published in The Astrophysical Journa
Complexity of decoupling and time-reversal for n spins with pair-interactions: Arrow of time in quantum control
Well-known Nuclear Magnetic Resonance experiments show that the time
evolution according to (truncated) dipole-dipole interactions between n spins
can be inverted by simple pulse sequences. Independent of n, the reversed
evolution is only two times slower than the original one. Here we consider more
general spin-spin couplings with long range. We prove that some are
considerably more complex to invert since the number of required time steps and
the slow-down of the reversed evolutions are necessarily of the order n.
Furthermore, the spins have to be addressed separately. We show for which
values of the coupling parameters the phase transition between simple and
complex time-reversal schemes occurs.Comment: Completely rewritten, new lower bounds on the number of time steps,
applications and references adde
Spin-1/2 particles moving on a 2D lattice with nearest-neighbor interactions can realize an autonomous quantum computer
What is the simplest Hamiltonian which can implement quantum computation
without requiring any control operations during the computation process? In a
previous paper we have constructed a 10-local finite-range interaction among
qubits on a 2D lattice having this property. Here we show that
pair-interactions among qutrits on a 2D lattice are sufficient, too, and can
also implement an ergodic computer where the result can be read out from the
time average state after some post-selection with high success probability.
Two of the 3 qutrit states are given by the two levels of a spin-1/2 particle
located at a specific lattice site, the third state is its absence. Usual
hopping terms together with an attractive force among adjacent particles induce
a coupled quantum walk where the particle spins are subjected to spatially
inhomogeneous interactions implementing holonomic quantum computing. The
holonomic method ensures that the implemented circuit does not depend on the
time needed for the walk.
Even though the implementation of the required type of spin-spin interactions
is currently unclear, the model shows that quite simple Hamiltonians are
powerful enough to allow for universal quantum computing in a closed physical
system.Comment: More detailed explanations including description of a programmable
version. 44 pages, 12 figures, latex. To appear in PR
Binarity as a key factor in protoplanetary disk evolution: Spitzer disk census of the eta Chamaeleontis cluster
The formation of planets is directly linked to the evolution of the
circumstellar (CS) disk from which they are born. The dissipation timescales of
CS disks are, therefore, of direct astrophysical importance in evaluating the
time available for planet formation. We employ Spitzer Space Telescope spectra
to complete the CS disk census for the late-type members of the ~8 Myr-old eta
Chamaeleontis star cluster. Of the 15 K- and M-type members, eight show excess
emission. We find that the presence of a CS disk is anti-correlated with
binarity, with all but one disk associated with single stars. With nine single
stars in total, about 80% retain a CS disk. Of the six known or suspected close
binaries the only CS disk is associated with the primary of RECX 9. No
circumbinary disks have been detected. We also find that stars with disks are
slow rotators with surface values of specific angular momentum j = 2-15 j_sun.
All high specific angular momentum systems with j = 20-30 j_sun are confined to
the primary stars of binaries. This provides novel empirical evidence for
rotational disk locking and again demonstrates the much shorter disk lifetimes
in close binary systems compared to single star systems. We estimate the
characteristic mean disk dissipation timescale to be ~5 Myr and ~9 Myr for the
binary and single star systems, respectively.Comment: Accepted by ApJ
Radiative transfer in very optically thick circumstellar disks
In this paper we present two efficient implementations of the diffusion
approximation to be employed in Monte Carlo computations of radiative transfer
in dusty media of massive circumstellar disks. The aim is to improve the
accuracy of the computed temperature structure and to decrease the computation
time. The accuracy, efficiency and applicability of the methods in various
corners of parameter space are investigated. The effects of using these methods
on the vertical structure of the circumstellar disk as obtained from
hydrostatic equilibrium computations are also addressed. Two methods are
presented. First, an energy diffusion approximation is used to improve the
accuracy of the temperature structure in highly obscured regions of the disk,
where photon counts are low. Second, a modified random walk approximation is
employed to decrease the computation time. This modified random walk ensures
that the photons that end up in the high-density regions can quickly escape to
the lower density regions, while the energy deposited by these photons in the
disk is still computed accurately. A new radiative transfer code, MCMax, is
presented in which both these diffusion approximations are implemented. These
can be used simultaneously to increase both computational speed and decrease
statistical noise. We conclude that the diffusion approximations allow for fast
and accurate computations of the temperature structure, vertical disk structure
and observables of very optically thick circumstellar disks.Comment: Accepted for publication in A&
RoboTAP: Target priorities for robotic microlensing observations
Context. The ability to automatically select scientifically-important transient events from an alert stream of many such events, and to conduct follow-up observations in response, will become increasingly important in astronomy. With wide-angle time domain surveys pushing to fainter limiting magnitudes, the capability to follow-up on transient alerts far exceeds our follow-up telescope resources, and effective target prioritization becomes essential. The RoboNet-II microlensing program is a pathfinder project, which has developed an automated target selection process (RoboTAP) for gravitational microlensing events, which are observed in real time using the Las Cumbres Observatory telescope network.
Aims. Follow-up telescopes typically have a much smaller field of view compared to surveys, therefore the most promising microlensing events must be automatically selected at any given time from an annual sample exceeding 2000 events. The main challenge is to select between events with a high planet detection sensitivity, with the aim of detecting many planets and characterizing planetary anomalies.
Methods. Our target selection algorithm is a hybrid system based on estimates of the planet detection zones around a microlens. It follows automatic anomaly alerts and respects the expected survey coverage of specific events.
Results. We introduce the RoboTAP algorithm, whose purpose is to select and prioritize microlensing events with high sensitivity to planetary companions. In this work, we determine the planet sensitivity of the RoboNet follow-up program and provide a working example of how a broker can be designed for a real-life transient science program conducting follow-up observations in response to alerts; we explore the issues that will confront similar programs being developed for the Large Synoptic Survey Telescope (LSST) and other time domain surveys
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