4,563 research outputs found
M-dwarf stellar winds: the effects of realistic magnetic geometry on rotational evolution and planets
We perform three-dimensional numerical simulations of stellar winds of
early-M dwarf stars. Our simulations incorporate observationally reconstructed
large-scale surface magnetic maps, suggesting that the complexity of the
magnetic field can play an important role in the angular momentum evolution of
the star, possibly explaining the large distribution of periods in field dM
stars, as reported in recent works. In spite of the diversity of the magnetic
field topologies among the stars in our sample, we find that stellar wind
flowing near the (rotational) equatorial plane carries most of the stellar
angular momentum, but there is no preferred colatitude contributing to mass
loss, as the mass flux is maximum at different colatitudes for different stars.
We find that more non-axisymmetric magnetic fields result in more asymmetric
mass fluxes and wind total pressures (defined as the sum of
thermal, magnetic and ram pressures). Because planetary magnetospheric sizes
are set by pressure equilibrium between the planet's magnetic field and , variations of up to a factor of in (as found in the
case of a planet orbiting at several stellar radii away from the star) lead to
variations in magnetospheric radii of about 20 percent along the planetary
orbital path. In analogy to the flux of cosmic rays that impact the Earth,
which is inversely modulated with the non-axisymmetric component of the total
open solar magnetic flux, we conclude that planets orbiting M dwarf stars like
DT~Vir, DS~Leo and GJ~182, which have significant non-axisymmetric field
components, should be the more efficiently shielded from galactic cosmic rays,
even if the planets lack a protective thick atmosphere/large magnetosphere of
their own.Comment: 16 pages, 9 figures, to appear in MNRA
DeepWalk: Online Learning of Social Representations
We present DeepWalk, a novel approach for learning latent representations of
vertices in a network. These latent representations encode social relations in
a continuous vector space, which is easily exploited by statistical models.
DeepWalk generalizes recent advancements in language modeling and unsupervised
feature learning (or deep learning) from sequences of words to graphs. DeepWalk
uses local information obtained from truncated random walks to learn latent
representations by treating walks as the equivalent of sentences. We
demonstrate DeepWalk's latent representations on several multi-label network
classification tasks for social networks such as BlogCatalog, Flickr, and
YouTube. Our results show that DeepWalk outperforms challenging baselines which
are allowed a global view of the network, especially in the presence of missing
information. DeepWalk's representations can provide scores up to 10%
higher than competing methods when labeled data is sparse. In some experiments,
DeepWalk's representations are able to outperform all baseline methods while
using 60% less training data. DeepWalk is also scalable. It is an online
learning algorithm which builds useful incremental results, and is trivially
parallelizable. These qualities make it suitable for a broad class of real
world applications such as network classification, and anomaly detection.Comment: 10 pages, 5 figures, 4 table
Magnetic field and wind of Kappa Ceti: towards the planetary habitability of the young Sun when life arose on Earth
We report magnetic field measurements for Kappa1~Cet, a proxy of the young
Sun when life arose on Earth. We carry out an analysis of the magnetic
properties determined from spectropolarimetric observations and reconstruct its
large-scale surface magnetic field to derive the magnetic environment, stellar
winds and particle flux permeating the interplanetary medium around Kappa1~Cet.
Our results show a closer magnetosphere and mass-loss rate of Mdot = 9.7 x
10^{-13} Msol/yr, i.e., a factor 50 times larger than the current solar wind
mass-loss rate, resulting in a larger interaction via space weather
disturbances between the stellar wind and a hypothetical young-Earth analogue,
potentially affecting the planet's habitability. Interaction of the wind from
the young Sun with the planetary ancient magnetic field may have affected the
young Earth and its life conditionsComment: 6 pages, 5 figures, Published at the Astrophysical Journal Letters
(ApJL): Manuscript #LET3358
Powerful Winds from Low-Mass Stars: V374 Peg
The rapid rotation (P=0.44 d) of the M dwarf V374Peg (M4) along with its
intense magnetic field point toward magneto-centrifugal acceleration of a
coronal wind. In this work, we investigate the structure of the wind of V374Peg
by means of 3D magnetohydrodynamical (MHD) numerical simulations. For the first
time, an observationally derived surface magnetic field map is implemented in
MHD models of stellar winds for a low mass star. We show that the wind of
V374Peg deviates greatly from a low-velocity, low-mass-loss rate solar-type
wind. We find general scaling relations for the terminal velocities, mass-loss
rates, and spin-down times of highly magnetized M dwarfs. In particular, for
V374Peg, our models show that terminal velocities across a range of stellar
latitudes reach ~(1500-2300) n_{12}^{-1/2} km/s, where n_{12} is the coronal
wind base density in units of 10^{12} cm^{-3}, while the mass-loss rates are
about 4 x 10^{-10} n_{12}^{1/2} Msun/yr. We also evaluate the angular-momentum
loss of V374Peg, which presents a rotational braking timescale ~28
n_{12}^{-1/2} Myr. Compared to observationally derived values from period
distributions of stars in open clusters, this suggests that V374Peg may have
low coronal base densities (< 10^{11} cm^{-3}). We show that the wind ram
pressure of V374Peg is about 5 orders of magnitude larger than for the solar
wind. Nevertheless, a small planetary magnetic field intensity (~ 0.1G) is able
to shield a planet orbiting at 1 AU against the erosive effects of the stellar
wind. However, planets orbiting inside the habitable zone of V374Peg, where the
wind ram pressure is higher, might be facing a more significant atmospheric
erosion. In that case, higher planetary magnetic fields of, at least, about
half the magnetic field intensity of Jupiter, are required to protect the
planet's atmosphere.Comment: 13 pages, 5 figures, 1 table. MNRAS in pres
Fibers and global geometry of functions
Since the seminal work of Ambrosetti and Prodi, the study of global folds was
enriched by geometric concepts and extensions accomodating new examples. We
present the advantages of considering fibers, a construction dating to Berger
and Podolak's view of the original theorem. A description of folds in terms of
properties of fibers gives new perspective to the usual hypotheses in the
subject. The text is intended as a guide, outlining arguments and stating
results which will be detailed elsewhere
Mitogenome Phylogenetics: The Impact of Using Single Regions and Partitioning Schemes on Topology, Substitution Rate and Divergence Time Estimation
The availability of mitochondrial genome sequences is growing as a result of recent technological advances in molecular biology. In phylogenetic analyses, the complete mitogenome is increasingly becoming the marker of choice, usually providing better phylogenetic resolution and precision relative to traditional markers such as cytochrome b (CYTB) and the control region (CR). In some cases, the differences in phylogenetic estimates between mitogenomic and single-gene markers have yielded incongruent conclusions. By comparing phylogenetic estimates made from different genes, we identified the most informative mitochondrial regions and evaluated the minimum amount of data necessary to reproduce the same results as the mitogenome. We compared results among individual genes and the mitogenome for recently published complete mitogenome datasets of selected delphinids (Delphinidae) and killer whales (genus Orcinus). Using Bayesian phylogenetic methods, we investigated differences in estimation of topologies, divergence dates, and clock-like behavior among genes for both datasets. Although the most informative regions were not the same for each taxonomic group (COX1, CYTB, ND3 and ATP6 for Orcinus, and ND1, COX1 and ND4 for Delphinidae), in both cases they were equivalent to less than a quarter of the complete mitogenome. This suggests that gene information content can vary among groups, but can be adequately represented by a portion of the complete sequence. Although our results indicate that complete mitogenomes provide the highest phylogenetic resolution and most precise date estimates, a minimum amount of data can be selected using our approach when the complete sequence is unavailable. Studies based on single genes can benefit from the addition of a few more mitochondrial markers, producing topologies and date estimates similar to those obtained using the entire mitogenome
Kondo Effects and Multipolar Order in the cubic PrTr2Al20 (Tr=Ti, V)
Our single crystal study reveals that PrTr2Al20 (Tr = Ti and V) provides the
first examples of a cubic {\Gamma}3 nonmagnetic ground doublet system that
shows the Kondo effect including a -ln T dependent resistivity. The {\Gamma}3
quadrupolar moments in PrV2Al20 induce anomalous metallic behavior through
hybridization with conduction electrons, such as T^{1/2} dependent resistivity
and susceptibility below ~ 20 K down to its ordering temperature T_O = 0.6 K.
In PrTi2Al20, however, quadrupoles are well-localized and exhibit an order at
T_O = 2.0 K. Stronger Kondo coupling in PrV2Al20 than in PrTi2Al20 suppresses
quadrupolar ordering, and instead promotes hybridization between the {\Gamma}3
doublet and conduction electrons, leading to most likely the quadrupolar Kondo
effect.Comment: 12 pages, 4 figure
A polynomial bound for untangling geometric planar graphs
To untangle a geometric graph means to move some of the vertices so that the
resulting geometric graph has no crossings. Pach and Tardos [Discrete Comput.
Geom., 2002] asked if every n-vertex geometric planar graph can be untangled
while keeping at least n^\epsilon vertices fixed. We answer this question in
the affirmative with \epsilon=1/4. The previous best known bound was
\Omega((\log n / \log\log n)^{1/2}). We also consider untangling geometric
trees. It is known that every n-vertex geometric tree can be untangled while
keeping at least (n/3)^{1/2} vertices fixed, while the best upper bound was
O(n\log n)^{2/3}. We answer a question of Spillner and Wolff [arXiv:0709.0170
2007] by closing this gap for untangling trees. In particular, we show that for
infinitely many values of n, there is an n-vertex geometric tree that cannot be
untangled while keeping more than 3(n^{1/2}-1) vertices fixed. Moreover, we
improve the lower bound to (n/2)^{1/2}.Comment: 14 pages, 7 figure
Magnetoelastic effects in Jahn-Teller distorted CrF and CuF studied by neutron powder diffraction
We have studied the temperature dependence of crystal and magnetic structures
of the Jahn-Teller distorted transition metal difluorides CrF and CuF
by neutron powder diffraction in the temperature range 2-280 K. The lattice
parameters and the unit cell volume show magnetoelastic effects below the
N\'eel temperature. The lattice strain due to the magnetostriction effect
couples with the square of the order parameter of the antiferromagnetic phase
transition. We also investigated the temperature dependence of the Jahn-Teller
distortion which does not show any significant effect at the antiferromagnetic
phase transition but increases linearly with increasing temperature for CrF
and remains almost independent of temperature in CuF. The magnitude of
magnetovolume effect seems to increase with the low temperature saturated
magnetic moment of the transition metal ions but the correlation is not at all
perfect
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