7,966 research outputs found
Cool Stars and Space Weather
Stellar flares, winds and coronal mass ejections form the space weather. They
are signatures of the magnetic activity of cool stars and, since activity
varies with age, mass and rotation, the space weather that extra-solar planets
experience can be very different from the one encountered by the solar system
planets. How do stellar activity and magnetism influence the space weather of
exoplanets orbiting main-sequence stars? How do the environments surrounding
exoplanets differ from those around the planets in our own solar system? How
can the detailed knowledge acquired by the solar system community be applied in
exoplanetary systems? How does space weather affect habitability? These were
questions that were addressed in the splinter session "Cool stars and Space
Weather", that took place on 9 Jun 2014, during the Cool Stars 18 meeting. In
this paper, we present a summary of the contributions made to this session.Comment: Proceedings of the 18th Cambridge Workshop on Cool Stars, Stellar
Systems, and the Sun, Eds G. van Belle & H. Harris, 13 pages, 1 figur
The solar wind in time – II. 3D stellar wind structure and radio emission
In this work, we simulate the evolution of the solar wind along its main-sequence lifetime and compute its thermal radio emission. To study the evolution of the solar wind, we use a sample of solar mass stars at different ages. All these stars have observationally reconstructed magnetic maps, which are incorporated in our 3D magnetohydrodynamic simulations of their winds. We show that angular-momentum loss and mass-loss rates decrease steadily on evolutionary time-scales, although they can vary in a magnetic cycle time-scale. Stellar winds are known to emit radiation in the form of thermal bremsstrahlung in the radio spectrum. To calculate the expected radio fluxes from these winds, we solve the radiative transfer equation numerically from first principles. We compute continuum spectra across the frequency range 100 MHz to 100 GHz and find maximum radio flux densities ranging from 0.05 to 2.2 μJy. At a frequency of 1 GHz and a normalized distance of d = 10 pc, the radio flux density follows 0.24 (Ω/Ω☉)0.9 (d/[10pc])-2μJy, where Ω is the rotation rate. This means that the best candidates for stellar wind observations in the radio regime are faster rotators within distances of 10 pc, such as κ1 Ceti (0.73 μJy) and χ1 Ori (2.2 μJy). These flux predictions provide a guide to observing solar-type stars across the frequency range 0.1-100 GHz in the future using the next generation of radio telescopes, such as ngVLA and Square Kilometre Array
Erratum: The solar wind in time II: 3D stellar wind structure and radio emission
This is an erratum to the paper ‘The solar wind in time - II: 3D stellar wind structure and radio emission’, which was published in MNRAS, 483(1), 873, 2019 (Ó Fionnagáin et al. 2019)
Refined conformal spectra in the dimer model
Working with Lieb's transfer matrix for the dimer model, we point out that
the full set of dimer configurations may be partitioned into disjoint subsets
(sectors) closed under the action of the transfer matrix. These sectors are
labelled by an integer or half-integer quantum number we call the variation
index. In the continuum scaling limit, each sector gives rise to a
representation of the Virasoro algebra. We determine the corresponding
conformal partition functions and their finitizations, and observe an
intriguing link to the Ramond and Neveu-Schwarz sectors of the critical dense
polymer model as described by a conformal field theory with central charge
c=-2.Comment: 44 page
Relations between classroom disciplinary problems and student motivation : Achievement as a potential mediator?
This study examined the relation between classroom disciplinary problems in language classes, student achievement, and three facets of student motivation: competence self-perceptions, test anxiety, and engagement. The analyses were conducted with the German sample from the Progress in International Reading Literacy Study (PIRLS) 2006 (N = 7899). The results demonstrated that discipline problems are directly and negatively related to achievement and to all motivation constructs considered. In most cases, the relation between classroom disciplinary problems and motivation constructs was mediated by verbal achievement. Boys were found to report more frequent discipline problems in classrooms than girls. This study contributes to research by assessing the impact of classroom disciplinary problems using doubly latent multilevel structural equation models in order to properly disaggregate effects occurring at the student, versus classroom level
Temperature and magnetic field dependences of the elastic constants of Ni-Mn-Al magnetic Heusler alloys
We report on measurements of the adiabatic second order elastic constants of
the off-stoichiometric NiMnAl single crystalline Heusler
alloy. The variation in the temperature dependence of the elastic constants has
been investigated across the magnetic transition and over a broad temperature
range. Anomalies in the temperature behaviour of the elastic constants have
been found in the vicinity of the magnetic phase transition. Measurements under
applied magnetic field, both isothermal and variable temperature, show that the
value of the elastic constants depends on magnetic order, thus giving evidence
for magnetoelastic coupling in this alloy system.Comment: 7 pages, 5 figures. Accepted for publication in Physical the Review
Second Low Temperature Phase Transition in Frustrated UNi_4B
Hexagonal UNi_4B is magnetically frustrated, yet it orders
antiferromagnetically at T_N = 20 K. However, one third of the U-spins remain
paramagnetic below this temperature. In order to track these spins to lower
temperature, we measured the specific heat C of \unib between 100 mK and 2 K,
and in applied fields up to 9 T. For zero field there is a sharp kink in C at
330 mK, which we interpret as an indication of a second phase
transition involving paramagnetic U. The rise in between 7 K and
330 mK and the absence of a large entropy liberated at may be due to a
combination of Kondo screening effects and frustration that strongly modifies
the low T transition.Comment: 4 pages, 4 figure
- …