From solar-like to anti-solar differential rotation in cool stars

Stellar differential rotation can be separated into two main regimes: solar-like when the equator rotates faster than the poles and anti-solar when the polar regions rotate faster than the equator. We investigate the transition between these two regimes with 3-D numerical simulations of rotating spherical shells. We conduct a systematic parameter study which also includes models from different research groups. We find that the direction of the differential rotation is governed by the contribution of the Coriolis force in the force balance, independently of the model setup (presence of a magnetic field, thickness of the convective layer, density stratification). Rapidly-rotating cases with a small Rossby number yield solar-like differential rotation, while weakly-rotating models sustain anti-solar differential rotation. Close to the transition, the two kinds of differential rotation are two possible bistable states. This study provides theoretical support for the existence of anti-solar differential rotation in cool stars with large Rossby numbers.Comment: 5 pages, 6 figures, accepted for publication in MNRA

Did the political climate exacerbate the pandemic in the U.S.?

The global pandemic that began in the United States in early 2020 continues to be a topic of controversy. The added aspect of affect polarization in the country’s political realm may have exacerbated the effects of COVID-19. In their published article in Nature Human Behaviour, Gollwitzer et. al. found that it was possible to link voting partisanship, physical distancing, and COVID-19 outcomes showing that a county’s partisanship might be used to predict the degree to which that county would socially distance and then, therefore, the rate of cases and fatalities in that error on a lagged timescale. This researcher attempted to replicate and validate the findings of an analysis conducted in the earliest months of the pandemic using approximately the same variables, models, and covariates, but over a longer span of time in the pandemic. Three possible mediator variables (physical distancing data, mask mandate data, and online sentiment data) were gathered and tested for usability in the main mediation analysis. Preliminary analysis of the data gathered did not support the assertion of sentiment or masking data would be useful to the mediation analysis due to insufficient data. Though the distancing data was significantly linked to partisanship to become a proxy, mixed models showed that pandemic dates after the period of the original analysis could not support physical distancing as a mediator for partisanship. Only the segment of the final dataset which matched the dates of the original work were processed through the same mediation analysis in STATA. Significant effects of partisanship on case growth rates were discovered, but not to the same degree as the original work

What controls the large-scale magnetic fields of M dwarfs?

Observations of active M dwarfs show a broad variety of large-scale magnetic fields encompassing dipole-dominated and multipolar geometries. We detail the analogy between some anelastic dynamo simulations and spectropolarimetric observations of 23 M stars. In numerical models, the relative contribution of inertia and Coriolis force in the global force balance -estimated by the so-called local Rossby number- is known to have a strong impact on the magnetic field geometry. We discuss the relevance of this parameter in setting the large-scale magnetic field of M dwarfs.Comment: 4 pages, 3 figures, conference proceeding, IAUS 302 'Magnetic Fields Throughout the Stellar Evolution', (26-30 Aug 2013, Biarritz, France

What controls the magnetic geometry of M dwarfs?

Context: observations of rapidly rotating M dwarfs show a broad variety of large-scale magnetic fields encompassing dipole-dominated and multipolar geometries. In dynamo models, the relative importance of inertia in the force balance -- quantified by the local Rossby number -- is known to have a strong impact on the magnetic field geometry. Aims: we aim to assess the relevance of the local Rossby number in controlling the large-scale magnetic field geometry of M dwarfs. Methods: we explore the similarities between anelastic dynamo models in spherical shells and observations of active M-dwarfs, focusing on field geometries derived from spectropolarimetric studies. To do so, we construct observation-based quantities aimed to reflect the diagnostic parameters employed in numerical models. Results: the transition between dipole-dominated and multipolar large-scale fields in early to mid M dwarfs is tentatively attributed to a Rossby number threshold. We interpret late M dwarfs magnetism to result from a dynamo bistability occurring at low Rossby number. By analogy with numerical models, we expect different amplitudes of differential rotation on the two dynamo branches.Comment: 4 pages, 4 figures, accepted for publication in A&

Antiferro-quadrupole state of orbital-degenerate Kondo lattice model with f^2 configuration

To clarify a key role of $f$ orbitals in the emergence of antiferro-quadrupole structure in PrPb$_{3}$, we investigate the ground-state property of an orbital-degenerate Kondo lattice model by numerical diagonalization techniques. In PrPb$_{3}$, Pr$^{3+}$ has a $4f^{2}$ configuration and the crystalline-electric-field ground state is a non-Kramers doublet $\Gamma_{3}$. In a $j$-$j$ coupling scheme, the $\Gamma_{3}$ state is described by two local singlets, each of which consists of two $f$ electrons with one in $\Gamma_{7}$ and another in $\Gamma_{8}$ orbitals. Since in a cubic structure, $\Gamma_{7}$ has localized nature, while $\Gamma_{8}$ orbitals are rather itinerant, we propose the orbital-degenerate Kondo lattice model for an effective Hamiltonian of PrPb$_{3}$. We show that an antiferro-orbital state is favored by the so-called double-exchange mechanism which is characteristic of multi-orbital systems.Comment: 3 pages, 3 figures, Proceedings of Skutterudite2007 (September 26-30, 2007, Kobe

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 $p_{\rm tot}$ (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 $p_{\rm tot}$, variations of up to a factor of $3$ in $p_{\rm tot}$ (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

A homomorphism between link and XXZ modules over the periodic Temperley-Lieb algebra

We study finite loop models on a lattice wrapped around a cylinder. A section of the cylinder has N sites. We use a family of link modules over the periodic Temperley-Lieb algebra EPTL_N(\beta, \alpha) introduced by Martin and Saleur, and Graham and Lehrer. These are labeled by the numbers of sites N and of defects d, and extend the standard modules of the original Temperley-Lieb algebra. Beside the defining parameters \beta=u^2+u^{-2} with u=e^{i\lambda/2} (weight of contractible loops) and \alpha (weight of non-contractible loops), this family also depends on a twist parameter v that keeps track of how the defects wind around the cylinder. The transfer matrix T_N(\lambda, \nu) depends on the anisotropy \nu and the spectral parameter \lambda that fixes the model. (The thermodynamic limit of T_N is believed to describe a conformal field theory of central charge c=1-6\lambda^2/(\pi(\lambda-\pi)).) The family of periodic XXZ Hamiltonians is extended to depend on this new parameter v and the relationship between this family and the loop models is established. The Gram determinant for the natural bilinear form on these link modules is shown to factorize in terms of an intertwiner i_N^d between these link representations and the eigenspaces of S^z of the XXZ models. This map is shown to be an isomorphism for generic values of u and v and the critical curves in the plane of these parameters for which i_N^d fails to be an isomorphism are given.Comment: Replacement of "The Gram matrix as a connection between periodic loop models and XXZ Hamiltonians", 31 page

An overview of the "Color Game" App project

The Color Game gaming app (2018–2019) invited players from all over the world to invent a visual language without words. Participants took part in a referential communication task where a Sender had to indicate a colour to a Receiver, with the help of black and white symbols. They could freely choose which other players they interacted with, and play repeatedly with their chosen contacts. This paper presents the Color Game dataset, accessible at https://osf.io/9yc25/, which records all interactions between app players. In its final cleaned-up version, the dataset contains 347,606 games by 2,535 players, from more than 100 different countries, speaking 80 different languages. This companion paper describes the app’s workings and history.1. General description 2. Preregistered predictions & projects 2.1. Preregistration process 2.2. The projects 2.2.1. FRIENDS (https://osf.io/y2vak/). 2.2.2. INFORMATION (https://osf.io/7y9pn/). 2.2.3. LANGUAGE (https://osf.io/a8bge/). 2.2.4. PRIORS (https://osf.io/dqhtv/). 2.2.5. SALIENCE (https://osf.io/f9xzq/) 2.2.6. TREES (https://osf.io/r7n32/). 2.3. Open-ended exploration 3. Open data & code 3.1. The Color Game dataset repository 3.2. Exclusion and inclusion criteria: preregistered rules 3.3. Exclusion and inclusion criteria: departures from the preregistered rules 3.4. Other datasets 3.5. Open code 4. Color Game deployment log 5. Descriptive and exploratory analyses 6. Acknowledgements 7. Creative Commons Licence 8. Data privacy Reference

The influence of shared visual context on the successful emergence of conventions in a referential communication task

Abstract Human communication is thoroughly context bound. We present two experiments investigating the importance of the shared context, that is, the amount of knowledge two interlocutors have in common, for the successful emergence and use of novel conventions. Using a referential communication task where black-and-white pictorial symbols are used to convey colors, pairs of participants build shared conventions peculiar to their dyad without experimenter feedback, relying purely on ostensive-inferential communication. Both experiments demonstrate that access to the visual context promotes more successful communication. Importantly, success improves cumulatively, supporting the view that pairs establish conventional ways of using the symbols to communicate. Furthermore, Experiment 2 suggests that dyads with access to the visual context successfully adapt the conventions built for one color space to another color space, unlike dyads lacking it. In linking experimental pragmatics with language evolution, the study illustrates the benefits of exploring the emergence of linguistic conventions using an ostensive-inferential model of communication.1. Introduction 1.1. Artificial language experiments and the emergence problem 1.2. Referential communication tasks and interaction 1.3. The current study 1.4. Ethical approval and preregistration 2. Experiment 1 2.1. Method 2.1.1. Participants 2.1.2. Materials 2.1.3. Procedure 2.1.4. Experimental task 2.2. Results 2.2.1. Does the shared visual context improve communicative success and do pairs improve over time? 2.2.2. Exploration of the questionnaires 2.3. Discussion 3. Experiment 2 3.1. Method 3.1.1. Participants 3.1.2. Materials 3.1.3. Procedure 3.2. Results 3.2.1. Can the results of Experiment 1 be replicated? 3.2.2. Does the shared visual context increase the number of conventions? 3.2.3. Are conventions developed by shared visual context pairs more generalizable? 3.3. Discussion 4. General discussion 5. Conclusio