Modeling the Rise of Fibril Magnetic Fields in Fully Convective Stars

Many fully convective stars exhibit a wide variety of surface magnetism, including starspots and chromospheric activity. The manner by which bundles of magnetic field traverse portions of the convection zone to emerge at the stellar surface is not especially well understood. In the Solar context, some insight into this process has been gleaned by regarding the magnetism as consisting partly of idealized thin flux tubes (TFT). Here, we present the results of a large set of TFT simulations in a rotating spherical domain of convective flows representative of a 0.3 solar-mass, main-sequence star. This is the first study to investigate how individual flux tubes in such a star might rise under the combined influence of buoyancy, convection, and differential rotation. A time-dependent hydrodynamic convective flow field, taken from separate 3D simulations calculated with the anelastic equations, impacts the flux tube as it rises. Convective motions modulate the shape of the initially buoyant flux ring, promoting localized rising loops. Flux tubes in fully convective stars have a tendency to rise nearly parallel to the rotation axis. However, the presence of strong differential rotation allows some initially low latitude flux tubes of moderate strength to develop rising loops that emerge in the near-equatorial region. Magnetic pumping suppresses the global rise of the flux tube most efficiently in the deeper interior and at lower latitudes. The results of these simulations aim to provide a link between dynamo-generated magnetic fields, fluid motions, and observations of starspots for fully convective stars.Comment: 20 pages, 15 figures, accepted to Astrophysical Journa

PERSISTENT POVERTY ACROSS THE RURAL-URBAN CONTINUUM

Persistent poverty is overwhelmingly rural and is very geographically concentrated. We have redefined the USDA ERS persistent poverty classification to include metropolitan counties meeting the 20 percent or higher poor criterion and we extend the time period through the 2000 Census. With this updated definition, there are 382 counties that have had poverty rates of 20 percent or more in each decennial census between 1960 and 2000. These persistent poverty counties are overwhelmingly rural (95 percent) and disproportionately rural (16 percent of nonmetro counties versus 2 percent of metro). The local economic environment in persistent poverty counties is much less favorable than in the nation as a whole. Per capita income is lower and unemployment rates higher in persistent poverty counties. Employment is more concentrated in services, extractive, construction/maintenance, and production/transportation occupations. Residents of persistent poverty counties tend to have lower education levels, and persistent poverty counties generally have larger shares of minority populations. The number of persistent poverty counties reduced considerably during the 1990s, but the "leavers" were disproportionately metropolitan, making persistent poverty increasingly a rural problem. Persistent poverty is overwhelmingly rural and it is very concentrated geographically. In this paper, we examine these striking regularities in U.S. economic geography, seeking to understand the causes and dynamics of poverty across the rural urban continuum. We also consider how alternative characterizations of "persistent poverty" and "rural and urban" might deepen our understanding of poverty and place. The paper has four sections. In the first, we examine how poverty and persistent poverty vary across the Rural Urban Continuum Codes and Urban Influence Codes developed by the USDA Economic Research Service (ERS). We start with the very useful "persistent poverty" classification developed by ERS that defines nonmetropolitan counties as persistent poverty counties if the poverty rate is 20 percent or higher in each decennial census between 1960 and 1990.We redefine persistent poverty to include metropolitan counties meeting the 20 percent or more poor criterion and we extend the time period through the 2000 Census. We also examine where poverty is concentrated in the United States and how persistent poverty varies across the new Core Based Statistical Area definitions for counties, developed recently by the Office of Management and Budget. In the second section, we examine how the demographic characteristics of the population vary across the rural-urban spectrum, comparing persistent poverty county demographics with those of all counties. The third section examines the dynamics of poverty and place. We examine the location, rurality and demographics of counties that escaped persistent poverty statues between 1990 and 2000, and how those characteristics compare to counties that remained in persistent poverty. We then identify the new entrants into high poverty since 1960. In the fourth section, we consider implications of reconceptualizing both "persistent poverty" and "rural and urban diversity". First, we explore the "persistent poverty" county classification, and how alternative definitions of persistent poverty counties might alter the conclusions one reaches about the geography of persistent poverty. We do this by exploring how defining persistent poverty with a different base year such as 1970 or 1980 affects the number of "persistent poverty" counties. We then explore what happens to "persistent poverty" if we raise the poverty threshold to 30 and 40 percent in defining persistent poverty counties. We then examine how conclusions about rural and urban persistent poverty change if one looks at poverty persistence in individual households rather than counties. Using PSID data, this analysis examines rates of persistent household poverty by looking at how the percent of households who remain in poverty for all 5 years during the 1993-98 period varies across central metro county to remote rural county continuum (an aggregation of Beale codes). Finally, we briefly explore how conclusions about the geography of poverty change if one divides metropolitan areas into "central city" and "suburb", and nonmetropolitan areas into "adjacent" and "nonadjacent."Food Security and Poverty,

Anti-solar differential rotation on the active sub-giant HU Virginis

Measuring surface differential rotation (DR) on different types of stars is important when characterizing the underlying stellar dynamo. It has been suggested that anti-solar DR laws can occur when strong meridional flows exist. We aim to investigate the differential surface rotation on the primary star of the RS CVn binary HU Vir by tracking its starspot distribution as a function of time. We also aim to recompute and update the values for several system parameters of the triple system HU Vir (close and wide orbits). Time-series high-resolution spectroscopy for four continuous months was obtained with the 1.2-m robotic STELLA telescope. Nine consecutive Doppler images were reconstructed from these data, using our line-profile inversion code iMap. An image cross-correlation method was applied to derive the surface differential-rotation law for HU Vir. New orbital elements for the close and the wide orbits were computed using our new STELLA radial velocities (RVs) combined with the RV data available in the literature. Photometric observations were performed with the Amadeus Automatic Photoelectric Telescope (APT), providing contemporaneous Johnson-Cousins $V$ and $I$ data for approximately 20 years. This data was used to determine the stellar rotation period and the active longitudes. We confirm anti-solar DR with a surface shear parameter $\alpha$ of -0.029 $\pm$ 0.005 and -0.026 $\pm$ 0.009, using single-term and double-term differential rotation laws, respectively. The best fit is achieved assuming a solar-like double-term law with a lap time of $\approx$ 400 d. Our orbital solutions result in a period of 10.387678 $\pm$ 0.000003 days for the close orbit and 2726 $\pm$ 7 d ($\approx$ 7.5 yr) for the wide orbit. A Lomb-Scarge (L-S) periodogram of the pre-whitened $V$-band data reveals a strong single peak providing a rotation period of 10.391 $\pm$ 0.008 d.Comment: Accepted for publication in A&

A possible solution of the grain boundary problem for applications of high-Tc superconductors

It is shown that the critical current density of high-Tc wires can be greatly enhanced by using a threefold approach, which consists of grain alignment, doping, and optimization of the grain architecture. According to model calculations, current densities of 4x10^6 A/cm2 can be achieved for an average grain alignment of 10 degree at 77K. Based on this approach, a road to competitive high-Tc cables is proposed.Comment: 3 pages, 5 figure

Theoretical limits on magnetic field strengths in low-mass stars

Observations have suggested that some low-mass stars have larger radii than predicted by 1-D structure models. Some theoretical models have invoked very strong interior magnetic fields (of order 1 MG or more) as a possible cause of such large radii. Whether fields of that strength could in principle by generated by dynamo action in these objects is unclear, and we do not address the matter directly. Instead, we examine whether such fields could remain in the interior of a low mass object for a significant time, and whether they would have any other obvious signatures. First, we estimate timescales for the loss of strong fields by magnetic buoyancy instabilities. We consider a range of field strengths and simple morphologies, including both idealized flux tubes and smooth layers of field. We confirm some of our analytical estimates using thin flux tube magnetohydrodynamic (MHD) simulations of the rise of buoyant fields in a fully-convective M-dwarf. Separately, we consider the Ohmic dissipation of such fields. We find that dissipation provides a complementary constraint to buoyancy: while small-scale, fibril fields might be regenerated faster than they rise, the dissipative heating associated with such fields would in some cases greatly exceed the luminosity of the star. We show how these constraints combine to yield limits on the internal field strength and morphology in low-mass stars. In particular, we find that for stars of 0.3 solar masses, no fields in flux tubes stronger than about 800 kG are simultaneously consistent with both constraints.Comment: 19 pages, 10 figures, accepted to Ap

Time-series Doppler images and surface differential rotation of the effectively-single rapidly-rotating K-giant KU Pegasi

According to most stellar dynamo theories, differential rotation (DR) plays a crucial role for the generation of toroidal magnetic fields. Numerical models predict surface differential rotation to be anti-solar for rapidly-rotating giant stars, i.e., their surface angular velocity could increase with stellar latitude. However, surface differential rotation has been derived only for a handful of individual giant stars to date. The spotted surface of the K-giant KU Pegasi is investigated in order to detect its time evolution and quantify surface differential rotation. We present altogether 11 Doppler images from spectroscopic data collected with the robotic telescope STELLA between 2006--2011. All maps are obtained with the surface reconstruction code iMap. Differential rotation is extracted from these images by detecting systematic (latitude-dependent) spot displacements. We apply a cross-correlation technique to find the best differential rotation law. The surface of KU Peg shows cool spots at all latitudes and one persistent warm spot at high latitude. A small cool polar spot exists for most but not all of the epochs. Re-identification of spots in at least two consecutive maps is mostly possible only at mid and high latitudes and thus restricts the differential-rotation determination mainly to these latitudes. Our cross-correlation analysis reveals solar-like differential rotation with a surface shear of $\alpha=+0.040\pm0.006$, i.e., approximately five times weaker than on the Sun. We also derive a more accurate and consistent set of stellar parameters for KU Peg including a small Li abundance of ten times less than solar.Comment: 13 pages, 12 figures, accepted for publication in A&

Editorial: Towards an Archaeology of Television

Over the last few years, ‘media archaeology’ has evolved from a marginal topic to an academic approach en vogue. Under its banner, conferences and publications bring together scholars from different disciplines who, revisiting the canon of media history and theory, emphasize the necessity for renewed historiographical narratives. Despite, or maybe because of profuse debates, media archaeology remains a loosely defined playground for researchers working at the intersection of history and theory. Far from offering uniform principles or constituting a homogeneous field, its prominent authors – Friedrich Kittler and Wolfgang Ernst, Siegfried Zielinksi, Jussi Parrika and Erkki Huhtamo, to name just a few – distinguish themselves by their heterogeneity regarding methodology and theoretical focus