Search results for "Line (formation)"
showing 4 items of 4 documents
Modeling X-ray emission from stellar coronae
2008
By extrapolating from observationally derived surface magnetograms of low-mass stars we construct models of their coronal magnetic fields and compare the 3D field geometry with axial multipoles. AB Dor, which has a radiative core, has a very complex field, whereas V374 Peg, which is completely convective, has a simple dipolar field. We calculate global X-ray emission measures assuming that the plasma trapped along the coronal loops is in hydrostatic equilibrium and compare the differences between assuming isothermal coronae, or by considering a loop temperature profiles. Our preliminary results suggest that the non-isothermal model works well for the complex field of AB Dor, but not for the…
Branch Points of Algebraic Functions and the Beginnings of Modern Knot Theory
1995
Many of the key ideas which formed modern topology grew out of “normal research” in one of the mainstream fields of 19th-century mathematical thinking, the theory of complex algebraic functions. These ideas were eventually divorced from their original context. The present study discusses an example illustrating this process. During the years 1895-1905, the Austrian mathematician, Wilhelm Wirtinger, tried to generalize Felix Klein's view of algebraic functions to the case of several variables. An investigation of the monodromy behavior of such functions in the neighborhood of singular points led to the first computation of a knot group. Modern knot theory was then formed after a shift in mat…
The Sun as a benchmark of flaring activity in stellar coronae
2009
The solar corona is a template to study and understand stellar activity. However the solar corona differs from that of active stars: the Sun has lower X‐ray luminosity, and on average cooler plasma temperatures. Active stellar coronae have a hot peak in their emission measure distribution, EM (T), at 8–20 MK, while the non‐flaring solar corona has a peak at 1–2 MK. In the solar corona significant amounts of plasma at temperature ∼10 MK are observed only during flares.To investigate what is the time‐averaged effect of solar flares we measure the disk‐integrated time‐averaged emission measure, EMF (T), of an unbiased sample of solar flares. To this aim we analyze uninterrupted GOES/XRS light …
Characterization of choline efflux from the perfused heart at rest and after muscarine receptor activation.
1986
The resting efflux of choline from perfused chicken hearts varied from 0.4 to 2.6 nmol/g min, but was constant for at least 80 min in the individual experiments. The rate of choline efflux was found to be equal to the rate of choline formation in the heart, which, from the following reasons, was essentially due to hydrolysis of choline phospholipids. Cardiac content of choline phospholipids (7,200 nmol/g) was much higher than that of acetylcholine (5.5 nmol/g). Resting release of acetylcholine was 0.016 nmol/g min and, after inhibition of cholinesterase, only about 0.1 nmol/g min. Resting efflux of choline was reduced by mepacrine, a phospholipase A2 inhibitor, by perfusion with a Ca2+-free…