36,929 research outputs found
Analysis of the acoustic cut-off frequency and HIPs in six Kepler stars with stochastically excited pulsations
Gravito-acoustic modes in the Sun and other stars propagate in resonant
cavities with a frequency below a given limit known as the cut-off frequency.
At higher frequencies, waves are no longer trapped in the stellar interior and
become traveller waves. In this article we study six pulsating solar-like stars
at different evolutionary stages observed by the NASA Kepler mission. These
high signal-to-noise targets show a peak structure that extends at very high
frequencies and are good candidates for studying the transition region between
the modes and the interference peaks or pseudo-modes. Following the same
methodology successfully applied on Sun-as-a-star measurements, we uncover the
existence of pseudo-modes in these stars with one or two dominant interference
patterns depending on the evolutionary stage of the star. We also infer their
cut-off frequency as the midpoint between the last eigenmode and the first peak
of the interference patterns. By using ray theory we show that, while the
period of one of the interference pattern is very close to half the large
separation the other, one depends on the time phase of mixed waves, thus
carrying additional information on the stellar structure and evolution.Comment: Accepted for publication in A&A. 14 pages, 28 figure
Inverse magnetic catalysis from the properties of the QCD coupling in a magnetic field
We compute the vacuum one-loop quark-gluon vertex correction at zero
temperature in the presence of a magnetic field. From the vertex function we
extract the effective quark-gluon coupling and show that it grows with
increasing magnetic field strength. The effect is due to a subtle competition
between the color charge associated to gluons and the color charge associated
to quarks, the former being larger than the latter. In contrast, at high
temperature the effective thermo-magnetic coupling results exclusively from the
contribution of the color charge associated to quarks. This produces a decrease
of the coupling with increasing field strength. We interpret the results in
terms of a geometrical effect whereby the magnetic field induces, on average, a
closer distance between the (electrically charged) quarks and antiquarks. At
high temperature, since the effective coupling is proportional only to the
color charge associated to quarks, such proximity with increasing field
strength makes the effective coupling decrease due to asymptotic freedom. In
turn, this leads to a decreasing quark condensate. In contrast, at zero
temperature both the effective strong coupling and the quark condensate
increase with increasing magnetic field. This is due to the color charge
associated to gluons dominating over that associated to quarks, with both
having the opposite sign. Thus, the gluons induce a kind of screening of the
quark color charge, in spite of the quark-antiquark proximity. The implications
of these results for the inverse magnetic catalysis phenomenon are discussed.Comment: Expanded discussion, references added. Version to appear in Phys.
Lett.
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