Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: A magnetic model for acoustic modes in roAp stars   The mechanism for excitation of p-modes in rapidly oscillating, peculiar A
(roAp, or cool chemically peculiar, CP) stars is unknown. Observations strongly
suggest that acoustic modes in roAp stars are causally linked to the stars'
magnetic field. We propose that small fluctuations in the shape of the mean
magnetic field drive magnetosonic waves, which are observed as p-modes in these
stars.
  The dynamic edge region of roAp stars is a force-free spherical shell. When
strongly coupled to the magnetic field, a force-free plasma can oscillate about
a minimum in its mean magnetic energy. We describe the stable eigenmodes for
this energy minimum in a spherical shell with an open boundary. The
wavenumbers, frequencies, and energies of resulting oscillations are consistent
with observations of p-modes in roAp stars. Our magnetic model for p-mode
oscillations in stars does not invoke convection or opacity mechanisms. We also
suggest the possibility of a nonlinear dynamo for such magnetic stars, which
lack the convection needed for the usual (-( dynamo.

32
The categories of this paper are: atom-ph
21
The categories of this paper are: adap-org
8
The categories of this paper are: hep-ex
22
The categories of this paper are: chao-dyn
35
The categories of this paper are: mtrl-th
18
The categories of this paper are: econ
7
The categories of this paper are: hep-th
27
The categories of this paper are: comp-gas
3
The categories of this paper are: physics
4
The categories of this paper are: cond-mat
12
The categories of this paper are: nucl-th
24
The categories of this paper are: dg-ga
1
The categories of this paper are: cs
29
The categories of this paper are: funct-an
13
The categories of this paper are: quant-ph
26
The categories of this paper are: bayes-an