82,864 research outputs found
The High Chromospheres of the Late A Stars
We report the detection of N V 1239 A transition region emission in HST/GHRS
spectra of the A7 V stars, Alpha Aql and Alpha Cep. Our observations provide
the first direct evidence of 1-3 x 10^5 K material in the atmospheres of normal
A-type stars. For both stars, and for the mid-A--type star Tau3 Eri, we also
report the detection of chromospheric emission in the Si III 1206 A line. At a
B-V color of 0.16 and an effective temperature of 8200 K, Tau3 Eri becomes the
hottest main sequence star known to have a chromosphere and thus an outer
convection zone. We see no firm evidence that the Si III line surface fluxes of
the A stars are any lower than those of moderately active, solar-type, G and K
stars. This contrasts sharply with their coronal X-ray emission, which is >100
times weaker than that of the later-type stars. Given the strength of the N V
emission observed here, it now appears unlikely that the X-ray faintness of the
A stars is due to their forming very cool, <= 1 MK coronae. An alternative
explanation in terms of mass loss in coronal winds remains a possibility,
though we conclude from moderate resolution spectra of the Si III lines that
such winds, if they exist, do not penetrate into the chromospheric Si
III--forming layers of the star, since the profiles of these lines are *not*
blueshifted, and may well be redshifted with respect to the star.Comment: LaTex, 12 pages, 3 Postscript figures, uses aaspp4, accepted by Ap
Rank one perturbations and the zeros of paraorthogonal polynomials on the unit circle
We prove several results about zeros of paraorthogonal polynomials using the
theory of rank one perturbations of unitary operators. In particular, we obtain
new details on the interlacing of zeros for successive POPUC.Comment: 8 page
SO(3)-Structures on 8-manifolds
We study Riemannian 8-manifolds with an infinitesimal action of SO(3) by
which each tangent space breaks into irreducible spaces of dimensions 3 and 5.
The relationship with quaternionic, almost product- and PSU-geometry is
thoroughly explained using representation-theoretical arguments.Comment: 19 pages, 2 figure
Complex homothetic foliations on K\"ahler manifolds
We obtain a local classification of complex homothetic foliations on Kaehler
manifolds by complex curves. This is used to construct almost Kaehler,
Ricci-flat metrics subject to additional curvature properties.Comment: Final version, up to typos. To appear in BLM
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