309 research outputs found
A study of the Mg II 2796.34 A emission line in late--type normal, and RS CVn stars
We carry out an analysis of the Mg II 2796.34 A emission line in RS CVn stars
and make a comparison with the normal stars studied in a previous paper (Paper
I). The sample of RS CVn stars consists of 34 objects with known HIPPARCOS
parallaxes and observed at high resolution with IUE. We confirm that RS CVn
stars tend to possess wider Mg II lines than normal stars having the same
absolute visual magnitude. However, we could not find any correlation between
the logarithmic line width log Wo and the absolute visual magnitude Mv (the
Wilson--Bappu relationship) for these active stars, contrary to the case of
normal stars addressed in Paper I. On the contrary, we find that a strong
correlation exists in the (Mv, log L) plane (L is the absolute flux in the
line). In this plane, normal and RS CVn stars are distributed along two nearly
parallel straight lines with RS CVn stars being systematically brighter by
about 1 dex. Such a diagram provides an interesting tool to discriminate active
from normal stars. We finally analyse the distribution of RS CVn and of normal
stars in the (log L, log Wo) plane, and find a strong linear correlation for
normal stars, which can be used for distance determinations.Comment: 10 pages, 7 figures, latex, to be published in A&
Multiwavelength optical observations of chromospherically active binary systems. III. High resolution echelle spectra from Ca II H&K to Ca II IRT
This is the third paper of a series aimed at studying the chromosphere of
active binary systems using the information provided for several optical
spectroscopic features. High resolution echelle spectra including all the
optical chromospheric activity indicators from the Ca II H & K to Ca II IRT
lines are analysed here for 16 systems. The chromospheric contribution in these
lines has been determined using the spectral subtraction technique. Very broad
wings have been found in the subtracted H_alpha profile of the very active star
HU Vir. These profiles are well matched using a two-component Gaussian fit
(narrow and broad) and the broad component can be interpreted as arising from
microflaring. Red-shifted absorption features in the H_alpha line have been
detected in several systems and excess emission in the blue wing of FG UMa was
also detected. These features indicate that several dynamical processes, or a
combination of them, may be involved. Using the E(H_alpha)/E(H_beta) ratio as a
diagnostic we have detected prominence-like extended material viewed off the
limb in many stars of the sample, and prominences viewed against the disk at
some orbital phases in the dwarfs OU Gem and BF Lyn. The He I D_3 line has been
detected as an absorption feature in mainly all the giants of the sample. Total
filling-in of the He I D_3, probably due to microflaring activity, is observed
in HU Vir. Self-absorption with red asymmetry is detected in the Ca II H & K
lines of the giants 12 Cam, FG UMa and BM CVn. All the stars analysed show
clear filled-in Ca II IRT lines or even notable emission reversal. The small
values of the E(8542)/E(8498) ratio we have found indicate Ca II IRT emission
arises from plage-like regions.Comment: Latex file with 38 pages, 25 figures. Full postscript (text and
figures) available at http://www.ucm.es/info/Astrof/pub_dmg.html To be
published in Astronomy & Astrophysics Supp.
The spinorial geometry of supersymmetric backgrounds
We propose a new method to solve the Killing spinor equations of
eleven-dimensional supergravity based on a description of spinors in terms of
forms and on the Spin(1,10) gauge symmetry of the supercovariant derivative. We
give the canonical form of Killing spinors for N=2 backgrounds provided that
one of the spinors represents the orbit of Spin(1,10) with stability subgroup
SU(5). We directly solve the Killing spinor equations of N=1 and some N=2, N=3
and N=4 backgrounds. In the N=2 case, we investigate backgrounds with SU(5) and
SU(4) invariant Killing spinors and compute the associated spacetime forms. We
find that N=2 backgrounds with SU(5) invariant Killing spinors admit a timelike
Killing vector and that the space transverse to the orbits of this vector field
is a Hermitian manifold with an SU(5)-structure. Furthermore, N=2 backgrounds
with SU(4) invariant Killing spinors admit two Killing vectors, one timelike
and one spacelike. The space transverse to the orbits of the former is an
almost Hermitian manifold with an SU(4)-structure and the latter leaves the
almost complex structure invariant. We explore the canonical form of Killing
spinors for backgrounds with extended, N>2, supersymmetry. We investigate a
class of N=3 and N=4 backgrounds with SU(4) invariant spinors. We find that in
both cases the space transverse to a timelike vector field is a Hermitian
manifold equipped with an SU(4)-structure and admits two holomorphic Killing
vector fields. We also present an application to M-theory Calabi-Yau
compactifications with fluxes to one-dimension.Comment: Latex, 54 pages, v2: clarifications made and references added. v3:
minor changes. v4: minor change
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