53,716 research outputs found
Minimal string-scale unification of gauge couplings
We look for the minimal particle content which is necessary to add to the
standard model in order to have a complete unification of gauge couplings and
gravity at the weakly coupled heterotic string scale. Using the current
precision electroweak data, we find that the presence of a vector-like fermion
at an intermediate scale and a non-standard hypercharge normalization are in
general sufficient to achieve this goal at two-loop level. If one requires the
extra matter scale to be below the TeV scale, then it is found that the
addition of three vector-like fermion doublets with a mass around 700 GeV
yields a perfect string-scale unification, provided that the affine levels are
kY=13/3, k2=1 and k3=2, as in the SU(5) X SU(5) string-GUT. Furthermore, if
supersymmetry is broken at the unification scale, the Higgs mass is predicted
in the range 125 GeV - 170 GeV, depending on the precise values of the top
quark mass and tan(beta) parameter.Comment: 12 pages, 2 figures, references updated and comments added, final
version to appear in Phys. Lett.
Planetary nebulae in the inner Milky Way: new abundances
The study of planetary nebulae in the inner-disk and bulge gives important
information on the chemical abundances of elements such as He, N, O, Ar, Ne,
and on the evolution of these abundances, which is associated with the
evolution of intermediate-mass stars and the chemical evolution of the Galaxy.
We present accurate abundances of the elements He, N, S, O, Ar, and Ne for a
sample of 54 planetary nebulae located towards the bulge of the Galaxy, for
which 33 have the abundances derived for the first time. The abundances are
derived based on observations in the optical domain made at the National
Laboratory for Astrophysics (LNA, Brazil). The data show a good agreement with
other results in the literature, in the sense that the distribution of the
abundances is similar to those works.Comment: Accepted for publication in RevMexAA (29 pages, 15 figures, 7 tables,
uses rmaa.cls
A scientific operations plan for the NASA space telescope
A ground system is described which is compatible with the operational requirements of the space telescope. The goal of the ground system is to minimize the cost of post launch operations without seriously compromising the quality and total throughput of space telescope science, or jeopardizing the safety of the space telescope in orbit. The resulting system is able to accomplish this goal through optimum use of existing and planned resources and institutional facilities. Cost is also reduced and efficiency in operation increased by drawing on existing experience in interfacing guest astronomers with spacecraft as well as mission control experience obtained in the operation of present astronomical spacecraft
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