14,852 research outputs found
The dependence of convective core overshooting on stellar mass: reality check, and additional evidence
Overshooting from the convective cores of stars more massive than about 1.2
M(Sun) has a profound impact on their subsequent evolution. And yet, the
formulation of the overshooting mechanism in current stellar evolution models
has a free parameter (f[ov] in the diffusive approximation) that remains poorly
constrained by observations, affecting the determination of astrophysically
important quantities such as stellar ages. In an earlier series of papers we
assembled a sample of 37 well-measured detached eclipsing binaries to calibrate
the dependence of f[ov] on stellar mass, showing that it increases sharply up
to a mass of roughly 2 M(Sun), and remains constant thereafter out to at least
4.4 M(Sun). Recent claims have challenged the utility of eclipsing binaries for
this purpose, on the basis that the uncertainties in f[ov] from the model fits
are typically too large to be useful, casting doubt on a dependence of
overshooting on mass. Here we reexamine those claims and show them to be too
pessimistic, mainly because they did not account for all available constraints
--- both observational and theoretical --- in assessing the true uncertainties.
We also take the opportunity to add semi-empirical f[ov] determinations for 13
additional binaries to our previous sample, and to update the values for 9
others. All are consistent with, and strengthen our previous conclusions,
supporting a dependence of f[ov] on mass that is now based on estimates for a
total of 50 binary systems (100 stars).Comment: 14 pages in emulateapj format, including figures and tables. Accepted
for publication in The Astrophysical Journal. One duplicate object has been
removed, and the tables and one figure have been update
On Detection Issues in the SC-based Uplink of a MU-MIMO System with a Large Number of BS Antennas
This paper deals with SC/FDE within a MU-MIMO system where a large number of
BS antennas is adopted. In this context, either linear or reduced-complexity
iterative DF detection techniques are considered. Regarding performance
evaluation by simulation, appropriate semi-analytical methods are proposed.
This paper includes a detailed evaluation of BER performances for uncoded
4-Quadrature Amplitude Modulation (4-QAM) schemes and a MU-MIMO channel with
uncorrelated Rayleigh fading. The accuracy of performance results obtained
through the semi-analytical simulation methods is assessed by means of parallel
conventional Monte Carlo simulations, under the assumptions of perfect power
control and perfect channel estimation. The performance results are discussed
in detail, with the help of selected performance bounds. We emphasize that a
moderately large number of BS antennas is enough to closely approximate the
SIMO MFB performance, especially when using the suggested low-complexity
iterative DF technique, which does not require matrix inversion operations. We
also emphasize the achievable "massive MIMO" effects, even for strongly
reduced-complexity linear detection techniques, provided that the number of BS
antennas is much higher than the number of antennas which are jointly employed
in the terminals of the multiple autonomous users.Comment: 7 pages, 4 figure
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