8,131 research outputs found
Dynamics of alliance formation and the egalitarian revolution
Arguably the most influential force in human history is the formation of
social coalitions and alliances (i.e., long-lasting coalitions) and their
impact on individual power. In most great ape species, coalitions occur at
individual and group levels and among both kin and non-kin. Nonetheless, ape
societies remain essentially hierarchical, and coalitions rarely weaken social
inequality. In contrast, human hunter-gatherers show a remarkable tendency to
egalitarianism, and human coalitions and alliances occur not only among
individuals and groups, but also among groups of groups. Here, we develop a
stochastic model describing the emergence of networks of allies resulting from
within-group competition for status or mates between individuals utilizing
dyadic information. The model shows that alliances often emerge in a phase
transition-like fashion if the group size, awareness, aggressiveness, and
persuasiveness of individuals are large and the decay rate of individual
affinities is small. With cultural inheritance of social networks, a single
leveling alliance including all group members can emerge in several
generations. Our results suggest that a rapid transition from a hierarchical
society of great apes to an egalitarian society of hunter-gatherers (often
referred to as "egalitarian revolution") could indeed follow an increase in
human cognitive abilities. The establishment of stable group-wide egalitarian
alliances creates conditions promoting the origin of cultural norms favoring
the group interests over those of individuals.Comment: 37 pages, 15 figure
PAH Formation in O-rich Planetary Nebulae
Polycyclic aromatic hydrocarbons (PAHs) have been observed in O-rich
planetary nebulae towards the Galactic Bulge. This combination of oxygen-rich
and carbon-rich material, known as dual-dust or mixed chemistry, is not
expected to be seen around such objects. We recently proposed that PAHs could
be formed from the photodissociation of CO in dense tori. In this work, using
VISIR/VLT, we spatially resolved the emission of the PAH bands and ionised
emission from the [SIV] line, confirming the presence of dense central tori in
all the observed O-rich objects. Furthermore, we show that for most of the
objects, PAHs are located at the outer edge of these dense/compact tori, while
the ionised material is mostly present in the inner parts of these tori,
consistent with our hypothesis for the formation of PAHs in these systems. The
presence of a dense torus has been strongly associated with the action of a
central binary star and, as such, the rich chemistry seen in these regions may
also be related to the formation of exoplanets in post-common-envelope binary
systems.Comment: 14, accepted for publication in the MNRAS Journa
Gas and dust from solar metallicity AGB stars
We study the asymptotic giant branch (AGB) evolution of stars with masses
between . We focus on stars with a solar chemical
composition, which allows us to interpret evolved stars in the Galaxy. We
present a detailed comparison with models of the same chemistry, calculated
with a different evolution code and based on a different set of physical
assumptions. We find that stars of mass experience hot
bottom burning at the base of the envelope. They have AGB lifetimes shorter
than yr and eject into their surroundings gas contaminated
by proton-capture nucleosynthesis, at an extent sensitive to the treatment of
convection. Low mass stars with become
carbon stars. During the final phases the C/O ratio grows to . We find
a remarkable agreement between the two codes for the low-mass models and
conclude that predictions for the physical and chemical properties of these
stars, and the AGB lifetime, are not that sensitive to the modelling of the AGB
phase. The dust produced is also dependent on the mass: low-mass stars produce
mainly solid carbon and silicon carbide dust, whereas higher mass stars produce
silicates and alumina dust. Possible future observations potentially able to
add more robustness to the present results are also discussed.Comment: 27 pages, 24 figures; accepted for publication in MNRA
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