1,148 research outputs found
The Origin and Universality of the Stellar Initial Mass Function
We review current theories for the origin of the Stellar Initial Mass
Function (IMF) with particular focus on the extent to which the IMF can be
considered universal across various environments. To place the issue in an
observational context, we summarize the techniques used to determine the IMF
for different stellar populations, the uncertainties affecting the results, and
the evidence for systematic departures from universality under extreme
circumstances. We next consider theories for the formation of prestellar cores
by turbulent fragmentation and the possible impact of various thermal,
hydrodynamic and magneto-hydrodynamic instabilities. We address the conversion
of prestellar cores into stars and evaluate the roles played by different
processes: competitive accretion, dynamical fragmentation, ejection and
starvation, filament fragmentation and filamentary accretion flows, disk
formation and fragmentation, critical scales imposed by thermodynamics, and
magnetic braking. We present explanations for the characteristic shapes of the
Present-Day Prestellar Core Mass Function and the IMF and consider what
significance can be attached to their apparent similarity. Substantial
computational advances have occurred in recent years, and we review the
numerical simulations that have been performed to predict the IMF directly and
discuss the influence of dynamics, time-dependent phenomena, and initial
conditions.Comment: 24 pages, 6 figures. Accepted for publication as a chapter in
Protostars and Planets VI, University of Arizona Press (2014), eds. H.
Beuther, R. S. Klessen, C. P. Dullemond, Th. Hennin
Finite-size effects and intermittency in a simple aging system
We study the intermittent dynamics and the fluctuations of the dynamic
correlation function of a simple aging system. Given its size and its
coherence length , the system can be divided into independent
subsystems, where , and is the dimension of space.
Each of them is considered as an aging subsystem which evolves according to
an activated dynamics between energy levels.
We compute analytically the distribution of trapping times for the global
system, which can take power-law, stretched-exponential or exponential forms
according to the values of and the regime of times considered. An effective
number of subsystems at age , , can be defined, which
decreases as increases, as well as an effective coherence length,
, where characterizes the trapping
times distribution of a single subsystem. We also compute the probability
distribution functions of the time intervals between large decorrelations,
which exhibit different power-law behaviours as increases (or
decreases), and which should be accessible experimentally.
Finally, we calculate the probability distribution function of the two-time
correlator.
We show that in a phenomenological approach, where is replaced by the
effective number of subsystems , the same qualitative behaviour
as in experiments and simulations of several glassy systems can be obtained.Comment: 15 pages, 6 figures, published versio
Tolerance without clonal expansion: Self-antigen-expressing B cells program self-reactive T cells for future deletion
B cells have been shown in various animal models to induce immunological tolerance leading to reduced immune responses and protection from autoimmunity. We show that interaction of B cells with naive T cells results in T cell triggering accompanied by the expression of negative costimulatory molecules such as PD-1, CTLA-4, B and T lymphocyte attenuator, and CD5. Following interaction with B cells, T cells were not induced to proliferate, in a process that was dependent on their expression of PD-1 and CTLA-4, but not CD5. In contrast, the T cells became sensitive to Ag-induced cell death. Our results demonstrate that B cells participate in the homeostasis of the immune system by ablation of conventional self-reactive T cells
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