8,761 research outputs found
Investigating the rotational evolution of young, low mass stars using Monte Carlo simulations
We investigate the rotational evolution of young stars through Monte Carlo
simulations. We simulate 280,000 stars, each of which is assigned a mass, a
rotational period, and a mass accretion rate. The mass accretion rate depends
on mass and time, following power-laws indices 1.4 and -1.5, respectively. A
mass-dependent accretion threshold is defined below which a star is considered
as diskless, which results in a distribution of disk lifetimes that matches
observations. Stars are evolved at constant angular spin rate while accreting
and at constant angular momentum when they become diskless. We recover the
bimodal period distribution seen in several young clusters. The short period
peak consists mostly of diskless stars and the long period one is mainly
populated by accreting stars. Both distributions present a long tail towards
long periods and a population of slowly rotating diskless stars is observed at
all ages. We reproduce the observed correlations between disk fraction and spin
rate, as well as between IR excess and rotational period. The period-mass
relation we derive from the simulations exhibits the same global trend as
observed in young clusters only if we release the disk locking assumption for
the lowest mass stars. We find that the time evolution of median specific
angular momentum follows a power law index of -0.65 for accreting stars and of
-0.53 for diskless stars, a shallower slope that results from a wide
distribution of disk lifetimes. Using observationally-documented distributions
of disk lifetimes, mass accretion rates, and initial rotation periods, and
evolving an initial population from 1 to 12 Myr, we reproduce the main
characteristics of pre-main sequence angular momentum evolution, which supports
the disk locking hypothesis. (abridged)Comment: 11 pages, 14 figures, accepted for publication in A&
Agent-based Sensor-Mission Assignment for Tasks Sharing Assets
(c) IFAAMASPeer reviewedPostprin
Warehouse Storing and Collecting of Parts
This report deals with reducing the high costs resulting from the wear and tear of the fork-lifts used to store or collect items in a warehouse. Two problems were identified and addressed separately. One concerns the way items should be stored or collected at storage locations on the shelves of one corridor. The other problem seeks for an efficient way to define which fork-lift should operate on each corridor, and the order by which the fork-lifts should visit the corridors.
We give to both problems formulations that fit in the framework of combinatorial optimization
Modular session types for objects
Session types allow communication protocols to be specified
type-theoretically so that protocol implementations can be verified by static
type checking. We extend previous work on session types for distributed
object-oriented languages in three ways. (1) We attach a session type to a
class definition, to specify the possible sequences of method calls. (2) We
allow a session type (protocol) implementation to be modularized, i.e.
partitioned into separately-callable methods. (3) We treat session-typed
communication channels as objects, integrating their session types with the
session types of classes. The result is an elegant unification of communication
channels and their session types, distributed object-oriented programming, and
a form of typestate supporting non-uniform objects, i.e. objects that
dynamically change the set of available methods. We define syntax, operational
se-mantics, a sound type system, and a sound and complete type checking
algorithm for a small distributed class-based object-oriented language with
structural subtyping. Static typing guarantees that both sequences of messages
on channels, and sequences of method calls on objects, conform to
type-theoretic specifications, thus ensuring type-safety. The language includes
expected features of session types, such as delegation, and expected features
of object-oriented programming, such as encapsulation of local state.Comment: Logical Methods in Computer Science (LMCS), International Federation
for Computational Logic, 201
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