3,633 research outputs found
Understanding angular momentum transport in red giants: the case of KIC 7341231
Context. Thanks to recent asteroseismic observations, it has been possible to
infer the radial differential rotation profile of subgiants and red giants.
Aims. We want to reproduce through modeling the observed rotation profile of
the early red giant KIC 7341231 and constrain the physical mechanisms
responsible for angular momentum transport in stellar interiors.
Methods. We compute models of KIC 7341231 including a treatment of shellular
rotation and we compare the rotation profiles obtained with the one derived by
Deheuvels et al. (2012). We then modify some modeling parameters in order to
quantify their effect on the obtained rotation profile. Moreover, we mimic a
powerful angular momentum transport during the Main Sequence and study its
effect on the evolution of the rotation profile during the subgiant and red
giant phases.
Results. We show that meridional circulation and shear mixing alone produce a
rotation profile for KIC 7341231 too steep compared to the observed one. An
additional mechanism is then needed to increase the internal transport of
angular momentum. We find that this undetermined mechanism has to be efficient
not only during the Main Sequence but also during the much quicker subgiant
phase. Moreover, we point out the importance of studying the whole rotational
history of a star in order to explain its rotation profile during the red giant
evolution.Comment: 8 pages, 8 figures, 5 table
Characterizing the propagation of gravity waves in 3D nonlinear simulations of solar-like stars
The revolution of helio- and asteroseismology provides access to the detailed
properties of stellar interiors by studying the star's oscillation modes. Among
them, gravity (g) modes are formed by constructive interferences between
progressive internal gravity waves (IGWs), propagating in stellar radiative
zones. Our new 3D nonlinear simulations of the interior of a solar-like star
allows us to study the excitation, propagation, and dissipation of these waves.
The aim of this article is to clarify our understanding of the behavior of IGWs
in a 3D radiative zone and to provide a clear overview of their properties. We
use a method of frequency filtering that reveals the path of {individual}
gravity waves of different frequencies in the radiative zone. We are able to
identify the region of propagation of different waves in 2D and 3D, to compare
them to the linear raytracing theory and to distinguish between propagative and
standing waves (g modes). We also show that the energy carried by waves is
distributed in different planes in the sphere, depending on their azimuthal
wave number. We are able to isolate individual IGWs from a complex spectrum and
to study their propagation in space and time. In particular, we highlight in
this paper the necessity of studying the propagation of waves in 3D spherical
geometry, since the distribution of their energy is not equipartitioned in the
sphere.Comment: 14 pages, 12 figues, accepted by Astronomy & Astrophysic
Dust heating by the interstellar radiation field in models of turbulent molecular clouds
We have calculated the radiation field, dust grain temperatures, and far
infrared emissivity of numerical models of turbulent molecular clouds. When
compared to a uniform cloud of the same mean optical depth, most of the volume
inside the turbulent cloud is brighter, but most of the mass is darker. There
is little mean attenuation from center to edge, and clumping causes the
radiation field to be somewhat bluer. There is also a large dispersion,
typically by a few orders of magnitude, of all quantities relative to their
means. However, despite the scatter, the 850 micron emission maps are well
correlated with surface density. The fraction of mass as a function of
intensity can be reproduced by a simple hierarchical model of density
structure.Comment: 32 pages, 14 figures, submitted to Ap
Calculating Cross Sections of Composite Interstellar Grains
Interstellar grains may be composite collections of particles of distinct
materials, including voids, agglomerated together. We determine the various
optical cross sections of such composite grains, given the optical properties
of each constituent, using an approximate model of the composite grain. We
assume it consists of many concentric spherical layers of the various
materials, each with a specified volume fraction. In such a case the usual Mie
theory can be generalized and the extinction, scattering, and other cross
sections determined exactly.
We find that the ordering of the materials in the layering makes some
difference to the derived cross sections, but averaging over the various
permutations of the order of the materials provides rapid convergence as the
number of shells (each of which is filled by all of the materials
proportionately to their volume fractions) is increased. Three shells, each
with one layer of a particular constituent material, give a very satisfactory
estimate of the average cross section produced by larger numbers of shells.
We give the formulae for the Rayleigh limit (small size parameter) for
multi-layered spheres and use it to propose an ``Effective Medium Theory''
(EMT), in which an average optical constant is taken to represent the ensemble
of materials.
Multi-layered models are used to compare the accuracies of several EMTs
already in the literature.Comment: 29 pages, 6 figures, accepted for publication in the Astrophysical
Journal (part 1, scheduled in Vol. 526, #1, Nov. 20
Infrared emission from interstellar dust cloud with two embedded sources: IRAS 19181+1349
Mid and far infrared maps of many Galactic star forming regions show multiple
peaks in close proximity, implying more than one embedded energy sources. With
the aim of understanding such interstellar clouds better, the present study
models the case of two embedded sources. A radiative transfer scheme has been
developed to deal with an uniform density dust cloud in a cylindrical geometry,
which includes isotropic scattering in addition to the emission and absorption
processes. This scheme has been applied to the Galactic star forming region
associated with IRAS 19181+1349, which shows observational evidence for two
embedded energy sources. Two independent modelling approaches have been
adopted, viz., to fit the observed spectral energy distribution (SED) best; or
to fit the various radial profiles best, as a function of wavelength. Both the
models imply remarkably similar physical parameters.Comment: 17 pages, 6 Figures, uses epsf.sty. To appear in Journal of
Astronophysics & Astronom
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