1,273 research outputs found
Self-consistent 2D models of fast rotating early-type star
This work aims at presenting the first two-dimensional models of an isolated
rapidly rotating star that include the derivation of the differential rotation
and meridional circulation in a self-consistent way.We use spectral methods in
multidomains, together with a Newton algorithm to determine the steady state
solutions including differential rotation and meridional circulation for an
isolated non-magnetic, rapidly rotating early-type star. In particular we
devise an asymptotic method for small Ekman numbers (small viscosities) that
removes the Ekman boundary layer and lifts the degeneracy of the inviscid
baroclinic solutions.For the first time, realistic two-dimensional models of
fast-rotating stars are computed with the actual baroclinic flows that predict
the differential rotation and the meridional circulation for intermediate-mass
and massive stars. These models nicely compare with available data of some
nearby fast-rotating early-type stars like Ras Alhague ( Oph), Regulus
( Leo), and Vega ( Lyr). It is shown that baroclinicity drives
a differential rotation with a slow pole, a fast equator, a fast core, and a
slow envelope. The differential rotation is found to increase with mass, with
evolution (here measured by the hydrogen mass fraction in the core), and with
metallicity. The core-envelope interface is found to be a place of strong shear
where mixing will be efficient.Two-dimensional models offer a new view of
fast-rotating stars, especially of their differential rotation, which turns out
to be strong at the core-envelope interface. They also offer more accurate
models for interpreting the interferometric and spectroscopic data of
early-type stars.Comment: 16 pages, 17 figures, to appear in Astronomy and Astrophysic
Physical processes leading to surface inhomogeneities: the case of rotation
In this lecture I discuss the bulk surface heterogeneity of rotating stars,
namely gravity darkening. I especially detail the derivation of the omega-model
of Espinosa Lara & Rieutord (2011), which gives the gravity darkening in
early-type stars. I also discuss the problem of deriving gravity darkening in
stars owning a convective envelope and in those that are members of a binary
system.Comment: 23 pages, 11 figure, Lecture given to the school on the cartography
of the Sun and the stars (May 2014 in Besan\c{c}on), to appear in LNP, Neiner
and Rozelot edts V2: typos correcte
An algorithm for computing the 2D structure of fast rotating stars
Stars may be understood as self-gravitating masses of a compressible fluid
whose radiative cooling is compensated by nuclear reactions or gravitational
contraction. The understanding of their time evolution requires the use of
detailed models that account for a complex microphysics including that of
opacities, equation of state and nuclear reactions. The present stellar models
are essentially one-dimensional, namely spherically symmetric. However, the
interpretation of recent data like the surface abundances of elements or the
distribution of internal rotation have reached the limits of validity of
one-dimensional models because of their very simplified representation of
large-scale fluid flows. In this article, we describe the ESTER code, which is
the first code able to compute in a consistent way a two-dimensional model of a
fast rotating star including its large-scale flows. Compared to classical 1D
stellar evolution codes, many numerical innovations have been introduced to
deal with this complex problem. First, the spectral discretization based on
spherical harmonics and Chebyshev polynomials is used to represent the 2D
axisymmetric fields. A nonlinear mapping maps the spheroidal star and allows a
smooth spectral representation of the fields. The properties of Picard and
Newton iterations for solving the nonlinear partial differential equations of
the problem are discussed. It turns out that the Picard scheme is efficient on
the computation of the simple polytropic stars, but Newton algorithm is
unsurpassed when stellar models include complex microphysics. Finally, we
discuss the numerical efficiency of our solver of Newton iterations. This
linear solver combines the iterative Conjugate Gradient Squared algorithm
together with an LU-factorization serving as a preconditionner of the Jacobian
matrix.Comment: 40 pages, 12 figures, accepted in J. Comput. Physic
Solar energetic electron events measured by MESSENGER and Solar Orbiter. Peak intensity and energy spectrum radial dependences: statistical analysis
Context/Aims: We present a list of 61 solar energetic electron (SEE) events
measured by the MESSENGER mission and the radial dependences of the electron
peak intensity and the peak-intensity energy spectrum. The analysis comprises
the period from 2010 to 2015, when MESSENGER heliocentric distance varied
between 0.31 and 0.47 au. We also show the radial dependencies for a shorter
list of 12 SEE events measured in February and March 2022 by spacecraft near 1
au and by Solar Orbiter around its first close perihelion at 0.32 au.
Results: Due to the elevated background intensity level of the particle
instrument on board MESSENGER, the SEE events measured by this mission are
necessarily large and intense; most of them accompanied by a CME-driven shock,
being widespread in heliolongitude, and displaying relativistic (1 MeV)
electron intensity enhancements. The two main conclusions derived from the
analysis of the large SEE events measured by MESSENGER, which are generally
supported by Solar Orbiter's data results, are: (1) There is a wide variability
in the radial dependence of the electron peak intensity between 0.3 au
and 1 au, but the peak intensities of the energetic electrons decrease
with radial distance from the Sun in 27 out of 28 events. On average and within
the uncertainties, we find a radial dependence consistent with . (2)
The electron spectral index found in the energy range around 200 keV
(200) of the backward-scattered population near 0.3 au measured by
MESSENGER is harder in 19 out of 20 (15 out of 18) events by a median factor of
20% (10%) when comparing to the anti-sunward propagating beam
(backward-scattered population) near 1 au.Comment: 20 pages, 13 figure
Asymptotic analysis of high-frequency acoustic modes in rapidly rotating stars
The asteroseismology of rapidly rotating pulsating stars is hindered by our
poor knowledge of the effect of the rotation on the oscillation properties.
Here we present an asymptotic analysis of high-frequency acoustic modes in
rapidly rotating stars. We study the Hamiltonian dynamics of acoustic rays in
uniformly rotating polytropic stars and show that the phase space structure has
a mixed character, regions of chaotic trajectories coexisting with stable
structures like island chains or invariant tori. In order to interpret the ray
dynamics in terms of acoustic mode properties, we then use tools and concepts
developed in the context of quantum physics. Accordingly, the high-frequency
acoustic spectrum is a superposition of frequency subsets associated with
dynamically independent phase space regions. The sub-spectra associated with
stable structures are regular and can be modelled through EBK quantization
methods while those associated with chaotic regions are irregular but with
generic statistical properties. The results of this asymptotic analysis are
successfully confronted with the properties of numerically computed
high-frequency acoustic modes. The implications for the asteroseismology of
rapidly rotating stars are discussed.Comment: 21 pages, 11 figures, accepted for publication in Astronomy and
Astrophysic
Unusually long path length for a nearly scatter-free solar particle event observed by Solar Orbiter at 0.43 au
Context: After their acceleration and release at the Sun, solar energetic particles (SEPs) are injected into the interplanetary medium and are bound to the interplanetary magnetic field (IMF) by the Lorentz force. The expansion of the IMF close to the Sun focuses the particle pitch-angle distribution, and scattering counteracts this focusing. Solar Orbiter observed an unusual solar particle event on 9 April 2022 when it was at 0.43 astronomical units (au) from the Sun.
//
Aims: We show that the inferred IMF along which the SEPs traveled was about three times longer than the nominal length of the Parker spiral and provide an explanation for this apparently long path.
//
Methods: We used velocity dispersion analysis (VDA) information to infer the spiral length along which the electrons and ions traveled and infer their solar release times and arrival direction.
//
Results: The path length inferred from VDA is approximately three times longer than the nominal Parker spiral. Nevertheless, the pitch-angle distribution of the particles of this event is highly anisotropic, and the electrons and ions appear to be streaming along the same IMF structures. The angular width of the streaming population is estimated to be approximately 30 degrees. The highly anisotropic ion beam was observed for more than 12 h. This may be due to the low level of fluctuations in the IMF, which in turn is very probably due to this event being inside an interplanetary coronal mass ejection The slow and small rotation in the IMF suggests a flux-rope structure. Small flux dropouts are associated with very small changes in pitch angle, which may be explained by different flux tubes connecting to different locations in the flare region.
//
Conclusions: The unusually long path length along which the electrons and ions have propagated virtually scatter-free together with the short-term flux dropouts offer excellent opportunities to study the transport of SEPs within interplanetary structures. The 9 April 2022 solar particle event offers an especially rich number of unique observations that can be used to limit SEP transport models
- …