47 research outputs found
Superconvergent Perturbation Method in Quantum Mechanics
An analogue of Kolmogorov's superconvergent perturbation theory in classical
mechanics is constructed for self adjoint operators. It is different from the
usual Rayleigh--Schr\"odinger perturbation theory and yields expansions for
eigenvalues and eigenvectors in terms of functions of the perturbation
parameter.Comment: 11 pages, LaTe
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
Presupernova Evolution of Rotating Massive Stars I: Numerical Method and Evolution of the Internal Stellar Structure
The evolution of rotating stars with zero-age main sequence (ZAMS) masses in
the range 8 to 25 M_sun is followed through all stages of stable evolution. The
initial angular momentum is chosen such that the star's equatorial rotational
velocity on the ZAMS ranges from zero to ~ 70 % of break-up. Redistribution of
angular momentum and chemical species are then followed as a consequence of
rotationally induced circulation and instablities. The effects of the
centrifugal force on the stellar structure are included. Uncertain mixing
efficiencies are gauged by observations. We find, as noted in previous work,
that rotation increases the helium core masses and enriches the stellar
envelopes with products of hydrogen burning. We determine, for the first time,
the angular momentum distribution in typical presupernova stars along with
their detailed chemical structure. Angular momentum loss due to (non-magnetic)
stellar winds and the redistribution of angular momentum during core hydrogen
burning are of crucial importance for the specific angular momentum of the
core. Neglecting magnetic fields, we find angular momentum transport from the
core to the envelope to be unimportant after core helium burning. We obtain
specific angular momenta for the iron core and overlaying material of
1E16...1E17 erg s. These values are insensitive to the initial angular
momentum. They are small enough to avoid triaxial deformations of the iron core
before it collapses, but could lead to neutron stars which rotate close to
break-up. They are also in the range required for the collapsar model of
gamma-ray bursts. The apparent discrepancy with the measured rotation rates of
young pulsars is discussed.Comment: 62 pages, including 7 tables and 19 figures. Accepted by Ap
CN and CH Band Strengths of Bright Giants in M3
CN and CH band strengths for ten bright red giants in M3 have been measured
from archival spectra obtained with the MMT. A CN-CH band strength
anticorrelation is confirmed for the program stars together with other stars
for which there is published data. This suggests an anticorrelation between
carbon and nitrogen abundances with a constant total abundance of carbon plus
nitrogen. However, stars which do not follow the CN-CH anticorrelation are also
found. The star III-77, which is found to have both strong CN and CH bands, is
the most peculiar among them. While three other stars, VZ194, VZ352, and
VZ1420, which show both weak CN and CH band strengths could be AGB stars.Comment: 10 pages, 5 figures, 1 tabl
Stellar Rotation in Young Clusters. II. Evolution of Stellar Rotation and Surface Helium Abundance
We derive the effective temperatures and gravities of 461 OB stars in 19
young clusters by fitting the H-gamma profile in their spectra. We use
synthetic model profiles for rotating stars to develop a method to estimate the
polar gravity for these stars, which we argue is a useful indicator of their
evolutionary status. We combine these results with projected rotational
velocity measurements obtained in a previous paper on these same open clusters.
We find that the more massive B-stars experience a spin down as predicted by
the theories for the evolution of rotating stars. Furthermore, we find that the
members of binary stars also experience a marked spin down with advanced
evolutionary state due to tidal interactions. We also derive non-LTE-corrected
helium abundances for most of the sample by fitting the He I 4026, 4387, 4471
lines. A large number of helium peculiar stars are found among cooler stars
with Teff < 23000 K. The analysis of the high mass stars (8.5 solar masses < M
< 16 solar masses) shows that the helium enrichment process progresses through
the main sequence (MS) phase and is greater among the faster rotators. This
discovery supports the theoretical claim that rotationally induced internal
mixing is the main cause of surface chemical anomalies that appear during the
MS phase. The lower mass stars appear to have slower rotation rates among the
low gravity objects, and they have a large proportion of helium peculiar stars.
We suggest that both properties are due to their youth. The low gravity stars
are probably pre-main sequence objects that will spin up as they contract.
These young objects very likely host a remnant magnetic field from their natal
cloud, and these strong fields sculpt out surface regions with unusual chemical
abundances.Comment: 50 pages 18 figures, accepted by Ap
Physics of rotation in stellar models
In these lecture notes, we present the equations presently used in stellar
interior models in order to compute the effects of axial rotation. We discuss
the hypotheses made. We suggest that the effects of rotation might play a key
role at low metallicity.Comment: 32 pages, 7 figures, lectures, CNRS school, will be published by
Springe
Evolution and Nucleosynthesis of Very Massive Stars
In this chapter, after a brief introduction and overview of stellar
evolution, we discuss the evolution and nucleosynthesis of very massive stars
(VMS: M>100 solar masses) in the context of recent stellar evolution model
calculations. This chapter covers the following aspects: general properties,
evolution of surface properties, late central evolution, and nucleosynthesis
including their dependence on metallicity, mass loss and rotation. Since very
massive stars have very large convective cores during the main-sequence phase,
their evolution is not so much affected by rotational mixing, but more by mass
loss through stellar winds. Their evolution is never far from a homogeneous
evolution even without rotational mixing. All VMS at metallicities close to
solar end their life as WC(-WO) type Wolf-Rayet stars. Due to very important
mass loss through stellar winds, these stars may have luminosities during the
advanced phases of their evolution similar to stars with initial masses between
60 and 120 solar masses. A distinctive feature which may be used to disentangle
Wolf-Rayet stars originating from VMS from those originating from lower initial
masses is the enhanced abundances of neon and magnesium at the surface of WC
stars. At solar metallicity, mass loss is so strong that even if a star is born
with several hundred solar masses, it will end its life with less than 50 solar
masses (using current mass loss prescriptions). At the metallicity of the LMC
and lower, on the other hand, mass loss is weaker and might enable star to
undergo pair-instability supernovae.Comment: 42 pages, 20 figures, Book Chapter in "Very Massive Stars in the
Local Universe", Springer, Ed. Jorick S. Vin
Asteroseismology and Interferometry
Asteroseismology provides us with a unique opportunity to improve our
understanding of stellar structure and evolution. Recent developments,
including the first systematic studies of solar-like pulsators, have boosted
the impact of this field of research within Astrophysics and have led to a
significant increase in the size of the research community. In the present
paper we start by reviewing the basic observational and theoretical properties
of classical and solar-like pulsators and present results from some of the most
recent and outstanding studies of these stars. We centre our review on those
classes of pulsators for which interferometric studies are expected to provide
a significant input. We discuss current limitations to asteroseismic studies,
including difficulties in mode identification and in the accurate determination
of global parameters of pulsating stars, and, after a brief review of those
aspects of interferometry that are most relevant in this context, anticipate
how interferometric observations may contribute to overcome these limitations.
Moreover, we present results of recent pilot studies of pulsating stars
involving both asteroseismic and interferometric constraints and look into the
future, summarizing ongoing efforts concerning the development of future
instruments and satellite missions which are expected to have an impact in this
field of research.Comment: Version as published in The Astronomy and Astrophysics Review, Volume
14, Issue 3-4, pp. 217-36
Interferometric Observations of Rapidly Rotating Stars
Optical interferometry provides us with a unique opportunity to improve our
understanding of stellar structure and evolution. Through direct observation of
rotationally distorted photospheres at sub-milliarcsecond scales, we are now
able to characterize latitude dependencies of stellar radius, temperature
structure, and even energy transport. These detailed new views of stars are
leading to revised thinking in a broad array of associated topics, such as
spectroscopy, stellar evolution, and exoplanet detection. As newly advanced
techniques and instrumentation mature, this topic in astronomy is poised to
greatly expand in depth and influence.Comment: Accepted for publication in A&AR