DS10 - Défi des autres savoirs

Stellar Evolution with Rapid Rotation – ESRR

Stellar Evolution with rapid rotation

The goal of the project is to build two-dimensional models of rapidly rotating stars including their temporal evolution. It also proposes to provide the necessary tools to interpret the interferometric, spectroscopic and seismic data obtained from this type of star.

Understanding rapidly rotating stars: an issue for the interpretation of observational data and for understanding the reionization of the Universe

The goal of this project is the construction of stellar models able to <br />manage both rapid rotation and secular time evolution, with a parallel <br />development of tools useful for spectroscopy, interferometry and <br />seismology allowing the full exploitation of the data available on <br />these stars. <br /> <br />It aims at boosting the scientific return of major projects in <br />Astrophysics such as the CoRoT (stellar seismology) or the Gaia <br />(fundamental parameters of stars) missions, or of large ground-based <br />instruments such as the VLTI (with GRAVITY or MATISSE), and finally in the <br />preparation of the PLATO mission (extra-solar planets) to obtain precise <br />stellar models. <br /> <br />The challenge is to succeed in modelling rapidly rotating stars <br />to understand their role in the evolution of galaxies or <br />planetary systems. It is recalled that more than half of the stars of <br />mass greater than 1.5 solar mass are rotating very fast. <br />This is the case of many nearby stars (Altair, Vega <br />for example) and for which reliable modelling is missing. It is <br />also the case of many massive stars (mass greater than 8 <br />solar masses) which are reputed to be the metal «factories« <br />of the Universe. Rotation plays a crucial role, particularly for <br />the first massive stars of the Universe, that is to say those that <br />led to its reionisation. Indeed, rotation is an essential parameter <br />that modulates the lifespan of stars and in the case of the <br />reionisation, it controls its duration.

To treat the effects of rotation of a star reliably
we must abandon the spherical symmetry of the current stellar models.
Rotation deforms the stars which are then of spheroidal shape, and
generates flows in the radiative zones
(baroclinic flows) which, in the absence of rotation, are in
hydrostatic balance. Modelling rotating stars requires
therefore to take into account large-scale flows
the impact of which is poorly understood today. Including such flows
requires models with at least two dimensions. The chosen method
therefore considers axisymmetric stellar models and allows the determination of
the differential rotation and meridian circulation.
These flows are therefore no longer ad hoc as in the
old 2D models but result from solving the equations of
hydrodynamics. This is the tour de force accomplished during the
realization of ESTER models by Espinosa Lara & Rieutord (2013).
Hence, we rely on these models to now take into account
the temporal evolution. When the temporal evolution is properly mastered
(this is one of the goals of the project), we will finally be able to
give the age of a star like Vega with better reliability.

The most spectacular results of the project (updated to
September 2019) concern the interpretation of observational data
of two nearby stars with very fast rotation.

The first is the star Sargas (theta Scorpii) where our modelling
allowed the interpretation of interferometric data from the
VLTI as well as high resolution spectroscopic data
(Domiciano de Souza et al., 2018). This modelling allowed us to
discover that Sargas is a giant star, whose radius is
30 times that of the sun, with a very fast rotation which is
exceptional for stars in this category. But its true
stage of evolution or age are still unclear and only
two-dimensional models, presently under development, will solve this
question.

The second star is Altair. Its modelling is the first
to integrate data from interferometry, spectroscopy
and seismology for a very fast rotating star. Our
completely original results (Bouchaud et al., 2019) give
precise values to several fundamental parameters of Altair. Furthermore
they allow for the first time an interpretation using
two-dimensional models of acoustic oscillation frequencies
of a rapidly rotating star. This modelling is quite
out of reach of traditional one-dimensional models.

Future developments of the project will attack the search of
a two-dimensional solution for convection zones in fast rotating
stars. The goal is to extend the two-dimensional modelling of rapidly
rotating stars towards the small masses, especially the solar-type stars,
and towards advanced stages including for example the evolution on
the branch of giants. Another perspective is the study of the so-called
first-generation stars where rotation most likely plays an important role,
especially in the capacity of massive stars of this era to reionise the
young Universe. Finally, a development to be realized is that of the
seismology of rapidly rotating early-type stars, which represent half
of these stars, because data of this type will be more and more
abundant in next years following the TESS and PLATO missions.

1. Domiciano de Souza A., Bouchaud K, Rieutord M., Espinosa Lara F. and Putigny B. (2018), ``The evolved fast rotator Sargas: stellar parameters and evolutionary status from VLTI/PIONIER and VLT/UVES«, in A&A 619, A167
2. Bouchaud K., Domiciano de

At some stage(s) of their evolution most stars face rapid rotation. The aim
of this project is the release of two-dimensional stellar models that
can deal with both fast rotation and time evolution. It also aims at
developing the tools needed to make science out of the spectroscopic,
interferometric and seismic data available for fast rotating stars. This
project leans on the successful development of the ESTER code, which
calculates the structure and the large-scale flows of a fast rotating
early-type star in two dimensions of space. This project also aims at
increasing the scientific impact of the large projects in astrophysics
like the space missions CoRoT (stellar seismology) or GAIA (fundamental
parameters of stars), and of large ground-based instruments at ESO-VLTI,
like GRAVITY or MATISSE. Finally, this project will contribute
to the preparation of the PLATO mission on exo-planets by producing very
precise stellar models needed for the determination of planets'
parameters.

Project coordinator

Monsieur Michel Rieutord (Institut de Recherches en Astrophysique et Planétologie)

The author of this summary is the project coordinator, who is responsible for the content of this summary. The ANR declines any responsibility as for its contents.

Partner

IRAP Institut de Recherches en Astrophysique et Planétologie
LAGRANGE (OCA/CNRS/UNS) LABORATOIRE LAGRANGE (OCA/CNRS/UNS)
LESIA Laboratoire d'études spatiales et d'instrumentation en astrophysique

Help of the ANR 263,341 euros
Beginning and duration of the scientific project: December 2016 - 48 Months

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