Finally a realistic model for Altaïr, a star with extreme rotation!

Altaïr, a bright star in the summer sky, is known for its very fast rotation. The centrifugal effect is such that its polar radius is 20% smaller than its equatorial radius. This large deviation from the sphere has made the modelling of Altaïr impossible by classical approaches in stellar physics. For the first time, its two-dimensional modeling could be performed, incorporating the most sophisticated observations. It reveals that Altaïr is a much younger star than previously estimated.

Altair (α Aquilae / α Aql) is the brightest star in the constellation of the Eagle. Located about 17 light years from Earth, it is also one of the closest stars visible to the naked eye. NASA/JPL/Caltech/Steve Golden

Located 17 light years from Earth, Altaïr is in our very near suburbs. A star of first magnitude in the constellation of the Eagle, it has attracted the attention of astronomers by its extreme speed of rotation, more than a hundred times faster than that of the Sun. Such rotation causes significant centrifugal flattening, and the star has therefore been a favourite target for near-infrared interferometers since G. van Belle first estimated its flattening in 2001. Successive improvements in interferometric techniques have subsequently led to a very good image of the surface of Altair.

The observation of Altaïr has thus made immense progress over the last two decades, which contrasts however with the weakness of the models describing it. One can however understand why: the models commonly used to describe stars are spherically symmetrical and have difficulty to take into account an important centrifugal flattening. The decisive progress made by a team of researchers from the Lagrange Laboratory (CNRS / Univ. Côte d’Azur / Obs. de la Côte d’Azur), the Laboratory for Space Studies and Instrumentation in Astrophysics (LESIA, Obs. de Paris-PSL / CNRS / Sorbonne Univ. / Univ. Paris Diderot) and the Institut de recherche en astrophysique et planétologie (IRAP, Univ. de Toulouse / CNRS / IRAP) was to use two spatial dimensions to model Altaïr.

This model was calculated by the ESTER code developed at IRAP, a code that incorporates all the effects of rapid rotation. Data from the PIONIER and GRAVITY interferometric instruments of the VLTI-ESO combined with older spectroscopic and seismological data then led to a concordance model that satisfies all known observational constraints. Thanks to the model, we now estimate the mass of Altaïr to be 1.86 times that of the Sun and give a new estimate of its age, which goes from 1 billion years according to spherical models to about 100 million years!

This new youth of Altaïr will have consequences on our knowledge of the formation of stars in the vicinity of the Sun. More broadly, this successful modeling opens the door to a finer understanding of the massive stars reputed to be the makers of the metals of the Universe.

Synthesis view of the Altaïr model. The interior shows its internal rotation speed with a core (in red) that rotates about 50% faster than its envelope (in green). The surface in different shades of blue illustrates the variation in temperature, and thus brightness, between the pole and the equator as observed with the interferometers. The sun is shown next to it at the same scale.

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  • Michel Rieutord,

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