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Be stars have a larger core than predicted

1er février 2012

Two continuous observations during 27 and 157 days by the CoRoT CNES space mission have allowed an international team led by an astronomer from the Paris Observatory, in collaboration with an astrophysicist from CEA-IRFU, to obtain an important result concerning rapidly-rotating, active, massive stars : their core are 20% larger than predicted. This is an important step for our understanding of the structure and evolution of such stars.

Once again, asteroseismology - this art of sounding stellar interiors by studying their global oscillations - opens a completely new window on the physical processes that transport energy from the nuclear core of these stars to their luminous atmosphere. A continuous observation of the light oscillations of two late Be stars by the CoRoT satellite during 27 and 157 days has revealed the size of their convective core, which appears larger than the one predicted by static models of such stars, also known as standard models. Those extremely rapidly rotating stars (their angular velocity is 20 times larger than the solar one, i.e. one rotation in 1.5 day, and their surfacic equatorial velocity is 140 times the solar one) are about 4 times more massive and 7 times larger than the Sun. This result constitutes an important step for the understanding of the structure and evolution of massive stars, which are at the origin of the heavy elements in the Universe, and particularly for the physics of Be stars. It can be explained by the internal dynamics related to the convection in the stellar core and to flows generated by the rapid rotation in their external envelope. This dynamics has been simultaneously understood and constrained thanks ro a combination of CoRoT observations with a study of the magnetism of these stars with the 2-meter Bernard Lyot Telescope at the Pic du Midi (France) and the modelling of their oscillations and of their deep hydrodynamics.

Massive stars rotating at critical speed

Compared to the Sun, Be stars have an "inverted" internal structure with a boiling nuclear convective core and a large external radiative envelope where energy is transported through radiation. Moreover, these stars are so rapidly rotating, that they are at 90% of the critical velocity at which gravity is not sufficient to ensure the equilibrium of the star. This simultaneously induces a strong flattening of the star as well as large-scale flows and turbulence in the radiative envelope that drive an important mixing of chemicals, which in turn modifies their internal structure, particularly the size of the convective core.

Figure 1 : Vue d’artiste d’une étoile Be en rotation rapide. L’axe de rotation est représenté en rouge. Le coeur convectif a un rayon plus important (en noir) que la valeur prédite par les modèles statiques de ces étoiles ne prenant pas en compte la rotation de l’étoile (en pointillés verts). Ceci est dû à la pénétration des mouvements convectifs dans l’enveloppe externe du fait de leur inertie (flèches noires) ainsi qu’aux flots de grande échelle (boucles rouges) et aux turbulences induites par la rotation rapide dans cette dernière. Le disque de décrétion de ces étoiles est représenté dans le plan équatorial.

The internal dynamics in action

Different types of oscillations are propagating in stellar interiors : acoustic waves due to compressibility and gravity waves due to Archimede’s force. In the case of the two late Be stars studied here, it is the gravity waves, which are strongly influenced by rapid rotation (they become gravito-inertial waves) that allow us to sound the internal structure down to the nuclear core. Surprise : comparing the oscillation spectrum observed with CoRoT with the computed theoretical one shows that the convective core is 1.25 times heavier and 1.2 times larger than predicted by standard static stellar models, which do not take into account the internal dynamics and in particular rotation. This means that the dynamical processes ignored in standard stellar models play a key role, especially at the border between the convective core and the radiative envelope. Using our knowledge about the internal hydrodynamics in massive stars and pulsation models that take into account their rapid rotation, scientists have been able to explain this disagreement between observations and static stellar models. First, they have shown that 2/3 of the observed non-standard extension is caused by the convective movements of the core that penetrate in the external radiative envelope because of their inertia. The remaining 1/3 is explained by the very slow internal flows and the turbulence sustained by the differential rotation in the external radiation zone. Moreover, thanks to complementary spectro-polarimetric observations with the Bernard Lyot Telescope at the Pic du Midi, they have shown that no magnetic field is detectable for these stars and constrained the maximum possible intensity of the undetected fossile magnetic field in the radiative envelope. They then concluded that the internal dynamics is dominated by rotation in the case of such rapidly rotating stars.

Figure 2 : Gauche : le satellite d’astérosismologie CoRoT ; Droite : le Télescope Bernard Lyot du Pic du Midi.

These results show once again how powerful asteroseismology is and how it allows us, combined with other observational technics such as spectro-polarimetric studies of stellar magnetic fields and numerical simulations of stellar internal flows, to give sharp constraints on the structure and evolution of stars. They also show how rapidly-rotating massive stars constitute an excellent laboratory to understand the impact of rotation on stellar evolution in general.

Reference

  • Seismic modelling of the late Be stars HD181231 and HD175869 observed with CoRoT : a laboratory for mixing processes, Neiner C., Mathis S., Saio H., Lovekin C., Eggenberger P., Lee U. A&A 2012, in presse

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