The subcritical baroclinic instability in local accretion disc models

摘要

Context. The presence of vortices in accretion discshas been debated for more than a decade. Baroclinic instabilities mightbe a way to generate these vortices in the presence of a radial entropygradient. However, the nature of these instabilities is still unclearand 3D parametric instabilities can lead to the rapid destruction ofthese vortices. Aims. We present new results exhibiting asubcritical baroclinic instability (SBI) in local shearing box models.We describe the 2D and 3Dbehaviour of this instability usingnumerical simulations and we present a simple analytical modeldescribing the underlying physical process. Methods. We investigate the SBI in local shearingboxes, using either the incompressible Boussinesq approximation or afully compressible model. We explore the parameter space varyingseveral local dimensionless parameters and we isolate the regimerelevant for the SBI. 3Dshearing boxes are also investigatedusing high resolution spectral methods to resolve both the SBI and3Dparametric instabilities. Results. A subcritical baroclinic instability isobserved in flows stable for the Solberg-Ho?land criterion using localsimulations. This instability is found to be a nonlinear(orsubcritical) instability, which cannot be described byordinary linear approaches. It requires a radial entropy gradientweakly unstable for the Schwartzchild criterion and a strong thermaldiffusivity (orequivalently a short cooling time).Incompressible simulations, the instability produces densitywaves which transport angular momentum outward with typically 10-3, the exact value depending on thebackground temperature profile. Finally, the instability survives in3D, vortex cores becoming turbulent due to parametric instabilities. Conclusions. The subcritical baroclinic instabilityis a robust phenomenon, which can be captured using local simulations.The instability survives in 3D thanks to a balance between the2DSBI and 3Dparametric instabilities. Finally, thisinstability can lead to aweak outwardtransport of angular momentum, due to the generation of density wavesby the vortices. Key words: accretion, accretion disks -instabilities - planets and satellites: formation - turbulence