Effect of stellar rotation on the development of post-shock instabilities during core-collapse supernovae

Abstract

Context. The growth of hydrodynamical instabilities is key to triggering a core-collapse supernova explosion during the phase of stalled accretion shock, immediately after the birth of a proto-neutron star (PNS). Stellar rotation is known to affect the standing accretion shock instability (SASI) even for small rotation rates, but its effect on the onset of neutrino-driven convection is still poorly known. Aims. We assess the effect of stellar rotation on SASI when neutrino heating is taken into account as well as the effect of rotation on neutrino-driven convection. The interplay of rotation with these two instabilities affects the frequency of the mode m = 2, which can be detected with gravitational waves at the onset of a supernova explosion. Methods. We used a linear stability analysis to study the dynamics of the accreting gas in the equatorial plane between the surface of the PNS and the stationary shock. We explored rotation effects on the relative strength of SASI and convection by considering a large range of specific angular momenta and neutrino luminosities. Results. The nature of the dominant non-axisymmetric instability developing in the equatorial post-shock region depends on both the convection parameter, χ, and the rotation rate. Equatorial convective modes with χ ≳ 5 are hampered by differential rotation. At smaller χ, however, mixed SASI-convective modes with a large angular scale, m = 1, 2, 3, can take advantage of rotation and become dominant for relatively low rotation rates, at which centrifugal effects are small. For rotation rates exceeding ∼30% of the Keplerian rotation at the PNS surface, a new instability regime is characterised by a frequency that, when measured in units of the post-shock velocity and radius, vsh/rsh, is nearly independent of the convection parameter, χ. A strong prograde m = 2 spiral dominates over a large parameter range and is favorable to the production of gravitational waves. In this regime, a simple linear relation exists between the oscillation frequency of the dominant mode and the specific angular momentum of the accreted gas. Conclusions. Three different regimes of post-shock instabilities can be distinguished depending on the rotation rate. For low rotation rates (less than 10% of the Keplerian rotation at the PNS surface), differential rotation has a linear destabilising effect on SASI and a quadratic stabilising or destabilising effect on the purely convective equatorial modes depending on their azimuthal wavenumber. Intermediate rotation rates (10% to 30% of the Keplerian rotation) lead to the emergence of mixed SASI-convection-rotation modes that involve large angular scales. Finally, strong rotation erases the influence of the buoyancy and heating rate on the instability. This independence allows for a reduction in the parameter space, which can be helpful for gravitational wave analysis.

A Parametric Study of the SASI Comparing General Relativistic and Nonrelativistic Treatments*

“…Other numerical studies focus on particular aspects of this instability, such as the hydrodynamics of the SASI (Ohnishi et al 2006;Sato et al 2009;Iwakami et al 2014), spiral modes (Blondin & Shaw 2007;Iwakami et al 2008;Fernández 2010), the spin-up of the possible remnant pulsar (Blondin & Mezzacappa 2007), the effect of nuclear dissociation (Fernández & Thompson 2009), saturation of the instability (Guilet et al 2010), the generation and amplification of magnetic fields (Endeve et al 2010(Endeve et al , 2012, the relative importance of the SASI and convection in CCSNe (Cardall & Budiardja 2015), the generation of, and impact on, gravitational waves by the SASI (Kotake et al 2007(Kotake et al , 2009Kuroda et al 2016Kuroda et al , 2017Andresen 2017;Andresen et al 2017Andresen et al , 2019Hayama et al 2018;O'Connor & Couch 2018a;Mezzacappa et al 2020Mezzacappa et al , 2023Drago et al 2023), and the effects of rotation (Yamasaki & Yamada 2005;Yamasaki & Foglizzo 2008;Buellet et al 2023;Walk et al 2023). Some of these studies included sophisticated microphysics, such as realistic equations of state (EoSs) and neutrino transport; however, with the exception of Kuroda et al (2017), none of these studies solved the general relativistic hydrodynamics (GRHD) equations, instead solving their nonrelativistic (NRHD) counterparts, some with an approximate relativistic gravitational potential.…”