Linear analysis of the standing accretion shock instability

Linear analysis of the standing accretion shock instability with non-spherical accretion flows Kazuya Takahashi with Yu Yamamoto, Shoichi Yamada (Waseda Univ. )

Outline Introduction Non-spherical structures in progenitors Standing Accretion Shock Instability (SASI) My research New points How? Results Summary

Introduction Standing Accretion Shock Instability (SASI): * One of the hopeful mechanisms for successful explosion of core-collapse supernovae * Multi-dimensional fluid instability, which enhances the neutrino heating because the fluid particles dwell the gain region much longer due to the complex flow patterns. (Iwakami et al. 2014) (Blonding et al. 2003)

Introduction The neutrino heating mechanism is thought to be the most promising one for core-collapse supernovae. However, successful explosions have not been obtained yet. Recently, the non-spherical structure in progenitors arises as a new key, which was caused by nuclear burnings in the shells and following violent convections (e. g. Arnett & Meakin 2011, Chatzopoulos et al. 2014). O burning Si burning Fe core (Arnett & Meakin 2011)

Introduction Actually, we recently showed by linear analysis that such a non-spherical fluctuations in convectiving shells can be amplified during the infall onto the stalled shock, which suggests that fluctuations in the progenitor may affect the shock dynamics (KT & Yamada 2014)

Introduction In most cases, however, numerical simulations are carried out with the steady upstream flows, partly because the star evolutions are calculated in 1 D and hence the progenitors have spherically symmetric structures. Adding perturbations by hand, Couch & Ott (2013, 2014) and Muller & Janka (2014) showed that perturbations in convecting shells can facilitate the shock revival even for a progenitor that fails to explode without the fluctuation. Muller & Janka (2014) reported that large scale perturbations (l = 1, 2) especially contribute the shock revival, which may have enhanced the Standing Accretion Shock Instability (SASI).

Introduction Previous analytical studies on SASI (e. g. a series of papers by Yamasaki & Yamada, or the group of Foglizzo) have assumed the spherically symmetric steady flows in front of the standing shock. We need the analytical study on SASI for non-steady and non-spherical upstream flows

How to analyze SASI Global linear analysis on the post-shock flow (cf. e. g. Yamasaki & Yamada 2007) We solved an eigenvalue problem. i. e. , We seek the eigenfunctions of the flow between the shock surface and the proto-neutron star surface. Outer Boundary → Inner Boundary → Connected by the Rankine-Hugoniot relations at the standing shock

How to analyze SASI Linearized equations: : vector that denotes perturbations : Matrices whose elements are the background quantities Initially, no perturbation: B. C. at the shock surface; connected by the Rankine-Hugoniot relations B. C. at the PNS surface; imposed : ONLY ONE condition is imposed

How to analyze SASI Laplace-transformation with respect to time: Laplace-transformed linearized equations: B. C. at the shock surface; connected by the Rankine-Hugoniot relations B. C. at the PNS surface; imposed (Laplace transformed) : ONLY ONE condition is imposed

How to analyze SASI B. C. at the shock surface; connected by the Rankine-Hugoniot relations 0. We know the upstream flows (given). 1. They are connected downstream with the linearized Rankine-Hugoniot relations provided a value of s and a one parameter is given: the perturbation of shock radius. Laplace-transformed linearized equations: 2. We solve the linearized cons. laws from the shock surface to PNS. B. C. at the PNS surface; imposed 3. Seek the parameter that gives the consistent value at PNS surface

How to analyze SASI B. C. at the shock surface; connected by the Rankine-Hugoniot relations 0. We know the upstream flows (given). 1. They are connected downstream with the linearized Rankine-Hugoniot relations provided a value of s and a one parameter is given: the perturbation of shock radius. Laplace-transformed linearized equations: For each value of s, we obtain the eigenvalue: Laplace- transformed value of 2. the Weshock solveradius. the linearized cons. Repeating this procedure a series of s, surface we obtain laws for from the shock to PNS. the evolution of the shock radius by performing the B. C. at the PNS surface; imposed inverse Laplace transform. 3. Seek the parameter that gives the consistent value at PNS surface

Formulation Basic equations that describe downstream flow: Continuity eq. Momentum eq. Energy eq. Eq. for electron number Neutrino transport: Light bulb + geometric factor cf. Onhishi et al. (2006), Scheck et al. (2006) Eo. S nucleon, nuclei (NSE, ideal Boltzmann gas), photons (ideal Bose gas), electrons and positrons (ideal Fermi gas)

Formulation Linearize the eqs. under the assumptions: * Background flow is spherically symmetric. * Decompose with the spherical harmonics Y_{lm} as our previous work And then Laplace-transform the linearized eqs. : whose elements are written by the background flow

Formulation Linearization: * Background flow is spherically symmetric * Unperturbed shock surface is spherical: * Non-spherical components are decomposed with Continuity eq. Momentum eq. Energy eq. Electron number

Formulation Laplace-transformed linearized eqs: Downstream flow across the shock is given by : perturbations in front of the shock (given) c: vector, R: matrix, whose components are written by the background flow One parameter family of

Formulation We solve the eingenvalue problem that is given by Basic equation Outer boundary (determined by one parameter) Inner boundary condition

Mode Analysis Im (oscillate frequency) × × × Re (Growth rate) SASI modes correspond to the poles of the Laplace transformed perturbed shock radius (= eigenvalue) in the complex plane

Mode Analysis Im (oscillate frequency) Along the path, we solve the eigenvalue problem for each s. ⇒ we can easily find the poles by seeing the rapid × changes of the eigenvalue around them. × × × search path Re (Growth rate) SASI modes correspond to the poles of the Laplace transformed perturbed shock radius (= eigenvalue) in the complex plane

Mode Analysis Im (oscillate frequency) Along the path, we solve the eigenvalue problem for each s. ⇒ we can easily find the poles by seeing the rapid × changes of the eigenvalue around them. × ×Re × search path Im Re (Growth rate) SASI modes correspond to the poles of the Laplace transformed perturbed shock radius (= eigenvalue) in the complex plane

Model setup Background steady flow: (The distance between the shock front and PNS ～ 50 km: rather small) Perturbations (2 patterns): (for every quantities: rho, v, etc. )

Results External forces do not affect the intrinsic SASI modes, which is obtained by the impulse force It only adds the simply oscillate mode whose frequency is the same as the input one. newly added mode by the external force Im(oscillate frequency) × × × Intrinsic modes × Re (Growth rate)

Results Why? G: Green function that describes the response to the impulsive force f : External force Im Mode of the Intrinsic eigenmode external force × × × × Re

Results About the intrinsic modes: We found 17 growing modes for the range of ω > 1 [ms]. (#modes for larger frequencies seem to be countably infinite) And we identified the most rapidly growing mode. The most rapidly-growing mode appears to be driven by the purely -acoustic cycle. One of the 4 advective-acoustic modes grows exponentially whereas the other modes are decaying ones. The growth rate of the advective-acoustic mode is, however, much smaller than the purely-acoustic-cycle modes.

Summary Oscillations do not affect the SASI activity, which only add a simple oscillate mode, as long as there are only growing or decaying modes. * The exponentially growing or decaying modes in SASI do NOT resonate with the simply oscillating force. * How about the evolutionary paths of each mode? For a background, where the distance between the shock front and PNS is rather small, the purely acoustic cycle appears to be more efficient than the advective-acoustic cycle. * How about other backgrounds, inner boundary conditions, or higher ells?

Appendix

Aim & Scope Investigating the impact of fluctuating upstream flows on shock dynamics (SASI) in core-collapse supernovae analytically in detail (linear analysis). Upstream (pre-shock flow) Consistently, we began the study of the growth and time-variability of fluctuation in super-sonic flows. Published (KT & Yamada 2014) Downstream (post-shock flow) Then, we investigate the impact on shock dynamics, SASI. In preparation

Aim & Scope Investigating the impact of fluctuating upstream flows on shock dynamics (SASI) in core-collapse supernovae analytically in detail (linear analysis). Upstream (pre-shock flow) Consistently, we began the study of the growth and time-variability of fluctuation in super-sonic flows. Published (KT & Yamada 2014, Ap. J) Downstream (post-shock flow) Then, we investigate the impact on shock dynamics, SASI. In preparation

Formulation Rankine-Hugoniot relations: U: conservative variable, F: corresponding flux, q: velocity of the shock, : jump across the shock n: normal vector of the shock surface Shock surface:

Yamamoto et al. (2013)