對磁聲波觸發前恆星核塌縮之模擬

Abstract

於此研究我們分析由環境中波狀擾動所觸發的前恆星核的塌縮。在前人的研究結果中，不考慮磁場及旋轉所造成的效應下，模擬結果顯示當波狀擾動以脈波的形式撞擊核心，脈波的振幅大於背景值的~2.5% 時，便足以在最適條件下造成核心塌縮。藉由將延伸至理想磁流體系統的鬆弛全差變遞減方法套用到 GAMER 程式碼中，我們得以考慮背景磁場對上述波致塌縮機制的影響。另外，考慮到系統中發生的加熱及冷卻效應，我們使用了兩種特別的狀態方程，它們最大的特點在於所牽涉的絕熱指數具有密度的相依性。 在背景磁場的影響下，進入完全鬆弛狀態的核心其形狀改變為一扁平橢球，而當我們不考慮磁場造成的效應，原本的球狀核心在鬆弛過程中不改變它的形狀。對於完全鬆弛的核心，我們用脈波撞擊之。當脈波到達核心時，它會在核心外表面產生向內傳播的橢圓震波，如果脈波夠強，這些震波的匯聚就足以造成核心的塌縮。對此機制來說，若使用第一種狀態方程，最適頻率落在 v/v0=4.5；而當我們使用第二種狀態方程，最適頻率則會減小至 v/v0=1.5。 當脈波以垂直背景磁場的方向撞擊核心，若要導致塌縮，脈波的振幅需有背景值的 16.5%，這比我們不考慮磁場效應的對應組還多了~30%。同時若脈波傳遞方向與背景磁場重合，波致塌縮機制的效率亦會顯著減小。最後，藉由將我們波致塌縮模擬的結果與前人相比較，兩者波致塌縮的過程差異讓我們對此機制有更深入的了解。

We analyze the collapse of isolated starless cores triggered by the impingement of wave-like perturbations. Previous work has shown that in the absence of the magnetic field and the initial rotation, a Gaussian sound pulse with its amplitude at least ~2.5% relative to the ambient gas can set off the collapse in the optimal case. By implementing the ideal MHD extension of relaxing TVD (RTVD) scheme into the GAMER code, we study this collapse mechanism regarding the magnetic interplay and the core rotation. To model the heating and cooling effects, we apply two types of the equation of state (EOS) that the adiabatic index is density-dependent. In the magnetic cases, the fully-relaxed core is a oblate spheroidal, while the spherical shape is retained in the non-magnetic case. Different Gaussian fast pulses are then launched toward the cores. On its arrival, the pulse generates inward spheroidal shocks on the core surface. The convergence of the shock fronts results in the core collapse provided the pulse is strong enough. The favorite frequency of the mechanism is v/v0=4.5 when the type 1 EOS is applied. In the cases of the type 2 EOS, the favorite frequency shifts to a smaller value, v/v0=1.5. For a pulse propagating perpendicular to the background magnetic field, the required pulse amplitude is 16.5% relative to the ambient, ~30% more than the corresponding value in the non-magnetic case. Meanwhile, the alignment between the pulse propagating direction and the magnetic field lines substantially decreases the efficiency of the material compression. Finally, we compare our results to the previous work to gain more insights into the nature of the wave-triggered collapse.