雙中子星合併之廣義相對論性模擬與千新星殘骸之局部混合

Binary Neutron Star Merger and the Mixing of Local Kilonova Ejecta

林家暉(Chia-Hui Lin)

指導教授 ： 朱有花

共同指導教授 ： 陳科榮

國立臺灣大學/理學院/物理學研究所/碩士(2019年)

https://doi.org/10.6342/NTU201904289

摘要

人類第一次重力波的偵測開啟了重力波天文學的世代，緻密天體的 研究因此快速地前進，其中我們對雙中子星合併最感興趣，除了它本 身豐富的物理涵蓋了四大交互作用，也因為它是重力波源並且有光學 對應體而能支援多波段觀測，該合併過程被認為是 R-過程 (快中子捕 獲過程) 元素的主要來源，其中不穩定之元素衰變會釋放電磁波，也就 形成千新星。
我們使用 Einsteintoolkit 來做雙中子星合併的三維相對論性磁流體力學 模擬，並且比較了不同的演化方式與初始設定，我們發現沒有考慮磁 場的合併會拋出較多物質，此外若模擬的雙中子星與我們的距離在五 百萬秒差距內則使用 LIGO 可以觀測到，為了與未來的觀測比較，我 們以另外一個程式 FLASH 來放大模擬尺度。 另外為了探討千新星拋出物質的混合，我們提出考慮兩層流體與不同 初始條件的模型，藉此我們探討了局部性混合的程度與不同物理量 (如 密度、溫度、速度、核反應放熱速率) 之間的關係，我們發現了超聲速 流體造成的特徵，且核反應放熱速率會影響該特徵。 總的來說，我們探討了廣域的雙中子星合併與局部的千新興殘骸混合， 在物理尺度下不同型態的混合值得未來的探索。

關鍵字

雙中子星 ； 廣義相對論 ； 千新星 ； 不穩定性 ； 超聲速流體

並列摘要

Binary neutron star (BNS) merger is rich in fundamental physics with gravitational wave (GW) signals and electro-magnetic(EM) counterparts, and it is believed to be the major site of producing r-process elements, and the ra- diative decay of r-process elements power the kilonovae.
In this thesis, I present the results of my 3D GRMHD simulations of BNS considering different numerical schemes with various initial setups by using a full GR code, Einsteintoolkit. My results suggest that the simulations with- out consideration of magnetic field produce more outflow of dynamic ejecta and the resulting gravitational wave signal can be detected by LIGO if the merger site is less than 5 Mpc. For making predictions for future observa- tions, I extend my simulations by using a multi-D hydro code, FLASH to produce synthetic observations of the kilonovae.
In order to investigate the ejecta of nuclear heating from the decay of r-process elements on the outflow. I also perform the local mixing of kilonova ejecta with 2-layer fluid model in different initial conditions. The relation of insta- bility growth rate and different physical variables such as velocity, nuclear heating rate, density and temperature is presented. The pattern of supersonic flow is found, and nuclear heating has some impact on the final mixing.
On conclusion, both the universal view of BNS and local view of mixing of kilonova ejecta are investgated. Testing various types of mixing with at phys- ical scale is worth of future exploration.

並列關鍵字

binary neutron star ； general relativity ； kilonova ； instability ； supersonic flow

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