Newsletter No. 478

4 478 • 19.5.2016 洞明集 In Plain View 恆星起源— 李華白的尋星之旅 (下圖,背景為位於智利的阿塔卡馬次毫米望遠鏡) Before a Star is Born: The Quest of Hua-bai Li for the Origin of the Stars (He's seen below in front of the Atacama Submillimeter Telescope in Chile) 中 大物理學系 李華白 教授率領的天文物理學家團隊, 發現磁場在高質量恆星的演化過程中起着關鍵性的 作用。 氣體微塵充塞宇宙,當這些星際物質受萬有引力影響積聚 到一定密度,恆星便有機會誕生。但宇宙中除了萬有引力之 外尚有其他的作用力,據李教授解釋,雲氣會被星際湍流打 散,而磁場則會把雲氣依磁場線分布排列,因為雲氣微粒中 含有離子。萬有引力、湍流與磁場之間的角力,一直以來都 是天文物理學家熱中研究討論的題目。 假如萬有引力主宰一切,那麼雲氣便會均速地自由聚降至一 個中心點,直至第一個核聚變出現。理論上,所有雲氣最終 都應該在這個自由聚降的過程之後(以一百萬年計)形成恆 星,但實證及統計數據卻顯示,只有很少部分的雲氣最終形 成恆星。 李教授和他的團隊感興趣的是磁場在高質量恆星的形成中 所扮演的角色。他們瞄準位於天蝎座的NGC6334—貓掌星 雲,因為這裏的雲氣質量是我們的太陽的二十萬倍,為高質 量恆星的誕生提供了肥美土壤,而且雖然離地球五千五百光 年,但在天文學角度來說算是相當近,令可靠的觀測得以順 利進行。 研究團隊從夏威夷與南極洲兩地的觀測站收集數據,繪製 出NGC6334不同標度的磁場線圖,審視其按磁場線排列的 雲氣分布。結論是,不論以何種標度分析,雲氣分布均以扁 平狀態呈現,而且垂直於磁場線,周邊的雲氣則凝成塊狀, 把磁場線微微挾起。 結果清楚顯示磁場在NGC6334中佔主導地位。磁場在這裏 既然扮演了恆星形成的主導力量,那麼萬有引力和湍流便需 角逐剩餘的影響力。萬有引力較強的話,雲氣的呈現形狀便 會更清晰,恆星形成的速度也會較快;反之,形狀便會模糊, 恆星形成的速度也較慢。 李教授的研究結果去年刊登在《自然》期刊。 李教授的另一項研究也印證了這個結果。他們研究三角座星 雲(也稱M33)的分子雲中的恆星形成,發現那裏的磁場排 序也跟整體的星雲磁場相同。 李教授大學時候在台灣唸數學和物理,其後在美國西北大學 取得博士學位,2013年來中大任教,並領導恆星形成的研究。 他的團隊模擬星際力學的成就,有賴中大物理學系內一部擁 有一千五百個中央處理器的高能電腦。物理系也正在建造一 個繪製磁場的儀器,李教授期待儀器建成後安裝在位於智 利阿塔卡馬的次毫米望遠鏡中,將有利他們探討恆星起源的 下一步工作。 A group of astrophysicists at CUHK led by Prof. Hua-bai Li of the Department of Physics has found that the magnetic field plays a significant role in the formation of massive stars. The universe is filled with gases and dusts which, when collapsing into sufficient density due to gravitational pull, form the celestial bodies such as our stars and planets. But there are other forces at work where stars are likely to be born. As Professor Li explains, turbulent cloud motions would disperse the gases while the magnetic field, or B-field in the astrophysicists’ parlance, would align the gas motion along the magnetic field lines because the particles themselves contain ions. The interplay of gravity, turbulence and B-field has long been a subject of rigorous research and debate among astrophysicists. If gravitation is the sole determinant, the collapse of the gases would be a uniform free-fall into a core which would trigger off the first nuclear fusion. In theory, all the gases should be turned into stars during the free-fall time in the order of one million years. But empirical and statistical data have suggested otherwise. Only a small percentage of gas will eventually form stars. Professor Li and his team examined the role B-field plays in the formation of massive stars. They chose to focus on NGC6334, also known as the Cat’s Paw Nebula, because the gas in this region has a total mass 200,000 times that of our sun and is a fertile ground for massive stars to sprout. Also, while its distance from the Earth is a staggering 5,500 lightyears away, in astronomical terms it is close enough for valid observations to be made. Using data gathered at various observatories in Hawaii and the Antarctica, the team was able to map out NGC6334’s magnetic field structure at a range of different scales and examine its cloud fragmentation with ordered B-fields. At all the different scales, the same pattern emerged: the cloud forms itself into a flattened structure perpendicularly to the B-fields, and at the end of the structure, the gas contracts into clumps, slightly pinching the B-field lines. The consistent pattern of gas fragmentation across all scales demonstrates the dominance of the B-fields in the region NGC6334. With the B-fields the primary ordering principle, gravity and turbulence come in to compete for second fiddle. If gravity prevails, the structure would be better defined and the star formation process will be more efficient. If turbulence prevails, the structure would be more tangled and theoretically it will take longer to form a star, These findings were published in Nature (Li, Yuen, Otto, Leung et al., ‘Self-similar fragmentation regulated by magnetic fields in a region forming massive stars’, Nature , vol. 520, 23 April 2015, pp. 518–521). The findings in NGC6334 were repeated in another study. Professor Li’s team zoomed in to study the star formation in the molecular clouds in the Triangulum Galaxy, also known as M33, and found the B-fields in these molecular clouds are ordered and aligned with the large-scale galactic B-fields. Professor Li studied mathematics and physics in university in his native Taiwan and later obtained a doctorate in physics from Northwestern University. He came to teach at CUHK in 2013 and lead the star-dust journey in stellar nativity. The team’s simulation of the interstellar dynamics has been greatly helped by the high-power computer at the Department of Physics with 1,500 CPU cores. Professor Li expects more data to come when a new B-field mapping instrument is built at the department and installed on the Atacama submillimeter telescopes in Chile.

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