原恆星系統L1551 IRS5及L1551 NE之分子雙極噴流

Abstract

L1551分子雲為一低質量恆星形成區域，且為距太陽系最近的恆星形成區域之一。觀測上顯示，此區域中存在許多不同演化階段的原恆星系統，意味著長時間以來，此區域皆為一活躍的恆星形成區域。我們使用次毫米微波陣列(SubMillimeter Array, SMA)觀測L1551分子雲中兩個原恆星系統—L1551 IRS5及L1551 NE—周圍區域的一氧化碳分子譜線，以探測該原恆星分子雙極噴流的物理性質。 L1551 IRS5是L1551分子雲中輻射熱光度最強的多重原恆星系統，且擁有巨大的分子雙極噴流。其大尺度分子雙極噴流呈東北(紅移端)—西南(藍移端)方向，各自向外延伸約1.5秒差距。我們藉由角分辨率約4角秒(約560天文單位) 的CO(2—1)分子譜線影像探測此噴流之中心約4000天文單位的區域。在此中心區域，我們分辨出三個不同噴流結構：(1) X形結構，自中心向外延伸約20角秒，其方位角與大尺度噴流相近，且有相同之紅藍移方向。此X形結構為中央高速噴流所開之錐狀空腔之臨邊增亮現象。(2) S形結構，自中心向外延伸約10角秒，其方位角與大尺度噴流相近，但紅藍移方向相反。此S形結構為新發現的分子雙極噴流。S形結構以及視線方向速度的變化顯示此噴流正在進動。此噴流可能是被此系統中一新發現的原恆星所驅使，且其環星盤與鄰近的原恆星有強烈潮汐作用而使環星盤進動。(3) 中央緊密結構，自中心向外延伸約1.4角秒，其方位角與大尺度噴流相近，且有相同之紅藍移方向。此結構為原恆星系統中心近期受中央高速噴流所驅使的分子氣體。 L1551 NE是L1551分子雲中輻射熱光度次強的多重原恆星系統，位於L1551 IRS5東北方2.5角分處，為L1551 IRS5的巨大分子雙極噴流影響。L1551 NE所驅使的分子雙極噴流尚未被確認。我們利用角分辨率約0.8角秒(約110天文單位)的CO(3—2)分子譜線影像探測此系統周圍較溫暖之氣體，首次成功拍攝下此系統分子雙極噴流的電波波段影像。此分子雙極噴流自中心延伸僅約8角秒，且其動力學年齡極短，顯示此為一年輕的原恆星系統，但其質量明顯小於其他位於同樣演化階段的分子雙極噴流。此現象指出，L1551 NE鄰近區域的氣體分子已被L1551 IRS5的強烈噴流掃除。另外，我們的一氧化碳分子譜線影像與紅外光觀測中之扇形反射星雲吻合，但卻未發現在紅外光波段同樣出現的束狀噴流。此現象暗示L1551 NE系統中的兩原恆星處在不同演化階段，驅使扇形反射星雲的原恆星較早演化或演化較快，其噴流將鄰近區域之分子氣體掃除。較晚演化的原恆星之束狀噴流驅使之分子噴流密度因此較低而無法觀測。

L1551 molecular cloud is located at a distance of ∼ 140 pc, in the constellation of Taurus, and is one of the nearest active low-mass star formation region. We used the Submillimeter Array (SMA) to image the CO molecular outflows from two multiple protostellar systems in L1551 region, L1551 IRS5 and L1551 NE, at an arcsecond angular resolution. We observed the CO(2–1) line emission in L1551 IRS5 system. First, we imaged the base of the outflow cavity and found signatures for multiple outflow events. We also found newly entrained gas in the cavity, indicating that the ambient gas is still falling into the cavity. Second, we discovered a precessing outflow, which has an opposite inclination to it of L1551 IRS5 large-scale outflow. This newly-found component is likely driven by the recently proposed candidate third protostar in L1551 IRS5 system. For the L1551 NE binary system, we shows a clear bipolar structure of its molecular outflow at the first time. We saw a cavity-like structure corresponding to the reflection nebula at infrered wavelength, but no jet-like structure, which is also seen at infrared. It is likely that, one of the binary launched the outflow first and cleaned ambient gas away, thus the outflow from another protostar entrained less gas and is unable to be detected. The mass of L1551 NE outflow and the dusty envelope are both by orders of magnitude smaller than those of other Class 0 protostars, suggesting the ambient gas has been swept up by the strong outflow from L1551 IRS5.