運用SMA與ALMA研究原恆星盤的形成與演化

Abstract

行星系統誕生於環繞著原恆星的盤狀結構。為了瞭解行星系統的形成機制，就必須研究此類盤狀結構是如何形成的。目前天文學家認為，此類盤狀結構是隨著原恆星的誕生一起形成的，而且其形成的機制與演化和角動量如何從周圍的氣體傳遞到盤狀結構上息息相關。因此，為了研究盤狀結構的形成與演化，我運用了 SMT, ASTE, SMA 和 ALMA 觀測了數個不同演化程度的原恆星，測量其周圍氣體的旋轉速度，研究了（1）從半徑 10,000 到 100 天文單位的角動量分佈，(2) 角動量分佈如何演化，（3）周圍氣體如何傳遞角動量給中央的盤狀結構。我的觀測結果顯示：（1）原恆星的周圍氣體有較小的角動量，而離原恆星較遠的氣體帶有比較大的角動量。（2）在比較老的原恆星周圍的氣體旋轉的速度比較快，並且遵循克卜勒運動；而在比較年輕的原恆星周圍的氣體，旋轉速度較慢，並且位於不同半徑的氣體都帶有相同的角動量。（3）原恆星演化到後期，周圍氣體塌縮的速率已經下降，並且周圍氣體只沿著數條氣流吸積到中央的盤狀結構上，與演化初期氣體會均向的流向中央有所不同。運用理論學家所提出的氣體塌縮理論，我建立了一套解析解的模型，計算了角動量分佈的演化，並利用這套模型解釋了上述的觀測結果。

Circumstellar disks around young stellar objects are sites of planet formation. It is intriguing to understand the formation process of such circumstellar disks. In the formation process of a low-mass star, a circumstellar disk is expected to form in the innermost (<100 AU) region of a protostellar envelope around a protostar at an early evolutionary stage, and envelope material infalls and feeds the central protostar and circumstellar disk. Formation and evolution of circumstellar disks are closely related to the mechanism of angular momentum transportation in protostellar envelopes from the envelope scale (thousands of AU) down to the disk scale (~100 AU). To study this mechanism, it is essential to reveal the kinematics of protostellar envelopes around representative protostellar sources and to compare the kinematics among sources at different evolutionary stages. Therefore, I have conducted SMT, ASTE, SMA and ALMA observations toward a sample of protostellar sources, and studied (1) the rotational motion on the scales from 10,000 AU to 100 AU around a Class 0 protostar B335, (2) evolution of the rotational motions on the scales of 1000-100 AU of a sample of Class 0 and I protostars, and (3) the connection between the disk and the protostellar envelope around a Class I protostar L1489 IRS. To study rotational motion from large to small scales in protostellar sources, I have conducted observations in the millimeter C18O (2-1) and submillimeter CS (7-6) lines with the SMT, ASTE and SMA toward a prototypical Class 0 protostar, B335. In B335, the C18O (2-1) emission traces the protostellar envelope on the scales from ~10,000 AU to a few hundred AU, while the CS (7-6) emission shows a compact envelope component with a size of ~800 AU surrounded by an east-west elongated outflow component with a size of ~3000 AU. On the scale of 10,000 AU, the C18O envelope exhibits rotational motion with a specific angular momentum of ~2 x 10^{-3} km/s pc (V ~ 0.04 km/s at a radius of 9000 AU), comparable to those of other NH3 dense cores. On the scale of a few hundred AU, the C18O envelope exhibits infalling motion but no signature of rotational motion (V < 0.04 km/s at a radius of 370 AU). The CS (7-6) line, having a higher upper energy level and a higher critical density than the C18O (2-1) line, can trace an inner dense and warm region around protostars, where rotational velocity is likely higher than that in an outer region. On the scale of ~100 AU, the CS envelope shows rotational motion (V = 0.11 km/s at a radius of 110 AU) but no signature of infalling motion. These results show that the specific angular momenta of the rotational motion in B335 decrease from radii of 10,000 AU to a few hundred AU, and the specific angular momenta on the scale of a few hundred AU are one to two orders of magnitude lower than those in other Class I and II sources. To study evolution of rotational motions of protostellar sources, I have conducted observations in the C18O (2-1) line with the SMA toward three Class 0, one Class 0/I, and two Class I protostars. My observational results show that two Class 0 sources, B335 and NGC 1333 IRAS 4B, do not exhibit detectable rotational motion on hundreds of AU scale, while L1527 IRS (Class 0/I) and L1448-mm (Class 0) exhibit rotational motions with radial profiles of V ~ r^{-1.0+/-0.2} and ~ r^{-1.0+/-0.1}, respectively. The other Class I sources, TMC-1A and L1489 IRS, exhibit the fastest rotational motions among the sample, and their rotational motions have flatter radial profiles of V ~ r^{-0.6+/-0.1} and ~ r^{-0.5+/-0.1}$, respectively. The rotational motions with the radial dependence of ~ r^{-1} can be interpreted as rotation with a conserved angular momentum in a dynamically infalling envelope, while those with the radial dependence of ~ r^{-0.5} can be interpreted as Keplerian rotation. To study the connection between circumstellar disks and their surrounding protostellar envelopes, I have conducted observations in the 1.3 mm continuum and the 12CO (2-1), C18O (2-1), and SO (5_6-4_5) lines with the ALMA toward a Class I protostar L1489 IRS. A circumstellar disk in Keplerian rotation around L1489 IRS is clearly identified in the 12CO and C18O emission, and the central protostellar mass is estimated to be 2.0 Msun. In addition, there are arm-like structures attached to the circumstellar disk, and their kinematics cannot be explained by the Keplerian rotation. These non-Keplerian structures could trace accretion flow following parabolic trajectories toward the disk. The SO emission primarily traces the transitional regions between the accretion flow and the disk, which could be due to the enhancement in the SO abundance in the regions of accretion shocks. From my observational results as well as those from literatures, I have found the kinematics of protostellar envelopes on 100-1000 AU scales around Class 0 and I protostars can be categorized into three groups, (1) infalling motion with little rotational motion around Class 0 protostars, (2) both infalling and rotational motions around Class 0 and I protostars, and (3) Keplerian rotation around Class I protostars. I propose that the three categories reflect the evolution sequence from infalling envelopes to formation of Keplerian disks. In an early stage of collapse of a dense core, the envelope material with a small angular momentum in the vicinity of the protostar collapses first, and the protostellar envelope on 100-1000 AU scales shows infalling motion but little rotational motion. As the expansion wave propagates outwardly, the envelope material with a larger angular momentum in an outer region start to collapse. As more angular momenta travel toward the center with the infalling motion, rotational velocities of the protostellar envelope on 100-1000 AU scales and the size of the central disk increase. With time, the protostellar envelope dissipates due to the mass ejection from outflows and the mass accretion onto the central protostar and disk. At a later evolutionary stage, the envelope material is infalling along few parabolic flows (not isotropically) due to the protostellar envelope is partially dissipated, and a Keplerian disk with an outer radius of hundreds of AU appears. Based on the inside-out collapse theory of protostellar envelopes, I have constructed an analytical model and computed evolution of radial profiles of rotational velocities, to interpret the observed results in the context of formation of large-scale (>100 AU) disk.