次毫米波段之原恆星光變性質分析

Abstract

恆星的形成與演化，在天文物理學上是一個重要的課題。為此我們投注心力，期待能夠了解氣體如何聚集並壓縮、進而形成原恆星，爾後演化至主序帶的整體過程。透過可見光以及紅外光波段的觀測，目前學術界已經架構出一套相對完整的模型，用於描述原恆星演化進入主序帶的過程。然而，針對恆星形成過程裡更初期的階段，卻仍然留有許多未定論。這是因為原恆星在其初形成時期，深處於厚重的雲氣與塵埃之中，無法直接利用可見光及紅外光等波段進行觀測。因此，透過觀測波段的選擇，如果是使用次毫米波段來進行觀測，因為此波段的輻射不會受到塵埃與氣體的遮擋 (消光)，我們便能夠更直接地觀測處於初生階段的原恆星。

JCMT Transient Survey 是一個使用次毫米波段觀測的長期監測計畫，此計畫的研究目標是探尋原恆星天體在其新生階段的「光變現象」。年輕恆星體如果在短時間內 (大約是月至年的時間長度) 變亮，此光變現象即是恆星形成假說裡「間歇性質量吸積」這類模型的有力證據。從 2020 年 2 月份開始，記錄至 2022 年 3 月為止，透過每月一次的固定量測，我們總共累積了 15 期針對 S255 天區的觀測資料。其中單次的觀測是以影像的方式記錄；透過影像分析，我們擷取影像中輻射強度的峰值位置，來代表原恆星天體。依照 Johnstone et al. (2018) 的分析方法來檢測 S255 天區，在 850 微米波段的觀測影像中，我們挑選出 51 個亮度超過 5 個觀測標準差的「次毫米天體」；經分析，並未找到單次隨機性的光強爆發現象，又或者是長週期性的亮度變化趨勢。為了解釋 S255 天區的監測結果：不同於計畫中其他鄰近的恆星形成區域 (古爾德帶)，在其設計之觀測視野內並未發現任何明顯的光變現象；為此我們設計了影像模擬實驗：將同計畫的其他天區放遠至與 S255 相當的距離。模擬的結果顯示，如果絕大多數的天體維持相等的光變幅度，將其置於更遠的距離下，觀測到的亮度變化量會減弱至同單次的觀測誤差相近之程度，以至於無法辦別天體是否真存有光變現象；而另一個次要的因素則是多個天體會因為解析度不足而混在一起，如各天體皆具有不同的光變幅度，此效應將加劇量化分析的難度。而針對大質量原恆星，如 S255-IR 及 S255-N 這類天體，因為於其他天區中並未有相似質量的天體，故無法與之前的研究結果直接相比較。於文獻中，S255-IR 是位於 S255天區的觀測視野內，已知具有明確亮度變化的天體；但單就目前現有的兩年監測結果，我們並未在這類大質量原恆星之中，觀察到任何明確的光變現象。

本碩士論文的架構將分為三部分，於第一章介紹恆星形成的概觀及研究動機。第二章呈現 JCMT Transient Survey 的實驗設計與結果分析。最末章則總結整體研究計畫並提出未來可行之延伸目標。

How do stars form? In addition, how human beings use their entire life as a snapshot to understand an almost eternity, the whole life of a star? It is a thrilling topic for me and the very beginning of this time-domain study.

Observations from the well-developed optical/IR bands have already established a clear paradigm of how young protostars get into the main sequence. However, how the protostar formation begins and how they evolve in the early stages of star formation still remain puzzling. As young protostars are enshrouded in their natal molecular cloud environment, the dusty surroundings just besiege most of the optical/IR emission. On the contrary, the submillimeter & millimeter observations are able to dive deep to disclose the baby stars in the protostellar stage.

The JCMT Transient Survey routinely monitors a sample of star-forming regions, including the S255 region. Toward S255, 15 epochs of 850-µm observations have been conducted since February 2020 till March 2022. Following the same analysis methodology introduced by Johnstone et al. (2018), we carried out the clump identification and searched for flux variability toward candidate protostellar clumps. We found no evident variable in either the secular or the stochastic type toward 51 clumps brighter than 60 mJy/beam, i.e. 5-sigma detection, in the 850-µm band.

To help interpret the result, we designed an image simulation by making use of the Herschel Gould Belt Survey (HGBS) images of the nearby low-mass star-forming regions and mimicking the SCUBA-2 observations of these regions but at farther distances of S255. We demonstrated that the types of variables with 10-50% fractional variation seen in the nearby star-forming regions would not be recognized if they all are shifted to the distance of S255. This is primarily because their variation amplitudes get buried beneath the single-epoch noise. Source merging then just exacerbates our detection limit of variability. On the other hand, these new observations toward S255 sample massive OB-type protostars (e.g. S255-IR & S255-N) and high column density cloud patches that have no correspondence in the nearby low-mass Gould Belt regions. In spite of the submillimeter variability of S255-IR reported by Liu et al. (2018), we find no evidence of variation either in the clumps at high-mass end in our Transient data.

The contents of the thesis are organized as the following. In Chapter 1, an introductory review and the research motivation are presented. I describe the JCMT Transient Survey and my analysis and interpretation of the survey data toward S255 in Chapter 2. In the last chapter, a summary is given accompanied by the future perspectives of this research.