恆星形成區的多波段觀測

Abstract

了解恆星如何形成一向是天文物理學中重要的課題。在恆星形成區中，有系統地研究剛形成的年輕恆星，有助於增進對恆星形成過程的了解，例如：恆星形成率及年輕恆星體在不同演化階段的生命期。這些參數，也有助於研究銀河系中恆星形成的物理機制。在過去數年中，我們利用不同波段的望遠鏡及數據資料，如：加法夏望遠鏡的紅外線相機、亞歷桑納電波天文台的次毫米波望遠鏡及史匹哲太空望遠鏡，研究蛇夫座及Sh 2-235分子雲中的恆星形成區。藉以研究低質量及大質量恆星形成區中的環境及恆星形成的活動。

我們根據年輕恆星體的光譜，加上一系列的過濾方法，發展了一個尋找年輕恆星體的新方法。我們利用SWIRE河外天體資料，及已知的年輕恆星體進行可靠度測試，該方法能夠偵測到92\%已知的年輕恆星體，而誤測率小於1\%。利用此方法，我們在蛇夫座分子雲中找到總數432個年輕恆星體，較c2D計畫增加45\%，光度較暗。Class I及Flat年輕恆星體數量的增加，說明兩者的演化生命期分別應為1.0及0.9百萬年。根據此生命期估計得到的每百萬年恆星形成率較低，但恆星形成效率較高。利用本研究估計所得的Kenncutt-Schmidt關係，與c2D計畫所得的結果一致。

我們也觀測兩個位於Sh 2-235分子雲中的大質量恆星區，利用統計方法選擇星團成員星並去除背景星，據以估計此星團的年齡及質量。Sh 2-233IR SW 及 Sh 2-233IR NE的年齡分別約為0.3及0.5百萬年，而G173.58+2.43得年齡約為0.8個百萬年。在Sh 2-233IR的核心，恆星形成效率可達40\%，而G173.58+2.43的平均恆星形成效率為20\%皆大於低質量恆星形成區。相較於Salpeter初始質量函數，兩個年輕星團的初始質量函數斜率較為水平，說明大質量恆星在兩區域的形成機率較高。兩恆星形成區的年齡，及初始質量函數的相似性，似乎說明兩者幾乎同時開始形成，且周圍的物理環境相當類似。Understating how stars are formed is an important topic in astrophysics. Detailed study of young stellar population in star-forming regions is important for understating physical process during star formation, such as star formation rate and lifetimes of young stellar objects (YSOs) in different evolutionary stages. These are key parameters to study the physical mechanism of star formation in Milky Way. In the past few years, we have carried out multi-wavelength observations, toward star forming regions in Ophiuchus cloud and Sh 2-235 complex, using CFHT/WIRCam and SMT of ARO, together with archived data from Spitzer. Our goal is to study the star formation activities in low-mass and massive star forming regions.

We have developed a new YSO identification method based on YSO SED fitting and source filters. A validity test using SWIRE and known YSOs is carried out. The method is able to recover 92% of c2D YSOs and the false detection rate is less than 1%. Based on this new method, we have detected 432 YSOs in Ophiuchus cloud, which is about 45% more than that of c2D project. The newly detected YSOs arefainter than c2D YSOs. The large increments of Class I and Flat YSOs implied that their evolutionary lifetimes should be 1.0 and 0.9 Myr, respective. The star formation efficiency (0.09) is higher than c2D estimation, while the star formation rate is lower because the lifetimes adopted is longer. Moreover, the Kenncutt-Schmidt relation derived using our value is consistent with c2D project.

Observations toward two massive star forming regions in Sh 2-235 complex are also carried out. By applying statistical subtraction of the background stars, we identified member sources and derived the age and mass of these groups. We derived ages of $\sim$0.3 and $\sim$0.5 Myr for Sh 2-233IR SW and Sh 2-233IR NE, respectively, while the age of G173.58+2.43 is ~0.8 Myr. The star formation efficiencies are found to be > 40% in the center of Sh 2-233IR and ~20% in G173.58+2.43. The slope of IMF of these two embedded clusters are both flatter than Salpeter IMFs, which may indicated that massive stars have efficiently formed in these regions. The similarities in ages and IMF slopes of star forming regions in Sh 2-235 complex may suggest these regions to be formed at similar epoch and ambient environments.