船底座星雲之多波段研究; A Multiwavelength Study of the Carina Nebula

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题名:	船底座星雲之多波段研究;A Multiwavelength Study of the Carina Nebula
作者:	高莎;Kausharbanu Sanchawala
贡献者:	天文研究所
关键词:	star formation;massive stars;X-ray;Near-infrared
日期:	2007-05-31
上传时间:	2009-09-22 10:12:54 (UTC+8)
出版者:	國立中央大學圖書館
摘要:	我們針對船底座星雲中央約400 平方角分的區域，進行多波段觀測研究，對象包括了 Trumpler (Tr) 14 與 Tr 16 兩個星團。我們結合了X 射線以及近紅外波段數據以指認此區域之前主序年輕恆星。一般恆星形成區含有大量塵埃，和可見光相比，近紅外觀測比較不受消光影響，因此紅外波段的顏色成為辨識前主序星體的重要工具。另一方面，X 波段能夠有效指認出弱線金牛座T 型星，同時不受場星樣本污染。我們利用三筆錢德拉太空X 射線望遠鏡的資料，辨識出大約800 顆X 射線源，極限通量達到 10-14 ergs cm-2 s-1. 我們的紅外觀測資料，在 J、H，以及 Ks 波段分別達到極限星等18.5、17.5，以及 16.5 等，這比之前任何船底座星雲的觀測還深了2 到3 個星等，我們因此找出了大約1000 顆可能的金牛座T 型（也就是第二與第三類）恆星，以及數十顆可能之原恆星（第一類）。我們在此區域辨認出的年輕恆星，是截至目前為止最完整的樣本。每個O 型恆星我們都偵測到X 射線，但早期B 型恆星則只偵測到10％。在某些中期與晚期B 型恆星當中，也偵測到X 射線，由於它們的X 射線通量與金牛座 T 型恆星相當，因此我們推斷這些中、晚期型恆星的X 射線，很有可能來自環繞在它們周圍的低質量伴星。在我們樣本中，大部分O 型恆星與早期B 型恆星的 X 射線光度與全波段光度皆遵循 Lx ∝ 10-7 Lbol 的關係，而且O 型恆星的X 射線的通量都沒有時間上的變化，這兩者皆吻合X 射線輻射來自恆星風震波的理論。我們的樣本中還包括16 顆以前沒有發現的OB 恆星，它們受到明顯的塵埃消光，因此之前的可見光觀測沒有能夠偵測到。前主序恆星與極度紅化天體 (H－Ks > 2) 的空間分佈顯示此區域有不同階段的恆星誕生活動，其中金牛座T 型星主要分佈在 Tr 14 與 Tr 16 兩個星團當中，而 Tr 14 包括更多的第二類星體，這表示 Tr 14 年輕得多。在Ks 波段觀測到 Tr14 的光度函數 (KLF) 呈現異常峰值，這是前主序恆星進行氘元素核反應所造成。相對而言，Tr 16 的 KLF 則依照預期，隨著星等增加緩慢上升。在 Tr 16 東南方，聚集了大量紅化明顯的星體，這些是此區域中更年輕的恆星族群。這個我們稱為 Tr 16-SE 的星群，包含了很多大質量恆星，其中一顆已知為O4 型恆星。另外在三個中紅外源附近，我們也發現證據，顯示恆星正在形成。這些都表示船底座星雲當中的大質量恆星，對於下一代恆星形成，有積極的誘發作用。 We carried out a multiwavelength study of the central ∼400 arcmin2 area of the Carina Nebula that covers the star clusters, Trumpler 14 (Tr 14), and Trumpler 16 (Tr 16). Our approach to identify the pre-main sequence (PMS) population of this massive star forming region is to combine the X-ray and near-infrared (NIR) wavelength information. NIR wavelengths, as compared to optical wavelengths, suffer much less problem due to extinctions which are typically high in a star-forming region and NIR colors of young stars provide a very useful tool to classify PMS objects. On the other hand, X-ray surveys provide clean samples of star forming regions as they suffer little contamination due to field stars and are the most efficient in identifying the weak-lined T Tauri population, which otherwise is difficult to be distinguished from unrelated field stars for its lack of classic signatures of youth, i.e., NIR excess, strong H emission, etc. We have used three archival Chandra datasets to derive a sample of ∼800 X-ray sources of this region, with a limiting X-ray flux of ∼ 10−14 ergs cm−2 s−1. With 10 limiting magnitudes of ∼18.5, 17.5, and 16.5 mag, in the J, H, and Ks bands, respectively, our NIR imaging observations are deeper by 2–3 magnitudes than any existing NIR observations of this large field in the Carina Nebula. We have identified ∼1000 T Tauri candidates (i.e. Class II and III candidates), and some dozen Class I candidates using the X-ray and NIR properties of the sources, thus producing the most comprehensive sample of the PMS candidates of this region. X-ray emission is detected from all the O stars and 10% of early-B stars in our field. We also detected X-ray emission from several mid- or late-B stars, with their X-ray luminosities being comparable to those typical of T Tauri candidates in our sample, providing a circumstantial evidence that the X-ray emission could come from a low/intermediate-mass PMS companion. Most O and early-B stars in our sample satisfy the canonical relation, LX ∝ 10−7Lbol, and none of the O stars are found to be variable in X-ray emission—both of these findings being consistent with the windshock model as the X-ray emission mechanism in these massive stars. We identified 16 candidate OB stars which have likely escaped previous detections due to larger extinctions they suffer in optical wavelengths. The spatial distribution of the PMS candidates and very red NIR sources (H − Ks > 2) suggests that we are apparently seeing star formation in different evolutionary stages. The T Tauri candidates are seen to be distributed in the direction of Tr 14 and Tr 16, with Tr 14 containing a much larger population of Class II candidates than Tr 16—implying a younger age for Tr 14. The Ks band luminosity function (KLF) of Tr 14 shows a sharp peak, likely due to the deuterium burning PMS stars—implying an age of 1–2 Myr for the cluster. In contrast, the KLF of Tr 16 is seen to rise smoothly until it turns over at Ks ∼17 mag due to the sensitivity of our data. Most of the red NIR sources, on the other hand, are seen to be concentrated to the south-east of Tr 16, where few T Tauri candidates are found, representing a still younger and probably the current generation of star birth. In particular, a compact, embedded group of ten X-ray sources, Tr 16-SE group, where many of these sources are massive star candidates, with one known O4 star, is discovered. The KLF of the Tr 16-SE group shows that the group has a much higher number density of sources relative to the field, indicating that it is indeed a bona fide young star group or cluster. We also find strong indications of ongoing cluster formation near three mid-infrared sources, where we see striking correlation of some very red NIR sources (H − Ks > 2) and hard X-ray sources.
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