孤立B[e]星的演化態及環星盤的特性;Evolution Status and Circumstellar Properties of Isolated B[e] stars

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题名:	孤立B[e]星的演化態及環星盤的特性;Evolution Status and Circumstellar Properties of Isolated B[e] stars
作者:	李建德;Lee,Chien-De
贡献者:	天文研究所
关键词:	發射線星;熱星;Be星;演化;紅外波段;次毫米波段;偏振;環星盤;emission-line stars;hot stars;Be stars;evolution;infrared;submillimeter;polarization;circumstellar disk
日期:	2016-07-29
上传时间:	2016-10-13 12:44:32 (UTC+8)
出版者:	國立中央大學
摘要:	我們研究八個演化態未定的B[e]型星,為了揭露它們環星塵埃的起源,本文的第一部份先確認目標星的演化狀態。我們進行兩項診斷分析,分別使用紅外波段以外的光譜能量分布,以及普查每個目標星的鄰近區域。接著第二部分,進一步利用偏振的特徵,來探討目標天體的環星盤的特性。相對於前主序赫比格星,這些B[e]星有相同高程度的近紅外超量輻射,意指溫熱塵埃的存在。然而在更長的波段輻射量卻大量減少,代表者包層大小非常致密。此外,它們孤立於恆星形成區之外,因此排除了前主序星的可能性。其中六顆目標星 , 包括了HD 50138、HD 45677、CD−24 5721、CD−49 3441、MWC 623以及HD 85567過去被認已為是FS CMa星,另外兩顆HD 181615/6以及HD 98922被新增到這個樣本裡。不同於赫比格星承襲原本分子雲氣裡頭的塵粒,我們認為這些孤立B[e]星的塵粒演化,已超過了前主序的階段,因此環星塵埃應該是原地形成的。特別的是,因為高度紅外超量的緣故,過去HD 98922被認為是年輕的赫比格星,我們提議它的演化應該在更成熟的狀態。在星球質量損失中,塵埃內而外的凝聚,所以其包層的空間被限制。這些星體的視線方向上,大多消光很低,因為各項相異的質量流,或是極小塵埃顆粒的異常光學特性。接著,我們展示兩顆B[e]星,HD 45677以及HD 501382,多個時期的光學偏振測量,觀測結果分別來自此研究以及文獻資料。修正目標前景的星際偏振後,波長相關的偏振特徵,暗示星盤內緣的主導者為氣體。另外,兩目標星的偏振觀測都出現了變化性,無論是長波段的偏振變化,或是整體波長相關的偏振特徵。這些觀測結果意味者,氣體與塵埃活動的交互作用,包括了B[e]星間歇性的質量損失、氣體密度的變化以及塵埃凝結。;We studied a sample of eight B[e] stars with uncertain evolutionary status to shed light on the origin of their circumstellar dust. In the first part of this work, we per- formed a diagnostic analysis on the spectral energy distribution beyond infrared wave- lengths, and conducted a census of neighboring region of each target to ascertain its evolutionary status. In the second part, we further studied the circumstellar disks of isolated B[e] stars characterized by polarization. In comparison to pre-main sequence Herbig stars, these B[e] stars show equally substantial excess emission in the near-infrared, indicative of existence of warm dust, but much reduced excess at longer wavelengths, so the dusty envelopes should be compact in size. Isolation from star-forming regions excludes the possibility of their pre- main sequence status. Six of our targets, including HD 50138, HD 45677, CD−24 5721, CD−493441, MWC623, and HD85567, have been previously considered as FSCMa stars, whereas HD 181615/6 and HD 98922 are added to the sample by this work. We argue that the circumstellar grains of these isolated B[e] stars, already evolved beyond the pre-main sequence phase, should be formed in situ. This is in contrast to Herbig stars, which inherit large grains from parental molecular clouds. It has been thought that HD 98922, in particular, is a Herbig star because of its large infrared excess, but we propose it being in a more evolved stage. Because dust condenses out of stellar mass loss in an inside-out manner, the dusty envelope is spatially confined, and anisotropic mass flows, or anomalous optical properties of tiny grains, lead to the generally low line-of-sight extinction toward these stars. Then, we present polarimetry of two isolated B[e] stars, HD 45677 and HD 50138, in optical wavelength obtained at several epochs including data from present work and literature. With a revised interstellar polarization correction, their characteristic wavelength-dependence of intrinsic polarization indicates a gas dominated disk. Additionally, these targets all exhibited significant variability. Either the additional polarization component in the long wavelengths or the variation of wavelength dependence were occasionally seen, in the sense that an interplay between sporadically mass loss, variation of gas density and dust condensation in the circumstellar envelope of B[e] stars.
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