- 學位論文
Identification of Herbig Ae/Be Stars in Open Clusters
Identification of Herbig Ae/Be Stars in Open Clusters
摘要
Herbig Ae/Be星是中質量的年輕星球，它們在主序前的演化速度快，使得我們所觀測到的數量，以及對這一類星球的了解，遠少於低質量年輕星球。在觀測上，Herbig Ae/Be星除了發射譜線，多數還具有紅外超量的特性；我們利用2MASS資料庫在疏散星團中挑選具有紅外超量的星球，並使用低色散光譜儀驗證這些Herbig Ae/Be候選星的譜型以及它們是否具有發射線。在觀測的11個候選星中，我們在星團Dolidze 16裡辨認出一個Herbig Ae星球，另外在其它星團裡則辨識出5個具有極大紅外超量的低質量年輕星球。最特別的乃是星團NGC 1857當中一顆具有Herbig Be星的特徵的B型星VES 888。我們的光度觀測顯示NGC 1857是個108.35年的中年星團，遠老於具有Herbig Be星之星團的年齡，因此推論VES 888為典型Be (classical Be) 恆星。它們的自轉接近碎裂速度(break-up velocity)，且其光譜具有發射線，然而這一類型星球多數沒有明顯的紅外超量，突顯了 VES 888的特殊性。在恆星的演化上，VES 888接近主序帶的轉折點；另外二個在NGC 1857裡的典型 Be 恆星僅有少量的紅外超量，位在相對於VES 888較遠離轉折點的主序帶上。典型Be恆星的紅外超量，在多數的例子中是來自於高溫的自由離子躍遷過程產生的輻射，似乎隨著恆星漸漸離開主序帶的過程中增加：恆星風將外殼層物質吹離表面，加以高速的自轉，使得物質堆積在盤狀結構上，吸收中心恆星的光線再往外輻射，造成明顯的紅外超量現象。
並列摘要
Herbig Ae/Be stars are intermediate-mass young stars. The fast evolution in the premain sequence phase causes a much less number of-and hence relatively poorer knowledge toward|Herbig Ae/Be stars than that of low-mass young stars. In addition to the emission lines in the spectra, the Herbig Ae/Be stars are characterized by infrared excess. We utilized the 2MASS database to identify Herbig Ae/Be candidates in open clusters, and carried out low-dispersed spectroscopic observations to study the nature of these candidate stars. Out of 11 observed sources, we have found 1 Herbig Ae star in the open cluster Dolidze 16, and in other clusters 5 extremely red low-mass young stars. Most notable is the star VES888 in NGC1857. Our optical photometry suggests that the age of the cluster is about 108:35 years, too old to host a Herbig Ae/Be star. It turns out that VES888 is a classical Be star, which as a group is characterized by rapid rotation close to break-up and emission lines in their spectra. Typically however the classical Be stars are lacking in infrared excess, hence making VES888 a rare case. VES888 is near the main sequence turn-off, whereas two other classical Be stars in the cluster, with much weaker IR excess, are both in earlier evolutionary stages. The infrared excess in classical Be stars, while in most cases arising from thermal free-free emission, seems to be escalated when the star is evolving off the main sequence to pu up the stellar envelope, in a disk-like geometry because of the rapid rotation, in which the dust grains condense to reprocess stellar light to be responsible for the strong circumstellar infrared emission.