仙王座β型變星,也稱為大犬座β型變星。這種變星的變光快速但幅度很小,亮度變化被認為是由於鐵在內部20萬K下的異常性質,造成表面的脈動。這些變星通常是光譜為B型的藍白色高溫恆星。不要與造父變星混淆,後者以造父一(仙王座δ)為原型命名,是亮超巨星。
性質
仙王座β型變星是質量介於7至20 M(太陽質量)的主序星。這些成員中有一些是天空中最亮的恆星,例如十字架三和馬腹一;角宿一也被歸類為仙王座β型變星,但在1970年神秘的停止了脈動[1]。通常,它們的亮度變化在0.01至0.3星等,週期0.1至0.3日(2.4至7.2小時)[1]。做為原型的上衛增一(仙王座β)視星等在 +3.16 到 +3.27 間變化,週期為4.57小時。亮度最亮的時刻發生在恆星最小且最熱的時刻。它們在紫外線上的亮度變化要大得多,可以達到1個星等[2]。少數恆星的變光週期被發現短於一小時,對應於基礎的徑向脈動週期的四分之一和基礎週期的八分之三。它們也有相對較低的振幅和相當窄的光譜類型B2-3 IV-V。它們被稱為短週期組,在GCVS的縮寫是BCEPS[3][4]。
仙王座β型變星的脈動式是由卡伯機制和p-模式脈動驅動。在恆星內部,溫度高達20萬K的深處,有豐富的鐵。鐵在如此的高溫下,不透明度將增加(不是減少),導致能量在數層之內累積。這會導致壓力的增加而反覆地向外推出與落回,一個循環的週期約數小時。這被稱為鐵凹凸或Z凹凸(Z習慣上是表示金屬量)[5]。這與慢脈動B星類似,顯示同樣是由鐵的不透明度變化驅動的p-模式脈動,但是有著較低的質量和較長的週期[6]。
觀測的歷史
美國天文學家Edwin Brant Frost在1902年發現仙王座β徑向速度的變化,最初的結論說是聯星的光譜。Paul Guthnick在1913年率先檢測到它的亮度變化[7]。不久之後,發現軍市一(大犬座β)和心宿一(天蝎座σ)也是變星[2], 維斯托·斯里弗在1940年指出,心宿一的徑向速度也有變化,還有R.D. Levee和奧托·斯特魯維分別在1952年和1955年推論出這是由於恆星的脈動[8]。 這種變星因為大犬座β星是20世紀末對其研究最密切的例子,因此通常稱為大犬座β型變星,然而它在南部低空的位置,意味著它的觀測經常會受到阻礙[9]。然而,因為仙王座β是第一顆被發現的這種類型變星,所以儘管有和造父變星相似而被混淆的風險,這一類型還是被稱為仙王座β型變星[2]。
塞西莉亞·佩恩-加波施金和Sergei Gaposchkin在他們1938顆變星的目錄中收錄了17顆可能是這類型的變星,但是將它們歸類為矮造父變星(盾牌座δ型變星)[10]。車府增十五(蝎虎座16)是在1952年之前被廣泛研究的另一這類型的變星[9]。在1966年,此類型變星的數量從18顆增加為41顆[11]。 奧托·斯特魯維在20世紀50年代廣泛的研究這種變星,然而在他逝世之後,研究就減少了[2]。
Christiaan L. Sterken和Mikolaj Jerzykiewicz 在1993年將59顆恆星歸類為仙王座β型變星,並將79顆列為疑似仙王座β型變星[12]。 Stankov在2005年列出93顆成員,77顆候選者,還有61顆可能不是或確定不是這類型的變星[13]。6顆,分別是天棓五(武仙座ι)、雙魚座53、九州殊口四(波江座ν)、壁宿一(飛馬座γ)、英仙座V354和白羊座53被並列為仙王座β型變星和SPB[14]。
仙王座β型變星列表
被排除或候選的仙王座β型變星列表
參考資料
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