經典造父變星

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經典造父變星 (也稱為第一星族造父變星第一型造父變星、或仙王座δ變星) 是造父變星的一種。它們是第一星族星,有著周期從數天至數月,質量是太陽4-20倍的變星[1],光度可以達到太陽的100,000倍[2]。經典造父變星是光譜類型在F6-K2的黃超巨星,在脈動週期中的半徑變化可以達到數百萬公里 (大約是船底座I的25%)[3][4]

經典脈動變星的光度和脈動週期之間存在著明確的關聯性[5][6],可靠的造父變星成為可行的標準燭光,建立起銀河和河外星系距離指標[7][8][9][10]。HST觀測經典造父變星使哈柏定律成為更為可靠的制約因素[7][8][10][11][12]。經典造父變星也用來闡明我們的銀河系,像是太陽位於銀河盤面的上方,以及銀河系在本地的螺旋結構[9]

銀河系內已知的經典造父變星超過700顆[13],在外星系也已經發現了數千顆。哈柏太空望遠鏡已經確認出距離一億光年的NGC 4603內的經典造父變星[14]

發現[编辑]

在1784年9月10日,愛德華·皮戈特檢測到天鷹座η的光度變化,這是第一顆被描述的經典造父變星。但是,這一種造父變星卻以幾個月後由约翰·古德利克發現的變星造父一為代表。造父一也是驗證周光關係時特別重要的一顆造父變星,因為他的距離是造父變星中最精確的,這要歸功於它的成員都在星團之中[15][16],並且可以從哈柏太空望遠鏡/依巴谷衛星得到可靠的視差[17]

周光關係[编辑]

經典造父變星的光度與它的變化週期有著直接的關係。脈動週期越長,光度也越明亮。這種周光關係是亨麗愛塔·勒維特在研究了麥哲倫雲的數千顆變星之後,於1908年發現的[18]。她於1912年與更進一步的證據發表[19]。一但校準了周光關係,只要知道脈動周期就可以建立給定的這顆造父變星的光度,然後從它的視星等就可以得知距離。從Ejnar Hertzsprung開始,20世紀有許多的天文學家都在校正周光關係[20]。校準期間的周光關係一直都有問題,不過本尼克等人在2007年使用哈柏太空望遠鏡精確的測量了10顆鄰近的經典造父變星的視差,確定了銀河系內穩定的周光關係[21]。同時,在2008年,ESO的天文學家使用迴光測量埋置在星雲中的船尾座 RS的距離,精確度達到只有1%的誤差[22]。但是,後者的發現一直是文獻中爭論的主題[21]

下面的經典造父變星周光關係公式,P是脈動週期, M_v絕對星等。這是建立在哈柏太空望遠鏡對鄰近的10顆造父變星以三角視差測距的結果:

 M_v = -2.43\pm0.12 (\log_{10}(P) - 1) - (4.05 \pm 0.02) \,

此處P的測量單位是天。 [17][21]。使用下列的公式也可以計算經典造父變星的距離:

 5\log_{10}{d}=V+ (3.34) \log_{10}{P} - (2.45) (V-I) + 7.52 \,. [21]
 5\log_{10}{d}=V+ (3.37) \log_{10}{P} - (2.55) (V-I) + 7.48 \,. [23]

IV分別表示近紅外線和視覺上 (可見光) 的平均星等。

以造父變星測量距離的不確定性[编辑]

綑綁在造父變星之間,對距離測量的不確定因素主要是:周光關係在不同通帶的本質,零點和斜率的關係這兩者都會衝擊到豐度,以及光度計的汙染 (混合)和衰減的變化規律 (通常是未知的),都會影響到造父變星測量的距離。所有這些主題在文獻中都有爭議 [24][2][8][11][25][26][27][28][29][30][31][32]

這些未解決的事項已經導致哈柏常數的值 (以經典造父變星建立) 在60 km/s/Mpc和80公里/s/Mpc之間不等[7][8][10][11][12],解決此一差異是天文學當今的首要問題之一,因為宇宙學的宇宙參數受到哈柏常數數值精確的制約和影響[10][12]

例子[编辑]

一些相當明亮的經典造父變星,展示出肉眼可以分辨的光度變化,包括:天鷹座η雙子座ζ劍魚座β,和據以命名的原型造父一。距離最近的經典造父變星是北極星 (勾陳一),雖然這顆恆星展示出許多特性,但是它的距離仍是一個爭議很多的議題[33]

相關條目[编辑]

參考資料[编辑]

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外部鏈結[编辑]

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