经典造父变星

经典造父变星(也称为第一星族造父变星第一型造父变星、或仙王座δ变星)是造父变星的一种。它们是第一星族星,有著周期从数天至数月,质量是太阳4-20倍的变星[1],光度可以达到太阳的100,000倍[2]。经典造父变星是光谱类型在F6-K2的黄超巨星,在脉动周期中的半径变化可以达到数百万公里(大约是船底座I的25%)[3][4]

经典脉动变星的光度和脉动周期之间存在著明确的关联性[5][6],可靠的造父变星成为可行的标准烛光,建立起银河和河外星系距离指标[7][8][9][10]哈伯太空望远镜观测经典造父变星使哈伯定律成为更为可靠的制约因素[7][8][10][11][12]。经典造父变星也用来阐明我们的银河系,像是太阳位于银河盘面的上方,以及银河系在本地的螺旋结构[9]

银河系内已知的经典造父变星超过700颗[13],在外星系也已经发现了数千颗。哈伯太空望远镜已经确认出距离一亿光年的NGC 4603内的经典造父变星[14]

发现

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在1784年9月10日,爱德华·皮戈特检测到天鹰座η的光度变化,这是第一颗被描述的经典造父变星。但是,这一种造父变星却以几个月后由约翰·古德利克发现的变星造父一为代表。造父一也是验证周光关系时特别重要的一颗造父变星,因为他的距离是造父变星中最精确的,这要归功于它的成员都在星团之中[15][16],并且可以从哈伯太空望远镜/依巴谷卫星得到可靠的视差[17]

周光关系

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经典造父变星的光度与它的变化周期有著直接的关系。脉动周期越长,光度也越明亮。这种周光关系是亨丽爱塔·勒维特在研究了麦哲伦云的数千颗变星之后,于1908年发现的[18]。她于1912年与更进一步的证据发表[19]。一旦校准了周光关系,只要知道脉动周期就可以建立给定的这颗造父变星的光度,然后从它的视星等就可以得知距离。从埃希纳·赫茨普龙开始,20世纪有许多的天文学家都在校正周光关系[20]。校准期间的周光关系一直都有问题,不过本尼克等人在2007年使用哈伯太空望远镜精确的测量了10颗邻近的经典造父变星的视差,确定了银河系内稳定的周光关系[21]。同时,在2008年,ESO的天文学家使用回光测量埋置在星云中的船尾座RS的距离,精确度达到只有1%的误差[22]。但是,后者的发现一直是文献中争论的主题[23]

下面的经典造父变星周光关系公式, 是脉动周期,  绝对星等。这是建立在哈伯太空望远镜对邻近的10颗造父变星以三角视差测距的结果:

 

此处 的测量单位是天。 [17][21]。使用下列的公式也可以计算经典造父变星的距离:

 [21]
 [24]

  分别表示近红外线和视觉上(可见光)的平均星等。

以造父变星测量距离的不确定性

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捆绑在造父变星之间,对距离测量的不确定因素主要是:周光关系在不同通带的本质,零点和斜率的关系这两者都会冲击到丰度,以及光度计的污染(混合)和衰减的变化规律(通常是未知的),都会影响到造父变星测量的距离。所有这些主题在文献中都有争议[25][2][8][11][26][27][28][29][30][31][32][33]

这些未解决的事项已经导致哈伯常数的值(以经典造父变星建立)在60 km/s/Mpc和80公里/s/Mpc之间不等[7][8][10][11][12],解决此一差异是天文学当今的首要问题之一,因为宇宙学的宇宙参数受到哈伯常数数值精确的制约和影响[10][12]

例子

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一些相当明亮的经典造父变星,展示出肉眼可以分辨的光度变化,包括:天鹰座η双子座ζ剑鱼座β,和据以命名的原型造父一。距离最近的经典造父变星是北极星(勾陈一),虽然这颗恒星展示出许多特性,但是它的距离仍是一个争议很多的议题[34]

相关条目

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参考资料

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外部链接

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