Evidence from zinc abundances for dust fractionation in chemically peculiar stars
Evidence from zinc abundances for dust fractionation in chemically peculiar stars
复制标题
化学奇特恒星中尘埃分馏的锌丰度证据
作者：
Van Winckel;J. Mathis;C. Waelkens
最近，有一小部分本质发光的低质量恒星被证明具有一种极其奇特的元素丰度模式。它们的光球碳、氮、氧和硫的丰度在太阳值的一个数量级之内，但所有其他通常丰富的金属都只有微量的存在；在两颗恒星中，铁的缺乏程度接近5个数量级。两种可能的解释是，低铁含量是原始的，意味着一个非常大的年龄，而CNO和S丰度是在进化过程中获得的，或者CNO和S丰度反映了最初的恒星组成，而低铁含量是尘埃形成的化学分离的结果。后一种假设之所以产生，主要是因为这些恒星的丰度模式与星际气体的丰度模式相似，在星际气体中，分解成尘埃起着重要作用，但人们不太容易理解，一个必然发生在星周包层中的过程如何能如此显著地影响光球的丰度。在这里，我们报告了在HD52961恒星中检测到相当数量的锌，并认为，因为锌只有在相当低的温度下才会凝结成尘埃，所以检测到接近正常数量的锌是分馏假说的令人信服的证据。
A SMALL number of intrinsically luminous, low-mass stars have recently been shown to have an extremely peculiar elemental abundance pattern. Their photospheric carbon, nitrogen, oxygen and sulphur abundances are within an order of magnitude of solar values, but all other normally abundant metals are present in only trace amounts; in two stars, iron is deficient by nearly five orders of magnitude. Two possible explanations are that the low iron content is primordial, implying a very great age, whereas the CNO and S abundances have been acquired during evolution, or that the CNO and S abundances reflect the initial stellar composition and the low iron content is the result of chemical separation by dust formation. The latter hypothesis arises mainly because the abundance pattern of these stars is similar to that of interstellar gas1,in which fractionation to dust plays an important part, but it is not easily understood how a process that must occur in the circumstellar envelope can so strikingly affect the photospheric abundances. Here we report the detection of appreciable amounts of zinc in the star HD52961, and argue that, because zinc will condense into dust only at rather low temperatures, its detection in near normal amounts is convincing evidence for the fractionation hypothesis.