Cygnus X-2, super-Eddington mass transfer, and pulsar binaries
Cygnus X-2, super-Eddington mass transfer, and pulsar binaries
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DOI：
10.1046/j.1365-8711.1999.02862.x
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发表时间：
1998-12
影响因子：
4.8
通讯作者：
Andrew King;Hans Ritter
中科院分区：
文献类型：
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作者：
Andrew King;Hans Ritter
我们考虑了天鹅座X-2中副星的不寻常的演化状态。光谱数据显示其质量较低（M2）。0:5 0:7 M()，但半径很大（R2）。7r（）和高亮度（L2）。150 L()。我们发现，这颗恒星非常类似于早期大质量案例B演化的遗迹，在此期间，中子星在其热时间尺度（10年）上抛出了大部分来自供体（初始质量M2i， 3:6 M()）的3 M（）。由于该系统太宽，无法由共同包膜演化产生，这有力地支持了中子星在超级爱丁顿传质过程中有效地抛出多余流入物的观点。天鹅座X-2的不同寻常之处在于，它的初始质量比qi / M2i=M1在接近qi的一个狭窄的临界范围内。2:6。较小的气导致长周期系统，前供体靠近Hayashi线，较大的气导致周期较短的脉冲星双星和相对较大的白矮星伴侣。后者自然地解释了几毫秒脉冲星双星中惊人的巨大伴星质量。因此，像天鹅座X-2这样的系统可能是形成脉冲星双星的重要通道。关键词：双星；近距离±星；演化±星；1 I N T R O D U C T I O N Cygnus X-2是一个具有长轨道周期的持久性x射线双星（P . 9:84 D; Cowley, Crampton & Hutchings 1979）。对明确的I型x射线爆发（small 1998）的观测表明，吸积成分是中子星而不是黑洞。Casares， Charles和Kuulkers（1998）发现的精确光谱信息，以及可以从中得出的参数，总结在表1中。质量比q ? M2=M1。0:34意味着质量传递使系统变宽，因此可能是由副恒星的膨胀驱动的。通常在长周期低质量x射线双星（lmxb）中，这是由于沿Hayashi线的次巨星次级核演化而发生的，典型的有效温度为Teff；2。3000±4000 K。然而，Casares等人的观察表明，这不是天鹅座X-2的情况。二次是在赫茨普朗隙（光谱型A9 III）：使用罗氏几何和斯特凡±玻尔兹曼定律给出L2。1 . 150l（含Teff）；7330 K（见表1）。此外，质量比q。0:34，假设主星是一颗中子星，因此服从M1 & 2m(;，这意味着次级星的质量较低（M2 ? qM1 & 0:68 M()）。相比之下，一颗孤立的a9iii恒星的质量约为4 M。最近，Orosz和Kuulkers（1999）对次级星系的椭球体变化进行了建模，从而得出了一个依赖于模型的倾角为i / 628.5 ^ 48，转化为组成质量M1 / 78 ^ 0:23 μ M（和M2 / 0:60 ^ 0:13 μ M）：在本文中，我们考虑了对天鹅座X-2中次级星系不寻常性质的解释。我们发现只有一种可行的可能性，即这颗恒星目前接近早期大质量案例B的质量转移结束，因此中子星以某种方式设法拒绝了过去转移给它的大部分质量（,3 M()）。为了支持这一观点，我们表明，这种类型的进化自然地解释了几毫秒脉冲星双星中惊人的大伴星质量。在本节中，我们考虑了关于天鹅座X-2中次级星系不寻常性质的四种可能的解释。我们将发现其中三个是站不住脚的，因此我们将集中于第四种可能性。2.1案例B传质开始时的正常恒星？最简单的解释是，在赫茨普龙±罗素（HR）图中，副星的位置正好是一颗普通恒星穿过赫茨普龙隙的位置。因为这样的恒星在核心不再燃烧氢，所以这是Kippenhahn & Weigert（1967，以下简称KW）定义的大质量B型质量传递。假设初始质量比为qi & 1，双星总是在传质过程中膨胀，传质过程发生在热时间尺度上。Kolb（1998）系统地研究了这种类型的进化
We consider the unusual evolutionary state of the secondary star in Cygnus X-2. Spectroscopic data give a low mass (M2 . 0:5 2 0:7 M() and yet a large radius (R2 . 7 R() and high luminosity (L2 . 150 L(). We show that this star closely resembles a remnant of early massive Case B evolution, during which the neutron star ejected most of the , 3 M( transferred from the donor (initial mass M2i , 3:6 M() on its thermal timescale , 10 yr. As the system is far too wide to result from common-envelope evolution, this strongly supports the idea that a neutron star efficiently ejects the excess inflow during super-Eddington mass transfer. Cygnus X-2 is unusual in having had an initial mass ratio qi M2i=M1 in a narrow critical range near qi . 2:6. Smaller qi lead to long-period systems with the former donor near the Hayashi line, and larger qi to pulsar binaries with shorter periods and relatively massive white dwarf companions. The latter naturally explain the surprisingly large companion masses in several millisecond pulsar binaries. Systems like Cygnus X-2 may thus be an important channel for forming pulsar binaries. Key words: binaries: close ± stars: evolution ± stars: individual: Cygnus X-2 ± pulsars: general ± X-rays: stars. 1 I N T R O D U C T I O N Cygnus X-2 is a persistent X-ray binary with a long orbital period (P 9:84 d; Cowley, Crampton & Hutchings 1979). The observation of unambiguous type I X-ray bursts (Smale 1998) shows that the accreting component is a neutron star rather than a black hole. The precise spectroscopic information found by Casares, Charles & Kuulkers (1998), and the parameters that can be derived from it, are summarized in Table 1. The mass ratio q M2=M1 . 0:34 implies that mass transfer widens the system, and is therefore probably driven by expansion of the secondary star. Normally in long-period low-mass X-ray binaries (LMXBs) this occurs because of the nuclear evolution of a subgiant secondary along the Hayashi line, with typical effective temperatures Teff;2 . 3000±4000 K. However, Casares et al.'s observations show that this cannot be the case for Cygnus X-2. The secondary is in the Hertzsprung gap (spectral type A9 III): use of Roche geometry and the Stefan±Boltzmann law gives L2 . 150 L( with Teff;2 . 7330 K (see Table 1). Moreover, the mass ratio q . 0:34, and the assumption that the primary is a neutron star and thus obeys M1 & 2 M(; implies that the secondary has a low mass (M2 qM1 & 0:68 M(). In contrast, an isolated A9 III star would have a mass of about 4 M(. More recently, Orosz & Kuulkers (1999) have modelled the ellipsoidal variations of the secondary and thereby derived a model-dependent inclination of i 628: 5 ^ 48 which translates into component masses M1 1:78 ^ 0:23M( and (M2 0:60 ^ 0:13M(: In this paper we consider explanations for the unusual nature of the secondary in Cygnus X-2. We find only one viable possibility, namely that this star is currently close to the end of early massive Case B mass transfer, and thus that the neutron star has somehow managed to reject most of the mass (, 3 M() transferred to it in the past. In support of this idea, we show that this type of evolution naturally explains the surprisingly large companion masses in several millisecond pulsar binaries. 2 M O D E L S F O R C Y G N U S X 2 In this section we consider four possible explanations for the unusual nature of the secondary in Cygnus X-2. We shall find that three of them are untenable, and thus concentrate on the fourth possibility. 2.1 A normal star at the onset of Case B mass transfer? The simplest explanation is that the position of the secondary in the Hertzsprung±Russell (HR) diagram is just that of a normal star crossing the Hertzsprung gap. Because such a star no longer burns hydrogen in the core, this is a massive Case B mass transfer as defined by Kippenhahn & Weigert (1967, hereafter KW). Provided that the initial mass ratio qi & 1 the binary always expands on mass transfer, which occurs on a thermal time-scale. Kolb (1998) investigated this type of evolution systematically and