Wind Roche-lobe overflow: Application to carbon-enhanced metal-poor stars
Wind Roche-lobe overflow: Application to carbon-enhanced metal-poor stars
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DOI：
10.1051/0004-6361/201220007
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发表时间：
2013-02
影响因子：
6.5
通讯作者：
C. Abate;O. Pols;R. Izzard;S. Mohamed;S. D. Mink
中科院分区：
文献类型：
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作者：
C. Abate;O. Pols;R. Izzard;S. Mohamed;S. D. Mink
碳增强贫金属星（CEMP）是银河系晕中极贫金属星的一个重要组成部分。大多数CEMP星也富含s过程元素，这些元素通常在双星系统中发现。这表明碳的富集是由于过去从渐近巨星分支（AGB）星星到低质量伴星的质量转移。二进制人口合成的模型是不能够复制所观察到的CEMP恒星的分数，而不调用非标准的核合成或初始质量函数的实质性变化。这被解释为模型中缺少物理成分的证据。最近的流体动力学模拟表明，有效的风的质量转移是可能的情况下，典型的AGB星的缓慢和密集的风通过一种机制，称为风罗奇叶溢出（WRLOF），这是典型的邦迪-霍伊尔-利特尔顿（BHL）吸积和罗奇叶溢出之间。WRLOF对质量传递的吸积效率和双星系统的角动量损失有影响。本工作的目的是了解WRLOF对CEMP星的总体影响。为了模拟人口的低金属丰度的二进制，我们结合了一个合成核合成模型与二进制人口合成代码。在这段代码中，我们实现了WRLOF机制。我们利用流体动力学模拟的结果来模拟WRLOF对吸积效率的影响，并通过假设一个简单的处方来考虑WRLOF对角动量损失的影响。这两个效应的结合扩大了成为CEMP星的系统的范围，使其朝着更长的初始轨道周期和更低质量的次级星发展。因此，我们的模型预测的CEMP星的数量增加了1：2 - 1：8的因素相比，早期的结果，考虑BHL处方。此外，产生更高的碳富集度，最终的轨道周期分布向更短的周期移动。
Carbon-enhanced metal-poor stars (CEMP) are observed as a substantial fraction of the very metal-poor stars in the Galactic halo. Most CEMP stars are also enriched in s-process elements and these are often found in binary systems. This suggests that the carbon enrichment is due to mass transfer in the past from an asymptotic giant branch (AGB) star on to a low-mass companion. Models of binary population synthesis are not able to reproduce the observed fraction of CEMP stars without invoking non-standard nucleosynthesis or a substantial change in the initial mass function. This is interpreted as evidence of missing physical ingredients in the models. Recent hydrodynamical simulations show that e cient wind mass transfer is possible in the case of the slow and dense winds typical of AGB stars through a mechanism called wind Roche-lobe overflow (WRLOF), which lies in between the canonical Bondi-Hoyle-Lyttleton (BHL) accretion and Roche-lobe overflow. WRLOF has an e ect on the accretion e ciency of mass transfer and on the angular momentum lost by the binary system. The aim of this work is to understand the overall e ect of WRLOF on the population of CEMP stars. To simulate populations of low-metallicity binaries we combined a synthetic nucleosynthesis model with a binary population synthesis code. In this code we implemented the WRLOF mechanism. We used the results of hydrodynamical simulations to model the e ect of WRLOF on the accretion e ciency and we took the e ect on the angular momentum loss into account by assuming a simple prescription. The combination of these two e ects widens the range of systems that become CEMP stars towards longer initial orbital periods and lower mass secondary stars. As a consequence the number of CEMP stars predicted by our model increases by a factor 1:2 1:8 compared to earlier results that consider the BHL prescription. Moreover, higher enrichments of carbon are produced and the final orbital period distribution is shifted towards shorter periods.