arXiv 1003.33402010-03-17astro-ph.SR

行星系统宿主HR\，8799：论其$λ$ Bootis性质

The planetary system host HR\,8799: On its $λ$ Bootis nature

A. Moya, P. J. Amado, D. Barrado, A. García Hernández, M. Aberasturi, B. Montesinos, F. Aceituno

备注：: 11 pages, 10 figures. Accepted for publication in MNRAS
DOI：: 10.1111/j.1365-2966.2010.16699.x

摘要：

HR\，8799是一颗$λ$牧夫座，$γ$剑鱼座恒星，拥有一个行星系统和一个有两个环的碎片盘。这使得这个系统成为一个非常有趣的研究目标。这项工作致力于确定这颗恒星的内部金属丰度，与其$λ$ Bootis性质有关（即，太阳表面轻元素丰度和太阳下表面重元素丰度），利用其$γ$剑鱼座脉动。这是获得这些信息的最准确的方法，也是第一次对行星系统的宿主恒星进行这样的研究。我们使用了平衡程序CESAM和非绝热脉动程序GraCo。我们应用频率比法（FRM）和时间相关对流理论（TDC）对模式识别、Brunt-Vaïsälä频率积分和模式不稳定性进行了估计，并对可能的模式进行了选择。当使用非地震学约束（即它在HR图中的位置）时，太阳丰度模型被丢弃。这一结果与解释$λ$ Bootis性质的主要假设之一相矛盾，即具有太阳丰度的恒星吸积/扩散气体。因此，根据这些结果，需要对这一假设进行修正。精确的内部化学混合过程似乎是必要的，以解释在太阳下内部金属丰度的恒星表面观察到的特殊丰度。利用FRM或不稳定性分析提供的星震学约束，可以非常准确地确定HR 8799的物理特征。然而，结果对倾角$i$的依赖性仍然存在。这个角度的确定，更精确的多色光度观测，以及高分辨率的光谱学可以确定这颗恒星的质量和金属丰度。

英文摘要：

HR\,8799 is a $λ$ Bootis, $γ$ Doradus star hosting a planetary system and a debris disk with two rings. This makes this system a very interesting target for asteroseismic studies. This work is devoted to the determination of the internal metallicity of this star, linked with its $λ$ Bootis nature (i.e., solar surface abundances of light elements, and subsolar surface abundances of heavy elements), taking advantage of its $γ$ Doradus pulsations. This is the most accurate way to obtain this information, and this is the first time such a study is performed for a planetary-system-host star. We have used the equilibrium code CESAM and the non-adiabatic pulsational code GraCo. We have applied the Frequency Ratio Method (FRM) and the Time Dependent Convection theory (TDC) to estimate the mode identification, the Brunt-Vaïsälä frequency integral and the mode instability, making the selection of the possible models. When the non-seismological constraints (i.e its position in the HR diagram) are used, the solar abundance models are discarded. This result contradicts one of the main hypothesis for explaining the $λ$ Bootis nature, namely the accretion/diffusion of gas by a star with solar abundance. Therefore, according to these results, a revision of this hypothesis is needed. The inclusion of accurate internal chemical mixing processes seems to be necessary to explain the peculiar abundances observed in the surface of stars with internal subsolar metallicities. The use of the asteroseismological constraints, like those provided by the FRM or the instability analysis, provides a very accurate determination of the physical characteristics of HR 8799. However, a dependence of the results on the inclination angle $i$ still remains. The determination of this angle, more accurate multicolour photometric observations, and high resolution spectroscopy can definitively fix the mass and metallicity of this star.

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