Global Disk Oscillations in Binary Be Stars
Global Disk Oscillations in Binary Be Stars
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DOI：
10.1093/pasj/61.1.57
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发表时间：
2008-09
影响因子：
2.3
通讯作者：
F. Oktariani;A. Okazaki
中科院分区：
文献类型：
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作者：
F. Oktariani;A. Okazaki
摘要：我们通过轨道间隔和双星质量比研究潮汐与伴星相互作用对双 Be 星周围圆盘中全局单臂振荡模式的影响。我们的模型考虑了有助于模态限制的三维效应，这是 Ogilvie (2008) 最近发现的。我们发现单臂振荡被很好地限制在圆盘大于几十个恒星半径的系统中。在这样的系统中，振荡周期几乎不依赖于二元参数。另一方面，在具有较小圆盘的系统中，模式限制不完全，振荡周期随着轨道间隔的增加和/或二元质量比的减小而增加。本征模对中心恒星的光谱类型不敏感。我们的研究结果表明，V/R振荡周期对轨道间隔和双星质量比的依赖性只能在短周期双星系统中观察到，并且对于轨道周期相似的系统，质量比较高的系统将表现出较短的V/R变化。 关键词：星：双星：一般，星：发射线，Be，星：振荡1。简介Be 星是非超巨星 B 型恒星，其光谱显示或在某个时间显示出一颗或多颗巴尔默线发射的恒星（Collins 1987）。 Be星是一颗快速旋转的B型恒星，作为中心恒星，具有两部分的星周包层、极风和赤道盘。极地风是一种低密度、快速流出的紫外线辐射。风结构可以通过线驱动风模型很好地解释（Castor et al. 1975；Friend & Abbott 1986）。另一方面，赤道盘是一个几何上薄的、高密度的等离子体，接近开普勒旋转（例如，Porter & Rivinius 2003），从中产生光学发射线和红外过量。尽管对于 Be 盘的形成仍然存在几种相互竞争的情况，但最有希望的一种情况是，该盘是由从中心恒星喷射出的气体的粘性衰减形成的（Lee 等人，1991 年；另见 Porter & Rivinius 2003 年，以及其中的参考文献）。许多 Be 恒星的光谱在几年到几十年内都表现出长期变化。其中一种现象称为长期 V/R 变化，即双峰发射线轮廓的紫色 (V) 和红色 (R) 峰相对强度之比的变化。对于孤立的 Be 星，长期 V/R 变化的周期通常在 5-10 年范围内。人们普遍认为，长期 V/R 变化归因于 Be 盘中的全局单臂（即方位波数 m=1）振荡（例如，Porter & Rivinius 2003）。它基于这样一个事实：在接近开普勒圆盘的单臂模式中，表现为非常缓慢旋转的微扰模式（例如，Kato 1983）。单臂振荡模型的早期版本（Okazaki 1991、1997；Papaloizou et al. 1992；Savonije & Heemskerk 1993）定性地很好地解释了观察到的 V/R 变化的特征。然而，由于缺乏将模态限制在盘内部的机制以及振荡周期对各种参数的高度敏感性，该模型的预测能力较差(Fiˇrt & Harmanec 2006)。最近，该模型取得了很大进展。 Ogilvie（2008）通过研究模态特性的三维效应解决了模态限制问题。虽然该模型的先前版本假设运动与高度无关，因为圆盘在几何上很薄，但他认为椭圆轨道周围垂直重力加速度的变化会激发偏心圆盘中的振荡垂直运动，这一点不应被忽视。 Ogilvie (2008) 表明，仅三维效应就允许受限的顺行模式。迄今为止，Be 盘中单臂振荡模式的理论研究大多局限于孤立的 Be 星。然而，根据当前的普查，双星系统中发现的 Be 星的比例约为 1/3 (Porter & Rivinius 2003)。伴星的存在很可能会改变 Be 盘中全局单臂振荡的一些特征。实际上，在由中子星和早期型Be星组成的Be/X射线双星中，V/R变化的时间尺度通常约为1年，这比典型的孤立Be星的V/R时间尺度要短得多(例如，Reig等，2005)。
AbstractWe study the effects of the tidal interaction with the companion, via orbital separation and binarymass ratio, on the global one-armed oscillation modes in disks around binary Be stars. Our model takesinto account the three-dimensional effect that contributes to the mode confinement, which was recentlyfound by Ogilvie (2008). We find that the one-armed oscillations are well confined in systems with diskslarger than a few tens of stellar radii. In such systems, the oscillation period depends little on the binaryparameters. On the other hand, in systems with smaller disks, where the mode confinement is incomplete,the oscillation period increases with increasing orbital separation and/or decreasing binary mass ratio. Theeigenmode is insensitive to the spectral type of the central star. Our results suggest that the dependenceof V/R oscillation period on the orbital separation and binary mass ratio should be observed only in shortperiod binary systems, and that, for systems with a similar orbital period, those with higher mass ratioswill show shorter V/R variations.Key words: stars: binaries: general, stars: emission-line, Be, stars: oscillations1. IntroductionBe stars are non-supergiant B-type stars whose spectrashow, or have at some time shown, one or more Balmerlines in emission (Collins 1987). A Be star is a rapidlyrotating, B-type star as the central star with a two-component circumstellar envelope, a polar wind and anequatorial disk. The polar wind is a low-density, fast out-flow emitting UV radiation. The wind structure is well ex-plained by the line-driven wind model (Castor et al. 1975;Friend & Abbott 1986). On the other hand, the equato-rial disk is a geometrically thin, high-density plasma innearly Keplerian rotation (e.g., Porter & Rivinius 2003),from which the optical emission lines and the IR excessarise. Although there are still several competing scenariosfor the Be disk formation, the most promising one is thescenario where the disk is formed by viscous decretion ofgas ejected from the central star (Lee et al. 1991; see alsoPorter & Rivinius 2003, and references therein).Many Be stars show long-term variations in their spec-tra over years to decades. One of these phenomena iscalled long-term V/R variations, which are variations ofthe ratio of relative intensity of violet (V) and red (R)peaks of a double-peaked emission line profile. The pe-riod of the long-term V/R variations is typically in therange 5-10yr for isolated Be stars. It is widely acceptedthat the long-term V/R variations are attributed to globalone-armed (i.e., the azimuthal wave number m=1) oscil-lations in the Be disk (e.g., Porter & Rivinius 2003). Itis based on the fact that in a nearly Keplerian disk aone-armed mode shows up as a very slowly revolving per-turbation pattern (e.g., Kato 1983). The early versionsof the one-armed oscillation model (Okazaki 1991, 1997;Papaloizou et al. 1992; Savonije & Heemskerk 1993) qual-itatively well explained the observed characteristics of theV/R variations. However, owing to the lack of the mech-anism for confining the modes to the inner part of thedisk and the high sensitivity of the oscillation period onvarious parameters, the model had little predictive power(Fiˇrt & Harmanec 2006).Recently, the model was advanced greatly. Ogilvie(2008) has solved the mode confinement problem by in-vestigating a three-dimensional effect on the mode char-acteristics. While previous versions of the model had as-sumed motions to be independent of height since the diskis geometrically thin, he argued that the variation of thevertical gravitational acceleration around an elliptical or-bit excites an oscillatory vertical motion in an eccentricdisk that should not be neglected. Ogilvie (2008) showedthat the three-dimensional effect alone allows confinedprograde modes.Hitherto, theoretical studies of one-armed oscillationmodes in Be disks are mostly limited to isolated Be stars.However, according to the current census, the fractionof Be stars found in binary systems is ∼ 1/3 (Porter &Rivinius 2003). The presence of the companion is mostlikely to change some features of the global one-armedoscillations in Be disks. Actually, in Be/X-ray binaries,which consist of a neutron star and an early-type Be star,the time-scale of V/R variations is frequently about 1yr,which is much shorter than the typical V/R time-scale forisolated Be stars (e.g., Reig et al. 2005).