RUI: White Dwarfs in Cataclysmic Variables

巨变星（cv）中的白矮星（WD）和相关的双星从一颗非常接近的类太阳恒星中积累（吸积）富氢气体。这些转移的气体携带着角动量，在WD周围形成了一个明亮的旋转吸积盘。这个吸积过程是高度爆炸性的，因为来自圆盘的物质首先剪切到WD表层，将质量、能量和自旋带入WD。人们对这一过程的物理性质了解甚少，直到最近才有可能对WD进行直接的光谱观测。然而，在矮新星爆发和类新星低亮度状态之间的静止间隔期间，当传质速率很低时，可以直接探测到WD辐射。这是因为内部吸积盘要么不太突出，要么不存在。模型大气将用于比较计算机生成的光谱与观测到的光谱，以确定WD表面温度、表面重力、旋转速率、旋转、加热、冷却和其吸积大气的化学丰度。为了解释这一扩展的经验CV WD特性，我们正在构建一个关于WD如何响应质量、能量和角动量注入的理论框架。该理论框架是基于使用计算机代码，在时变吸积条件下产生一维和二维随时间变化的演化WD模型序列。理论模型以及推导出的CV WDs的经验性质将回答以下问题：(1)通过光子向下照射、压缩和盘状物质剪切混合进入WDs的吸积加热的物理性质、能量预算、加热深度和开尔文时间是什么？(2)通过吸积盘气体的角动量转移，WD发生了多少自旋？(3)当只在WD赤道区域添加物质时，随纬度和深度的加热与球形吸积相比如何，即一维吸积研究的准确性如何？吸积带是否形成，如果形成，它的表面积、深度和自旋时间尺度是多少？(4)通过切向盘吸积形成的吸积带能否解释在大多数cv中观察到的边界层亮度弱或缺失（吸积气体首先与WD相互作用的地方）？(5)自CV演化开始以来，WD发生了多少吸积加热？典型的简历寿命是多少？cv是否在观测到的2 - 3小时的轨道周期间隙中进化？(6)观测到的CV WD旋转速度分布的物理解释是什么？它们的冷却速率与单一白矮星相比如何？非磁性cv中的WDs与磁性cv中的WDs冷却方式不同吗？(7)从它们的表面温度、光度、旋转速度和化学丰度模式与吸积速率、CV亚型和轨道周期的关系来看，CV WD特性和演化的全局图景是什么？在这个项目中进行的研究结果将提供对灾难性变星系统中切向（盘）吸积的物理见解。该结果也可能在其他吸积双星和盘状吸积天体的中心致密天体（赤道）吸积流的更广泛背景下提供重要应用。

Abstract - Sion AST 99-01955The white dwarf (WD) star in cataclysmic variables (CVs) and related binaries accumulates (accretes) hydrogen-rich gas from a very close sun-like star. This transferred gas, which carries angular momentum, forms a bright, swirling accretion disk around the WD. This accretion process is highly explosive as matter from the disk shears first into the WD surface layers bringing mass, energy and spin into the WD. The physics of this process is very poorly understood and until recently no direct spectroscopic observations of the WD were possible. However, during the quiescent intervals between the outbursts of dwarf novae and the low brightness states of nova-like variables, when the mass transfer rate is very low, the WD radiation can be directly detected. This is because the inner accretion disk is either much less prominent or absent. Model atmospheres will be used to compare computer-generated spectra with observed spectra to determine WD surface temperatures, surface gravities, rotation rates, spinup, heating, cooling, and chemical abundances of their accreted atmospheres. In order to interpret this expanding body of empirical CV WD properties, a theoretical framework on how the WD responds to the injection of mass, energy and angular momentum is being constructed. This theory framework is based upon the use of computer codes that produce time-dependent evolutionary WD model sequences in one dimension and two dimensions under conditions of time-variable accretion. The theoretical models, together with the derived empirical properties of CV WDs, will provide answers to the following questions:(1) What are the physics, energy budget, heating depth and Kelvin time of accretion heating by downward photon irradiation, compression, and shear mixing of disk matter into the WD?(2) How much spinup of the WD occurs through angular momentum transfer by the accreting disk gas?(3) When matter is added only in the equatorial region of the WD, how does the heating with latitude and with depth compare with spherical accretion, i.e. how accurate are the 1D accretion studies? Does an accretion belt form and, if so, what is its surface area, depth, and spindown timescale?(4) Do accretion belts formed via tangential disk accretion explain the weak or missing boundary layer luminosity (where the accreting gas first interacts with the WD), that is observed in most CVs?(5) How much accretion heating of the WD has occurred since the onset of CV evolution? What are typical CV lifetimes? Do CVs evolve across the observed orbital period gap between periods of 2 and 3 hours?(6) What is the physical interpretation of the observed distribution of CV WD rotational velocities? How do their cooling rates compare with single white dwarfs? Do WDs in non-magnetic CVs cool differently than magnetic ones? (7) What is the global picture of CV WD properties and evolution from the standpoint of their surface temperatures, luminosities, rotational velocities, and chemical abundance patterns versus accretion rate, CV subtype and orbital period?The results from the research carried out in this project will provide physical insights on tangential (disk) accretion in cataclysmic variable star systems. The results may also provide important applications in the broader context of (equatorial) accretion flows onto central compact objects in other accreting binaries and disk accreting objects.