White Dwarfs in Cataclysmic Variables: Probes of Evolution and Accretion Physics

激变变星是一个紧凑的双星(周期不到12小时)，其中主星(白矮星)吸积物质和来自填充其罗氏叶的次星(主序星)的角动量。在非磁性系统中，物质通过围绕白矮星的吸积盘以连续或零星的速度转移。持续的低速率吸积(静止)每隔几周到几个月被几天到几周的强烈吸积(爆发)中断，每隔几千年被一次热核爆炸(一种经典的新星事件)中断。因此，这些系统中的白矮星是激变演化和吸积物理的探测器，因为它们带有长期吸积和爆炸性热核历史的热、化学和旋转印记。在这里，锡恩教授将在合作者和学生的陪同下，研究巨变双星中的白矮星，并使用多组分合成光谱拟合来测量它们的表面温度、自转速度和吸积大气的化学成分。这将使他们能够探索这些系统中的年龄、进化史、时间平均吸积率、白矮星质量和核心温度，以及质量传递驱动机制。特别是，对光球层化学丰度的测量将检验吸积和扩散理论，而热核处理元素的过量可能表明史前新星爆炸或来自原本质量更大的次生物质的处理过的核心物质。白矮星自转速度在周期间隙上下的分布是理解吸积过程中角动量转移物理的关键，这里将对此进行探索。通过这个项目，他们还将扩大白矮星的样本，这些白矮星的表面温度已知，涵盖了所有灾难性的可变亚型。他们将进行随时间变化的吸积演化模拟，从而为确定白矮星质量、限制白矮星自转速度以及确定长期吸积加热对白矮星的热影响提供一种新的独立手段。理解激变变星中吸积的后果是全球理解整个宇宙(例如中子星和黑洞周围)中与吸积有关的现象的第一步，这些现象很难被观测到。此外，新星爆炸喷出的物质进入星际介质，因此在激变变数中白矮星外层包层的组成，告诉我们哪些元素(金属)有望在星际介质中循环使用，以供下一代恒星使用。此外，一些激变的可变系统可能是Ia型超新星的前身，我们对这些吸积的白矮星的理解的变化也转化为Ia型超新星可能产生的种群规模的变化。在整个项目中，本科生将在数据的分析和合成光谱建模中发挥关键作用。他们还将通过在专业会议上提交海报论文和共同撰写经评审的出版物，参与传播成果。

A cataclysmic variable is a compact binary (with a period less than twelve hours) in which the primary (a white dwarf) accretes matter and angular momentum from the secondary star (a main sequence star) filling its Roche-lobe. In non-magnetic systems, the matter is transferred, at continuous or sporadic rates, by means of an accretion disk around the white dwarf. Ongoing accretion at a low rate (quiescence) is interrupted every few weeks to months by intense accretion (outburst) of days to weeks, and every few thousand years by a thermonuclear explosion (a classical nova event). Thus the white dwarfs in these systems are probes of cataclysmic evolution and accretion physics because they bear the thermal, chemical and rotational imprint of their long-term accretion and explosive thermonuclear history. Here, Professor Sion, accompanied by collaborators and students, will examine white dwarfs in cataclysmic binaries and make measurements of their surface temperatures, rotation rates and chemistry of their accreted atmospheres, using multi-component synthetic spectral fitting. This will allow them to probe the age, evolutionary history, time-averaged accretion rate, white dwarf mass and core temperature, and the mass transfer driving mechanisms in these systems. In particular, measurements of photospheric chemical abundances will test accretion and diffusion theories, while overabundances of thermonuclear-processed elements could indicate pre-historical nova explosions or processed core material from an originally more massive secondary. The distribution of white dwarf rotation rates above and below the period gap, which will be explored here, is key to understanding the physics of angular momentum transfer during accretion. Through this project they will also enlarge the sample of white dwarfs with known surface temperatures across all cataclysmic variable subtypes. They will carry out evolutionary accretion simulations with time-variable accretion and thus provide a new and independent means of determining white dwarf masses, constraining white dwarf rotation velocities, and determining the thermal impact that prolonged accretion heating has on the white dwarfs. An understanding of the consequences of accretion in cataclysmic variables is the first step in a global understanding of accretion-related phenomena throughout the universe (e.g. around neutron stars and black holes) which cannot be easily observed. Also the ejected material into the interstellar medium from novae explosions, and hence the composition of the outer envelopes of white dwarfs in cataclysmic variables, tells us about which elements (metals) are expected to be recycled in the interstellar medium for the next generation of stars. In addition, some cataclysmic variable systems are possible progenitors for Type Ia supernovae and changes in our understanding of these accreting white dwarfs translates into change in the possible size of the population from which Type Ia supernovae arise. Throughout this project, undergraduate students will play key roles in the analysis and synthetic spectral modeling of the data. They will also participate in the dissemination of the results via presentation of poster papers at professional meetings and co-authoring refereed publications.