富金属白矮星的强吸积斑
Scars of Intense Accretion Episodes at Metal-Rich White Dwarfs
摘要：
对DBZ型白矮星的时间平均吸积率的重新评估表明，历史上的时间平均吸积率明显高于目前在DAZ型白矮星上观察到的吸积率。DAZ白矮星观测到的持续的瞬时吸积速率（通常超过1 e8 g/s）与DBZ恒星在过去1 e5 - 1 e6年中推断的吸积速率之间的差异可能是几个数量级，因此必须由数十至数百年的高速率事件造成，因此它们至今仍未被发现。本文探讨的可能性，这种短暂的，强烈的吸积事件的气相材料可以解释现有的数据。对于星周气体的合理假设，吸积速率接近或超过1 e15 g/s是可能的，类似于在静态激变变星中观察到的速率，并且可能通过未来的X射线任务或宽场监测设施检测到。容易发生这种快速吸积的气体碎片可能在潮汐破裂事件之后立即通过碰撞和升华而丰富，或者如果额外的物体撞击现存的磁盘。在气体产生事件之间的长时间内，颗粒盘物质以坡印廷-罗伯逊拖曳率或接近坡印廷-罗伯逊拖曳率的速度吸积，这与尘埃DAZ白矮星推断的速率一致。在这张照片中，没有红外线过量的温暖DAZ恒星的吸积速率与未被检测到的颗粒盘一致。这一总体情况对DBZ白矮星的吸积和小行星碎片的衍生化学成分的准稳态模型有影响。
英文摘要：
A re-evaluation of time-averaged accretion rates at DBZ-type white dwarfs points to historical, time-averaged rates significantly higher than the currently observed episodes at their DAZ counterparts. The difference between the ongoing, instantaneous accretion rates witnessed at DAZ white dwarfs, which often exceed 1e8 g/s, and those inferred over the past 1e5-1e6 yr for the DBZ stars can be a few orders of magnitude, and therefore must result from high-rate episodes of tens to hundreds of years so they remain undetected to date. This paper explores the likelihood that such brief, intense accretion episodes of gas-phase material can account for existing data. For reasonable assumptions about the circumstellar gas, accretion rates approaching or exceeding 1e15 g/s are possible, similar to rates observed in quiescent cataclysmic variables, and potentially detectable with future x-ray missions or wide-field monitoring facilities. Gaseous debris that is prone to such rapid accretion may be abundant immediately following a tidal disruption event via collisions and sublimation, or if additional bodies impinge upon an extant disk. Particulate disk matter accretes at or near the Poynting-Robertson drag rate for long periods between gas-producing events, consistent with rates inferred for dusty DAZ white dwarfs. In this picture, warm DAZ stars without infrared excesses have rates consistent with accretion from particulate disks that remain undetected. This overall picture has implications for quasi-steady state models of accretion and the derived chemical composition of asteroidal debris in DBZ white dwarfs.
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