Anomalous Pre-Main Sequence Lithium Depletion

长期以来，在低质量恒星中，大气中锂的存在一直被用作年轻的标志；它既被用来识别它们，也被用来估计它们的年龄。基本物理原理是，收缩的原星在中间加热，最终达到约300万K的Li点火温度。一旦发生这种情况，对流混合确保李灿迅速耗尽，直到所有Li都消失或辐射核(在质量较高的恒星中)发展起来，并停止向地表(如果对流带底部低于300万K)输送贫Li物质。上述理论预测了Li丰度与年龄之间的一一对应关系。但这在实践中是看不到的--存在散射，而这种散射似乎取决于旋转。速度较快的旋转者保持Li的时间更长。已经提出的一种观点是，初始条件起作用；一些恒星是在一系列“幕式吸积”事件中形成的。如果这种吸积是“冷的”(恒星迅速地辐射出大部分吸积能量)，那么原恒星可能会被吸积“挤压”，达到比预期更高的中心温度。这可能导致Li的异常亏损，也可能影响原恒星的自转。为了寻找Li异常亏损的证据，我们需要大样本恒星的光谱，其中的成员关系可以根据恒星的化学特征来建立。这样的机会出现在盖亚-ESO调查与盖亚天体测量相结合的时候。盖亚视差和自行与GES径向速度和重力指数相结合，可以用来选择年轻星团和协会中成员的清洁样本。然后，GES的数据可以用来寻找锂异常低的候选天体。PHD项目是为了定量评估遭受过度锂缺乏(或者确实是出乎意料的富锂)的PMS恒星的比例，并对周期性吸积模型进行关键测试。很可能需要额外的数据来确认锂元素贫乏的候选人的年轻和候选人。或者我们只能对它们的数量设置上限。该项目的一个可能的扩展将是识别在其他性质方面异常的星系团成员(来自Gaia天体计量学和Gaia-ESO光谱)，如HR图位置、自转(使用TESS任务的周期)、磁活动或化学丰度。就像爱丁顿曾经说过的(关于白矮星)：“奇怪的物体坚持显示出一种与它们的光度完全不一致的光谱类型，最终可能会教给我们比按照规则辐射的主机更多的东西。”

The presence of lithium in their atmospheres has long been used as asignature of youth in low-mass stars; it is used both to identify themand as a means of estimating their age. The basic physics is that a contracting protostar heats up in themiddle and eventually reaches the Li ignition temperature of about 3million K. Once this happens, convective mixing ensures that Li can bedepleted rapidly until all the Li has gone or a radiative core develops(in higher mass stars) and halts the transport of Li-depleted materialto the surface (if the convection zone base is cooler than 3 million K).The above theory predicts a one-to-one mapping between Li abundancemass and age. But this is not seen in practice - there is a scatter andthat scatter appears to depend on rotation. Faster rotators preservetheir Li for longer.An idea that has been put forward is that the initial conditions play arole; that some stars are built up in a series of "episodic accretion"events. If that accretion is "cold" (the star radiates away most of theaccretion energy promptly), then the protostar may be "crushed" by theaccretion and attain greater central temperatures than expected. Thismay result in anomalous Li depletion and may also affect the rotationof the protostar.To search for evidence of anomalous Li depletion we need spectroscopyof large samples of stars, where membership can be establishedindependently of the chemical characteristics of the star. Such anopportunity arises with the Gaia-ESO survey combined with Gaiaastrometry. The Gaia parallaxes and proper motions, combined with GESradial velocities and gravity indices can be used to select cleansample of members in young clusters and associations. The GES data can then be used to find candidate objects with anomalously low Li.The PhD project is to quantitatively assess the fraction of PMS stars that sufferexcessive Li depletion (or indeed are unexpectedly lithium-rich) and to provide acritical test of episodic accretion models..It is likely that additional data twill be needed to confirm the youth of and candidate Li-poor candidates. Or it may be that we can only put upper limits on their number.A likely extension to the project will be to identify cluster members (from Gaia astrometryplus Gaia-ESO spectroscopy) that are anomalous with respect to other properties such as HR diagram position, rotation (using periods from the TESS mission), magnetic activity or chemical abundances. As Eddington once said (about white dwarfs) : "Strange objects, which persist in showing a type of spectrum entirely out of keeping with their luminosity, may ultimately teach us more than a host which radiate according to rule."