Anomalous Pre-Main Sequence Lithium Depletion

在低质量恒星的大气层中存在锂元素一直被认为是年轻恒星的标志;它既被用来识别它们，也被用来估计它们的年龄。基本的物理原理是，收缩的原恒星在中间加热，最终达到Li的点火温度约300万K。一旦发生这种情况，对流混合确保Li可以迅速耗尽，直到所有的Li消失或辐射核心发展（在更高质量的恒星中），并停止Li耗尽物质向表面的传输（如果对流区底部温度低于300万K）。但这在实际中并不常见--有一种散射，这种散射似乎取决于旋转.有一种观点认为，初始条件起着作用，一些恒星是在一系列的“偶发性吸积“事件中形成的。如果吸积是“冷的”（星星迅速辐射掉大部分的吸积能量），那么原星星可能会被吸积“压碎”，并达到比预期更高的中心温度。这可能会导致异常的锂耗尽，也可能会影响原星星的旋转。为了寻找异常锂耗尽的证据，我们需要大样本恒星的光谱，其中的成员关系可以独立于星星的化学特征来建立。这样的机会出现与Gaia-ESO调查结合Gaiaastrometry。Gaia的弛豫和自行，结合GES的径向速度和重力指数，可以用来在年轻的星团和星协中选择干净的成员样本。GES的数据可以用来寻找候选对象与异常低Li。博士项目是定量评估PMS星的比例sufferescentive锂耗尽（或确实是出乎意料的锂丰富），并提供情节吸积模型的严格测试。很可能需要更多的数据来确认Li-poor候选人的年轻程度。这个项目的一个可能的扩展将是识别星系团成员（从盖亚天体测量加上盖亚-ESO光谱学），这些成员在其他属性方面是异常的，例如HR图位置，旋转（使用TESS使命的周期），磁活动或化学丰度。正如爱丁顿曾经说过的（关于白色矮星）：“奇怪的物体，持续显示出一种与它们的光度完全不一致的光谱，最终可能教会我们比一个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."