Anomalous Pre-Main Sequence Lithium Depletion

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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."