Anomalous Pre-Main Sequence Lithium Depletion

The presence of lithium in their atmospheres has long been used as a
signature of youth in low-mass stars; it is used both to identify them
and as a means of estimating their age.

The basic physics is that a contracting protostar heats up in the
middle and eventually reaches the Li ignition temperature of about 3
million K. Once this happens, convective mixing ensures that Li can be
depleted rapidly until all the Li has gone or a radiative core develops
(in higher mass stars) and halts the transport of Li-depleted material
to the surface (if the convection zone base is cooler than 3 million K).

The above theory predicts a one-to-one mapping between Li abundance
mass and age. But this is not seen in practice - there is a scatter and
that scatter appears to depend on rotation. Faster rotators preserve
their Li for longer.

An idea that has been put forward is that the initial conditions play a
role; that some stars are built up in a series of "episodic accretion"
events. If that accretion is "cold" (the star radiates away most of the
accretion energy promptly), then the protostar may be "crushed" by the
accretion and attain greater central temperatures than expected. This
may result in anomalous Li depletion and may also affect the rotation
of the protostar.

To search for evidence of anomalous Li depletion we need spectroscopy
of large samples of stars, where membership can be established
independently of the chemical characteristics of the star. Such an
opportunity arises with the Gaia-ESO survey combined with Gaia
astrometry. The Gaia parallaxes and proper motions, combined with GES
radial velocities and gravity indices can be used to select clean
sample 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 suffer
excessive Li depletion (or indeed are unexpectedly lithium-rich) and to provide a
critical 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 astrometry
plus 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."