Wide Binary Observing Proposal (twins and not-twins)

Starting point:

 I would like to spend some time at the MW  meeting talking and thinking
about a possible proposal for FEROS on the 2.2m (deadline end of the month);
the goal (for me) is to see whether there is a scientifically complelling case, and
whether anyone's interested in collaborating.
The science goal would be to exploit a set of wide binaries from Gaia
(derived by Kareem and Haijun) as a laboratory of what can work (or not)
in chemichal tagging; and learn about  the largest separation binaries while we are at it...

Parent sample, and its subsample:

The starting point is the following sample of wide (100AU-50,000AU) binaries,
selected as pairs of stars of the same proper motion and parallax (contaminant cleaned).

Parent wide-binary sample from Gaia, with the apparent magnitude difference shown as a function of physical separation. The most striking feature (but not the only feature if interest here) is the sequence of bins (identical magnitudes/masses). The slight asymmetry (around 0) in DeltaG arises from the definition of component 1 and 2; and is uninteresting.

Let's call the binaries, where the two components are within 0.2mag, twins.

Same sample as above, but shown here as a function of apparent magnitude. Only stars G<12 feros="" font="" lend="" m.="" spectroscopy="" straightforward="" themselves="" to="" with="">

• 

  Same sample as above, restricted to the binaries where both components are brighter than 12mag

Science questions:

Let's presume the members of the pairs are sibblings, stars born at the same time in the same place from the "same" material (and let's question and observationally probe this assumption in a minute).

• twin wide binaries may be the "easiest" case to check how identical photospheric abundances are among siblings; the fact that these sibblings are twins eliminates or reduces the spectral differences arising from Teff, or logg as a complication. Are these identical in spectral space? This can probably be asked in a data driven way. 
• Are there exceptions to "undistinguishably identical abundances"? Can we explain them?
• If there are non-identical abundances, does their incidence depend on separation?
• non-twin sibblings provides us with stars born at the same time from the same material, but with different masses.
• their abundance differences (on the MS) would inform us about atomic diffusion (heavy element settling)
• provide us with numerous empirical examples how spectra look like thatdiffere mostly because of logg,Teff; this is particularly important in the regime where spectral models  are problematic, such as Teff<3000k .="" aspects="" but:="" can="" li="" nbsp="" these="" two="" untangle="" we="">

The data at the same time would provide a means to check how clean that sample is, whether it contains any hiearchical triplets etc...

Observations:

At FEROS one gets full-coverage optical Echelle spectra 3600-9000A with S/N~50 AT G=12 (solar type star) in 10min, according to

 http://www.eso.org/observing/etc/bin/gen/form?INS.NAME=FEROS++INS.MODE=spectro

So, this makes spectroscopy of 100 pairs straightforward (50-80h), of 1000 pairs feasible; but that would need serious justification.