will chemical tagging work?

Chemical tagging is the name given to the idea that we can match up stars in abundance space (detailed chemical properties) as well as kinematic space to figure out the origin and common orbits of stars in the Milky Way. Because it would be so valuable to figure out that different stars shared a common origin at formation (for things like orbit inference), chemical tagging could enormously improve the precision of any dynamical or galaxy-formation information coming from next-generation surveys.

In the many conversations I have seen about chemical tagging, arguments break out about whether it is possible to measure the chemical abundances of stars of different temperatures and surface gravities comparably. That is: Can we figure out that this F star has the same abundances as this other K star? Or this red giant and this main-sequence star? And it is certainly not clear: Chemical abundances are not measured at enormous precision and there are many possible biases, sources of variance, and systematic error.

My proposal is that we ask these questions not in the space of the outputs of chemical-abundance models but rather in the space of stellar spectroscopy observables. The question becomes not are the models good enough? but rather is there information in the data? And there needs to be information sufficient to distinguish thousands (yes that is the goal) of chemically distinct sub-populations.

If there is sufficient information, then in the dozens-to-hundreds-of-dimensions space of all possible absorption-line measurements (plus stellar temperature), do we see thousands of distinct families of (possibly very complex) one-dimensional loci (each locus being a birth-mass-sequence at fixed chemical abundances and age)? The idea would be to do this purely in the space of spectra but—probably necessarily—relying heavily on models to guide the eye (or really guide the code) where to look.

I have discussed this with Ken Freeman (ANU) and Mike Blanton (NYU), but as far as I know, no-one is working on it. Blanton had the great idea that we don't really need to make spectral features before starting. The question does the distribution of stellar spectra split up into many tiny, thin, curvy lines in spectrum space? can be asked with just well-calibrated spectra. And we have lots of those!


  1. ps. I mention my conversation with Freeman here.

  2. A question I have not seen properly addressed is what the expected distribution of points in an N-dimensional abundance space should look like. Many discussions seem to not take into account the fact that, say, the r-process abundance may not vary independently of the other abundances, but rather the abundance of stars may be simply in a 2 or 3 parameter family. This would greatly reduce the filling fraction of abundance space, and therefore our ability to distinguish between stars born in different molecular clouds.