On the relative abundances of elements, and isotopes: how successfully do you think the effects of cosmic ray spallation have been modeled? For example, that such a process alters many ratios is certain, but modeling their effects would depend on good estimates of the intensity of the ‘cosmic radiation’ (and, to some extent, its energy spectrum), wouldn’t it?

How well constrained is this, particularly in GMCs and star-forming regions?

Then there’s the jets from AGNs, which produce the giant clouds of plasma which radio astronomers so often see (as ‘double lobes’ and ‘hourglass’ morphologies). Sometimes these jets ‘hit’ nearby galaxies and globular clusters; sure there’s often very little in the way of ISM in these, but what effects does being blasted by an AGN jet have on the relative abundances of elements and isotopes? In Seyferts, these jets certainly exist, but are – per current thinking – stopped by the denser ISM; the intensity of the ‘cosmic radiation’ is surely far greater there than that which we experience here, waaay out in a disk, and the effects of cosmic spallation correspondingly greater.