Why should we care about the X-ray burst ashes?

The astrophysical rapid proton capture (rp-)process was proposed as the power source of type I X-ray bursts on accreting neutron stars shortly after they were discovered a little over 40 years ago. Since that time, many nuclear physics measurements have been performed to improve calculations of the X-ray burst light curve. So much progress has been made that most of the key nuclear reactions thought to impact the light curve have been, or will soon be, constrained. However, in recent years it has been shown that the nuclear abundances resulting from X-ray burst burning, known as the ashes, are important in their own right. The ashes influence the thermal and structural composition of the accreted neutron star outer layers, which in turn impact calculations of other accreting neutron star observables that are used as unique probes of ultradense matter. I will discuss this recently emphasized avenue of rp-process studies, emphasizing recent and near-future stable and radioactive ion beam measurements.