We solve for the evolution of the vertical extent of the convective region of a neutron star atmosphere during a Type I X-ray burst .
The convective region is well-mixed with ashes of nuclear burning due to the short turbulent mixing time scale and its extent determines the rise time of the burst light curve .
Using a full nuclear reaction network , we show that the maximum vertical extent of the convective region during photospheric radius expansion ( RE ) bursts can be sufficiently great that : ( 1 ) some ashes of burning are ejected by the radiation driven wind during the RE phase and , ( 2 ) some ashes of burning are exposed at the neutron star surface following the RE phase .
Depending on the ignition conditions , ashes with mass number in the range A \sim 30 - 60 are mixed in with the ejected material .
As the ejected material cools during the RE phase some of the ejected heavy-element ashes cease to be fully ionized .
In addition , those ashes that remain bound to the neutron star will temporarily reside in the photosphere after it has settled back down to the neutron star surface .
Some of these surface ashes are of high enough proton number Z that they are not fully ionized .
We calculate the expected column density of ejected and surface ashes in hydrogen-like states and determine the equivalent widths of the resulting photoionization edges from both the wind and neutron star surface .
We find that these can exceed 100 eV and are potentially detectable .
A detection would probe the nuclear burning processes and might enable a measurement of the gravitational redshift of the neutron star .
In addition , we find that in bursts with pure helium burning layers , protons from ( \alpha , p ) reactions cause a rapid onset of the ^ { 12 } C ( p , \gamma ) ^ { 13 } N ( \alpha , p ) ^ { 16 } O reaction sequence .
The sequence bypasses the relatively slow ^ { 12 } C ( \alpha , \gamma ) ^ { 16 } O reaction and leads to a sudden surge in energy production that is directly observable as a rapid ( \sim \textrm { ms } ) increase in flux during burst rise .