Models of Type I X-Ray Bursts in 4U 1820-30

I present ignition models for Type I X-ray bursts and superbursts from the ultracompact binary 4U 1820-30. A pure helium secondary is usually assumed for this system, although some evolutionary models predict a small amount of hydrogen (around 10% by mass). I show that even this small amount of hydrogen significantly affects the expected Type I burst properties. Using the 2-4 hour recurrence times observed during periods of regular bursting, I determine the accretion rate onto the star for different compositions, a new constraint on evolutionary models. I show that models with hydrogen predict 10% variations in burst fluence with recurrence time, which should in future allow the accreted composition to be determined. For superbursts, I show that the expected recurrence times are 1-2 years for pure He accretion, and 5-10 years if hydrogen is present. These results emphasise that a combined analysis of normal Type I X-ray bursts and superbursts is a promising way to probe the thermal structure and composition of the neutron star's outer layers.