Nova
The last shout of a dead star
Stars with a mass between ~3 to 8 solar mass end their life by leaving a compact C+O or O+Ne white dwarf. Theoretically, white dwarf can last forever. If the white dwarf has a companion star, the white dwarf can revive!
Depending on the distance from its companion star, matter can transfer from the companion star to the more compact white dwarf. The H-rich matter accumulates and triggers unstable H-burning. This creates a thermonuclear runaway of the H-rich surface and the consequent outburst. I am interested in understanding the ourburst process, its dependence on the white dwarf and the observable.
nucleosynthetic signature
Chemical isotope pairs such as 12C/13C, 14N/15N and 16O/17O are measured in nearby novae, such as planetary nebula K4-47 and Nova Ophiuchus 2017. They provide direct constraints on the possible nova models which may reproduce a similar set of isotope abundance ratios. The figure here shows an example. The cyan box corresponds to the measured abundance of the planetary nebular K4-47. Our C+O white dwarf model on the higher mass side can fit the lower 14N/15N ratio observed. Multiple constraints can pin down the exact mass and related physical processes during outburst.
GAmma-ray signature
Compared to classical supernovae, novae are much dimmer and eject much lower mass to the surrounding. However, they occur much more often and can be found in the Galaxy. This provides ample observational opportunities to directly compare theory with observations. In particular, the nova outburst produces radioactive isotopes such as 7Be, 13N, 18F and 22Na. They emit high energy gamma-ray photons which may directly escape from the white dwarf.
I built a Monte-Carlo radiative transfer code to predict the associated signals for these outburst. Here I show the typical gamma-ray spectra from an O+Ne white dwarf. We observe the time-dependence of the spectra. At early time, we see the 478 keV line (7Be) and later the 1275 keV line (22Na) becomes prominent.