Dredge-up

Dredge-up refers to a period in the evolution of a star where a surface convection zone extends down to the layers where material has undergone nuclear fusion. As a result, the fusion products are mixed into the outer layers of the stellar atmosphere and these nuclides can appear in the spectrum of the star.

The first dredge-up occurs when a main sequence star enters the red giant branch (RGB). As a result of the convective mixing, the outer atmosphere will display the spectral signature of hydrogen fusion: the ¹²C/¹³C and C/N ratios are lowered, and the surface abundances of lithium and beryllium may be reduced.

The second dredge-up occurs in stars with 4 – 8 solar masses of material. When helium fusion comes to an end at the core, convection mixes the products of the CNO cycle.^[1] This second dredge up results in an increase in the surface abundance of ⁴He and ¹⁴N, while the amount of ¹²C and ¹⁶O decreases.^[2]

The third dredge-up occurs after a massive star enters the asymptotic giant branch (AGB) and a flash occurs along a helium burning shell. This dredge-up causes helium, carbon and the s-process products to be brought to the surface. The result is an increase in the abundance of carbon relative to oxygen, which can create a carbon star.^[2]

The names of the dredge-ups are set by the evolutionary and structural state of the star in which each occurs, not by the sequence experienced by the star. As a result, lower-mass stars experience the first and third dredge-ups in their evolution but not the second.

References

1. ^ Lambert, D. L. (1992). "Observational Effects of Nucleosynthesis in Evolved Stars". In Mike G. Edmunds and Roberto J. Terlevich. Elements and the Cosmos. University of Cambridge. pp. 92–109. ISBN 052141475X. 
2. ^ ^{a b} Kwok, Sun (2000). The origin and evolution of planetary nebulae. Cambridge University Press. p. 199. ISBN 0521623138.