MolPlan: New cradles of molecules in interstellar space: planetary nebulæ
When low- to middleweight stars like our Sun approach the end of their lives, they first become red giants and eventually small white dwarf stars. In doing so, the old giants cast off their outer layers of gas and dust into space, creating beautiful planetary nebulae (PNe). During the PN stage the remaining stellar core is a hot white dwarf, whose intense UV radiation should destroy most molecules that had previously been ejected, and hamper the formation of new molecules in the nebula. In recent years however, it has become clear that the molecular content of PNe is not as sparse as originally thought. The 2014 detection of OH+ in the Helix nebula with the Herschel satellite was the first clear evidence for the formation of molecular ions in PNe. This has created many scientific questions regarding the survival and formation of molecules in PNe. From observations we know that the molecules reside in or near dense clumps where they are shielded from the UV radiation of the central star. It is in such clumps that molecules may survive the PN phase and new molecules may be formed. Such clumps are observed in a number of highly evolved PNe with central stars where the nuclear burning has stopped and the luminosity of the central star has dropped significantly. The origin of these clumps is heavily debated. Many scientists argue that they are remnants of an early phase of the evolution before the central star started ionizing the nebula. However, such clumps are not identifiable in young PNe. This seems contradictory. Hence we propose to quantitatively investigate a new physical instability which may lead to the formation of clumps in recombining ionized gas around cooling white dwarfs. We will use the PDR code Cloudy to derive the temporal evolution of the physical conditions in a recombing plasma. This model will then be used in a theoretical analysis to determine if the gas is unstable in the way we proposed and what the time scale would be to form the globules. Finally, we will use Cloudy to create a model of the advection flow off the globules in the Helix nebula by matching it to the Herschel spectrum.