Wednesday, April 1, 2015

Nebulae from Destructed Giant Planets

A white dwarf is basically the leftover core of a star that has shed its outer layers. The large amount of mass that is lost as a star evolves into a white dwarf can destabilise a planetary system around the star. This can potentially send planets towards the star where they either collide with the star or become tidally disrupted. If a gas giant planet meets such a fate, it can drive the formation of what is known as a “real planetary nebula”.

The planetary nebula Sharpless 2-71, as imaged by the Gemini Multi-Object Spectrograph on Gemini North in Hawaii. Image credit: Gemini Observatory/AURA.

A gas giant planet is comprised mostly of hydrogen and helium, with a dense solid core in the middle. If it is orbiting a white dwarf and happens to be perturbed into an orbit which brings it too close to the white dwarf, it can become tidally disrupted or even collide with the white dwarf. Such an event strips away the hydrogen and helium envelope of the planet. Roughly half of the stripped material forms an accretion disk around the white dwarf and the other half gets flung out of the system.

The accreted hydrogen undergoes nuclear burning on the surface of the white dwarf. This causes part of the accreted hydrogen to re-inflate into a red giant envelope ~100 times the Sun’s radius around the white dwarf. For a newly formed white dwarf with 0.6 times the Sun’s mass, a red giant envelope can be inflated with an accreted mass of only ~0.001 times the Sun’s mass (i.e. about the mass of Jupiter). For an old and cool white dwarf of the same mass, a red giant envelope can be inflated with an even lower accreted mass of only ~0.0001 times the Sun’s mass (i.e. ~1/10th the mass of Jupiter).

Subsequently, part of the red giant envelope is blown away in the form of a wind to form a nebula. The central white dwarf, still very hot from all the accretion and nuclear burning, heats up and ionizes the nebula. This causes the nebula to glow. As a result, such a nebula whose material came from the destruction of a planet, is known as a “real planetary nebula”. By contrast, typical planetary nebulae are formed from ionized gas ejected from old red giant stars at the last stages of their evolution.

- Bear & Soker (2015), “Planetary systems and real planetary nebulae from planets destruction near white dwarfs”, arXiv:1502.07513 [astro-ph.SR]
- Corradi et al. (2015), “Binarity and the abundance discrepancy problem in planetary nebulae”, arXiv:1502.05182 [astro-ph.SR]