Astroonomers Explain Orion Nebula's Age Paradox

The study will soon be published in the scientific journal Astronomy & Astrophysics.

The Orion Nebula is a large star-forming region visible to the naked eye. Some of the young stars in the nebula are surrounded by discs of gas and dust in which planets form. Radiation from the massive central star, 1 Ori C1 (Theta-1 Orionis C1), gradually strips material from these discs, giving them shapes that resemble tadpoles.

The discs are losing mass so rapidly that, given the age of the stars (around one million years) they should have disappeared long ago. The large gas shell surrounding the central star poses another puzzle. The star sets the surrounding gas in motion, causing the shell to expand at around 13 kilometres per second. Extrapolating backwards from this speed suggests that the shell formed only a few hundred thousand years ago. The stars are therefore much older than both the gas shell and the dust discs (the 'tadpoles') appear to be. This is the age paradox of the Orion Nebula.

A simulated stellar collision

Leiden astronomer Maite Wilhelm and her colleagues propose a new scenario that could explain both puzzles. They suggest that 1 Ori C1, which has a mass around 33.5 times that of the Sun, formed when a binary star system merged. A binary system consists of two stars that orbit a common centre of mass under the influence of their mutual gravity. The researchers suspect that a third star disrupted the system, knocking one of the stars out of its stable orbit and eventually causing the two stars to collide.

If the two stars did merge a few hundred thousand years ago, the resulting star's luminosity (the amount of light it emits) must have increased sharply from that point onwards. The researchers used computer simulations to investigate whether this scenario could explain the observations.

Simulations support the scenario

The simulations showed that the scenario is plausible. Before the collision, the two stars emitted much less radiation, allowing the surrounding dust discs to survive for most of their lifetime. Only after the merger did the star's radiation increase sharply, triggering the rapid evaporation of the discs.

The rapid expansion of the gas shell, however, requires an additional explanation. The merger alone is estimated to provide thirty times more driving force, whereas a factor of one hundred is needed to explain the observed expansion. The researchers suspect that cool gas from the evaporating dust discs also plays a role. As this gas mixes with the hot gas inside the expanding bubble, it could provide the additional driving force needed.

The proposed scenario could also account for two other unusual features of 1 Ori C1: its observed tilted magnetic field and a slight excess of nitrogen in the star.

'For the first time, we are seeing the consequences of a major stellar collision in our own cosmic neighbourhood.'

More stellar collisions in our cosmic neighbourhood previously thought

Co-author Simon Portegies Zwart is enthusiastic about the findings. 'This is the first time we have witnessed the consequences of a major stellar collision in a our own cosmic neighbourhood. It also shows that stellar collisions may be much more common than we previously thought, and that they can have far-reaching consequences.'

Maite Wilhelm adds: 'Our idea shows that subtle effects in triple-star systems can have a major impact on the environments in which stars form, and potentially even on planets that are still taking shape.'