Stellar Collision Could Solve the Orion Nebula's Age Paradox, Leiden Astronomers Propose
Planet-forming disks in the famous nebula look far younger than their stars. A merger of two stars a few hundred thousand years ago, triggered by a third, may explain why.
The Orion Nebula is a large star-forming region visible to the naked eye, and it has carried a puzzle for decades. Many of its young stars, which are around 1 million years old, are surrounded by disks of gas and dust where planets form. Yet those disks are being destroyed so quickly that, by any normal accounting, they should have vanished long ago.
Radiation from the nebula's massive central star, known as 1 Ori C1 (Theta-1 Orionis C1), strips material from the disks, giving them tadpole-like shapes. A second clue deepens the mystery. The star's winds drive a large shell of gas outward at about 13 kilometers per second, or 8 miles per second. Running that speed backward suggests the shell formed only a few hundred thousand years ago. The stars, in other words, appear much older than both the shell and the disks. Astronomers call this the age paradox of the Orion Nebula.
Leiden astronomer Maite Wilhelm and colleagues now propose one event that could explain both. They suggest 1 Ori C1, which has about 33.5 times the mass of the sun, was created when two stars in a binary system merged. A third star, they suspect, disturbed the pair, knocking one out of its stable orbit and eventually sending the two into each other.
If the merger happened a few hundred thousand years ago, the resulting star would have become much brighter from that point on. Computer simulations by the team show the scenario is plausible. Before the collision, the two stars gave off far less radiation, so the surrounding dust disks could survive for most of their lives. After the merger, the new star's radiation spiked and began evaporating the disks rapidly.
The shell's fast expansion needs a bit more help. The merger alone is estimated to supply 30 times the necessary driving force, but about 100 times is needed. The researchers suspect cool gas from the evaporating disks mixes with the hot gas inside the expanding bubble and adds the missing push.
The idea also accounts for two other oddities of 1 Ori C1: its tilted magnetic field and a slight excess of nitrogen in the star, both of which fit a merger history.
Co-author Simon Portegies Zwart called the result significant. "This is the first time we have witnessed the consequences of a major stellar collision in our own cosmic neighborhood," he said. "It also shows that stellar collisions may be much more common than we previously thought, and that they can have far-reaching consequences."
Wilhelm said the work shows how subtle effects inside triple-star systems can reshape the environments where stars are born, and potentially even planets still taking shape. The study, "A triple-induced merger in θ1 solves the proplyd lifetime problem and the curiously young shell of M42," is published in Astronomy & Astrophysics (DOI 10.1051/0004-6361/202662018). It is a proposal backed by simulations rather than a direct observation of the merger, so astronomers will look for further evidence in the nebula's stars and gas.
