Protostar grows despite ionizing ultraviolet heating

Japanese astronomers have discovered that the giant embryonic star is still expanding despite the emission of hot gas plumes. This observation can help understand why stars grow to such large sizes.

Young protostars are gaining weight by collecting matter from a dense disk of gas and dust that revolves around them, scientists explained. But as soon as they grow beyond a certain size, further growth is inhibited by the light that they emit. This can happen when ultraviolet light removes electrons from the atoms of the surrounding disk to create a hot ionized plasma. It evaporates from the star – this process is called photoevaporative outflow.

Theoretical calculations have shown that this and related factors are too weak to stop the buildup. But, to confirm this, there is insufficient observational evidence, not least because massive protostars are rare and very distant from Earth.

Researchers at the RIKEN Star and Planet Laboratory have studied the protostar known as G45.47 + 0.05 using the ALMA radio observatory in Chile and the VLA radio observatory in New Mexico. They searched for radio waves and microwaves emitted when an electron falls between two energy levels in a hydrogen atom and when electrons scatter positive ions without trapping them – two signs that the gas is being ionized.

Researchers have identified these signals in an hourglass-shaped area that extends outward from the protostar. Their observations showed that gas reaches a temperature of about 10,000 degrees Celsius and moves at a speed of about 30 kilometers per second. This suggests that the hourglass-shaped region is filled with ionized gas, which was launched from the protostar disk by ionization with light.

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