Study Reveals Major Difference Between Jets Ejected by Low- and High-Mass Protostars

Astronomers exploring the fast-moving jet of material expelled by a huge, still-forming young star have discovered a significant difference between that jet and those expelled by less-massive young stars.

The researchers made this finding by using the Karl G. Jansky Very Large Array (VLA) of the U.S. National Science Foundation to produce the most comprehensive image yet of the inner region of this jet emerging from a giant young star.

Both high- and low-mass young stars, also called protostars, force jets outward perpendicular to a material disk that is closely revolving around the star. In stars that have masses analogous to the Sun, these jets are focused or narrowed, much tightly close to the star in a process known as collimation.

Since the majority of high-mass protostars are located very far, examining the regions proximal to them has been quite hard, and hence astronomers were uncertain if this was the case with them.

A research group noted a giant protostar known as Cep A HW2, situated around 2,300 light-years from Earth in the constellation Cepheus. It is believed that Cep A HW2 will develop into a new star about 10 times bigger than the Sun.

The latest VLA images revealed the finest details to be ever seen in such an object, offering astronomers their initial view of the innermost part of the jet, a part that is almost as long as the diameter of the Solar System.

What we saw is very different from what usually is seen in the jets from low-mass stars.

Adriana Rodriguez-Kamenetzky, National Autonomous University of Mexico

Observations in lower-mass protostars have revealed the jets to be collimated as close to the star as just a few times the distance between the Earth and Sun.

But in Cep A HW2, “We see not a single jet, but two things—a wide-angle wind originating close to the star, then a highly-collimated jet some distance away,” said Alberto Sanna from the Osservatorio Astronomico di Cagliari (INAF) in Italy. The collimated jet begins at a distance from the star similar to the distance from the Sun to Neptune or Uranus.

According to the astronomers, this latest finding raises two major possibilities.

First and foremost, the same mechanism could be working in both low-mass and high-mass protostars; however, the collimation distance could be established by the mass, taking place farther away in larger systems.

Another possibility is that high-mass stars may generate just the wide-angle wind observed in Cep A HW2, with collimation only coming when the flow is restricted by physical conditions around the star.

That case would point to a major difference in the mechanisms at work in protostars of different masses. Answering this question is important to understanding how stars of all masses form.

Carlos Carrasco-Gonzalez, Study Lead, National Autonomous University of Mexico

Gonzalez and his collaborators have reported their findings in the Astrophysical Journal.