Astronomers using NSF’s Karl G. Jansky Very Large Array (VLA) have observed the fast-moving jet of material ejected by Cep A HW2, a massive protostar located 2,283 light-years away in the Cepheus A star-forming region.
An artist’s conception of Cep A HW2, showing a wide-angle wind originating close to the star and an accretion disk, with a much narrower jet farther away. Image credit: Bill Saxton, NRAO, AUI & NSF.
Low- and high-mass protostars propel jets outward perpendicular to a disk of material closely orbiting the star.
In stars with masses similar to the Sun, these jets are narrowed, or focused, relatively tightly near to the star in a process called collimation.
Because most high-mass protostars are more distant, studying the regions close to them has been more difficult, so astronomers were unclear if this was the case with them.
Cep A HW2 is expected to develop into a new star about 10 times more massive than the Sun.
The new VLA images of the system showed the finest detail yet seen in such an object, giving the astronomers their first view of the innermost portion of the jet, a portion roughly 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,” said Dr. Adriana Rodriguez-Kamenetzky, an astronomer at the National Autonomous University of Mexico.
“In lower-mass protostars, observations have shown the jets to be collimated as close to the star as only a few times the Earth-Sun distance.”
“In Cep A HW2, however, 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,” added Dr. Alberto Sanna, an astronomer at the Osservatorio Astronomico di Cagliari.
The collimated jet starts at a distance from the star comparable to the distance from the Sun to Uranus or Neptune.
VLA image of the jet from Cep A HW2, with surrounding area shown in the image from the NASA/ESA Hubble Space Telescope; circles indicate location of the accretion disk, not seen in this image. Image credit: Carrasco-Gonzalez et al. / Bill Saxton, NRAO, AUI & NSF / STScI.
“The discovery raises two main possibilities,” the astronomers said.
“First, the same mechanism could be at work in both high-mass and low-mass protostars, but the collimation distance could be determined by the mass, occurring farther away in more-massive systems.”
“The second possibility is that high-mass stars might produce only the wide-angle wind seen in Cep A HW2, with collimation only coming when physical conditions around the star restrict the flow.”
“That case would point to a major difference in the mechanisms at work in protostars of different masses,” said Dr. Carlos Carrasco-Gonzalez, an astronomer at the National Autonomous University of Mexico.
“Answering this question is important to understanding how stars of all masses form.”
The findings will be published in the Astrophysical Journal Letters.
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Carlos Carrasco-González et al. 2021. Zooming into the Collimation Zone in a Massive Protostellar Jet. ApJL, in press; arXiv: 2106.01235
