A team of astronomers has determined how the gas flow from a massive infant star is launched. The researchers used the Atacama Large Millimeter/submillimeter Array (ALMA) to observe the 10-solar-mass protostar Orion KL Source I in the Kleinmann-Low Nebula and obtained clear evidence of rotation in the outflow.

Artists impression of Orion KL Source I. The massive protostar is surrounded by a disk of gas and dust. The outflow is launched from the surface of the outer disk. Image credit: ALMA / ESO / NAOJ / NRAO.

Stars form from gas and dust floating in interstellar space. But, astronomers do not yet fully understand how it is possible to form the massive stars seen in space.

One key issue is gas rotation. The parent cloud rotates slowly in the initial stage and the rotation becomes faster as the cloud shrinks due to self-gravity.

Stars formed in such a process should have very rapid rotation, but this is not the case. The stars observed in the Universe rotate more slowly.

How is the rotational momentum dissipated? One possible scenario involves that the gas emanating from protostars.

If the gas outflow rotates, it can carry rotational momentum away from the system.

Astronomers have tried to detect the rotation of the outflow to test this scenario and understand its launching mechanism.

In a few cases signatures of rotation have been found, but it has been difficult to resolve clearly, especially around massive protostars.

Orion KL Source I observed with ALMA. The massive protostar is located in the center and surrounded by a gas disk (red). A bipolar gas outflow is ejected from the protostar (blue). Image credit: ALMA / ESO / NAOJ / NRAO / Hirota et al.

Dr. Tomoya Hirota, an astronomer at the National Astronomical Observatory of Japan (NAOJ) and SOKENDAI, and colleagues observed a protostar called Orion KL Source I in the Kleinmann-Low Nebula, the most active part of the Orion Nebula complex.

Thanks to its close vicinity and ALMAs advanced capabilities, Dr. Hirota and co-authors were able to reveal the nature of the outflow from Orion KL Source I.

We have clearly imaged the rotation of the outflow. In addition, the result gives us important insight into the launching mechanism of the outflow, Dr. Hirota said.

The new ALMA observations beautifully illustrate the rotation of the outflow: it rotates in the same direction as the gas disk surrounding the star; this strongly supports the idea that the outflow plays an important role in dissipating the rotational energy.

Furthermore, ALMA clearly shows that the outflow is launched not from the vicinity of Orion KL Source I itself, but rather from the outer edge of the disk. This morphology agrees well with the magnetocentrifugal disk wind model.

The findings appear today in the journal Nature Astronomy.

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Hirota et al. Disk-Driven Rotating Bipolar Outflow in Orion Source I. Nature Astronomy, published online June 12, 2017

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ALMA Observes Massive Protostar in Kleinmann-Low Nebula - Sci-News.com