Images revealed new details about the birthplaces of planets

ALMA and VLA observe hundreds of planet-forming disks around infant stars.

Using two of the most powerful radio telescopes in the world, astronomers have created more than three hundred images of planet-forming disks around very young stars in the Orion Clouds. These pictures uncover new insights concerning the origin of planets and the earliest stages of star formation.

Astronomers used the National Science Foundation’s Karl G. Jansky Very Large Array (VLA) and the Atacama Large Millimeter/submillimeter Array (ALMA) telescopes. They pointed them to a star-forming region called the Orion Molecular Clouds. Both telescopes peered through the dense clouds and observed hundreds of protostars and their disks in various stages of their formation.

John Tobin of the National Radio Astronomy Observatory (NRAO) in Charlottesville, Virginia, said“This survey revealed the average mass and size of these very young protoplanetary disks. We can now compare them to older disks that have been studied intensively with ALMA as well.”

Very young stars, also called protostars, form in clouds of gas and dust in space. The first step in the formation of a star is when these dense clouds collapse due to gravity. As the cloud collapses, it begins to spin—forming a flattened disk around the protostar. Material from the disk continues to feed the star and make it grow. Eventually, the left-over material in the disk is expected to form planets.

Many aspects of these first stages of star formation, and how the disk forms, are still unclear.

The survey revealed that very young disks can be similar in size, but are on average, much more massive than older disks.

This image shows the Orion Molecular Clouds, the target of the VANDAM survey. Yellow dots are the locations of the observed protostars on a blue background image made by Herschel. Side panels show nine young protostars imaged by ALMA (blue) and the VLA (orange). Credit: ALMA (ESO/NAOJ/NRAO), J. Tobin; NRAO/AUI/NSF, S. Dagnello; Herschel/ESA
This image shows the Orion Molecular Clouds, the target of the VANDAM survey. Yellow dots are the locations of the observed protostars on a blue background image made by Herschel. Side panels show nine young protostars imaged by ALMA (blue) and the VLA (orange). Credit: ALMA (ESO/NAOJ/NRAO), J. Tobin; NRAO/AUI/NSF, S. Dagnello; Herschel/ESA

Tobin said, “When a star grows, it eats away more and more material from the disk. This means that younger disks have a lot more raw material from which planets could form. Possibly bigger planets already start to form around very young stars.”

Nicole Karnath of the University of Toledo, Ohio (now at SOFIA Science Center) said, “Among hundreds of survey images, four protostars looked different than the rest and caught the scientists’ attention. These newborn stars looked very irregular and blobby. We think that they are in one of the earliest stages of star formation, and some may not even have formed into protostars yet.”

“It is special that the scientists found four of these objects. We rarely find more than one such irregular object in one observation. We are not entirely sure how old they are, but they are probably younger than ten thousand years.”

To be defined as a typical (class 0) protostar, stars should have not only a flattened rotating disk surrounding them but also an outflow—spewing away material in opposite directions—that clears the dense cloud surrounding the stars and makes them optically visible. This outflow is crucial because it prevents stars from spinning out of control while they grow. But when precisely these outflows start to happen, it is an open question in astronomy.

One of the infant stars in this study, called HOPS 404, has an outflow of only two kilometers (1.2 miles) per second (a typical protostar-outflow of 10-100 km/s or 6-62 miles/s). It is a big puffy sun that is still gathering a lot of mass, but just started its outflow to lose angular momentum to be able to keep growing,” explained Karnath. “This is one of the smallest outflows that we have seen, and it supports our theory of what the first step in forming a protostar looks like.”

Tobin said, “The combined use of ALMA and the VLA has given us the best of both worlds. Thanks to these telescopes, we start to understand how planet formation begins.”

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