Study Reveals New Details about the Earliest Stages of Star Formation

Using two of the world’s most intense radio telescopes, an international team of astronomers has successfully created over 300 images of planet-creating disks around extremely young stars in the Orion Clouds.

VANDAM survey. ALMA and the VLA observed more than 300 protostars and their young protoplanetary disks in Orion. This image shows a subset of stars, including a few binaries. The ALMA and VLA data complement each other: ALMA sees the outer disk structure (visualized in blue), and the VLA observes the inner disks and star cores (orange). Image Credit: ALMA (ESO/NAOJ/NRAO), J. Tobin; NRAO/AUI/NSF, S. Dagnello.

Such images uncover innovative details about the planets’ birthplaces and also about the earliest stages of the formation of stars.

In the universe, a majority of the stars are accompanied by planets. Such planets are born in gas and dust rings known as protoplanetary disks. These disks surround even extremely young stars.

For astronomers, it is important to know when exactly these disks begin to form and what do they actually look like. However, nascent stars are extremely faint, and added to that, thick clouds of gas and dust surround these stars in stellar nurseries. Only very sensitive radio telescope arrays will be able to detect the small disks surrounding the newborn stars amidst the thickly packed material in these clouds.

For this latest study, astronomers pointed the Atacama Large Millimeter/submillimeter Array and Karl G. Jansky Very Large Array (VLA) of the National Science Foundation to an area in space in which several stars are born. This region is referred to as the Orion Molecular Clouds.

Known as VLA/ALMA Nascent Disk and Multiplicity (VANDAM), the survey is the largest to be ever performed on young stars and their disks. Extremely young stars, also known as protostars, develop in clouds of dust and gas in space. When these thick clouds collapse as a result of gravity, this triggers the formation of a star.

When the cloud collapses, it starts to rotate and creates a flattened disk around the protostar. From the disk, the material continues to feed the star and allows it to grow. Ultimately, the remaining material in the disk is predicted to form planets.

Several aspects of these initial stages of the formation of stars, and the way the disk develops, still continue to be vague. However, this latest survey offers a few missing clues as the ALMA and VLA looked through the thick clouds and visualized an unlimited number of protostars and their disks in numerous stages of their formation.

Young Planet-Forming Disks

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.

John Tobin, Team Lead, National Radio Astronomy Observatory

The National Radio Astronomy Observatory is located in Charlottesville, Virginia.

What Tobin and his group discovered is that extremely young disks can be analogous in size, but on average, they are relatively larger when compared to older disks.

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,” added Tobin.

Four Special Protostars

Four protostars appeared different from the remaining stars in an unlimited number of survey images, and this caught the attention of the researchers.

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.

Nicole Karnath, SOFIA Science Center

Earlier, Karnath was a team member at the University of Toledo in Ohio.

It is unique that the researchers identified four of these celestial objects. “We rarely find more than one such irregular object in one observation,” Karnath added, who employed these four nascent stars to suggest a schematic pathway for the earliest phases of the formation of stars. “We are not entirely sure how old they are, but they are probably younger than ten thousand years.”

To be described as a standard (class 0) protostar, stars should possess a flattened rotating disk enclosing them and should also have an outflow—discharging material in reverse directions—that clears the thick cloud enclosing the stars and renders them optically visible. This outflow is very significant because it avoids stars from spinning out of control while they are growing. However, it is not clear as to when exactly these outflows begin to occur and continues to remain an open mystery in the field of astronomy.

HOPS 40—one of the nascent stars in this study—has an outflow of just 2 km (1.2 mi) per second (a standard protostar-outflow of 10 to 100 km/second or 6 to 62 miles/second).

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. 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.

Nicole Karnath, SOFIA Science Center

Combining ALMA and VLA

The exceptional sensitivity and resolution offered by both the VLA and ALMA were important to interpret both the inner and outer areas of protostars as well as their disks in this latest survey.

The ALMA can investigate the thick dusty material surrounding the protostars in exquisite detail; however, the images obtained from the VLA made at longer wavelengths were more crucial to figure out the internal structures of the youngest protostars at scales that are smaller than the solar system.

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

The National Radio Astronomy Observatory is a facility of the National Science Foundation, managed under a cooperative agreement by Associated Universities, Inc.


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