Using NSF’s Karl G. Jansky Very Large Array (VLA) and the Atacama Large Millimeter/submillimeter Array (ALMA), astronomers have captured images of 328 protoplanetary disks around very young stars (protostars) in the Orion Molecular Clouds, a giant star-forming region approximately 1,400 light-years from Earth in the constellation Orion.
This composite 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 ESA’s Herschel Space Observatory. Side panels show nine young protostars imaged by ALMA (blue) and the VLA (orange). Image credit: ALMA / ESO / NAOJ / NRAO / J. Tobin / AUI / NSF / S. Dagnello / Herschel / ESA.
Protostars form in clouds of gas and dust in space. The first step in the formation 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 about these first stages of star formation, and how the disk forms, are still unclear.
But a new survey called VLA/ALMA Nascent Disk and Multiplicity (VANDAM) provides some missing clues as the VLA and ALMA peered through the dense clouds and observed hundreds of protostars and their disks in various stages of their formation.
“The VANDAM survey revealed the average mass and size of these very young protoplanetary disks,” said Dr. John Tobin, an astronomer at the National Radio Astronomy Observatory.
“We can now compare them to older disks that have been studied intensively with ALMA as well.”
The astronomers found that very young disks can be similar in size, but are on average much more massive than older disks.
“When a star grows, it eats away more and more material from the disk,” Dr. Tobin said.
“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.”
This schematic shows a proposed pathway (top row) for the formation of protostars, based on four very young protostars (bottom row) observed by VLA (orange) and ALMA (blue). Step 1 represents the collapsing fragment of gas and dust. In step 2, an opaque region starts to form in the cloud. In step 3, a hydrostatic core starts to form due to an increase in pressure and temperature, surrounded by a disk-like structure and the beginning of an outflow. Step 4 depicts the formation of a class 0 protostar inside the opaque region, that may have a rotationally supported disk and more well-defined outflows. Step 5 is a typical class 0 protostar with outflows that have broken through the envelope (making it optically visible), an actively accreting, rotationally supported disk. In the bottom row, white contours are the protostar outflows as seen with ALMA. Image credit: ALMA / ESO / NAOJ / NRAO / N. Karnath / AUI / NSF / B. Saxton / S. Dagnello.
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,” said Dr. Nicole Karnath, a researcher at the University of Toledo.
“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 10,000 years.”
To be defined as a typical (class 0) protostar, stars should not only have 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 important, because it prevents stars from spinning out of control while they grow. But when exactly these outflows start to happen is an open question in astronomy.
One of the infant stars in the study, HOPS 404, has an outflow of only 2 km/s (1.2 miles/s) — a typical protostar has an outflow of 10-100 km/s (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,” Dr. Karnath said.
“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.”
The results appear in the Astrophysical Journal.
_____
John J. Tobin et al. 2020. The VLA/ALMA Nascent Disk and Multiplicity (VANDAM) Survey of Orion Protostars. II. A Statistical Characterization of Class 0 and Class I Protostellar Disks. ApJ 890, 130; doi: 10.3847/1538-4357/ab6f64
