Free Astronomy Magazine May-June 2020

MAY-JUNE 2020 T his 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 ac- tively accreting, rotationally supported disk. In the bottom row, white contours are the protostar outflows as seen with ALMA. [ALMA (ESO/NAOJ/NRAO), N. Karnath; NRAO/AUI/NSF, B. Saxton and S. Dagnello] 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. 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.” Among hundreds of survey images, four protostars looked different than the rest and caught the scien- tists’ attention. “These newborn stars looked very irregular and blobby,” said team member Nicole Karnath of the University of Toledo, Ohio (now at SOFIA Science Center). “We think that they are in one of the earliest stages of star forma- tion 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,” added Karnath, who used these four infant stars to propose a schematic path- way for the earliest stages of star formation. “We are not entirely sure how old they are, but they are prob- ably younger than ten thousand years.” To be defined as a typical (class 0) protostar, stars should not only have a flattened rotating disk surround- ing 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 pre- vents stars from spinning out of con- trol while they grow. But when exactly these outflows start to hap- pen, is an open question in astron- omy. 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,” ex- plained 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.” The exquisite resolution and sensi- tivity provided by both ALMA and the VLA were crucial to understand both the outer and inner regions of protostars and their disks in this sur- vey. While ALMA can examine the dense dusty material around proto- stars in great detail, the images from the VLA made at longer wave- lengths were essential to under- stand the inner structures of the youngest protostars at scales smaller than our solar system. “The com- bined use of ALMA and the VLA has given us the best of both worlds,” said Tobin. “Thanks to these tele- scopes, we start to understand how planet formation begins.” !