Star formation has a lot of complex physics that feed into it. Classical models used something equivalent to a “collapse” of a cloud of gas by gravity, with a star being birthed in the middle. More modern understandings show a feature called a “streamer”, which funnels gas and dust to proto-stars from the surrounding disc of material. But our understanding of those streamers is still in its early stages, like the stars they are forming. So a new paper published in Astrophysical Journal Letters by Pablo Cortes of the National Radio Astronomy Observatory (NRAO) and his co-authors is a welcome addition to the literature - and it shows a unique feature of the process for the first time.
That feature is based on the Alfvén Mach Number, which describes the ratio of the speed of a flow of gas to the “Alfvén Speed”, which is the speed at which magnetic waves travel through a plasma. A streamer is made up of both neutral “gas”, which makes up the majority of it, and ionized “plasma” that make up a small fraction of it. Magnetic fields can only exert force on the plasma, but since the gas and plasma are coupled inside a streamer, the gas regularly runs into the charged plasma, keeping the whole system locked into a kind of “highway” bounded on either side by massive magnetic fields.
To analyze these streamers, the authors used data from the Atacama Large Millimeter/submillimeter Array (ALMA) that was collected as part of the ALMA Perseus Polarization Survey (ALPPS) dataset that looked at 38 young stellar objects. Two of those early stellar objects were hosted in a “protostar” known as SVS 13A. It actually consists of two separate binary protostars (VLA 4A and VLA 4B) that are orbiting each other, but are also surrounded by a massive circumbinary disc of gas and dust.
Lecture on the formation of Protostars. Credit - Protostar_and_Planets_VI YouTube Channel / Mark KrumholzOne of the most notable features of that disc are the “spiral arms” that make it look similar to the Milky Way. In this case, one of those spirals forms a “streamer” that is directly feeding the young stars at the center of the system. Monitoring that streamer at different wavelengths and with different polarizations allowed the researchers to note a few important facts about it.
First, the gas in the streamer was flowing subsonically, meaning it was flowing smoothly, almost equivalent to a laminar flow in water on Earth. This was in contrast to the predicted wild flow of gas bumping off of charged ions and into other molecules. Having seen that gas feature - they next needed to look at what the magnetic field was doing to the plasma. They had to rule out other potential sources of polarization of the light, such as “self-scattering” of light off of dust and “mechanical alignment” of the gas flows.
They used a technique called “Alignment Measure” to match up the magnetic field to the direction the gas was flowing, and it came out almost perfectly, showing that the magnetic fields were indeed causing the flow to be contained. With that evidence in hand, they moved on to calculating the Alfvén Mach Number. Their evidence points clearly to the fact that the streamer is “sub-Alfvénic” - in other words the magnetic energy of the flow is the dominant determining path over the kinetic energy of the gas inside it.
Video explaining ALMA's data archive. Credit - European ALMA Regional Centre NetworkCombining that with the subsonic speed the gas is traveling, and the whole streamer appears as a relatively structured, well organized system to funnel gas, dust, and plasma into the budding stars. While this set of circumstances isn’t necessarily representative for all young star systems, the fact that the researchers were able to find this at all means that it is at least possible to have a well-organized flow of material to young stars, rather than the catastrophic collapse of an orbital disc or a turbulent rush of particles in a streamer. Further research will be needed to show if this orderly state is common or not, but at least now we have proof for the first time that it could be.
Learn More:
P. C. Cortés et al - First Results from ALPPS: A Sub-Alfvénic Streamer in SVS 13A
UT - High-Mass Stars Are Fed By Elongated Streamers Of Gas
UT - The Keen-Eyed Vera Rubin Observatory Has Discovered A Massive Stellar Stream