Wednesday, September 8, 2021

Scientists study young star to understand what the Sun was like in its "infancy"

 The Sun, a star that has existed for 4.6 billion years, did not always look the same as it does today.  And while astronomers have a good theoretical model to explain the evolution of our sun, they need to back up predictions with actual observations.  That's exactly what one team did in its new study, looking at a young star named GM Aurigae.

 This was probably the best observation ever made of a protoplanetary disk colliding with a young star.  The details of this process are important for understanding what the Sun was like in youth, because these collisions have some consequences and implications.

 Stars are born out of clouds of gas and dust, but the cloud in question isn't fully utilized—it forms a disk of matter that keeps rotating around the newborn star.  From this rotating ring, called the protoplanetary disk, are born planets, asteroids, comets and moons.  But there's a lot more going on in this evolving system.

 Computer simulation of a protoplanetary disk and magnetic field lines (multicolored) in a young star.

 The innermost contents of the protoplanetary disk are channeled by the star's magnetic field, so that matter is carried to the stellar surface.  This was observed in the study of the star GM Aurigae — its magnetic field, in a way, helped stellar gravity to attract gas and dust.  There, where matter collided with the star's surface, what astronomers call a “hot spot” is born, because the material is so hot after impact.

 This dynamic appears to affect the star's brightness, as the team observed GM Aurigae's visible light peak just about a day after ultraviolet light.  It is possible that this happened because the source of visible light (a particular point on the star) and ultraviolet (another point on the star) entered and left the field of view as it rotated.  This is because regions of stellar surfaces can have different temperatures and move at different speeds as the star rotates.

 Approximately 2 million years old, GM Aurigae has a mass similar to that of the Sun and is about 420 light-years away from us in the constellation Auriga.  Proximity and size help astronomers to learn more about this star and to induce its characteristics that would also be present in the Sun when young.  But for a precise result, several observatories were used, including Hubble, Swift and TESS.

  Hot spot simulation and its density variations and different temperatures.  Below, the graph reveals less dense (solid line) and denser (dotted line) regions of the hot spot, which vary as the star rotates.

 Finally, the team compared the observations with computer models that describe the accumulation of matter in stars, and found that the hot spot varies in density from the center to the edge (although it is called a “hot spot”, it is a relatively large area in the stellar surface).  Hotspot areas with different densities also have different temperatures.  This explains why ultraviolet light peaks before visible light (different temperatures have different wavelengths).

 These findings confirm theoretical predictions, according to the team.  "This result teaches us more about what our Sun was like when we were young," said Catherine Espaillat, one of the authors of the article published in Nature magazine. "Now our Sun has sunspots, dark areas where the surface temperature is cooler. When the Sun was young, it also had hot spots".

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