A recent study has revealed that a giant young star, known as HW2, is experiencing an exceptionally rapid growth rate, accumulating mass from a surrounding disk of gas at a rate equivalent to about two Jupiter masses every year. This remarkable finding provides the clearest evidence yet for how massive stars can achieve their enormous sizes.
Here's a breakdown of the key aspects of this discovery:
- The Star: HW2 is a young, massive star located approximately 2,300 light-years away in the Cepheus A star-forming region. It already has a mass about 16 times that of our Sun. Such massive stars are relatively rare but play a crucial role in the cosmos by producing heavy elements and eventually ending their lives in powerful supernova explosions.
- The Discovery: Using the Very Large Array (VLA) radio telescopes, astronomers were able to observe a dense ring of hot ammonia gas orbiting HW2. This ring is identified as part of an accretion disk, a rotating structure of gas and dust that feeds material onto the central star.
- The Growth Rate: The study determined that gas is collapsing inward from this disk onto HW2 at an astonishing rate of about two thousandths of a solar mass per year. To put this in perspective, this is roughly equivalent to two Jupiter masses being added to the star every year, making it one of the highest stellar growth rates ever directly observed.
- The Mechanism: The hot ammonia gas in the accretion disk, heated to hundreds of kelvins, emits radio waves that allowed the scientists to track its movement. The observations show that the gas is not only rotating around the star but also falling onto it at near free-fall speeds, a balance between gravity and centrifugal forces. This confirms that disk accretion is the mechanism fueling the growth of this massive young star.
- Disk Asymmetry: Interestingly, the researchers found that the accretion disk is not uniform. One side is denser and shows more turbulence, suggesting that external streams of gas, known as streamers, might be feeding fresh material into the disk, sustaining the high growth rate.
- Significance: This discovery resolves a long-standing debate in astrophysics about how massive stars accumulate their mass. Previously, some theories suggested that these giants might form through the chaotic merging of smaller stars. However, this new evidence strongly supports the idea that accretion disks play a key role even for the most massive stars.
General Information about Star Formation and Accretion:
Stars are born within vast clouds of gas and dust called molecular clouds. Gravity causes these clouds to collapse, and as a dense core forms, it begins to accrete surrounding material. This infalling material often forms a rotating disk around the young star, known as an accretion disk or protoplanetary disk.
Accretion disks play a crucial role in the early stages of star formation:
- Feeding the Star: The disk acts as a conduit, channeling material onto the growing protostar at its center. The rate at which mass is accreted can vary significantly depending on the star's age and mass, as well as the properties of the disk. Young stars typically have higher accretion rates.
- Angular Momentum Transport: The rotation of the collapsing cloud results in angular momentum. The accretion disk helps to transport this angular momentum outward, allowing material to spiral inward onto the star.
- Planet Formation: Protoplanetary disks are also the birthplaces of planets. Dust and gas within the disk collide and stick together, gradually growing into larger bodies called planetesimals, which eventually coalesce to form planets.
The study of accretion processes in young stars like HW2 is vital for understanding not only how stars form and evolve but also the conditions under which planetary systems arise. The extreme growth rate observed in HW2 highlights the dynamic and complex processes at play in the early lives of massive stars. Scientists plan to continue observing HW2 with other powerful telescopes like ALMA to further investigate the disk's properties and predict the star's future evolution.