Scientists have long known that stars of so-called intermediate mass, i.e., heavier than the sun but still far from being supergiants, accumulate mass extremely rapidly during their formation. And now they have managed to examine this process in detail.
How are planets formed in a protoplanetary disk?
Stars form in massive clouds of gas called molecular clouds. During their formation, they accumulate gas from these clouds, and as the stars rotate, gas and dust accumulate in a rotating disk around the star called a protoplanetary disk. As the name suggests, this is where planets form by accumulating material from the disk.
This is a simple explanation of a highly complex process that is the subject of ongoing research in astronomy. Decades of research have confirmed the basic hypothesis that stars gradually accumulate material from these disks, with some of it being blown away by the star’s radiation and some forming planets.
Over time, the protoplanetary disk dissipates, the star stops accumulating material and reaches its final mass, and planet formation ceases.
However, it is difficult to look into these environments to understand what is happening. The entire process is hidden behind a thick layer of gas and dust, and much of what is known is based partly on observations and partly on theory. Therefore, this explanation does not apply to all stars.
Research on the formation of intermediate-mass stars
It works for stars roughly the same size as the Sun, but it does not work for intermediate-mass stars. When astronomers observe intermediate-mass stars with masses between 1.5 and 4 solar masses, they find that they accumulate mass faster than expected.
New research shows that instead of decreasing with age, intermediate-mass stars experience rapid growth spurts later in their formation. Paradoxically, this does not hinder planet formation, but actually allows gas giants such as Jupiter to grow.
The study, titled “Evolution of the Accretion Rate of Young Intermediate-mass Stars: Implications for Disk Evolution and Planet Formation” is published in The Astronomical Journal, with Sean Brittain as the lead author. Brittain is a professor in the Department of Physics and Astronomy at Clemson University in South Carolina.
The study focuses on two types of stars: T Tauri stars, which are medium-mass stars, and Herbig stars. T Tauri stars are young, pre-main sequence stars, while Herbig stars are what T Tauri stars evolve into (IMTTS). Herbig stars are closer to being on the main sequence, but they are still forming and are still in their disks. They are more massive and hotter than the Sun.
Accretion of material by intermediate-mass protostars
When stars accrete material, they also emit energy, and this is the focus of the new study. By measuring the energy emitted, astrophysicists can determine the growth rate of a star.
“When material falls onto a star, a lot of energy is released. Just like when you drop a chair, it will make a noise or even break. In the case of material being accreted, the energy released is much greater. We can see this as extra radiation coming from the system, and this allows us to determine the rate at which the stars grow in mass,” said Brittain.
By observing Herbig stars, researchers have found that the rate of accretion increases with age as stars reach maturity.
“This implied that the disks surrounding these stars must start out to be very massive indeed. This would pose a problem because such massive disks would be unstable and break up before planets even have the chance to be formed,” said Rene’ Oudmaijer, a member of the team from the Royal Observatory of Belgium.
Instability of massive protoplanetary disks
It may seem that massive disks contribute to the formation of planets, but in fact, the opposite is true. When the mass of a disk is about 10% or more of the mass of its star, it becomes unstable. It can quickly break up into clusters, preventing the gradual formation of planets due to collisions and accretion.
When researchers observed IMTTS, they found that their accretion rates were more than 10 times lower than those of their more developed counterparts, Herbig stars.
“Instead of higher accretion rates, we found values that were up to 30 times lower than those of the Herbig stars. In a way, this would solve the mass problem, as the disk does not need to be so massive to begin with,” said Gwendolyn Meeus of the Autonomous University of Madrid in Spain.
Consequently, the decrease in accretion rates for aging Herbig stars indicates massive disks that are unstable, and instability hinders planet formation. Even lower accretion rates for IMTTS seem to solve the mass problem, since the disk can initially be smaller and still allow gas giants to form.
But this raises another problem: older stars accumulate mass much faster than their younger counterparts. Theory suggests that this cannot be true; in fact, it makes no sense. Why would a star accumulate less mass when more is available, and more mass when less is available?
Explanation of the riddle
Researchers began developing a model that could explain this. Their model shows that accretion is caused by part of the star’s radiation, specifically far ultraviolet (FUV) radiation.
“We put forward a physically plausible scenario that accounts for the systematic increase of stellar accretion based on the increase of the effective temperature of the stars as they evolve towards the zero-age main sequence,” the researchers write in their paper.
Here is their solution: more evolutionarily advanced Herbig stars are hotter than their younger IMTTs counterparts, and this higher temperature causes the star to emit more in the far ultraviolet (FUV). “Thus, the luminosity of the far-ultraviolet (FUV) radiation will increase by orders of magnitude. We propose that this increase drives a higher stellar accretion rate,” the authors explain.
FUV ionizes more gas in the disk, which creates greater accretion onto the star. This occurs due to magnetorotational instability (MRI), the main force that causes accretion onto the star.
Stars are surrounded by their magnetic fields, and neutral gas interacts poorly with them. But ionized gas does interact. It responds to the magnetic field lines around stars. When gas becomes highly ionized, it essentially creates turbulence in the star’s magnetic field lines. Turbulence transfers angular momentum from the disk outward.
This means that the inner regions of the disk have weaker angular momentum, allowing the star to accrete material more quickly.
In my opinion, this solves the problem of giant planet formation around medium-mass stars. The increased heat of Herbig stars promotes rapid accretion, and a more massive disk is not necessary.
“The understanding that hotter stars emit more ultraviolet radiation than cooler stars has been known for well over 100 years, and the expectations that the ionization of the disk plays an important role in the accretion process have been around for decades,” Brittain said.
This work represents the next step in scientific progress, building on these fundamental ideas and demonstrating that these systems really have an unexpected late surge of growth.
Planet formation around Herbig stars
These results are also consistent with modern observations of Herbig stars. Observations using ALMA and the SPHERE instrument on the Very Large Telescope show widespread spiral arms in their disks. Various explanations have been proposed for these spiral structures, including gravitational waves or perturbations from companions such as brown dwarfs, stellar companions, or planets. But their less evolved counterparts, IMTTS, do not have such structures.
In the conclusion of their scientific article, the authors write that “… spiral structure among Herbig stars is more plausibly a signpost of gas giant planet formation.”
“Finally, our model allows for disks with sufficient mass to form planets around Herbig stars to persist for several Myr, even at the high accretion rates observed in this evolutionary state, they conclude.
According to phys.org
