Computer simulations suggest that the amplification of magnetic fields in stellar collisions may play an important role in the formation of a particular subset of stars in clusters. Blue straggler stars in clusters appear not only bluer, but also younger than other cluster members. One proposed explanation for their apparently different ages is that they are the result of stellar collisions. However, this would require the resulting star to spin down efficiently without losing too much mass. Scientists at the Max Planck Institute for Astrophysics have now shown, using sophisticated 3D simulations, that the energy of the magnetic field is greatly amplified in the collisions of low-mass stars, providing a potentially efficient spin-down mechanism.
Clusters of stars, containing hundreds of thousands of stars that formed around the same time and from the same molecular cloud, provide astronomers with an excellent laboratory for studying how stars of similar age, composition and mass evolve over time. However, one particular subset, the 'blue stragglers', pose a challenge: they appear bluer and brighter than the other cluster members, and therefore appear to be younger. Why don't they age like typical cluster stars?
The answer could be that they actually formed later than the other stars in stellar collisions and thus gained mass. However, since most collisions between two low-mass stars are off-axis (rather than perfectly head-on), the resulting massive star would rotate rapidly and lose most of its mass during the spin-down to a stable state – unless the spin-down is efficient. While many proposed spin-down mechanisms require magnetic fields, it has remained unclear for more than two decades whether they actually exist and whether they have the strength to play a significant role.
A team at the Max Planck Institute for Astrophysics (MPA) has now presented sophisticated 3D moving-mesh magnetohydrodynamical simulations of collisions between low-mass main-sequence stars, which show that the magnetic field energy is amplified by a factor of up to 10 billion during collisions. At the core of the merged star, the magnetic field can reach 100 million Gauss (for comparison, the magnetic field in sunspots can reach up to 5000 Gauss). "Our simulations showed that the magnetic field in stellar collisions can be amplified, which is a promising sign for an effective spin-down mechanism," says MPA postdoctoral researcher Taeho Ryu, who led the study. "This amplification is independent of collision parameters, so it could happen every time two stars collide in a cluster."
The simulations also show a flattened, rotating gas structure around the collision, which could indicate the formation of a disk. Magnetic braking and an effect called "disk locking" could further facilitate the spin-down. "Our next step will be to actually follow the long-term evolution after the collision to see how these stars evolve over millions or billions of years and whether they really end up as the blue straggler stars that we observe," adds Ryu.
This animation shows the same simulation as the figure above. The left panel shows the evolution of the density, the right panel the evolution of the magnetic field strength as two stars of 0.7 and 0.6 solar masses collide.
Contact:
Taeho Ryu
Postdoc
2358
tryu@mpa-garching.mpg.de
Original publication
Ryu, Taeho; Sills, Alison; Pakmor, Ruediger; de Mink, Selma; Mathieu, Robert
Magnetic Field Amplification during Stellar Collisions between Low-mass Stars
ApJ, Volume 980, Issue 2, id.L38, 11 pp.
Source | DOI
