University of Surrey’s Professor Mark Gieles and colleagues may have found a solution to a problem that has perplexed astronomers for several decades: why are stars in Milky Way’s globular clusters made of material different to other stars? In a new study, the team introduces novel actors to the equation — supermassive stars.
This Hubble image shows a globular cluster called NGC 6397. Image credit: NASA / ESA / T. Brown, S. Casertano & J. Anderson, STScI.
Globular clusters are collections of around a hundred thousand stars, held together by their mutual gravitational attraction in a spherical shape a few hundred light-years across.
It is thought that every galaxy has a population of globular clusters. Some, like our own Milky Way Galaxy, have a few hundred, while elliptical galaxies can have several thousand.
Globular clusters are generally very ancient objects formed around the same time as their host galaxy. To date, no new star formation has been observed within a globular cluster.
Since the 1960s, it has been known that most stars in globular clusters contain different chemical elements than all other stars in the Milky Way — these could not have been produced in the stars themselves because the required temperatures are about 10 times higher than the temperatures of these stars.
Professor Gieles and co-authors argue that a supermassive star, with a mass that is tens of thousands times the mass of the Sun, formed at the same time as the globular clusters.
At that time, these clusters were filled with dense gas out of which the stars were forming.
As the stars collect more and more gas, they get so close to each other that they could physically collide and form a supermassive star in a runaway collision process.
The supermassive star was hot enough to produce all the observed elements and ‘pollute’ the other stars in the cluster with the peculiar elements we observe today.
“What is truly novel in our model is that the formation of the supermassive stars and the globular clusters are intimately linked, and this new mechanism is the first model that can form enough material to pollute the cluster, and with the correct abundances of different elements, which has been a long-standing challenge,” Professor Gieles said.
The researchers propose various ways to test this new model of globular clusters and supermassive star formation with existing and upcoming telescopes, which can peer deep into the regions where the globular clusters formed, when the Universe was very young.
The team’s work appears in the Monthly Notices of the Royal Astronomical Society.
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Mark Gieles et al. 2018. Concurrent formation of supermassive stars and globular clusters: implications for early self-enrichment. MNRAS 478 (2): 2461-2479; doi: 10.1093/mnras/sty1059
