Using state of the art computer simulations, a team of astronomers from the University of Bonn in Germany have found the first evidence that the way in which stars form depends on their birth environment. The team, based at the University of Bonn in Germany, publish their results in the journal Monthly Notices of the Royal Astronomical Society.
Stars are thought to form in interstellar space from dark clouds of gas and dust. Their properties are expected to depend on the conditions of their dusty birth environment, in the same way that the temperature and constitution of clouds on Earth determines whether we experience drizzly weather, rain with large or small droplets, or a hail shower. In contrast, until now stars have appeared to unexpectedly form in the same manner everywhere. “Sites of star formation are the bad weather regions in a galaxy and the forming stars are, in a very rough analogy, like the raindrops condensing out of this material”, comments team member Prof. Dr. Pavel Kroupa.
The group of scientists now have evidence that the mass distribution of stars does indeed depend on the environment in which they form. “Surprisingly, this evidence does not come to us from young regions of ongoing star formation, but from a very old class of objects, so called globular star clusters”, says Dr. Michael Marks, lead author of the new paper. “The number of observed stars less massive than our Sun in globular clusters is at odds with their structure.”
Globular clusters are massive congregations of thousands stars surrounding our Galaxy, the Milky Way. Star formation in these clusters ceased billions of years ago. “Nevertheless, using our simulations we found that the connection between star formation and birth environment can be understood when invoking a process that occurs very early in the life of any cluster, called residual-gas expulsion”, continues Marks.
Once a star completes its formation it starts to shine and eventually the radiation coming from the cluster of freshly-hatched stars quickly drives out the gas from which they formed. The region of star birth is then destroyed, leaving behind stars of different masses. “This process leads to expansion of the whole aggregate of stars with the accompanying stripping of some of the stars from the cluster by the gravitational attraction of the young Milky Way. The faster the gas is blown out the stronger is the expansion and the more stars are removed”, Kroupa explains. He adds, “The imprint of this process is still visible in the present-day mass distribution”. This means that careful observations of present-day stellar populations in globular clusters allow their initial star content to be reconstructed.