How nature shapes the birth of stars

The team, based at the University of Bonn in Germany, publish their results in the 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.

The astronomers find globular clusters must have formed with many more massive stars than are counted in individual star forming regions today. "Otherwise the star birth region a globular cluster formed from is not destroyed quickly enough and the subsequent expansion is too weak to remove enough stars from the cluster," says Marks. "If this had happened the distribution of masses of stars we see today would be quite different." The deduced differences in the number of massive stars having formed in globular clusters depending on the cloud conditions is indeed in agreement with theoretical expectation.

According to their results, differences in the initial star content appear only when conditions in the star birth regions are very extreme compared to those we see today. "We do not observe these extreme environments in the present day, but these may have well been frequent when globular clusters were born around 12 billion years ago," Marks states. Their work predicts that stars form in the same way, with the same range of masses, in different sites in the present day Milky Way.

Kroupa summarises their results. "With this work, we might have uncovered the long expected systematic differences in the star formation process." The Bonn astronomers now plan to use further simulations to study the effect of these differences on the long-term evolution of globular star clusters.