Simulating the Birth of Massive Stars
New computer simulations of the flow of ionized gas around a massive young star appear to explain successfully some of the classical puzzles associated with massive star formation. The colors are coded to represent the velocity of an ionized wind, with red for gas moving away from the viewer and blue for gas moving towards the viewer. The star itself is located at the mark.
Astronomers have made great strides recently in understanding how modest stars -- those like the sun or smaller -- are formed. Stars form from giant clouds of gas and dust in space as the matter in these clouds gradually becomes denser and denser, compressed by the influence of its own gravity. The formation of the most massive stars, however, poses a puzzle. Even before they reach their final masses, these stars become so hot and luminous that they ionize the surrounding material, producing bright bubbles of ionized gas that, incidentally, are often more easily spotted than are the stars themselves. The puzzle is: how can a massive protostar continue to grow to its final size if it is surrounded by a hot bubble?
A few years ago, SAO astronomer Eric Keto proposed a new hypothesis for the evolution of massive protostars, suggesting that the gravitational field around a massive protostar may be strong enough to overcome the effects of an ionized gas bubble. Now, together with SAO astronomer Roberto Galvan-Madrid and three colleagues (including Thomas Peters who recently joined SAO), he has published the first computer simulations of this effect. The simulations are also among the first to include ionization and thus to allow for structures to develop fully in three-dimensions rather than with the imposed spatial symmetries of the simpler calculations.
The numerical experiments confirm the previous hypothesis, but also result in several significant new conclusions. The simulations show that many types of bubbles can be produced in response to the local conditions of a cloud, as are observed, and moreover predict the formation and growth of multiple new stars, each in its own dense fragment of the larger-scale cloud. While the first star forms in the center of the flow, the outlying stars eventually limit its growth by attracting all the cloud matter to themselves in a process the authors call "fragmentation-induced starvation." New stars that form even further out then starve this second generation. These results, together with others from their simulations, appear to offer realistic answers to many of the outstanding puzzles of massive star formation in clusters.