The birth and death of stars
Abstract
We present analytical models for two violent phases of stellar evolution whose effects upon the interstellar medium are most profound: supernova explosions and protostellar outflows. Using models for outflows, we analyze the efficiency and dynamics of a forming stellar cluster. The distribution of ejecta that expand away from a core- collapse supernova can be predicted once one knows the velocity of the blastwave shock that crossed the stellar envelope. We identify a new model for this shock's velocity that is simple and accurate, with which we construct detailed models relating stellar structures to their ejecta distributions. The shock model also permits us to calculate the observable features of shock emergence, including the energy and duration of the soft- X-ray burst, the upper limit of ejecta velocities, and the mass (if any) of relativistic ejecta. The latter may help to understand weak gamma-ray bursts from supernovae. We also use the structural similarity of red and blue supergiants to approximate their ejecta with simpler, less flexible models. We show that hydromagnetic winds from accreting protostars will assume a common force distribution at large distances, whether or not the wind emanates from a narrow region of disk radii. In any power-law density distribution, such a wind sweeps ambient gas into thin shells whose features match those commonly observed in bipolar molecular outflows, regardless how the driving wind's intensity varies over time. This implies that prompt entrainment, not turbulence, is responsible for these features. This model predicts the rate of mass ejection from a star-forming region, and thus the efficiency with which a star cluster can form. Using the energy injection and mass ejection implied by this model, we address the dynamical evolution of a dense clump inside a molecular cloud as it creates a cluster of low-mass stars. We use the virial equation of motion and assume that star formation is limited by ambipolar diffusion. For lower pressures than within starburst nuclei, an equilibrium state is possible if turbulence does not decay extremely rapidly. However, clumps tend to oscillate about their equilibria, and this can cause star formation to proceed in bursts.
- Publication:
-
Ph.D. Thesis
- Pub Date:
- 1999
- Bibcode:
- 1999PhDT........11M
- Keywords:
-
- SUPERNOVAE;
- GAMMA RAY BURSTS;
- FLUID DYNAMICS;
- SHOCK WAVES;
- INTERSTELLAR MEDIUM;
- STAR FORMATION;
- PROTOSTELLAR OUTFLOWS;
- STAR FORMATION EFFICIENCY;
- STELLAR CLUSTERS;
- Physics: Astronomy and Astrophysics, Physics: Fluid and Plasma