Harri A. T. Vanhala

Numerical Simulations of Triggered Star Formation. I. Collapse of Dense Molecular Cloud Cores

Results from numerical simulations of shock waves impacting molecular cloud cores are presented. The three-dimensional smoothed particle hydrodynamics code used in the calculations includes effects from a varying adiabatic exponent, molecular, atomic, and dust cooling, as well as magnetic pseudofluid. The molecular cloud cores are assumed to be embedded in background cloud material and to have evolved into their preimpact state under ambipolar diffusion. The shock wave is assumed to be locally plane parallel and steady. The results are sensitive to the thermodynamics employed in the calculations, because it determines the shock structure and the stability of the core. Shocks with velocities in the range of 20-45 km s-1 are capable of triggering collapse, while those with lower speeds rarely do. The results also depend on the properties of the preimpact core. Highly evolved cores with high initial densities are easier to trigger into collapse, and they tend to collapse to a single point. Less evolved cores with lower densities and larger radii may fragment during collapse and form binaries.

Injection of Radioactivities into the Forming Solar System

Meteorite studies have revealed the presence of short-lived radioactivities in the early solar system. The current data suggest that the origin of at least some of the radioactivities requires contribution from recent nucleosynthesis at a stellar site. This sets a strict time limit on the time available for the formation of the solar system and argues for the theory of the triggered origin of the solar system. According to this scenario, the formation of our planetary system was initiated by the impact of an interstellar shock wave on a molecular cloud core. The shock wave originated from a nearby explosive stellar event and carried with it radioactivities produced in the stellar source. In addition to triggering the collapse of the molecular cloud core, the shock wave also deposited some of the freshly synthesized radioactivities into the collapsing system. The radioactivities were then incorporated into the first solar system solids, in this manner leaving a record of the event in the meteoritic material. The viability of the scenario can be investigated through numerical simulations studying the processes involved in mixing shock-wave material into the collapsing system. The high-resolution calculations presented here show that injection occurs through Rayleigh-Taylor instabilities, the injection efficiency is approximately 10%, and temporal and spatial heterogeneities in the abundances of the radioactivities may have existed at the time of their arrival in the forming solar system.

Injection of Radioactivities into the Presolar Cloud: Convergence Testing

According to the hypothesis of the triggered origin of the solar system, the formation of our planetary system was initiated by the impact of an interstellar shock wave on a molecular cloud core. The shock wave originated from a nearby explosive stellar event and carried with it radioactivities produced in the stellar source. In addition to triggering the collapse of the molecular cloud core, the shock wave also deposited some of the freshly synthesized radioactivities into the collapsing system. These radioactivities were then incorporated into the first solar system solids, in this manner leaving a record of the event in the meteoritic material. The viability of the scenario can be assessed by numerical simulations studying the processes involved in injecting shock wave material into the collapsing system. Calculations performed at different resolutions confirm the previously suggested conclusions: injection occurs through Rayleigh-Taylor instabilities, the injection efficiency is approximately 10%, and temporal and spatial heterogeneities in the abundances of the radioactivities in the early solar system are possible. The results are used to estimate dilution factors for different stellar sources.

TRIGGERING COLLAPSE OF THE PRESOLAR DENSE CLOUD CORE AND INJECTING SHORT-LIVED RADIOISOTOPES WITH A SHOCK WAVE. I. VARIED SHOCK SPEEDS

The discovery of decay products of a short-lived radioisotope (SLRI) in the Allende meteorite led to the hypothesis that a supernova shock wave transported freshly synthesized SLRI to the presolar dense cloud core, triggered its self-gravitational collapse, and injected the SLRI into the core. Previous multidimensional numerical calculations of the shock-cloud collision process showed that this hypothesis is plausible when the shock wave and dense cloud core are assumed to remain isothermal at ~10 K, but not when compressional heating to ~1000 K is assumed. Our two-dimensional models with the FLASH2.5 adaptive mesh refinement hydrodynamics code have shown that a 20 km s?1 shock front can simultaneously trigger collapse of a 1 M ? core and inject shock wave material, provided that cooling by molecular species such as H2O, CO, and H2 is included. Here, we present the results for similar calculations with shock speeds ranging from 1?km?s?1 to 100?km?s?1. We find that shock speeds in the range from 5?km?s?1 to 70?km?s?1 are able to trigger the collapse of a 2.2 M ? cloud while simultaneously injecting shock wave material: lower speed shocks do not achieve injection, while higher speed shocks do not trigger sustained collapse. The calculations continue to support the shock-wave trigger hypothesis for the formation of the solar system, though the injection efficiencies in the present models are lower than desired.

Simultaneous Triggered Collapse of the Presolar Dense Cloud Core and Injection of Short-Lived Radioisotopes by a Supernova Shock Wave

Cosmochemical evidence for the existence of short-lived radioisotopes (SLRIs) such as26Al and60Fe at the time of the formation of primitive meteorites requires that these isotopes were synthesized in a massive star and then incorporated into chondrites within ~106 yr. A supernova shock wave has long been hypothesized to have transported the SLRIs to the presolar dense cloud core, triggered cloud collapse, and injected the isotopes. Previous numerical calculations have shown that this scenario is plausible when the shock wave and dense cloud core are assumed to be isothermal at ~10 K, but not when compressional heating to ~1000 K is assumed. We show here for the first time that when calculated with the FLASH2.5 adaptive mesh refinement (AMR) hydrodynamics code, a 20 km s−1 shock wave can indeed trigger the collapse of a 1 M☉ cloud while simultaneously injecting shock wave isotopes into the collapsing cloud, provided that cooling by molecular species such as H2O, CO2, and H2 is included. These calculations imply that the supernova trigger hypothesis is the most likely mechanism for delivering the SLRIs present during the formation of the solar system.

Injection of newly synthesized elements into the protosolar cloud

The relatively high initial abundance of the short–lived radioisotope 26Al in calcium–aluminium–rich refractory inclusions found in meteorites is inconsistent with forming the 26Al by irradiation in the solar nebula, unless the inclusions are shielded from irradiation by a more volatile mantle. Nucleosynthesis of the 26Al in a stellar source, such as a supernova, remains a likely alternative explanation, coupled with rapid injection of the newly synthesized 26Al into the protosolar cloud. In order to retain the live 26Al, the protosolar cloud must then collapse to form the solar nebula in less than 1 Myr. These requirements lead to the hypothesis of the supernova–triggered collapse of the protosolar cloud and injection of supernova shock wave material into the cloud. Theoretical models of the interaction of interstellar shock waves with target protosolar clouds show that a distant supernova can both trigger collapse and inject ca. 10% of the shock wave material incident on the cloud through Rayleigh–Taylor fingers.

The triggered origin of the solar system

The scenario of the triggered origin of the solar system suggests that the formation of our planetary system was initiated by the impact of an interstellar shock wave on a molecular cloud core. The strength of this scenario lies in its ability to explain the presence of short-lived radionuclides in the early solar system. According to the proposal, the radioactivities were produced in a stellar source, transported into the molecular cloud core by a shock wave and mixed into the collapsing system during the interaction between the shock wave and the core. We examine the viability of the scenario by presenting results from recent numerical simulations. The calculations show that molecular cloud cores can be triggered into collapse by the impact of a shock wave propagating at the velocity of 10–45 km s−1. Some of the shock wave material incident on the core, typically 10–20%, can be injected into the collapsing system. The time scale of the process is ∼104–105 years, sufficiently short for the survival of the short-lived radioactivities. The simulations therefore confirm the viability of the scenario of the triggered origin of the solar system.