Master's Thesis Defense by Emil Pedersen – Niels Bohr Institute - University of Copenhagen

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Master's Thesis Defense by Emil Pedersen

Title: Evolution, formation and feedback of massive stars in a realistic GMC environment

Abstract:

The aim of this thesis is to investigate the evolution of high-mass stars starting from prestellar cores and until they die in a supernova. The formation of stars has always been a central part of astrophysics and has been studied for decades, however it is only recently that technology has allowed us to apply more advanced stellar evolution models.

In this work I use the stellar evolution code MESA to simulate the protostellar evolution with a patch to enable the inclusion of dynamic accretion and calculate the fraction of energy absorbed by the star from the accretion luminosity, thus enabling the implementation of accretion profiles. I test models with and without accretion, to verify that the models work in a classical non-accreting frame as well as investigate what the implications is for the evolution and lifetime of high-mass stars.

The accretion profiles comes from a simulation of the outer Galactic disk and is carried out in the 3D magnetohydrodynamical code Ramses, which is able to capture the unsteady nature of accretion caused by various effects in a molecular cloud, such as if the star resides within density filaments in the cloud being fed by turbulence driven accretion flows.

High-mass stars are one of the fundamental engines driving the evolution of the universe as the supernovae explosions enriches the interstellar medium with heavy elements thus playing an instrumental role in the gas-star life-cycle, while also driving large scale turbulent motions in molecular clouds. The classical approach in stellar evolution treats stars to have accreted all of their available mass from the ambient reservoir when they are born. This assumption does not hold true for high-mass stars, as their Kelvin-Helmholtz timescale is smaller than the accretion time, meaning they will contract to main sequence and ignite hydrogen burning while still accreting mass. Because the rate of nuclear processes is proportional to the temperature, the gradual building of the star causes the nuclear fuel consumption rate to increase gradually, in contrast to the classical treatment where a 40 M_sun star is burning as a 40 M_sun star throughout its entire life. As the consumption of nuclear fuel is tied to the lifetime, this in turn means an increase in lifetime. For stars above 50 M_sun I found an increase in lifetime by more than 50% when compared to the classical models. Furthermore, I found that the classical approach of determining a stars lifetime based on its mass, no longer holds true, as there is a significant variation in the lifetime of stars with similar masses. This variation is an effect of the dynamic accretion rate, as some stars will reach higher masses faster than others, thus burning their fuel more rapidly.

 

A prolonged lifetime of high-mass stars also has implications for the delay-time distribution and supernova feedback in dense molecular clouds. The impact of supernovae feedback depends weakly on the ambient density, however it depends strongly on the distance to the molecular cloud, thus making the lifetime an important input parameter for stellar feedback models used in simulations of star formation etc.

 

Finally, the increased lifetime of high-mass stars has implications for the rate of O-star formation in the Milky Way, which is determined through observations of HII or UCHII regions. Thus an increase in lifetime of 50% would imply a reduction of O-star formation rate of 33%.

Supervisor: Troels Haugbølle