Date of Award

12-2012

Document Type

Thesis

Degree Name

Master of Science (MS)

Legacy Department

Physics

Advisor

Hartmann, Dieter

Committee Member

Leising , Mark

Committee Member

Sosolik , Chad

Abstract

The aim of this work is to evaluate the '26Al method' used to determine the Galactic Star Formation Rate (SFR) and compare it to other alternative methods. 26Al is a radioactive isotope produced mainly in massive star winds and in the ensuing core collapse supernova explosion. The radioactive 26Al decays with a life time of 106 years by emitting γ- ray photons in 1.808 MeV band. The 26Al method involves using the Galactic 26Al radioactive flux as a tracer. This approach based on the γ- ray line measurements does not suffer from extinction and small number statistics.
To evaluate the 26Al method, we model the spatial distribution of massive stars by Monte-Carlo methods and simulate the kinematics of radioactive 26Al produced in massive star winds and supernovae explosions. The wind from a massive star leads to the formation of a hot bubble around the star and we use a wind/ISM interaction model to simulate this interaction of the hot bubble and the ISM. A Supernova explosion inside this wind bubble results in the production of additional 26Al, the distribution of which is modeled using a supernova expansion model.
Thus, this work builds on the previous work (Diehl et al. 2006) by including the 26Al contribution from massive star winds to the total budget of the Galactic 26Al. We find that the results from our simulation are consistent with observations. We conclude that the addition of 26Al from massive star winds results in a lowering of the estimated SFR by 20%. We further infer that a donut with arms distribution model (free-electron density model of only the thin disk and spiral arms) reflects the real distribution of Galactic 26Al and we derive a SFR of ~ 3.5 + 1.7 Mʘ yr-1 which is consistent with Diehl et. Al (2006).
The uncertainties associated with the 26Al method are largely due to uncertainty in the possible sources of 26Al and the yields associated with each source. All other SFR estimating methods suffer from varying degrees of selection affects. There are systematic errors owing to incomplete catalogs, Galactic visual extinction and small number statistics. Thus, the 26Al method is a direct means of deriving the Galactic SFR that is not prone to selection affects.

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