Avalanches in stars and at finite temperature

Sheikh, Mohammed Azeem

Permalink

https://hdl.handle.net/2142/105628 Copy

Description

Title

Avalanches in stars and at finite temperature

Author(s)

Sheikh, Mohammed Azeem

Issue Date

2019-07-11

Director of Research (if dissertation) or Advisor (if thesis)

Dahmen, Karin A

Doctoral Committee Chair(s)

Weaver, Richard L

Committee Member(s)

• Weissman, Michael B
• Cooper, Stephen L

Department of Study

Physics

Discipline

Physics

Degree Granting Institution

University of Illinois at Urbana-Champaign

Degree Name

Ph.D.

Degree Level

Dissertation

Keyword(s)

Avalanches, Finite Temperature, Stars, Creep, Tabby's Star, Kepler

Abstract

We study two related but distinct aspects of avalanches in physical systems. The first is the study of avalanches that we have observed in stars. We apply results from the mean field avalanche model to observations made by the \emph{Kepler} spacecraft and the VIRGO instrument, looking at several stars including our own Sun and Tabby's star. In this examination, we use the stars' light curve, their integrated flux as a function of time, to extract avalanche information. Dimming events on the Sun are fairly well understood, and we find that there is scaling even in the Sun's data, likely caused by sunspots or combinations of such spots. We also look at Tabby's star, where the anomalous dimming has not been explained, and show that there is also avalanche scaling seen in this extraordinary star. We then look at avalanches at finite but low temperature in plastic deformation. The slow plastic deformation of materials under stress, known as creep motion, has long been studied in material's science. We hypothesize that at low temperatures, this deformation is the result of temperature activated avalanches. In order to explore this idea, we develop an extension of the mean field model to incorporate temperature. This model poses a problem since it requires exponentially many evaluations of rate constants when simulated using a kinetic monte carlo algorithm. We solve this problem by using a recursive strategy to pair down the number of evaluations and effectively choose the appropriate rate constants. Finally, we evaluate theoretically the interevent time distribution between these thermally activated avalanches. We identify high and low temperature regimes, at which the character of the distributions changes dramatically. We use simulations to verify our results, and connect them to experimental efforts currently underway to determine these distributions.

Graduation Semester

2019-08

Type of Resource

text

Permalink

http://hdl.handle.net/2142/105628

Copyright and License Information

Copyright 2019 by Mohammed Azeem Sheikh. All rights reserved.

Owning Collections