INTERSTELLAR SILICATE DUST: MODELING AND GRAIN ALIGNMENT by Indrajit Das A ...

174 Pages·2013·1.12 MB·English

INTERSTELLAR SILICATE DUST: MODELING AND GRAIN ALIGNMENT
by
Indrajit Das
A Dissertation
Submitted to the
Graduate Faculty
of
George Mason University
In Partial fulfillment of
The Requirements for the Degree
of
Doctor of Philosophy
Physics
Committee:
Dr. Joseph Weingartner, Dissertation Director
Dr. Shobita Satyapal, Committee Member
Dr. Mario Gliozzi, Committee Member
Dr. PadmanabhanSeshaiyer,CommitteeMember
Dr. Michael Summers, Director, School of
Physics, Astronomy and Computational
Sciences
Dr. Richard Diecchio, Interim Associate
Dean for Student and Academic Affairs,
College of Science
Dr. Peggy Agouris, Interim Dean, College
of Science
Date: Fall Semester 2013
George Mason University
Fairfax, VA
Interstellar Silicate Dust: Modeling and Grain Alignment
A dissertation submitted in partial fulfillment of the requirements for the degree of
Doctor of Philosophy at George Mason University
By
Indrajit Das
Master of Science
George Mason University, 2008
Bachelor of Engineering
Bengal Engineering and Science University, 1997
Director: Dr. Joseph Weingartner, Associate Professor
School of Physics, Astronomy and Computational Sciences (SPACS)
Fall Semester 2013
George Mason University
Fairfax, VA
Copyright c 2013 by Indrajit Das
(cid:13)
All Rights Reserved
ii
Dedication
I dedicate this dissertation to my parents and sisters.
iii
Acknowledgments
First and foremost, I would like to thank Dr. Joseph Weingartner for his wonderful and
immense company on my side. He has always been there for me not only as an academic
mentor but also as a great friend. Can’t thank him more. I would also like to thank
my dissertation committee members - Dr. Shobita Satyapal, Dr. Mario Gliozzi and Dr.
Padmanabhan Seshaiyer for their guidance and helpful comments.
iv
Table of Contents
Page
List of Tables . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . viii
List of Figures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . x
Abstract . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . xiv
1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
1.1 Dust, Polarization, and Magnetic Fields . . . . . . . . . . . . . . . . . . . 1
1.2 Constraints on Silicate Dust Models with 10 µm Feature . . . . . . . . . 3
1.2.1 Motivation from Min et al (2007) . . . . . . . . . . . . . . . . . . 4
1.3 Gaussian Random Sphere . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
1.4 Grain Alignment Theory . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
2 Interstellar Silicate Dust Modeling - Constraints on shape and mineralogy . . . 12
2.1 Replicating Min et al (2007) results . . . . . . . . . . . . . . . . . . . . . . 12
2.1.1 Convergence calculations with DDA method . . . . . . . . . . . . 12
2.1.2 Running DDSCAT for 10 µm feature . . . . . . . . . . . . . . . . 14
2.1.3 Codes and data files used . . . . . . . . . . . . . . . . . . . . . . . 15
2.2 Material Compositions and their optical properties . . . . . . . . . . . . . 16
2.2.1 10 µm feature for different compositions . . . . . . . . . . . . . . . 16
2.3 Observed Data and Fitting Algorithm . . . . . . . . . . . . . . . . . . . . 32
2.4 Beyond Min et al (2007) . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37
2.5 Densities used for materials . . . . . . . . . . . . . . . . . . . . . . . . . 38
2.6 Abundance constraints . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39
2.7 Stoichiometric Constraints . . . . . . . . . . . . . . . . . . . . . . . . . . 40
2.8 Results and discussions . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44
2.8.1 Fitting 10 µm extinction feature . . . . . . . . . . . . . . . . . . . 44
2.8.2 Varying Mg/Si abundance ratio . . . . . . . . . . . . . . . . . . . . 49
2.8.3 With Fe-oxide inclusions . . . . . . . . . . . . . . . . . . . . . . . . 52
2.8.4 Fitting with 10 µm Polarization feature . . . . . . . . . . . . . . . 54
2.9 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 57
3 Mechanical Torques . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 61
v
3.1 Physical properties of a GRS grain . . . . . . . . . . . . . . . . . . . . . . 61
3.2 Collisions of Gas Particles with the Grain . . . . . . . . . . . . . . . . . . 63
3.3 Torque Due to Incoming and Reflected Atoms . . . . . . . . . . . . . . . . 65
3.4 Mechanical Torque Due to Outgoing Atoms/Molecules . . . . . . . . . . 69
3.5 Total Mechanical Torque . . . . . . . . . . . . . . . . . . . . . . . . . . . . 72
3.6 Rotational Averaging . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 72
3.7 Drag Torque . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 74
3.8 Extreme Subsonic Limit . . . . . . . . . . . . . . . . . . . . . . . . . . . . 76
3.9 Extreme Supersonic Limit . . . . . . . . . . . . . . . . . . . . . . . . . . . 80
3.10 Mechanical Force . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 82
3.11 Computational Approach . . . . . . . . . . . . . . . . . . . . . . . . . . . 86
3.11.1 Defining and Characterizing the GRS . . . . . . . . . . . . . . . . 86
3.11.2 Incoming Trajectories . . . . . . . . . . . . . . . . . . . . . . . . . 86
3.11.3 Arrival at the Grain Surface and Reflection . . . . . . . . . . . . . 88
3.11.4 Integrals over the Reduced Speed . . . . . . . . . . . . . . . . . . . 89
3.11.5 Characterization of the Grain Surface . . . . . . . . . . . . . . . . 89
3.11.6 Torque Evaluations . . . . . . . . . . . . . . . . . . . . . . . . . . . 90
3.11.7 Code Verification: Spherical Grains. . . . . . . . . . . . . . . . . . 91
3.12 Results. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 92
3.12.1 Grain Shape . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 92
3.12.2 Arrival Rate and Torques . . . . . . . . . . . . . . . . . . . . . . . 94
4 Grain Dynamics and Alignment . . . . . . . . . . . . . . . . . . . . . . . . . . 107
4.1 Equations of Motion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 107
4.2 Stationary Points . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 112
4.3 Crossover Points . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 114
4.4 Results. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 115
4.5 Conclusions and Future Work . . . . . . . . . . . . . . . . . . . . . . . . . 122
A An Appendix . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 128
A.1 Details of solid GRS# 1 . . . . . . . . . . . . . . . . . . . . . . . . . . . . 129
A.2 Details of solid GRS# 2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . 129
A.3 Details of vacuum included GRS# 1 . . . . . . . . . . . . . . . . . . . . . 132
A.4 Details of vacuum/Fe-oxide included GRS# 2 . . . . . . . . . . . . . . . . 132
B An Appendix . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 145
B.1 Torque Due to Incoming Atoms . . . . . . . . . . . . . . . . . . . . . . . 145
B.2 Torque Due to Outgoing Particles . . . . . . . . . . . . . . . . . . . . . . 146
vi
B.3 Extreme Subsonic Limit . . . . . . . . . . . . . . . . . . . . . . . . . . . . 146
B.4 Drag Force in the Extreme Subsonic Limit . . . . . . . . . . . . . . . . . 148
vii
List of Tables
Table Page
2.1 Sets of grain material mass densities (gcm−3) . . . . . . . . . . . . . . . 39
2.2 Mass percentages of different compositions used for the best χ2 fits for
four different shapes fitted with extinction feature for Mg/Si = 1.22. . . . 44
2.3 Mass percentages of different compositions used for the best χ2 fits for
four different shapes fitted with polarization feature. Mg/Si = 1.2. . . . . 59
3.1 GRS Expansion Coefficients . . . . . . . . . . . . . . . . . . . . . . . . . 93
3.2 GRS Derived Quantities . . . . . . . . . . . . . . . . . . . . . . . . . . . 94
4.1 Adopted Parameter Values for Grain Rotational Dynamics . . . . . . . . 115
A.1 Spherical Harmonic Coefficients for solid GRS# 1 . . . . . . . . . . . . . 128
A.2 Spherical Harmonic Coefficients for solid GRS# 2 . . . . . . . . . . . . . 130
A.3 Spherical Harmonic Coefficients for vacuum/Oxide included GRS# 1 . . . 131
A.4 Spherical Harmonic Coefficients for inclusion#1 for vacuum/Oxide in-
cluded GRS# 1 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 133
A.5 Spherical Harmonic Coefficients for inclusion# 2 for vacuum/Oxide in-
cluded GRS# 1 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 134
A.6 Spherical Harmonic Coefficients for inclusion# 3 for vacuum/Oxide in-
cluded GRS# 1 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 135
A.7 Spherical Harmonic Coefficients for inclusion # 4 for vacuum/oxide in-
cluded GRS# 1 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 136
A.8 Spherical Harmonic Coefficients for inclusion # 5 for vacuum/Oxide in-
cluded GRS# 1 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 137
A.9 Spherical Harmonic Coefficients for vacuum/Oxide included GRS# 2 . . . 139
A.10 Spherical Harmonic Coefficients for inclusion#1 for vacuum/Oxide in-
cluded GRS# 2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 140
A.11 Spherical Harmonic Coefficients for inclusion# 2 for vacuum/Oxide in-
cluded GRS# 2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 141
viii
A.12 Spherical Harmonic Coefficients for inclusion# 3 for vacuum/Oxide in-
cluded GRS# 2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 142
A.13 Spherical Harmonic Coefficients for inclusion # 4 for vacuum/oxide in-
cluded GRS# 2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 143
A.14 Spherical Harmonic Coefficients for inclusion # 5 for vacuum/Oxide in-
cluded GRS# 2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 144
ix