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Taylor Tobin Thesis Defense

Event Type
Athol Kemball
134 Astronomy
Jul 15, 2019   10:00 am  
Originating Calendar
Astronomy Graduate Program Calendar

The  detailed  mechanisms  responsible  for  the  polarization  of  SiO  masers  toward  late-type evolved stars continues to be the subject of debate.  Possible primary sources include the local  magnetic  field  or  anisotropic  pumping,  while  additional  polarization  may  arise  due to conversion from linear to circular modes through scattering or Faraday rotation.  Some maser features display an internal rotation in their electric vector position angle (EVPA) by 𝚫χ∼π/2 across their extent;  such features can provide robust constraints on SiO maser polarization theories as they allow inference of the angle, θ, between the magnetic field and the line of sight.  Masers remain an important high-resolution probe of the near-circumstellar environment  of  these  objects  and  reducing  uncertainties  in  maser  polarization  theory  is critical to our understanding of the astrophysics of these regions. In  this  thesis defense, I will describe two research studies of SiO EVPA reversal features.  The first of these (Tobin et al., 2018; 2019) concerns the analysis of a single SiO ν = 1, J = 1−0 maser feature displaying an EVPA reversal of 𝚫χ > π/2 toward the Mira variable, TX Cam, as observed by the Very Long Baseline Array in five epochs forming part of a prior larger imaging sequence.  While we find that the fractional linear polarization profile, ml(θ), across the EVPA rotation feature is consistent with the asymptotic theoretical maser polarization solution derived by Goldreich et al. (1973) for saturated masers in the limit of small Zeeman splitting,  with θ passing  through  the  critical  Van  Vleck  angle,  the  corresponding  EVPA profile χ(θ)  rotates  too  smoothly  to  arise  from  this  mechanism  alone.  I will discuss this and other results, as well as our investigation of possible sources of this discrepancy.  The second research study concerns development of a new theoretical formalism for maser polarization radiative transport more general than several previous approaches and specifically including optional Faraday rotation.  This formalism uses the radiative transfer solution method of Landi Degl’Innocenti (1987) to integrate the Stokes {I,Q,U,V} parameters.  This new formalism allows a more complete analysis of the observational results discussed above. Initial results will be presented from a one-dimensional numerical implementation of this maser polarization radiative transfer code. The two studies described here provide important  new  constraints  on  maser  polarization  theory  and  open  new  observational  and theoretical avenues for further exploration of this area of research.

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