Modeling CN Zeeman Effect Observations of the Envelopes of a Low-Mass Protostellar Disk and a Massive Protostar

• Main Authors: Mazzei, Renato, Li, Zhi-Yun, Chen, Che-Yu, Tu, Yisheng, Fissel, Laura, Klein, Richard I
• Format: Journal Article
• Language: English
• Published: 01-12-2023
• Subjects: Physics - Astrophysics of Galaxies; Physics - Solar and Stellar Astrophysics
• DOI: 10.48550/arxiv.2312.00884

We use the POLARIS radiative transfer code to produce simulated circular
polarization Zeeman emission maps of the CN $J = 1 - 0$ molecular line
transition for two types of protostellar envelope magnetohydrodynamic
simulations. Our first model is a low mass disk envelope system (box length $L
= 200\text{ au}$), and our second model is the envelope of a massive protostar
($L = 10^4\text{ au}$) with a protostellar wind and a CN enhanced outflow
shell. We compute the velocity-integrated Stokes $I$ and $V$, as well as the
implied $V/I$ polarization percentage, for each detector pixel location in our
simulated emission maps. Our results show that both types of protostellar
environment are in principle accessible with current circular polarization
instruments, with each containing swaths of envelope area that yield percentage
polarizations that exceed the 1.8\% nominal sensitivity limit for circular
polarization experiments with the Atacama Large Millimeter/submillimeter Array
(ALMA). In both systems, high polarization ($\gtrsim$1.8\%) pixels tend to lie
at an intermediate distance away from the central star and where the
line-center opacity of the CN emission is moderately optically thin ($\tau_{LC}
\sim 0.1-1$). Furthermore, our computed $V/I$ values scale roughly with the
density weighted mean line-of-sight magnetic field strength, indicating that
Zeeman observations can effectively diagnose the strength of envelope-scale
magnetic fields. We also find that pixels with large $V/I$ are preferentially
co-located where the absolute value of the velocity-integrated $V$ is also
large, suggesting that locations with favorable percentage polarization are
also favorable in terms of raw signal.