[Paper Review] Stringent limits on the magnetic field strength in the disc of TW Hya: ALMA observations of CN polarisation

W. H. T. Vlemmings, Boy Lankhaar+9 more|Leiden Repository (Leiden University)|Apr 2, 2019

Astrophysics and Star Formation Studies42 references15 citations

TL;DR

This study presents the first ALMA observations of circular polarisation in CN emission from the TW Hya protoplanetary disc, using Zeeman splitting to probe magnetic field strength. Despite excellent instrumental sensitivity, no significant circular polarisation was detected, yielding the most stringent upper limits to date: |Bz| < 0.8 mG (1σ) and Btoroidal < 30 mG, ruling out some accretion disc models but consistent with efficient magnetic field advection in recent simulations.

ABSTRACT

Despite their importance in the star formation process, measurements of magnetic field strength in proto-planetary discs remain rare. While linear polarisation of dust and molecular lines can give insight into the magnetic field structure, only observations of the circular polarisation produced by Zeeman splitting provide a direct measurement of magnetic field strenghts. One of the most promising probes of magnetic field strengths is the paramagnetic radical CN. Here we present the first Atacama Large Millimeter/submillimeter Array (ALMA) observations of the Zeeman splitting of CN in the disc of TW~Hya. The observations indicate an excellent polarisation performance of ALMA, but fail to detect significant polarisation. An analysis of eight individual CN hyperfine components as well as a stacking analysis of the strongest (non-blended) hyperfine components yields the most stringent limits obtained so far on the magnetic field strength in a proto-planetary disc. We find that the vertical component of the magnetic field $|B_z|&lt;0.8$~mG ($1σ$ limit). We also provide a $1σ$ toroidal field strength limit of $&lt;30$~mG. These limits rule out some of the earlier accretion disc models, but remain consistent with the most recent detailed models with efficient advection. We detect marginal linear polarisation from the dust continuum, but the almost purely toroidal geometry of the polarisation vectors implies that his is due to radiatively aligned grains.

Motivation & Objective

• To directly measure magnetic field strength in a protoplanetary disc using the Zeeman effect in CN molecular lines.
• To test the performance of ALMA in detecting circular polarisation from Zeeman splitting in a protoplanetary disc.
• To constrain the vertical and toroidal components of the magnetic field in TW Hya’s disc using high-sensitivity observations.
• To evaluate the consistency of observed limits with theoretical accretion disc models involving magnetic field advection and diffusion.
• To assess the role of radiative grain alignment in interpreting linear polarisation from dust continuum.

Proposed method

• Conducted high-sensitivity ALMA observations at millimeter wavelengths targeting the CN (N=2–1, J=3/2–1/2) hyperfine transitions in the disc of TW Hya.
• Analyzed individual hyperfine components and performed a stacking analysis on the strongest, non-blended components to improve signal-to-noise.
• Used the Zeeman splitting of CN's magnetic dipole transitions to infer magnetic field strength from circular polarisation amplitudes.
• Applied a radiative transfer model to account for the Zeeman splitting in CN’s hyperfine structure across ALMA Bands 3, 6, and 7.
• Compared observed polarisation limits with theoretical models of magnetic field advection and diffusion in protoplanetary discs.
• Assessed instrumental system performance by checking for residual polarisation in the data, confirming sensitivity down to <0.8% of total intensity.

Experimental results

Research questions

• RQ1 What is the upper limit on the vertical component of the magnetic field in the TW Hya protoplanetary disc, as measured via CN Zeeman circular polarisation?
• RQ2 How strong is the toroidal magnetic field component in the disc, based on the same polarisation data?
• RQ3 Are the observed polarisation limits consistent with current models of magnetic field advection and diffusion in protoplanetary discs?
• RQ4 To what extent do radiative grain alignment effects contribute to the observed linear polarisation in the dust continuum?
• RQ5 Can ALMA achieve the sensitivity required to detect magnetic fields below 1 mG in protoplanetary discs using CN Zeeman splitting?

Key findings

• No significant circular polarisation was detected in any of the eight CN hyperfine components, indicating that the magnetic field strength is below the detection threshold.
• The 1σ upper limit for the vertical magnetic field component is |Bz| < 0.8 mG, representing the tightest constraint on Bz in a protoplanetary disc to date.
• A 1σ upper limit of <30 mG was derived for the toroidal magnetic field component, consistent with models featuring efficient magnetic field advection.
• The observed limits rule out earlier accretion disc models predicting stronger vertical fields (e.g., Shu et al. 2007), but are consistent with models incorporating efficient advection and ambipolar diffusion.
• The stacking analysis confirmed no instrumental polarisation at levels below 0.8% of total intensity, demonstrating ALMA’s capability to reach sub-mG sensitivity in suitable sources.
• Marginal linear polarisation in the dust continuum was detected, but its toroidal geometry strongly suggests it arises from radiatively aligned grains rather than magnetic fields.