# Erratum: Magnetic topologies of young suns: the weak-line T Tauri stars TWA 6 and TWA 8A

C. A. Hill, C. P. Folsom, J. -F. Donati, G. J. Herczeg, G. A. J. Hussain, S. H.

P. Alencar, S. G. Gregory

► To cite this version:

C. A. Hill, C. P. Folsom, J. -F. Donati, G. J. Herczeg, G. A. J. Hussain, et al.. Erratum: Magnetic topologies of young suns: the weak-line T Tauri stars TWA 6 and TWA 8A. 2019, pp.5526-5527. 10.1093/mnras/stz1254 . insu-03674427

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# Erratum: Magnetic topologies of young suns: the weak-line T Tauri stars TWA 6 and TWA 8A

by C. A. Hill<sup>1,1*</sup> C. P. Folsom,<sup>1</sup> J.-F. Donati,<sup>1</sup> G. J. Herczeg,<sup>2</sup> G. A. J. Hussain,<sup>1,3</sup> S. H. P. Alencar,<sup>4</sup> S. G. Gregory<sup>5</sup> and the MaTYSSE collaboration

**Key words:** errata, addenda – techniques: polarimetric – stars: formation – stars: imaging – stars: individual: TWA 6 – stars: individual: TWA 8A – magnetic fields.

This is an erratum to the paper 'Magnetic topologies of young suns: the weak-line T Tauri stars TWA 6 and TWA 8A' that was published in MNRAS, 484, 5810–5833 (2019).

We correct several typographical errors in Table 3 of Hill et al. (2019), and update Fig. 11. These corrections do not change the conclusions of the work.

The first two columns of values in Table 3, labelled 'Stokes I only' and 'Stokes V only' incorrectly included values in the rows from '-2 kG' down to '-20 kG'. As these bins of magnetic field strength were not included in the models, there should be no values in these rows. This was a simple typographical error, with values from adjacent columns incorrectly placed here. As a consequence, the mean magnetic field strengths $\langle|B_f|\rangle$ quoted at the bottom of Table 3 are incorrect. We have also corrected the value of $\langle|B_f|\rangle$ for the fit using both Stokes I and V, as previously the uncertainty was calculated as the sum of the individual uncertainty on each bin, but now we add the uncertainties in quadrature, yielding a more precise value of $\langle|B_f|\rangle = 5.9 \pm 0.2$ kG (as compared to the previous value of 6.0 $\pm$ 0.5 kG). This same quantity is also stated in Section 5.5 paragraph 7, Section 7 paragraph 6, and in the abstract. The updated values of $\langle|B_f|\rangle$ do not change the conclusions of this work. All values quoted in this erratum take precedence over those stated in Hill et al. (2019).

We also update the lower panel in Fig. 11, as the fractional filling factor from direct spectral fitting for the 0 kG bin is more appropriately taken as 1 minus the sum of the non-zero bins, equal to 0.897 $\pm$ 0.033, rather than the value of 0.017 $\pm$ 0.016 which was unchanged after subtracting the negative fields from the positive fields. This agrees well with the results from ZDI, with a filling factor of 0.8 for the 0 kG bin. The marginally higher fraction from direct spectral fitting likely stems from an ambiguity between the 0 and 2 kG bins.

<table><caption>**Table 3.** Best fit parameters from direct spectral fitting of TWA 8A. The first and second columns respectively give the results of fitting Stokes I and V separately (using our third model) for the spectrum taken on 27 Mar 2015. Parameters with no error bars for the V fit were held fixed. The third column shows the results of fitting Stokes I and V simultaneously, where we present the mean over the 10 nights that could be reliably telluric-corrected, with error bars given as the standard deviations. Values for fits to individual nights are presented in Table C2.</caption><thead><tr><th></th><th>Stokes I only<br>27 Mar 2015</th><th>Stokes V only<br>27 Mar 2015</th><th>Stokes I and V<br>mean</th></tr></thead><tbody><tr><td>v sin i (km s<sup>-1</sup>)</td><td>4.77 &plusmn; 0.23</td><td>4.77</td><td>4.82 &plusmn; 0.16</td></tr><tr><td>v<sub>mic</sub> (km s<sup>-1</sup>)</td><td>1.15 &plusmn; 0.08</td><td>1.15</td><td>1.08 &plusmn; 0.05</td></tr><tr><td>[Ti/H]</td><td>-7.006 &plusmn; 0.017</td><td>-7.01</td><td>-6.976 &plusmn; 0.022</td></tr><tr><td>0 G</td><td>0.002 &plusmn; 0.084</td><td>0.859 &plusmn; 0.014</td><td>0.017 &plusmn; 0.016</td></tr><tr><td>+2 kG</td><td>0.312 &plusmn; 0.067</td><td>0.023 &plusmn; 0.010</td><td>0.161 &plusmn; 0.016</td></tr><tr><td>+5 kG</td><td>0.483 &plusmn; 0.044</td><td>0.057 &plusmn; 0.007</td><td>0.245 &plusmn; 0.019</td></tr><tr><td>+10 kG</td><td>0.090 &plusmn; 0.020</td><td>0.016 &plusmn; 0.005</td><td>0.055 &plusmn; 0.004</td></tr><tr><td>+15 kG</td><td>0.060 &plusmn; 0.013</td><td>0.041 &plusmn; 0.005</td><td>0.047 &plusmn; 0.003</td></tr><tr><td>+20 kG</td><td>0.053 &plusmn; 0.011</td><td>0.004 &plusmn; 0.004</td><td>0.035 &plusmn; 0.004</td></tr><tr><td>-2 kG</td><td>-</td><td>-</td><td>0.155 &plusmn; 0.010</td></tr><tr><td>-5 kG</td><td>-</td><td>-</td><td>0.209 &plusmn; 0.007</td></tr><tr><td>-10 kG</td><td>-</td><td>-</td><td>0.041 &plusmn; 0.005</td></tr><tr><td>-15 kG</td><td>-</td><td>-</td><td>0.010 &plusmn; 0.003</td></tr><tr><td>-20 kG</td><td>-</td><td>-</td><td>0.027 &plusmn; 0.004</td></tr><tr><td>⟨|B<sub>f</sub>|⟩ (kG)</td><td>5.9 &plusmn; 0.4</td><td>1.2 &plusmn; 0.1</td><td>5.9 &plusmn; 0.2</td></tr></tbody></table>

<footnote n="1"><sup>1</sup>IRAP, Université de Toulouse, CNRS, UPS, CNES, 14 Avenue Edouard Belin, Toulouse, F-31400, France</footnote>

<footnote n="2"><sup>2</sup>Kavli Institute for Astronomy and Astrophysics, Peking University, Yi He Yuan Lu 5, Haidian Qu, Beijing 100871, China</footnote>

<footnote n="3"><sup>3</sup>ESO, Karl-Schwarzschild-Str. 2, D-85748 Garching, Germany</footnote>

<footnote n="4"><sup>4</sup>Departamento de Física - ICFx-UFMG, Av. Antônio Carlos, 6627, 30270-901 Belo Horizonte, MG, Brazil</footnote>

<footnote n="5"><sup>5</sup>University of St Andrews, St Andrews, KY16 9SS, UK</footnote>

<footnote>* E-mail: colinalastairhill@gmail.com</footnote>
<figure><img src="image_3.png" /></figure>

**Figure 11.** The distribution of surface magnetic field strengths for TWA 8A, as determined from ZDI and direct spectral fitting of Ti I lines. Blue bars show the fraction of the total mapped magnetic field strength from ZDI, for fields of a given bin. Top panel: Comparison between the magnetic field strengths determined from fitting Stokes *I* data. Red circles show the mean filling factors for each field strength using our third model to simultaneously fit Stokes *I* and *V* spectra (see Section 5.5). Black circles show the combined filling factors for both the positive and negative fields. Thus, one can directly compare the recovered field strengths for Stokes *I* data from ZDI and direct spectral fitting by comparing the blue bars and the black circles, respectively. One can see that a significantly larger fraction of higher-strength fields are recovered by direct spectral fitting, as compared to that from ZDI (see discussion in Section 5.5). Bottom panel: Comparison between the magnetic field strengths determined from fitting Stokes *V* data. Black circles show the resulting filling factors after subtracting the contributions of the negative fields from those of the positive fields. Note that the filling factor for the 0 kG bin is calculated as 1 minus the sum of the non-zero bins, so the sum total of all fractions is unity. As Stokes *V* profiles are sensitive to the sign of the line-of-sight component of *B*, significant cancellation of fields may occur, and so we must compare our fits with ZDI to Stokes *V* profiles, to these black circles. In this case, we see that ZDI recovers a similar fraction of field strengths for the 5 and 10 kG bins, significantly less for the 15 and 20 kG bins, but significantly more for the 2 kG bin (see discussion in Section 5.5). ZDI also recovers a smaller fraction for the 0 kG bin, likely due to degeneracy between the 0 and 2 kG fields in direct spectral fitting.

## REFERENCES

Hill C. A., Folsom C. P., Donati J. F., Herczeg G. J., Hussain G. A. J., Alencar S. H. P., Gregory S. G., 2019, MNRAS, 484, 5810

This paper has been typeset from a T\*EX/L\*EX file prepared by the author.