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Appendix A: Imaginary part of the visibilities

Figure A.1 shows the imaginary part of the visibilities at 689 GHz (~450 μm, Band 9) and at 336 Gz (~870 μm, Band7) for SR 21 and HD 135344B. Non-zero imaginary visibilities indicate the presence of an azimuthal asymmetry (e.g. Isella et al. 2013). For HD 135344B, there are significant variations from zero at both frequencies. For SR 21, the non-zero values are marginal in Band 7, but are significant in Band 9 data.

	Fig. A.1 Imaginary part of the visibilities at 689 GHz (~450 μm, Band 9) and at 336 GHz (~870 μm, Band7) for SR 21 (top panel) and HD 135344B (bottom panel).
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Appendix B: Residual maps

Figure B.1 illustrates the residual maps for both sources after subtracting the best-fit models (Table 2) to the Band 7 data (336 GHz). For SR 21 the best-fit is described by a ring model, whereas for HD 135344B it is a vortex model. The residuals for HD 135344B show a spiral structure, as also suggested by Pérez et al. (2014) in Band 9 (689 GHz).

	Fig. B.1 Residual maps after subtracting the best-fit models from Table 2, contours start at ± 3σ and space by 3σ steps.
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Appendix C: Uncertainties in the azimuthal variations of the spectral index

	Fig. C.1 Calculated spectral index (αmm) assuming different alignments of the images for SR 21 (top panel) and HD 135344B (bottom panel).
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The calculation of the spectral index (α_mm) from the observations depends considerably on how the two images are

overlapped. To demonstrate that the apparent azimuthal variations remain uncertain, we shifted the alignment of the Band 7 and Band 9 images of SR21 and HD135344B by the positional uncertainty. Figure C.1 shows α_mm by shifting 20 mas in both vertical and horizontal direction. The only reliable variation of α_mm is in the radial direction for SR 21, which remains significant independent of the alignment. In HD 135344B, there is a hint of high values of α_mm opposite to the azimuthal asymmetry, which also remains independent of the alignment.

© ESO, 2015