This article has an erratum: [https://doi.org/10.1051/0004-6361/201526027e]
The solid black line shows the co-added spectra from the four epochs of 2014 observations and a single epoch of 2012 observations of the erupting M31N 2008-12a, centred on the Hα emission line, (the peak redwards of Hα is the He i (6678 Å) emission). The solid grey line indicates the best-fitting model spectrum expected by a freely expanding ejecta with a bi-polar morphology, see the text for further details and discussion.
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