H fine-structure in SN 1987A within the first 111 days*

Abstract

We present flux-calibrated spectra of the Hα region for SN 1987A at 3 Å resolution, covering the period 1987 February 25 to June 14, the first 111 days of SN 1987A. We find that Hα in SN 1987A shows an undisturbed P Cyg-type profile until March 15, superimposed by variable fine-structure after that date, developing within a few days into a blueshifted peak-like flux excess at –5000 km s−1 and a redshifted flux deficiency at +4400 km s−1. Both features move towards smaller radial velocity with time. They contain roughly 7 per cent of the totally absorbed and emitted Hα flux, respectively. Contributions to the observed Hα profile by line blends can be safely excluded, so that the observed line profile structure is very likely to be caused by scattered Hα photons. Similar fine-structure is also observed at Na I-D and at Paschen-α. We discuss our observations in terms of possible relations to speckle observations (‘mystery spot’) and of models involving a jet or interactions with surrounding pre-existing material. All these models are excluded by the high degree of symmetry of the fine-structure and by the fact that the redshifted component appears as flux deficit. Instead, we propose density inhomogeneities in the inner regions of the envelope as origin, being invisible before March 15 and visible afterwards due to the retreat of the photosphere relative to the faster moving layers of the ejected envelope.