SALT HRS discovery of the binary nucleus of the Etched Hourglass Nebula MyCn 18

Abstract

The shaping of various morphological features of planetary nebulae (PNe) is increasingly linked to the role of binary central stars. Identifying a binary within a PN offers a powerful tool with which to directly investigate the formation mechanisms behind these features. The Etched Hourglass Nebula, MyCn 18, is the archetype for several binary-linked morphological features, yet it has no identified binary nucleus. It has the fastest jets seen in a PN of 630 km s${}^{-1}$, a central star position offset from the nebula centre, and a bipolar nebula with a very narrow waist. Here we report on the Southern African Large Telescope (SALT) High Resolution Spectrograph (HRS) detection of radial velocity variability in the nucleus of MyCn 18 with an orbital period of $18.15\pm 0.04$ days and a semi-amplitude of $11.0\pm 0.3$ km s${}^{-1}$. Adopting an orbital inclination of $38\pm 5$ deg and a primary mass of $0.6\pm 0.1$ $M_{\odot }$ yields a secondary mass of $0.19\pm 0.05$ $M_{\odot }$ corresponding to an M5V companion. The detached nature of the binary rules out a classical nova (CN) as the origin of the jets or the offset central star as hypothesised in the literature. Furthermore, scenarios that produce the offset central star during the AGB and that form narrow waist bipolar nebulae result in orbital separations 80--800 times larger than observed in MyCn 18. The inner hourglass and jets may have formed from part of the common envelope ejecta that remained bound to the binary system in a circumbinary disk, whereas the offset central star position may best be explained by proper motion. Detailed simulations of MyCn 18 are encouraged that are compatible with the binary nucleus to further investigate its complex formation history.

arXiv comment13 pages, 5 figures, 2 tables. Accepted for publication in PASA

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