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 ± 0.04 days and a semi-amplitude of 11.0 ± 0.3 km s −1 . Adopting an orbital inclination of 38 ± 5 deg and a primary mass of 0.6 ± 0.1 M yields a secondary mass of 0.19 ± 0.05 M 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.

On the post-common-envelope central star of the planetary nebula NGC 2346

“…In summary, we confirm the central star of NGC 2346 to be one of the longest period post-CE systems known (see e.g. Rebassa-Mansergas et al 2012;Sowicka et al 2017;Miszalski et al 2018). Furthermore, the secondary of the system is found to be sub-giant which, in spite of the larger uncertainties involved, must be one of, if not the most, massive post-CE secondaries known (Davis et al 2010;Zorotovic et al 2010).…”

Energy budget and core-envelope motion in common envelope evolution

“…For the MyCn 18 system, Miszalski et al 2018 obtain primary and secondary masses of 0.6 ± 0.1 M and 0.19 ± 0.05 M respectively, whereas our particle masses are M 1,c = 0.4 M and M 2 = 1 M . Observations provide only a plane-of-the-sky projection, and thus a minimum of the full 3D offset, which would require a larger offset velocity.…”

Close Binaries and the Abundance Discrepancy Problem in Planetary Nebulae

“…However, the likelihood of the two longest period binaries having the two lowest adfs by chance alone if the two properties were uncorrelated is just under 1%. A likely third such object is MyCn 18, for which [26] recently reported a binary period of 18.15 days, while [27] measured an adf of 1.8. The probability of the three lowest adfs coinciding with the three longest periods by chance is much less than 1% and strengthens the hypothesis that only when the binary period is shorter than a threshold value of around 1.15 days will the PN exhibit an elevated or extreme adf.…”