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Register a new userSALT HRS discovery of the binary nucleus of the Etched Hourglass Nebula MyCn 18
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Authors
Brent Miszalski
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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 $18.15pm0.04$ days and a semi-amplitude of $11.0pm0.3$ km s$^{-1}$. Adopting an orbital inclination of $38pm5$ deg and a primary mass of $0.6pm0.1$ $M_odot$ yields a secondary mass of $0.19pm0.05$ $M_odot$ 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.
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EGB6 is an extended, faint old planetary nebula (PN) with an enigmatic nucleus. The central star (PG0950+139) is a hot DAOZ-type white dwarf (WD). An unresolved, compact emission knot was discovered to be located 0.166 away from the WD and it was shown to be centered around a dust-enshrouded low-luminosity star. It was argued that the dust disk and evaporated gas (photoionized by the hot WD) around the companion are remnants of a disk formed by wind material captured from the WD progenitor when it was an asymptotic giant branch (AGB) star. In this paper, we assess the hot WD to determine its atmospheric and stellar parameters. We performed a model-atmosphere analysis of ultraviolet (UV) and optical spectra. We found Teff = 105,000 +/- 5000 K, log g = 7.4 +/- 0.4, and a solar helium abundance (He = 0.25 +/- 0.1, mass fraction). We measured the abundances of ten more species (C, N, O, F, Si, P, S, Ar, Fe, Ni) and found essentially solar abundance values, indicating that radiation-driven wind mass-loss, with a theoretical rate of log(dot-M/M_sun/yr) = -11.0 (+1.1)(-0.8) prevents the gravitational separation of elements in the photosphere. The WD has a mass of M/M_sun = 0.58 (+0.12)(-0.04) and its post-AGB age (log(t_evol/yr) = 3.60 (+1.26)(-0.09)) is compatible with the PN kinematical age of log(t_PN}/yr) = 4.2. In addition, we examined the UV spectrum of the hot nucleus of a similar object with a compact emission region, TOL26 (PN G298.0+34.8), and found that it is a slightly cooler DAOZ WD (Teff about 85,000 K), but this WD shows signatures of gravitational settling of heavy elements.
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We present a detailed investigation of SBS1150+599A, a close binary star hosted by the planetary nebula PN G135.9+55.9 (TS01, Stasinska et al, 2009). The nebula, located in the Galactic halo, is the most oxygen-poor one known to date and is the only one known to harbor a double degenerate core. We present XMM-Newton observations of this object, which allowed the detection of the previously invisible component of the binary core, whose existence was inferred so far only from radial velocity and photometric variations. The parameters of the binary system were deduced from a wealth of information via three independent routes using the spectral energy distribution (from the infrared to X-rays), the light and radial velocity curves, and a detailed model atmosphere fitting of the stellar absorption features of the optical/UV component. We find that the cool component must have a mass of 0.54+/-0.2 Msun, an average effective temperature, Teff, of 58000+/-3000 K, a mean radius of 0.43+/-0.3 Rsun, a gravity log g=5.0+/-0.3, and that it nearly fills its Roche lobe. Its surface elemental abundances are found to be: 12 + log He/H = 10.95+/-0.04 dex, 12 + log C/H = 7.20+/-0.3 dex, 12 + log N/H < 6.92 and 12 + log O/H < 6.80, in overall agreement with the chemical composition of the planetary nebula. The hot component has Teff = 160-180 kK, a luminosity of about ~10e4 Lsun and a radius slightly larger than that of a white dwarf. It is probably bloated and heated as a result of intense accretion and nuclear burning on its surface in the past. The total mass of the binary system is very close to Chandrasekhar limit. This makes TS01 one of the best type Ia supernova progenitor candidates. We propose two possible scenarios for the evolution of the system up to its present stage.
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