Appraising the Necklace: A post-common-envelope carbon dwarf inside an apparently carbon-poor planetary nebula

TL;DR

This study investigates the Necklace planetary nebula, a post-CE system hosting a dwarf carbon (dC) star companion, by combining HST UV spectroscopy with multi-band light and radial-velocity curve modelling to constrain the central binary and nebular chemistry. Despite the dC nature of the companion, the inner nebula appears not carbon-rich, challenging simple enrichment scenarios; the authors explore C/O via direct ionic sums and sophisticated ICFs drawn from 3MdB Cloudy models and an ANN-based regressor, finding C/O consistently below unity. The central-star analysis suggests a 1.5–2.0 M$_\odot$ progenitor that became carbon-rich late in its evolution, with a highly inflated dC companion likely accreting substantial carbon-rich material prior to the CE event; a reconciliation with the carbon-poor inner nebula remains uncertain, potentially involving carbon-rich dust or chemical inhomogeneity. Overall, the Necklace provides a unique probe of CE evolution, dC star formation, and nebular chemistry in post-CE planetary nebulae, motivating deeper UV/IR studies and spatially resolved abundance mapping.

Abstract

Context: The Necklace nebula is a bipolar, post-common-envelope planetary nebula, the central star of which has been shown to have a dwarf carbon star companion. Aims: We aim to understand the origins of the Necklace and its dwarf carbon central star. Methods: We study the carbon abundance of the nebula through far ultraviolet spectroscopy obtained with the Hubble Space Telescope. Furthermore, through simultaneous modelling of multiband light and velocity curves, we attempt to constrain the parameters of the central star system. Results: Puzzlingly, we find that the region of the inner nebula observed with the Hubble Space Telescope is seemingly not carbon-rich, at odds with the dwarf carbon star nature of the companion of the central star. The initial mass of the nebular progenitor was likely very close to the limit to become carbon-rich, perhaps experiencing a very late thermal pulse. The dwarf carbon star companion is found to be significantly inflated with respect to that expected for an isolated main sequence star of the same mass. Conclusions: The properties of the central binary are consistent with the progenitor having become carbon-rich and its companion having accreted a significant amount of that carbon-enriched material. However, it is unclear how this evolutionary hypothesis can be reconciled with the inner nebula potentially being carbon poor.

Figures (6)

• Figure 1: HST images of the Necklace. The field of view of each panel is 25$\times$25 arcsec$^2$. North is up and east is left. The colour-composite image (upper left) comprises Red: [N ii], Green: H$\alpha$ and Blue: [O iii]. In the [O iii] image, the position and size of the circular COS aperture is shown in red, while the "inner nebula" region from c11 is shown in blue.
• Figure 2: Phase-folded light curves along with the associated phoebe2 fits.
• Figure 3: Radial velocity curve of the secondary star of the Necklace for different ions listed in Table \ref{['T-wave']} The emission lines used are listed in Table \ref{['T-wave']}. The underlaid dashed lines are the phoebe2 model RVs ($K_2\approx$77.5 km s$^{-1}$). The grey points on the [C iv] plot are tentatively associated with the hot primary (see text), while the lighter grey curve on that same subplot are the phoebe2 model RVs for that component.
• Figure 4: COS-HST spectrum of the Necklace. The most remarkable features are labeled.
• Figure 5: WISE imagery of the Necklace highlighting the clear excess in both bands 3 and 4, possibly attributable to carbonaceous dust, although the estimated dust mass based on IRAS photometry is on the order of $1\times10^{-5}$ M$_\odot$ only. North is up, East is left, and all images measure 180"$\times$180" and the location of the Necklace is marked with a red circle.