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Register a new userTwos company, threes a crowd: SALT reveals the likely triple nature of the nucleus of the extreme abundance discrepancy factor planetary nebula Sp 3
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The substantial number of binary central stars of planetary nebulae (CSPNe) now known ($sim$50) has revealed a strong connection between binarity and some morphological features including jets and low-ionisation structures. However, some features and asymmetries might be too complex or subtle to ascribe to binary interactions alone. A tertiary component, i.e. a triple nucleus, could be the missing ingredient required to produce these features. The only proven triple, NGC 246, is insufficient to investigate the shaping role of triple nuclei, but one straight-forward way to identify more triples is to search for binaries in nuclei with known visual companions. Here we report on the SALT HRS discovery of a 4.81 d orbital period in the CSPN of Sp 3 which has a visual companion 0.31 away. The spectroscopic distance of the visual companion agrees with distance estimates to the nebula, the GAIA DR2 parallax of the central star, and the gravity distance of the central star. This supports a physical association between the visual companion and the 4.81 d binary, making the nucleus of Sp 3 a likely triple. We determine $T_mathrm{eff}=68^{+12}_{-6}$ kK, $log g=4.6pm0.2$ cm s$^{-2}$ and $v_mathrm{rot}=80pm20$ km s$^{-1}$ for the primary from NLTE model atmosphere analysis. The peculiar nebula presents an apparent bipolar morphology, jets and an unexpected `extreme oxygen abundance discrepancy factor (adf) of 24.6$^{+4.1}_{-3.4}$. The adf is inconsistent with the purported trend for longer orbital period post-CE PNe to exhibit normal adfs, further highlighting selection effects in post-CE PNe. The Type-I nebular abundances of Sp 3, whose origin is often tied to more massive progenitors, are incongruous with the likely Galactic Thick Disk membership of Sp 3, possibly suggesting that rotation and binarity may play an important role in the AGB nucleosynthesis of PNe. (abridged)
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Classical barium stars are binary systems which consist of a late-type giant enriched in carbon and slow neutron capture (s-process) elements and an evolved white dwarf (WD) that is invisible at optical wavelengths. The youngest observed barium stars are surrounded by planetary nebulae (PNe), ejected soon after the wind accretion of polluted material when the WD was in its preceeding asymptotic giant branch (AGB) phase. Such systems are rare but powerful laboratories for studying AGB nucleosynthesis as we can measure the chemical abundances of both the polluted star and the nebula ejected by the polluter. Here we present evidence for a barium star in the PN Hen 2-39. The polluted giant is very similar to that found in WeBo 1. It is a cool (Teff=4250 +/- 150 K) giant enhanced in carbon ([C/H]=0.42 +/- 0.02 dex) and barium ([Ba/Fe]=1.50 +/- 0.25 dex). A spectral type of C-R3 C_24 nominally places Hen 2-39 amongst the peculiar early R-type carbon stars, however the barium enhancement and likely binary status mean that it is more likely to be a barium star with similar properties, rather than a true member of this class. An AGB star model of initial mass 1.8 Msun and a relatively large carbon pocket size can reproduce the observed abundances well, provided mass is transferred in a highly conservative way from the AGB star to the polluted star (e.g. wind Roche-lobe overflow). The nebula exhibits an apparent ring morphology in keeping with the other PNe around barium stars (WeBo 1 and A 70) and shows a high degree of ionization implying the presence of an invisible hot pre-WD companion that will require confirmation with UV observations. In contrast to A 70, the nebular chemical abundance pattern is consistent with non-Type I PNe, in keeping with the trend found from nebular s-process studies that non-Type I PNe are more likely to be s-process enhanced. (abridged)
It has recently been noted that there seems to be a strong correlation between planetary nebulae with close binary central stars, and highly enhanced recombination line abundances. We present new deep spectra of seven objects known to have close binary central stars, and find that the heavy element abundances derived from recombination lines exceed those from collisionally excited lines by factors of 5-95, placing several of these nebulae among the most extreme known abundance discrepancies. This study nearly doubles the number of nebulae known to have a binary central star and an extreme abundance discrepancy. A statistical analysis of all nebulae with measured recombination line abundances reveals no link between central star surface chemistry and nebular abundance discrepancy, but a clear link between binarity and the abundance discrepancy, as well as an anticorrelation between abundance discrepancies and nebular electron densities: all nebulae with a binary central star with a period of less than 1.15 days have an abundance discrepancy factor exceeding 10, and an electron density less than $sim$1000 cm$^{-3}$; those with longer period binaries have abundance discrepancy factors less than 10 and much higher electron densities. We find that [O~{sc ii}] density diagnostic lines can be strongly enhanced by recombination excitation, while [S~{sc ii}] lines are not. These findings give weight to the idea that extreme abundance discrepancies are caused by a nova-like eruption from the central star system, occurring soon after the common-envelope phase, which ejects material depleted in hydrogen, and enhanced in CNONe but not in third-row elements.
EGB 6 is a faint, large, ancient planetary nebula (PN). Its central star, a hot DAOZ white dwarf (WD), is a prototype of a rare class of PN nuclei associated with dense, compact emission-line knots. The central star also shows excess fluxes in both the near- (NIR) and mid-infrared (MIR). In a 2013 paper, we used Hubble Space Telescope (HST) images to show that the compact nebula is a point-like source, located 0.16 (~118 AU) from the WD. We attributed the NIR excess to an M dwarf companion star, which appeared to coincide with the dense emission knot. We now present new ground-based NIR spectroscopy, showing that the companion is actually a much cooler source with a continuous spectrum, apparently a dust-enshrouded low-luminosity star. New HST images confirm common proper motion of the emission knot and red source with the WD. The I-band, NIR, and MIR fluxes are variable, possibly on timescales as short as days. We can fit the spectral-energy distribution with four blackbodies (the WD, a ~1850 K NIR component, and MIR dust at 385 and 175 K). Alternatively, we show that the NIR/MIR SED is very similar to that of Class 0/I young stellar objects. We suggest a scenario in which the EGB 6 nucleus is descended from a wide binary similar to the Mira system, in which a portion of the wind from an AGB star was captured into an accretion disk around a companion star; a remnant of this disk has survived to the present time, and is surrounded by gas photoionized by UV radiation from the WD.
Recent studies have indicated that triple star systems may play a role in the formation of an appreciable number of planetary nebulae, however only one triple central star is known to date (and that system is likely too wide to have had much influence on the evolution of its component stars). Here, we consider the possibility that Sh 2-71 was formed by a triple system which has since broken apart. We present the discovery of two regions of emission, seemingly aligned with the proposed tertiary orbit (i.e. in line with the axis formed by the two candidate central star systems previously considered in the literature). We also perform a few simple tests of the plausibility of the triple hypothesis based on the observed properties (coordinates, radial velocities, distances and proper motions) of the stars observed close to the projected centre of the nebula, adding further support through numerical integrations of binary orbits responding to mass loss. Although a number of open questions remain, we conclude that Sh 2-71 is currently one of the best candidates for planetary nebula formation influenced by triple-star interactions.
Recent work (Corradi et al. 2015, Jones et al. 2016) has shown that the phenomenon of extreme abundance discrepancies, where recombination line abundances exceed collisionally excited line abundances by factors of 10 or more, seem to be strongly associated with planetary nebulae with close binary central stars. To further investigate, we have obtained spectra of a sample of nebulae with known close binary central stars, using FORS2 on the VLT, and we have discovered several new extreme abundance discrepancy objects. We did not find any non-extreme discrepancies, suggesting that a very high fraction of nebulae with close binary central stars also have an extreme abundance discrepancy.
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