[Paper Review] Post-red-giant-branch Planetary Nebulae

David Jones, T. C. Hillwig+1 more|arXiv (Cornell University)|Apr 13, 2023

Stellar, planetary, and galactic studies4 citations

TL;DR

This paper reviews observational and theoretical evidence that some planetary nebulae (PNe) originate from common envelope (CE) events during the red giant branch (RGB) phase, not just the asymptotic giant branch (AGB). It identifies eight strong candidates for post-RGB central stars in PNe, using multi-band light curves, radial velocity curves, and non-LTE spectral modelling, with key results showing that some systems have masses and luminosities consistent with post-RGB evolution despite discrepancies in derived parameters that may reveal flaws in current post-CE evolutionary models.

ABSTRACT

Common envelope events have been associated with the formation of a planetary nebulae since its proposition more than forty five years ago. However, until recently there have been doubts as to whether a common envelope while the donor is ascending the red giant branch, rather than the subsequent asymptotic red giant branch, would result in a planetary nebula. There is now strong theoretical and observational evidence to suggest that some planetary nebulae are, indeed, the products of common envelope phases which occurred while the nebular progenitor was on the red giant branch. The characterisation of these systems is challenging but has the potential to reveal much about the common envelope -- a critical evolutionary phase in the formation of a plethora of interesting astrophysical phenomena.

Motivation & Objective

• To investigate whether planetary nebulae (PNe) can form via common envelope (CE) events during the red giant branch (RGB) phase, not just the asymptotic giant branch (AGB).
• To identify and characterise post-RGB central stars in PNe, which are critical for testing CE evolution models.
• To resolve discrepancies between mass estimates from light/radial velocity curves and those inferred from Kiel diagram loci, which may indicate flaws in post-CE evolutionary tracks.
• To explore the link between RGB CE events and extreme abundance discrepancies observed in some PNe.
• To assess the viability of post-RGB central stars as probes of the poorly understood common envelope phase in binary evolution.

Proposed method

• Simultaneous modelling of multi-band light curves and radial velocity curves for binary central stars in PNe to derive orbital parameters and stellar masses.
• Non-LTE (NLTE) spectral modelling of hot central stars to determine effective temperature and surface gravity with high precision.
• Comparison of derived stellar parameters (Teff, log g, L) with theoretical post-RGB and post-AGB evolutionary tracks on the Kiel diagram.
• Use of orbital period distributions of post-CE binaries (both in PNe and outside) to test consistency with post-RGB formation pathways.
• Analysis of nebular morphology and abundance discrepancy factors (e.g., in Abell 46) as indirect indicators of RGB CE origin.
• Cross-verification of spectroscopic parameters with photometric and dynamical modelling to resolve inconsistencies in mass estimates.

Experimental results

Research questions

• RQ1 Can common envelope events occurring during the red giant branch (RGB) produce observable planetary nebulae?
• RQ2 Are there observable post-RGB central stars in planetary nebulae, and how can they be identified?
• RQ3 Why do some central stars show a discrepancy between masses derived from light/radial velocity curves and those inferred from their position on the Kiel diagram?
• RQ4 Do RGB common envelope events explain the extreme abundance discrepancies observed in some planetary nebulae?
• RQ5 How do the orbital period distributions of post-CE binaries in PNe compare to those of naked post-CE binaries, and what does this imply for post-RGB formation?

Key findings

• Eight strong candidates for post-RGB central stars in planetary nebulae have been identified, including Abell 46, ESO 330-9, HaTr 4, HaTr 7, HaWe 13, Hf 2-2, Ou 5, and PN G283.7−05.1.
• The central star of PN G283.7−05.1 has a definitively post-RGB mass of 0.34±0.05 M⊙, though its luminosity (log L ~ 3.5) is too high for a post-RGB star.
• Abell 46 and Ou 5 show consistent surface gravity and temperature with post-RGB tracks for lower masses than those derived from light and radial velocity curve modelling.
• ESO 330-9 and HaTr 7 have NLTE-derived parameters that match both post-RGB evolutionary tracks and light/radial velocity curve solutions.
• Theoretical simulations now suggest that CE events on the RGB can produce observable PNe down to a minimum central star mass of ~0.25 M⊙, revising the previous estimate of >0.4 M⊙.
• The consistency between orbital period distributions of post-CE binaries in PNe and those outside PNe supports the idea that post-RGB systems are part of the broader post-CE population.