[Paper Review] Planetary nebulae that might have been shaped by a triple stellar system

Ealeal Bear, Noam Soker|arXiv (Cornell University)|Jun 27, 2016

Stellar, planetary, and galactic studies3 citations

TL;DR

This study proposes that approximately 13–21% of planetary nebulae (PNe) were shaped by triple stellar systems, based on morphological asymmetries in resolved images. Using a weighted classification system (eta_t=1, eta_l=0.67, eta_m=0.33) for non-axial and non-mirror symmetric structures, the authors identify a significant fraction of PNe as likely or possibly influenced by tertiary stars in their progenitor systems.

ABSTRACT

We examine the images of hundreds of planetary nebulae (PNe) and find that about one in six PNe might have been shaped by an interacting triple stellar progenitor. For resolved images we classify PNe by one of four categories. (1) PNe that show no need for a tertiary star to account for their morphology. (2) PNe whose structure posses a pronounced departure from axial-symmetry and/or mirror-symmetry. We classify these as `having a triple stellar progenitor'. (3) PNe whose morphology possesses departure from axial-symmetry and/or mirror-symmetry, but not as pronounced as in the previous class, and are classified as `likely shaped by triple stellar system'. (4) PNe with minor departure from axial-symmetry and/or mirror symmetry that could have been as well caused by an eccentric binary system or the inter-stellar medium. These are classified as `maybe shaped by a triple stellar system'. Given a weight eta_t=1, eta_l=0.67, eta_m=0.33 to classes 2, 3 and 4, respectively, we find that about 13 - 21% of PNe have been shaped by triple stellar systems. Although in some evolutionary scenarios not all three stars survive the evolution, we encourage the search for a triple stellar systems at the center of some PNe.

Motivation & Objective

• To investigate whether the morphological asymmetries observed in planetary nebulae (PNe) can be attributed to the influence of a triple stellar progenitor system.
• To classify PNe based on their symmetry properties to distinguish those requiring a tertiary star for morphological explanation.
• To quantify the fraction of PNe shaped by triple stellar systems using a weighted classification scheme.
• To encourage observational searches for triple stellar systems at the centers of PNe with asymmetric morphologies.

Proposed method

• Analyzing resolved images of hundreds of planetary nebulae to assess deviations from axial- and mirror-symmetry.
• Classifying PNe into four morphological categories: (1) symmetric and explainable without a tertiary star; (2) strongly asymmetric (classified as 'having a triple stellar progenitor'); (3) moderately asymmetric ('likely shaped by triple system'); (4) weakly asymmetric ('maybe shaped by triple system').
• Assigning weight factors to each classification category: eta_t=1 for class 2, eta_l=0.67 for class 3, and eta_m=0.33 for class 4.
• Calculating the total fraction of PNe shaped by triple systems by summing weighted contributions across the three ambiguous classes.
• Using the weighted sum to estimate the statistical likelihood that triple stellar systems influenced the nebulae's shaping, even if not all three stars survive the evolution.

Experimental results

Research questions

• RQ1 What fraction of planetary nebulae exhibit morphological features that require a triple stellar system for explanation?
• RQ2 How can the degree of asymmetry in planetary nebulae be quantitatively classified to distinguish between binary and triple progenitor influences?
• RQ3 To what extent can morphological asymmetries in PNe be attributed to a tertiary star, even if the system's final configuration does not retain all three stars?
• RQ4 Which morphological features in PNe are most indicative of a triple stellar progenitor system?

Key findings

• Approximately 13–21% of planetary nebulae are estimated to have been shaped by triple stellar systems, based on weighted classification of morphological asymmetries.
• About one in six planetary nebulae shows morphological features that suggest the influence of a tertiary star in their progenitor system.
• The classification system assigns the highest weight (eta_t=1) to PNe with pronounced asymmetries, indicating strong evidence for a triple progenitor.
• PNe with moderate asymmetries (class 3) receive a moderate weight (eta_l=0.67), suggesting a likely, but less certain, role of triple systems.
• PNe with minor asymmetries (class 4) are assigned a low weight (eta_m=0.33), indicating a possible but less probable influence from triple systems.
• The study encourages continued observational efforts to detect triple stellar systems at the centers of PNe with asymmetric morphologies.