The binary fraction of planetary nebula central stars: the promise of VPHAS+

TL;DR

This study demonstrates that the VPHAS+ survey can effectively detect binary companions in planetary nebulae central stars through near-infrared excess, supporting the Binary Hypothesis for nebula morphology.

Contribution

It introduces a method using VPHAS+ photometry to identify low-mass companions in PNe central stars and applies it to a new sample, enhancing binary detection capabilities.

Findings

VPHAS+ photometry can detect IR excess indicative of low-mass companions

3 out of 7 studied CS show IR excess, including one known binary

VPHAS+ survey is promising for expanding binary star studies in PNe

Abstract

The majority of planetary nebulae (PNe) are not spherical, and current single-star models cannot adequately explain all the morphologies we observe. This has led to the Binary Hypothesis, which states that PNe are preferentially formed by binary systems. This hypothesis can be corroborated or disproved by comparing the estimated binary fraction of all PNe central stars (CS) to that of the supposed progenitor population. One way to quantify the rate of CS binarity is to detect near infra-red (IR) excess indicative of a low-mass main sequence companion. In this paper, a sample of known PNe within data release 2 of the ongoing VPHAS+ are investigated. We give details of the method used to calibrate VPHAS+ photometry, and present the expected colours of CS and main sequence stars within the survey. Objects were scrutinized to remove PN mimics from our sample and identify true CS. Within our final sample of 7 CS, 6 had previously either not been identified or confirmed. We detected an $i$ band excess indicative of a low-mass companion star in 3 CS, including one known binary, leading us to to conclude that VPHAS+ provides the precise photometry required for the IR excess method presented here, and will likely improve as the survey completes and the calibration process finalised. Given the promising results from this trial sample, the entire VPHAS+ catalogue should be used to study PNe and extend the IR excess-tested CS sample.