Novel constraints on companions to the Helix nebula central star

Abstract

ABSTRACT The Helix is a visually striking and the nearest planetary nebula, yet any companions responsible for its asymmetric morphology have yet to be identified. In 2020, low-amplitude photometric variations with a periodicity of 2.8 d were reported based on Cycle 1 TESS observations. In this work, with the inclusion of two additional sectors, these periodic light curves are compared with lcurve simulations of irradiated companions in such an orbit. Based on the light-curve modelling, there are two representative solutions: (i) a Jupiter-sized body with 0.102 R$_\odot$ and an arbitrarily small orbital inclination $i=1^{\circ }$, and (ii) a 0.021 R$_\odot$ exoplanet with $i\approx 25^{\circ }$, essentially aligned with the Helix nebular inclination. Irradiated substellar companion models with equilibrium temperature 4970 K are constructed and compared with existing optical spectra and infrared photometry, where Jupiter-sized bodies can be ruled out, but companions modestly larger than Neptune are still allowed. Additionally, any spatially unresolved companions are constrained based on the multiwavelength, photometric spectral energy distribution of the central star. No ultracool dwarf companion earlier than around L5 is permitted within roughly 1200 au, leaving only faint white dwarfs and cold brown dwarfs as possible surviving architects of the nebular asymmetries. While a planetary survivor is a tantalizing possibility, it cannot be ruled out that the light-curve modulation is stellar in nature, where any substellar companion requires confirmation and may be possible with JWST observations.