Long and short term variability of the possible nascent planetary nebula IRAS 22568+6141: A late thermal pulse?

TL;DR

This study investigates the decades-long variability of IRAS 22568+6141, a low-ionisation planetary nebula, revealing its recent fading and shock-driven excitation, likely caused by a late thermal pulse event.

Contribution

It provides new spectroscopic and photometric data showing the nebula's variability and suggests a late thermal pulse as the underlying cause, which is a novel interpretation for this object.

Findings

Brightness increased around 1990 and has been fading since.

Emission lines show shock excitation and significant variability.

The object likely experienced a late thermal pulse causing its changes.

Abstract

IRAS 22568+6141 has been classified as a low-ionisation planetary nebula (PN) and presents non-thermal radio continuum emission, which could be a signature of nascent PNe. We present intermediate-resolution long-slit spectra obtained in 2021 and 2023, high-resolution long-slit spectra taken in 2023, and a light curve at the $r$-filter between 1953 and 2019, that reveal changes in IRAS 22568+6141 with timescales of decades and a few years. The object underwent an energetic event around 1990 that suddenly increased its brightness which has been fading since then. A comparison with a published spectrum from 1988 shows an increase of the H$\beta$ flux in 2021 by factor of $\simeq$6 and the [O III] emission lines that were absent in 1988. Between 2021 and 2023 the H$\beta$ flux decreased by a factor of $\simeq$1.7, and the [O III] emission lines almost vanished. These results and the variability observed in other emission lines indicate that IRAS 22568+6141 is recombining and cooling down between 2021 and 2023, and probably since 2005, as suggested by archival radio continuum and mid-IR observations. The intermediate- and high-resolution spectra show that the excitation of the emission lines is dominated by shocks in 2021 and 2023, and, probably, also in 1988, which may be related to the non-thermal radio continuum emission from the object. Although the variability might be due to changes in the physical conditions in the shocks or in a nova-like eruption, it accommodates better to that expected from a late thermal pulse, which is further suggested by a comparison with other similar objects. New observations and monitoring in the coming years are crucial to corroborate the origin of the variability.