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

¹ Instituto de Astrofísica de Andalucía IAA–CSIC, Glorieta de la Astronomía s/n, 18008 Granada, Spain
² Instituto de Astronomía, Universidad Nacional Autónoma de México, Apdo. Postal 877, 22800 Ensenada, B.C., Mexico
³ Instituto de Ciencias Físicas, Universidad Nacional Autónoma de México, Av. Universidad s/n, 62210 Cuernavaca, Morelos, Mexico
⁴ Observatorio Astronómico Nacional, Instituto de Astronomía, Universidad Nacional Autónoma de México, Apdo. Postal 106, 22800 Ensenada, B.C., Mexico

^★ Corresponding author; rcala@iaa.es

Received: 30 July 2024
Accepted: 21 October 2024

Abstract

IRAS 22568+6141 has been classified as a low-excitation planetary nebula (PN) and presents non-thermal radio continuum emission, which could be a signature of a 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. They all 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 in the Hβ flux in 2021 by factor of ≃6 and the [O III] emission lines, which were absent in 1988. Between 2021 and 2023, the Hβ flux decreased by a factor of ≃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 was recombining and cooling 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 was 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 better accommodates 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.