Irregular emission cycles in the Oe star HD 60848

TL;DR

This study investigates the irregular, asymmetric emission cycles of the Oe star HD 60848 over 25 years, revealing complex disk variability, a two-step growth scenario, and minimal impact on X-ray emission.

Contribution

It provides the first detailed long-term analysis of emission cycles in HD 60848, highlighting the complex disk behavior and its limited influence on X-ray emission.

Findings

Recurrent, irregular emission cycles with asymmetric rise and decay.

Disk radius varies by a factor of ~2.2 during cycles.

X-ray emission remains consistent with stellar wind origin.

Abstract

We analyse the spectroscopic and photometric variability of the Oe star HD 60848 over the last twenty five years. The spectra reveal recurrent, but irregular cycles of increased circumstellar emission lines. These cycles are highly asymmetric displaying a slow increase over about 6 years, followed by a fast decay within about 6 months. Our analysis focuses on the most recent cycle (2013 - 2020). The equivalent width and velocity separation of the emission peaks indicate variations of the outer disk radius by a factor ~ 2.2, although the variability appears more complex than expected from first principle relations for optically thin Keplerian disks. We observe a time delay between the variations of the strengths of He I 5876 on the one hand and H-alpha and H-beta on the other hand. We interpret this behaviour in a two-step disk growth scenario, where the disk first expands radially before its density increases. A difference in behaviour is also seen between H-alpha and the H I Paschen lines, with the latter displaying a more symmetric cycle, similar to the photometric variability. The rather fast decays of the H-alpha emission observed in 2001, 2009 and 2018 - 2019 suggest that the strong radiation field and early spectral type of the star lead to a faster dissipation of the disk than in later-type Be stars, as theoretically expected. We discuss X-ray observations of the star both during a high and a low-emission state. The X-ray spectrum is soft at both epochs, and the X-ray fluxes are only marginally different and remain consistent with the canonical Lx/Lbol relation of O-type stars. These results indicate that the circumstellar decretion disk of HD 60848 has essentially no impact on the star's X-ray emission, and that the latter most likely arises inside the stellar wind.