Investigation of the pulsations, outbursts, and evolution of the yellow hypergiants
Authors: van Genderen A. M. , Lobel A. , Timmerman R. , Deul E. R. , Vos A. , Nieuwenhuijzen H. , van Ballegoij E. J. , Sblewski M. , Henry G. W. , Blown E. , Di Scala G. 
Published: February 2025

In this predominantly photometric study, we investigate various types of photometric and physical instabilities in the yellow hypergiant (YHG) ρ Cas as well as its evolution between 1885 and 2023. The second yellow hypergiant we study is YHG HR 8752, which recently became stable. The other two yellow hypergiants are HR 5171A and HD 179821. We constructed temperature calibration relations based on Teff and simultaneously observed BV photometry, which exclusively for ρ Cas enabled us to discuss dependencies between Teff and the selective continuum opacity variations in B and V. An important goal of ours was to find correlations between various variable astrophysical parameters and the onset of recurring outburst events. The latter events also represent a stellar pulsation, but of a quite different type than the ordinary stellar pulsations. We present an in-depth photometric analysis of the unsteady long-term variations (LTV) of the median (B − V)obs of all ordinary pulsations and the outbursts of ρ Cas between 1962 and 2020. For this purpose, we introduce a new photometric parameter as well as four new temperature calibration relations. They improve our understanding of the photometric and physical properties of the outburst events and of ordinary pulsation sequences and reveal clear correlations between Teff and the variability in the selective continuum opacity. In the Appendices we also discuss HR 5171A and HD 179821 as well as a number of arresting features observed in HR 8752, which became a stable star between 1996 and 2017. The construction of four temperature calibration relations based on available Teff values and simultaneously observed BV parameters was essential and so was the introduction of a new photometric parameter for measuring selective continuum opacities. We discuss the improved insight into the dependencies between Teff, the selective continuum opacity, and the (B − V)obs variability. We locate ρ Cas in the Hertzsprung–Russell diagram (HRD) and visualize the selection of cyclic tracks during its ordinary pulsations and outburst events. Next, we present the highlights of our research results through a four-paneled diagram representing the correlations between variations in Teff/selective continuum opacity, the median (B − V)obs, and the stellar radius as well as the trends in the ordinary quasi-periods, which include the onset of outbursts. The derived minimum and maximum photospheric radii of ρ Cas during the outbursts of 1986, 2000, and 2013 agree within estimated errors with the radii during the 2000 outburst measured from distance independent radial velocity observations. This underlines the reliability of our new temperature calibration relations. Based on an analysis of early observations from 1885 to 1963, combined with more recent data from 1963 to 2023, we find that ρ Cas underwent six outbursts over 138 years. From ‘young’ to ‘old’, these outbursts occurred in 1895, 1905, 1946, 1986, 2000, and 2013. The time intervals amount to 10, 41, 40, 14 and 13 years, respectively. The Teff of outburst maxima and minima increases from the young to old events. We propose that ρ Cas is on the verge of copying the behaviour of HR 8752, namely, becoming more stable in the next decades. In addition, ρ Cas will evolve along a blue track in the HRD potentially into a second region of dynamical instability, but after a state of stability without ordinary pulsations and outbursts; however, this will only occur after 2045. All five of its normal outburst events revealed a relatively low Teff for large selective continuum opacity, contrary to the rather abnormal sixth and latest outburst event in 2013 at a larger Teff by ∼1000 K but for smaller selective continuum opacities.