Sarah Martell

Although red clump stars function as reliable standard candles, their surface characteristics (i.e. Teff, log g, and [Fe/H]) overlap with those of red giant branch stars, which are not standard candles. Recent results have revealed that spectral features containing carbon (e.g. CN molecular bands) carry information correlating with the ‘gold-standard’ asteroseismic classifiers that distinguish red clump from red giant branch stars. However, the underlying astrophysical processes driving the correlation between these spectroscopic and asteroseismic quantities in red giants remain inadequately explored. This study aims to enhance our understanding of this ‘spectro-seismic’ effect by refining the list of key spectral features predicting red giant evolutionary state. In addition, we conduct further investigation into those key spectral features to probe the astrophysical processes driving this connection. We employ the data-driven The Cannon algorithm to analyse high-resolution (R ∼ 80 000) Veloce Rosso spectra from the Anglo-Australian Telescope for 301 red giant stars (where asteroseismic classifications from the TESS mission are known for 123 of the stars). The results highlight molecular spectroscopic features, particularly those containing carbon (e.g. CN), as the primary indicators of the evolutionary states of red giant stars. Furthermore, by investigating CN isotopic pairs (that is, 12C14N and 13C14N), we find suggestions of statistically significant differences in the reduced equivalent widths of such lines, suggesting that physical processes that change the surface abundances and isotopic ratios in red giant stars, such as deep mixing, are the driving forces of the ‘spectro-seismic’ connection of red giants.

We present time-series linear-polarization observations of the bright O4 supergiant ζ Puppis. The star is found to show polarization variation on time-scales of around an hour and longer. Many of the observations were obtained contemporaneously with Transiting Exoplanet Survey Satellite (TESS) photometry. We find that the polarization varies on similar time-scales to those seen in the TESS light curve. The previously reported 1.78-d photometric periodicity is seen in both the TESS and polarization data. The amplitude ratio of photometry to polarization is ∼9 for the periodic component and the polarization variation is oriented along position angle ∼70°–160°. Higher frequency stochastic variability is also seen in both data sets with an amplitude ratio of ∼19 and no preferred direction. We model the polarization expected for a rotating star with bright photospheric spots and find that models that fit the photometric variation produce too little polarization variation to explain the observations. We suggest that the variable polarization is more likely the result of scattering from the wind, with corotating interaction regions producing the periodic variation and a clumpy outflow producing the stochastic component. The Hα emission line strength was seen to increase by 10 per cent in 2021 with subsequent observations showing a return to the pre-2018 level.