Short-term Variability of Evolved Massive Stars with TESS. II. A New Class of Cool, Pulsating Supergiants

Dorn-Wallenstein, Trevor Z.; Levesque, Emily M.; Neugent, Kathryn F.; Davenport, James R. A.; Morris, Brett M.; Gootkin, Keyan (2020). Short-term Variability of Evolved Massive Stars with TESS. II. A New Class of Cool, Pulsating Supergiants. Astrophysical journal, 902(1), p. 24. Institute of Physics Publishing IOP 10.3847/1538-4357/abb318

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Massive stars briefly pass through the yellow supergiant (YSG) phase as they evolve redward across the H-R
diagram and expand into red supergiants (RSGs). Higher-mass stars pass through the YSG phase again as they
evolve blueward after experiencing significant RSG mass loss. These post-RSG objects offer us a tantalizing
glimpse into which stars end their lives as RSGs and why. One telltale sign of a post-RSG object may be an
instability to pulsations, depending on the star’s interior structure. Here we report the discovery of five YSGs with
pulsation periods faster than 1 day, found in a sample of 76 cool supergiants observed by the Transiting Exoplanet
Survey Satellite at a two-minute cadence. These pulsating YSGs are concentrated in an H-R diagram region not
previously associated with pulsations; we conclude that this is a genuine new class of pulsating star, fast yellow
pulsating supergiants (FYPSs). For each FYPS, we extract frequencies via iterative prewhitening and conduct a
time-frequency analysis. One FYPS has an extracted frequency that is split into a triplet, and the amplitude of that
peak is modulated on the same timescale as the frequency spacing of the triplet; neither rotation nor binary effects
are likely culprits. We discuss the evolutionary status of FYPS and conclude that they are candidate post-RSGs. All
stars in our sample also show the same stochastic low-frequency variability found in hot OB stars and attributed to
internal gravity waves. Finally, we find four α Cygni variables in our sample, of which three are newly discovered.

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