Short Term Variability of Evolved Massive Stars with TESS II: A New Class of Cool, Pulsating Supergiants. (arXiv:2008.11723v1 [astro-ph.SR])
<a href="http://arxiv.org/find/astro-ph/1/au:+Dorn_Wallenstein_T/0/1/0/all/0/1">Trevor Z. Dorn-Wallenstein</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Levesque_E/0/1/0/all/0/1">Emily M. Levesque</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Neugent_K/0/1/0/all/0/1">Kathryn F. Neugent</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Davenport_J/0/1/0/all/0/1">James R. A. Davenport</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Morris_B/0/1/0/all/0/1">Brett M. Morris</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Gootkin_K/0/1/0/all/0/1">Keyan Gootkin</a>
Massive stars briefly pass through the yellow supergiant (YSG) phase as they
evolve redward across the HR 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 tess at two-minute cadence. These pulsating YSGs
are concentrated in a HR diagram region not previously associated with
pulsations; we conclude that this is a genuine new class of pulsating star,
Fast Yellow Pulsating Supergiants (FYPS). 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
(SLFV) found in hot OB stars and attributed to internal gravity waves. Finally,
we find four $alpha$ Cygni variables in our sample, of which three are newly
discovered.
Massive stars briefly pass through the yellow supergiant (YSG) phase as they
evolve redward across the HR 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 tess at two-minute cadence. These pulsating YSGs
are concentrated in a HR diagram region not previously associated with
pulsations; we conclude that this is a genuine new class of pulsating star,
Fast Yellow Pulsating Supergiants (FYPS). 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
(SLFV) found in hot OB stars and attributed to internal gravity waves. Finally,
we find four $alpha$ Cygni variables in our sample, of which three are newly
discovered.
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