Kavli Affiliate: George Ricker
| First 5 Authors: Rahul Jayaraman, Gerald Handler, Saul Rappaport, Jim Fuller, Donald W. Kurtz
| Summary:
Tidally tilted pulsators (TTPs) are an intriguing new class of oscillating
stars in binary systems; in such stars, the pulsation axis coincides with the
line of apsides, or semi-major axis, of the binary. All three TTPs discovered
so far have been $delta$~Scuti stars. In this Letter, we report the first
conclusive discovery of tidally tilted pulsations in a subdwarf B (sdB) star.
HD 265435 is an sdB–white dwarf binary with a 1.65-hr period that has been
identified and characterized as the nearest potential Type Ia supernova
progenitor. Using TESS 20-s cadence data from Sectors 44 and 45, we show that
the pulsation axis of the sdB star has been tidally tilted into the orbital
plane and aligned with the tidal axis of the binary. We identify 31 independent
pulsation frequencies, 27 of which have between 1 and 7 sidebands separated by
the orbital frequency ($nu_{rm orb}$), or multiples thereof. Using the
observed amplitude and phase variability due to tidal tilting, we assign $ell$
and $m$ values to most of the observed oscillation modes and use these mode
identifications to generate preliminary asteroseismic constraints. Our work
significantly expands our understanding of TTPs, as we now know that (i) they
can be found in stars other than $delta$~Scuti pulsators, especially
highly-evolved stars that have lost their H-rich envelopes, and (ii) tidally
tilted pulsations can be used to probe the interiors of stars in very tight
binaries.
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