The Abrupt Resumptions of Pulsations in α Cygni (Deneb)

Abstract

Paddock’s 1927–1935 radial velocities of α Cygni (Deneb) show semi-regular pulsations with a dominant period of about 12 days. Lucy found many periods of lesser amplitude. In Paddock’s data and subsequent 1956 data from Abt, after the large-amplitude pulsations seemed to damp down, abruptly new sets of pulsations started within a fraction of a day. Five of those resumptions occurred with intervals averaging 72.4 ± 0.3 days. These resumptions seem to start at any phase during the pulsations. We are unaware of this behavior in any other star. We also find evidence of this behavior in the 1977–1982 radial velocity data of Parthasarathy & Lambert and the 1997–2001 photometry/radial velocity data of Richardson et al., as well as in photometric data from the NASA Transiting Exoplanet Survey Satellite (TESS) spacecraft and ground-based visual observations in the AAVSO International Database. The ground-based data have too infrequent sampling intervals to confirm the ∼70-day resumption intervals or to pinpoint the day and phase of abrupt amplitude increase. The TESS data, with 2-minute cadence, shows one instance of pulsation resumption, but does not have a long enough time series to confirm a 70-day interval. Without any evidence for duplicity, we cannot explain this behavior as being due to a companion in a highly eccentric orbit. α Cygni is one of the Luminous Blue Variables (supergiants) and these resumptions may be related to the microvariations produced in convective layers below their atmospheres, pulsation-driven shocks and rarefactions, or pulsation-convection interactions. We encourage further observations by ground-based observers and using the TESS spacecraft to confirm the ∼70-day resumption interval and lack of correlation with pulsation phase in both radial velocity and light curve data.