TESS observations of non-Be fast rotators

¹ Groupe d’Astrophysique des Hautes Energies, STAR, Université de Liège, Quartier Agora (B5c, Institut d’Astrophysique et de Géophysique), Allée du 6 Août 19c, 4000 Sart Tilman, Liège, Belgium
² Royal Observatory of Belgium, Avenue Circulaire/Ringlaan 3, 1180 Brussels, Belgium
³ LESIA, Paris Observatory, PSL University, CNRS, Sorbonne University, Université Paris Cité, 5 place Jules Janssen, 92195 Meudon, France

Received: 3 June 2024
Accepted: 8 July 2024

Abstract

Context. The variability of fast-rotating Oe/Be stars has been reported in detail in recent years. However, much less is known about the behaviour of fast-rotating OB stars without known decretion disks, and hence it is difficult to identify the commonalities and differences in the photometric variability of these two populations, especially with regards to their pulsational properties and their link with the presence of circumstellar material.

Aims. Via an in-depth literature search, we identified a set of fast-rotating (vsin(i) > 200 km s⁻¹) early B-type stars not known to have disks. TESS and Kepler light curves were built for 58 stars that appear isolated (no bright neighbour within 1′ and no known companion) to avoid contamination of the light curves. Frequency spectra were calculated and then analysed to determine the noise level and the presence of significant signals above the noise.

Methods. Red noise is detected in all targets, without obvious correlations between noise and stellar parameters. Long-term changes are much less frequent than in Be stars, with only 12% of our targets having the variability below 0.5 d⁻¹ dominating their frequency spectrum. In contrast, strong frequency groups are detected in about a third of targets, as in Be stars. These groups generally occur in pairs with harmonic frequencies, as is usually seen in Be stars, but with the first group more often displaying larger amplitudes. Finally, the most frequent variability is due to isolated frequencies in the 0.5–6. d⁻¹ range (which is found in two-thirds of cases and dominates the spectra in 42% of the sample). Higher-frequency signals (up to 40 d⁻¹) are sometimes also detected but rarely (only 12% of stars) appear as the strongest ones of the frequency spectra. Overall, fast-rotating B-type stars, with or without disks, display similar photometric properties, except as regards their longer-term behaviour.