Filling the gap between synchronized and nonsynchronized sdBs in shortperiod sdBV+dM binaries with TESS: TIC 137608661, a new system with a welldefined rotational splitting
Abstract
TIC 137608661/TYC 454426581/FBS 0938+788 is a new sdBV+dM reflectioneffect binary discovered by the TESS space mission with an orbital period of 7.21 h. In addition to the orbital frequency and its harmonics, the Fourier transform of TIC 137608661 shows many gmode pulsation frequencies from the subdwarf B (sdB) star. The amplitude spectrum is particularly simple to interpret as we immediately see several rotational triplets of equally spaced frequencies. The central frequencies of these triplets are equally spaced in period with a mean period spacing of 270.12 s, corresponding to consecutive l = 1 modes. From the mean frequency spacing of 1.25 μHz we derive a rotation period of 4.6 d in the deep layers of the sdB star, significantly longer than the orbital period. Among the handful of sdB+dM binaries for which the sdB rotation was measured through asteroseismology, TIC 137608661 is the nonsynchronized system with both the shortest orbital period and the shortest core rotation period. Only NY Vir has a shorter orbital period but it is synchronized. From a spectroscopic followup of TIC 137608661 we measure the radial velocities of the sdB star, determine its atmospheric parameters, and estimate the rotation rate at the surface of the star. This measurement allows us to exclude synchronized rotation also in the outer layers and suggests a differential rotation, with the surface rotating faster than the core, as found in few other similar systems. Furthermore, an analysis of the spectral energy distribution of TIC 137608661, together with a comparison between sdB pulsation properties and asteroseismic models, gives us further elements to constrain the system.
 Publication:

Monthly Notices of the Royal Astronomical Society
 Pub Date:
 April 2022
 DOI:
 10.1093/mnras/stac160
 arXiv:
 arXiv:2201.06559
 Bibcode:
 2022MNRAS.511.2201S
 Keywords:

 asteroseismology;
 stars: horizontal branch;
 stars: individual: TIC 137608661;
 stars: oscillations (including pulsations);
 Astrophysics  Solar and Stellar Astrophysics
 EPrint:
 Accepted for publication in MNRAS Main Journal