Letter to the Editor

Discovery of a substellar companion in the TESS light curve of the δ Scuti/γ Doradus hybrid pulsator HD 31221

¹ Konkoly Observatory, Research Centre for Astronomy and Earth Sciences, ELKH, Konkoly-Thege Miklós út 15-17, 1121 Budapest, Hungary
² MTA-ELTE Exoplanet Research Group, Szent Imre h. u. 112, 9700 Szombathely, Hungary
³ ELTE Eötvös Loránd University, Doctoral School of Physics, Pázmány Péter sétány 1/A, 1117 Budapest, Hungary
e-mail: xilard1@gothard.hu
⁴ CSFK, MTA Centre of Excellence, Konkoly Thege Miklós út 15-17, 1121 Budapest, Hungary
⁵ ELTE Eötvös Loránd University, Gothard Astrophysical Observatory, Szent Imre h. u. 112, 9700 Szombathely, Hungary
⁶ MTA-ELTE Lendület “Momentum” Milky Way Research Group, Hungary
⁷ ELKH-SZTE Stellar Astrophysics Research Group, Szegedi út, Kt. 766, 6500 Baja, Hungary
⁸ Deutsches Zentrum für Luft- und Raumfahrt, Institute of Planetary Research, Rutherfordtstrasse 2, 12489 Berlin, Germany
⁹ Department of Physics, Engineering and Astronomy, Stephen F. Austin State University, 1936 North St., Nacogdoches, TX 75962, USA
¹⁰ MTA CSFK Lendület Near-Field Cosmology Research Group, Konkoly-Thege Miklós út 15-17, 1121 Budapest, Hungary
¹¹ ELTE Eötvös Loránd University, Institute of Physics, Pázmány Péter sétány 1/A, 1117 Budapest, Hungary
¹² Dept. Astrofísica, Universidad de La Laguna (ULL), 38206 La Laguna, Tenerife, Spain
¹³ Instituto de Astrofísica de Canarias (IAC), 38200 La Laguna, Tenerife, Spain

Received: 9 December 2022
Accepted: 4 May 2023

Abstract

Context. Close-in, sub-stellar companions to δ Scuti type stars present a highly suitable testbed for examining how planetary-mass objects can influence stellar pulsations.

Aims. We aim to constrain the mass of HD 31221 b, probe its atmosphere, and demonstrate how it affects the pulsational pattern of its host, HD 31221.

Methods. We made use of the available data from the short-cadence Transiting Exoplanet Survey Satellite (TESS). We modeled the nine observed transits and the out-of-phase variations, including Doppler beaming, ellipsoidal variations, and the reflection effect. We also incorporated ground-based photometry from the MuSCAT2 imager installed at the 1.52 m Telescopio Carlos Sanchez in the Teide Observatory, Spain, as well as speckle interferometry from the Southern Astrophysical Research telescope.

Results. We found HD 31221 b to have an orbital period of 4.66631 ± 0.00011 days, with a radius of 1.32 ± 0.14 R_J and a mass of 11.5 ± 10.3 M_J (from the ellipsoidal effect), making it consistent with either a brown dwarf or a giant planet. As HD 31221 is a rapid rotator (v sin I_⋆ = 175.31 ± 1.74 km s⁻¹), we deduced the spin-orbit misalignment to be λ = −121.6 ± 14.4° and I_⋆ = 55.9 ± 11.3°. The phase curve is dominated by the reflection effect, with a geometric albedo of 1.58 ± 0.50. We also found evidence that HD 31221 is a δ Scuti/γ Doradus hybrid pulsator. There are three cases for which the 3rd, 85th, and 221st orbital harmonics almost exactly coincide with peaks in the Fourier spectrum of the star, hinting at tidally perturbed stellar oscillations.

Conclusions. HD 31221 b is the third substellar object that is found to be disrupting the pulsations of its host, following HAT-P-2 and WASP-33. Additional photometric observations by CHEOPS and/or PLATO can be used to further constrain its mass and provide a more in-depth analysis of its atmosphere.