A closer look at the WISPIT 2 host star : Evidence of a spectroscopic binary

Context. While hundreds of protoplanetary discs have been studied in great detail, the detection of protoplanets that are still embedded in their native discs remains rare. WISPIT 2 is only the second laboratory allowing us to directly study planet formation while in progress. The recently discovered system hosts two giant protoplanets in a multi-ringed disc. Aims. We characterise the WISPIT 2 host star spectroscopically to determine its stellar properties, accretion rate, and inner disc diagnostics to provide a more complete picture of the system. Methods. We present optical and near-infrared spectroscopic observations obtained with the ESO VLT/X-shooter and 2.2 m/FEROS instruments. We modelled the stellar spectrum to determine the spectral type and effective temperature, analysed the emission lines to estimate the accretion rate, and searched for evidence of a close stellar companion using radial velocity measurements. Results. Our observations reveal that WISPIT 2 is a spectroscopic binary. The binary has a period of (4.8 ± 0.1) days, which corresponds to a semi-major axis of 0.072 au or 15.54 R⊙, assuming co-planarity with the disc and a circular orbit. The binary system consists of a ∼0.97 M⊙ primary of spectral type K3 (Teff ∼ 4700 K) and a ∼0.33 M⊙ secondary (mass ratio ∼0.34). We detect weak Hα emission below the typical chromospheric level, indicating little to no ongoing accretion onto the young stars, consistent with an accretion rate of ≲2 × 10−11 M⊙ yr−1. Conclusions. This discovery makes the WISPIT 2 disc the first circumbinary system with directly imaged protoplanets and establishes this system as a unique benchmark for studying planet formation and disc evolution around binary stars.