A quiet host in an active planet-forming disk: Optical spectroscopy of WISPIT 2

Abstract

Context. WISPIT 2 is a young pre-main-sequence star hosting a multi-ringed transition disk and two directly imaged protoplanets. One of them, WISPIT 2b, has been confirmed to be accreting. This makes WISPIT 2 the closest known analogue to PDS 70; however, prior to this work, no optical spectrum of the central star had been published. Population-synthesis models predict that stellar accretion can be strongly suppressed once two or more giant planets have formed, so characterising the atmospheric and accretion properties of such hosts is essential. Aims. We present the first optical spectrum of WISPIT 2 and use it to derive the host atmospheric parameters, test its youth spectroscopically, and constrain its stellar accretion state relative to PDS 70 and to the embedded planets. Methods. We analysed low-resolution HFOSC spectra (R ∼ 1200 for Grism 7 and R ∼ 2200 for Grism 8) from the 2-m Himalayan Chandra Telescope with iSpec, validating the pipeline at HFOSC resolution against Gaia FGK Benchmark Stars and K-type pre-main-sequence templates from the public Manara et al. library. The Hα residual filling was measured relative to synthetic photospheric profiles and compared with the expected chromospheric-noise level. Results. Hα remains in net absorption, but is partially filled in by emission; the weak residual filling (1.5─2.0σ) lies ∼1 dex below the chromospheric-noise level, consistent with chromospheric emission and no detectable accretion. We place a 95% upper limit Ṁacc < 3.6 × 10−11 M⊙ yr−1, below even the lowest epoch of PDS 70's monitoring and implying a host-to-planet ratio Ṁ★/Ṁ2b ≲ 18. We also measure Teff = 4551 ± 150 K and a first, low-resolution, model-dependent global metallicity [M/H] = −0.17 ± 0.16; the surface gravity and Li I equivalent width (EW) support the pre-main-sequence nature of the host. Conclusions. Both known double-protoplanet hosts, PDS 70 and WISPIT 2, therefore appear to show strongly suppressed or undetectable stellar accretion. Larger spectroscopic samples are needed to test whether this is common in multi-protoplanet transition disks.