2026A&A...706A.312S

Context. Studying young proto-planetary discs is essential for understanding planet formation, but traditional angular differential imaging introduces self-subtraction artefacts that make their small-scale structure difficult to interpret. We present high-resolution total- and polarised-intensity K_s-band images of the LkCa 15 system that are free of such artefacts.
Aims. LkCa 15 is a young proto-planetary system with a ∼160 au disc and previous claims of two protoplanet candidates at 15 and 18 au. We aim to analyse the LkCa 15 proto-planetary disc using high-contrast imaging to search for super-Jupiter planets beyond 20 au and to characterise the dust distribution and grain composition.
Methods. We used near-simultaneous reference-star differential imaging (RDI, 'star-hopping') to obtain self-subtraction-free K_s-band images beyond 0.1″. We first modelled the K_s-band total- and polarised-intensity images together with ALMA submillimetre continuum maps using RADMC-3D and a two grain-size (micron and millimetre) compact olivine model. Residual mismatches in the near-IR then motivated us to extract the scattering phase function, S (θ), and polarised fraction, P(θ), from the SPHERE data and compare them with aggregate-scattering models, which pointed to porous CAHP grains in the surface layer and led us to recompute the NIR scattered-light models with CAHP.
Results. Our initial two grain-size (micron and millimetre) olivine model roughly reproduces the observed NIR and ALMA disc morphology, with a flared micron surface layer from ∼25-85 au (H/R ∼ 0.08 at 50 au; surface gap ∼35-40 au) and a millimetre mid-plane ring from ∼55-130 au with a gap at ∼75-100 au, for i ∼ 50° and PA ∼ 61°. The near-IR data, however, are less forward-scattering than the model. From the phase functions, we find that S (θ) rises by ∼5× from θ ∼ 90° to θ ∼ 35°, while P(θ) shows a broad sub-Rayleigh peak with P_max ∼ 0.35 near θ ∼ 90°. These analyses disfavour compact olivine Mie spheres and are better matched by porous aggregates (CAHP-128-100 nm), so we recomputed the NIR scattered-light models with CAHP-128-100 nm grains in the surface layer (retaining compact millimetre grains for the ALMA continuum), which improves the match to the K_s-band morphology and polarisation. From the number ratio between the 12 μm and 2 mm grains, we inferred a size-distribution slope of ζ ∼ -2.3. Although no new candidate planets were detected, we estimated upper mass limits: beyond 200 au, planets more massive than ∼1.5 M_J are unlikely, while in the inner disc planets up to ∼3.6 M_J could remain undetected.
Conclusions. The star-hopping RDI data, together with phase-function diagnostics and RADMC-3D modelling with compact olivine and porous CAHP grains, allow us to reproduce the main observed features of the LkCa 15 system. The number ratio between the 12 μm and 2 mm olivine grains further shows that micron-sized grains are under-abundant relative to size distributions in the ISM or debris discs, providing new insights into grain growth and dust dynamics in gas-rich proto-planetary discs.