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 Kₛ-band images of the LkCa 15 system that are free of such artefacts. 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. We used near-simultaneous reference-star differential imaging (RDI, `star-hopping') to obtain self-subtraction-free Kₛ-band images beyond 0. 1 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 (þeta), and polarised fraction, P (þeta), 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. We first modelled the Kₛ-band total- and polarised-intensity images together with ALMA submillimetre continuum maps using RADMC-3D 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 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 and mathrm PA The near-IR data, however, are less forward-scattering than the model. From the phase functions, we find that S (þeta) rises by sim5 from þeta to þeta while P (þeta) shows a broad sub-Rayleigh peak with P_ near þeta 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 nanometre grains in the surface layer (retaining compact millimetre grains for the ALMA continuum), which improves the match to the Kₛ-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 MJ are unlikely, while in the inner disc planets up to ∼3. 6 MJ could remain undetected. The star-hopping RDI data, together with phase-function diagnostics and 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. RADMC-3D
Swastik et al. (Tue,) studied this question.