Imaging the LkCa 15 system in polarimetry and total intensity without self-subtraction artefacts

Abstract

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 Ks-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 Ks-band images beyond 0.1″. We first modelled the Ks-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 Pmax ~ 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 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. 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.