What you see is not necessarily what you get: Interpreting near-infrared scattering phase functions of debris discs

Context. Scattering phase functions (SPFs) derived from resolved scattered-light images of debris discs are widely used to infer dust grain properties, often through parametric descriptions such as the Henyey-Greenstein (HG) phase function. However, the extent to which the inferred scattering behaviour reflects the intrinsic dust properties, rather than projection effects, disc geometry, or methodological choices, remains uncertain. Aims. We investigate how reliably scattering phase functions and HG asymmetry parameters can be recovered from scattered-light images and identify the regimes in which geometric and methodological effects introduce significant biases. Methods. For the first time, we performed a systematic test of the reliability of SPF determinations using a physically motivated forward-modelling framework that combines dust-scattering calculations, size-dependent grain dynamics, and ray-tracing imaging to generate synthetic total-intensity scattered-light observations. Because the intrinsic SPFs are known a priori, surface-brightness phase functions extracted from the images can be directly compared to the input scattering behaviour. We explored a grid of grain size distributions, disc inclinations, and opening angles, and we fitted two-component HG functions to evaluate how reliably the forward-scattering parameter g_ Even under idealised conditions with perfect knowledge of the disc geometry, the extracted phase functions can differ substantially from the intrinsic SPFs. First, the limited range of accessible scattering angles plays a dominant role, as the smallest angles, where the strongest forward-scattering peaks occur, are generally unobservable. As a result, the apparent scattering behaviour does not vary monotonically with grain size: Large grains may not appear as forward-scattering, while grains comparable to the observing wavelength can appear comparatively more anisotropic. Second, projection effects, line-of-sight mixing, and SPF-extraction choices further modify the recovered phase functions. Consequently, the fitted HG asymmetry parameter g_ Scattering phase functions and HG asymmetry parameters derived from scattered-light images should be interpreted as effective observation-dependent quantities rather than direct proxies of dust properties. Robust interpretation of debris-disc images therefore requires forward modelling that accounts for projection effects, limited scattering-angle coverage, and observational biases. traces the underlying dust properties. Results. depends strongly on viewing geometry and methodology and does not uniquely trace grain properties. Conclusions.