Euclid Q1 reveals spatial variations of the extinction law in the dense cloud LDN 1641

Abstract

Dust extinction laws are essential for precision photometry and provide a direct probe of grain properties, but their behaviour in dense molecular clouds remains poorly constrained at high extinction. Using Euclid Quick Data Release 1 (Q1) imaging of the Orion A dark cloud Lynds Dark Nebula 1641 (LDN 1641), we measured the extinction law from the broad Visible Instrument (VIS) band and the Near-Infrared Spectrometer and Photometer (NISP) $Y$, $J$, and $H$ bands along sightlines reaching $A_V\sim 30$ mag towards the cloud core. We derived colour-excess ratios $E(λ-H)/E(Y-H)$ from linear fits to colour--colour diagrams of $(λ-H)$ versus $(Y-H)$ and converted them into relative extinctions, $A_λ/A_H$. The near-infrared extinction in LDN 1641 is well described by a power law, $A_λ\propto λ^{-α}$, with $α=1.57 \pm 0.06$, corresponding to $A_{\rm VIS}/A_H=4.23 \pm 0.24$, $A_Y/A_H=2.13 \pm 0.18$, and $A_J/A_H=1.47 \pm 0.11$. We further find significant spatial variations: $α$ changes by up to $27\%$, with systematically smaller values and therefore flatter extinction curves in higher-extinction regions. This flattening is consistent with an enhanced large-grain population and supports substantial grain growth from the diffuse outskirts to the dense core of a single molecular cloud.