Probing accretion and stellar properties in the Orion Nebula with VLT/X-Shooter^⋆

¹ European Southern Observatory, Karl-Schwarzschild-Strasse 2, 85748 Garching bei München, Germany
² Institute of Theoretical Physics and Astrophysics, Masaryk University, Kotlářská 2, Brno 611 37, Czech Republic
³ University Observatory, Faculty of Physics, Ludwig-Maximilians-Universität München, Scheinerstr 1, D-81679 Munich, Germany
⁴ Exzellenzcluster “Origins”, Boltzmannstr. 2, D-85748 Garching, Germany
⁵ Max-Planck-Institut für Extraterrestrische Physik, Giessenbachstra.e 1, 85748 Garching, Germany
⁶ School of Cosmic Physics, Dublin Institute for Advanced Studies, 31 Fitzwilliam Place, Dublin 2, Ireland
⁷ Space Research Institute, Austrian Academy of Sciences, Schmiedlstr. 6, 8042 Graz, Austria

^⋆⋆ Corresponding author: lara.alvopiscarreta@eso.org

Received: 12 May 2025
Accepted: 6 September 2025

Abstract

Context. Multiple photometric studies have reported the presence of seemingly older accreting pre-main-sequence (PMS) stars in optical colour-magnitude diagrams (CMDs). These sources appear bluer than the majority of cluster members, leading to older isochronal age estimates.

Aims. We investigated this phenomenon in the Orion Nebula, which harbours a subset of stars that show infrared excess detected by Spitzer (which indicates the presence of protoplanetary discs) and Hα excess emission (which traces ongoing mass accretion), yet seem to have significantly older isochronal ages (≳10 Myr) than the bulk population (∼1−3 Myr) in the r, (r − i) CMD. This raises the question of whether these stars are truly older or whether their photometric properties are affected by observational biases or other physical processes.

Methods. We performed a detailed spectroscopic analysis of 40 Orion Nebula stars using VLT/X-Shooter, covering CMD-based isochronal ages from 1 to over 30 Myr. We derived extinction values, stellar properties, and accretion parameters by modelling the ultraviolet excess emission via a multi-component fitting procedure. The sample spans spectral types from M4.5 up to K6, and masses in the range ∼0.1−0.8 M_⊙.

Results. We demonstrate that when extinction and, more importantly, accretion effects are accurately constrained, the stellar luminosity and effective temperature of the majority of the seemingly old stars become consistent with a younger population (∼1−5 Myr). This is supported by strong lithium absorption (EW_Li ≳ 400 mÅ), which corroborates their youth, and by the accretion-to-stellar luminosity ratios (L_acc/L_⋆) typical for young, accreting stars. Three of these sources, however, remain old even after our analysis, despite showing signatures consistent with ongoing accretion from a protoplanetary disc. More generally, our analysis indicates that excess continuum emission from accretion shocks affects the placement of PMS stars in the CMD, displacing sources towards bluer optical colours.

Conclusions. This study highlights the critical role of accretion in shaping stellar property estimates (including age) derived from optical CMDs and emphasises the need to carefully account for accretion effects when interpreting age distributions in star-forming regions. Understanding these biases is essential for accurately constraining the early evolution of PMS stars.