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

TL;DR

This study investigates apparent age spreads among disc-bearing PMS stars in the Orion Nebula by combining VLT/X-Shooter spectroscopy with a self-consistent FRAPPE multicomponent fitting approach to separate photospheric, extinction, and accretion contributions. It finds that most stars labeled as CMD-old become consistent with a younger population ($\sim$1–5 Myr) once accretion-related continuum excess and extinction are properly accounted for, an interpretation supported by lithium lines and by $L_{\rm acc}/L_{\star}$ ratios typical of young accretors. Three objects (#30, #38, #39) remain old even after the analysis, suggesting genuine age differences or geometric effects in extreme cases. The work further demonstrates that accretion continuum flux can shift PMS stars toward bluer optical colours in CMDs, highlighting the need to incorporate accretion physics when deriving ages from isochrone fitting in star-forming regions, and it places Orion’s accretion properties within the context of other nearby SFRs.

Abstract

Multiple photometric studies have reported the presence of seemingly older accreting pre-main sequence stars (PMS) in optical colour-magnitude diagrams (CMDs). We investigate this phenomenon in the Orion Nebula, which harbors a subset of stars that show infrared excess detected by Spitzer and Halpha excess emission, yet display significantly older isochronal ages (>10 Myr) compared to the bulk population (~1-3 Myr) in the r, (r-i) CMD. We perform 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 derive extinction values, stellar properties, and accretion parameters by modeling the ultraviolet excess emission through a multicomponent fitting procedure. The sample spans spectral types from M4.5 up to K6, and masses in the range ~0.1-0.8 Msun. We demonstrate that, when extinction and, most 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, which corroborates their youth, and by the accretion-to-stellar luminosity ratios 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. This study highlights the critical role of accretion in shaping the stellar properties estimates (including age) derived from optical CMDs and emphasizes the need to carefully account for accretion effects when interpreting age distributions in star-forming regions.

Figures (12)

• Figure 1: Position of the targets in the sky. The X-Shooter sample is marked as yellow stars, and sources with isochronal ages $\gtrsim$ 10 Myr in the CMD are pinpointed with green circles. The position of $\theta^{1}$ Ori C is marked with a cyan circle. The background is a DSS2-infrared image McLean_2000.
• Figure 2: r, (r$-$i) OmegaCAM colour-magnitude diagram for the X-Shooter sample (yellow stars) with sources $\gtrsim$10 Myr (isochrone highlighted in blue) pinpointed with green circles (CMD$-$old). Sources with a disc within 1 degree from the center of the Orion Nebula Cluster according to Spitzer photometry Megeath_2012 are displayed as dark gray circles. Only sources brighter than 17.5 mag are shown. The solid lines show the location of 0.1, 0.5, 1, 3, 10, 20, and 30 Myr PISA PMS isochrones PISA_isochrones.
• Figure 3: HR diagram of our sample (yellow stars) with sources appearing older than 10 Myr in the CMD (CMD$-$old; see Figure \ref{['fig:cmd']}) highlighted with green circles. Among these sources, those remaining older than 10 Myr after estimating stellar parameters with FRAPPE (CMD$+$HRD$-$old) are marked with dark blue circles. The 0.1, 0.5, 1, 3, 5, 10 and 20 Myr PISA isochrones PISA_isochrones are shown as solid black lines, while the 0.2, 0.4, 0.6 and 0.8 M$_{\odot}$ mass tracks are presented as dashed gray lines.
• Figure 4: EW$_{\rm Li}$ vs. T$_{\rm eff}$ for the veiling-corrected sample as obtained with FRAPPE. The marker scheme is the same as in Figure \ref{['fig:HRD']}. The pink pentagons are EW$_{\rm Li}$ measurements performed on X-Shooter spectra of Class II sources located in Lupus from Biazzo_2017. The mean EW$_{\rm Li}$ values and associated standard deviations in the T$_{\rm eff}$ range between 4200 K down to 3000 K are plotted in yellow and pink for the Orion Nebula and Lupus, respectively.
• Figure 5: Comparison of accretion parameters as a function of stellar properties between our X-Shooter sample of Orion Nebula sources and the sample of different nearby SFRs (transparent circles) presented in Manara_2023_PPVII. The marker scheme for our sample is the same as in Figure \ref{['fig:HRD']}. Left panel: Accretion luminosity as a function of stellar luminosity as obtained using FRAPPE. The black lines indicate $L_{\rm acc}$ equal to 1$L_{\star}$, 0.1$L_{\star}$ and 0.01$L_{\star}$. The error bars indicate the typical uncertainties on the stellar and accretion luminosities ($\sim$0.2 dex and 0.25 dex, respectively). Right panel: Mass accretion rate as a function of stellar mass as obtained using FRAPPE. Literature targets with arrows pinpoint upper limits. The error bars indicate the typical uncertainties on the mass accretion rate and stellar mass ($\sim$0.35 dex and 0.1 dex, respectively).