Far-ultraviolet flux distribution in Orion and its relation to stellar accretion

Technical Summary: Far-ultraviolet flux distribution in Orion and its relation to stellar accretion

Problem Statement
The physical and chemical evolution of protoplanetary discs is significantly shaped by external ultraviolet (UV) radiation from massive O and B stars. While numerical models and observations suggest that extreme-ultraviolet (EUV) and far-ultraviolet (FUV) photons drive external photoevaporation, reducing disc mass, size, and lifetime, the observational constraints remain incomplete. In the solar neighborhood (<2 kpc), FUV fluxes are typically moderate ($10^2 - 10^4 G_0$), making direct detection of photoevaporative winds difficult. Conversely, studying regions with higher fluxes often involves distant star-forming regions where astrometric uncertainties and sensitivity limits complicate analysis. Orion, the closest region hosting active star formation and young OBA stars, offers a unique laboratory to bridge this gap. However, accurately determining the FUV flux at the position of individual stars is challenging due to the complex three-dimensional (3D) geometry of stellar clusters and uncertainties in parallax measurements. Furthermore, the link between external FUV radiation and the evolution of inner disc regions (probed by stellar accretion) remains poorly understood, with contradictory observational results.

Methodology
The authors employed a multi-step approach to map the FUV flux distribution and correlate it with stellar accretion:

1. Stellar Population and Sub-clustering: Using Gaia DR3 data, the authors selected a sample of stars within a $6^\circ$ radius of the Orion Nebula Cluster (ONC) core. They applied the OPTICS clustering algorithm to the 6D phase space (Right Ascension, Declination, proper motions, parallax, and parallax-over-error) to identify 22 main sub-clusters.
2. FUV Flux Calculation: To address parallax uncertainties, the authors adopted the method of Anania et al. (2025b). Instead of relying solely on 3D distances derived from potentially uncertain parallaxes, they utilized the 2D geometry of the sub-clusters. Assuming isotropy, they derived the 3D local density function ($\hat{\rho}_{pairs}$) from the 2D projected density function ($\hat{\Sigma}_{pairs}$) using Abel's inversion. This allowed them to infer the best-estimate 3D separation between target stars and OBA-type stars (both cluster members and field stars) to compute the FUV flux ($F_{FUV}$) and its uncertainty.
3. Accretion Dataset: The study utilized stellar accretion luminosities ($L_{acc}$) inferred from H$\alpha$ emission in Gaia XP spectra (Delfini et al., 2025) for 1,200 Orion members. To account for observational incompleteness, the authors modeled a detection curve based on stellar luminosity to estimate limit accretion luminosities for non-detections.
4. Population Synthesis Modeling: The authors updated the Diskpop code to include external photoevaporation, using the FRIEDv2 grid for mass loss rates. They simulated a population of 7,500 viscous discs evolving under constant FUV fluxes sampled from the derived Orion distribution. Two scenarios were tested: photoevaporation acting from $t=0$ and delayed by 1 Myr to mimic early dust shielding.

Key Contributions

• FUV Flux Catalog: The paper provides a publicly available table of FUV fluxes (and uncertainties) for approximately 8,600 stars in Orion, covering a wide range of fluxes from $1$ to $10^5 G_0$.
• Methodological Advancement: The study demonstrates a robust statistical method for computing FUV fluxes in clustered environments by leveraging 2D cluster geometry to mitigate 3D parallax uncertainties, offering a tool for future target selection.
• Accretion vs. Environment Analysis: It presents a systematic comparison of Gaia-based accretion luminosities against the local FUV radiation field across different age bins in Orion.

Results

• Flux Distribution: The majority of the Orion stellar population (~68%) is weakly irradiated ($< 10^2 G_0$). Approximately 35% of the sample is intermediately irradiated ($10^2 - 10^4 G_0$), while only ~5% is exposed to strong FUV fluxes ($> 10^4 G_0$).
• Accretion Trends: Gaia-based $L_{acc}$ decreases with age. The detection fraction of H$\alpha$ emission declines more rapidly in regions with strong FUV fluxes ($\gtrsim 10^2 G_0$) compared to weaker flux regions.
• Model Comparison:
  • The population synthesis model broadly reproduces the trend of declining detection fractions in high-flux environments, suggesting external photoevaporation may efficiently deplete discs.
  • However, the model overestimates the detection fraction of accretors across all age bins.
  • The model fails to reproduce the observed population of young (<2 Myr), highly irradiated ($> 10^4 G_0$) strong accretors ($L_{acc} > 10^{-2} L_\odot$). Even introducing a 1 Myr delay in photoevaporation (to account for dust shielding) does not fully recover the observed high luminosities.

Significance and Claims
The authors conclude that while their results are broadly consistent with the hypothesis that external photoevaporation depletes discs in strongly irradiated environments, the evidence is not yet sufficient to definitively confirm this mechanism solely based on accretion luminosity trends. The lack of a tight correlation between $L_{acc}$ and FUV flux suggests that accretion luminosity is not a preferred tracer of external photoevaporation in the explored flux range.

The paper emphasizes that the tools provided for computing FUV fluxes are essential for future observations targeting specific flux regimes. However, the authors remain modest regarding the interpretation of the accretion data, noting that:

1. Gaia-based accretion measurements suffer from large uncertainties and noise (e.g., nebular background H$\alpha$).
2. The discrepancy between models and observations (overestimated detection fractions and unexplained young strong accretors) may stem from incomplete modeling of cluster dynamics, variable turbulence, or initial conditions, rather than a failure of the photoevaporation physics itself.
3. Firm conclusions require additional spectroscopic measurements of stellar and disc properties (mass, radius, disc mass) across the full FUV flux range in Orion to disentangle the effects of internal and external processes.

The study highlights Orion as a critical region for investigating intermediate FUV fluxes ($10^2 - 10^4 G_0$), a regime less accessible in nearby regions like Lupus or Taurus, but notes that regions with extreme fluxes ($> 10^4 G_0$) are better studied in more distant, massive star-forming regions.