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Abstract Most stars are born in stellar clusters, and their protoplanetary disks, which are the birthplaces of planets, can, therefore, be affected by the radiation of nearby massive stars. However, little is known about the chemistry of externally irradiated disks, including whether or not their properties are similar to the so-far better-studied isolated disks. Motivated by this question, we present ALMA Band 6 observations of two irradiated Class II protoplanetary disks in the outskirts of the Orion Nebula Cluster to explore the chemical composition of disks exposed to (external) far-ultraviolet (FUV) radiation fields: the 216-0939 disk and the binary system 253-1536A/B, which are exposed to radiation fields of 102–103times the average interstellar radiation field. We detect lines from CO isotopologues, HCN, H2CO, and C2H toward both protoplanetary disks. Based on the observed disk-integrated line fluxes and flux ratios, we do not find significant differences between isolated and irradiated disks. The observed differences seem to be more closely related to the different stellar masses than to the external radiation field. This suggests that these disks are far enough away from the massive Trapezium stars, that their chemistry is no longer affected by external FUV radiation. Additional observations toward lower-mass disks and disks closer to the massive Trapezium stars are required to elucidate the level of external radiation required to make an impact on the chemistry of planet formation in different kinds of disks.
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This content will become publicly available on February 13, 2026
Modeling the Impact of Moderate External UV Irradiation on Disk Chemistry
Abstract The chemistry within a protoplanetary disk is greatly affected by external radiation from the local stellar environment. Previous work has focused on extreme radiation fields, representative of the center of something like the Orion Nebula Cluster. However, even in such environments, many disks exist at the edges of a cluster where the lower stellar density leads to radiation fields weaker by orders of magnitude compared to the center. We present new chemical models of a T Tauri disk in the presence of a moderately increased interstellar radiation field (ISRF). Such an environment has a background UV strength 10–100 times higher than the galactic average ISRF. Moderate radiation fields are among the most prevalent disk-harboring environments and have interesting implications for the chemistry of the outer disk radii. We find that the external UV radiation creates an outer ionization front that impacts the cold disk chemistry to varying degrees, depending on outer disk structure. Certain molecules like C+, N2H+, C, and CS are more strongly impacted by the ISRF in their abundance, column density, and observable emission. Other abundant species like HCO+and CO are less affected by the external UV flux in the outer disk under such moderate UV conditions. Further, we demonstrate that the chemistry occurring in the inner tens of au is relatively unchanged, which suggests that even in moderately externally irradiated disks the inner disk chemistry may be more similar to isolated disks like those in, e.g., the Taurus and Lupus star-forming regions.
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- Award ID(s):
- 2205698
- PAR ID:
- 10626759
- Publisher / Repository:
- IOPP
- Date Published:
- Journal Name:
- The Astrophysical Journal
- Volume:
- 980
- Issue:
- 2
- ISSN:
- 0004-637X
- Page Range / eLocation ID:
- 189
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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