Chemistry in externally FUV irradiated disks in the outskirts of the Orion Nebula

TL;DR

This study investigates the chemical composition of externally FUV-irradiated protoplanetary disks in the Orion Nebula, finding that their chemistry resembles that of isolated disks when sufficiently distant from massive stars.

Contribution

First observational analysis of chemical composition in externally irradiated disks in Orion, showing external FUV radiation has limited impact at certain distances.

Findings

Detected key molecules in irradiated disks.

No significant chemical differences from isolated disks.

Chemistry influenced more by stellar mass than external radiation.

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 (ONC) to explore the chemical composition of disks exposed to (external) FUV radiation fields: the 216-0939 disk and the binary system 253-1536A/B, which are exposed to radiation fields of $10^2-10^3$ times the average interstellar radiation field. We detect lines from CO isotopologues, HCN, H$_2$CO, and C$_2$H 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 towards 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.