Chemical Evolution and Inheritance in Young Stellar Objects in the MonR2 Molecular Cloud

Young stellar objects (YSOs) forming across the Galaxy are windows into protoplanetary chemistry, and are analogues for the early solar nebula. High-resolution near-IR observations of the CO rovibrational absorption bands taken with ground-based telescopes have enabled precise evaluation of carbon and oxygen isotopes for solar-type YSO environments, informing models of protoplanetary and prebiotic chemistry. Massive YSOs provide additional insight into rare environments of star and planet formation, and chemistry that cannot be easily probed with dimmer, low-mass YSOs. Here we present new observations of YSOs in the MonR2 star-forming complex, ~778 pc from the Sun and one of the closest regions of massive star formation. Contemporaneous observations of YSOs within this cloud further provide unique opportunities to evaluate protoplanetary chemistry and chemical inheritance in systems sharing a common origin. Using the iSHELL spectrograph (R ~78,000-88,000) on NASA’s IRTF, we observed the 0.65" binary MonR2 IRS3 (A,B), and the compact, massive YSO, MonR2 IRS2, detecting the four isotopologues of CO,¹²C¹⁶O (K) and ¹³C¹⁶O, ¹²C¹⁸O, ¹²C¹⁷O (M), from which precise isotope ratios were derived. Our results for oxygen are summarized in Fig. 1. We find significant ¹⁶O-excesses relative to ¹⁸O and ¹⁷O along the mass-independent (slope=1) fractionation line for two epochs of MonR2 IRS3 (B): 2019— ¹⁶O/¹⁸O = 830 ± 40 (1σ), ¹⁶O/¹⁷O = 3640 ± 220; and 2021— ¹⁶O/¹⁸O = 1040 ± 40, ¹⁶O/¹⁷O = 3560 ± 310. For the one epoch (2019) so far observed for MonR2 IRS2, we find: ¹⁶O/¹⁸O = 830 ± 25, ¹⁶O/¹⁷O = 3690 ± 130. Elevated ¹²C/¹³C ratios (120 ± 6 to 160 ± 10) are also found in these same targets. MonR2 IRS3 (A) does not show ¹⁶O-excesses but is close to the mass-independent line, and reveals lower ¹²C/¹³C ratios for both epochs (~90 to 110). We see little variation between epochs for MonR2 IRS3 (A,B) suggesting that oxygen signatures in the binary are robust over short periods. The mass-independent ¹⁶O-excesses for MonR2 IRS3 (B) [possibly without a disk] and for MonR2 IRS2 [possibly with a warped disk and ionizing wind], along with high ¹²C/¹³C, are consistent with signatures of CO self-shielding in these YSOs. MonR2 IRS3 (A) [possibly having a disk] does not show signatures of CO self-shielding but is close to the mass-independent line for oxygen. We thus far conclude that gravitationally bound YSOs may not follow similar chemical evolutionary pathways. Further, signatures of CO self-shielding in massive YSOs may be inherited from the parent cloud rather than originate in the disk itself, followed by mixing as the YSO evolves.