Impact of accretion variability on the CO emission of a disk around a very low-mass star with JWST

Abstract

Context: Very low-mass stars (M_⋆<0.2M_⊙) are known to host a larger fraction of terrestrial planets compared to higher mass stars. Studying the structure of their protoplanetary disk is therefore crucial to understanding the formation of Earth-like planets. Aims: In this work, we aim to characterize the innermost regions of the disk around a known very low-mass star, 2MASS J16053215-1933159 (J1605 hereafter), by analyzing new JWST/NIRSpec observations taken in 2024, two years after the first observation with JWST/MIRI in 2022. We intend to investigate the atomic and molecular emission in both spectra to establish a global view of the system and study its variability. Methods: After identifying both the fundamental band of CO and HI recombination emission lines from both spectra corrected from the contribution of the stellar photosphere, we used the HI lines to derive the stellar accretion rate at each epoch. The CO emission was fitted with a slab model to derive the gas characteristics. Results: We find that between the two epochs (2022 and 2024), the CO flux is reduced by a factor of ∼ 3, the flux of the only HI line seen by both NIRSpec and MIRI (the HI 10-6 line) is reduced by a factor of sim13, and the total continuum-density flux is reduced by ∼ 15 %. The accretion rate derived from individual HI lines is consistent with a decrease of a factor of sim13 in two years. After correcting the spectrum from the CO absorption present in the J1605 stellar photosphere, we find that the decrease in the disk's CO emission is consistent with a CO gas of the same column density and temperature at each epoch, but originating from a smaller emitting area when the accretion rate is low (NIRSpec's epoch). While warm hydrocarbons (≃ 500 K) are detected with MIRI-MRS (2022) with extremely high column densities of ̊m C_2H_2, no hydrocarbon features are seen with NIRSpec (2024). We propose that non-Local Thermal Equilibrium (LTE) effects quench the near-IR emission of hydrocarbons in the warm reservoir. Taking the newly detected hot CO (∼ 800 K) as a reference and assuming LTE, we constrained the column density of ̊m C_2H_2, HCN, and ̊m CH_4 to be less than 30% of CO in the hot reservoir. This value suggests that the C/O in the inner disk is unlikely to be much larger than unity. Conclusion: The variation of the accretion luminosity of the very low-mass star J1605 correlates with the variation of its fundamental CO luminosity similarly to T-Tauri stars, supporting the idea that very low-mass stars could be seen as scaled-down versions of T-Tauri stars. The detection of the emission of the fundamental band of CO in disks around cold objects (very low-mass stars, brown dwarfs, and giant planets) is challenging because of the presence of CO absorption in their photosphere. Future observations (e.g., with ELT/METIS) looking for such emission will require robust ways to constrain the photosphere emission and follow the time variability of the sources.