A transition from H$_2$O to C$_2$H$_2$ dominated spectra with decreasing stellar luminosity

TL;DR

This study uses JWST-MIRI mid-infrared spectroscopy to reveal that disks around very low-mass stars show a higher ratio of C$_2$H$_2$ to H$_2$O, indicating a shift towards hydrocarbon-rich chemistry with decreasing stellar luminosity.

Contribution

It demonstrates a clear anti-correlation between C$_2$H$_2$/H$_2$O flux ratio and stellar luminosity, highlighting the impact of host star properties on disk chemistry and composition.

Findings

Disks around VLMS have higher C$_2$H$_2$/H$_2$O flux ratios than those around higher-mass stars.

VLMS disks show similar H$_2$O emission temperatures to T Tauri disks, suggesting oxygen depletion is not the main cause.

Carbon enhancement and hydrocarbons are more prevalent in disks around VLMS, affecting planet formation environments.

Abstract

The chemical composition of the inner regions of disks around young stars will determine the properties of planets forming there. Many disk physical processes drive the chemical evolution, some of which depend on/correlate with the stellar properties. We aim to explore the connection between stellar properties and inner disk chemistry, using mid-infrared spectroscopy. We use JWST-MIRI observations of a large, diverse sample of sources to explore trends between C$_2$H$_2$ and H$_2$O. Additionally, we calculate the average spectrum for the T Tauri ($M_{*}$$>$0.2 $M_{\odot}$) and very low-mass star (VLMS, $M_{*}$$\leq$0.2 $M_{\odot}$) samples and use slab models to determine the properties. We find a significant anti-correlation between the flux ratio of C$_2$H$_2$/H$_2$O and the stellar luminosity. Disks around VLMS have significantly higher $F_{\rm{C_2H_2}}$/$F_{\rm{H_2O}}$ flux ratios than their higher-mass counterparts. We also explore trends with the strength of the 10 $\mu$m silicate feature, stellar accretion rate, and disk dust mass, all of which show correlations with the flux ratio, which may be related to processes driving the carbon-enrichment in disks around VLMS, but also have degeneracies with system properties. Slab model fits to the average spectra show that the VLMS H$_2$O emission is quite similar in temperature and column density to a warm ($\sim$600 K) H$_2$O component in the T Tauri spectrum, indicating that the high C/O gas phase ratio in these disks is not due to oxygen depletion alone. Instead, the presence of many hydrocarbons, including some with high column densities, points to carbon enhancement in the disks around VLMS. The observed differences in the inner disk chemistry as a function of host properties are likely to be accounted for by differences in the disk temperatures, stellar radiation field, and the evolution of dust grains.