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

TL;DR

The paper reveals a robust anti-correlation between the C$_2$H$_2$/H$_2$O flux ratio and stellar luminosity in protoplanetary disks, with very low-mass stars and brown dwarfs showing hydrocarbon-rich, carbon-enhanced inner disks while TTauri disks remain H$_2$O-rich. Using JWST-MIRI MRS data for a large, diverse sample and 0D LTE slab modeling, the authors show TTauri spectra require multiple H$_2$O components, whereas VLMS spectra are dominated by C$_2$H$_2$ with a rich hydrocarbon inventory at cold temperatures. The results imply inner-disk C/O gas-phase ratios exceeding unity in VLMS, driven by carbon enrichment rather than pure oxygen depletion, and link these chemical differences to disk temperature, radiation field, and dust evolution. These findings have significant implications for planet formation chemistry and motivate future multi-wavelength, time-domain, and spatially resolved studies to connect outer-disk processes with inner-disk composition. ==

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 $μ$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.

Figures (9)

• Figure 1: The average T Tauri (top) and VLMS (bottom) sample JWST spectra. The detected molecular species are highlighted. The C$_2$H$_2$ and H$_2$O regions (yellow and blue, respectively) show the regions over which the fluxes are integrated. Most of the unlabeled lines in the T Tauri average are various H$_2$O transitions.
• Figure 2: The relationship between the flux ratio of C$_2$H$_2$ to H$_2$O as a function of stellar luminosity. Objects with $M_{*}$$>$0.2 $M_{\odot}$ are colored in blue and represent our T Tauri sample, while those with stellar masses below 0.2 $M_{\odot}$ are our VLMS sample and are shown in red. The two outliers around $L_{*}$$\sim$1 $L_{\odot}$ are DL Tau and V1094 Sco, which will be analyzed in detail in Tabone et al. (in prep.). This trend is statistically significant with a $p-$value of 1.3$\times$10$^{-7}$ and a correlation coefficient of -0.77. Upper/lower limits (downward/upward triangles) are the 3$\sigma$ limits. Error bars are smaller than the points for most targets.
• Figure 3: Left: $F_{\rm{C_2H_2}}$/$F_{\rm{H_2O}}$ as a function of the strength of the 10 $\mu$m silicate feature (stronger silicate features have higher values). For four of the VLMS, there is either no 10 $\mu$m emission or the emission is coming at least partially from C$_2$H$_4$, therefore we adopt feature strength of 1 for these sources (denoted by open points). Two outliers at a 10 $\mu$m band strength of 3 and 3.8 are the transitional disks LkCa 15 and PDS 70 and have been removed for clarity. Middle: The relationship between $F_{\rm{C_2H_2}}$/$F_{\rm{H_2O}}$ and $\dot{M}$. Right: The relationship between $F_{\rm{C_2H_2}}$/$F_{\rm{H_2O}}$ and ${M_{\rm{dust}}}$. The PCCs and $p-$values can be found for each panel. All of the relationships are statistically significant ($p-$value$<$0.05); however the correlations are not as strong as the $F_{\rm{C_2H_2}}$/$F_{\rm{H_2O}}$ vs. stellar luminosity relationship. Rotated triangular markers for the VLMS sample indicate lower limits on $F_{\rm{C_2H_2}}$/$F_{\rm{H_2O}}$ and upper limits on ${M_{\rm{dust}}}$. Error bars are smaller than the points for most targets.
• Figure 4: The average spectra for disks in our T Tauri sample (top) and VLMS sample (bottom) in black, compared to the best-fit model in red. The model components are shown below each for reference.
• Figure 5: The best-fit slab model parameters for the average VLMS spectrum (left circles) and T Tauri spectrum (right squares) for the different molecules. The H$_2$O component for the T Tauri spectrum is the intermediate ($T\sim600$) component. Temperature and column density are shown on the top on the left and right, respectively. The equivalent emitting radius is shown on the bottom left and that radius normalized to the H$_2$O snowline for each subsample is shown on the bottom right. Only the optically thick C$_2$H$_2$ component is shown for the VLMS in the radii plots, as the radius is unconstrained in the optically thin case. Error bars are determined from the 1$\sigma$ confidence intervals and in some cases are degenerate between parameters. For example, in the case of optically thin emission, the column density and equivalent emitting radii are degenerate. See Figures \ref{['fig: chi2 ttauri']} and \ref{['fig: chi2 vlms']} for the $\chi^2$ maps for the T Tauri and VLMS fits, respectively.