JWST Reveals Carbon-rich Chemistry in a Transitional Disk

TL;DR

JWST-MIRI MRS observations reveal a carbon-rich transitional disk around RX J1615.3-3255, with strong emissions from H2O, HCN, C2H2, 12CO2, 13CO2, OH, and 13C12CH2, in stark contrast to GM Aur, which shows primarily H2O and OH. LTE slab modeling yields a CO2/H2O column density ratio of about 2.15 in J1615, indicating enhanced carbon-bearing gas, and detects a hot water reservoir near the star in J1615 that GM Aur lacks. The two disks share similar stellar properties, yet differ in accretion rate and inner-disk dust processing; J1615 has larger, more crystalline grains and lower accretion, conditions that may shield carbon-bearing species and favor their observability. The results expand the catalog of carbon-rich protoplanetary environments and suggest that a combination of accretion-driven UV fields and inner-disk dust evolution shapes the MIR carbon chemistry in transitional disks. This work highlights the chemical diversity of planet-forming regions and motivates broader JWST surveys to understand the prevalence and origins of carbon-rich gas in disks around solar-mass stars.

Abstract

We present JWST-MIRI Medium Resolution Spectrometer (MRS) observations of the Classical T Tauri stars GM Aur and RX J1615.3-3255 (J1615), both hosting transitional disks. Despite their similar stellar and disk properties, the two systems differ strikingly in their carbon-bearing molecular emission. Using local thermodynamic equilibrium (LTE) slab models to analyze spectral lines within the 13.6-17.7 micron wavelength range, we find that J1615 exhibits strong emission from H2O, HCN, C2H2, 12CO2, 13CO2, OH, and 13C12CH2, whereas GM Aur shows only H2O and OH. We measure the accretion rates of both objects using contemporaneous optical spectra and find that J1615's accretion rate is lower than that of GM Aur. We constrain the properties of the dust in both disks using SED modeling and find elevated amounts of crystalline silicates and larger dust grains in the disk of J1615. The enhanced carbon emission in J1615 may result from a combination of lower accretion rate and larger and more processed dust grains in the inner disk, conditions that together may allow carbon-rich gas to persist and be detected. These results expand the sample of protoplanetary disks around solar-mass stars with strong CO2 and C2H2 emission and identify J1615 as a carbon-rich transitional disk, providing new insights into the chemical diversity of planet-forming environments.

Figures (12)

• Figure 1: The JWST-MIRI spectra for transitional disks GM Aur (blue) and J1615 (black). Bright atomic and molecular hydrogen lines are labeled with grey dashed lines. Molecular ions are labeled in black. Atomic and molecular features analyzed in this work are denoted with colored text. Wavelengths beyond 20 $\mu$m are omitted, but are shown in Figure \ref{['fig:variability']}.
• Figure 2: The $13.6-17.7$$\mu$m range of the GM Aur spectrum. The continuum-subtracted JWST-MIRI data (black, top panel) is overlaid with the total modeled emission (red, top panel) from molecules (bottom panel) H$_2$O (blue) and OH (pink). Slab model fits were also performed for HCN, C$_2$H$_2$, $^{12}$CO$_2$, $^{13}$CO$_2$, and $^{13}$C$^{12}$CH$_{2}$, however, H$_2$O and OH were the only positive detections, while the remaining species did not match our detection criteria (see Section \ref{['slabmodels']}). The detected bright H$\;$, H$_2$, and atomic emission lines are labeled with dashed lines.
• Figure 4: LTE slab model fits (blue) of the $\sim6.8-7.5$$\mu$m bending mode of water in the disks of GM Aur and J1615 (JWST-MIRI spectrum in black). Bright atomic and molecular hydrogen and other atomic and ionic emission lines are labeled with grey dashed lines. The horizontal blue lines are the fitting windows used to calculate $\chi^2$ (see Appendix \ref{['chi2s']}). As stated in Section \ref{['h2o']}, the presence of water lines at these wavelengths in J1615 may indicate a water reservoir closer to the central star.
• Figure 5: The $9 - 10$$\mu$m and $10.1 - 11.5$$\mu$m segments of the JWST-MIRI MRS spectra of GM Aur (blue) and J1615 (black). H$\;$ and H$_2$ lines are labeled with vertical gray dashed lines. The location of OH emission lines are indicated with vertical pink lines alongside their associated quantum number. $A'$ and $A"$ symmetry lines are labeled, with the former being a strong indicator of prompt emission. The higher detection rate of OH lines in GM Aur over J1615 (see Section \ref{['oh']}) suggests that the photodissociation of H$_2$O into OH is more important in the disk of GM Aur than J1615.
• Figure 6: SED disk models for GM Aur (left) and J1615 (right). The total model (blue) consists of the photosphere (dotted gray line), outer disk (dashed gray lines), and the optically thin inner dust component (solid gray line). Best-fit parameters can be found in Section \ref{['sed']} and Table \ref{['table:bfoptthick']}. Photometry is de-reddened using the Mathis-1999 extinction law with $A_{V} = 0.0$ for J1615 and $A_{V} = 0.6$ for GM Aur Manara-etal-2014. Labels refer to the observing mission/instrument and their references are included in Table \ref{['table:PhotometrySources']} in Appendix \ref{['photo']}.