MINDS. Cha Hα 1, a brown dwarf with a hydrocarbon-rich disk

TL;DR

The study uses JWST/MIRI MRS spectroscopy to characterize the inner disk of the brown dwarf Cha Hα 1, revealing a strong 10 μm silicate feature and dust dominated by large ~4 μm amorphous grains with modest crystalline forsterite. A rich molecular inventory is detected, including C$_2$H$_2$, C$_6$H$_6$, C$_3$H$_4$, C$_4$H$_2$, C$_2$H$_4$, C$_2$H$_6$, CH$_4$, CH$_3$, CO$_2$, $^{13}$CO$_2$, HCN, H$_2$, and H$_2$O, while CO and OH are absent, pointing to a carbon-rich inner disk with C/O > 1. The authors model the data with dust continuum fitting and 0D slab models to constrain molecular temperatures, column densities, and emitting radii, finding hydrocarbons and H$_2$ emission originate from a compact region within ~0.1 au and temperatures ~225–450 K, while CO$_2$ traces slightly cooler/outer gas. The results indicate a diverse chemistry unique among BD disks and provide a valuable benchmark for models of disk chemistry and planet formation around substellar objects. The presence of water alongside hydrocarbons and a high C/O ratio in Cha Hα 1 offers important constraints for future theoretical and observational studies of BD disk evolution and planet formation.

Abstract

Context. Recent JWST observations have shown that brown dwarfs (BD) are chemically rich, offering valuable insights into giant planet formation. Aims. As part of the MIRI mid-INfrared Disk Survey (MINDS) JWST guaranteed time program, we aim to characterize the gas and dust composition of the disk around the brown dwarf [NC98] Cha HA 1, hereafter Cha H$α$ 1, in the mid-infrared. Methods. We obtain data from the MIRI Medium Resolution Spectrometer (MRS) from 4.9 to 28$μ$m. We use the dust fitting tool DuCK to investigate the dust composition and grain sizes while we identify and fit molecular emission using slab models. Results. Compared with disks around very low mass stars, clear silicate emission features are seen in this BD disk. In addition, JWST reveals a plethora of hydrocarbons, including C$_2$H$_2$, $^{13}$CCH$_2$, CH$_3$, CH$_4$, C$_2$H$_4$, C$_4$H$_2$, C$_3$H$_4$, C$_2$H$_6$, and C$_6$H$_6$ which suggest a disk with a gas C/O > 1. Additionally, we detect CO$_2$, $^{13}$CO$_2$, HCN, H$_2$, and H$_2$O. CO and OH are absent from the spectrum. The dust is dominated by large $\sim$4 $μ$m size amorphous silicates (MgSiO$_3$). We infer a small dust mass fraction ($>$10$\%$) of 5 $μ$m size crystalline forsterite. We do not detect polycyclic aromatic hydrocarbons. Conclusions. Cha H$α$ 1 shows the most diverse chemistry seen to date in a BD protoplanetary disk, consisting of a strong dust feature, 12 carbon-bearing molecules plus H$_2$, and water. The diverse molecular environment offers a unique opportunity to test our understanding of BD disks chemistry and how it affects the possible planets forming in them.

Figures (14)

• Figure 1: MIRI Cha H$\alpha$ 1 reduced spectra. Blue, yellow, and red colors denote the SHORT, MEDIUM and LONG bands respectively for each channel of the ddither approach. The gray line shows the sdither reduction and the blue spectrum is the final flux-corrected one in Ch. 4. The green line corresponds to the Spitzer Cha H$\alpha$ 1 low resolution spectrum obtained from CASSIS Lebouteiller11.
• Figure 2: Comparison between spectra before and after the residual gas subtraction. The blue line is the rebinned ($R \sim$ 500) MINDS data, and the orange line is the first dust fitting. The green line corresponds to the C$_2$H$_2$ subtracted data.
• Figure 3: Dust fitting results after subtracting gas emission around 14 $\mu$m. The shaded area represents 1$\sigma$ uncertainties. The bottom panel shows the residuals of the fit. The effect of subtracting the C$_2$H$_2$ molecular emission can be seen at $\sim$14 $\mu$m.
• Figure 4: $\chi^2$ maps for HCN, CO$_2$ + $^{13}$CO$_2$, C$_6$H$_6$, C$_2$H$_4$, C$_2$H$_6$, CH$_4$, and water. The color scale shows the $\chi^2_{min}$/$\chi^2$. The best fit model ($\chi^2_{min}$/$\chi^2$ = 1) is marked with a red circle. A red contour denotes the emitting region radius as listed in Table \ref{['Tab:molecules']}. Gray contours show the emitting radii in au while white contours represent the 1$\sigma$, 2$\sigma$, and 3$\sigma$ levels.
• Figure 5: Cha H$\alpha$ 1 spectrum, with the JWST-MIRI continuum subtracted data (black) compared to the stacked emission from the slab models for C$_2$H$_2$ + $^{13}$CCH$_2$, HCN, CO$_2$ + $^{13}$CO$_2$, C$_6$H$_6$, C$_3$H$_4$, C$_4$H$_2$, C$_2$H$_4$, C$_2$H$_6$, CH$_4$, H$_2$O, and CH$_3$. The parameters of the slab models shown can be found in Table \ref{['Tab:molecules']} and \ref{['Tab:App:molecules']}. The H$_2$ lines are also marked. Horizontal lines represent the windows in which the $\chi^2$ for each fitting has been evaluated. Note that the y axis are different for each panel.