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A&A
Volume 703, November 2025
Article Number A18
Number of page(s) 16
Section Stellar structure and evolution
DOI https://doi.org/10.1051/0004-6361/202555621
Published online 03 November 2025

© The Authors 2025

Licence Creative CommonsOpen Access article, published by EDP Sciences, under the terms of the Creative Commons Attribution License (https://creativecommons.org/licenses/by/4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

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1. Introduction

Brown dwarfs (BDs) are often referred to as “failed stars” because they lack sufficient mass (≤0.075 M⊙ for solar metallicity) to sustain the nuclear fusion of hydrogen into helium in their cores. The presence of warm dusty disks around these dim objects is well known (Comeron et al. 1998; Natta & Testi 2001; Natta et al. 2002) based on near- and mid-infrared studies. These disks can result in the formation of Earth-sized or smaller planets (Apai et al. 2005; Payne & Lodato 2007; Daemgen et al. 2016). Chauvin et al. (2004) were the first to report a giant planetary candidate around a BD. In the following years, other BDs with Jupiter-mass planets were detected (e.g., Todorov et al. 2010; Han et al. 2013; Jung et al. 2018; Barrado Y Navascués et al. 2007).

The study of the properties of disks around young BDs provides important clues on the formation of the planetary systems forming around them. Though usually smaller in size and mass (Hendler et al. 2017), BD protoplanetary disks are similar to the disks around their young stellar counterparts, such as T Tauri stars (TTSs), in many ways. Both BD disks and TTS disks are typically flat (Scholz et al. 2007), with evidence of dust grain growth in their inner regions (Sterzik et al. 2004; Apai et al. 2005; Meru et al. 2013; Pinilla et al. 2013). Some BD disks also show signs of inner disk dust clearing (Muzerolle et al. 2006; Rilinger et al. 2019). Furthermore, the fraction of BDs observed to host disks is similar to the TTS disk fraction, possibly indicating a shared formation mechanism (Monin et al. 2010), and outflows have been observed in these faint objects (Whelan et al. 2007), which indicate similar accretion and ejection mechanisms in BDs as in their higher mass counterparts. However, some aspects of disks around BDs are different from those around TTSs and Herbig Ae/Be stars (intermediate mass pre-main sequence stars, 2–8 M⊙, the higher mass analogs to the T Tauri stars). They have a lower accretion rate (∼10−12 M⊙yr−1; Herczeg et al. 2009), a tendency to show a flatter disk geometry inferred from Spitzer Space Telescope observations and spectral energy distribution (SED) modeling (e.g., Apai et al. 2005; Guieu et al. 2007; Scholz et al. 2007; Pascucci et al. 2009), and a longer lifetime (Barrado y Navascués & Martín 2003; Carpenter et al. 2006; Barrado y Navascués et al. 2007). The dust in BD disks seems to undergo more efficient radial drift than in higher mass systems (Pinilla et al. 2013). The disk chemistry and physical evolution also show important differences as a function ofstellar mass, temperature, and radiation field (e.g., different dust populations at the observed radii or different molecular abundances in the disk atmospheres; Pascucci et al. 2009). This is important for planet formation models since the different chemistry of materials in the disk may also influence the bulk composition and volatile content of the forming planets.

Thanks to Spitzer, the Herschel Space Observatory, and the Atacama Large Millimeter Array, the past few years have seen a dramatic progress in understanding disks around the lowest mass stars and BDs (e.g., Pascucci et al. 2013; Riviere-Marichalar et al. 2015; Testi et al. 2016; Kurtovic et al. 2021). However, our understanding of planet formation, chemical composition, disk structure, and evolution around BDs lags behind that of their higher mass counterparts, such as T Tauri and Herbig Ae/Be stars. The latter are easier to characterize due to their brighter nature and have been studied in larger numbers. The sensitivity of the James Webb Space Telescope (JWST) enables the characterization of the disks around BDs in a wavelength range previously inaccessible, revealing insights into physical processes of the gas and dust. Very few BD disks have been observed to date with the JWST. Perotti et al. (2025) have presented the Mid-InfraRed Instrument (MIRI; Rieke et al. 2015; Wright et al. 2015, 2023) spectrum of a water rich, highly inclined BD disk that appears to be poor in hydrocarbons. In contrast, TWA 27 A has a disk with no water or silicate emission but is rich in hydrocarbons (Patapis et al. 2025). Similarly, recent JWST observations of very low mass stars (VLMS; M < 0.3 M⊙) have shed new light on the chemical composition of their disks, revealing a complex chemistry rich in hydrocarbons, including some isotopologues Tabone et al. (2023), Arabhavi et al. (2024), Kanwar et al. (2024a). These disks around VLMS, including the disk around TWA 27 A, display strong hydrocarbon emissions (in some cases even a hydrocarbon pseudo-continuum) with very few oxygen carriers, suggesting a high volatile carbon-to-oxygen (C/O) ratio. A larger sample of observed objects and extension of the sample to lower masses will provide information about which is the norm among BD disks (see Arabhavi et al. 2025a for a study on trends in the inner disk gas of VLMS disks) and will help investigate the various conditions under which planets may form.

The Cycle 1 JWST/MIRI Guaranteed Time Observation (GTO) program MINDS (MIRI mid-INfrared Disk Survey1) is aimed at investigating the chemical inventory in the terrestrial planet-forming zone of disks from Herbig Ae/Be stars to BDs (Henning et al. 2024; Kamp et al. 2023). As part of this program, we present JWST spectroscopic observations of the disk around the BD [NC98] Cha HA 1 (hereafter Cha Hα 1), which has been characterized in detail in Bayo et al. (2017). Cha Hα 1 is a member of the roughly 1.4–2.4 Myr-old Chamaeleon I South star forming region at a distance of 191.8 − 2.8 + 2.4 $ ^{+2.4}_{-2.8} $ pc (Galli et al. 2021). It is a BD with spectral type M7.5 and mass 0.04–0.05 M⊙ (Comerón et al. 2000) that is still accreting at a low rate (Ṁacc = 10−11.69 M⊙year−1; Manara et al. 2016). It is the first BD detected in X-rays (Neuhauser & Comeron 1998). Its disk has been modeled in the past using Infrared Space Observatory (ISO) data up to 14.3 μm, which required an optically thick, flared disk to account for the mid-IR flux (Natta & Testi 2001). Spitzer low resolution spectra show the detection of C2H2 in emission (Pascucci et al. 2009).

This work is organized as follows: In Sect. 2, we describe the observation and data reduction. In Section 3 and 4 we describe the dust fitting and slab model retrievals used in this work. In Section 5 we present our results on the dust content and molecular inventory. Finally we discuss our findings and present some conclusions in Sections 6 and 7.

2. Observations and data reduction

Cha Hα 1 was observed with the MIRI on board the JWST on 15 August 2022 as part of the GTO program MINDS (ID: 1282, PI: Thomas Henning). The Medium Resolution Spectroscopy (MRS; Wells et al. 2015; Argyriou et al. 2023) mode was used, which involves four Integral Field Units (IFUs): channel 1 (4.9–7.65 μm), channel 2 (7.51–11.71 μm), channel 3 (11.55–18.02 μm), and channel 4 (17.71–27.9 μm). Each channel is composed of three sub-bands, SHORT (A), MEDIUM (B), and LONG (C) leading to a total of 12 wavelength bands (see Figure 1). Observations were performed with the target acquisition option using the same target and in FASTR1 readout mode with a point source 4-point dither pattern in the positive direction. The total exposure time per sub-band was 1232.12 s.

thumbnail Fig. 1.

MIRI Cha Hα 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 Hα 1 low resolution spectrum obtained from CASSIS (Lebouteiller et al. 2011).

We implemented a hybrid pipeline for the reduction of our MIRI/MRS data. The MINDS pipeline is publicly available2, and for this reduction, we used version 1.0.3. The pipeline relies on the standard JWST pipeline (v1.14.1; Bushouse et al. 2023) using CRDS context jwst_1254.pmap, and is complemented with routines from the Vortex Image Processing (VIP) package (Gomez Gonzalez et al. 2017; Christiaens et al. 2023) to derive optimal-quality output spectra through background subtraction, bad pixel correction, and centroiding options not available in the standard pipeline. The pipeline is structured around three main stages that are the same as in the JWST pipeline, namely Detector1, Spec2, and Spec3. Stray light is corrected after the first stage (Detector1). From there, we considered two different approaches for background subtraction. In one case, to remove the background, we carried out a direct pairwise dither subtraction (hereafter referred to as ddither). This method is more suited to fainter sources where the resulting PSF overlap is minimal and reduces the noise level producing a higher signal-to-noise ratio (S/N) spectrum. However, the flux is lower than expected at the longest wavelengths due to some self-subtraction between minimally overlapping PSFs. In the second approach (hereafter referred to as sdither) we estimate and remove the background leveraging the four-point dither pattern to obtain a first guess on the background map, then we refine it using a median-filter which both smooths the background estimate and removes residual star signals from it. The result of this method is noisier and, as can be seen in Figure 1, there are some jumps. This is due to some structured bright background noise in the spectral frames of the long-ward end of the bands of channel 4 (and even in the LONG sub-band of channel 3). In the spectral frames of the beginning of each band the background seems to be well subtracted. Thus, we assumed that the beginning of each band of channel 4 in the sdither approach marks the absolute flux. We kept our higher S/N ddither reduction corrected in channel 4 to match the flux level at the beginning of each sdither band and linked it to the next band. In any case, the correction was only done for channel 4 and does not affect any molecular discussion since we have not found any molecule at wavelengths longer than ∼17 μm.

Spec2 was then used with default parameters, but the stray light correction and background subtraction were skipped. The outlier detection step in Spec3 is skipped, and replaced by a custom VIP-based bad pixel correction routine applied before Spec3. The bad pixels are identified through sigma-filtering, and corrected with a Gaussian kernel. VIP-based routines are then also used for the identification of the star centroid in the spectral cubes produced at stage 3. The centroid location is identified with a 2D Gaussian fit in a weighted mean image for each band’s spectral cube, where the weights are set to be proportional to a proxy of the S/N of the source in each channel.

The identified centroid locations are subsequently used for aperture photometry: the spectrum is extracted by summing the signal in a 1.5 full width at half maximum (FWHM) aperture centered on the source, 2 FWHM for the ddither reduction, where the FWHM is equal to 1.22 λ/D, with λ the wavelength and D ∼ 6.5m the diameter of the telescope. Aperture correction factors are applied to account for the flux loss as presented in Argyriou et al. (2023).

The reduced spectra, using both background subtraction methods, are presented in Fig. 1. The different bands of the 4 channels in the ddither approach are highlighted in colors. We take the continuum-level from the sdither approach, which is more accurate, and the higher S/N from the ddither approach. To do so, we use a smooth polynomial to place the high S/N spectrum at the correct absolute flux level in a per band basis. The result is the blue line labeled as “corrected” in Fig. 1. Due to the faintness of the target, the S/N is poorer in Ch. 4 and we decide to cut the spectrum at 26 μm. The spectral resolving power across the 5–25 μm spans from R ∼ 3500 at shorter wavelengths to R ∼ 1500 at longer wavelengths. The low resolution R ∼ 60–127) Spitzer Cha Hα 1 spectrum downloaded from the Combined Atlas of Sources with Spitzer IRS Spectra (CASSIS; Lebouteiller et al. 2011) is also shown for comparison. The flux agreement of both sets of data is excellent.

3. Dust fitting

The mid-IR excess emission observed in a young object arises from the hotter inner disk surrounding it. The shape of the continuum and the optically thin dust emission features generated by the dust grains in the disk surface layers can be used to probe the size and composition of the dust in the disk and therefore also the grain growth and crystallization (e.g., Bouwman et al. 2001). That is, the degree of dust processing in protoplanetary disks can be estimated using silicate emission features present in the MIRI spectrum.

To fit the dust emission, we used the Dust Continuum Kit (DuCK; Kaeufer et al. 2024), a dust fitting tool based on a two-layer disk model, as described by Juhász et al. (2009). This allows us to calculate flux contributions from various components, including the central star, inner rim, optically thin disk surface, and optically thick mid plane. In the fitting process, we use absorption efficiencies (Q curves) of different dust species obtained with the Gaussian random field method (GRF; Min et al. 2007) which generates individual grain shapes based on a Gaussian random field and calculates the average opacity over a range of individual particle shapes. We used a library of GRF Q curves including amorphous silicates MgOlivine (Mg2SiO4, Henning & Stognienko 1996), MgPyroxene (MgSiO3, Dorschner et al. 1995), and SiO2 (Spitzer & Kleinman 1960) and crystalline silicates forsterite (Servoin & Piriou 1973) and enstatite (Jaeger et al. 1998). We used grain sizes of 0.1, 1, 2, 3, 4, and 5 μm. The fitting process follows Kaeufer et al. (2024) and is described in detail in Jang et al. (2024) with the only difference being that in the case of Cha Hα 1 we allowed for a temperature gradient in the disk surface instead of using a single temperature.

To fit the observed spectrum, we rebinned the MIRI data to have a constant spectral resolving power (R ∼ 500) to avoid the narrow gas emission lines (blue line in Figures 2 and 3). Data beyond 26 μm is neglected due to significant noise. Given the gas-rich spectrum, particularly in the 14 μm wavelength range, we took additional steps to try to suppress the impact of gas emission on dust fitting. We first fitted the spectrum, including the gas emission, and it resulted in a residual of approximately 2.5 mJy at the peak of the gas emission (orange dashed line in Figure 2). Then the model is shifted down 0.3 mJy to make the data and model agree at 13 μm (green dot-dashed line in Figure 2). Finally, the observed flux around 14 μm was subtracted from the shifted model. Spectra before and after the subtraction are compared in Fig. 2, and the final fit is shown in Fig. 3.

thumbnail Fig. 2.

Comparison between spectra before and after the residual gas subtraction. The blue line is the rebinned (R ∼ 500) MINDS data, and the orange line is the first dust fitting. The green line corresponds to the C2H2 subtracted data.

thumbnail Fig. 3.

Dust fitting results after subtracting gas emission around 14 μm. The shaded area represents 1σ uncertainties. The bottom panel shows the residuals of the fit. The effect of subtracting the C2H2 molecular emission can be seen at ∼14 μm.

4. Slab model fitting

In order to provide an inventory of molecules in the main planet-forming region of the disk of Cha Hα 1, that is inside a few au, we use zero-dimensional (0D) slab models calculated with the PRODIMO code (Woitke et al. 2009; Kamp et al. 2017; Woitke et al. 2016). The 0D slab model fitting is a simplified way to describe a layer (or slab) of material with uniform properties, that is, it assumes a homogeneous column of gas that radiates with a fixed temperature and a constant column density. A grid of slab models, one for each of the molecules investigated, was generated in Arabhavi et al. (2024) following previously described methods (Tabone et al. 2023; Grant et al. 2023). We use the synthetic spectra produced in Arabhavi et al. (2024) where line positions, Einstein A coefficients, statistical weights, and partition functions, were taken from the HITRAN 2020 database (Gordon et al. 2022), the GEISA database (Delahaye et al. 2021) and Arabhavi et al. (2024) for C3H4 and C6H6. An overview of the molecular data used for calculating the mid-IR spectra is shown in Table 2 of Henning et al. (2024). We note that we use the LTE treatment for the molecules presented in that table. These synthetic spectra are available for a wide range of ambient conditions (e.g., gas temperature, gas column density). In particular, we sample the temperature in steps of 25 K from 25 K to 1500 K and column densities in steps of 1/6 dex from 1014 cm−2 to 1024.5 cm−2. For C6H6 and C3H4 the temperature is limited to ≤600 K because of incompleteness of the laboratory data (Arabhavi et al. 2024).

The slab models outlined above provide total line fluxes in a certain wavelength range per molecule and account for the distance to Cha Hα 1 (191.8 pc, Galli et al. 2021). For comparison with the observed data the MIRI spectrum needs to be continuum subtracted. The continuum and continuum subtracted spectrum are shown in Figures A.1 and B.1. See Appendix A for a detailed description on the continuum definition. We then convolve the slab models with the appropriate spectral resolution of the instrument, in this case R ∼ 2500–3500 depending on the channel (Labiano et al. 2021; Pontoppidan et al. 2024). Finally, the convolved model spectrum is resampled to have the same wavelength grid as the observed spectrum.

We employ χ2 minimization with the prodimopy (v2.1.4)3 tool to identify the best fitting slab model. Our investigation encompasses the parameter space of column density (N), temperature (T), and the equivalent radius of the emitting area (R). The emitting area radius acts as a scaling factor for achieving the absolute flux level while the column density and temperature capture the relative flux levels of lines or the spectral shape. We define very restricted wavelength windows for the χ2-minimization trying to avoid the contribution from other molecules while still containing features that help to constrain the fit (e.g., optically thin and temperature-sensitive lines). These windows are shown in Figure 5. The fitting is done in an iterative way, one molecule at a time, to further reduce contamination from other species going from the strongest line emission contributor to the weakest, subtracting the fit from the spectrum for each molecule before starting the fitting of the next molecule. However, we note that some molecules present quite some overlap in their model spectrum which we do not take into account in our fitting procedure. This procedure results in some differences to a simultaneous fit of all molecules but when no pseudo-continua exists there is strong overlap (Kaeufer et al. 2024). In cases where the emission overlap is complete (C3H4 and C4H2, and C2H2 – see Sect. 5) we have fitted the two molecules emitting in the same area simultaneously. In these cases the free parameters are N1 × T1 × R1 × N2 × T2 × R2.

In that way, the order for the fitting has been C2H2 – HCN – CO2 – C6H6 – C3H4 – C4H2 – C2H4 – C2H6 – CH4 – H2O. The χ2 maps are presented in Figure 4 and show the 1σ-, 2σ-, and 3σ- contours. However, these are not a good approximation of the error because of the stepwise procedure used and the molecular emission overlap. In Table 1 we show the estimated rms noise level, σ of each band where molecular emission was found. To estimate this uncertainty we use the JWST Exposure Time Calculator (ETC4) to obtain an estimate of S/N as described in Jang et al. (2025). Another way to measure the errors of the derived parameters is to repeat the fitting in a different order. In this way we find changes in the derived temperatures and column densities of 75 K and 0.5 orders of magnitude respectively. Finally we used the following criteria to determine if a molecule detection is robust: If at least two features are detected, if the main feature is stronger than the 3σ-uncertainty, and if the same molecule is detected when we change the order of the fitting, we considered it a firm detection. If any of those criteria failed we declared the detection tentative. We chose to require at least two features for a robust detection to adopt a conservative approach, and we did not impose a three-feature requirement because for some molecules, such as for C6H6, the third feature may be challenging to detect within the explored range of temperature and column density. We note that even in the case of a firm detection, there are cases where the parameters cannot be well constrained because the fit is degenerate (see Fig. 4). In these cases, we do not provide a value for the parameters and instead just list the molecule as “Detected”.

thumbnail Fig. 4.

The χ2 maps for HCN, CO2 + 13CO2, C6H6, C2H4, C2H6, CH4, and water. The color scale shows the χmin2/χ2. The best fit model (χmin2/χ2 = 1) is marked with a red circle. A red contour denotes the emitting region radius as listed in Table 2. Gray contours show the emitting radii in au while white contours represent the 1σ, 2σ, and 3σ levels.

Table 1.

Median continuum flux, estimated S/N from the ETC, and 1σ- noise for each band where molecules are detected.

We performed the exercise of determining different continuum levels and repeat the molecular fits for each of them in order to evaluate the differences. The same molecules were detected in all cases. The results of this test can be seen in Appendix B.

Table 2.

Temperatures, column densities, and emitting radii of the detected molecules obtained from the slab model fits.

5. Results

The first thing to note is the good agreement of the JWST spectrum with the Spitzer data (Fig. 1), taken in 2005, showing the absence of detectable variability in this wavelength range. This is in contrast to the finding of aperiodic variability with amplitude of ∼0.4 mag at optical wavelengths which is attributed to accretion and disk properties (Cody & Hillenbrand 2014).

The much higher spectral resolution of MIRI-MRS compared to previous Spitzer data reveals a plethora of molecular features (Fig. 5). C2H2 emission at 13.7 μm is particularly strong, consistent with earlier findings that this molecules emission is enhanced in disks around BDs and VLMS (Pascucci et al. 2009). The other molecular features can be ascribed to several hydrocarbon molecules (C6H6, C3H4, C4H2, C2H4, C2H6, CH4, and CH3) plus CO2, HCN, H2, and H2O. In addition, one 13C-substituted isotopologue each for acetylene and carbon dioxide (hereafter 13CCH2 and 13CO2) have been found. The multitude of detected carbon bearing molecules points to a carbon-rich inner disk environment as has been seen in TWA 27 A (Patapis et al. 2025) and in some VLMS (Tabone et al. 2023; Arabhavi et al. 2024; Kanwar et al. 2024a). However this is the only case in which water is clearly detected in addition to all the hydrocarbons.

thumbnail Fig. 5.

Cha Hα 1 spectrum, with the JWST-MIRI continuum subtracted data (black) compared to the stacked emission from the slab models for C2H2 + 13CCH2, HCN, CO2 + 13CO2, C6H6, C3H4, C4H2, C2H4, C2H6, CH4, H2O, and CH3. The parameters of the slab models shown can be found in Tables 2 and C.1. The H2 lines are also marked. Horizontal lines represent the windows in which the χ2 for each fitting has been evaluated. We note that the y axis is different for each panel.

5.1. Dust content

The dust grains typical to the interstellar medium (ISM) are sub-micron (∼0.1 μm) sized, amorphous silicate grains (e.g., Kemper et al. 2004; Bouwman et al. 2001; Apai et al. 2004) which show a single-peaked, relatively sharp and strong, unstructured feature peaking at 9.8 μm. In contrast, what we see in the Cha Hα 1 spectrum is a broader and flat topped emission feature typical of larger amorphous grains indicating that some dust processing has already happened within the very first million years of disk formation. Less prominent forsterite features can be seen around 16 μm. No PAH features are found in agreement with possible PAH destruction by EUV or X-ray photons, as discussed by Siebenmorgen & Krügel (2010).

In agreement to what can be extracted from looking at the spectrum, our dust fitting reveals that dust is dominated by large ∼4 μm sized amorphous silicates (> 50% mass fraction of MgSiO3). This shows that grain growth is already underway at the age of ∼1.5 Myr as has been seen in previous works (Kessler-Silacci et al. 2007; Furlan et al. 2009; Ribas et al. 2017; Grant et al. 2018). We also find a lower mass fraction of smaller grains of MgOlivine. Notably, amorphous SiO2 is detected only marginally. For crystalline silicates, we only see large grains of forsterite (13% of ∼5 μm size) which have very weak and flat features. No enstatite was detected in contrast to what is seen in other BD disks (Apai et al. 2005; Merín et al. 2007).

5.2. Molecular inventory

The best fit models for each molecule are presented in Figure 5 together with the windows used for the χ2 minimization. Zoom-ins of some molecules can be seen in Figures 6–8. The best-fitting parameters are summarized in Table 2.

thumbnail Fig. 6.

Zoom-in of the 13.7 μm region. The upper panel shows the fit using two C2H2 slab models with optically thick (dark brown) and thin (light brown) emission. Two peaks at 13.695 μm and 13.732 μm associated with 13CCH2 can be clearly distinguished. The lower panel shows the best fit when using just one slab model (T = 325 K, N = 3.1 × 1018 cm−2, and R = 0.07 au).

C2H2 and 13CCH2: As mentioned before, the Q branch of C2H2 at 13.7 μm associated with the ν5 bending mode is the most prominent molecular feature in the entire MIRI-MRS spectrum. This is also the only feature that has been seen in previous low resolution data from Spitzer. At a spectral resolving power of ∼2500, MIRI-MRS also reveals the shortward peaks at 13.63 and 13.68 μm due to hot bands that were blended with the main peak in lower resolution Spitzer spectra. Previous observations of VLMS have shown that, in some cases, an optically thick component forms a pseudo-continuum of molecular emission (e.g., Tabone et al. 2023; Arabhavi et al. 2024; Kanwar et al. 2024a). Thus we treated the C2H2 molecule as if it was two different species fitted simultaneously but we limit the parameter space of the column density of the first model to larger column densities (N > 1019 cm−2, which is when the molecular pseudo-continuum appears for the range of temperatures explored) and the second model to lower column densities. We assume that the temperature is the same for both models and let it vary between 250 and 450 K. Figure 6 shows that the overall shape of the 13.7 μm feature can be well fitted with a model with a larger column density which reproduces well the molecular pseudo-continuum and another model with lower column density which reproduces the peaks of the Q-branches. The lower panel Fig. 6 shows, for comparison, the best fit model when using just one slab for the fit. In addition, two peaks at 13.695 and 13.732 μm associated with 13CCH2 can be clearly seen. Our fit does include the contribution of 13CCH2 assuming a C2H2/13CCH2 ratio of 35 (Woods & Willacy 2009) but the models cannot fully reproduce the observed spectrum due to incomplete molecular data. Proper modeling must await more complete molecular spectroscopy for this C2H2 isotopologue.

HCN: The ro-vibrational band from the fundamental ν2 bending mode of HCN at 14.0 μm and the hot band at 14.3 μm are both easily distinguished in Fig. 5 and are well fit by a slab of gas at a warmer temperature (in the range 300–800 K). However we note that there are strong P-branch emission lines of C2H2 in the same wavelength range as HCN and C2H2 has been fitted first and the result subtracted from the MIRI spectrum. We have done the exercise of fitting first HCN and the obtained results differ by 50–100 K, a smaller error than the degeneracy that can be seen in the χ2 map in Fig. 4.

Pascucci et al. (2009) found that the the emission ratio of HCN relative to C2H2 in the disk surface of cool stars was weaker compared to the T Tauri disks. They could not detect HCN in the Spitzer spectrum of Cha Hα 1 but, with the MIRI spectrum, we can confirm that the C2H2 /HCN line flux ratio is larger than unity as expected for cool stars instead of smaller than unity as seen in T Tauri stars. The HCN emission in Cha Hα 1 is much larger, relative to C2H2 than in the VLMS 2MASS J16053215-1933159 (hereafter J1605; Tabone et al. 2023) or ISO-ChaI 147 (Arabhavi et al. 2024) that have a stronger molecular continuum and higher stellar luminosities.

CO2 and 13CO2: Figure 5 includes the fit to the CO2 bending mode at 14.98 μm that is clearly detected. The Q branch of 13CO2 at 15.4 μm is also clearly detected. The isotopologue 13CO2 has been also recently detected in other protoplanetary disks (e.g., Grant et al. 2023; Arabhavi et al. 2024; Kanwar et al. 2024a; Vlasblom et al. 2024). We use a slab model with both species included using the standard CO2/13CO2 ratio of 70 from the interstellar medium (Milam et al. 2005). The best fit indicates cooler gas at 175 K and larger radii (∼0.15 au) than the other molecules. The χ2 map in Fig. 4 shows that the gas temperature is well constrained below 250 K.

C6H6: This is the largest molecule that we have detected. We identified two features around 14.85 μm to be Q branches of the fundamental and hot bending mode ν4 of benzene, C6H6, presented in Fig. 5. Their relative intensity is sensitive to temperature and indicates gas at T = 225 K. However, we note that in this case, even when C6H6 is clearly detected, the column density is degenerate (see Fig. 4) and our best fit value shown in Table 2 is likely an upper limit.

C3H4 and C4H2: The CH bending mode ν9 at ∼15.80 μm for C3H4 is tentatively detected (Fig. 7) while the CH bending mode ν5 at 15.92 μm for C4H2 stands out more clearly. Since the emission of both species almost completely overlap, we fitted both molecules simultaneously leaving the column density as a free parameter and letting the temperature vary between 175 and 350 K. In Fig. 7, we show the MIRI spectrum in the range of wavelengths where C3H4 and C4H2 emit. In the case of C3H4, the strength of the features to fit are similar to the residuals and, according to our criteria, we consider it a tentative detection. For C4H2 there are several features and the strength of the main one is over 3σ; so we consider it a real detection.

thumbnail Fig. 7.

Zoom-ins of slab model fits of C3H4 and C4H2, C2H4, C2H6, and H2O. We note that the slab models in the first panel are stacked one on top of the other. The parameters of the slab models shown can be found in Table C.1

C2H4: A zoom-in of the 10–11 μm region can be seen in Fig. 7 showing the feature at 10.53 μm. We note that while the χ2 map in Fig. 4 is quite well constrained, this spectral region can be affected by the broad silicate emission, and thus the uncertainty in the derived parameters can be higher that that obtained from the χ2 maps. In fact, the slab model fits well the main peaks of C2H4 but following our conservative approach, and because there are multiple unidentified wiggles between the five peaks of C2H4 that are almost as bright than few C2H4 features, we claim a tentative detection.

C2H6: The emission of C2H6 can be seen in Fig. 7 where the line at 12.279 μm is H2 S(2). We set a window to evaluate the fit so it avoids this line. However, since the emission of this molecule is of order of the residuals and it is affected by the emission of C2H2 and C2H4, we consider it a tentative detection.

CH4: Figure 5 shows the Q branch of the ν4 mode of CH4 around 7.66 μm and Figure 8 is a zoom-in of that region. CH4 was previously seen in the GV Tau N (0.8 M⊙) T Tauri disk in absorption (Gibb & Horne 2013) and more recently in emission in the very low mass disks of J1605 (tentative detection: Tabone et al. 2023), ISO-ChaI 147 (Arabhavi et al. 2024), Sz28 (Kanwar et al. 2024a), and TWA 27 A (Patapis et al. 2025). In the case of ISO-ChaI 147, CH4 is the molecule with the largest column density and it forms a molecular continuum. We note that C2H2 and HCN also have some emission lines in this region, but cannot reproduce by themselves this broad feature.

thumbnail Fig. 8.

Zoom-in of the 7.5 μm region. Stacked slab model fits of HCN, CH4, C2H2, C2H6, and H2O are shown. The dark and light brown colors for C2H2 + 13CCH2 represent the optically thick and thin models, respectively.

H2O: Figure 7 shows the detection of the ro-vibrational band of water in the 6 μm region. For this detection we have used the detection criteria used in Arabhavi et al. (2025b): a detection is real if there are at least 5 lines detected in the wavelength range between 6.5 and 6.7 μm. The fit is degenerate and, while we can provide a probable temperature of T = 500 K, the lines in the region are not sensitive to column density. This is the first BD disk for which ro-vibrational water is detected and the only source for which several hydrocarbons have been detected along with water (Arabhavi et al. 2025b). We do not detect the pure rotational water emission seen in some VLMS between 16 and 18 μm.

CH3: Figure 5 shows our CH3 detection. We used the molecular data from Helmich (1996); however, it is incomplete and we could not fit the emission from this molecule in the same way as we did for the other species. We show a model with 400 K to confirm the detection.

H2: In addition to the many other features, several H2 pure rotational (ν = 0 – 0) lines are clearly identified in the MIRI-MRS spectrum (see Figs. 5 and 9), in particular, S(1)-S(3) and S(5) are clearly detected, while S(4) is only tentative. The emitting region seems to be unresolved; however some extended emission is barely noticeable in Figure D.1. Hence, the emission is most probably coming from the very inner, warm region of the disk.

thumbnail Fig. 9.

Zoom-in of the positions of the H2 pure rotational lines in the Cha Hα 1 MIRI spectrum. The position of the H2 lines are marked with dashed lines. The red profiles show line fitting using a Gaussian function. The emission from other molecules is marked in color in each panel, and it is based on slab models reported in the Tables 2 and C.1.

As can be seen in Figure 9, S(4) and S(5) are heavily contaminated by the emission of other molecules, mainly CH4 and water, respectively. S(3) also falls on top of the C2H4 emission. We note that while we plot a slab model of the C2H4 emission in Figure 9, mid panel, the fit of the model was not well constrained. The same is applicable to the S(2) transition (that falls on top of the C2H6 emission). An in depth modeling of these emissions is needed in order to separate the emission of H2 and calculate accurate fluxes.

To measure preliminary line fluxes we follow Franceschi et al. (2024) who performed a similar analysis for J1605. The H2 lines were fitted in the continuum subtracted spectrum using a Gaussian profile. This was done using the Levenberg-Marquardt least-squares minimization. The H2 line flux was then calculated by integrating the Gaussian component. The line profiles and the best-fit profiles are shown in Fig. 9, and the line parameters are reported in Table 3.

Table 3.

Measured parameters of the identified pure rotational molecular hydrogen lines.

Finally, there are clear emission features in Fig. 5 at ∼10 μm and long ward of the CH3 (∼16.5 μm) emission, which correspond to emission that we have not been able to ascribe to any molecule.

6. Discussion

6.1. Dust in the disk of Cha Hα 1

The features present in the MIRI spectrum probe dust emission originating essentially from large silicate dust grains located in the optically thin surface layers of the inner disk indicating that grain growth is already underway at an age of ∼1.5 Myr. The dominant grain size is found to be ∼4 μm which is in the larger side if we compare it with the sample of T Tauri stars from several associations studied by Oliveira et al. (2011). If we compare Cha Hα 1 with the other disks around BDs which have been observed with MIRI, that is TWA 27 A (Patapis et al. 2025) and J0438 (Perotti et al. 2025), the first thing to note is the difference in the dust features. TWA 27 A shows a complete absence of silicate features implying that either it is dust-depleted or the grains have grown to large sizes (> 5 μm). In either case the dust opacity will be very low and we can see deeper into the disk, closer to the mid plane. J0438 has a highly inclined disk, and thus some of the silicate features turn into absorption, resulting in a smaller 10 μm feature. Apart from the combined (absorption and emission) silicate feature, the overall spectrum shape is smooth, indicating dominant amorphous silicates and no significant crystalline features. Trying to find a similar spectral shape to that of Cha Hα 1, we also compared it with Sz28 (Kanwar et al. 2024a; Kaeufer et al. 2024). Sz28 is a VLMS (M5.5) with a MIRI spectrum and it does show a 10 μm feature though less prominent than that of Cha Hα 1. Its dust emission is also dominated by large (∼5 μm) grains. This aligns with the finding that the region of a protoplanetary disk traced by the 10 μm silicate emission features is linked to the luminosity of the central object (Kessler-Silacci et al. 2007). Consequently, the grain sizes inferred from these features may primarily reflect the location of the silicate emission zone, rather than the overall properties of the disk’s silicate dust. In VLMs and BDs, this implies we are observing dust from a region closer to the star compared to their higher mass counterparts. The crystallinity fraction derived is in the range of that observed by Oliveira et al. (2011). Sz28 shows a larger fraction (20% vs. 13% for Cha Hα 1) than Cha Hα 1. In addition they find a different population of crystalline dust in the warm zone (≤1 AU) emitting at λ ∼ 10 μm and the colder region emitting at λ > 16 μm. We do not see any evidence, when fitting the dust, of an inhomogeneous composition of the dust. Further discussion on dust properties for a sample of VLMSs and BDs which include Cha Hα 1 can be found in Jang et al. (2025).

As a protoplanetary disk evolves, dust grains are believed to grow in size. As they become larger, they are more strongly influenced by gravity, causing them to settle toward the disk’s mid plane and deplete from the upper disk layers (Dullemond & Dominik 2004). During this process, the strength of the silicate emission feature diminishes. In this scenario, Cha Hα 1 may be slightly less evolved than Sz28, still with a flared disk in which dust settling into the mid plane is still in its initial steps. TWA 27 A would be in a more evolved stage when substantial grain growth and dust sedimentation have occurred in the disks.

6.2. Volatile content

The large number of carbon-bearing molecules (nine hydrocarbons plus HCN) detected in Cha Hα 1 points to a carbon-rich inner disk environment. Nevertheless, we also find the presence of H2O, CO2, and 13CO2. The presence of water lines at ∼7 μm points to a reservoir of hot water in the inner disk.

Regarding the temperature differences, hydrocarbons in general have temperatures in the 225–450 K range as have been seen in other MINDS VLM objects and points to a common reservoir for all of them. They emit from a confined area within 0.1 au from the host star while CO2 seems to be emitted from slightly further out or deeper in the disk. On the contrary, HCN is much warmer, probably closer or higher in the disk. This trend in the CO2 is seen also in Sz28 and other VLMS (Arabhavi et al. 2024). Tabone et al. (2023) suggest that a gap in the disk might prevent O-rich icy grains from moving inward, resulting in a high C/O ratio in the inner disk, which enables strong mid-IR emission. If we accept this idea, the outer disk could still maintain a standard C/O ratio, potentially explaining the observed CO2 emission and the fact that it is systematically cooler than the hydrocarbons (at least for these objects). Recently, Kanwar et al. (2024b, 2025) were able to reproduce the molecular emission from different species simultaneously with a thermo-chemical disk model with a gap. In that model, hydrocarbons come from inside the gap, while CO2 emission comes from outside the gap.

If we compare Cha Hα 1 with the other BD disks observed with MIRI, that is J0438 and TWA 27 A, J0438 is peculiar because it resembles the spectra of higher mass T Tauri disks with a volatile reservoir lacking hydrocarbons (except for acetylene, C2H2) and dominated by water. TWA 27 A is hydrocarbon-rich, all detected molecules contain carbon, while CO2 is the only oxygen-bearing molecule. The diversity of hydrocarbons indicates that the C/O ratio is larger than unity. This is also the case of Sz28. In both cases C2H2 has a very high column density and forms a molecular pseudo continuum (as happens with Cha Hα 1) with column densities higher than that of Cha Hα 1. The diversity of molecules in the three objects (TWA 27 A, Sz28, and Cha Hα 1), is similar and all of them show a carbon rich inner disk; however, the dominant species differ. For example TWA 27 A shows C2H4 as the dominant hydrocarbon while in Sz28 this molecule is not detected and for Cha Hα 1 it is only a tentative detection. Among the four objects Cha Hα 1 is the most diverse in molecular species. It presents a similar amount of hydrocarbon molecules detected and, in addition, it also has water. However, we note that Cha Hα 1 is the only source for which the spectrum at short wavelengths, < 7 μm, is not dominated by stellar photospheric absorption (that could mask the presence of warm water in the other sources). In addition, (Arabhavi et al. 2025b) show that bright hydrocarbon emission can outshine emission from large column densities of water.

Mah et al. (2024) found that in VLMSs, very short viscous timescales and close-in ice lines contribute to a high gaseous C/O ratio after about 2 Myr. By including disk substructures, they observed that for low viscosities, the inner disk can remain water-rich for extended periods (at least 5 Myr), regardless of the gap’s depth or formation timing. This could explain water-rich sources like J0438. If, after 2 Myr, the disk starts shifting toward a high C/O ratio, objects like Cha Hα 1–with faint water signatures, various hydrocarbons, and shallow molecular continuum–could be in the midst of this transition. Sz28 and TWA 27 A would be more evolved disks with a high C/O ratio. Lienert et al. (in prep) explore the case when the gap in the disk is due to internal photo evaporation and their model can reproduce a C/O > 1 ratio as inferred in VLMS and BDs when using a reduced photo evaporative mass loss rate, which lead to delayed gap opening times.

6.3. H2 emission

We can use the estimated fluxes to build a rotational diagram (Goldsmith & Langer 1999) to estimate the emitting gas temperature assuming that the H2 emission is in local thermodynamical equilibrium (LTE). Following Franceschi et al. (2024) we obtain the rotation diagram presented in Figure 10. This figure has been produced using the tool pdrtpy, introduced in Berné et al. (2022), Pound & Wolfire (2023). Since the emitting radius of the H2 emission cannot be directly estimated due to its optically thin nature, we assume the H2 emission has the same emitting radius as the C2H2 optically thicker emission, 0.06 au. Using this estimate for the emitting radius, we found a temperature of 566 ± 110 K. This temperature is consistent with the of H2 emission coming from the same region of the disk as hydrocarbons. In addition, the emission is observed at a velocity of 20 ± 6 km s−1 consistent with the rest velocity of the star (16.35 ± 0.63, Joergens 2006). However, due to the uncertainties of the line intensities because of the underlying emission of other molecules, further investigation will have to wait for a proper modeling of all molecules involved. H2 pure rotational lines are found to be relatively weak toward T Tauri stars with respect to emission from other molecules and atoms. This emission was first observed in BD disks by Pascucci et al. (2013) who detected either the S(1) or the S(2) transitions in half of their BD sample with Spitzer/IRS spectra. Among the BD disks with a MIRI spectrum, the three of them show H2 pure rotational emission. J0438 presents five pure rotational bands of H2 with marginally extended emission for the H2 S(1), S (3) and S (5) lines. The authors attribute this emission to a disk wind and/or to the disk surface. For TWA 27 A only H2 S(1) is clearly detected.

thumbnail Fig. 10.

Rotation diagram of H2 pure rotational transitions. A linear fit performed on the estimated fluxes is shown as a single component for the emitting gas with a temperature of about 570 K.

7. Conclusions

We have used JWST to probe the inner, dense, and warm region of the disk around the BD Cha Hα 1, where small planets may form. We have used the 5–26 μm spectrum to investigate the dust and gas emission and we draw the following conclusions:

  • Cha Hα 1 presents a strong silicate feature. The dust is primarily composed of large (∼4 μm) amorphous grains, indicating that grain growth has already begun by ∼1.5 Myr, consistent with previous studies. Amorphous SiO2 is barely detected. For crystalline silicates, only large grains of forsterite are observed. Unlike other BD disks, there is no reliable detection of enstatite. No PAH features are found either.

  • The spectrum exposes numerous molecular features, with particularly strong emission of the organic molecule C2H2 at 13.7 μm. Additional molecular features include various hydrocarbons (C6H6, C3H4, C4H2, C2H4, C2H6, CH4, and CH3), as well as CO2, HCN, H2, and H2O. Two isotopologues, 13CCH2 and 13CO2, have also been identified. The diversity of carbon-bearing molecules suggests a carbon-rich inner disk environment, as observed in the BD TWA 27 A (Patapis et al. 2025) and some VLMSs (Tabone et al. 2023; Arabhavi et al. 2024; Kanwar et al. 2024a). However, this is the only case where warm water is detected alongside all the hydrocarbons. We do not detect CO or OH.

  • The hydrocarbons exhibit temperatures between 225–450 K, suggesting they originate from a shared reservoir. They emit from a compact region within 0.1 au of the host star. In contrast, CO2 appears to emit from farther out and/or deeper within the disk, while HCN is significantly warmer, likely indicating its emitting area is closer to the star or higher up in the disk.

  • Several H2 pure rotational lines are clearly identified. The gas temperature and velocity point towards the emission most probably coming from the very inner, warm surface of the disk, the same region of the disk as hydrocarbons.

  • Based on models by Mah et al. (2023) and Kanwar et al. (2024a) we hypothesize that Cha Hα 1–with faint water signatures, various hydrocarbons, shallow molecular continuum in addition to a strong silicate feature– could be transitioning from a very young disk rich in water to a more evolved disk with a high C/O ratio.

The spectrum revealed by JWST/MIRI for Cha Hα 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 H2, and water. The rich molecular environment presents a unique opportunity to test our understanding of disk chemistry and its influence on potential planet formation. Gaining insight into the reservoirs of the detected molecules can further illuminate the composition of future planets forming around BDs.

Acknowledgments

We are grateful to the referee, Dr. Ralf Siebenmorgen, for his useful comments and suggestions to the original manuscript. This work is based on observations made with the NASA/ESA/CSA James Webb Space Telescope. The data were obtained from the Mikulski Archive for Space Telescopes at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS 5-03127 for JWST. These observations are associated with program #1282. The specific observations analyzed can be accessed via doi: 10.17909/1wqf-3m04. The following National and International Funding Agencies funded and supported the MIRI development: NASA; ESA; Belgian Science Policy Office (BELSPO); Centre Nationale d’Etudes Spatiales (CNES); Danish National Space Centre; Deutsches Zentrum fur Luft- und Raumfahrt (DLR); Enterprise Ireland; Ministerio De Economía y Competividad; Netherlands Research School for Astronomy (NOVA); Netherlands Organisation for Scientific Research (NWO); Science and Technology Facilities Council; Swiss Space Office; Swedish National Space Agency; and UK Space Agency. M.M.C. and D.B. have been funded by grants No. PID2019-107061GB-C61 and PID2023-150468NB-I00 by the Spain Ministry of Science, Innovation/State Agency of Research MCIN/AEI/ 10.13039/501100011033 and by “ERDF A way of making Europe. A.C.G. acknowledges support from PRIN-MUR 2022 20228JPA3A “The path to star and planet formation in the JWST era (PATH)” funded by NextGeneration EU and by INAF-GoG 2022 “NIR-dark Accretion Outbursts in Massive Young stellar objects (NAOMY)” and Large Grant INAF 2022 “YSOs Outflows, Disks and Accretion: towards a global framework for the evolution of planet forming systems (YODA)”. G.P. gratefully acknowledges support from the Carlsberg Foundation, grant CF23-0481 and from the Max Planck Society. E.v.D. acknowledges support from the ERC grant 101019751 MOLDISK and the Danish National Research Foundation through the Center of Excellence “InterCat” (DNRF150). T.H. and K.S. acknowledge support from the European Research Council under the Horizon 2020 Framework Program via the ERC Advanced Grant Origins 83 24 28. I.K., A.M.A., and E.v.D. acknowledge support from grant TOP-1 614.001.751 from the Dutch Research Council (NWO). I.K., and J.K. acknowledge funding from H2020-MSCA-ITN-2019, grant no. 860470 (CHAMELEON). T.K. acknowledges support from STFC Grant ST/Y002415/1. B.T. is a Laureate of the Paris Region fellowship program, which is supported by the Ile-de-France Region and has received funding under the Horizon 2020 innovation framework program and Marie Sklodowska-Curie grant agreement No. 945298. O.A. and V.C. acknowledge funding from the Belgian F.R.S.-FNRS. I.A., D.G. and B.V. thank the Belgian Federal Science Policy Office (BELSPO) for the provision of financial support in the framework of the PRODEX Programme of the European Space Agency (ESA). L.C. acknowledges support by grant PIB2021-127718NB-I00, from the Spanish Ministry of Science and Innovation/State Agency of Research MCIN/AEI/10.13039/501100011033. T.P.R acknowledges support from ERC grant 743029 EASY. D.R.L. acknowledges support from Science Foundation Ireland (grant number 21/PATH-S/9339). M.T., M.V. and A.D.S acknowledge support from the ERC grant 101019751 MOLDISK. VC thanks the Belgian F.R.S.-FNRS, and the Belgian Federal Science Policy Office (BELSPO) for the provision of financial support in the framework of the PRODEX Programme of the European Space Agency (ESA) under contract number 4000142531.

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Appendix A: Continuum definition

For gas fitting using the 0D slab models, the continuum must be more accurately determined than what a dust fitting tool–constrained by its opacity and grain size assumptions–can provide. Therefore, we did not use the output from the dust retrieval code shown in Figure 3 directly, as its continuum slightly overestimates the observed data at several wavelengths. Additionally, we suspected the presence of a molecular pseudo-continuum. To address this, we adopted a continuum determination procedure based on the method described in Temmink et al. (2024), ensuring a more reproducible and physically consistent continuum estimate. This procedure estimates the dust continuum emission using the pybaselines package (Erb 2022). Initially, a dust continuum level is estimated using a Savitzky-Golay filter with a third order polynomial. Then, all emission lines extending above 2σ of the standard deviation are masked until only the continuum remained. We explored different widths of the Savitzky-Golay filter. Larger widths would not trace well the area around 10 μm, due the shape of the silicate emission bump. Thus we used a small width (blue dashed line in Figure A.1); however subtracting the continuum defined with the narrow width subtracts also the molecular pseudo-continuum. In order to retain the molecular-pseudo-continuum we defined two windows around 7.7 and 14 μm and use a cubic-spline function to interpolate between the ends of those windows. The resulting continuum is the blue line in Figure A.1.

thumbnail Fig. A.1.

Continuum determination is shown in the top panel. The lower panels are zooms of the regions around 7.7 μm (left) and 14 μm (right). The red line represents the dust fit performed in Section 3. The dashed blue line is the continuum resulting from using a small width Savitzky-Golay filter. The final masked continuum adopted in this work is the solid blue line while the windows masked are represented by horizontal lines at the bottom of each panel

Appendix B: Continuum test

Because the continuum definition can be highly subjective when relying on a visual identification of line-free regions, we opted to choose a more reproducible approach following the method described in Temmink et al. (2024). Nevertheless, during the initial fitting, the presence of a molecular pseudo-continuum became evident, needing the use of the windowing technique outlined in Appendix A to fit the spectral features. However, the definition of this window still involves a degree of subjectivity. In order to investigate the impact in our results when using different continuum definitions we explored several window sizes in the spectral region around 14 μm. The results corresponding to the largest window tested are presented in Figure B.1, which shows both the continuum and continuum subtracted spectra, and in Figure B.2, which displays the fit of C2H2. We present the results obtained for the fitting with the tested continuum in Table B.1 in order to compare them with the parameters obtained in this work. As a result from this test we find that the same molecules are detected, or tentatively detected, in all cases and the maximum change in the parameters is 100 K and 1 order of magnitude in temperatures and column densities respectively.

Table B.1.

Comparison of parameters derived in this work with the ones obtained from the continuum test.

thumbnail Fig. B.1.

Continuum test. The upper panel shows the continuum used in this work (blue line) compared to the continuum defined with the largest window tested (green line). The windows used in each case are shown as horizontal lines. The lower panel shows the continuum subtracted spectra with the same code of colors.

thumbnail Fig. B.2.

Continuum test. Comparison of the C2H2 slab model result from the best fit used in this work (upper panel) and using the continuum shown in green in Fig. A.1 (lower panel). Both panels show a fit using two C2H2 slab models with higher (dark brown) and lower (light brown) column densities.

Appendix C: Molecules with no quantitative information

For some molecules, when slab model fits were attempted, the remaining degeneracies in column density and temperature were too significant to allow for a meaningful analysis. Those molecules are labeled as "Detected" or "Tentative detection" in Table 2. We provide in Table C.1 the parameters of the slab models used to plot the emission of these molecules in this work. We note that in some cases, such as water, the degeneracy is very large (see the χ2 maps in Fig. 4). In the case of CH3 we did not fit the molecule due to limitations defined in Sect. 5.2.

Table C.1.

Temperatures, column densities, and emitting radii of the slab models used to show the "detected" and "tentatively detected" molecules in the spectrum of Cha Hα 1.

Appendix D: H2 emission

The emission of the S(1) and S(2) H2 lines may be somewhat extended. In Fig. D.1 we show IFU aligned images of the H2 S(1) line. The panel on the left shows a wavelength with no emission, the central panel shows the emission at the H2 S(1) line wavelength and in the right panel we see the difference between both images. A structure is hinted in the lower right part of the image. Another indication that the structure may be real is that these lines appear much stronger (while all other lines in the spectrum stay the same) if we use a background subtraction method such as sdither. In this case we leverage the four-point dither pattern to obtain a first guess on the background map, then refine it using a median filter which both smooths the background estimate and removes residual star signals from it. When using our preferred background subtraction method, ddither, we performed a direct pairwise dither subtraction, and thus, we could be removing part of this emission making the line in the spectrum to show fainter.

thumbnail Fig. D.1.

IFU aligned images of the H2 S(1) line. The panels on the left show wavelengths with no emission at 17.009 μm (top) and 12.829 μm (bottom). The right panels show the emission at the line rest-wavelengths for H2 S(1) at the top and H2 S(2) at the bottom.

All Tables

Table 1.

Median continuum flux, estimated S/N from the ETC, and 1σ- noise for each band where molecules are detected.

In the text
Table 2.

Temperatures, column densities, and emitting radii of the detected molecules obtained from the slab model fits.

In the text
Table 3.

Measured parameters of the identified pure rotational molecular hydrogen lines.

In the text
Table B.1.

Comparison of parameters derived in this work with the ones obtained from the continuum test.

In the text
Table C.1.

Temperatures, column densities, and emitting radii of the slab models used to show the "detected" and "tentatively detected" molecules in the spectrum of Cha Hα 1.

In the text

All Figures

thumbnail Fig. 1.

MIRI Cha Hα 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 Hα 1 low resolution spectrum obtained from CASSIS (Lebouteiller et al. 2011).
In the text
thumbnail Fig. 2.

Comparison between spectra before and after the residual gas subtraction. The blue line is the rebinned (R ∼ 500) MINDS data, and the orange line is the first dust fitting. The green line corresponds to the C2H2 subtracted data.
In the text
thumbnail Fig. 3.

Dust fitting results after subtracting gas emission around 14 μm. The shaded area represents 1σ uncertainties. The bottom panel shows the residuals of the fit. The effect of subtracting the C2H2 molecular emission can be seen at ∼14 μm.
In the text
thumbnail Fig. 4.

The χ2 maps for HCN, CO2 + 13CO2, C6H6, C2H4, C2H6, CH4, and water. The color scale shows the χmin2/χ2. The best fit model (χmin2/χ2 = 1) is marked with a red circle. A red contour denotes the emitting region radius as listed in Table 2. Gray contours show the emitting radii in au while white contours represent the 1σ, 2σ, and 3σ levels.
In the text
thumbnail Fig. 5.

Cha Hα 1 spectrum, with the JWST-MIRI continuum subtracted data (black) compared to the stacked emission from the slab models for C2H2 + 13CCH2, HCN, CO2 + 13CO2, C6H6, C3H4, C4H2, C2H4, C2H6, CH4, H2O, and CH3. The parameters of the slab models shown can be found in Tables 2 and C.1. The H2 lines are also marked. Horizontal lines represent the windows in which the χ2 for each fitting has been evaluated. We note that the y axis is different for each panel.
In the text
thumbnail Fig. 6.

Zoom-in of the 13.7 μm region. The upper panel shows the fit using two C2H2 slab models with optically thick (dark brown) and thin (light brown) emission. Two peaks at 13.695 μm and 13.732 μm associated with 13CCH2 can be clearly distinguished. The lower panel shows the best fit when using just one slab model (T = 325 K, N = 3.1 × 1018 cm−2, and R = 0.07 au).
In the text
thumbnail Fig. 7.

Zoom-ins of slab model fits of C3H4 and C4H2, C2H4, C2H6, and H2O. We note that the slab models in the first panel are stacked one on top of the other. The parameters of the slab models shown can be found in Table C.1
In the text
thumbnail Fig. 8.

Zoom-in of the 7.5 μm region. Stacked slab model fits of HCN, CH4, C2H2, C2H6, and H2O are shown. The dark and light brown colors for C2H2 + 13CCH2 represent the optically thick and thin models, respectively.
In the text
thumbnail Fig. 9.

Zoom-in of the positions of the H2 pure rotational lines in the Cha Hα 1 MIRI spectrum. The position of the H2 lines are marked with dashed lines. The red profiles show line fitting using a Gaussian function. The emission from other molecules is marked in color in each panel, and it is based on slab models reported in the Tables 2 and C.1.
In the text
thumbnail Fig. 10.

Rotation diagram of H2 pure rotational transitions. A linear fit performed on the estimated fluxes is shown as a single component for the emitting gas with a temperature of about 570 K.
In the text
thumbnail Fig. A.1.

Continuum determination is shown in the top panel. The lower panels are zooms of the regions around 7.7 μm (left) and 14 μm (right). The red line represents the dust fit performed in Section 3. The dashed blue line is the continuum resulting from using a small width Savitzky-Golay filter. The final masked continuum adopted in this work is the solid blue line while the windows masked are represented by horizontal lines at the bottom of each panel
In the text
thumbnail Fig. B.1.

Continuum test. The upper panel shows the continuum used in this work (blue line) compared to the continuum defined with the largest window tested (green line). The windows used in each case are shown as horizontal lines. The lower panel shows the continuum subtracted spectra with the same code of colors.
In the text
thumbnail Fig. B.2.

Continuum test. Comparison of the C2H2 slab model result from the best fit used in this work (upper panel) and using the continuum shown in green in Fig. A.1 (lower panel). Both panels show a fit using two C2H2 slab models with higher (dark brown) and lower (light brown) column densities.
In the text
thumbnail Fig. D.1.

IFU aligned images of the H2 S(1) line. The panels on the left show wavelengths with no emission at 17.009 μm (top) and 12.829 μm (bottom). The right panels show the emission at the line rest-wavelengths for H2 S(1) at the top and H2 S(2) at the bottom.
In the text

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