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MINDS. Anatomy of a water-rich, inclined, brown dwarf disk: lack of abundant hydrocarbons

Giulia Perotti, Nicolás T. Kurtovic, Thomas Henning, Göran Olofsson, Aditya M. Arabhavi, Kamber Schwarz, Jayatee Kanwar, Roy van Boekel, Inga Kamp, Ilaria Pascucci, Ewine F. van Dishoeck, Manuel Güdel, Pierre-Olivier Lagage, David Barrado, Alessio Caratti o Garatti, Adrian M. Glauser, F. Lahuis, Valentin Christiaens, Riccardo Franceschi, Danny Gasman, Sierra L. Grant, Hyerin Jang, Till Kaeufer, Maria Morales-Calderón, Milou Temmink, Marissa Vlasblom

Abstract

2MASS J04381486+2611399 (or J0438) is one of the few young brown dwarfs (BD) with a highly inclined ($i\!\sim\!70^\circ$) disk. Here we report results from JWST-MIRI MRS, HST-ACS and ALMA Band 7 observations. Despite its late spectral type (M7.25), the spectrum of J0438 resembles those of inner disks around earlier-type stars (K1-M5, T Tauri stars), with a volatile reservoir lacking hydrocarbons (except for acetylene, C$_2$H$_2$) and dominated by water. Other identified species are H$_2$, CO$_2$, HCN, [Ar$^{+}$], and [Ne$^{+}$]. The dominance of water over hydrocarbons is driven by multiple factors such as disk dynamics, young disk age, low accretion rate and possible inner disk clearing. J0438 appears highly dynamic, showing a seesaw-like variability and extended emission in H$_2 \,\,\, S$(1), $S$(3), $S$(5), [Ne$^{+}$] and CO ($J=3-2$). Interestingly, the CO emission reaches up to 400 au from the brown dwarf, suggesting ongoing infalling/outflowing activity impacting the disk chemistry. These observations underscore the combined power of MIRI, HST and ALMA in characterizing the chemical diversity and dynamics of brown dwarf disks.

MINDS. Anatomy of a water-rich, inclined, brown dwarf disk: lack of abundant hydrocarbons

Abstract

2MASS J04381486+2611399 (or J0438) is one of the few young brown dwarfs (BD) with a highly inclined () disk. Here we report results from JWST-MIRI MRS, HST-ACS and ALMA Band 7 observations. Despite its late spectral type (M7.25), the spectrum of J0438 resembles those of inner disks around earlier-type stars (K1-M5, T Tauri stars), with a volatile reservoir lacking hydrocarbons (except for acetylene, CH) and dominated by water. Other identified species are H, CO, HCN, [Ar], and [Ne]. The dominance of water over hydrocarbons is driven by multiple factors such as disk dynamics, young disk age, low accretion rate and possible inner disk clearing. J0438 appears highly dynamic, showing a seesaw-like variability and extended emission in H(1), (3), (5), [Ne] and CO (). Interestingly, the CO emission reaches up to 400 au from the brown dwarf, suggesting ongoing infalling/outflowing activity impacting the disk chemistry. These observations underscore the combined power of MIRI, HST and ALMA in characterizing the chemical diversity and dynamics of brown dwarf disks.

Paper Structure

This paper contains 26 sections, 18 figures, 3 tables.

Table of Contents

  1. Introduction
  2. Observations and data reduction
  3. The mid-infrared spectrum of J0438
  4. Dust continuum
  5. Line detections
  6. Non-detections
  7. Slab modelling
  8. Discussion
  9. Why is the emission of hydrocarbons absent in J0438?
  10. Line fluxes ratios
  11. Disk inclination
  12. Accretion luminosity and millimeter flux
  13. Age
  14. Mass accretion rate
  15. Sub-structures
  16. ...and 11 more sections

Figures (18)

  • Figure 1: JWST-MIRI MRS spectrum of the young J0438 disk (black). The spectrum is dominated by amorphous silicate absorption and emission bands typical of a highly inclined disk configuration. A very weak CO$_2$ ice absorption feature is tentatively detected (see Appendix \ref{['app:cont_sub']}, Fig. \ref{['fig:co2_ice']}) along with inner disk gas reservoir (Table \ref{['tab:fluxes']}) composed by molecular hydrogen (H$_2$), water (H$_2$O), carbon dioxide (CO$_2$), hydrogen cyanide (HCN), acetylene (C$_2$H$_2$) and the singly ionized cations of argon and neon ([Ar$^{+}$], [Ne$^{+}]$; for zoom-ins see Appendices \ref{['app:stellar_contamination']} and \ref{['app:variability']}). The comparison between the JWST-MIRI and the Spitzer-IRS low- (SL, LL) and high- (SH) resolution spectra reveal continuum and line variability (grey and teal). The Spitzer-IRS spectra were taken from CASSIS Lebouteiller2011Lebouteiller2015 and Pascucci2013.
  • Figure 2: Comparison between the total synthetic (red shaded region) and the continuum-subtracted JWST-MIRI spectrum (black). The total synthetic spectrum is a composite of water, C$_2$H$_2$, HCN, CO$_2$, $^{13}$CO$_2$ and OH slab models plotted in several colours (see Section \ref{['sec:slab_modelling']} for more details).
  • Figure 3: Comparison between CO$_2$, HCN and C$_2$H$_2$ flux ratios and H$_2$O flux for a sample of G, K, M-star disks. The values for G, K and early M-star disks are scaled from banzatti2020Xie2023 and for mid-to-late M-star disks from Pascucci2013 and Arabhavi2025b and color-coded according to the spectral type (SpTy): from G (blue) to M (red). J160532 (M4.5), Sz 114 (M5), Sz 28 (M5.25), ISO-ChaI 147 (M.5.75) and J0438 (M7.25) are labelled. Arrows are upper limits. J1605 is not included in the middle panel because HCN was not detected in this disk tabone2023. We also highlighted RY Tau (K1), a T Tauri disk with a disk inclination comparable to J0438 ($i\sim65^\circ$). For the comparisons, the water complex at $17.22~\mu$m is used and the fluxes are scaled to 140 pc using updated Gaia DR3 distances collected in Manara2023. Adapted from Xie2023.
  • Figure 4: Comparison between flux at 0.89 mm scaled to 140 pc and accretion luminosity. The value are color-coded according to the spectral type (SpTy) from G (blue) to M (red). The symbols corresponding to J160532 (M4.5), Sz 114 (M5), Sz 28 (M5.25), ISO-ChaI 147 (M5.75) and J0438 (M7.25) are surrounded by black edges. The values are from Pascucci2009aPascucci2016banzatti2020Xie2023Franceschi2024. Arrows represent upper limits. J0438 shows weak accretion luminosity and millimeter flux in agreement with late M-star disks. Adapted from Xie2023.
  • Figure 5: Schematic illustration of the inner disk around the young brown dwarf J0438. The inner disk of J0438 is characterized by faint water vapour and not by hydrocarbon-rich gas despite of the late spectral type (M7.25). The possible presence of an inner hole at $\sim0.28~$au reported by Luhman2007 and the low mass accretion rate ($\sim 10^{-11} M_\odot$ yr$^{-1}$; Muzerolle2005) are likely responsible for slowing down the inward drift of icy dust grains/pebbles and hence for prolonging a water-rich phase for this disk. ALMA observations resolved extended emission from the BD disk suggesting ongoing outflowing/infalling activity. Image credit: A. Houge.
  • ...and 13 more figures