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Discovery of an Accretion Burst in a Free-Floating Planetary-Mass Object

V. Almendros-Abad, A. Scholz, B. Damian, R. Jayawardhana, A. Bayo, L. Flagg, K. Muzic, A. Natta, P. Pinilla, L. Testi

Abstract

We report the discovery of a long-lasting burst of disk accretion in Cha J11070768-7626326 (Cha 1107-7626), a young, isolated, 5-10 M$_{\mathrm{Jupiter}}$ object. In spectra taken with XSHOOTER at ESO's Very Large Telescope as well as NIRSPEC and MIRI on the James Webb Space Telescope, the object transitions from quiescence in April-May 2025 to a strongly enhanced accretion phase in June-August 2025. The line flux changes correspond to a 6-8-fold increase in the mass accretion rate, reaching $10^{-7}$ M$_{\mathrm{Jupiter}}$yr$^{-1}$, the highest measured in a planetary-mass object. During the burst, the H$α$ line develops a double-peaked profile with red-shifted absorption, as observed in stars and brown dwarfs undergoing magnetospheric accretion. The optical continuum increases by a factor of 3-6; the object is $\sim$1.5-2 mag brighter in the R-band during the burst. Mid-infrared continuum fluxes rise by 10-20%, with clear changes in the hydrocarbon emission lines from the disk. We detect water vapour emission at 6.5-7 $μm$, which were absent in quiescence. By the end of our observing campaign, the burst was still ongoing, implying a duration of at least two months. A 2016 spectrum also shows high accretion levels, suggesting that this object may undergo recurring bursts. The observed event is inconsistent with typical variability in accreting young stars and instead matches the duration, amplitude and line spectrum of an EXor-type burst, making Cha1107-7626 the first substellar object with evidence of a potentially recurring EXor burst.

Discovery of an Accretion Burst in a Free-Floating Planetary-Mass Object

Abstract

We report the discovery of a long-lasting burst of disk accretion in Cha J11070768-7626326 (Cha 1107-7626), a young, isolated, 5-10 M object. In spectra taken with XSHOOTER at ESO's Very Large Telescope as well as NIRSPEC and MIRI on the James Webb Space Telescope, the object transitions from quiescence in April-May 2025 to a strongly enhanced accretion phase in June-August 2025. The line flux changes correspond to a 6-8-fold increase in the mass accretion rate, reaching Myr, the highest measured in a planetary-mass object. During the burst, the H line develops a double-peaked profile with red-shifted absorption, as observed in stars and brown dwarfs undergoing magnetospheric accretion. The optical continuum increases by a factor of 3-6; the object is 1.5-2 mag brighter in the R-band during the burst. Mid-infrared continuum fluxes rise by 10-20%, with clear changes in the hydrocarbon emission lines from the disk. We detect water vapour emission at 6.5-7 , which were absent in quiescence. By the end of our observing campaign, the burst was still ongoing, implying a duration of at least two months. A 2016 spectrum also shows high accretion levels, suggesting that this object may undergo recurring bursts. The observed event is inconsistent with typical variability in accreting young stars and instead matches the duration, amplitude and line spectrum of an EXor-type burst, making Cha1107-7626 the first substellar object with evidence of a potentially recurring EXor burst.

Paper Structure

This paper contains 13 sections, 4 figures.

Table of Contents

  1. Introduction
  2. Observations
  3. X-SHOOTER
  4. JWST
  5. Line emission
  6. Evolution of the line spectrum
  7. Double-peaked H$\alpha$
  8. Accretion rate changes
  9. Spectral energy distribution
  10. Continuum
  11. Hydrocarbon lines
  12. Silicate feature
  13. Summary and outlook

Figures (4)

  • Figure 1: Line profile changes in H$\alpha$ from VLT/XSHOOTER (left) and Paschen $\gamma$ from both XSHOOTER and JWST (right) seen in VLT/XSHOOTER observations of Cha1107-7626. The eight epochs are colour-coded and clearly show the evolution of the burst.
  • Figure 2: Medium-resolution G140H and G235H JWST NIRSPEC spectra from 14/08/2025 (black line) together with the PRISM spectrum from the same epoch (red line). The strongest emission lines detected are identified.
  • Figure 3: Time evolution of the mass accretion rate across all observations seen in H$\alpha$ (blue circles), Paschen $\gamma$ (orange squares). Black diamonds show the average over the following available tracers at each epoch: Paschen $\delta$, Paschen $\beta$, Brackett $\gamma$, and H$\alpha$, each shown with small light gray circles. A small offset in date has been applied for clarity. The Pa$\gamma$ and H$\alpha$ accretion rate error bars represent the uncertainties from the line flux measurements only. Systematic uncertainties in the accretion rate, primarily from the adopted $L_{\rm line}$–$L_{\rm acc}$ relation and stellar parameters, are not shown; these amount to $\sim$0.4 dex.
  • Figure 4: Time evolution of the spectral energy distribution for Cha1107-7626, as observed with JWST NIRSPEC-PRISM and MIRI-LRS, in August 2024 and August 2025. Bottom left and right panels show a zoom to the optical and mid-infrared, respectively. The main mid-infrared features discussed in Section \ref{['sec:sed']} are indicated in the bottom right panel.