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Accretion Properties of the Young Brown Dwarf 2MASS J08440915-7833457

Toni V. Panzera, Laura S. Flagg, Margaret A. Mueller, Christopher M. Johns-Krull, Gregory J. Herczeg

TL;DR

This work investigates magnetospheric accretion in a young brown dwarf by combining UV and optical observations of 2MASS J08440915-7833457. Through FUV line-profile analyses (C IV, Si IV, N V) and H2 fluorescence modeling, alongside optical continuum fitting with a hydrogen slab, the study derives stellar parameters, inner-disk radii, and an accretion rate. It finds that C IV emission in this brown dwarf is narrower than typical CTTSs, and the L_CIV/L_acc relation extrapolated from higher masses overestimates Ṁacc at the BD/planetary boundary, suggesting BD-specific shock structures. H2 emission originates inside the co-rotation radius, indicating disc truncation interior to corotation and a truncated inner disk. Overall, the results extend accretion understanding into the lowest mass regime and highlight the need for mass- and age-dependent calibrations of UV accretion diagnostics for brown dwarfs and forming planets.

Abstract

We present HST-COS FUV and -STIS optical observations towards the young accreting brown dwarf 2MASS-J08440915-7833457 (J0844) from the ULLYSES DDT Program. We analyse hot FUV lines such C IV, Si IV, and N V, as well as fluorescent emission from H2. Despite evidence for accretion, the C IV line profiles are narrower than in typical classical T Tauri stars (CTTSs), resembling weak-lined T Tauri stars more closely. Additionally, the C IV integrated line flux does not follow the level expected of an accreting object in the magnetically saturated regime. However, comparing J0844 to appropriate low mass analogs, J0844 does show excess C IV emission characteristic of accretion, suggesting the magnetic saturation level may need to be redefined for the lowest mass objects. The C IV/Si IV emission line ratio is found to be 20, which is higher than most CTTSs, with a few exceptions (e.g., TW Hya). We fit the STIS optical spectrum to calculate an accretion rate, which we find to be 4.2 x 10-11 Msol/yr. The accretion rate found based on the empirical LCIV-Macc relationship is twoorders of magnitude higher, suggesting this relationship may not hold at the lowest masses. We find the H2 emission appears to originate within the co-rotation radius, pointing to either disc truncation well inside the co-rotation radius or additional sources of H2 emission that we do not consider (e.g., from the accretion flow itself). These data provide an extension of our current understanding of accretion and inner disc conditions to the relatively unexplored lowest mass regime.

Accretion Properties of the Young Brown Dwarf 2MASS J08440915-7833457

TL;DR

This work investigates magnetospheric accretion in a young brown dwarf by combining UV and optical observations of 2MASS J08440915-7833457. Through FUV line-profile analyses (C IV, Si IV, N V) and H2 fluorescence modeling, alongside optical continuum fitting with a hydrogen slab, the study derives stellar parameters, inner-disk radii, and an accretion rate. It finds that C IV emission in this brown dwarf is narrower than typical CTTSs, and the L_CIV/L_acc relation extrapolated from higher masses overestimates Ṁacc at the BD/planetary boundary, suggesting BD-specific shock structures. H2 emission originates inside the co-rotation radius, indicating disc truncation interior to corotation and a truncated inner disk. Overall, the results extend accretion understanding into the lowest mass regime and highlight the need for mass- and age-dependent calibrations of UV accretion diagnostics for brown dwarfs and forming planets.

Abstract

We present HST-COS FUV and -STIS optical observations towards the young accreting brown dwarf 2MASS-J08440915-7833457 (J0844) from the ULLYSES DDT Program. We analyse hot FUV lines such C IV, Si IV, and N V, as well as fluorescent emission from H2. Despite evidence for accretion, the C IV line profiles are narrower than in typical classical T Tauri stars (CTTSs), resembling weak-lined T Tauri stars more closely. Additionally, the C IV integrated line flux does not follow the level expected of an accreting object in the magnetically saturated regime. However, comparing J0844 to appropriate low mass analogs, J0844 does show excess C IV emission characteristic of accretion, suggesting the magnetic saturation level may need to be redefined for the lowest mass objects. The C IV/Si IV emission line ratio is found to be 20, which is higher than most CTTSs, with a few exceptions (e.g., TW Hya). We fit the STIS optical spectrum to calculate an accretion rate, which we find to be 4.2 x 10-11 Msol/yr. The accretion rate found based on the empirical LCIV-Macc relationship is twoorders of magnitude higher, suggesting this relationship may not hold at the lowest masses. We find the H2 emission appears to originate within the co-rotation radius, pointing to either disc truncation well inside the co-rotation radius or additional sources of H2 emission that we do not consider (e.g., from the accretion flow itself). These data provide an extension of our current understanding of accretion and inner disc conditions to the relatively unexplored lowest mass regime.
Paper Structure (15 sections, 9 equations, 12 figures, 12 tables)

This paper contains 15 sections, 9 equations, 12 figures, 12 tables.

Table of Contents

  1. Introduction
  2. Observations
  3. Analysis
  4. Stellar Parameters
  5. The Far-Ultraviolet (FUV) Spectrum
  6. Molecular Hydrogen
  7. Double Gaussian Fit
  8. Stellar and Disc Component Fit
  9. The Optical Spectrum
  10. Results and Discussion
  11. Hot Gas Tracers
  12. Warm Gas Tracer: H$_2$
  13. Accretion
  14. $L_\mathrm{CIV}/L_\mathrm{acc}$
  15. Summary

Figures (12)

  • Figure 1: The ultraviolet spectrum of J0844. The hot FUV lines are indicated, as well as the prominent molecular hydrogen lines. The prominent emission lines at $\lambda 1216$ Å and $\lambda 1302$ Å are the Ly$\alpha$ and O I airglow lines respectively.
  • Figure 2: Left panel: Portion of the lightcurve of J0844 as observed with TESS, showing cyclic variations attributed to rotation. Right panel: Power spectrum obtained through a LombScargle periodogram analysis (2018ascl.soft12013L2018AJ....156..123A) of the left panel. The grey dashed line marks the power spectrum peak, which corresponds to a period of 1.42 days (Table \ref{['tesslog']} and \ref{['tab:periods']}.)
  • Figure 3: Fits to the C IV doublet. The left panel is the first line of the doublet, at $\lambda$1548.2 Å. The right panel is the second line of the doublet, at $\lambda$1550.8 Å. The zero velocity on each panel corresponds to these rest wavelengths. The green and blue curves are the two Gaussian components, and the red curve is the best fit (addition of components convolved with the LSF).
  • Figure 4: Fits to the Si IV doublet. The left panel is the first line of the doublet, at $\lambda$1393.8 Å. The right panel is the second line of the doublet, at $\lambda$1402.8 Å. The zero velocity on each panel corresponds to these rest wavelengths. The blue curve is the C IV template. The H$_2$ lines (green curves) lie at rest wavelengths of $\lambda$1393.73, 1393.96 Å (left panel), and $\lambda$1402.65 Å (right panel). These lines were assumed to be present based on the H$_2$ emission observed in TW Hya (herczeg_farultraviolet_2002). The red curve is the best fit. Given the relatively high noise levels in this region, these data have been binned to 10 km s$^{-1}$ instead of 5 km s$^{-1}$ as is the case for all other emission line plots.
  • Figure 5: Double Gaussian fit to the coadded H$_2$ line profile. The blue curve is the narrow component, and the green curve is the broad component. The red curve is the best fit (addition of components convolved with the LSF). The narrow component has $v = 12.8$$\pm$ 6.0 km s$^{-1}$ and FWHM = 24.2 $\pm$ 9.6 km s$^{-1}$. The broad component has $v = 14.0$$\pm$ 10.7 km s$^{-1}$ and FWHM = 126.5 $\pm$ 23.5 km s$^{-1}$. The fit has $\chi^2 = 29.6$, and $\chi^2_\mathrm{red} = 0.4$.
  • ...and 7 more figures