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Gray Spectral Variability in Three Brown Dwarfs Observed by HST/WFC3 Time-Series Observations

Madalyn F. Chapleski, Yifan Zhou

TL;DR

This work analyzes time-series spectra from HST/WFC3 for three variable L/T transition brown dwarfs plus a well-known benchmark, focusing on the 1.10–1.65 $\mu$m range to probe cloud and chemical inhomogeneities. By fitting a two-component heterogeneous atmosphere drawn from the SONORA Diamondback grid, the authors quantify how patchy clouds and temperature contrasts drive the observed variability, using $F_{\rm total}=(1-\alpha)F_{\rm base}+\alpha F_{\rm patch}$ and a nested-sampling fit for $T_{\mathrm{eff}}$, $f_{\mathrm{sed}}$, and related parameters, followed by $\chi^2$ minimization for patch properties. The results reveal object-dependent variability mechanisms: 2MASS J2139 is dominated by cloud-thickness changes with a large patch fraction, J1629 and J0758 favor small cooler patches with thicker clouds, and J1126 shows a different driver with identical $f_{\mathrm{sed}}$ between base and patch. Color–magnitude analyses in synthetic HST bands indicate largely gray variability with diverse trajectories, implying multiple processes (clouds, chemistry, possibly high-altitude hazes) shape L/T transition variability and emphasizing the need for broad-wavelength, multi-object studies to constrain atmospheric models for brown dwarfs and exoplanet analogs.

Abstract

The L/T transition is a critical evolutionary stage for brown dwarfs and self-luminous giant planets. L/T transition brown dwarfs are more likely to be spectroscopically variable, and their high-amplitude variability probes distributions in their clouds and chemical makeup. This paper presents Hubble Space Telescope Wide Field Camera 3 spectral time series data for three variable L/T transition brown dwarfs and compares the findings to the highly variable benchmark object 2MASS J2139. All four targets reveal significant brightness variability between 1.1 to 1.65 micron but show a difference in wavelength dependence of the variability amplitude. Three of our targets do not show significant decrease in variability amplitude in the 1.4 $μ$m water absorption band commonly found in previous studies of L/T transition brown dwarfs. Additionally, at least two brown dwarfs have irregular-shaped, non-sinusoidal light curves. We create heterogeneous atmospheric models by linearly combining SONORA Diamondback model spectra, comparing them with the observations, and identifying the optimal effective temperature, cloud opacity, and cloud coverage for each object. Comparisons between the observed and model color-magnitude variations that trace both spectral windows and molecular features reveal that the early- T dwarfs likely possess heterogeneous clouds. The three T dwarfs show different trends in the same color-magnitude space which suggests secondary mechanisms driving their spectral variability. This work broadens the sample of L/T transition brown dwarfs that have detailed spectral time series analysis and offers new insights that can guide future atmospheric modeling efforts for both brown dwarfs and exoplanets.

Gray Spectral Variability in Three Brown Dwarfs Observed by HST/WFC3 Time-Series Observations

TL;DR

This work analyzes time-series spectra from HST/WFC3 for three variable L/T transition brown dwarfs plus a well-known benchmark, focusing on the 1.10–1.65 m range to probe cloud and chemical inhomogeneities. By fitting a two-component heterogeneous atmosphere drawn from the SONORA Diamondback grid, the authors quantify how patchy clouds and temperature contrasts drive the observed variability, using and a nested-sampling fit for , , and related parameters, followed by minimization for patch properties. The results reveal object-dependent variability mechanisms: 2MASS J2139 is dominated by cloud-thickness changes with a large patch fraction, J1629 and J0758 favor small cooler patches with thicker clouds, and J1126 shows a different driver with identical between base and patch. Color–magnitude analyses in synthetic HST bands indicate largely gray variability with diverse trajectories, implying multiple processes (clouds, chemistry, possibly high-altitude hazes) shape L/T transition variability and emphasizing the need for broad-wavelength, multi-object studies to constrain atmospheric models for brown dwarfs and exoplanet analogs.

Abstract

The L/T transition is a critical evolutionary stage for brown dwarfs and self-luminous giant planets. L/T transition brown dwarfs are more likely to be spectroscopically variable, and their high-amplitude variability probes distributions in their clouds and chemical makeup. This paper presents Hubble Space Telescope Wide Field Camera 3 spectral time series data for three variable L/T transition brown dwarfs and compares the findings to the highly variable benchmark object 2MASS J2139. All four targets reveal significant brightness variability between 1.1 to 1.65 micron but show a difference in wavelength dependence of the variability amplitude. Three of our targets do not show significant decrease in variability amplitude in the 1.4 m water absorption band commonly found in previous studies of L/T transition brown dwarfs. Additionally, at least two brown dwarfs have irregular-shaped, non-sinusoidal light curves. We create heterogeneous atmospheric models by linearly combining SONORA Diamondback model spectra, comparing them with the observations, and identifying the optimal effective temperature, cloud opacity, and cloud coverage for each object. Comparisons between the observed and model color-magnitude variations that trace both spectral windows and molecular features reveal that the early- T dwarfs likely possess heterogeneous clouds. The three T dwarfs show different trends in the same color-magnitude space which suggests secondary mechanisms driving their spectral variability. This work broadens the sample of L/T transition brown dwarfs that have detailed spectral time series analysis and offers new insights that can guide future atmospheric modeling efforts for both brown dwarfs and exoplanets.

Paper Structure

This paper contains 15 sections, 2 equations, 9 figures.

Table of Contents

  1. Introduction
  2. Observations and Data Reduction
  3. Spectral Variability Characterization
  4. Spectral Time Series
  5. Light Curve Analysis
  6. Atmospheric Modeling
  7. Model Methods and Assumptions
  8. Fitting the Base Model
  9. Fitting the Max/Min Flux Ratio
  10. Color Magnitude Evolution Analysis
  11. Comparing to Models on Color Magnitude Spaces
  12. Variability In- and Out- of the Water Absorption Band
  13. Comparing to an Empirical Color-Magnitude Diagram
  14. Summary
  15. Appendix A: Corner Plots

Figures (9)

  • Figure 1: Spectral variability detected in all three new targets (first three columns) and verified in the comparison source 2MASS J2139 (the last column). Top panel: Normalized minimum (blue) and maximum (red) spectra with uncertainties. They are normalized to the highest flux value of each target's maximum spectrum. Transmission curves for the F127M, F139M, and F153M filters used for synthetic photometry in later sections are shown for 2MASS J2139. Bottom panel: Maximum-to-minimum flux ratios with uncertainties. Target names are shown in the top panel title. The dip in flux ratio at $\sim$1.35 $\mu$m for 2MASS J1126 is a systematic feature due to the pointing drift of the telescope and should not be interpreted as a normal signal.
  • Figure 2: Normalized light curves in four photometric bands: G141 broadband (1.10--1.65 , black), F127M (blue), F139M (red), and F153M (green). Light curves are vertically offset for clarity: 0.05 flux units for the three leftmost targets (2MASS J1629, 2MASS J0758, and 2MASS J1126) and 0.15 flux units for 2MASS J2139 due to its larger variability amplitude.
  • Figure 3: Top panel: The best-fit model spectrum $F_\mathrm{base}$ (red) and the average observed spectrum (black) of each object. The $T_\mathrm{eff}$ and $f_\mathrm{sed}$ values that make up each model can be found in Table \ref{['tab:modelfits']}. Bottom panel: The observed ratio and uncertainties (black) and the model ratio (red). The model ratio is calculated as $F_\mathrm{base}/F_\mathrm{total}$ for all objects. We masked out the region where the flux has a steep slope ($1.32-1.35 \mu$m) for all objects.
  • Figure 4: The minimum $\chi^2$ values as functions of the $f_\mathrm{sed}$ vs. $T_\mathrm{eff}$ for $F_\mathrm{patch}$. The overall minimum $\chi^2$ values for the map is highlighted using a black rectangle, and the highest and lowest $\chi^2$ values are represented by the reddest and bluest colors, respectively.
  • Figure 5: Color magnitude diagram for F127M-F153M vs. F127M and the evolution of each target plotted with their respective lines of best fit and model tracks derived from SONORA Diamondback spectral grid. The labeled dashed lines represent constant $T_\mathrm{eff}$, and the blue solid line in both panels corresponds to no sedimentation parameter (cloud-free) and the orange solid line corresponds to $f_{sed}$=1 (thick clouds). The best-fitting lines only indicate the directions of the color-magnitude variations, not the amplitudes. 2MASS J1126, J1629, J0758, and J2139 are shown in orange, red, blue, and green, respectively.
  • ...and 4 more figures