[Paper Review] Long-term, Multiwavelength Light Curves of Ultra-Cool Dwarfs: II. The evolving Light Curves of the T2.5 SIMP 0136 & the Uncorrelated Light Curves of the M9 TVLM 513

Bryce Croll, Philip S. Muirhead+4 more|arXiv (Cornell University)|Sep 12, 2016

Stellar, planetary, and galactic studies5 citations

TL;DR

This study presents 17 nights of near-infrared photometry of the T2.5 brown dwarf SIMP 0136, revealing rapid, rotation-period-scale evolution in its light curve amplitude—from >6% to <1% in days—indicating that spectral interpretations based on single-rotation observations may reflect only ephemeral snapshots. For the M9 dwarf TVLM 513, simultaneous J- and I-band photometry confirms variability is due to clouds or aurorae, not starspots.

ABSTRACT

We present 17 nights of ground-based, near-infrared photometry of the variable L/T transition brown dwarf SIMP J013656.5+093347 and an additional 3 nights of ground-based photometry of the radio-active late M-dwarf TVLM 513-46546. Our TVLM 513-46546 photometry includes 2 nights of simultaneous, multiwavelength, ground-based photometry, in which we detect obvious J-band variability, but do not detect I-band variability of similar amplitude, confirming that the variability of TVLM 513-46546 most likely arises from clouds or aurorae, rather than starspots. Our photometry of SIMP J013656.5+093347 includes 15 nights of J-band photometry that allow us to observe how the variable light curve of this L/T transition brown dwarf evolves from rotation period to rotation period, night-to-night and week-to-week. We estimate the rotation period of SIMP J013656.5+093347 as 2.406 +/- 0.008 hours, and do not find evidence for obvious differential rotation. The peak-to-peak amplitude displayed by SIMP J013656.5+093347 in our light curves evolves from greater than 6% to less than 1% in a matter of days, and the typical timescale for significant evolution of the SIMP J013656.5+093347 light curve appears to be approximately &lt;1 to 10 rotation periods. This suggests that those performing spectrophotometric observations of brown dwarfs should be cautious in their interpretations comparing the spectra between a variable brown dwarf's maximum flux and minimum flux from observations lasting only approximately a rotation period, as these comparisons may depict the spectral characteristics of a single, ephemeral snapshot, rather than the full range of characteristics.

Motivation & Objective

• To investigate the long-term evolution of photometric variability in ultra-cool dwarfs, particularly at the L/T transition.
• To determine whether short-duration spectrophotometric observations capture the full range of spectral characteristics in variable brown dwarfs.
• To distinguish between starspot, cloud, and auroral origins of variability in late-type dwarfs using multiwavelength photometry.
• To measure the timescale of light curve evolution in L/T transition brown dwarfs for the first time with robust data.
• To assess the reliability of spectral comparisons between maximum and minimum flux states in variable brown dwarfs.

Proposed method

• Conducted 17 nights of ground-based near-infrared photometry on the T2.5 brown dwarf SIMP J013656.5+093347, with 15 nights in J-band.
• Performed simultaneous J- and I-band photometry on the M9 dwarf TVLM 513-46546 over two nights to compare variability amplitudes across bands.
• Used Fourier analysis and phase-folded light curves to determine the rotation period of SIMP 0136 with high precision (2.406 ± 0.008 hours).
• Tracked night-to-night and week-to-week changes in peak-to-peak amplitude to quantify light curve evolution timescales.
• Compared J-band and I-band variability amplitudes in TVLM 513 to rule out starspot-driven variability.
• Evaluated the consistency of photometric periods across nights to constrain differential rotation.

Experimental results

Research questions

• RQ1 What is the timescale for significant evolution in the light curve of the L/T transition brown dwarf SIMP 0136?
• RQ2 Does the variability in the M9 dwarf TVLM 513 arise from starspots, clouds, or aurorae, as indicated by multiwavelength photometry?
• RQ3 To what extent do single-rotation observations of variable brown dwarfs represent the full range of spectral characteristics?
• RQ4 Is there evidence for differential rotation in SIMP 0136 based on night-to-night period stability?
• RQ5 How does the amplitude of variability in SIMP 0136 evolve from one rotation period to the next over days?

Key findings

• The rotation period of SIMP J013656.5+093347 is measured as 2.406 ± 0.008 hours with no significant night-to-night period variation.
• The peak-to-peak amplitude of SIMP 0136's J-band light curve evolves from greater than 6% to less than 1% within just a few days.
• The timescale for significant light curve evolution in SIMP 0136 is as short as one rotation period and as long as approximately one day.
• No significant I-band variability was detected in TVLM 513-46546 despite clear J-band variability, ruling out starspots as the primary cause.
• The variability in TVLM 513 is most likely driven by clouds or aurorae, not starspots, based on multiwavelength photometric consistency.
• The study demonstrates that spectrophotometric comparisons between maximum and minimum flux states in variable brown dwarfs may reflect only a single, transient snapshot, not the full spectral range.