[Paper Review] Long-term, multiwavelength light curves of ultra-cool dwarfs: I. An interplay of starspots & clouds likely drive the variability of the L3. 5 dwarf 2MASS 0036+ 18

Bryce Croll, Philip S. Muirhead+6 more|arXiv (Cornell University)|Sep 12, 2016

Stellar, planetary, and galactic studies3 citations

TL;DR

This study presents 120 days of multiwavelength photometry of the L3.5 brown dwarf 2MASS 0036+18, revealing a rotation period of 3.080 ± 0.001 hr with minimal light curve evolution. The lack of significant phase shifts across optical and near-infrared wavelengths, combined with decreasing variability amplitude at longer wavelengths, indicates that starspots—possibly 100 K hotter or cooler than the 1700 K photosphere—are the primary driver of variability, though complex interplay with thick, multi-layered clouds is also likely.

ABSTRACT

We present multi-telescope, ground-based, multiwavelength optical and near-infrared photometry of the variable L3.5 ultra-cool dwarf 2MASSW J0036159+182110. We present 22 nights of photometry of 2MASSW J0036159+182110, including 7 nights of simultaneous, multiwavelength photometry, spread over ~120 days allowing us to determine the rotation period of this ultra-cool dwarf to be 3.080 +/- 0.001 hr. Our many nights of multiwavelength photometry allow us to observe the evolution, or more specifically the lack thereof, of the light curve over a great many rotation periods. The lack of discernible phase shifts in our multiwavelength photometry, and that the amplitude of variability generally decreases as one moves to longer wavelengths for 2MASSW J0036159+182110, is generally consistent with starspots driving the variability on this ultra-cool dwarf, with starspots that are ~100 degrees K hotter or cooler than the ~1700 K photosphere. Also, reasonably thick clouds are required to fit the spectra of 2MASSW J0036159+182110, suggesting there likely exists some complex interplay between the starspots driving the variability of this ultra-cool dwarf and the clouds that appear to envelope this ultra-cool dwarf.

Motivation & Objective

• To characterize the long-term photometric variability of the L3.5 brown dwarf 2MASS 0036+18 across multiple wavelengths.
• To determine whether starspots, cloud opacity variations, or auroral activity are the dominant source of variability.
• To assess the stability and evolution of the light curve over more than 100 rotation periods.
• To constrain the presence of clouds and their role in modulating atmospheric temperature contrasts.

Proposed method

• Conducted 22 nights of ground-based photometry using multiple telescopes across optical and near-infrared bands.
• Performed simultaneous multiwavelength observations on 7 nights to enable direct comparison of light curve phases.
• Measured the rotation period via light curve folding and phase analysis over ~120 days.
• Fitted the spectral energy distribution to infer cloud properties and atmospheric temperature structure.
• Analyzed variability amplitude trends with wavelength to distinguish between starspot and cloud-driven models.
• Evaluated flare rates and excluded transiting planetary companions via photometric monitoring.

Experimental results

Research questions

• RQ1 What is the dominant physical mechanism driving the photometric variability of the L3.5 brown dwarf 2MASS 0036+18?
• RQ2 Do multiwavelength light curves exhibit significant phase shifts, and what does this imply about the nature of the inhomogeneities?
• RQ3 How do the amplitude and morphology of the light curve evolve over multiple rotation periods?
• RQ4 To what extent do clouds and starspots coexist and interact in shaping the observed variability?
• RQ5 Is the variability consistent with a single mechanism, or is a combination of starspots and multi-layered clouds required?

Key findings

• The rotation period of 2MASS 0036+18 is precisely measured as 3.080 ± 0.001 hr over 120 days of monitoring.
• No significant phase shifts are observed between multiwavelength light curves, favoring starspots or multi-layered clouds over single-layer cloud opacity variations.
• Variability amplitude decreases with increasing wavelength, consistent with starspots that are ~100 K hotter or cooler than the 1700 K photosphere.
• Spectral fitting indicates that significant, reasonably thick clouds envelope the brown dwarf, suggesting a complex interplay with surface inhomogeneities.
• The light curve shows minimal evolution from rotation period to period, contrasting with the rapid variability evolution seen in L/T transition dwarfs.
• The lack of optical/near-infrared flares and absence of transiting planets rule out those mechanisms as the source of variability.