[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 studies4 citations

TL;DR

This study presents long-term, multiwavelength photometry of the L3.5 brown dwarf 2MASS 0036+18, revealing a rotation period of 3.080 ± 0.001 hr. 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—primarily drive the variability, though thick clouds likely coexist, suggesting a complex interplay between starspots and cloud structures in shaping its light curve.

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

• Investigate the physical origin of photometric variability in the L3.5 brown dwarf 2MASS 0036+18.
• Determine whether starspots or cloud inhomogeneities are the dominant driver of variability in early L-dwarfs.
• Assess the long-term stability of the light curve over multiple rotation periods to distinguish between transient cloud features and persistent starspots.
• Constrain the atmospheric properties of 2MASS 0036+18, particularly cloud opacity and temperature structure, using spectral fitting.
• Evaluate the potential for planetary transits or flares in the system using high-cadence photometric monitoring.

Proposed method

• Conducted 22 nights of ground-based photometry using multiple telescopes across optical and near-infrared bands.
• Performed simultaneous multiwavelength photometry on 7 nights to enable direct comparison of variability phases across wavelengths.
• Measured the rotation period by phase-folding light curves over ~120 days of baseline observations.
• Fitted the observed spectral energy distribution to constrain cloud thickness and atmospheric temperature structure.
• Analyzed multiwavelength light curves for phase shifts and amplitude variations to distinguish between starspot and cloud-driven variability.
• Used Hα detection data (Pineda et al., 2016) to support the presence of magnetic activity consistent with starspots.

Experimental results

Research questions

• RQ1 What is the dominant physical mechanism—starspots or inhomogeneous cloud cover—driving the photometric variability of the L3.5 dwarf 2MASS 0036+18?
• RQ2 Do multiwavelength light curves of 2MASS 0036+18 exhibit significant phase shifts, and what does this imply about the vertical structure of atmospheric inhomogeneities?
• RQ3 How stable is the light curve of 2MASS 0036+18 over multiple rotation periods, and does this stability favor starspots over transient cloud features?
• RQ4 To what extent do the observed variability amplitudes and their wavelength dependence support a starspot model with temperature contrasts of ~100 K?
• RQ5 Can the presence of thick clouds, as inferred from spectral fitting, be reconciled with the lack of significant multiwavelength phase shifts?

Key findings

• The rotation period of 2MASS 0036+18 is measured as 3.080 ± 0.001 hours from 120 days of photometric monitoring.
• No significant phase shifts were detected between optical and near-infrared light curves, indicating that the source of variability is likely coherent across multiple atmospheric pressure levels.
• The amplitude of photometric variability decreases with increasing wavelength, a signature consistent with starspots that are ~100 K hotter or cooler than the 1700 K photosphere.
• Spectral fitting indicates that the atmosphere of 2MASS 0036+18 is enveloped by reasonably thick clouds, suggesting a complex interplay between clouds and starspots.
• The lack of significant light curve evolution over multiple rotation periods suggests that the dominant variability mechanism is stable, consistent with long-lived starspots or uniformly distributed cloud features.
• The absence of strong phase offsets and the presence of Hα emission (Pineda et al., 2016) support the hypothesis that starspots are the primary driver of variability, though clouds may play a secondary role.