Cloud Atlas: Discovery of Rotational Spectral Modulations in a Low-mass, L-type Brown Dwarf Companion to a Star

TL;DR

This paper reports the discovery of rotational spectral modulations in the L6-type brown dwarf LP261-75B, revealing insights into its cloud properties through time-resolved spectroscopy, marking a significant addition to the limited known cases.

Contribution

First detection of rotational spectral modulations in an L6-type brown dwarf companion, expanding understanding of atmospheric cloud properties in such objects.

Findings

Gray spectral variations with weak wavelength dependence

Rotational period approximately 4.78 hours

Unusual light curve with three peaks

Abstract

Observations of rotational modulations of brown dwarfs and giant exoplanets allow the characterization of condensate cloud properties. As of now rotational spectral modulations have only been seen in three L-type brown dwarfs. We report here the discovery of rotational spectral modulations in LP261-75B, an L6-type intermediate surface gravity companion to an M4.5 star. As a part of the Cloud Atlas Treasury program we acquired time-resolved Wide Field Camera 3 grism spectroscopy (1.1--1.69~$\mu$m) of LP261-75B. We find gray spectral variations with the relative amplitude displaying only a weak wavelength dependence and no evidence for lower-amplitude modulations in the 1.4~$\mu$m water band than in the adjacent continuum. The likely rotational modulation period is 4.78$\pm$0.95 h, although the rotational phase is not well sampled. The minimum relative amplitude in the white light curve measured over the whole wavelength range is 2.41$\pm$0.14\%. We report an unusual light curve with seemingly three peaks approximately evenly distributed in rotational phase. The spectral modulations suggests that the upper atmosphere cloud properties in {LP261-75B} are similar to two other mid-L dwarfs of typical infrared colors, but differ from that of the extremely red L-dwarf WISE0047.