Letter to the Editor

Detection of J-band photometric periodicity in the T8 dwarfs 2MASS J09393548-2448279 and EQ J1959-3338

¹ Centro de Astrobiología, CSIC-INTA, Camino Bajo del Castillo s/n, 28692 Villanueva de la Cañada, Madrid, Spain
² Department of Physics and Astronomy, The University of Western Ontario, 1151 Richmond St, London, Ontario N6A 3K7, Canada
³ Department of Physics and Astronomy, Institute for Earth and Space Exploration, The University of Western Ontario, 1151 Richmond St, London, Ontario N6A 3K7, Canada
⁴ Departament d’Astronomia i Astrofísica, Universitat de València, C. Dr. Moliner 50, 46100 Burjassot, València, Spain

^⋆ Corresponding author: pamiles@cab.inta-csic.es

Received: 8 April 2025
Accepted: 24 April 2025

Abstract

Aims. We aim to study the near-infrared variability of the T8 dwarfs 2MASS J09393548-2448279 and EQ J1959-3338 by analyzing their J-band photometric signal, which can provide new insights into the atmospheric dynamics of cold brown dwarfs.

Methods. We used FLAMINGOS-2 on the Gemini South telescope to perform J-band differential photometry continuously over 4 h for each target. The resulting light curves have a cadence of 20 s and a photometric uncertainty of 2–4 mmag.

Results. We detect periodic variability in both T8 dwarfs, with an amplitude of 16.6 ± 0.9 mmag and a period of 1.364 ± 0.012 h for EQ J1959-3338, which we attribute to rotational modulation. For 2MASS J09393548-2448279, we observe an amplitude of 4.6 ± 0.4 mmag and a period of 1.733 ± 0.040 h, though this periodicity could represent a fraction of a longer period.

Conclusions. With the detection of variability in 2MASS J09393548-2448279 and EQ J1959-3338, the number of known variable T8 dwarfs has doubled, making them prime candidates for infrared space-based monitoring and radio observations to investigate atmospheric dynamics and the influence of the magnetic field in very cool atmospheres.