A deep staring campaign in the σ Orionis cluster

Variability in substellar members^⋆,⋆⋆

¹ Department of Physics and Astronomy, York University, Toronto, ON M3J 1P3, Canada
e-mail: pmelliott0@gmail.com
² SUPA, School of Physics and Astronomy, University of St Andrews, North Haugh, St Andrews, Fife KY16 9SS, UK
³ Thüringer Landessternwarte Tautenburg, Sternwarte 5, 07778 Tautenburg, Germany

Received: 11 April 2017
Accepted: 8 August 2017

Abstract

Context. The young star cluster near σ Orionis is one of the primary environments to study the properties of young brown dwarfs down to masses comparable to those of giant planets.

Aims. Deep optical imaging is used to study time-domain properties of young brown dwarfs over typical rotational timescales and to search for new substellar and planetary-mass cluster members.

Methods. We used the Visible Multi Object Spectrograph (VIMOS) at the Very Large Telescope (VLT) to monitor a 24′× 16′ field in the I-band. We stared at the same area over a total integration time of 21 h, spanning three observing nights. Using the individual images from this run we investigated the photometric time series of nine substellar cluster members with masses from 10 to 60 M_Jup. The deep stacked image shows cluster members down to ≈5 M_Jup. We searched for new planetary-mass objects by combining our deep I-band photometry with public J-band magnitudes and by examining the nearby environment of known very low mass members for possible companions.

Results. We find two brown dwarfs, with significantly variable, aperiodic light curves, both with masses around 50 M_Jup, one of which was previously unknown to be variable. The physical mechanism responsible for the observed variability is likely to be different for the two objects. The variability of the first object, a single-lined spectroscopic binary, is most likely linked to its accretion disc; the second may be caused by variable extinction by large grains. We find five new candidate members from the colour-magnitude diagram and three from a search for companions within 2000 au. We rule all eight sources out as potential members based on non-stellar shape and/or infrared colours. The I-band photometry is made available as a public dataset.

Conclusions. We present two variable brown dwarfs. One is consistent with ongoing accretion, the other exhibits apparent transient variability without the presence of an accretion disc. Our analysis confirms the existing census of substellar cluster members down to ≈7 M_Jup. The zero result from our companion search agrees with the low occurrence rate of wide companions to brown dwarfs found in other works.