A deep staring campaign in the σ Orionis cluster

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 hours, 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.

Wide σ Orionis binaries resolved by UKIDSS

“…J. S. Béjar, in prep. ) or VIMOS (Elliott et al 2017). Except for the innermost arcminute in the cluster centre, which is affected by the intense glare of the bright eponymous σ Ori trapezium, both the UKIDSS and Mayrit catalogues nicely complement each other for wide multiplicity studies.…”

“…An exhaustive analysis of multiplicity of much fainter brown dwarfs and planetary‐mass objects in σ Orionis would require a new deeper dataset, such as VIRCAM (Peña Ramírez et al 2012; V. J. S. Béjar, in prep.) or VIMOS (Elliott et al 2017). Except for the innermost arcminute in the cluster center, which is affected by the intense glare of the bright eponymous σ Ori trapezium, both the UKIDSS and Mayrit catalogs nicely complement each other for wide multiplicity studies.…”

The one that got away: a unique eclipse in the young brown dwarf Roque 12

“…The most prominent one might be Boyajian's Star found in the Kepler data set (Boyajian et al 2016), but there are other examples at younger ages (Stauffer et al 2017), see also Scholz et al (2019) for a discussion. A brown dwarf in σ Orionis might also belong into that category (Elliott et al 2017). Among Pleiades members, a few examples of such disk-less 'dippers' have been identified (Rebull et al 2016).…”

Stars and brown dwarfs in the sigma Orionis cluster. IV. IDS/INT and OSIRIS/GTC spectroscopy and Gaia DR2 astrometry

“…In the last lustrum, the Mayrit catalogue has been used for searching for pulsations and photometric variability in substellar objects (Cody & Hillenbrand 2014;Elliott et al 2017), extending spectroscopic surveys to the lowest masses and the largest separations to the cluster centre (Koenig et al 2015;Zapatero Osorio et al 2017;Cottle et al 2018), characterising discs with theoretical models and ALMA and Spitzer observations (Álvarez-Meraz et al 2017;Ansdell et al 2017;Winter et al 2018;Pérez-Blanco et al 2018), identifying largescale Herbig-Haro jets driven by proto-brown dwarfs (Riaz et al 2017(Riaz et al , 2019, and carefully analysing the massive multiple stellar system that gives the name to the cluster (Simón-Díaz et al 2015;Schaefer et al 2016), not counting the studies performed by the first author and his colleagues on multiplicity (Caballero 2014;Caballero et al 2016Caballero et al , 2018 and parallaxes and proper motions (Caballero 2017(Caballero , 2018.…”