The ESO SupJup Survey

Springer Link (Chiba Institute of Technology)

Abstract

Context. The distinct formation pathways proposed for directly imaged exoplanets and isolated brown dwarfs might leave imprints in the inherited chemical composition. Elemental and isotopic tracers could help inform the suspected histories, but this requires a careful characterisation of the sub-stellar atmospheres. In particular, objects at the L-T transition exhibit signs of dynamics that can drive their atmospheres out of chemical equilibrium. Aims. In this work, we studied the nearest L-T brown dwarfs, Luhman 16A and B, to assess the chemical disequilibrium in their atmospheres. We also investigated the elemental and isotopic compositions in the context of their probable formation history within the Oceanus moving group. Methods. As part of the ESO SupJup Survey, we obtained spatially resolved CRIRES+ K-band spectra of the binary. These high-resolution observations were analysed using an atmospheric retrieval framework that couples the radiative transfer code petitRADTRANS to the MultiNest sampling algorithm. Results. We detect and retrieve the abundances of 12CO, H2O, CH4, NH3, H2S, HF, and the 13CO isotopologue. We find that both atmospheres are in chemical disequilibrium with somewhat stronger vertical mixing in Luhman 16A compared to B (Kzz,A ~ 108.7 vs Kzz,B ~ 108.2 cm2 s−1). The tested chemical models, free-equilibrium and disequilibrium chemistry, yield consistent mixing ratios and agree with earlier work at shorter wavelengths. The free-chemistry gaseous C/O ratios show evidence of oxygen trapping in silicate-oxide clouds. While the C/O ratios are consistent with the solar composition, the metallicities are modestly enhanced with [C/H] ~ 0.15. The carbon isotope ratios are measured at 12C/13CA = 74−2+2 and 2C/13CB = 74−3+3. Conclusions. The coincident constraints of metallicities and isotopes across the binary provide further evidence in favour of a common formation. The 12C/13C ratios are aligned with the present-day interstellar medium but lower than the Solar System value. This suggests a recent inheritance and corroborates the relatively young age (~500 Myr) of Luhman 16A and B as members of the Oceanus moving group.

Authors 11

1. S. de Regt Aachen Leiden Observatory

   Leiden University · Leiden Observatory

   Affiliation as printed

   Leiden Observatory , Leiden University , PO Box 9513 , 2300 RA , Leiden , The Netherlands

2. Ignas A. G. Snellen Aachen Leiden Observatory

   Leiden University · Leiden Observatory

   Affiliation as printed

   Leiden Observatory , Leiden University , PO Box 9513 , 2300 RA , Leiden , The Netherlands

3. D. González Picos
4. Siddharth Gandhi

   University of Warwick

   Affiliation as printed

   Centre for Exoplanets and Habitability , University of Warwick , Gibbet Hill Road , Coventry CV4 7AL , UK

   Department of Physics , University of Warwick , Coventry CV4 7AL , UK

5. Natalie Grasser Aachen Leiden Observatory

   Leiden University · Leiden Observatory

   Affiliation as printed

   Leiden Observatory , Leiden University , PO Box 9513 , 2300 RA , Leiden , The Netherlands

6. A. Y. Kesseli

   Institut für Palliative Care · Infrared Processing and Analysis Center

   Affiliation as printed

   IPAC , Mail Code 100-22 ,

7. Rico Landman Aachen Leiden Observatory

   Leiden University · Leiden Observatory

   Affiliation as printed

   Leiden Observatory , Leiden University , PO Box 9513 , 2300 RA , Leiden , The Netherlands

8. P. Mollière

   Max Planck Institute for Astronomy

   Affiliation as printed

   Max-Planck-Institut für Astronomie , Königstuhl 17 , 69117 Heidelberg , Germany

9. E. Nasedkin

   California Institute of Technology · Trinity College Dublin

   Affiliation as printed

   Caltech , 1200 E. California Boulevard , Pasadena , CA 91125 , USA

   School of Physics , Trinity College Dublin , University of Dublin , Dublin 2 , Ireland

10. T. Stolker Aachen Leiden Observatory

    Leiden University · Leiden Observatory

    Affiliation as printed

    Leiden Observatory , Leiden University , PO Box 9513 , 2300 RA , Leiden , The Netherlands

11. Y. Zhang

    California Institute of Technology

    Affiliation as printed

    Department of Astronomy , California Institute of Technology , Pasadena , CA 91125 , USA