A New Grid of Model Atmospheres for Metal-poor Ultracool Brown Dwarfs

Abstract

Abstract We present a new grid of model atmospheres and synthetic spectra down to extremely metal-poor ([M/H] ≥ −3.0), ultracool (T eff ≥ 900 K) brown dwarfs based on the PHOENIX code (Hauschildt et al. 1997). The grid is publicly available and continuously updated as new models are computed. The models are aimed at characterizing metal-poor halo brown dwarfs identified in surveys such as Backyard Worlds: Planet 9 (Kuchner et al. 2017; Schneider et al. 2020) and brown dwarfs in globular clusters (Dieball et al. 2019). Our setup includes the formation and depletion by gravitational settling of condensate clouds as defined for the BT-Settl grid of Allard et al. (2012), using the version 15.5 branch of PHOENIX, which was split off from the main branch (Barman et al. 2011; Husser et al. 2013). Our calculations employ the Allard & Homeier cloud formation model (Helling et al. 2008; Allard et al. 2012).