A new set of atmosphere and evolution models for cool T–Y brown dwarfs and giant exoplanets

Abstract

We present a new set of solar metallicity atmosphere and evolutionary models for very cool brown dwarfs and self-luminous giant exoplanets, which we term ATMO 2020. Atmosphere models are generated with our state-of-the-art 1D radiative-convective equilibrium code ATMO, and are used as surface boundary conditions to calculate the interior structure and evolution of 0.001 − 0.075 M objects. Our models include several key improvements to the input physics used in previous models available in the literature. Most notably, the use of a new H-He equation of state including ab initio quantum molecular dynamics calculations has raised the mass by ∼ 1 − 2% at the stellar-substellar boundary and has altered the cooling tracks around the hydrogen and deuterium burning minimum masses. A second key improvement concerns updated molecular opacities in our atmosphere model ATMO, which now contains significantly more line transitions required to accurately capture the opacity in these hot atmospheres. This leads to warmer atmospheric temperature structures, further changing the cooling curves and predicted emission spectra of substellar objects. We present significant improvement for the treatment of the collisionally broadened potassium resonance doublet, and highlight the importance of these lines in shaping the red-optical and near-infrared spectrum of brown dwarfs. We generate three different grids of model simulations, one using equilibrium chemistry and two using non-equilibrium chemistry due to vertical mixing, all three computed self-consistently with the pressure-temperature structure of the atmosphere. We show the impact of vertical mixing on emission spectra and in colourmagnitude diagrams, highlighting how the 3.5 − 5.5 µm flux window can be used to calibrate vertical mixing in cool T-Y spectral type objects.

The ExoMolOP database: Cross sections andk-tables for molecules of interest in high-temperature exoplanet atmospheres

“…Lacis & Oinas (1991); Pierrehumbert (2010)), and is extensively used for radiative transfer calculations in the context of planetary and substellar atmospheres (see, e.g. Irwin et al 2008;Showman et al 2009;Freedman et al 2008Freedman et al , 2014Lee et al 2019;Mollière et al 2015;Amundsen et al 2014;Sharp & Burrows 2007;Malik et al 2017;Drummond et al 2016;Phillips et al 2020). K-tables are generally considered faster (and more accurate for the same R = λ ∆λ ) than cross sections, but they also come with their own assumptions and therefore limitations (see, e.g.…”

“…There are tools available online, such as the exo-k library 1 (Leconte et al, in prep), which enable conversion between different formats, some of which are those used in this work. Many other works have computed opacities for use in radiative-transfer retrieval and atmospheric modelling codes; see, for example, Showman et al (2009), Freedman et al (2008), Freedman et al (2014), Lee et al (2019), Amundsen et al (2014), Kempton et al (2017), Grimm & Heng (2015), Malik et al (2019), Kitzmann et al (2020), Allard et al (2012), Sharp & Burrows (2007), Line et al (2013), Gandhi & Madhusudhan (2017), Phillips et al (2020), Jørgensen (1998), Kurucz & Bell (1995).…”

Direct imaging of sub-Jupiter mass exoplanets with James Webb Space Telescope coronagraphy

“…Specifically, throughout this study we focus our efforts on the TW Hya Association (TWA) (Kastner et al 1997;Gagné et al 2018a) and the Pictoris Moving Group ( PMG) (Zuckerman et al 2001;Gagné et al 2018a). Both of these moving groups occupy a unique region of parameter space, with ages old enough that planetary formation processes have largely ended due to disk clearing (Haisch et al 2001), ages young enough that any potentially formed planets have retained a significant amount of heat from their initial gravitational contraction and are therefore more luminous (Baraffe et al 2003;Phillips et al 2020), and distances close enough to more favourably probe the innermost architectures of planetary systems through direct imaging. Although many other moving groups fulfil one or even two of these qualities (Gagné et al 2018a), currently TWA and PMG present the best opportunity to fulfil all three at once and are hence chosen for this investigation.…”

“…Furthermore, recent work has suggested that the high nucleation energy barrier of Na 2 S strongly inhibits its formation and therefore its inclusion as a dominant cloud species may not be strictly accurate (Gao et al 2020). For these reasons, and as none of the ATMO models of Phillips et al (2020) include the effects of cloud opacity, we select the solar metallicity petitCODE models without any cloud opacity and retain model consistency between mass ranges.…”

“…Furthermore, new and sophisticated exoplanet atmosphere and evolutionary models have been produced (e.g. Linder et al 2019;Phillips et al 2020), enabling a more accurate determination of their predicted luminosities. Recently, Perrin et al (2018) provided a significant, target unspecific, update to the contrast predictions for JWST, Sallum & Skemer (2019) gave an account of the non-redundant masking and kernel phase capabilities of JWST, and finally Brande et al (2020) have explored the feasibility of JWST mid-infrared coronagraphic imaging of field-aged exoplanets around the nearest stars.…”