[Paper Review] The Mid-Infrared Spectra of Brown Dwarfs

D. Saumon, Mark S. Marley+1 more|arXiv (Cornell University)|Oct 28, 2003

Astronomy and Astrophysical Research2 references3 citations

TL;DR

This paper presents synthetic mid-infrared (5–20 μm) spectra of brown dwarfs using radiative-convective equilibrium models with cloud sedimentation and non-equilibrium chemistry, predicting that SIRTF/IRS observations will reveal NH₃ in T dwarfs despite vertical depletion, detect silicate clouds near 10 μm in mid-L dwarfs, and constrain non-equilibrium CO/CH₄ and N₂/NH₃ chemistry, while potentially identifying stratospheric emission from trace species like CO₂ and HCN.

ABSTRACT

We present an analysis of brown dwarf model spectra in the mid-infrared spectral region (5 - 20 microns), in anticipation of data obtained with the Space Infrared Telescope Facility. The mid-infrared spectra of brown dwarfs are in several ways simpler than those in the near-infrared and yet provide powerful diagnostics of brown dwarf atmospheric physics and chemistry, especially when combined with ground-based data. We discuss the possibility of detection of new molecular species and of the silicate cloud, predict strong observational diagnostics for non-equilibrium chemistry between CO and CH4, and N2 and NH3, and speculate on the possibility of discovering brown dwarf stratospheres.

Motivation & Objective

• To predict mid-infrared spectral diagnostics of brown dwarfs in anticipation of SIRTF/IRS observations.
• To assess the detectability of molecular species such as NH₃, CO₂, H₂S, and deuterated compounds like CH₃D in mid-IR spectra.
• To evaluate the impact of non-equilibrium chemistry on CO, CH₄, N₂, and NH₃ abundances in brown dwarf atmospheres.
• To determine the observational potential of IRAC photometry and IRS spectroscopy for constraining atmospheric structure and composition.
• To explore the possibility of detecting brown dwarf stratospheres via emission from trace species in the mid-IR.

Proposed method

• Modeling brown dwarf atmospheres using radiative-convective equilibrium with cloud sedimentation (f_sed = 3) and condensate opacity from Fe, MgSiO₃, Al₂O₃, H₂O, and NH₃.
• Computing high-resolution synthetic spectra using molecular opacities for H₂O, CH₄, CO, NH₃, H₂S, PH₃, TiO, VO, CrH, FeH, CO₂, HCN, C₂H₂, C₂H₄, C₂H₆, and alkali metals.
• Incorporating non-equilibrium chemistry via vertical mixing coefficients (K_zz = 10² to 10⁴ cm²/s) to simulate CO/CH₄ and N₂/NH₃ disequilibrium.
• Calculating column densities and detection thresholds (ε) to assess the required abundance enhancement for detectability of trace species.
• Simulating IRAC photometry (3.6–8.0 μm) and IRS spectroscopy (5.3–19.5 μm) to evaluate observational utility.
• Analyzing brightness temperature (T_br) profiles to determine photospheric pressure levels and spectral energy distribution completeness.

Experimental results

Research questions

• RQ1 Can mid-infrared spectra from SIRTF/IRS detect NH₃ in T dwarfs despite strong vertical depletion due to atmospheric mixing?
• RQ2 What is the detectability threshold for CO₂, H₂S, and deuterated species like CH₃D in mid-IR spectra of low-gravity, high-metallicity brown dwarfs?
• RQ3 How do non-equilibrium abundances of CO and CH₄ shift the positions of their mid-IR absorption bands compared to equilibrium chemistry?
• RQ4 Can silicate cloud features near 10 μm be detected in mid-L dwarfs, and what constraints do they place on cloud vertical structure?
• RQ5 Is there observational evidence for brown dwarf stratospheres via emission from trace species like CO₂, HCN, or HCO in the mid-IR?

Key findings

• The 10.5 μm NH₃ band appears at T_eff ≤ 1000 K for K_zz ≥ 10² cm²/s, despite equilibrium abundance profiles, due to vertical transport effects.
• The 7.8 μm CH₄ band appears at T_eff ≈ 1400 K for K_zz = 10² cm²/s and 1200 K for K_zz = 10⁴ cm²/s, significantly cooler than the 1600 K equilibrium transition.
• NH₃ can be depleted by over an order of magnitude at T_eff = 800 K, yet its 10–11 μm features remain strong due to flat abundance profiles.
• CO₂ is marginally detectable at 15 μm in models with T_eff = 1200 K and log g = 4, requiring higher metallicity and lower gravity for detection.
• CH₃D is the most detectable deuterated species, but detection requires an enhancement factor of >250 over equilibrium abundance at T_eff ≥ 600 K.
• A strong PH₃ band at 4.3 μm falls within the IRAC band 2 (4.5 μm), enabling photometric detection of PH₃ in cool brown dwarfs.