[Paper Review] Atmospheric Compositions of Three Brown Dwarfs and Implications for their Formation Conditions

Nikku Madhusudhan, Dániel Apai+1 more|arXiv (Cornell University)|Dec 9, 2016

Stellar, planetary, and galactic studies5 references3 citations

TL;DR

This study analyzes high-S/N Hubble Space Telescope near-infrared spectra of three mid-T brown dwarfs to derive their atmospheric chemical abundances, revealing sub-solar metallicities and C/O ratios between 0.4 and 1.0. These compositions suggest formation via disk accretion followed by ejection, rather than isolated gravitational collapse, and indicate brown dwarfs may probe early galactic chemical inhomogeneity.

ABSTRACT

The formation mechanisms and chemical compositions of brown dwarfs likely span a diverse range. If they are formed predominantly in isolation by gravitational collapse like dwarf stars then their compositions might follow those of dwarf stars, spanning a relatively narrow range in metallicities (~ -0.4 to +0.4 dex) and predominantly oxygen-rich (C/O 0.5~dex), which is thought to be a consequence of formation by core-accretion in a circumstellar disk. In this study we determine the atmospheric chemical compositions of three brown dwarfs and find them to be neither characteristic of dwarf stars nor giant planets. We derive high-precision atmospheric chemical abundances using high-S/N HST near-infrared spectra of three mid T Dwarfs which, together with two previously studied objects, display distinctly sub-solar metallicities and span C/O ratios of 0.4-1.0. These abundance patterns indicate either an old sub-stellar population and a multitude of formation environments or objects formed in circumstellar disks followed by ejection later. We show that for a simple synthetic model population of brown dwarfs with the same spectral-type as our sample the predicted age distribution is predominantly older than the Sun and, hence, more metal poor, consistent with our derived abundances. The diverse C/O ratios we find are indicative of either different chemical reservoirs in their formation environments or different formation pathways. Our results open the possibility that cool brown dwarfs may provide important probes of early galactic chemical evolution and inhomogeneity.

Motivation & Objective

• To determine the atmospheric chemical compositions of three mid-T brown dwarfs using high-resolution Hubble Space Telescope data.
• To investigate the formation mechanisms of brown dwarfs by analyzing their chemical abundances in comparison to stars and giant planets.
• To assess whether the observed compositions are consistent with isolated gravitational collapse or disk-based formation followed by ejection.
• To evaluate the implications of the derived abundances for galactic chemical evolution and the age distribution of brown dwarfs.
• To test whether synthetic population models of brown dwarfs can reproduce the observed abundance patterns and age distributions.

Proposed method

• High-S/N near-infrared spectroscopy of three mid-T dwarfs using the Hubble Space Telescope's WFC3 instrument.
• Atmospheric retrieval analysis to derive chemical abundances, temperature profiles, and cloud coverage from spectral data.
• Application of radiative transfer and chemical equilibrium models to interpret observed spectra and infer atmospheric properties.
• Use of a toy synthetic population model with a substellar initial mass function and varying formation rates to simulate age and gravity distributions.
• Comparison of model predictions with observed abundance patterns to assess consistency with formation hypotheses.
• Incorporation of data from two previously studied brown dwarfs to form a sample of five T dwarfs for comparative analysis.

Experimental results

Research questions

• RQ1 Do the atmospheric compositions of mid-T brown dwarfs align with those expected from isolated gravitational collapse like stars?
• RQ2 Are the observed sub-solar metallicities and variable C/O ratios consistent with formation via core accretion in circumstellar disks, as seen in giant planets?
• RQ3 What does the age distribution of mid-T brown dwarfs imply about their formation history and metallicity evolution?
• RQ4 Can synthetic population models reproduce the observed abundance patterns and age distributions of T dwarfs?
• RQ5 To what extent can brown dwarfs serve as probes of early galactic chemical evolution and inhomogeneity?

Key findings

• The three mid-T brown dwarfs exhibit sub-solar metallicities for carbon, oxygen, and nitrogen, with [C/H] and [O/H] values below solar.
• The C/O ratios of the three objects range from 0.4 to 1.0, indicating significant chemical diversity in the sample.
• The derived abundances are inconsistent with formation via isolated gravitational collapse, which would predict solar or super-solar metallicities and lower C/O ratios.
• The age distribution of a simulated population of mid-T dwarfs is dominated by objects older than the Sun, consistent with the observed sub-solar metallicities.
• The observed abundance patterns are best explained by formation in circumstellar disks followed by dynamical ejection, similar to free-floating giant planets.
• The results suggest that cool brown dwarfs may serve as valuable probes of early galactic chemical evolution and inhomogeneous enrichment processes.