Tuesday, July 29, 2014

Observations of a Low-Gravity Brown Dwarf

Brown dwarfs occupy the mass range between the most massive planets and the least massive stars. They are not massive enough to sustain hydrogen fusion in their cores and so they cool gradually with time. As a brown dwarf cools, it contracts, evolving from a low to a high “surface” gravity. The word “surface” is shown in quotation because a brown dwarf does not have a solid surface. Instead, a brown dwarf’s “surface” simply refers to its observable photosphere. Brown dwarfs are gaseous throughout. They are primarily composed of hydrogen and helium, with trace amounts of heavier elements.

Figure 1: Artist impression of a brown dwarf that is still glowing red-hot from heat acquired during its formation.

For a young brown dwarf that has yet to cool and contract to its final radius, it can display low-gravity features that can be identified from observations in the near-infrared waveband. This is because the low-gravity atmosphere of a young brown dwarf drives the formation of thicker than normal clouds in the brown dwarf’s photosphere. The thicker clouds give rise to a redder near-infrared spectrum because shorter wavelength light (i.e. bluer light) is attenuated and scattered by clouds more than longer wavelength light (i.e. redder light). As a result, a brown dwarf with a redder near-infrared spectrum can signify its youthfulness.

Gagné et al. (2014) present the discovery of SIMP J2154-1055 - an L4 spectral type brown dwarf displaying signs of low-gravity in its near-infrared spectrum. SIMP J2154-1055 has a redder near-infrared spectrum compared with other known L4 brown dwarfs. This is consistent with the presence of thicker than normal clouds in a low-gravity photosphere. SIMP J2154-1055 has a good probability of being part of the Argus Association, a loose group of stars with similar ages. If it is indeed a member of the Argus Association, SIMP J2154-1055 would be roughly 30 to 50 million years old and its mass would be ~10 times the mass of Jupiter, indicating that SIMP J2154-1055 is a relatively young and low-mass brown dwarf.

Figure 2: Near-infrared spectrum of SIMP J2154-1055 compared with other known L4 brown dwarfs. SIMP J2154-1055 is the reddest L4 brown dwarf yet identified. All spectra are normalized to their median in the 1.27 to 1.33 μm range. Gagné et al. (2014).

Gagné et al. (2014), “SIMP J2154-1055: A New Low-Gravity L4β Brown Dwarf Candidate Member of the Argus Association”, arXiv:1407.5344 [astro-ph.SR]