Quasi-Molecular K-H[2 ]Absorption As An Alternative To The Resurgence Of CaH Bands In The Spectra Of T-Type Dwarfs: Is The Cloud-Clearing Scheme At Stake?

Details Quasi-Molecular K-H[2 ]Absorption As An Alternative To The Resurgence Of CaH Bands In The Spectra Of T-Type Dwarfs: Is The Cloud-Clearing Scheme At Stake? Quasi-Molecular K-H[2 ]Absorption As An Alternative To The Resurgence Of CaH Bands In The Spectra Of T-Type Dwarfs: Is The Cloud-Clearing Scheme At Stake?

Aug 2007

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2007iaus..240..332a&link_type=abstract

Binary Stars as Critical Tools and Tests in Contemporary Astrophysics, International Astronomical Union. Symposium no. 240, he

Astronomy and Astrophysics

Astrophysics

Scientific paper

As brown dwarfs cool off with time, their atmospheres become denser and more transparent, allowing the emitted thermal flux to escape from deeper atmospheric layers. Burgasser et al. (2002) have investigated and classified the red spectra of T dwarfs in a spectral sequence where a resurgence of the hydride bands, after disappearing in the M to L spectral transition, occur between the late L to T before disappearing again in the late T dwarfs. CaH for example is identified in mid-T dwarfs at around 0.7μm (Burgassser 2003). The authors explain this resurgence by a cloud-clearing scheme where holes would allow to see the CaH from deeper enriched layers, while it is settled out from the uppermost atmospheric layers seen on the rest of the brown dwarf surface. We present the first synthetic spectra of T dwarfs including a semi-classical modelling of the pressure broadening of alkalis lines (Na I D, Li I, K I, Rb I, and Cs I fundamental resonance doublets) by molecular hydrogen and helium, the most important species in these atmospheres. We compare the models to the T dwarfs red optical spectra of Burgasser et al. (2003) and we find that the 0.7μm feature has been wrongly identified to CaH. In particular, the very strong KI resonance transition doublet at 0.77μm explains by itself this absorption feature by producing a quasi-molecular satellite absorption feature at this wavelength. The strength of this satellite is very sensitive to the density of perturbers in the lower photosphere and to the background opacity provided by the Na I D red wing, which explains naturally both its apparition in late L dwarfs and its vanishing in late T dwarfs. We find in conclusion that no cloud-clearing scheme or non-equilibrium processes is necessary to explain this absorption feature, and the evolution of the red optical spectrum of T dwarfs. And this should teach us caution about these atmospheres often too enthusiastly considered planetary. MHR 3D convection models are nevertheless underway to estimate the likelihood of cloud-clearings in late L and T dwarfs.

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