Physical Properties of Gliese 229B Based on Newly Determined Carbon and Oxygen Abundances of Gliese 229A

TL;DR

This study refines the understanding of Gliese 229B's physical properties by analyzing its spectrum based on newly determined carbon and oxygen abundances of its primary star, Gliese 229A, leading to improved models of its temperature and gravity.

Contribution

It introduces a method to derive elemental abundances from molecular lines in M dwarfs and applies it to better constrain the physical parameters of Gliese 229B.

Findings

Best-fit models suggest Teff around 800-850K and logg around 4.75-5.0.

Metallicity differences of 0.1 dex have minimal impact on spectra.

Age constraints help narrow down the model parameters.

Abstract

Recently Tsuji & Nakajima (2914) and Tsuji, Nakajima & Takeda (2015) have developed a method of molecular line spectroscopy of M dwarfs with which Carbon and oxygen abundances are derived respectively from CO and H2O lines in the K band. They applied this method to Gl229A, the primary star of the brown dwarf companion, Gl229B. The derived abundances of Gl229A are logAc=-3.27+/-0.07 and logAo=-3.10+/-0.02, which are close to the classical values of the solar abundances of carbon and oxygen. We generate model spectra of Gl229B for the metallicity of Gl229A as well as for the classical solar metallicity. We find that the differences of the resulting spectra are not so large for the differences of the metallicity of 0.1 dex or so, but we now discuss the spectrum of Gl229B on the basis of the reliable metallicity. From the literature, the lower limit to the age of Gl229A is found to be 0.3Gyr. From the kinematics of Gl229A, we evaluate the upper limit to the age of Gl229A to be 3.0Gyr. The observed and model spectra are compared and goodness of fit is obtained in the range of model parameters, 750<Teff<1000K, and 4.5<logg<5.5. Among the candidates that satisfy the age constraint, the best combinations of model parameters are (Teff,logg)=(800K,47.5) and (850K,5.0), while acceptable combinations are (750K,4.75),(850K,4.75) and (900K,5.0).