Measuring the Physical Properties of the Coldest Brown Dwarfs with SpeX Adam J. Burgasser (MIT) Adam Burrows (U. Arizona) J. Davy Kirpatrick (IPAC/Caltech) “A Method for Determining the Physical Properties of the Coldest Known Brown Dwarfs” Burgasser, Burrows & Kirkpatrick 2006, ApJ, 639, 1095

T dwarfs are the coldest, lowest luminosity brown dwarfs currently known. With effective temperatures T eff < 1200 K (as low as 700 K), these objects are distinguished by the presence of H 2 O and CH 4 absorption bands in their near infrared spectra, and blue J-K colors arising from strong collision-induced H 2 absorption. They have spectral properties similar to hot extrasolar planets. Modeling of T dwarf spectra is hampered by deep and overlapping molecular bands, a hinderance in the determination of the physical properties for these sources, including, mass, age and composition. Such parameters are crucial for understanding the origins and evolution of brown dwarfs.

We searched for gravity and metallicity features in low resolution near infrared T dwarf spectra obtained with SpeX. In this set of T6-T6.5 dwarfs (T eff ≈ 1000 K), we see that the 1.05 and 2.1µm peaks clearly vary in peak strength while most other features are identical.

We see similar variations in the 1.05 and 2.1µm peaks of theoretical spectral models, arising from differences in gravity and metallicity. H 2 O and CH 4 bands vary primarily with T eff. However, these models generally provide a poor match to the data, largely due to problems with CH 4 opacities. How can we use the models to derive T eff, log g and [M/H]?

Ideally, one could use spectral indices sampling T eff, log g and [M/H] features to derive physical properties. Yet the mismatch of the spectral models requires calibration. This can be done using data for the widely- separated brown dwarf companion Gliese 570D, which has a well-constrained age (2-5 Gyr) and metallicity ([M/H] ≈ 0). Semi-empirical analysis yields T eff ≈ 800 K and log g ≈ 5.1 for this brown dwarf.

With this calibration in hand, we used two spectral indices (K/H and H 2 O-J) with differing dependencies on T eff and log g to derive these parameters for 16 late-type field T dwarfs. In most cases, well-defined determinations of T eff and log g were possible. The two exceptions 2MASS and 2MASS appear to be colder than 700 K, the minimum T eff of our models. These may be the coldest brown dwafs currently known.

For one T dwarf, 2MASS , metallicity effects were also apparent. This plot compares the same index fits but for -0.5 < [M/H] < 0.0. The solutions are degenerate since we are using two indices, but a best fit appears to be in the [M/H] = -0.3 to -0.1 range. This is the first T dwarf with a quantitative determination of its metallicity.

Determination of T eff and log g yields masses, radii and ages for individual field brown dwarfs, paving the way for direct measurements of the field substellar luminosity and mass functions, the age distribution of brown dwarfs, identification of coeval low mass moving groups, and correlations between metallicity and age. These are all key ingredients in our understanding of the formation and evolution of the Galaxy and the local Solar environment. Our procedure can be generalized to other spectral types. Furthermore, by basing our technique on low resolution near infrared spectra, it can be applied to statistically large samples of intrinsically faint sources as will soon be uncovered by UKIDSS and PanStarrs.