THE UNITED KINGDOM INFRARED TELESCOPE
ANNUAL REPORT
1995 AND 1996

2. Scientific Results during 1995 and 1996

2.2. Selected Scientific Results

2.2.6. CGS4 Reveals the Unique Nature of Brown Dwarf Gl 229B

T.R. Geballe (JAC)

Brown dwarfs are objects with masses between those of the least massive stars (objects which sustain thermonuclear reactions in their cores) and objects several times more massive than Jupiter. The upper mass limit thus has a fairly rigorous specification, and is calculated to be in the range of 60-80 times the mass of Jupiter (0.06-0.08 M). The lower mass range is poorly defined; an object of several times Jupiter's mass might be regarded as a planet if it were orbiting a star, but might be considered a brown dwarf if it were an isolated object.

With one exception objects presently proposed as brown dwarfs have masses close to the upper limit and temperatures of about 2000-3000K. Arguments that these candidates are indeed brown dwarfs often hinge on fine details of their spectra, which generally resemble those of red dwarfs, as well as on estimates of their luminosities and ages. Often the arguments and claims are controversial.

Gl 229B is different. The discovery of this object (Nakajima et al. 1995) approximately 8'' from the rather dim nearby star Gl 229A (formerly known as Gl 229) was followed by (i) the determination that it was in orbit about Gl 229A, and (ii) the determination that its effective temperature was only around 1000K. The temperature of Gl 229B and its distance imply a very low luminosity (), immediately identifying it as a cool brown dwarf. Almost all of the radiation from Gl 229B is emitted at infrared wavelengths. Oppenheimer er al (1995) obtained a low resolution near-infrared (1-2.5m) spectrum of Gl 229B at Palomar Observatory which revealed a number of strong absorption bands, similar in wavelength and profile to the methane bands that dominate the spectrum of Jupiter. Chemical equilibrium calculations (Tsuji, Ohnaka, and Aoki 1995) show that methane is highly abundant in hydrogen-rich atmospheres at these temperatures.

Late in 1995 a 1-2.5m spectrum of Gl 229B was obtained at UKIRT by Geballe et al. (1996, Figure 9, upper panel) with significantly higher resolution and signal-to-noise ratio than the Palomar spectrum. This spectrum, obtained using CGS4 and extracted with care from the glare of the approximately ten thousand times times brighter Gl 229A, provides a considerably more detailed view of Gl 229B than the Palomar spectrum and will be studied for years to come.

  
Figure 9: Upper panel: Spectrum of the cool brown dwarf Gl 229B obtained with CGS4 in late 1995 (Geballe et al. 1996). The intensity scale is logarithmic. Spectral features in the 1.12-1.14, 1.36-1.42, 1.81-1.93, and 2.48-2.52 m intervals may not be real. Lower panels: sections of the spectrum, with comparison spectra of Titan (courtesy of T. Owen) and of water vapor opacity at a pressure of 1 bar and a temperature of 700K (courtesy of R.S. Freedman, D. Schwenke, and D. Saumon).

Examination of the spectrum shows many wavelength intervals in which absorption is strong. Many of these absorptions closely resemble those seen in the spectra of all of the giant gaseous planets, which are largely due to absorption by methane. Somewhat suprisingly, among solar system objects the best match to portions of the Gl 229B spectrum dominated by methane is the spectrum of Titan (e.g., Figure 9, lower left). The earlier low resolution spectrum of Gl 229B by Oppenheimer et al. (1995) was ambiguous as to the presence of water vapor. However, when the UKIRT spectrum is compared to the spectra of all of the giant outer planets and Titan, it is immediately apparent that the 1.5-1.6m continuum is badly eaten away on its short wavelength side, implying absorption by HO (Figure 9 lower left). The UKIRT spectrum, with its higher resolution, also is able to demonstrate the presence of water unequivocably. As illustrated in Figure 9 (lower right), there is an excellent detailed match between the spectrum of Gl 229B near 2.0m and models of the HO spectrum. The narrow features at 1.5-1.6m in Gl 229B (Figure 9, lower left) also match those predicted for water vapor.

The UKIRT 1-2.5m spectrum of Gl 229B contains a wealth of information about conditions and abundances in the atmosphere of a cool brown dwarf. It samples a regime of density and temperature that never has been observed previously by astronomers. Detailed analysis of the spectrum is under way by several research groups. During Semester 96B low resolution 3-5m spectra of Gl 229B were obtained at UKIRT and higher resolution spectra of selected spectral intervals in the 1-2.5m region were obtained at UKIRT during 98A.

References

Geballe, T.R., Kulkarni, S.R., Woodward, C.E., and Sloan, G.C. 1996, ApJ, 467, L101
Oppenheimer, B.R., Kulkarni, S.R., Matthews, K., and Nakajima, T. 1995 Science, 270, 147
Nakajima, T., Oppenheimer, B.R., Kulkarni, S.R., Golimowski, D.A., Matthews, K., and Durrance
1995, Nature, 378, 463
Tsuji, T., Ohnaka, K., and Aoki, W. 1995, in ``The Bottom of the Main Sequence - and Beyond'',
ed. C.G. Tinney (Berlin: Springer), 45.