3. Infrared spectroscopy
With magnitude photometric accuracy at its limits, the DENIS survey data by itself cannot provide unbiased samples. Near infrared spectroscopy was thus obtained at the 3.9-m Anglo-Australian Telescope, on the nights of 1996 April 9 and 10 (UT) and 1996 October 21 and 22 (UT). On both runs the Infra-Red Imaging Spectrograph (IRIS - Allen et al. 1993) was used in its cross-dispersed HK echelle mode. This provides complete wavelength coverage from 1.438 - 2.536 m, at a resolution of , and a dispersion of . A slit of width 1.4" and length 13" was used.
Figure 2 shows the resulting spectra. Because the AAT is a low-altitude site, observations through the atmospheric water vapour bands were impossible. Outside these regions, the spectra show broad stellar H2 O absorption bands characteristic of low temperature atmospheres. Other typical cool atmosphere features include: CO bandheads at 2.3-2.4 m; and numerous spectral lines of neutral metals - in particular Na I m and Ca I m.
The appearance of the spectrum of DENIS-P J1058.7-1548 is similar to that of GD165B, while both DENIS-P J1228.2-1547 and J0205.4-1159 are later. DENIS-P J0205.4-1159 is the coolest of the three, and only Gl 229B has a later spectral type. It is by a significant margin the coldest isolated object identified to date. Its spectrum shows evidence for the onset of absorption by methane at 2.22 m. This feature is present in both of the independent spectra which were averaged to produce Figure 2- leaving us confident of the feature's reality. Given the presence of methane in the even colder atmosphere of Gl 229B (Allard et al. 1996), its association with this feature in DENIS-P J0205.4-1159 seems reasonable. This would imply a photospheric temperature of ≲ 1500K (cf. Tsuji et al. 1994, Figure 3), which is definitely substellar.
Jones et al. (1994) have shown that L and/or Teff information can be obtained for late-type dwarfs using features in their infrared spectra. In particular, the strength of H2 O (as measured by the slope of the pseudo-continuum in regions of stellar H2 O absorption) is a sensitive measure. We have used both literature data and our own observations of known late dwarfs to calibrate an empirical relation between these slopes and Mk. In essence, we use the H2O absorption strength, much like a broad band colour, as an estimator of Teff. Moreover, brown dwarfs, as they age, slide along an extension of the main sequence in the H-R diagram (D'Antona & Mazzitelli 1985) - the luminosity spread in this main sequence "extension" due to mass differences is magnitude, which is similar to that seen due to metallicity for stars on the main sequence (eg. Tinney et al 1995, Fig 3). So in the absence of parallaxes or atmospheric models, infrared spectra can provide luminosity information in the same way that colours do for stars on the main sequence. In particular, we derive the following luminosity estimates (Delfosse et al 1997); DENIS-P J1228.2-1547: M , DENIS-P J1058.7-1548: M , DENIS-P J0205.4-1159: M . These compare with MK = 11.7 0.2 for GD 165B (Dahn private communication). So, DENIS-P J1228.2-1547 and DENIS-P J0205.4-1159 are of lower luminosity than GD 165B, and only Gl 229B (MK = 15.5, Matthews et al. 1996) has a lower luminosity.
© European Southern Observatory (ESO) 1997
Online publication: April 6, 1998