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Astron. Astrophys. 327, L25-L28 (1997)

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4. Masses

Recently, Martín et al. (1997b, hereafter MBDF) and Tinney et al. (1997, hereafter TDF) have independently obtained high resolution optical spectra of DENIS-P J1228.2-1547 and J1058.7-1548 - TDF using the Anglo-Australian Telescope; MBDF using Keck I. They both find a very strong 670.8 nm lithium line in the DENIS-P J1228.2-1547 spectrum, and set an upper limit for DENIS-P J1058.7-1548.

Since these very cool dwarfs have been fully convective for extended periods, the lithium detection proves beyond reasonable doubt that the mass of DENIS-P J1228.2-1547 is lower than the lithium burning threshold of [FORMULA] 0.06 solar masses (Nelson, Rappaport, Chiang 1993), and a fortiori that it is a brown dwarf.

The status of DENIS-P J1058.7-1548 and DENIS-P J0205.4-1159 is less clear cut, as one has to rely on model and age-dependent effective temperature arguments. For an age of a few Gyr current models place the transition between stars and brown dwarfs at [FORMULA] and a spectral type later than M10 (Baraffe & Chabrier 1996, Chabrier et al. 1996), though the recent inclusion of dust formation in atmospheric models (Jones & Tsuji, 1997, Allard 1997a) may change this limit. All three DENIS objects clearly have infrared spectral types later than M10V, as also found in TDF from optical spectra. Dusty atmospheric models provide a best estimate of 1800K for the effective temperature of GD 165B (Allard 1997b, Kirkpatrick et al. 1997b). Given the similarity of its infrared spectrum with that of GD 165B, the effective temperature of DENIS-P J1058.7-1548 is close to 1800 K. The effective temperatures of DENIS-P J1228.2-1547 and DENIS-P J0205.4-1159 are clearly lower than 1800 K. The 2.2 [FORMULA] feature in DENIS-P J0205.4-1159, if indeed due to methane, implies Teff ≲ 1500K.

Taken together, the data and the available models imply that DENIS-P J1228.2-1547 and DENIS-P J0205.4-1159 are brown dwarfs. DENIS-P J1228.2-1547 certainly is, given its Li detection. Given its later spectral type and probable CH4 detection, DENIS-P J0205.4-1159 must be even cooler, and most likely of lower mass. It will be searched for lithium as soon as it becomes observable later in 1997. DENIS-P J1058.7-1548 is probably also a brown dwarf, but given the uncertainties in theoretical models, we consider its status - like GD 165B - more uncertain.

Scaling the detection of these three objects to the full survey, DENIS will detect a few hundred brown dwarfs. Once their distances are measured, they will unambiguously establish the luminosities appropriate to brown dwarfs and will define the brown dwarf sequence in the HR diagram. We are measuring the parallaxes of the three objects discussed here, and a parallax follow-up of the brown dwarfs in the full survey is planned. This will provide much needed contraints for brown dwarf interior and atmospheric models. Exciting times are ahead for brown dwarf research.

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© European Southern Observatory (ESO) 1997

Online publication: April 6, 1998