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Astron. Astrophys. 345, L55-L57 (1999)

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4. Spectral analysis

The spectral type of APMPM J0237-5928 has been determined using the spectral features described in Kirkpatrick, Henry & McCarthy (1991) and in Reid, Hawley & Gizis (1995). The ratio A from Kirkpatrick, Henry & McCarthy (1991) is 1.39 and the TiO5 index of Reid, Hawley & Gizis (1995) is 0.32. Both indices indicate most clearly the spectral type in the respective papers and both determine APMPM J0237-5928 to be an M5 star. According to Bessel (1995), a typical M5 star has an absolute R magnitude of 13.21. Without extinction, this places APMPM J0237-5928 with the APM measured [FORMULA] at a distance of 11.0 pc. Using the absolute R magnitude of 12.58 given by Leggett (1992) for M5 stars, we get a distance of 14.5 pc.

We have also used the so-called "NextGen" grid of model atmospheres for M dwarfs (Hauschildt, Allard & Baron 1999) available via anonymous ftp from the authors for determining the effective temperature and a scaled metallicity. We used several spectral ranges to compare the observed spectrum with models for various effective temperatures (2900 K, 3000 K, 3100 K, 3200 K) and metallicities (-0.5, 0.0). We kept log(g) fixed at 5.0 since log(g) influences most prominently high resolution line profiles (Schweitzer et al. 1996). The parameters for the best fitting models are [FORMULA] K and solar metallicity. The best fit was decided by a [FORMULA] technique and by eye. The 2900 K models have kindly been made available by P. H. Hauschildt (private communication) and contain the same input physics as the NextGen models. The derived stellar parameters are typical of an M5 dwarf and in agreement with other recent studies (e.g. Leggett et al. 1996)

It is, however, important to mention that the NextGen grid is already superseded by newer developments for the coolest atmospheres in this grid (Hauschildt, 1999, private communication). The use of these NextGen models enabled us to compare the derived stellar parameters with the evolutionary models of Baraffe et al. (1998) who also used the NextGen grid of model atmospheres. The comparison yields under the assumption of APMPM J0237-5928 having reached the main sequence a mass of [FORMULA] [FORMULA] and an R magnitude of [FORMULA]. Without extinction, this places APMPM J0237-5928 at a distance of [FORMULA] pc.

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© European Southern Observatory (ESO) 1999

Online publication: April 28, 1999