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Astron. Astrophys. 348, 524-532 (1999)

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1. Introduction

Barnes & Evans (1976) showed that a tight relation exists between the visual surface brightness and the colour of giants. Such a relation allows to determine the angular diameter and, if the distance is known, the radius of a star from photometric data alone (Lacy 1977, Dumm & Schild 1998). Lacy assumed that the Barnes-Evans relation derived for giants holds also for dwarfs, but this has never been actually proved.

For giants, additional measurements of the angular diameters have become available in recent years (Dumm & Schild 1998, Dyck et al. 1998, and references therein). Angular diameters of dwarfs can so far not be measured directly, but can be derived from bolometric fluxes and temperatures or, more accurately, from flux scaling of model atmospheres to the low-resolution optical/IR overall spectral energy distributions. Leggett et al. (1996, hereafter L96) have applied this method to 16 M-dwarfs using the advanced NextGen M-dwarf model atmospheres of Hauschildt et al. (1999). There is excellent agreement between the L96 radii and those predicted by recent stellar models (Baraffe et al. 1998, hereafter BCAH98; see the comparisons made in L96 and in Beuermann et al. 1998, henceforth Paper I). This convergence of theory and observation is generally regarded as a breakthrough and an important step towards a temperature and radius scale of stars on the lower main sequence, although there is still some concern about the remaining differences (e.g. Clemens et al. 1998).

In this paper, we use the results of L96 to derive the surface brightness of M-dwarfs. We then show that the visual surface brightness vs. Cousins [FORMULA] relationships for M-dwarfs differs from that of M-giants in a colour-dependent way and find a close agreement between observationally determined and theoretically predicted gravity dependencies (BCAH98, Alibert et al. 1999, Hauschildt et al. in preparation). We, furthermore, show that M-dwarfs display a metallicity dependence of the surface brightness in the infrared K-band which also agrees with that predicted. The good agreement between theory and observation increases our confidence in the derived Barnes-Evans relations and the implied radius scale of M-dwarfs.

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© European Southern Observatory (ESO) 1999

Online publication: July 26, 1999
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