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Astron. Astrophys. 360, L39-L42 (2000)

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4. The mass of CoD-33°7795 B

Based on its spectral type and magnitude (H[FORMULA] mag, L99), CoD-33°7795 B would be located at [FORMULA] pc, if it would be main-sequence dwarf ([FORMULA] mag, Kirkpatrick & McCarthy 1994). From the six objects with [FORMULA] mag found within 5 pc around the Sun, we can estimate the probability for chance alignment of CoD-33°7795 B within [FORMULA] around star A to be [FORMULA]. Given the very sparse space density of T Tauri stars in the TWA region, the probability for CoD-33°7795 B to be a free-floating young TWA brown dwarf, unrelated to star A, is of the same order. Thus, there is a high probability that component B is a physical companion to star A.

CoD-33°7795 A is a spectroscopic binary (W99). For an equal-mass binary at [FORMULA] pc, the age is [FORMULA] Myrs (Weintraub et al. 2000). We can assume the same age for its companion. Hence, for its young age and spectral type, CoD-33°7795 B is below the sub-stellar limit according to different sets of tracks and isochrones (e.g. Baraffe et al. 1998). Hence, it is a brown dwarf.

The mass of each component in the spectroscopic binary CoD-33°7795 A, assuming that both components have equal masses, is [FORMULA] [FORMULA] (Weintraub et al. 2000 using Baraffe et al. 1998 tracks). Thus, the separation [FORMULA] at [FORMULA] pc distance corresponds to a projected separation of [FORMULA] AU and to an orbital period of [FORMULA] yrs. Assuming a circular orbit viewed pole-on, we expect [FORMULA] mas/yr orbital motion.

The location of object B relative to star A in the FORS2 image is [FORMULA] deviant from the HST images (Fig. 3b): object B lies [FORMULA] west of star A. This can be interpreted as first indication for orbital motion after the two year epoch difference. The alternative interpretation that object B is a fast moving foreground star, is extremly unlikely (see above). If this slight deviation indeed is orbital motion, the inclination is not edge-on, because we see motion in the plane of the sky. Given the good seeing and image quality at the VLT, the errors in the location of object B relative to star A should improve, if one can obtain unsaturated images. Then, one can detect curvature in the orbit within a few years.

Given the young age and spectral type M8.5 to M9 of CoD-33°7795 B, its effective temperature - using the scale intermediate between giant and dwarfs provided by Luhman (1999) - is [FORMULA] K, where the error comes from the error in the Luhman scale and the spectral type ([FORMULA] sub-types). This results in a bolometric luminosity of [FORMULA] ([FORMULA][FORMULA] (using [FORMULA] mag).

Comparing these numbers with theoretical models, we can estimate its mass: From the Burrows et al. (1997) models, we obtain [FORMULA] [FORMULA]. According to Baraffe et al. (1998), the object is located on the 10 Myrs isochrone (co-eval with the primary) with a mass of [FORMULA] [FORMULA]. With the new Chabrier et al. (2000) models, the companion has a mass of [FORMULA] [FORMULA] for an age of 1 to 20 Myrs. Overall, a range of [FORMULA] to 40 [FORMULA] is reasonable. All those models, however, are uncertain at the young age of our object.

Because CoD-33°7795 A is a spectroscopic binary and because it may soon be possible to detect orbital motion of the companion brown dwarf, masses and/or mass ratios might be determined soon. Finally, all three objects should be co-eval, so that this triple system will be a good test case for theoretical evolutionary tracks and isochrones.

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© European Southern Observatory (ESO) 2000

Online publication: August 23, 2000