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Astron. Astrophys. 354, L9-L12 (2000)

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4. Interpretation: Background or companion?

If TWA-7B orbits TWA-7, then they should form a common proper motion pair. In Fig. 2 we show the 1[FORMULA] error ellipses of the positions of TWA-7B relative to TWA-7 as obtained at the three different epochs. Taking into account the known proper motion of TWA-7, these error ellipses overlap, consistent with TWA-7B being a background object, which did not move relative to TWA-7. However, due to the large errors in the NIC2 and NTT astrometry, the data are also not inconsistent with TWA-7B being a companion. Possible orbital motion of TWA-7B could be in the direction opposite to the proper motion. A final decision should be possible after a few more years. The angular separation of [FORMULA] at a distance of [FORMULA] pc corresponds to a projected separation of [FORMULA] AU and an orbit period of [FORMULA] yrs for a circular orbit.

The color of TWA-7B is H-K =[FORMULA] mag, the absolute magnitudes at [FORMULA] pc are M[FORMULA] and M[FORMULA] mag (for negligible absorption). The errors in absolute magnitudes are mainly due to the error in distance, while apparent magnitudes are more precise. While the ground-based color does not strongly constrain the spectral type, the HST F090M-F165M-color ([FORMULA] mag) is consistent with a late K- or early M-type object, so that TWA-7B could be an unrelated background object. However, the NICMOS quantum efficiency is very low in the F090M-filter (10 to 20[FORMULA]) leading to large and uncertain color terms, and this (intermediate-band, [FORMULA]m) filter is more sensitive to spectral features than broad-band filters, so that the magnitude for this filter may be more uncertain. Also, because a young planet should be variable in the infrared due to many impacts of planetesimals and comets, one should be careful with comparing magnitudes obtained at different epoches.

The galactic model by Wainscoat et al. (1992) allows us to compute the density of sources in the direction of TWA-7. From their cumulative star counts for H[FORMULA] mag, we obtained a probability of [FORMULA] for finding one object within a [FORMULA] radius circle. Even when considering that we observed four stars, where we found one such faint object within [FORMULA], the probability for TWA-7B being an unrelated background object is small. Because the PSF of TWA-7B is consistent with a point-source rather than extended, it is unlikely that it is a background galaxy.

According to Burrows et al. (1997), the ground-based H-K color is consistent with an object with effective temperature T[FORMULA] K and surface gravity [FORMULA] g/s2. These values are consistent with an object with a mass of [FORMULA]M[FORMULA] and an age of [FORMULA] yrs, which is also the age of TWA-7 and the other TWA members. With K=[FORMULA] mag, [FORMULA] pc distance, and B.C.[FORMULA]= 2 mag, we obtained a bolometric luminosity for TWA-7B of [FORMULA] L/L[FORMULA]. An effective temperature of [FORMULA] K is similar to those of known old T-dwarfs (Burgasser et al. 1999), so that the spectrum of TWA-7B should also show strong methane absorption features. If TWA-7B is a planet, would be just [FORMULA] times more distant than the outermost jovian planet in the solar system and just [FORMULA] times more massive than the most massive planet in the solar system, so that such an object may be less surprising than the very close-in 51 Peg-type planets.

Considering all arguments, it is possible that the HST and NTT images of TWA-7B presented here are the first direct images of an extra-solar planet, but it seems more likely that this particular object is an unrelated background object. In any case, we have shown that ground-based direct imaging detection of extra-solar planets at 2.5" separation from a young star is possible with current technology (e.g. with SHARP at NTT).

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© European Southern Observatory (ESO) 2000

Online publication: January 31, 2000