3 Results

After the standard reduction process (sky substraction, flatfielding and bad pixel removal), we applied an optimum PSF-substraction technique developed by Pantin et al. (2000). A faint object is located in the east direction of Gl 86 (see Fig. 1) and can already be seen in the non-PSF substracted images. But applying the PSF subtraction technique it is much easier to identify, because the residual light in the wings of the coronographic image of Gl 86 is efficiently reduced.

Figure 1: Gl 86 in K band after applying the optimum PSF subtraction of Pantin et al. (2000). The PSF reference star HD 13424 was observed about 20 min after Gl 86. The total integration time on Gl 86 and on HD 13424 was 6 min each. The found companion is located between the two bars. Note also the Airy ring around the companion

3.1 Astrometry

 

 

Table 2: Astrometric data of the detected object relative to Gl 86. The first column shows the date of observation, the $\Delta \alpha$ and $\Delta \delta$ denote the measured offsets in right ascention and declination of the companion relative to Gl 86. $\Delta \alpha ^*$ and $\Delta \delta ^*$ denote the expected relative position of the object if unrelated to Gl 86 by using its proper motion

Date	$\Delta \alpha$ [mas]	$\Delta \delta$ [mas]	$\Delta \alpha ^*$ [mas]	$\Delta \delta ^*$ [mas]
08.09.2000	$1510 \pm25$	$853 \pm6$	1510	853
10.11.2000	$1522 \pm3$	$789 \pm20$	929.1	962.2
12.12.2000	$1508 \pm10$	$851 \pm13$	638.6	1019.5

Gl 86 is a high proper motion star with $\mu_{\alpha}^*=2092.7$ mas/yr and $\mu_{\delta}=654.8$ mas/yr as measured by Hipparcos. We would therefore expect the object to move 9.08 mas/day in western and 1.79 mas/day in southern direction relative to Gl 86 if it is an unrelated background star. The high proper motion of this star enables therefore to check for the binding status within the relatively short time baseline of three months.

We define the position of Gl 86 by fitting circular isophotes to the non PSF subtracted, coronographic, image. The center of the circle is determined at different isophot values, and agrees well within an error of less than about half a pixel size (i.e. $\le$ $0\hbox{$.^{\prime\prime}$ }025$). The position of the faint object is then easily found relative to this central position by fitting a Gaussian to it in the PSF subtracted image.

Applying this procedure to our datasets we find that the distance between Gl 86 and the faint object does not change significantly during our time baseline (see Table 2). We conclude that this object is indeed a gravitationaly bound companion to Gl 86 at a projected distance of $r=1\hbox{$.^{\prime\prime}$ }72 \pm 0\hbox{$.^{\prime\prime}$ }03$ and a position angle ${\rm PA}=119\pm 1 ^\circ$. From now on, we will call this object Gl 86B.

Figure 2: Color-Magnitude diagram of Gl 86B. The DUSTY model track of a 1 Gyr old object is indicated by the solid line (dots mark from right to left the models with 0.06, 0.07, 0.072, 0.075, 0.08, 0.09 and 0.1 ${M}_{\odot }$) and the dashed line represents the COND model (from Chabrier et al. 2000) of the same age (the numbers and crosses at the COND track give the mass in ${M}_{\odot }$ and the temperature). Late M dwarfs are marked by small stars and are taken from Leggett et al. (1998). Some BD companions with known absolute photometry are marked by triangles and other BDs as polygons (data from Dahn et al. 2000; Goldman et al. 1999; Kirkpatrick et al. 1999). Gl 86B apparently falls in a region not covered by the two extreme model cases: dusty or dust free atmosphere

3.2 Photometry

Gl 86B is faint, and to derive its photometry is not without difficulties. In order to reduce the large gradient in the local background of the object, caused by the residual wing left from the occulting mask, we used the PSF-subtracted images for aperture photometry. This method turned out to be very robust and converges at a certain aperture size where the background is still flat enough and not affected by the increasing noise residuals towards the mask. Having at least two data cubes per filter during different nights, we find that the measured flux of Gl 86B does not vary by more than about 20% between these images. Conservatively, we estimate our photometric error to be within 0.2-0.3 mag. We find the following magnitudes for Gl 86B: $J=14.7~(\pm0.2)$, $H=14.4~(\pm0.2)$, $K=13.7~(\pm0.2)$ and $K_{{\rm S}}=14.2~(\pm0.3)$. As the distance of Gl 86 is measured by Hipparcos a distance modulus of 0.19 can be adopted for this object. We also observed Gl 86B using the Circular Variable Filter (CVF) mode of SHARP II+ which is a narrow-band filter system allowing to select the central wavelength thus giving a resolving power of R=60. We planned to test the presence of methane absorption bands (Brandner et al. 1997), but the low count-rates, and the unavailability of PSF reference star observations did not allow us to derive reliable flux ratios for the three CFV bands chosen. Thus these observations have been used for astrometric purpose only. The magnitude difference between Gl 86 and the companion is more than 9 mag in K band. This and their small separation explain why Sterzik, Marchis & Kürster could not detect this object in their images which were taken without a coronographic mask.

Figure 3: Gl 86B placed into the Fig. 8 of Kirkpatrick et al. (2000). It shows that Gl 86B falls also in the $K_{\rm S}$ band between the known L and T dwarfs. The dashed line represents the measured $K_{\rm S}$ magnitude of Gl 86B with its error interval (dotted lines). The points with error bars represent known M and L dwarfs and are fitted by the solid line representing a second-order polynomial fit. The known T-dwarfs Gliese 229B and Gliese 570 D are also indicated and are more than 2.5 mag fainter than the latest L-dwarfs (after Kirkpatrick et al. 2000)