2012A&A...540A.131V

Spectral characterization of sub-stellar companions is essential to understand their composition and formation processes. However, the large contrast ratio of the brightness of each object to that of its parent star limits our ability to extract a clean spectrum, free from any significant contribution from the star. During the development of the long slit spectroscopy (LSS) mode of IRDIS, the dual-band imager and spectrograph of SPHERE for the Very Large Telescope (VLT), we proposed a data analysis method to estimate and remove the contributions of the stellar halo and speckles to the spectra. This method has never been tested on real data because of the lack of instrumentation capable of combining adaptive optics (AO), coronagraphy, and LSS. Nonetheless, a similar attenuation of the star can be obtained using a particular observing configuration where the slit is positioned on the faint companion while keeping the bright primary outside. Test data were acquired with this slit configuration using the AO-assisted spectrograph VLT/NACO. We obtained new J- and H-band spectra of SCR J1845-6357 B, a T6 companion to a nearby (3.85±0.02pc) M8 star. This system is a well-suited benchmark as it is relatively wide (∼1.0'') with a modest contrast ratio (∼4mag), and a previously published JHK spectrum is available for reference. We demonstrate that (1) our method is efficient at estimating and removing the stellar contribution, (2) it allows us to properly recover the overall spectral shape of the companion spectrum, and (3) it is essential to obtain an unbiased estimation of physical parameters. We also show that the slit configuration associated with this method allows us to use long exposure times with high throughput producing high signal-to-noise ratio data. However, the signal of the companion gets over-subtracted, particularly in our J-band data, compelling us to use a fake companion spectrum to estimate and compensate for the loss of flux. Our spectral analysis provides an estimation of T_eff=1000±100K leading to R=0.7±0.1R_Jup, a value that closely agrees with evolutionary models for ages older than 1.5Gyr. Finally, we report a new astrometric measurement of the position of the companion (sep=0.817'', PA=227.92deg), which has undergone a significant proper motion since the previous measurement.