Direct evidence of a sub-stellar companion around CT Chamaeleontis

Aims. In our ongoing search for close and faint companions around T Tauri stars in the Chamaeleon star-forming region, we here present observations of a new common proper motion companion to the young T-Tauri star and Chamaeleon member CT Cha and discuss its properties in comparison to other young, low-mass objects and to synthetic model spectra from different origins. Methods. Common proper motion of the companion and CT Cha was confirmed by direct Ks-band imaging data taken with the VLT Adaptive Optics (AO) instrument NACO in February 2006 and March 2007, together with a Hipparcos binary for astrometric calibration. An additional J-band image was taken in March 2007 to obtain color information for a first classification of the co mpanion. Moreover, AO integral field spectroscopy with SINFONI in J, a nd H+K bands was obtained to deduce physical parameters of the companion, such as temperature and extinction. Relative flu x calibration of the bands was achieved using photometry from the NACO imaging data. Results. We found a very faint (Ks= 14.9 mag, Ks0 = 14.4 mag) object, just∼ 2.67 ′′ northwest of CT Cha corresponding to a projected separation of∼ 440 AU at 165± 30 pc. We show that CT Cha A and this faint object form a common proper motion pair and that the companion is by≥ 4σ significance not a stationary background object. The near-i nfrared spectroscopy yields a temperature of 2600± 250 K for the companion and an optical extinction of AV= 5.2± 0.8 mag, when compared to spectra calculated from Drift-Phoenix model atmospheres. We demonstrate the validity of the model fits by comparison to several other well-know n young sub-stellar objects. Conclusions. We conclude that the CT Cha companion is a very low-mass member of Chamaeleon and very likely a physical companion to CT Cha, as the probability for a by chance alignment is≤ 0.01. Due to a prominent Pa-β emission in the J-band, accretion is probably still ongoing onto the CT Cha companion. From temperature and luminosity (log(Lbol/L⊙)= ‐2.68± 0.21), we derive a radius of R= 2.20 +0.81 −0.60 RJup. We find a consistent mass of M= 17± 6 MJup for the CT Cha companion from both its luminosity and temperature when placed on evolutionary tracks. Hence, the CT Cha companion is most likely a wide brown dwarf companion or possibly even a planetary mass object.