The stellar content of M17 has been investigated by multicolor photometry and spectroscopy. Various independent estimates yield a distance of 2:1 AE 0:2 kpc. The ratio of total-to-selective extinction is R ¼ 3:9. Within a projected area of 3:6 ; 3:7 pc, there are several thousand stars. About 74% of them show infrared excess suggesting the presence of dense circumstellar material; the excess frequency is higher for fainter stars. The number of spectroscopically classified exciting stars could be enlarged from 13 to 46. The two central O4 stars are both spectroscopic binaries; multiplicity of other early O-type stars could also be established, increasing the number of high-mass stars even further. Our data suggest at least two episodes of star formation: There are about 500 ZAMS sources (2 < A V < 7)-among them many spectroscopically classified OB stars and a significant fraction of lower mass sources with infrared excess ($25%) and X-ray emission ($6%). About 3350 heavily reddened sources with 10 < A V < 40) are most likely deeply embedded pre-main-sequence objects with an age of less than 5 ; 10 5 yr. This group contains about 47% sources with infrared excess and 12% X-ray emitters. Cluster members later than about A0 have not yet reached the main sequence. In addition, a group of 647 protostellar candidates (1:5 < K À L < 6:9) has been detected in the cluster center as well as in the northern and southwestern bar. This population of accreting protostars argues in favor of ongoing star formation triggered by the central O stars in M17.

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 companion. Moreover, AO integral field spectroscopy with SINFONI in J, and H + K bands was obtained to deduce physical parameters of the companion, such as temperature and extinction. Relative flux calibration of the bands was achieved using photometry from the NACO imaging data. Results. We found a very faint (Ks = 14.9 mag, Ks 0 = 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-infrared spectroscopy yields a temperature of 2600 ± 250 K for the companion and an optical extinction of A V = 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-known 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(L bol /L ) = -2.68 ± 0.21), we derive a radius of R = 2.20 +0.81 −0.60 R Jup . We find a consistent mass of M = 17 ± 6 M Jup 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.

The formation of massive planetary or brown dwarf companions at large projected separations from their host star is not yet well understood. In order to put constraints on formation scenarios we search for signatures in the orbit dynamics of the systems. We are specifically interested in the eccentricities and inclinations since those parameters might tell us about the dynamic history of the systems and where to look for additional low-mass sub-stellar companions. For this purpose we utilized VLT/NACO to take several well calibrated high resolution images of 6 target systems and analyze them together with available literature data points of those systems as well as Hubble Space Telescope archival data. We used a statistical Least-Squares Monte-Carlo approach to constrain the orbit elements of all systems that showed significant differential motion of the primary star and companion. We show for the first time that the GQ Lup system shows significant change in both separation and position angle. Our analysis yields best fitting orbits for this system, which are eccentric (e between 0.21 and 0.69), but can not rule out circular orbits at high inclinations. Given our astrometry we discuss formation scenarios of the GQ Lup system. In addition, we detected an even fainter new companion candidate to GQ Lup, which is most likely a background object. We also updated the orbit constraints of the PZ Tel system, confirming that the companion is on a highly eccentric orbit with e>0.62. Finally we show with a high significance, that there is no orbital motion observed in the cases of the DH Tau, HD 203030 and 1RXS J160929.1-210524 systems and give the most precise relative astrometric measurement of the UScoCTIO 108 system to date.