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. We are studying the extremely young cluster of M 17 to investigate the birth of high-mass stars and the initial mass function. Methods. Deep JHKL imaging and K-band spectroscopy from the VLT of 201 stars toward the cluster is presented.Results. The majority of 104 stars show the CO band-head in absorption. Half of them emit X-rays and/or have infrared excess, indicative of very young objects. Their intrinsic IR luminosity is compatible with intermediate and high-mass pre-main sequence stars. Nine additional stars have the CO feature in emission, while sixty sources are lacking any stellar spectral feature due to veiling by circumstellar dust. Conclusions. We suggest that CO absorption is -as in the case of low-mass stars -also a common feature during the early evolution of stars with higher masses. According to model calculations the observed CO absorption is most likely a sign of heavily accreting protostars with mass accretion rates above 10 −5 M yr −1 .

We investigate the morphology and the evolutionary stage of the hypercompact H ii region M17-UC1 using observations at infrared wavelengths and NIR radiative transfer modeling. For the first time, this region is resolved into two emission areas separated by a dark lane reminiscent of an obscuring silhouette caused by a circumstellar disk. So far, the observational data as well as model calculations suggest that M17-UC1 is surrounded by a disk of cool dust. This direct detection of a circumstellar disk candidate around a hypercompact H ii region is in agreement with the expectations of the disk accretion model for high-mass star formation.

Abstract. We combined the ISOCAM Parallel Mode Survey at 6.7 µm (LW2 filter) with the Two Micron All Sky Survey in order to obtain a powerful tool to search for AGN independent of dust extinction. Using moderate colour criteria H − K > 0.5 and K − LW2 > 2.7 we have selected a sample of 77 AGN candidates in an effective area of ∼10 square degrees. By means of optical spectroscopy we find 24 (∼30%) type-1 QSOs at redshifts 0.1 < z < 2.3; nine of them have z > 0.8. About one third of the ISO-2MASS QSOs show so red optical colours, that they are missed in optical and UV AGN surveys like SDSS, 2DF, or HES. With a surface density of about 2 deg −2 down to R < 18 mag the ISO-2MASS QSOs outnumber the 1.35 deg −2 of the SDSS quasar survey by 50%; we find a combined optical-IR QSO surface density of 2.7 deg −2 . Since only two of the ISO-2MASS QSOs have also J − K > 2, the inclusion of the ISO mid-infrared photometry significantly extends the capabilities of the pure 2MASS red AGN survey. We suggest that the newly found red AGN resemble young members of the quasar population, and that quasars spend much of their lifetime in a dust enshrouded phase.

While the formation of low-mass stars has become a well-studied process, it is still difficult to verify a similar evolutionary sequence for massive stars. Although several young stages from massive starless cores to massive protostellar candidates with jets and outflows have been observed, massive star/disk systems whose properties can be inferred uniquely are rare. The final stage of this sequence, i.e., a newborn massive star that is still surrounded by a remnant disk, is missing. This is probably a consequence of the rapid evolution of these systems and the early destruction of the disk in the vicinity of a massive star. We report on an optically visible young massive star (IRS 15) within M17 that displays a huge IR excess. This fortunate coincidence offers the rare opportunity to investigate the star as well as its circumstellar environment in great detail. We have performed both optical and infrared photometry and spectroscopy of the stellar source; in addition, its circumstellar environment has been investigated by mid-infrared imaging. Our data suggest that IRS 15 is a star of about surrounded by a huge remnant 26 M , disk of about half a Jupiter mass of dust. From this, we corroborate that massive stars can form by disk accretion and conclude that also their circumstellar disks evolve like those of low-mass stars.