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.

We have observed the central region of the IR-dark cloud filament associated with IRAS 18507+0121 at millimeter wavelengths in CO(J=1−0), 13 CO(J=1−0), and C 18 O(J=1−0) line emission and with the Spitzer Space Telescope at mid-IR wavelengths. Five massive outflows from two cloud cores were discovered. Three outflows are centered on or near an Ultracompact Hii (UC Hii) region (G34.4+0.23) while the remaining two outflows originate from the millimeter core G34.4+0.23 MM. Modeling of the SEDs of the mid-IR sources identified 31 young stellar objects in the filament with a combined stellar mass of ∼ 127 ± 27 M . An additional 22 sources were identified as probable cluster members based on the presence of strong 24 µm emission. The total star formation efficiency in the G34.4 cloud filament is estimated to be ∼ 7% while the massive and intermediate mass star formation efficiency in the entire cloud filament is estimated to be

The formation of low-mass stars like our Sun can be explained by the gravitational collapse of a molecular cloud fragment into a protostellar core and the subsequent accretion of gas and dust from the surrounding interstellar medium. Theoretical considerations suggest that the radiation pressure from the protostar on the in-falling material may prevent the formation of stars above ten solar masses through this mechanism, although some calculations have claimed that stars up to 40 solar masses can in principle be formed via accretion through a disk. Given this uncertainty and the fact that most massive stars are born in dense clusters, it was suggested that high-mass stars are the result of the runaway merging of intermediate-mass stars. Here we report observations that clearly show a massive star being born from a large rotating accretion disk. The protostar has already assembled about 20 solar masses, and the accretion process is still going on. The gas reservoir of the circumstellar disk contains at least 100 solar masses of additional gas, providing sufficient fuel for substantial further growth of the forming star.

Abstract. We present CS (2-1) and CS (3-2) observations of the molecular gas associated with the Galactic starburst template NGC 3603, over an area of 5. 8 × 16. 7, with the OB cluster at the center. Total velocity integrated maps and channel maps give insight into the spatial distribution and the kinematic structure of the dense gas in the giant molecular cloud from which the starburst cluster originated. We identify 13 molecular clumps with radii less than 0.8 pc and derive upper limits for their virial masses as well as lower limits for their H 2 column densities:One of the clumps, MM 11, clearly stands out with a mass and column density 4 times higher than average. The CS (3-2) / CS (2-1) map shows higher intensity ratios to the south of the OB cluster than to the north (0.80 ± 0.08 versus 0.32 ± 0.11), which indicates a substantial difference in the physical conditions (either opacities or excitation temperatures) of the molecular gas. From the average of the line peak velocities, 14.2 ± 1.6 km s −1 , we deduce a kinematic distance of 7.7 ± 0.2 kpc for NGC 3603. We estimate the star formation efficiency ( > ∼ 30%) of the central part of the NGC 3603 H  region. If we assume the age of the OB cluster to be less than 3 Myr and the star formation rate to be larger than 1.3 × 10 −3 M yr −1 , the derived timescale for gas removal (τ ∼ 6 Myr) can explain why the starburst cluster itself is nearly void of interstellar material. The remnant clump MM 1 appears to constitute the head of a prominent pillar which still becomes dispersed by ionizing radiation and stellar winds originating from the massive stars of the cluster. Because some of the molecular clumps are associated with near and mid infrared sources as well as OH, H 2 O and CH 3 OH maser sources we conclude that star formation is still going on within NGC 3603.

We have observed the massive star-forming region IRAS 18507+0121 at millimeter wavelengths in 3 mm continuum emission, H 13 CO + ( J ¼ 1 0) and SiO (v ¼ 0, J ¼ 2 1) line emission, and at near-infrared (NIR) wavelengths between 1.2 and 2.1 m. Two compact molecular cores are detected: one north and one south, separated by $40 00 . The northern molecular core contains a newly discovered, deeply embedded, B2 protostar surrounded by several hundred solar masses of warm gas and dust, G34.4+0.23 MM. Based on the presence of warm dust emission and the lack of detection at NIR wavelengths, we suggest that G34.4+0.23 MM may represent the relatively rare discovery of a massive protostar (e.g., analogous to a low-mass ''Class 0'' protostar). The southern molecular core is associated with an NIR cluster of young stars and an ultracompact H ii region, G34.4+0.23, with a central B0.5 star. The fraction of NIR stars with excess infrared emission indicative of circumstellar material is greater than 50%, which suggests an upper limit on the age of the IRAS 18507+0121 starforming region of 3 Myr.