High resolution (lambda / Delta-lambda = 50,000) K-band spectra of massive, embedded, young stellar objects are presented. The present sample consists of four massive young stars located in nascent clusters powering Galactic giant H II regions. Emission in the 2.3 micron 2--0 vibrational--rotational bandhead of CO is observed. A range of velocity broadened profiles seen in three of the objects is consistent with the emission arising from a circumstellar disk seen at various inclination angles. Br gamma spectra of the same spectral and spatial resolution are also presented which support an accretion disk or torus model for massive stars. In the fourth object, Br emission suggesting a rotating torus is observed, but the CO profile is narrow, indicating that there may be different CO emission mechanisms in massive stars and this is consistent with earlier observations of the BN object and MWC 349. To--date, only young massive stars of late O or early B types have been identified with clear accretion disk signatures in such embedded clusters. Often such stars are found in the presence of other more massive stars which are revealed by their photospheric spectra but which exhibit no disk signatures. This suggests the timescale for dissipating their disks is much faster than the less massive OB stars or that the most massive stars do not form with accretion disks.Comment: 28 pages, 10 Figures, accepted for publication in the Astrophysical Journa

We present a near‐infrared study of the stellar content of 35 H ii regions in the Galactic plane, 24 of which have been classified as giant H ii regions. We have selected these optically obscured star‐forming regions from the catalogues of Russeil, Conti & Crowther and Bica et al. In this paper, we have used the near‐infrared domain J‐, H‐ and Ks‐band colour images to visually inspect the sample. Also, we have used colour‐colour and colour–magnitude diagrams to indicate ionizing star candidates, as well as the presence of young stellar objects such as classical T Tauri stars and massive young stellar objects (MYSOs). We have obtained Spitzer Infrared Array Camera images for each region to help further characterize them. Spitzer and near‐infrared morphology were used to place each cluster in an evolutionary phase of development. Spitzer photometry was also used to classify the MYSOs. A comparison of the main sequence in the colour–magnitude diagrams for each observed cluster was used to infer whether or not the cluster kinematic distance is consistent with brightnesses of the stellar sources. We find qualitative agreement for a dozen of the regions, but about half the regions have near‐infrared photometry that suggests they may be closer than the kinematic distance. A significant fraction of these already have spectrophotometric parallaxes that support smaller distances. These discrepancies between kinematic and spectrophotometric distances are not a result of the spectrophotometric methodologies, as independent non‐kinematic measurements are in agreement with the spectrophotometric results. For instance, the trigonometric parallaxes of star‐forming regions were collected from the literature and show the same effect of smaller distances when compared to the kinematic results. In our sample of H ii regions, most of the clusters are evident in the near‐infrared images. Finally, it is possible to distinguish among qualitative evolutionary stages for these objects.

We present the results from a survey, designed to investigate the accretion process of massive young stellar objects (MYSOs) through near infrared narrow band imaging using the H 2 ν=1-0 S(1) transition filter. A sample of 353 Massive Young Stellar Object (MYSO) candidates was selected from the Red MSX Source survey using photometric criteria at longer wavelengths (infrared and submillimeter) and chosen with positions throughout the Galactic Plane. Our survey was carried out at the SOAR Telescope in Chile and CFHT in Hawaii covering both hemispheres. The data reveal that extended H 2 emission is a good tracer of outflow activity, which is a signpost of accretion process on young massive stars. Almost half of the sample exhibit extended H 2 emission and 74 sources (21%) have polar morphology, suggesting collimated outflows. The polar-like structures are more likely to appear on radio-quiet sources, indicating these structures occur during the pre-UCH ii phase. We also found an important fraction of sources associated with fluorescent H 2 diffuse emission that could be due to a more evolved phase. The images also indicate only ∼23% (80) of the sample is associated with extant (young) stellar clusters. These results support the scenario in which massive stars are formed by accretion disks, since the merging of low mass stars would not produce outflow structures.

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We present K-band spectra of the near infrared counterparts to IRS 2E and IRS 2W which is associated with the ultracompact H ii region W51d, both of them embedded sources in the Galactic compact H ii region W51 IRS 2. The high spatial resolution observations were obtained with the laser guide star facility and Near-infrared Integral Field Spectrograph (NIFS) mounted at the Gemini-North observatory. The spectrum of the ionizing source of W51d shows the photospheric features N iii (21155 ) in emission and He ii (21897 ) in absorptionÅ A which lead us to classify it as a young O3 type star. We detected CO overtone in emission at 23000 in the A spectrum of IRS 2E, suggesting that it is a massive young object still surrounded by an accretion disk, probably transitioning from the hot core phase to an ultracompact H ii region.