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VLT K-band spectroscopy of massive stars deeply embedded in IRAS sources with UCHII colours
We have obtained high resolution (R = 10 000) K-band spectra ofcandidate young massive stars deeply embedded in (ultra-) compact H IIregions (UCHIIs). These objects were selected from a near-infraredsurvey of 44 fields centered on IRAS sources with UCHII colours. Often,the near-infrared counterpart of the IRAS source is a young embeddedcluster hosting massive stars. In these clusters, three types of objectsare identified. The first type (38 objects) consists of "naked" OB starswhose K-band spectra are dominated by photospheric emission. We classifythe K-band spectra of the OB-type cluster members using near-infraredclassification criteria. A few of them have a very early (O3-O4 V)spectral type, consistent with a young age of the embedded clusters. Thespectral classification provides an important constraint on the distanceto the embedded cluster. The ionising power of the population thusderived is compared to the information obtained from the infrared andradio flux of these sources. In most cases these two differentdeterminations of the ionising flux are consistent, from which weconclude that we have identified the ionising star(s) in about 50% ofthe embedded clusters. The second type (7 objects) are point sourcesassociated with UCHII radio emission, that exhibit nebular emissionlines in the near-infrared. Six of the objects in this group produce HeI emission indicative of an embedded O-type star. These objects are moreembedded than the OB stars and probably do not dominate the infraredflux as measured by IRAS. They may emit the bulk of their reprocessed UVradiation at mm wavelengths. The third type (20 objects) ischaracterised by broad (100-200 km s-1) Brγ emissionand no photospheric absorption profiles. Bik et al. (2005, A&A,submitted) show that these objects are massive YSO candidates surroundedby dense circumstellar disks.
| The distance and structure of the Coalsack. I - Photometric data
The results of UBV and H-beta photometry for 284 stars of spectral typesB to F located in the area of the dark cloud Coalsack are presented.Most of the present stars are brighter than 10.0 mag and are classifiedin the MK system of Houk and Cowley (1975). For each star, the bestvalues for the extinction and distance modulus are provided, withtypical estimated errors of + or - 0.08 mag for the extinction and + or- 0.32 mag for the distance modulus. Possible misclassifications andspectral peculiarities are noted for several of the stars.
| Stromgren and H-beta photometry of stars earlier than G0 in the Southern Coalsack direction
Four-color and H-beta photometry have been obtained for 236 starsearlier than G0 in order to study the color excess distribution in theSouthern Coalsack direction. The beta histogram of the stars reveals arelative absence of stars in the range of A4 to A9. The (b-y)distribution for B-type stars suggests the existence of three differenttypes of stars located at different distances.
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Observation and Astrometry data
| Constellation: | Σταυρός Νότιος |
| Right ascension: | 12h40m08.60s |
| Declination: | -63°27'12.1" |
| Apparent magnitude: | 8.963 |
| Distance: | 507.614 parsecs |
| Proper motion RA: | -11.4 |
| Proper motion Dec: | 0.9 |
| B-T magnitude: | 9.238 |
| V-T magnitude: | 8.986 |
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