VLT K-band spectroscopy of massive stars deeply embedded in IRAS sources with UCHII colours

¹ Astronomical Institute “Anton Pannekoek”, University of Amsterdam, Kruislaan 403, 1098 SJ Amsterdam, The Netherlands e-mail: abik@eso.org
² European Southern Observatory, Karl-Schwarzschild Strasse 2, 85748 Garching-bei-München, Germany
³ University of Cincinnati, Cincinnati, OH 45221-0011, USA

Received: 22 December 2004
Accepted: 18 March 2005

Abstract

We have obtained high resolution () K-band spectra of candidate young massive stars deeply embedded in (ultra-) compact  regions (UCHIIs). These objects were selected from a near-infrared survey of 44 fields centered on IRAS sources with UCHII colours. Often, the near-infrared counterpart of the IRAS source is a young embedded cluster hosting massive stars. In these clusters, three types of objects are identified. The first type (38 objects) consists of “naked” OB stars whose K-band spectra are dominated by photospheric emission. We classify the K-band spectra of the OB-type cluster members using near-infrared classification criteria. A few of them have a very early (O3-O4 V) spectral type, consistent with a young age of the embedded clusters. The spectral classification provides an important constraint on the distance to the embedded cluster. The ionising power of the population thus derived is compared to the information obtained from the infrared and radio flux of these sources. In most cases these two different determinations of the ionising flux are consistent, from which we conclude that we have identified the ionising star(s) in about 50% of the embedded clusters. The second type (7 objects) are point sources associated with UCHII radio emission, that exhibit nebular emission lines in the near-infrared. Six of the objects in this group produce  emission indicative of an embedded O-type star. These objects are more embedded than the OB stars and probably do not dominate the infrared flux as measured by IRAS. They may emit the bulk of their reprocessed UV radiation at mm wavelengths. The third type (20 objects) is characterised by broad (100-200 km s^-1) Brγ emission and no photospheric absorption profiles. Bik et al. (2005, A&A, submitted) show that these objects are massive YSO candidates surrounded by dense circumstellar disks.