VLT K-band spectroscopy of massive young stellar objects in (ultra-)compact HII regions

Free access article

Issue		A&A Volume 455, Number 2, August IV 2006


Page(s)		561 - 576
Section		Interstellar and circumstellar matter
DOI		http://dx.doi.org/10.1051/0004-6361:20042403

A&A 455, 561-576 (2006)
DOI: 10.1051/0004-6361:20042403

VLT K-band spectroscopy of massive young stellar objects in (ultra-)compact HII regions

A. Bik^{1, 2}, L. Kaper¹ and L. B. F. M. Waters^{1, 3}

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, Garching-bei-München, 85748, Germany
³ Instituut voor Sterrenkunde, Katholieke Universiteit Leuven, Celestijnenlaan 200B, 3001 Heverlee, Belgium

(Received 22 November 2004 / Accepted 10 May 2006 )

Abstract
High-quality K-band spectra of strongly reddened point sources, deeply embedded in (ultra-)compact $\ion{H}{ii}$ regions, have revealed a population of 20 young massive stars showing no photospheric absorption lines, but sometimes strong Br $\gamma$ emission. The Br $\gamma$ equivalent widths occupy a wide range (from about 1 to over 100 Å); the line widths of 100-200 km s^-1 indicate a circumstellar rather than a nebular origin. The K-band spectra exhibit one or more features commonly associated with massive young stellar objects (YSOs) surrounded by circumstellar material: a very red colour $(J-K) \ga 2$ , CO bandhead emission, hydrogen emission lines (sometimes doubly peaked), and $\ion{Fe}{ii}$ and/or $\ion{Mg}{ii}$ emission lines. The large number of objects in our sample allows a more detailed definition and thorough investigation of the properties of the massive YSOs. In the (K, J-K) colour-magnitude diagram (CMD) the massive YSO candidates are located in a region delimited by the OB zero-age main sequence, Be stars, Herbig Ae and Be stars, and B[e] supergiants. The massive YSO distribution in the CMD suggests that the majority of the objects are of similar spectral type as the Herbig Be stars, but some of them are young O stars. The spectral properties of the observed objects do not correlate with the location in the CMD. The CO emission must come from a relatively dense ( ${\sim} 10^{10}~ \mathrm{cm}^{-3}$ ) and hot ( $T\sim 2000{-}5000$ K) region, sufficiently shielded from the intense UV radiation field of the young massive star. The hydrogen emission is produced in an ionised medium exposed to UV radiation. The best geometrical solution is a dense and neutral circumstellar disk causing the CO bandhead emission, and an ionised upper layer where the hydrogen lines are produced. We present arguments that the circumstellar disk is more likely a remnant of the accretion process than the result of rapid rotation and mass loss such as in Be/B[e] stars.

Key words: infrared: stars -- stars: formation -- stars: early-type -- stars: circumstellar matter -- stars: pre-main sequence

What is OpenURL?

The OpenURL standard is a protocol for transmission of metadata describing the resource that you wish to access. An OpenURL link contains article metadata and directs it to the OpenURL server of your choice. The OpenURL server can provide access to the resource and also offer complementary services (specific search engine, export of references...). The OpenURL link can be generated by different means.

If your librarian has set up your subscription with an OpenURL resolver, OpenURL links appear automatically on the abstract pages.
You can define your own OpenURL resolver with your EDPS Account. In this case your choice will be given priority over that of your library.
You can use an add-on for your browser (Firefox or I.E.) to display OpenURL links on a page (see http://www.openly.com/openurlref/). You should disable this module if you wish to use the OpenURL server that you or your library have defined.