EDP Sciences
Free access
Volume 405, Number 1, July I 2003
Page(s) 175 - 188
Section Formation, structure and evolution of stars
DOI http://dx.doi.org/10.1051/0004-6361:20030631

A&A 405, 175-188 (2003)
DOI: 10.1051/0004-6361:20030631

A VLT spectroscopic study of the ultracompact H II region G29.96-0.02

N. L. Martín-Hernández1, A. Bik2, L. Kaper2, A. G. G. M. Tielens1, 3 and M. M. Hanson4

1  Kapteyn Institute, PO Box 800, 9700 AV Groningen, The Netherlands
2  Astronomical Institute Anton Pannekoek, University of Amsterdam, Kruislaan 403, 1098 SJ Amsterdam, The Netherlands
3  SRON, National Institute for Space Research, PO Box 800, 9700 AV Groningen, The Netherlands
4  Department of Physics, University of Cincinnati, Cincinnati, OH 45221, USA

(Received 24 March 2003 / Accepted 15 April 2003 )

A high quality, medium-resolution K-band spectrum has been obtained of the ultracompact $\ion{H}{ii}$ region G29.96-0.02 with the Very Large Telescope (VLT). The slit was positioned along the symmetry axis of the cometary shaped nebula. Besides the spectrum of the embedded ionizing O star, the long-slit observation revealed the rich emission-line spectrum produced by the ionized nebula with sub-arcsec spatial resolution. The nebular spectrum includes Br $\gamma$, several helium emission lines and a molecular hydrogen line. A detailed analysis is presented of the variation in strength, velocity and width of the nebular emission lines along the slit. The results are consistent with previous observations, but the much better spatial resolution allows a critical evaluation of models explaining the cometary shape of the nebula. Our observations support neither the wind bow shock model nor the champagne flow model. The measured line ratios of the nebular hydrogen and helium lines are compared to predictions from case B recombination-line theory. The results indicate an electron temperature between 6400 and 7500 K, in good agreement with other determinations and the Galactocentric distance of 4.6 kpc. The He +/H + ratio is practically constant over the slit; we argue that He is singly ionized throughout the nebula. We review the various observational constraints on the effective temperature of the ionizing star and show that these are in agreement with its K-band spectral type of O5-O6 V.

Key words: infrared: ISM -- ISM: lines and bands -- stars: early type -- $\ion{H}{ii}$ regions -- ISM: individual: G29.96-0.02 -- ISM: kinematics and dynamics

Offprint request: N. L. Martín-Hernández, leticia.martin@obs.unige.ch

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