EDP Sciences
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Volume 412, Number 1, December II 2003
Page(s) 185 - 198
Section Formation, structure and evolution of stars
DOI http://dx.doi.org/10.1051/0004-6361:20031372

A&A 412, 185-198 (2003)
DOI: 10.1051/0004-6361:20031372

Dusty ring nebulae around new candidate Luminous Blue Variables

J. S. Clark1, M. P. Egan2, P. A. Crowther1, 3, D. R. Mizuno4, V. M. Larionov5, 6 and A. Arkharov7

1  Department of Physics and Astronomy, University College London, Gower Street, London, WC1E 6BT, UK
2  US Air Force Research Laboratory: MDA/AS, 7100 Defence Pentagon, Washington, DC 20301-7100, USA
3  Department of Physics and Astronomy, University of Sheffield, Hounsfield Road, Sheffield, S3 7RH, UK
4  Institute for Scientific Research, Boston College, 140 Commonwealth Avenue, Chestnut Hill, MA 02467-2862, USA
5  Astronomical Institute of St. Petersburg University, St. Petersburg, Petrodvorets, Universitetsky pr. 28, 198504 St. Petersburg, Russia
6  Isaac Newton Institute of Chile, St. Petersburg Branch, Russia
7  Central Astronomical Observatory, 196140 St. Petersburg, Russia

(Received 2 June 2003 / Accepted 2 September 2003)

We report on the discovery of a further two ring nebulae in the Midcourse Space Experiment (MSX) Galactic Plane Survey; G24.73+0.69 and G26.47+0.02 . Morphologically, both appear similar to the nebulae found around the Luminous Blue Variable (LBV) candidates G79.29+0.46 and Wra 17-96 . A central, unresolved point source was identified in both cases - positional coincidence with the star StRS 237 was found for G26.47+0.02 , while no optical counterpart could be identified for G24.73+0.69 . However, subsequent near IR broadband imaging of the G24.73+0.69 field identified a very red - $(J-K) \sim$ 2 mag - stellar counterpart to the central object. Near-IR spectroscopy of both objects reveal rich emission line spectra dominated by H I, He I and low excitation metals, suggesting classification as luminous B supergiants and revealing a striking superficial similarity to the other MSX ring sources and known LBVs. We utilised a NLTE model atomsphere code to model the  K band spectra and near-IR spectral energy distributions of the central stars in order to determine their physical parameters. Adopting a distance, d= 5.2 kpc to G24.73+0.69 yields a temperature, T= 12 kK, luminosity, log ( L/ $L_{\odot}$)  = 5.6 and mass loss rate, $\dot{M}=1\times 10^{-5}~M_{\odot}$ yr -1. G26.47+0.02 appears to be a more extreme object; adopting d= 6.5 kpc results in T= 17 kK, log ( L/ $L_{\odot}$)  = 6.0 and $\dot{M}=9\times 10^{-5}$  $M_{\odot}$ yr -1, placing it at the Humphreys-Davidson limit for massive stellar objects. Analysis of the spatially resolved mid-IR fluxes of both objects reveal extended periods of enhanced mass loss, resulting in comparatively low mass nebulae, with chemistries dominated by O-rich dust (with a population of small Fe grains existing co-spatially with the silicate dust). Comparison to the other MSX ring nebulae sources reveals a homogeneous group of objects, with both stellar and nebular properties consistent with known LBVs. With both spectroscopic and/or photometric variability observed for those sources with multiepoch observations, we propose a close affinity between both classes of object and suggest that long term monitoring of the MSX sources will reveal them to be bona fide LBVs.

Key words: stars: emission line, Be -- stars: circumstellar matter -- stars: winds, outflows

Offprint request: J. S. Clark, jsc@star.ucl.ac.uk

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