Volume 412, Number 1, December II 2003
|Page(s)||185 - 198|
|Section||Interstellar and circumstellar matter|
|Published online||25 November 2003|
Dusty ring nebulae around new candidate Luminous Blue Variables
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
Corresponding author: J. S. Clark, firstname.lastname@example.org
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 – 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, 5.2 kpc to G24.73+0.69 yields a temperature, 12 kK, luminosity, log (L/) = 5.6 and mass loss rate, yr-1. G26.47+0.02 appears to be a more extreme object; adopting 6.5 kpc results in 17 kK, log (L/) = 6.0 and 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
© ESO, 2003
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