Dusty ring nebulae around new candidate Luminous Blue Variables

Details Dusty ring nebulae around new candidate Luminous Blue Variables Dusty ring nebulae around new candidate Luminous Blue Variables

Dec 2003

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2003a%26a...412..185c&link_type=abstract

Astronomy and Astrophysics, v.412, p.185-198 (2003)

Astronomy and Astrophysics

Astrophysics

40

Stars: Emission Line, Be, Stars: Circumstellar Matter, Stars: Winds, Outflows

Scientific paper

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) ~ 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/Lsun) = 5.6 and mass loss rate, dot {M}=1x 10-5 Msun 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/Lsun) = 6.0 and dot {M}=9x 10-5 Msun 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.

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