Dusty ring nebulae around new candidate Luminous Blue Variables

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/ …)  = 5.6 and mass loss rate, M = 1*10^-5 M yr^-1 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)  = 6.0 and …  M 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.