Dusty ring nebulae around new candidate Luminous Blue Variables

Abstract

Abstract. 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 casespositional 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 , He  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 ) = 5.6 and mass loss rate,Ṁ = 1 × 10 −5 M 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 andṀ = 9 × 10 −5 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.

Bolometric luminosity variations in the luminous blue variable AFGL2298

“…Clark et al 2005b, and references therein). Of these, the presence of high mass circumstellar nebulae surrounding a subset of B hypergiants is particularly suggestive of enhanced mass loss in the past, possibly associated with giant eruptions (Ueta et al 2001;Clark et al 2003b;Smith & Owocki 2006). AFGL 2298 (=IRAS 18576+0341), observed as part of a long term spectroscopic and photometric monitoring campaign of (candidate) LBVs, has been found to be highly variArticle published by EDP Sciences Where not shown, error-bars are smaller than the size of the respective symbols.…”

The molecular gas around the luminous blue variable star G24.73+0.69

“…This new distance led to a substantial reduction in the calculated intrinsic luminosity of G24. Clark et al (2003) quoted log(L /L ) = 5.6 for a distance of 5.2 kpc. Taking a distance of 3.5 kpc, the luminosity drops to log(L /L ) ∼ 5.25, which would make it one of the faintest LBVs yet identified (see for comparison the HR diagram of Clark et al 2009), thus a possible post RSG/YHG object.…”

“…The ring nebula around the LBV star G24.73+0.69 (hereafter G24) was discovered by Clark et al (2003) in the mid-IR. Figure 1 displays the emission in the Spitzer-IRAC 8 μm band toward G24, which clearly shows the nebula around the central massive LBV star.…”

“…All IRDCs are considered as the precursors of protostars, being a signature of active star formation (Rathborne et al , 2007. To explain the origin of the IR shells, Clark et al (2003) argued that the inner one (i.e. the ring nebula) originates in material ejected from the central star during the LBV phase.…”

“…Using the rotational model of Fich et al (1989) and adopting a velocity of +42 km s −1 as a systemic velocity for the molecular shell, we obtained kinematic distances of either 3.5 kpc or 12 kpc. Lacking any additional discriminator between the two kinematic distances and taking into account that a distance of 5.2 kpc is quoted as an upper limit for G24 (Clark et al 2003), we adopted a distance of 3.5 kpc as the most plausible value for the LBV star and the molecular shell. This new distance led to a substantial reduction in the calculated intrinsic luminosity of G24.…”