*4.2. Dust Nebulae*

With the SPITZER MIPSGal survey more than 400 small (∼ 1) single bubbles were detected in 24 μm emission [66,67]. An extended sample was even derived using citizen-science and machine-learning methods [68]. For most of these bubbles no optical counterpart is known, making them heavy obscured gas and/or pure dust bubbles. Some of the bubbles contain central, NIR bright stars (some even faintly visible in the optical), while others do not show central sources at all, not even in SPITZER IRAC images 3.6 & 4.5 μm from the GLIMPSE surveys. The nature of these small bubbles were an enigma, until first classification spectra of some the bright central sources were taken [69,70]. Several of those turned out to be massive evolved stars, like blue supergiants, LBV candidates and Wolf-Rayet stars. Others were red supergiants and AGB stars. The nature and origin of the emission of the bubbles however remained uncertain. It could be hot dust, or MIR lines of ionized gas, or both. Taking SPITZER IRS spectra of several of the bubble revealed that all cases exist [71,72]: bubbles for which no central stars are detected seems to be dominated by line emission (mostly the high ionization [OIV] *λ*25.9 μm line), and are therefore most likely planetary nebulae. Bubbles with NIR visible central stars that show dust dominated IRS spectra are even less frequent. Stellar NIR spectroscopic classification again prove that the central stars are dominated by evolved massive stars [73] of various types, with several Wolf-Rayet stars, e.g., [74,75], and a number of LBV candidates or related stars [76–78]. Two of these candidates can now be seen as established LBVs (see Table 2, WS1 [79] and MN48 [80]. Also, several previously known LBVs show MIR nebulae, e.g., MWC 930 [81] or HR Car [82].

Still there are some problems with interpreting the small MIR bubbles and especially the LBV candidate interpretation. Most of these bubbles are round/spherical, e.g., [69], and bipolar structures are rare among the 24 μm bubbles. While this is consistent with the morphology of circumstellar gas nebulae of Wolf-Rayet stars, it seems to contradict the results found for LBV nebulae [52] which have as reported above a preference for bipolar morphologies. From the 24 μm images alone it is not clear, whether the nebulae are (a) dust only (b) partly dust, partly gas, or (c) dominated by ionized gas. The distribution and kinematics of the dust and gas are often different for circumstellar nebulae of massive stars, as e.g., shown for the nebula of the classical LBV AG Carinae [61]. Gail et al. [83] were among the first to investiage the problem of dust formation in an CNO precessed material like LBV envelops. Later hydrodynamical simulations of gas and dust nebulae, e.g., [84], show the small dust grains follow the gas quite well, but the larger grains show their own unique distribution. The morphologies problem may therefore be dominantly a wavelength bias. Last but not least one can speculate about the bubbles being signposts of a more general, previously overlooked short high mass-loss phase in the evolution of many massive stars. We are currently running a program with the LBT infrared spectrograph LUCI to classify more of the central stars using HK band spectroscopy.
