Warm dust around blue hypergiants: Mid-infrared imaging of the luminous blue variable HD 168625

We present the first high spatial resolution images of thermal emission from the nebula surrounding the Galactic luminous blue variable (LBV) HD 168625. Four images, centered at 4.7, 10.1, 11.6, and 19.9 mu m, have been obtained at UKIRT with MAX, the new mid-IR camera of the Max-Planck-Institut fur Astronomie. A Bry image taken at UKIRT with IRCAM3 is also presented. On the basis of the available spectrophotometric data and of our photometry of the central star, we revise the distance currently assumed for this source from 2.2 to 1.2 kpc. The dust emission, which is optically thin and therefore sensitive to the mass distribution, clearly indicates the bipolar structure of the nebula. The majority of the infrared (IR) flux comes from a geometrically thin layer of dust tracing the outer edges of the nebular lobes. An inner shell or ring along the equatorial plane is also detected. From the mid-IR spectral energy distribution we constrain the composition and temperature of the grains. Through a simple geometrical model we estimate the dust density and total mass in the shell. Considering the structure of the nebula from an evolutionary point of view, we investigate the formation mechanism (novalike ejection or interacting stellar winds) and the recent past of the LBV precursor. We find that the interaction of the present wind with a slower dusty wind ((M) over dot similar or equal to 2 x 10(-5) M-. yr(-1), upsilon similar or equal to 10 km s(-1)) ejected by the stellar precursor in a previous phase is by far the most viable formation mechanism for the nebula around HD 168625. Finally, the general occurrence of a redward evolution for the LBV precursors is discussed.