Dust in R71: first detection of crystalline silicates in the LMC

Abstract

We present infrared spectroscopy taken with the Infrared Space Observatory (ISO) of the Luminous Blue Variable (LBV) R71 in the Large Magellanic Cloud (LMC). The spectrum shows clear evidence for the presence of crystalline olivine at 23.5 mu m. This is the first detection of circumstellar crystalline silicates outside our galaxy. In addition, we identify emission at 6.2, 7.7 and possibly 8.6 mu m from C-rich small grains (PAHs). The presence of C-rich grains is not expected in an environment where C/O is less than 1. We fit the dust spectrum using a radiative transfer model and find a dust mass of 0.02 M_sun. R71 was probably a Red Supergiant when it produced the dust shell and had a time-averaged mass loss rate of the order of 7*E(-4) M_sun yr(-1) for a gas/dust ratio of 100. based on observations obtained with ISO, an ESA project with instruments funded by ESA Member states (especially the PI countries: France, Germany, the Netherlands and the United Kingdom) with the participation of ISAS and NASA