Overview

A normal main-sequence or red giant star possesses a photosphere, an outer layer of the stellar atmosphere that generates the optical photons that we observe and which is gravitationally bound to the star. Physical conditions in the layer above the photosphere vary greatly among mainsequence stars of different mass and among giant stars, many of which undergo large amplitude photospheric pulsations. Many stars possess chromospheres and, perhaps, also coronae. Mass motions are complicated with some matter falling back onto the photosphere and some being ejected to infinity. It is somewhere in this unsettled region that the outflowing circumstellar shell (CS) begins. For most stars, the circumstellar matter is at least partially transparent and the photospheric spectrum may be observed at wavelengths characteristic of the photospheric temperature. For a few stars with very large mass loss rates, such as IRC + 10216 and CRL 3068, the CS is sufficiently opaque that the photosphere is largely invisible from near infrared to ultraviolet wavelengths. For these stars we see a false photosphere at infrared wavelengths. That is, dust grains that have formed in the outflowing circumstellar gas have sufficient opacity to absorb essentially all of the true photospheric radiation. We then see a cool, roughly blackbody, emission spectrum characteristic of the temperature of dust grains in the inner portions of the CS.