Circumstellar Infrared Emission. I: The Circumstellar Origin of Interstellar Dust
Authors: Woolf Neville J. 
Published: July 2016

Infrared astronomy has in the past decade emerged from being a part-time occupation of a few astronomers. Three major subdivisions of research have become apparent, solar system, galactic and extragalactic studies. In each of these fields infrared studies have made unique contributions. Planets emit the bulk of their radiation in the infrared, and infrared studies are essential to study planetary thermal problems. Many extragalactic objects have been found to emit astonishingly large fluxes in the infrared.In galactic astronomy the current major contribution of infrared studies has been to act as a bridge between two separate disciplines, stellar astronomy, and studies of the interstellar medium. Infrared studies have proved invaluable for studying star birth and star death. Both of these phases had previously seemed mysterious and invisible. And indeed they were not visible, because they occurred shrouded in dust that blocked transmission of visible rays. However, the dust that is merely opaque in the visible, is self-luminous in the infrared, and so in the midst of this optical darkness there has appeared a great infrared light.At this time, we have progressed further with the study of star death than of star birth. The ejected matter from dying stars carries the dust shroud with it into space, and so the gas and dust become part of the interstellar medium. This process is clearly significant for understanding the composition and origin of interstellar dust.Because star death and birth are embedded in dust, there has developed a separate interest in explaining the physical processes at work in these dust clouds. This study explains processes of optical circumstellar absorption lines, intrinsic polarization of cool star light, and stellar molecular masers.Perhaps what these two paragraphs have just said is that our conceptual scheme of separating stellar astronomy and interstellar astronomy still acts as such a division that the infrared astronomer needs to present different aspects of this one topic, circumstellar infrared emission to different audiences. Such an opportunity has been given to the author in that he has been asked to give within a few weeks two talks. The first of these reviews is being presented at IAU Symposium # 52 on Interstellar Dust and Related Topics. The second is being given at the summer meeting of the Astronomical Society of the Pacific which has a symposium on Circumstellar Dust.The two reviews have been made complementary. The first of these is primarily an observational study. It shows the infrared observations of stellar and interstellar dust, and in a qualitative way shows that one gives rise to the other. The second review is theoretical and attempts to place the first study in its theoretical context. It deals almost exclusively with the stellar and circumstellar parts of the topic. Together they present one man's view of Circumstellar Infrared Emission.The literature relevant to this topic is voluminous. There have been false leads, dead ends, and irrelevant detail. This review has attempted to follow a thread through this detail, and to expose the skeleton of a scheme for understanding the processes at work. Such a review is intrinsically more dangerous, more likely to become obsolete than a comprehensive one. However, by carrying the seeds of its own destruction it seems to offer a greater opportunity for the growth of astronomy.