The Search for Infrared Protostars

Abstract

Protostars are the Holy Grail of infrared astronomy. In the 15 years since the discovery of the BN object in Orion by Becklin & Neugebauer (1967) and the strong infrared emission from R Mon by Mendoza (1966), infrared astronomers have devoted much of their efforts toward trying to understand the birth of stars. They have often hoped, and sometimes believed, that their studies would yield an unambiguous example of a protostar in Spitzer’s (1948) sense of an isolated interstellar cloud undergoing inexorable gravitational contraction to form a single star. These hopes were strongly encouraged by Larson’s (1969) theoretical models, which predicted that protostars would pass through a phase of high infrared luminosity during their accretion stage. The fact that no conclusive identification of such an object has yet been made stems not from a lack of candidate infrared sources, but from the difficulty of establishing the evolutionary status of the 30 or more objects that have already been found associated with dense molecular clouds. This review describes the searches that have been made for infrared sources in star-forming regions and the detailed studies that have been made of these sources and of the interstellar matter in their immediate vicinity. The emphasis is on those objects manifesting themselves almost exclusively at infrared wavelengths, rather than on the presumably more evolved objects such as compact H II regions and young, visible stars with infrared excesses. Recent work in these latter fields has been described by Habing & Israel (1979), Cohen & Kuhi (1979), and Hyland (1981). Other useful review articles can found in the conference proceedings on infrared astronomy (Wynn-Williams & Cruikshank 1981), interstellar molecules (Andrew 1980), and on various aspects of star formation (Gehrels 1978, Appenzeller et al. 1980, Roger Dewdney 1982). Infrared techniques are described by Soifer & Pipher (1978). The objects attracting the most attention from infrared astronomers almost all have bolometric luminosities in excess of 103 L®. Current theoretical rood-