Tracing protostellar evolution by observations of ices

The energetic phenomena associated with protostars alter the physical and chemical state of the material in circumstellar envelopes and disks. In particular, the structure and composition of icy mantles on dust grains can potentially be strongly influenced by processes such as thermal heating, ultraviolet irradiation, cosmic rays, and shocks. In this paper we discuss the evidence for these processes by analyzing observations of ice bands in the infrared spectra of high and low-mass protostars. ISO observations have shown that thermal processing, i.e. sublimation and crystallization, of ices plays an important role in the envelopes of high-mass protostars. In fact, the ice bands may be used as a tracer of the evolution of the protostar. Toward low-mass protostars, the unprecedented sensitivity and spectral resolution of the NIRSPEC spectrometer at the Keck II telescope allow a detailed analysis of ice band profiles. The first observations of ices in circumstellar disks around low-mass protostars are discussed. We show that the apolar, volatile, CO ices in the upper layers of the slightly inclined edge-on circumstellar disk of L1489 IRS have evaporated. The energetic formation of new molecules, such as CO_2 and 'XCN', does not occur, however. New insights into the nature of apolar ices are evident from the newly detected band of ^(13)CO. In this paper we also present complementary observations obtained with (sub-)millimeter single dish telescopes and interferometers along the same 'icy' lines of sight. These are essential to characterize the line of sight conditions and determine the location of the ices observed in a pencil absorption beam