Temperature Fluctuations and Very Cold Dust

Abstract

The small heat capacity of very small interstellar grains results in temperature spiking on absorption of ultraviolet photons from the interstellar radiation field. These grains cool by emission of infrared radiation at wavelengths that are out of equilibrium with thermal emission at the average grain temperature. One effect of this is the appearance of excess emission at short (∼ 3μm) and middle (12 – 25μm) infrared wavelengths which signals the presence of grains that have been transiently heated to high temperatures. Such grains will, however, cool to temperatures that are considerably lower than the average temperature between high temperature spikes and will spend the majority of the time at these low temperatures. As a result, they will contribute excess emission at very long infrared wavelengths and this emission will be seen in IRAS sources at mm wavelengths.