2012ApJ...758...38K

We present Spitzer IRS spectroscopy of CO₂ ice bending mode spectra at 15.2 µm toward 19 young stellar objects (YSOs) with luminosity lower than 1 L_☉(3 with luminosity lower than 0.1 L_☉). Ice on dust grain surfaces can encode the history of heating because pure CO₂ ice forms only at elevated temperature, T > 20 K, and thus around protostars of higher luminosity. Current internal luminosities of YSOs with L < 1L_☉ do not provide the conditions needed to produce pure CO₂ ice at radii where typical envelopes begin. The presence of detectable amounts of pure CO₂ ice would signify a higher past luminosity. Many of the spectra require a contribution from a pure, crystalline CO₂component, traced by the presence of a characteristic band splitting in the 15.2 µm bending mode. About half of the sources (9 out of 19) in the low-luminosity sample have evidence for pure CO₂ice, and 6 of these have significant double-peaked features, which are very strong evidence of pure CO₂ice. The presence of the pure CO₂ ice component indicates that the dust temperature, and hence luminosity of the central star/accretion disk system, must have been higher in the past. An episodic accretion scenario, in which mixed CO-CO₂ ice is converted to pure CO₂ ice during each high-luminosity phase, explains the presence of pure CO₂ice, the total amount of CO₂ice, and the observed residual C¹⁸O gas.