CO2 Ice toward Low-luminosity, Embedded Protostars: Evidence for Episodic Mass Accretion via Chemical History

TL;DR

This study uses Spitzer IRS spectroscopy to detect pure CO2 ice in low-luminosity protostars, providing evidence that these stars experienced episodic luminosity bursts in their past, which affected their chemical composition.

Contribution

It presents the first evidence of pure CO2 ice in low-luminosity protostars, supporting episodic accretion models of star formation through chemical history analysis.

Findings

Approximately half of the low-luminosity sources show pure CO2 ice.

Presence of pure CO2 ice indicates past higher luminosity episodes.

Supports episodic accretion as a key process in protostar evolution.

Abstract

We present Spitzer IRS spectroscopy of CO2 ice bending mode spectra at 15.2 micrometer toward 19 young stellar objects with luminosity lower than 1 Lsun (3 with luminosity lower than 0.1 Lsun). 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 < 1 Lsun 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 CO2 component, traced by the presence of a characteristic band splitting in the 15.2 micrometer bending mode. About half of the sources (9 out of 19) in the low luminosity sample have evidence for pure CO2 ice, and six of these have significant double-peaked features, which are very strong evidence of pure CO2 ice. 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 CO2 ice, the total amount of CO2 ice, and the observed residual C18O gas.