Formation of interstellar solid CO₂ after energetic processing of icy grain mantles

¹ INAF – Osservatorio Astrofisico di Catania, via Santa Sofia 78, 95123 Catania, Italy e-mail: mepalumbo@oact.inaf.it
² INAF – Osservatorio Astronomico di Capodimonte, via Moiarello 16, 80131 Napoli, Italy

Received: 12 March 2008
Accepted: 26 September 2008

Abstract

Context. Space infrared observations with ISO-SWS and Spitzer telescopes have clearly shown that solid carbon dioxide (CO₂) is ubiquitous and abundant along the line of sight to quiescent clouds and star forming regions. Due to the CO₂ low gas-phase abundance, it is suggested that CO₂ is synthesized on grains after energetic processing of icy mantles and/or surface reactions.

Aims. We study quantitatively the abundance of carbon dioxide synthesized from ice mixtures of astrophysical relevance induced by ion irradiation at low temperature. We compare the CO₂ stretching and bending-mode band profiles observed towards some young stellar objects (YSOs) for which infrared spectra exist.

Methods. Using a high vacuum experimental setup, the effects induced by fast ions ( keV) on several ice mixtures of astrophysical interest are investigated. Chemical and structural modifications of the ice samples that form new molecular species are analyzed using infrared spectroscopy. The formation cross section of solid CO₂ is estimated from the increase in column density as a function of the dose fitting of experimental data with an exponential curve.

Results. Our laboratory experiments showed that carbon dioxide is formed after irradiation of ice mixtures containing C- and O-bearing molecules. Furthermore, when the same amount of energy is released into the icy sample, a larger amount of CO₂ is formed in H₂O-rich mixtures in agreement with previous studies. We also found that the CO₂ stretching and bending mode band profiles depend on the mixture and temperature of the ice sample. We found that the amount of carbon dioxide formed after ion irradiation can account for the observed carbon dioxide towards YSOs. Furthermore, we discovered that laboratory spectra are a good spectroscopic analogue of the interstellar features.

Conclusions. Even if the comparison between laboratory and observed spectra presented here cannot be considered unique and complete, our results quantitatively support the hypothesis that interstellar solid CO₂ forms after ion irradiation and UV photolysis of icy mantles.