ISO-SWS observations of interstellar solid13CO2: Heated ice and the Galactic12C/13C abundance ratio

Academic Article

Abstract

  • We present observations of the stretching mode of 13CO2 ice along 13 lines of sight in the Galaxy, using the Short Wavelength Spectrometer on board of the Infrared Space Observatory (ISO-SWS). Remarkable variations are seen in the absorption band profile in the different lines of sight. The main feature is attributed to 13CO2 mixed with polar molecules such as H2O, and CH3OH. The high-mass protostars GL 2136, GL 2591, S 140 : IRS1, and W 3 : IRS5 show an additional narrow substructure at 2282 cm-1 (4.382 μm), which we attribute to a polar, CH3OH-containing CO2 ice, that experienced heating. This heating effect is sustained by a good correlation of the strength of the substructure with dust and CO gas temperatures along the line of sight, and anti-correlation with ice abundances. Thus, our main conclusion is that interstellar CO2 ices around luminous protostars are subjected to, and altered by, thermal processing and that it may reflect the evolutionary stage of the nearby protostar. In contrast, the ices around low mass protostars and in a quiescent cloud in our sample do not show signs of thermal processing. Furthermore, we determine for the first time the Galactic 12C/13C ratio from the solid state as a function of Galacto-centric radius. The 12CO2/13CO2 ratio for the local ISM (69±15), as well as the dependence on Galacto-centric radius, are in good agreement with gas phase (C18O, H2CO) studies. For the few individual objects for which gas phase values are available, the 12C/13C ratios derived from CO2 tend to be higher compared to CO studies (albeit with ∼ 2.5 σ significance only). We discuss the implications of this possible difference for the chemical origin of interstellar CO2.
  • Authors

    Author List

  • Boogert ACA; Ehrenfreund P; Gerakines PA; Tielens AGGM; Whittet DCB; Schutte WA; Van Dishoeck EF; De Graauw T; Decin L; Prusti T
  • Start Page

  • 349
  • End Page

  • 362
  • Volume

  • 353
  • Issue

  • 1