From the spectra displayed in Fig. 1, we derive the following 2 upper limits on the optical depth of the fundamental transition of solid O2 at 6.45 : 0.2 for R CrA IRS 2 and 0.1 for NGC 7538 IRS9, W3 IRS5 and S140 IRS1.
The integrated cross section of solid O2 has been previously estimated as cm/molecule in multicomponent mixtures (Ehrenfreund et al. 1992). New measurements indicate that the cross section in apolar ices can be as small as cm/molecule. To calculate the upper limits on the column density of solid O2 we adopt a cross section of cm (measured for a CO:O2=2:1 ice mixture). The upper limits on O2 column densities are thus cm-2 for R CrA IRS2 and cm-2 for NGC 7538 IRS9, W3 IRS5 and S140 IRS1.
Another method of constraining the abundance of solid O2 is the analysis of the solid CO absorption profile. Fig. 2 shows the CO band toward R CrA IRS2 as observed with the cooled grating spectrometer CGS4 on the United Kingdom Infrared Telescope (Chiar et al. 1998). Saturation of the CO band results in large error bars near its peak position. Toward R CrA IRS2, the CO band is very narrow, FWHM= 3.2 cm-1. Its profile is consistent with pure CO (Chiar et al. 1998) and ice mixtures of O2: CO = 1:2 or O2:CO = 20:1 (Ehrenfreund et al. 1997, Elsila et al. 1997). However, at very large quantities of O2 the CO feature shows a distinctive redshift to 2135.8 cm-1, which is inconsistent with the observations of R CrA IRS2. In a more complex mixture of CO:O2:CO2:N2 = 100:500: 50:100 (Elsila et al. 1997) the position falls close (2141.6 cm-1) but the width of the profile (6.8 cm-1) is inconsistent with the observations, see Fig. 2. An abundance of solid O2 which exceeds the abundance of solid CO is not favored by theoretical models, taking into account recent values for the abundance of gas-phase oxygen and carbon (Tielens & Hagen 1982, Meyer et al. 1998, Cardelli et al. 1996). From the data presented in Fig. 2 we conclude that a mixture with O2/CO = 70% results in a band wider than the observed CO profile of R CrA IRS2. Furthermore, the position of the CO band profile is inconsistent with a mixture CO:O2=1:1. We thus derive an upper limit of 50% O2 ice with respect to CO toward R CrA IRS2.
Elsila et al. (1997) provided a reasonable fit in band position (2142 cm-1) to the CO band of NGC 7538 IRS9 observed by Tielens et al. (1991) with a CO:O2:CO2:N2 = 100:500:50:100 mixture. However, the fit was done in transmittance. In optical depth it is apparent that the bandwidth of the mixture (FWHM=7.1 cm-1) is much larger than the observed bandwidth (FWHM=4.75 cm-1). The difference is caused by saturation effects owing to the large optical depth of the interstellar CO feature. Chiar et al. (1998) used a two-component fit without any solid O2 to match the profile of the CO band in NGC 7538 IRS9. Additional measurements listed in their Table 5 show that a number of good fits to the apolar CO component can be achieved for O2-rich mixtures where O2/CO = 50-100%. We therefore adopt an upper limit of O2/CO=1, which is still consistent with theoretical models. This abundance translates to 19% O2 relative to H2O ice toward this source, and accounts for less than 3% of the total interstellar oxygen budget. Due to the larger width of the CO band toward NGC 7538 IRS9 (4.75 cm-1), the presence of O2 ice can not be as well constrained as for R CrA IRS2.
© European Southern Observatory (ESO) 1999
Online publication: June 17, 1999