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Astron. Astrophys. 346, L57-L60 (1999)

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2. Observations

Because of the low evaporation temperatures of apolar ices ([FORMULA]20 K), their detections in the interstellar medium are limited to regions which contain a large amount of cold, quiescent material, far away from protostars. Our search for solid O2 was therefore directed toward lines of sight with large apolar CO ice abundances. Chiar et al. (1998) have recently summarized solid CO observations for a large number of regions. On the basis of their Fig. 7, two lines of sight stand out as excellent candidates: R CrA IRS2 and NGC 7538 IRS9. R CrA is located in the Corona Australis complex, which, like Taurus, appears cold in nature. Toward R CrA IRS2 the amount of apolar CO ice is more than 50% that of water ice, the highest solid CO:H2O ratio ever measured (Chiar et al. 1998). Toward NGC 7538 IRS9, the abundance of apolar CO ice is 19% relative to water ice (Chiar et al. 1998). Gas and solid-state measurements indicate a cold cloud component (Mitchell et al. 1990, Chiar et al. 1998), where solid O2 may also be present in abundance. We also present the ISO-SWS observations of two deeply embedded protostellar objects: W3 IRS5 and S140 IRS1. The higher temperature of these clouds is not favorable for the presence of solid O2, but both sources have high fluxes in the 6.2-6.6 µm region, yielding higher signal-to-noise in their ISO spectra.

The ISO-SWS observations of these sources were performed in the AOT6 observation mode. The integration times in the 6.2-6.6 µm region were 2400 s for R CrA IRS2 and 750 s for NGC 7538 IRS9, W3 IRS5 and S140 IRS1. The data were reduced with the SWS Interactive Analysis package (de Graauw et al. 1996a). The standard product generation steps were done, with manual dark current subtraction and exclusion of data points that suffered from uncorrected events in the read-out electronics (visible as signal jumps) to obtain the best S/N possible.

The SWS spectra have been divided by the local continuum level around 6.45 µm. The low value of S/N = 10 in the final spectrum of R CrA IRS2 (Fig. 1) is due to the low brightness of the source (5.5 Jy at 6.45 [FORMULA]). At these flux levels the detector noise and signal drifts dominate the S/N of SWS observations in this spectral region. The S/N ratio is 20 in the final spectra of NGC 7538 IRS9, W3 IRS5 and S140 IRS1. NGC 7538 IRS9 is brighter than R CrA IRS2 around 6.45 [FORMULA] (33 Jy), but here the spectrum is dominated by the long-wavelength wing of the solid H2O bending mode located at 6.0 µm.

[FIGURE] Fig. 1. ISO-SWS spectra of R CrA IRS2, NGC 7538 IRS9, W3 IRS5 and S140 IRS1 between 6.3 and 6.6 µm compared to that of solid O2 in the laboratory, whose fundamental transition falls at 6.45 µm.

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© European Southern Observatory (ESO) 1999

Online publication: June 17, 1999