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

4. Discussion

Table 1 lists the column densities of important oxygen-bearing species and the total hydrogen column density (H and H₂) in R CrA IRS2 and NGC 7538 IRS9. The hydrogen column density of the interstellar medium is often derived from the visual extinction. In dense clouds, the grain sizes are known to be larger than in the diffuse medium so that the amount of visual extinction per unit mass is larger than in the diffuse medium. Observations of the field star Elias 16 behind the Taurus dense cloud show a A_v/(9.7) ratio of 31.2, as compared to 18.5 for the diffuse medium towards CygOB#12 (Whittet et al. 1988, Whittet et al. 1997). Since the grain sizes in R CrA IRS2 are expected to be comparable, we assume that the same factor of 1.69 times the A_v/N_H ratio in the diffuse ISM can be assumed. With an A_v value of 35 (Chiar et al. 1998), we thus derive a total H column density of  cm^-2 for R CrA IRS2. The A_v of NGC 7538 IRS9 derived from the depth of the 9.7 µm feature (Willner et al. 1982) is insensitive to the grain size. The inferred column density is  cm^-2 (Chiar et al. 1998).

Table 1. Column densities and abundances per 10⁶ n_H of oxygen bearing species in R CrA IRS2 and NGC 7538 IRS9 compared to the O⁰ abundance in the diffuse ISM.
References :
(a) Tanaka et al. 1994; (b) Allamandola et al. 1992; (c) Chiar et al. 1998; (e) Harju et al. 1993; (f) Mitchell et al. 1990; (g) van Dishoeck & Helmich 1996; (h) Keane & Schutte, in preparation; (i) Schutte et al. 1996; (j) Gerakines et al. 1999; (k) Boogert et al. 1999; (m) Marechal et al. 1997; (n) Olofsson et al. 1998

The column density of CO gas of R CrA IRS2 is based on the C¹⁸O 1-0 data by Harju et al. (1993). We derive N(C¹⁸O)= cm^-2 for typical dark cloud conditions. For a normal ¹⁶O/¹⁸O ratio of 500, this implies a column density of gaseous CO of 1.6 cm^-2. The CO gas column density towards NGC 7538 IRS9 was determined by Mitchell et al. (1990) from the ¹³CO IR absorption. From the ISO-SWS spectrum of R CrA IRS2 we determined an upper limit of 10¹⁸ cm² for hot H₂O, assuming a line width 3 km s^-1. The upper limit on the H₂O gas column density in NGC 7538 comes from the ISO spectra. The column densities of the CO and H₂O ice are determined from ground based spectra. The ISO spectrum of NGC 7538 IRS9 also revealed other oxygen bearing ice species like OCN^- and HCOOH while the presence of CH₃OH was known from ground based spectra.

The recent balloon experiment PIROG 8 searched for the 425 GHz gas-phase O₂ line toward NGC 7538 IRS9 and W51 (Olofsson et al. 1998). No emission could be detected, and the inferred upper limits on the O₂/CO ratio are 0.04 and 0.05 (3 ) for these regions. This value leads to an upper limit on the O₂ gas column density in NGC 7538 IRS9 of cm^-2. Marechal et al. (1997) searched for gas-phase ¹⁶O¹⁸O in dark clouds. For regions in L134 which are comparable to R CrA IRS2, they give an upper limit of O₂/CO  0.15, which would correspond to an O₂ upper limit of cm^-2 for R CrA IRS2.

The upper limits on solid O₂ from the CO profile deconvolution found in this paper are 6.0  cm^-2 in R CrA IRS2 and 1.2 cm^-2 in NGC 7538 IRS9. These limits are also consistent with those derived from the 6.45 µm feature. Table 1 contains the abundances of oxygen bearing species in R CrA IRS2 and NGC 7538 IRS9. We compare these values with the abundance of gaseous O⁰ in the diffuse ISM (Meyer et al. 1998), which is the oxygen available to form these species in dense clouds. Not listed are the dust species (silicates, oxides) which are assumed to contain the same amount of oxygen in both diffuse and dense clouds. We estimate that in R CrA IRS2 at least 27% ( 13%) of the oxygen is unaccounted for. More significant is that at least 43% ( 6%) of the oxygen seems to be missing in NGC 7538 IRS9. Recent Kuiper Airborne Observatory and ISO observations of the [OI] 63 µm line indicate that up to 40% of the oxygen could be in atomic form (Poglitsch et al. 1996, Baluteau et al. 1997). However, these observations trace only the foreground material and not the dense cloud. Some of the missing oxygen in the two protostellar objects discussed here might be in atomic oxygen, but further observational evidence is needed.

© European Southern Observatory (ESO) 1999

Online publication: June 17, 1999
helpdesk.link@springer.de