Astron. Astrophys. 322, 962-974 (1997)

5. CO isotope ratio, excitation, and conversion factor

Comparison between the carbon isotope ratios observed in CO and in the gas phase generally should be a powerful diagnostic: the fractionation reaction of ¹² CO and (Watson, Anicich, and Huntress 1976), with an exothermicity corresponding to 35 K, should act powerfully to enhance the abundance of ¹³ CO at low temperatures (Crutcher and Watson 1981). Knowledge of the ¹² CO/¹³ CO isotope ratio could serve to discriminate between low and high temperature solutions at the HCO emission peak, because only the low-temperature, high-density, very optically thick solutions for CO are compatible with ¹² CO/¹³ CO ratios substantially larger than those given by the profile integrals directly ( 30-50). Ordinarily one might rely on Copernicus or HST CO absorption line data to settle this matter but questions of the proper wavelength assignments of some rotation-vibration bands (Haridass and Huber 1994) have resulted in values ranging from N(¹² CO)/N(¹³ CO) = 55 (Wannier et al. 1982) and 82 (Lyu et al. 1994) to 167 (Lambert et al. 1994).

Toward the star in the J=1-0 line, Wilson et al. (1992) find (¹² CO)/ (¹³ CO) 110, superseding the older value of from Langer, Glassgold and Wilson (1987). We reported (Liszt 1992, 1993) that this ratio is surprisingly high ( 30) at the emission peaks to the North and South: the more sensitive new ¹³ CO data shown in Fig. 7 and summarized in Table 2 confirm this (see also Kopp et al. 1996). To the South, in the higher velocity line, the intensity ratios are and . To the North, they are and . Clearly the isotopic intensity ratio varies widely over the central , with a very strong peak toward the star. Unless the optical depth is much lower there, the isotopic abundance ratio must also change by roughly similar amounts.

Fig. 7. Line profiles of 13 CO J=1-0 emission observed at 4 positions directly North and South of Oph. The channel spacing and resolution are 0.13 km s-1.

¹² CO J=1-0 emission toward Oph cannot be too optically thin unless it is very different from that seen in absorption. Adopting the column density and rotational populations recently measured in uv -absorption leads to values for the integrated optical depth of the J=1-0 line which are 0.9-1.4 km s^-1. If the optical depth is distributed over an intrinsic profile which resembles the CO emission (which itself is very similar to the absorption-line profiles of species besides CH ), the peak optical depth is 1.2-2.0. The higher optical depth corresponds to an excitation temperature of 4.5 K and a 1.2 K emission line brightness, the lower to an excitation temperature slightly over 6 K and a CO J=1-0 emission line which is perhaps somewhat brighter ( K) than observed. For the excitation solution with , the isotopic abundance ratio should be (only) 40% larger than the intensity ratio.

At the position South, the linewidths (FWHM) of the stronger J=1-0 components are km s^-1 and km s^-1 for ¹² CO and ¹³ CO respectively. If the weaker line is optically thin, this difference in linewidth can be explained entirely if the line center optical depth in ¹² CO is in the range 2.0-2.5. This scant difference in width precludes heavy saturation of the ¹² CO profile and eliminates those excitation solutions in which the temperature is low and the carbon isotope ratio is as large as, say, 167 (Lambert et al. 1994) ². We conclude that the emission measurements are consistent with the very large ¹² CO/¹³ CO ratio found in optical absorption toward the star, but that such a large ratio is not characteristic of the immediately adjacent gas sampled in emission away; the ¹² CO optical depth is too small.

The ¹³ CO lines are strong enough in some positions so that might hope to detect C¹⁸ O. If the ¹³ CO/C¹⁸ O intensity ratio were found to be unusually small, this might support a supra-terrestrial ¹² C/¹³ C isotope ratio. A search for C¹⁸ O by Kopp et al. (1996) yielded N(¹³ CO)/N(C¹⁸ O) 5 at one position. Our data (Table 2) South of the star yield a value for the ratio of line strengths at the 3- level. Given the low optical depths in both lines, this intensity ratio cannot differ greatly from that of the abundances themselves.

5.1. CO excitation and the CO- conversion factor in diffuse gas

The weak rotational excitation and low thermal pressure  K , inferred from the small CO J=1-0 excitation temperature toward the star (Smith, Krishna Swamy, and Stecher 1978; Liszt 1979) are confirmed by mm-wave emission observations of the unseen CN (Crane et al. 1989) and CO J=3-2 lines (van Dishoeck and Black 1991) but not by a comparison of the ¹² CO J=2-1 and J=1-0 emission lines (Crutcher and Federman 1987). Although there is a fair range of ratios in the literature, the thermal pressure inferred from a comparison of the two lowest CO lines is typically p/k = K. Similar or slightly higher pressures may be derived from the uv absorption-line column densities of Lambert et al. (1994) who found excitation temperatures of 3.9 , 4.6 and 6.3 K for the lowest three rotation transitions. These excitation temperatures would produce an easily-detectable ( K) J=3-2 line, in substantial disagreement with the results of van Dishoeck and Black (1991). Much weaker excitation and lower gas pressure are implied by the analysis of other uv absorption-line data of Lyu et al. (1994).

It seems undeniable that the CO profiles have wide swings in integrated intensity under conditions where neither nor N(CO) changes much. Toward the star, N()/ K km s^-1 (K km s^-1)^-1, which is 2-3 times larger than typically-employed values. However, at the emission peak to the South, the CO is four times stronger and the CO intensity- column density ratio is unexceptional or even somewhat low. This suggests that the common value is established fairly soon after CO emission turns on (Liszt 1982), even while N(CO)/N(C ) .

© European Southern Observatory (ESO) 1997

Online publication: June 5, 1998

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