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

6. Summary

Mapping of the large-scale emission patterns around Oph shows that the CO emission components to the North and South of the star separately `turn on' over narrow ranges of the CH line profile integral. We ascribe the rapid brightening of the CO line to the effects of a partial C CO conversion (Liszt 1992, 1993; Kopp et al. 1996) in a nearly-fixed total gas column, which in turn lends confidence to the assumption that CH traces the column density (Mattila 1986; Magnani and Onello 1995).

For OH no such conclusion is reached, as our profiles fail to show not only the pre-shock gas at 4.5 km s^-1 discussed by Crutcher (1979), but most of the stronger CO component as well. We attribute this to a more widely-identified problem with OH excitation in the regime of the C CO conversion (Dickey, Kazès, and Crovisier 1981; Liszt and Lucas 1996), whereby the excitation temperature is unaccountably small (see also Roueff 1996). However the excitation of CH is probably no better understood; its (typical) negative excitation temperature deduced toward the star should probably have been squelched by electron impact excitation. OH and CH emission vary proportionally and inversely for the stronger and weaker CO components, respectively.

Four spectra of HCO emission show that it resembles CO and CH in its behaviour, peaking in both components, one on either side of the star. Given the close relationship between HCO and OH in models of the chemistry, and as observed directly (Lucas and Liszt 1996; Liszt and Lucas 1996) there is no obvious explanation as to why the OH behaviour is so different in the northern and southern regions. Clearly there is a serious problem in understanding either the chemistry or the excitation of this molecule.

A search for CS, CN, HCN, and H emission to the South of the star, where HCO is strong, detected only HCN. We infer that the molecular abundances are not necessarily much larger than would be found toward the star, unless the gas is much colder to the South. We could not confirm the earlier detection of CS emission toward the star by Drdla et al. (1989). Limits on N(CS) at the Southern emission peak are typically an order of magnitude below those set optically by Snow (1976).

At four positions from the star, we found that the isotopic intensity ratio (¹² CO)/ (¹³ CO) is 30-50; such values are quite large considering the 3-6 K ¹² CO brightnesses, but still much smaller than the ratio found toward the star by Wilson et al. (1992). Given that the ¹² CO optical depth is in the range 1-2 both toward the star and to the South, large variations in the CO isotopic abundance ratio are indicated. The relative abundance of ¹³ CO increases as the CO lines brighten generally at outlying positions.

One of the most interesting questions is the existence of a strong minimum in the CO emission and ¹³ CO relative abundance very near the position of the star; ratios as large as N(¹² CO)/N(¹³ CO) = 167 are seen nowhere else, even at similarly-low N(¹² CO) = (Lucas and Liszt 1997). The star is also the approximate center of a pattern of interesting kinematic symmetry, but, owing to the existence of a large surrounding HII region, the absorbing gas parcels are often considered to be quite distant from it. Then, the observed symmetries and other seemingly-unique behaviour are entirely accidental, like weather. On the other hand, if the star and gas were close, the star's local space velocity of some 100 km s^-1 would move it past the clouds at the rate of 1 pc () every years. A time-series of absorption profiles could yield valuable insights into the small-scale structure of the absorbing gas (Liszt 1992).

Oph is known to have a substantial bow shock (van Buren and McCray 1988; Fig. 8), and it is tempting to attribute this to interaction with neutral gas. Such shocks are actually quite common (ibid), but, if the star and neutral gas are not too distant from each other, there are a variety of simple geometries which yield some aspects of the observations-in this case, a CO minimum near the position of the star and strong one-sided increase away from it-with a minimum of contrivance. We rely on the fact that CO, which is on the verge of becoming strongly self-shielding toward the star, is expected to be strongly affected by even slight shifts in the photoionization rate and extinction (Kopp et al. 1996).

Fig. 8. 60µ IRAS image of the region around Oph showing its bow shock. The position of the star is marked by a cross. corresponds to 2.44 pc at a distance of 140 pc.

In Fig. 9 we show a case where a bright star, contributing much of the local uv -photoionization, is viewed through a thin, undisturbed slab of gas. The extinction across the near (to us) face of the slab, measured to the star, follows a cosecant law and is much higher at than at o or a. Coupled with the inverse-square dependence of the radiation field ( is much more distant from the star than either o or a) the photoionizing flux across the front face of the cloud varies greatly and a strong, one-sided maximum would presumably be produced in CO. Much stronger emission would be expected at than at a, even though both are seen equidistant from the star.

Fig. 9. A viewing geometry which creates a one-sided maximum in CO as the result of increased extinction.

© European Southern Observatory (ESO) 1997

Online publication: June 5, 1998

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