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

1. Introduction

The diffuse or optical absorption line clouds toward the nearby (140 pc) O9 star Oph offer a unique opportunity: in the apparent absence of dense gas beyond the star, the denser occulting material is clearly visible and easily isolated in radiofrequency molecular emission. The optically-determined column density and rotational excitation temperature of CO (Wannier, Jenkins and Penzias 1982, Lyu, Smith, and Bruhweiler 1994, Lambert et al. 1994) predict a line brightness of 1-2 K at 2.6mm, as observed (Liszt 1992, 1993; Kopp et al. 1996). For OH (Crutcher 1979, Roueff 1996) or CH (Lien 1984; but see below) the optically-determined column densities and radio line brightness together imply typical excitation temperatures.

Radiofrequency molecular emission and optical absorption line measurements provide complementary information. Column densities, which are difficult to derive from the former, are often cleanly-determined from the latter. Line profiles, on the other hand, have until recently been obtained at much higher resolution using radiofrequency heterodyne techniques. In one interesting case, recognition of the existence of a very narrow neutral-bearing component seen in K I (Hobbs 1973) and CO (Liszt 1979), having b km s^-1, probably caused a substantial downward revision of the very large carbon depletion factors which were at first derived toward Oph (Morton 1975). But the molecular emission lines around Oph are considerably broader than those arising from truly dark clouds.

At optical wavelengths, line profiles of many atomic and molecular species can now be obtained at sub-km s^-1 resolution (Lambert, Sheffer, and Crane 1990, Crawford et al. 1994, Barlow et al. 1995, Crawford 1996). Even so, the emission observations offer three opportunities to complement the optical work. First, the gas may be traced over its spatial extent, so that we can see how the absorption line material, in otherwise anonymous diffuse clouds ¹, relates to the overall spatial distribution. Second, the column density sensitivity of the mm-wave emission observations may under some circumstances substantially exceed that of the optical data, owing to the increased sensitivity of new receivers and the possibility of molecular rotational excitation by ambient electrons in diffuse gas. Last, it is possible to search in emission for species like HCO (Liszt and Lucas 1994) which have either unknown or inaccessible optical spectra (Koch, van Hemert, and van Dishoeck 1995).

The appearance of the CO emission seen around Oph has a very particular but not well understood behaviour whereby one or the other of the two dense kinematic components toward the star brightens within - (projected distance 1 pc) by very large factors (Liszt 1992, ; Kopp et al. 1996). The star sits in a clear minimum in the integrated emission. However, given the very small amount of the total carbon column density which is in CO toward the star, N(C ) (Morton 1975; Cardelli et al. 1993), N(CO) = (Wannier et al. 1982; Lyu et al. 1994; Lambert et al. 1994), the increase in CO brightness above 6 K probably does not signify a similarly large increase in the extinction or total column density. Indeed, the highest CO column density which can be derived at the position of maximum CO brightness is still nearly an order of magnitude less than N(C ) toward the star (Liszt 1993; Kopp et al. 1996) and the more likely value for N(CO) is not more than twice what is seen toward the star. The much stronger CO emission exemplifies what can happen in the regime where CO first becomes self-shielding (ibid) with only a slight C -CO conversion. Of course it is remarkable that 6 K CO emission might arise from a column density which represents such a small fraction of the carbon, and at = 1 mag.

In this work, we have extended the large-scale mapping to species beside ¹² CO, namely CH, OH, and (to a lesser extent) HCO and ¹³ CO. We have also attempted to extend the recent detection of 3mm HCO emission around Oph to other species, CN, CS, HCN, and H. Of these, CN of course has a known column density in absorption N(CN) = (van Dishoeck and Black 1989) and has been sought but not seen at very low levels in mm-wave emission toward Oph (Crane et al. 1989), consistent with its use as an independent indicator of the temperature of the cosmic microwave background radiation. We saw HCN at the HCO emission peak, and deduce a column density comparable to or perhaps slightly greater than that of HCO . A limit on the column density of CS was set from Copernicus data (N(CS) ; Snow 1976) and can probably be improved now; we cannot confirm the detection of CS (J=2-1) emission reported toward Oph by Drdla, Knapp, and van Dishoeck (1989).

In Sect. 2 we discuss the new data taken for this work. In Sect. 3 we discuss maps of OH, CH, and CO and demonstrate an interesting series of relationships among the emission brightnesses of these three molecules. In Sect. 4 we derive column densities and column density limits for mm-wave emission species in the material around Oph. In Sect. 5 we discuss the isotopic abundances measured in CO emission and absorption. This work is summarized briefly in Sect. 6.

© European Southern Observatory (ESO) 1997

Online publication: June 5, 1998

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