An Initial Study of the Carbon Monoxide Ion (CO$^+$) in Photodissociation Regions
Session 84 -- Star Forming Clouds
Display presentation, Wednesday, 11, 1995, 9:20am - 6:30pm

## [84.13] An Initial Study of the Carbon Monoxide Ion (CO$^+$) in Photodissociation Regions

William B. Latter (National Radio Astronomy Observatory), Christopher K. Walker (Steward Observatory)

The recent first detection of the carbon monoxide ion in the interstellar medium (Latter, Walker, \& Maloney 1993, ApJL, 419, L97) has opened up new avenues for the understanding of processes in photodissociation regions (PDR); especially star formation regions. PDR are regions of gas and dust where the effects of UV photons bear the strongest influence on the ionization structure and chemistry. We report the first use of CO$^+$ as a molecular diagnostic of the interstellar medium; in particular, a region in M17SW that is dominated by a strong flux of ultraviolet photons. Strip maps of the strongest observable transition of CO$^+$, $N=2 \to 1,\ J = 5/2 - 3/2$ ($\nu = 236.063$ GHz), and HCO$^+$ $J=3 \to 2$ ($\nu = 267.558$ GHz) and $J = 1 \to 0$ ($\nu = 89.189$ GHz) emission have been made across the M17SW PDR using the Caltech Submillimeter Observatory and NRAO 12m telescope. These data are discussed and compared to chemical models of photodissociation regions. Since the central stars of young planetary nebulae are strong sources of ultraviolet radiation surrounded by a molecular envelope, some planetary nebulae can be described as compact PDR. New observations have also been made toward the planetary nebula NGC 7027, confirming the presence of CO$^+$ in that object. The chemistry of CO$^+$ in these regions is discussed and principle formation routes are described. It is evident from these data that the general description of M17SW as a clumpy'' photodissociation region viewed approximately edge-on to the line of sight is correct.