Latter, Walker, and Maloney 1993, ApJ Letters, 419, L97

Detection of the Carbon Monoxide Ion (CO⁺) in the Interstellar Medium and a Planetary Nebula

 Christopher K. Walker
Steward Observatory

 Philip R. Maloney
U. Colorado

 1993, ApJ Letters, 419, L97

We report the detection of the carbon monoxide ion (CO⁺) in the interstellar medium (M17SW) and a planetary nebula (NGC 7027. These detections are based on observations of three millimeter and submillimeter transitions in M17SW and one in NGC 7027. Chemical models suggest that CO⁺ should be most abundant where complex molecules are least likely to be present. In our search for CO⁺ we therefore minimized the chance of confusion while maximizing the probability of detection by observing regions whose chemistry is dominated by the effects of ultraviolet radiation.

Confirmation spectra of CO⁺ in the planetary nebula NGC 7027 with the NRAO 12m (bottom) and the Caltech Submillimeter Observatory (top).
(Latter, Walker, and Maloney 1995, BAAS, 26, 1458)

An Initial Study of the Carbon Monoxide Ion (CO⁺) in Photodissociation Regions

William B. Latter
National Radio Astronomy Observatory - Tucson

 Christopher K. Walker
Steward Observatory

 Philip R. Maloney
U. Colorado

 1995 Bull. American Astron. Soc., 26, 1458

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 -> 1, J = 5/2 - 3/2 (nu = 236.063 GHz), and HCO⁺ J=3 -> 2 (nu = 267.558 GHz) and J = 1 -> 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.

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