The Nature of the [ITAL]IRAS[/ITAL] Ring G159.6−18.5 in Perseus and Its Exciting Star HD 278942

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Abstract
We discuss an extended feature in the Perseus molecular cloud complex, which is most prominent in the IRAS database as an almost complete ring of radius 075 but is also clearly seen in optical surveys and in radio continuum emission. While earlier identified as a supernova remnant, we argue that the feature is probably an H II region, based on new interferometric radio continuum data at 408 and 1420 MHz, diffuse Hα emission, and the identification of HD 278942 as an O9.5–B0 V star located at the geometric center of the IRAS ring. The spectral index of the radio continuum emission is consistent with an optically thin H II region. However, an origin of the radio continuum emission as at least partially caused by synchrotron emission from the interaction region between the stellar wind and the remaining molecular cloud cannot be ruled out. We present visual photometry, spectroscopy, and polarimetry of the star HD 278942, which we believe to be the source of the excitation for the H II region and the stellar wind. While coincident with an IRAS point source, no compact radio continuum source is seen at the location of the star. Null results for CO J = 1–0 emission different from the ambient cloud are reported. The possibility that HD 278942 is a main-sequence star with a circumstellar disk is suggested.