Search for Calibrators for the Submillimeter Array: I. High-Mass Star Forming Regions

We present initial results of an ongoing search for interferometric calibrators at submillimeter (sub-mm) wavelengths with the Submillimeter Array (SMA). Powerful radio galaxies are commonly used as calibrators at centimeter and millimeter wavelengths, but many are not strong enough to serve as calibrators at sub-mm wavelengths because of their rapidly declining flux densities toward shorter wavelengths. The inability to find a calibrator close to the target source may limit or even prevent us from imaging many interesting sources at sub-mm wavelengths. Here, we investigate whether high-mass protostellar objects and ultracompact H II regions can serve as useful calibrators for the SMA. The dust emission associated with these objects makes them among the brightest sub-mm sources in the sky. Our observations at 0.85 mm (345 GHz) with an angular resolution of ∼3 reveal that although a large fraction of the dust emission originates from an extended “halo” component, a compact unresolved component often remains that when sufficiently strong may serve as a useful calibrator. These observations also provide a first glimpse at the small-scale distribution of dust around ultracompact H II regions and high-mass protostellar objects at submm wavelengths. We discuss the origin of the core-halo structure seen in many sources, and conclude with suggestions for future searches for calibrators with the SMA. Subject headings: techniques: interferometric, H II regions, radio continuum: ISM Institute of Astronomy and Astrophysics, Academia Sinica, P.O. Box 23-141, Taipei 106, Taiwan, R.O.C. Institute of Astronomy, National Central University, 300 Jungda Rd, Jungli City, Taoyuan 320, Taiwan, R.O.C. Institute of Astronomy and Astrophysics, Academia Sinica, 645 North A’ohoku Place, Hilo, HI 96720 Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138