DECIPHERING THE IONIZED GAS CONTENT IN THE MASSIVE STAR-FORMING COMPLEX G75.78+0.34

Abstract

We present sub-arcsecond observations toward the massive star forming region G75.78+0.34. We used the Very Large Array to study the centimeter continuum and H2O and CH3OH maser emission, and the Owens Valley Radio Observatory and Submillimeter Array to study the millimeter continuum and recombination lines (H40alpha and H30alpha). We found radio continuum emission at all wavelengths, coming from three components: (1) a cometary ultracompact (UC) HII region with an electron density 3.7x10^4 cm^{-3}, excited by a B0 type star, and with no associated dust emission; (2) an almost unresolved UCHII region (EAST), located 6 arcsec to the east of the cometary UCHII region, with an electron density 1.3x10^5 cm^{-3}, and associated with a compact dust clump detected at millimeter and mid-infrared wavelengths; and (3) a compact source (CORE), located 2 arcsec to the southwest of the cometary arc, with a flux density increasing with frequency, and embedded in a dust condensation of 30 Msun. The CORE source is resolved into two compact and unresolved sources which can be well-fit by two homogeneous hypercompact HII regions each one photo-ionized by a B0.5 ZAMS star, or by free-free radiation from shock-ionized gas resulting from the interaction of a jet/outflow system with the surrounding environment. The spatial distribution and kinematics of water masers close to the CORE-N and S sources, together with excess emission at 4.5 mum and the detected dust emission, suggest that the CORE source is a massive protostar driving a jet/outflow.