We present 1.3 and 3.6？cm radio continuum images and a HCO+ spectrum of the massive star forming region G75.78+0.34 obtained with the Very Large Array (VLA) and with the Berkley Illinois Maryland Association (BIMA) interferometer. Three structures were detected in the continuum emission: one associated with the well-known cometary H ？？ region, plus two more compact structures located at 6′′ east and at 2′′ south of cometary H ？？ region. Using the total flux and intensity peak we estimated an electron density of ≈1.5 × 104？cm？3, an emission measure of ≈6 × 107？cm？6？pc, a mass of ionized gas of ≈3？M⊙, and a diameter of 0.05？pc for the cometary H ？？ region, being typical values for an ultracompact H ？？ region. The HCO+ emission probably originates from the molecular outflows previously observed in HCN and CO. 1. Introduction H ？？ regions are classified as ultracompact, compact, and classical according to their sizes, ionized gas masses, densities, and emission measures (e.g., [1]). Classical H ？？ regions such as the Orion Nebulae have sizes of ~10？pc, densities of ~100？cm3, ~105？M⊙ of ionized gas, and EM ~ 102？pc？cm6. Compact H ？？ regions have densities 5 × 103？cm3, sizes of 0.5？pc, ionized gas mass of ~1？M⊙, and EM 107？pc？cm？6 while ultracompact H ？？ regions have sizes of 0.1？pc, and thermal electron densities of 104？cm3, mass of ionized gas of ~10？2？M⊙, and EM 107？pc cm？6 [2, 3]. The study of the physical properties of H ？？ regions and their classification is a fundamental key to understand how they evolute and how stars form. The radio continuum emission at centimeter wavelengths traces the emission of the ionized gas. On the other hand, the study of the molecular gas next to H ？？ regions provides significant information about these objects and issues of star formation theory applied to these objects. So far, most of the studies of the molecular gas are based on CO emission, which has a low dipole moment, implying that low rotational transitions do not trace dense gas, while molecules with higher dipole moments can be used to observe high density gas. The high density cores (n ~ 104-105？cm3) can be studied from mm and submm line emission of the HCN and HCO+ molecules [4]. In this work, we use 1.3 and 3.6？cm Very Large Array images to study the physical conditions of the gas in the ultracompact H ？？ region G75.78+0.34, as well as Berkley Illinois Maryland Association (BIMA) interferometric data at 3？mm. This object presents a well-known molecular outflow observed in CO [5, 6] and HCN [7] is located in the giant molecular cloud ON2 and was firstly
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