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We have conducted a 3.6 cm radio-continuum survey with the VLA in its compact D-array to look for extended radio-continuum emission around known ultracompact H II regions. We Ðnd extended emission in 12 out of 15 Ðelds. The extended emission ranges in size up to 5@, with typical diameters of 60A, or 2 pc. In some cases, the extended component produces an order of magnitude more Ñux than the ultracompact component. It is possible that the extended emission is from H II regions not connected with the ultracompact H II regions, but on the basis of the morphology of the emission, we believe that it is possible or even likely that the ultracompact and extended emission are directly connected in about half of the sources. The possible consequences of directly connected extended emission are profound. We may be forced to reconsider the deÐnition of an ultracompact H II region and consider models in which dense, ultracompact ionized components are embedded in less dense, more extended regions. Subject headings : H II regions È radio continuum : ISM

We present a high-sensitivity radio continuum survey at 6 and 1.3 cm using the Karl G. Jansky Very Large Array toward a sample of 58 high-mass star-forming regions. Our sample was chosen from dust clumps within infrared dark clouds with and without IR sources (CMC-IRs and CMCs, respectively), and hot molecular cores (HMCs), with no previous, or relatively weak radio continuum detection at the 1 mJy level. Due to the improvement in the continuum sensitivity of the Very Large Array, this survey achieved map rms levels of ∼3-10μJybeam −1 at subarcsecond angular resolution. We extracted 70 continuum sources associated with 1.2 mm dust clumps. Most sources are weak, compact, and prime candidates for high-mass protostars. Detection rates of radio sources associated with the millimeter dust clumps for CMCs, CMC-IRs, and HMCs are 6%, 53%, and 100%, respectively. This result is consistent with increasing high-mass star formation activity from CMCs to HMCs. The radio sources located within HMCs and CMC-IRs occur close to the dust clump centers, with a median offset from it of 12,000 au and 4000 au, respectively. We calculated 5-25 GHz spectral indices using power-law fits and obtained a median value of 0.5 (i.e., flux increasing with frequency), suggestive of thermal emission from ionized jets. In this paper we describe the sample, observations, and detections. The analysis and discussion will be presented in Paper II.

Fifty-four ultracompact (UC) H ii regions in the GLIMPSE survey region (jbj < 1 and 30 < l < 70 ) were observed in H 2 CO and H110 using the 305 m Arecibo telescope. By analyzing H 2 CO absorption against the UC H ii region continuum emission, we resolve the distance ambiguity toward 44 sources. This determination is critical to measure global physical properties of UC H ii regions (e.g., luminosity, size, mass) and properties of the Galaxy (e.g., spiral structure, abundance gradients). We find that the distribution of UC H ii regions in this survey is consistent with a '' local spur,'' the Perseus, Sagittarius, and Scutum arms as delineated by Taylor & Cordes. However, departures from model velocities produce distance uncertainties only slightly smaller than the proposed arm separations.

We report results of an extensive observational campaign of the 6 cm formaldehyde maser in the young massive stellar object IRAS 18566+0408 (G37.55+0.20) conducted from 2002 to 2009. Using Arecibo, VLA, and GBT, we discovered quasi-periodic formaldehyde flares (P ∼ 237 days). Based on Arecibo observations, we also discovered correlated variability between formaldehyde (H 2 CO) and methanol (CH 3 OH) masers. The H 2 CO and CH 3 OH masers are not spatially coincident, as demonstrated by different line velocities and high angular resolution MERLIN observations. The flares could be caused by variations in the infrared radiation field, possibly modulated by periodic accretion onto a young binary system.