Velocity shifts and differential broadening of radio recombination lines are used to estimate the densities and velocities of the ionized gas in several hypercompact and ultracompact H ii regions. These small H ii regions are thought to be at their earliest evolutionary phase and associated with the youngest massive stars. The observations suggest that these H ii regions are characterized by high densities, supersonic flows, and steep density gradients, consistent with accretion and outflows that would be associated with the formation of massive stars.

We present the analysis of four 3.6 cm radio continuum archival observations of Orion obtained using the Very Large Array in its A configuration, with 0B3 angular resolution. The observations were made during the period 1994-1997. In a region of 4 0 ; 4 0 , we detect 77 compact radio sources. Of these sources, 54 are detected in one or more of the individual observations, and 36 of these show time variability (by more than 30%) between the observed epochs. A deep image made from averaging all data shows an additional 23 faint sources, in the range of 0.1 to 0.3 mJy. Of the 77 sources, 39 are new centimeter detections. However, only 9 of the 77 sources do not have a previously reported counterpart at near-infrared, optical, or X-ray wavelengths. In particular, we detect three faint sources in the OMC-1S region that may be related to the sources that power the multiple outflows that emanate from this part of the Orion Nebula.

We present a complete sample of molecular clumps containing compact and ultracompact (UC) H ii regions between ℓ = 10 • and 60 • and |b| < 1 • , identified by combining the the ATLASGAL sub-mm and CORNISH radio continuum surveys with visual examination of archival infrared data. Our sample is complete to optically thin, compact and UC H ii regions driven by a zero age main sequence star of spectral type B0 or earlier embedded within a 1,000 M ⊙ clump. In total we identify 213 compact and UC H ii regions, associated with 170 clumps. Unambiguous kinematic distances are derived for these clumps and used to estimate their masses and physical sizes, as well as the Lyman continuum fluxes and sizes of their embedded H ii regions. We find a clear lower envelope for the surface density of molecular clumps hosting massive star formation of 0.05 g cm −2 , which is consistent with a similar sample of clumps associated with 6.7 GHz masers. The mass of the most massive embedded stars is closely correlated with the mass of their natal clump. Young B stars appear to be significantly more luminous in the ultraviolet than predicted by current stellar atmosphere models. The properties of clumps associated with compact and UC H ii regions are very similar to those associated with 6.7 GHz methanol masers and we speculate that there is little evolution in the structure of the molecular clumps between these two phases. Finally, we identify a significant peak in the surface density of compact and UC H ii regions associated with the W49A star-forming complex, noting that this complex is truly one of the most massive and intense regions of star formation in the Galaxy.

We present high angular resolution (∼ 1 ′′ ), sensitive CO(2-1) line observations of the region OMC1 South in the Orion Nebula made using the Submillimeter Array (SMA). We detect the CO(2-1) high velocity outflow that was first found by Rodríguez-Franco et al. (1999a) with the IRAM 30 m. Our observations resolve the outflow, whose velocity-integrated emission has a deconvolved width of 0. "89 ± 0. "06 (390 AU) and a projected length of ∼ 48 ′′ (21,000 AU) with very high redshifted and blueshifted gas with velocities of about ±80 km s −1 . This outflow is among the most collimated (∼ 3 • ) and youngest outflows (600 yr) that have been reported. The data show that this collimated outflow has been blowing in the same direction during the last 600 yr. At high velocities, the CO(2-1) outflow traces an extremely collimated jet, while at lower velocities the CO emission traces an envelope possibly produced by entrainment of ambient gas. Furthermore, we also detect for the first time a millimeter wavelength continuum source possibly associated with a class I protostar that we suggest could be the possible exciting source for this collimated outflow. However, the bolometric luminosity of this source appears to be far too low to account for the powerful molecular outflow.Subject headings: stars: pre-main sequence -ISM: jets and outflows -ISM: individual: (Orion-S, OMC1-S, M42) -

We present sensitive 1.3 cm radio continuum observations of the region OMC1 South (OMC-1S) in Orion using the Very Large Array in its B configuration. We detect eleven radio sources clustered in a 30 ′′ × 30 ′′ region, of which only three had been detected previously at radio wavelengths in deep 3.6 cm observations. The eight new radio sources are compact (θ s ≤ 0. ′′ 1) and we set lower limits to their spectral indices, α > 0.8 ± 0.3 (with S ν ∝ ν α ), that suggest that they may be optically-thick H II regions. However, one of the new sources exhibits significant circular polarization, indicating that gyrosynchrotron emission with large positive spectral indices may be an alternative explanation. Furthermore, we find that four other sources are associated with infrared sources of low bolometric luminosity that cannot drive an H II region. Finally, two of the sources previously detected at 3.6-cm are angularly resolved in the 1.3 cm image and their major axes have position angles that align well with large scale outflows emanating from OMC-1S. The radio source 143-353 has a major axis with a position angle consistent with those of the HH 202 and HH 528 flows, while the radio source 134-411 has a major axis with a position angle consistent with that of the low-velocity molecular outflow associated with the far-infrared source FIR 4.