High Resolution Observations of the Massive Protostar in IRAS18566+0408

We report 3 mm continuum, CH3CN(5-4) and 13CS(2-1) line observations with CARMA, in conjunction with 6 and 1.3 cm continuum VLA data, and 12 and 25 micron broadband data from the Subaru Telescope toward the massive proto-star IRAS18566+0408. The VLA data resolve the ionized jet into 4 components aligned in the E-W direction. Radio components A, C, and D have flat cm SEDs indicative of optically thin emission from ionized gas, and component B has a spectral index alpha = 1.0, and a decreasing size with frequency proportional to frequency to the -0.5 power. Emission from the CARMA 3 mm continuum, and from the 13CS(2-1), and CH3CN(5-4) spectral lines is compact (i.e. < 6700 AU), and peaks near the position of VLA cm source, component B. Analysis of these lines indicates hot, and dense molecular gas, typical for HMCs. Our Subaru telescope observations detect a single compact source, coincident with radio component B, demonstrating that most of the energy in IRAS18566+0408 originates from a region of size < 2400 AU. We also present UKIRT near-infrared archival data for IRAS18566+0408 which show extended K-band emission along the jet direction. We detect an E-W velocity shift of about 10 km/sec over the HMC in the CH3CN lines possibly tracing the interface of the ionized jet with the surrounding core gas. Our data demonstrate the presence of an ionized jet at the base of the molecular outflow, and support the hypothesis that massive protostars with O-type luminosity form with a mechanism similar to lower mass stars

Broadband VLA Spectral Line Survey of a Sample of Ionized Jet Candidates

The study of the interaction between ionized jets, molecular outflows and their environments is critical to understanding high-mass star formation, especially because jets and outflows are thought to be key in the transfer of angular momentum outwards from accretion disks. We report a low-spectral resolution VLA survey for hydrogen radio recombination lines, OH, NH$_3$, and CH$_3$OH lines toward a sample of 58 high-mass star forming regions that contain numerous ionized jet candidates. The observations are from a survey designed to detect radio continuum; the novel aspect of this work is to search for spectral lines in broadband VLA data (we provide the script developed in this work to facilitate exploration of other datasets). We report detection of 25$\,$GHz CH$_3$OH transitions toward ten sources; five of them also show NH$_3$ emission. We found that most of the sources detected in CH$_3$OH and NH$_3$ have been classified as ionized jets or jet candidates and that the emission lines are coincident with, or very near ($\lesssim 0.1$ pc) these sources, hence, these molecular lines could be used as probes of the environment near the launching site of jets/outflows. No radio recombination lines were detected, but we found that the RMS noise of stacked spectra decreases following the radiometer equation. Therefore, detecting radio recombination lines in a sample of brighter free-free continuum sources should be possible. This work demonstrates the potential of broadband VLA continuum observations as low-resolution spectral line scans.Comment: 38 pages, 19 figures. Accepted for publication in The Astrophysical Journal Supplement Serie

Weak and Compact Radio Emission in Early High-Mass Star Forming Regions: II. The Nature of the Radio Sources

In this study we analyze 70 radio continuum sources associated with dust clumps and considered to be candidates for the earliest stages of high-mass star formation. The detection of these sources was reported by Rosero et al. (2016), who found most of them to show weak (${\scriptstyle <}$1 mJy) and compact (${\scriptstyle <}\,$0.6$^{\prime \prime}$) radio emission. Herein, we used the observed parameters of these sources to investigate the origin of the radio continuum emission. We found that at least $\sim 30\%$ of these radio detections are most likely ionized jets associated with high-mass protostars, but for the most compact sources we cannot discard the scenario that they represent pressure-confined HII regions. This result is highly relevant for recent theoretical models based on core accretion that predict the first stages of ionization from high-mass stars to be in the form of jets. Additionally, we found that properties such as the radio luminosity as a function of the bolometric luminosity of ionized jets from low and high-mass stars are extremely well-correlated. Our data improve upon previous studies by providing further evidence of a common origin for jets independently of luminosity.Comment: Accepted for publication in the Ap

Weak and Compact Radio Emission in Early High-Mass Star Forming Regions: I. VLA Observations

We present a high sensitivity radio continuum survey at 6 and 1.3$\,$cm using the Karl G. Jansky Very Large Array towards 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, 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 VLA, this survey achieved map rms levels of $\sim$ 3-10 $\mu$Jy beam$^{-1}$ at sub-arcsecond angular resolution. We extracted 70 centimeter continuum sources associated with 1.2$\,$mm dust clumps. Most sources are weak, compact, and are prime candidates for high-mass protostars. Detection rates of radio sources associated with the mm 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 4,000$\,$AU, respectively. We calculated 5 - 25$\,$GHz spectral indices using power law fits and obtain 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.Comment: Accepted for publication in the ApJ

Excited Hydroxyl Outflow in the High-Mass Star-Forming Region G34.26+0.15

G34.26+0.15 is a region of high-mass star formation that contains a broad range of young stellar objects in different stages of evolution, including a hot molecular core, hyper-compact HII regions and a prototypical cometary ultra-compact HII region. Previous high-sensitivity single dish observations by our group resulted in the detection of broad 6035 MHz OH absorption in this region; the line showed a significant blue-shifted asymmetry indicative of molecular gas expansion. We present high-sensitivity Karl G. Jansky Very Large Array (VLA) observations of the 6035 MHz OH line conducted to image the absorption and investigate its origin with respect to the different star formation sites in the region. In addition, we report detection of 6030 MHz OH absorption with the VLA and further observations of 4.7 GHz and 6.0 GHz OH lines obtained with the Arecibo Telescope. The 6030 MHz OH line shows a very similar absorption profile as the 6035 MHz OH line. We found that the 6035 MHz OH line absorption region is spatially unresolved at $\sim 2$" scales, and it is coincident with one of the bright ionized cores of the cometary HII region that shows broad radio recombination line emission. We discuss a scenario where the OH absorption is tracing the remnants of a pole-on molecular outflow that is being ionized inside-out by the ultra-compact HII region.Comment: 19 pages, 6 figures. Accepted for publication in The Astrophysical Journa

High Resolution, Wide Field, Narrow Band, Snapshot Imaging

We investigate the imaging performance of an interferometric array in the case of wide field, high resolution, narrow band, snapshot imaging. We find that, when uv-cell sizes are sufficiently small (ie. image sizes are sufficiently large), each instantaneous visibility record is gridded into its own uv-cell. This holds even for dense arrays, like the core of the next generation VLA. In this particular, application, Uniform weighting of the gridded visibilities approaches Natural weighting, with its often deleterious consequences on the resulting synthesized beam. For a core-dominated array, we show that the resulting image noise is highly correlated on scales comparable to the spatial frequencies of the core baselines. In general, this study accentuates the fact that, for imaging applications that require high resolution (Plains array and greater), many of the core antennas can be employed as a separate subarray for low resolution science, without sacrificing the quality of the high resolution science.Comment: 18 pages; Next Generation VLA Memo No. 78; https://ngvla.nrao.edu/page/memos#gen-mem