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Gas-phase abundance ratios between \ce{C2H4O2} isomers methyl formate (MF), glycolaldehyde (GA), and acetic acid (AA) are typically on the order of 100:10:1 in star-forming regions. However, an unexplained divergence from this neat relationship was recently observed towards a collection of sources in the massive protocluster NGC 6334I; some sources exhibited extreme MF:GA ratios, producing a bimodal behavior between different sources, while the MF:AA ratio remained stable. Here, we use a three-phase gas-grain hot-core chemical model to study the effects of a large parameter space on the simulated \ce{C2H4O2} abundances. A combination of high gas densities and long timescales during ice-mantle desorption ($\sim$125--160~K) appears to be the physical cause of the high MF:GA ratios. The main chemical mechanism for GA destruction occurring under these conditions is the rapid adsorption and reaction of atomic H with GA on the ice surfaces before it has time to desorb. The different binding energies of MF and GA on water ice are crucial to the selectivity of the surface destruction mechanism; individual MF molecules rapidly escape the surface when exposed by water loss, while GA lingers and is destroyed by H. Moderately elevated cosmic-ray ionization rates can increase absolute levels of COM production in the ices and increase the MF:GA ratio, but extreme values are destructive for gas-phase COMs. We speculate that the high densities required for extreme MF:GA ratios could be evidence of COM emission dominated by COMs desorbing within a circumstellar disk.

At centimeter wavelengths, single-dish observations have suggested that the Sagittarius (Sgr) B2 molecular cloud at the Galactic Center hosts weak maser emission from several organic molecules, including CH$_2$NH, HNCNH, and HCOOCH$_3$. However, the lack of spatial distribution information of these new maser species has prevented us from assessing the excitation conditions of the maser emission as well as their pumping mechanisms. Here, we present a mapping study toward Sgr B2 North (N) to locate the region where the complex maser emission originates. We report the first detection of the Class I methanol (CH$_3$OH) maser at 84 GHz and the first interferometric map of the methanimine (CH$_2$NH) maser at 5.29 GHz toward this region. In addition, we present a tool for modeling and fitting the unsaturated molecular maser signals with non-LTE radiative transfer models and Bayesian analysis using the Markov-Chain Monte Carlo approach. These enable us to quantitatively assess the observed spectral profiles. The results suggest a two-chain-clump model for explaining the intense CH$_3$OH Class I maser emission toward a region with low continuum background radiation. By comparing the spatial origin and extent of maser emission from several molecular species, we find that the 5.29 GHz CH$_2$NH maser has a close spatial relationship with the 84 GHz CH$_3$OH Class I masers. This relationship serves as observational evidence to suggest a similar collisional pumping mechanism for these maser transitions., Comment: 17 figures

We use both chirped-pulse Fourier transform and frequency modulated absorption spectroscopy to study the rotational spectrum of 2-methoxyethanol in several frequency regions ranging from 8.7-500 GHz. The resulting rotational parameters permitted a search for this molecule in Atacama Large Millimeter/submillimeter Array (ALMA) observations toward the massive protocluster NGC 6334I as well as source B of the low-mass protostellar system IRAS 16293-2422. 25 rotational transitions are observed in the ALMA Band 4 data toward NGC 6334I, resulting in the first interstellar detection of 2-methoxyethanol. A column density of $1.3_{-0.9}^{+1.4} \times 10^{17}$ cm$^{-2}$ is derived at an excitation temperature of $143_{-39}^{+31}$ K. However, molecular signal is not observed in the Band 7 data toward IRAS 16293-2422B and an upper limit column density of $2.5 \times 10^{15}$ cm$^{-2}$ is determined. Various possible formation pathways--including radical recombination and insertion reactions--are discussed. We also investigate physical differences between the two interstellar sources that could result in the observed abundance variations., Comment: Accepted for publication in The Astrophysical Journal Letters

Much of the information gleaned from observations of star-forming regions comes from the analysis of their molecular emission spectra, particularly in the radio regime. The time-consuming nature of fitting synthetic spectra to observations interactively for such line-rich sources, however, often results in such analysis being limited to data extracted from a single-dish observation or a handful of pixels from an interferometric observation. Yet, star-forming regions display a wide variety of physical conditions that are difficult, if not impossible, to accurately characterize with such a limited number of spectra. We have developed an automated fitting routine that visits every pixel in the field of view of an ALMA data cube and determines the best-fit physical parameters, including excitation temperature and column densities, for a given list of molecules. In this proof-of-concept work, we provide an overview of the fitting routine and apply it to 0".26, 1.1 km s$^{-1}$ resolution ALMA observations of two sites of massive star-formation in NGC 6334I. Parameters were found for 21 distinct molecules by generating synthetic spectra across 7.48 GHz of spectral bandwidth between 280 and 351 GHz. Spatial images of the derived parameters for each of the > 8000 pixels are presented with special attention paid to the C$_2$H$_4$O$_2$ isomers and their relative variations. We highlight the greater scientific utility of the column density and velocity images of individual molecules compared to traditional moment maps of single transitions., Comment: 40 pages, 71 figures, accepted for publication in The Astrophysical Journal

We present a study of the physical and chemical properties of the Extended Green Object (EGO) G19.01$-$0.03 using sub-arcsecond angular resolution Atacama Large Millimeter/submillimeter Array (ALMA) 1.05mm and Karl G. Jansky Very Large Array (VLA) 1.21cm data. G19.01$-$0.03 MM1, the millimetre source associated with the central massive young stellar object (MYSO), appeared isolated and potentially chemically young in previous Submillimeter Array observations. In our $\sim0.4''$-resolution ALMA data, MM1 has four low-mass millimetre companions within 0.12pc, all lacking maser or outflow emission, indicating they may be prestellar cores. With a rich ALMA spectrum full of complex organic molecules, MM1 does not appear chemically young, but has molecular abundances typical of high-mass hot cores in the literature. At the 1.05mm continuum peak of MM1, $\mathrm{N}(\mathrm{CH}_{3}\mathrm{OH})=(2.22\pm0.01)\times10^{18}$cm$^{-2}$ and $T_{\mathrm{ex}} = 162.7\substack{+0.3 \\ -0.5}$K based on pixel-by-pixel Bayesian analysis of LTE synthetic methanol spectra across MM1. Intriguingly, the peak CH$_{3}$OH $T_{\mathrm{ex}}=165.5\pm0.6$ K is offset from MM1's millimetre continuum peak by $0.22''\sim880$au, and a region of elevated CH$_{3}$OH $T_{\mathrm{ex}}$ coincides with free-free VLA 5.01cm continuum, adding to the tentative evidence for a possible unresolved high-mass binary in MM1. In our VLA 1.21cm data, we report the first NH$_{3}$(3,3) maser detections towards G19.01$-$0.03, along with candidate 25GHz CH$_{3}$OH $5(2,3)-5(1,4)$ maser emission; both are spatially and kinematically coincident with 44GHz Class I CH$_{3}$OH masers in the MM1 outflow. We also report the ALMA detection of candidate 278.3GHz Class I CH$_{3}$OH maser emission towards this outflow, strengthening the connection of these three maser types to MYSO outflows., Comment: 24 pages, 15 figures, 7 tables, Accepted for publication in MNRAS

We describe the calibration and imaging heuristics developed and deployed in the ALMA interferometric data processing pipeline, as of ALMA Cycle 9. The pipeline software framework is written in Python, with each data reduction stage layered on top of tasks and toolkit functions provided by the Common Astronomy Software Applications package. This framework supports a variety of tasks for observatory operations, including science data quality assurance, observing mode commissioning, and user reprocessing. It supports ALMA and VLA interferometric data along with ALMA and NRO45m single dish data, via different stages and heuristics. In addition to producing calibration tables, calibrated measurement sets, and cleaned images, the pipeline creates a WebLog which serves as the primary interface for verifying the data quality assurance by the observatory and for examining the contents of the data by the user. Following the adoption of the pipeline by ALMA Operations in 2014, the heuristics have been refined through annual development cycles, culminating in a new pipeline release aligned with the start of each ALMA Cycle of observations. Initial development focused on basic calibration and flagging heuristics (Cycles 2-3), followed by imaging heuristics (Cycles 4-5), refinement of the flagging and imaging heuristics with parallel processing (Cycles 6-7), addition of the moment difference analysis to improve continuum channel identification (2020 release), addition of a spectral renormalization stage (Cycle 8), and improvement in low SNR calibration heuristics (Cycle 9). In the two most recent Cycles, 97% of ALMA datasets were calibrated and imaged with the pipeline, ensuring long-term automated reproducibility. We conclude with a brief description of plans for future additions, including self-calibration, multi-configuration imaging, and calibration and imaging of full polarization data., Comment: Accepted for publication by Publications of the Astronomical Society of the Pacific, 65 pages, 20 figures, 10 tables, 2 appendices. Small corrections and additions from the proof stage have been applied

Using the Atacama Large Millimetre/submillimeter Array (ALMA) and the Karl G. Jansky Very Large Array (VLA), we observed the Extended Green Object (EGO) G19.01$-$0.03 with sub-arcsecond resolution from 1.05 mm to 5.01 cm wavelengths. Our $\sim0.4''\sim1600$ AU angular resolution ALMA observations reveal a velocity gradient across the millimetre core MM1, oriented perpendicular to the previously known bipolar molecular outflow, that is consistently traced by 20 lines of 8 molecular species with a range of excitation temperatures, including complex organic molecules (COMs). Kinematic modelling shows the data are well described by models that include a disc in Keplerian rotation and infall, with an enclosed mass of $40-70 \mathrm{M}_{\odot}$ (within a 2000 AU outer radius) for a disc inclination angle of $i=40^{\circ}$, of which $5.4-7.2 \mathrm{M}_{\odot}$ is attributed to the disc. Our new VLA observations show that the 6.7 GHz Class II methanol masers associated with MM1 form a partial ellipse, consistent with an inclined ring, with a velocity gradient consistent with that of the thermal gas. The disc-to-star mass ratio suggests the disc is likely to be unstable and may be fragmenting into as-yet-undetected low mass stellar companions. Modelling the centimetre--millimetre spectral energy distribution of MM1 shows the ALMA 1.05 mm continuum emission is dominated by dust, whilst a free-free component, interpreted as a hypercompact HII region, is required to explain the VLA $\sim$5 cm emission. The high enclosed mass derived for a source with a moderate bolometric luminosity ($\sim$10$^{4} \mathrm{L}_{\odot}$) suggests that the MM1 disc may feed an unresolved high-mass binary system., Comment: 15 pages, 8 figures, 5 tables, Accepted for publication in MNRAS

Following an eruptive accretion event in NGC6334I-MM1, flares in the various maser species, including water masers, were triggered. We report the observed relative proper motion of the highly variable water masers associated with the massive star-forming region, NGC6334I. High velocity H$_2$O maser proper motions were detected in 5 maser clusters, CM2-W2 (bow-shock structure), MM1-W1, MM1-W3, UCHII-W1 and UCHII-W3. The overall average of the derived relative proper motion is 85 km s$^{-1}$. This mean proper motion is in agreement with the previous results from VLA multi-epoch observations. Our position and velocity variance and co-variance matrix analyses of the maser proper motions show its major axis to have a position angle of $-$79.4$^\circ$, cutting through the dust cavity around MM1B and aligned in the northwest-southeast direction. We interpret this as the axis of the jet driving the CM2 shock and the maser motion. The complicated proper motions in MM1-W1 can be explained by the combined influence of the MM1 northeast-southwest bipolar outflow, CS(6-5) north-south collimated bipolar outflow, and the radio jet. The relative proper motions of the H$_2$O masers in UCHII-W1 are likely not driven by the jets of MM1B protostar but by MM3-UCHII. Overall, the post-accretion burst relative proper motions of the H$_2$O masers trace shocks of jet motion., Comment: 18 pages, 6 figures, accepted for publication by The Astrophysical Journal

A diverse array of science goals require accurate flux calibration of observations with the Atacama Large Millimeter/Submillimeter array (ALMA), however, this goal remains challenging due to the stochastic time-variability of the ``grid'' quasars ALMA uses for calibration. In this work, we use 343.5 GHz (Band 7) ALMA Atacama Compact Array observations of four bright and stable young stellar objects over 7 epochs to independently assess the accuracy of the ALMA flux calibration and to refine the relative calibration across epochs. The use of these four extra calibrators allow us to achieve an unprecedented relative ALMA calibration accuracy of $\sim 3\%$. On the other hand, when the observatory calibrator catalog is not up-to-date, the Band 7 data calibrated by the ALMA pipeline may have a flux calibration poorer than the nominal 10\%, which can be exacerbated by weather-related phase decorrelation when self-calibration of the science target is either not possible or not attempted. We also uncover a relative flux calibration uncertainty between spectral windows of 0.8\%, implying that measuring spectral indices within a single ALMA band is likely highly uncertain. We thus recommend various methods for science goals requiring high flux accuracy and robust calibration, in particular, the observation of additional calibrators combined with a relative calibration strategy, and observation of solar system objects for high absolute accuracy.

Amide molecules produced in space could play a key role in the formation of biomolecules on a young planetary object. However, the formation and chemical network of amide molecules in space is not well understood. In this work, ALMA observations are used to study a number of amide(-like) molecules toward the high-mass star-forming region NGC 6334I. The first detections of cyanamide (NH$_{2}$CN), acetamide (CH$_{3}$C(O)NH$_{2}$) and N-methylformamide (CH$_{3}$NHCHO) are presented for this source. These are combined with analyses of isocyanic acid (HNCO) and formamide (NH$_{2}$CHO) and a tentative detection of urea (carbamide; NH$_{2}$C(O)NH$_{2}$). Abundance correlations show that most amides are likely formed in related reactions occurring in ices on interstellar dust grains in NGC 6334I. However, in an expanded sample of sources, large abundance variations are seen for NH$_{2}$CN that seem to depend on the source type, which suggests that the physical conditions within the source heavily influence the production of this species. The rich amide inventory of NGC 6334I strengthens the case that interstellar molecules can contribute to the emergence of biomolecules on planets., Comment: Accepted for publication in The Astrophysical Journal

We report the results of a pilot program for a Green Bank Telescope (GBT) MUSTANG Galactic Plane survey at 3 mm (90 GHz), MGPS90. The survey achieves a typical $1\sigma$ depth of $1-2$ mJy beam$^{-1}$ with a 9" beam. We describe the survey parameters, quality assessment process, cataloging, and comparison with other data sets. We have identified 709 sources over seven observed fields selecting some of the most prominent millimeter-bright regions between $0\deg < \ell < 50\deg$ (total area $\approx 7.5 \deg^2$). The majority of these sources have counterparts at other wavelengths. By applying flux selection criteria to these sources, we successfully recovered several known hypercompact HII (HCHII) regions, but did not confirm any new ones. We identify 126 sources that have mm-wavelength counterparts but do not have cm-wavelength counterparts and are therefore candidate HCHII regions; of these, 10 are morphologically compact and are strong candidates for new HCHII regions. Given the limited number of candidates in the extended area in this survey compared to the relatively large numbers seen in protoclusters W51 and W49, it appears that most HCHII regions exist within dense protoclusters. Comparing the counts of HCHII to ultracompact HII (UCHII) regions, we infer the HCHII region lifetime is 16-46% that of the UCHII region lifetime. We additionally separated the 3 mm emission into dust and free-free emission by comparing with archival 870 $\mu$m and 20 cm data. In the selected pilot fields, most ($\gtrsim80$%) of the 3 mm emission comes from plasma, either through free-free or synchrotron emission., Comment: Accepted to ApJS. Data publicly released, with links in paper. Reposted to fix missing figure labels

The rotational spectrum of the most stable conformer of ethylene glycol (HO(CH$_2$)$_2$OH) has been recorded between 360-890 GHz using a frequency-modulation sub-millimeter spectrometer. The refinement and extension of the spectroscopic parameters over previous efforts provides predicted catalog frequencies for ethylene glycol with sufficient accuracy for comparison to high-frequency astronomical data. The improvement in the cataloged line positions, and the need for improved accuracy enabled by high-frequency laboratory work, is demonstrated by an analysis of ethylene glycol emission at 890 GHz in the high-mass star-forming region NGC 6334I in ALMA Band 10 observations. The need for accurate rotational spectra at sub-millimeter wavelengths/THz frequencies is discussed., Comment: accepted in the Journal of Physical Chemistry A

A long standing problem in astrochemistry is the inability of many current models to account for missing sulfur content. Many relatively simple species that may be good candidates to sequester sulfur have not been measured experimentally at the high spectral resolution necessary to enable radioastronomical identification. On the basis of new laboratory data, we report searches for the rotational lines in the microwave, millimeter, and sub-millimeter regions of the sulfur-containing hydrocarbon HCCSH. This simple species would appear to be a promising candidate for detection in space owing to the large dipole moment along its $b$-inertial axis, and because the bimolecular reaction between two highly abundant astronomical fragments (CCH and SH radicals) may be rapid. An inspection of multiple line surveys from the centimeter to the far-infrared toward a range of sources from dark clouds to high-mass star-forming regions, however, resulted in non-detections. An analogous search for the lowest-energy isomer, H$_2$CCS, is presented for comparison, and also resulted in non-detections. Typical upper limits on the abundance of both species relative to hydrogen are $10^{-9}$-$10^{-10}$. We thus conclude that neither isomer is a major reservoir of interstellar sulfur in the range of environments studied. Both species may still be viable candidates for detection in other environments or at higher frequencies, providing laboratory frequencies are available., Comment: Accepted in the Astrophysical Journal

The relative column densities of the structural isomers methyl formate, glycolaldehyde, and acetic acid are derived for a dozen positions towards the massive star-forming regions MM1 and MM2 in the NGC 6334I complex, which are separated by $\sim$4000 AU. Relative column densities of these molecules are also gathered from the literature for 13 other star-forming regions. In this combined dataset, a clear bi-modal distribution is observed in the relative column densities of glycolaldehyde and methyl formate. No such distribution is evident with acetic acid. The two trends are comprised of star-forming regions with a variety of masses, suggesting that there must be some other common parameter that is heavily impacting the formation of glycolaldehyde. This is indicative of some demonstrable differentiation in these cores; studying the abundances of these isomers may provide a clue as to the integral chemical processes ongoing in a variety of protostellar environments., Comment: Accepted for publication in the Astrophysical Journal

Vibrationally excited states of detected interstellar molecules have been shown to account for a large portion of unidentified spectral lines in observed interstellar spectra toward chemically rich sources. Here, we present the first interstellar detection of the first and second vibrationally excited torsional states of acetic acid ($v_\mathrm{t} = 1, 2$) toward the high-mass star-forming region NGC 6334I. The observations presented were taken with the Atacama Large Millimeter/submillimeter Array in bands 4, 6, and 7 covering a frequency range of 130 - 352 GHz. By comparing a single excitation temperature model to the observations, the best-fit excitation temperature and column density are obtained to be 142(25) K and $1.12(7) \times 10^{17} \mathrm{cm^{-2}}$ respectively. Based on the intensity maps of the vibrationally excited CH$_3$COOH transitions, we found that the CH$_3$COOH emissions are compact and concentrated toward the MM1 and MM2 regions with a source size smaller than 2 arcsec. After locating the emission from different CH$_3$COOH transitions, which cover a large range of excitation energies, we are able to explain the variation of the CH$_3$COOH emission peak within the MM2 core by invoking continuum absorption or outflows., Comment: 15 pages, 5 figures

The bright maser emission produced by several molecular species at centimeter to long millimeter wavelengths provides an essential tool for understanding the process of massive star formation. Unimpeded by the high dust optical depths that affect shorter wavelength observations, the high brightness temperature of these emission lines offers a way to resolve accretion and outflow motions down to scales below $\sim$1 au in deeply embedded Galactic star-forming regions at kiloparsec distances. The recent identification of extraordinary accretion outbursts in two high-mass protostars, both of which were heralded by maser flares, has rapidly impacted the traditional view of massive protostellar evolution, leading to new hydrodynamic simulations that can produce such episodic outbursts. In order to understand how these massive protostars evolve in response to such events, larger, more sensitive ground-based centimeter wavelength interferometers are needed that can simultaneously image multiple maser species in the molecular gas along with faint continuum from the central ionized gas. Fiducial observations of a large sample of massive protostars will be essential in order to pinpoint the progenitors of future accretion outbursts, and to quantify the outburst-induced changes in their protostellar photospheres and outflow and accretion structures. Knowledge gained from these studies will have broader impact on the general topic of accretion onto massive objects., Comment: Science white paper submitted to the Astro2020 Decadal Survey. arXiv admin note: substantial text overlap with arXiv:1806.06981

Massive young stellar objects are known to undergo an evolutionary phase in which high mass accretion rates drive strong outflows. A class of objects believed to trace this phase accurately is the GLIMPSE Extended Green Object (EGO) sample, so named for the presence of extended 4.5 $\mu$m emission on sizescales of $\sim$0.1 pc in \textit{Spitzer} images. We have been conducting a multi-wavelength examination of a sample of 12 EGOs with distances of 1 to 5 kpc. In this paper, we present mid-infrared images and photometry of these EGOs obtained with the SOFIA telescope, and subsequently construct SEDs for these sources from the near-IR to sub-millimeter regimes using additional archival data. We compare the results from greybody models and several publicly-available software packages which produce model SEDs in the context of a single massive protostar. The models yield typical \rstar\/$\sim$10 \rsun, \tstar\/$\sim$10$^3$ to 10$^4$ K, and \lstar\/$\sim$1~$-$~40~$\times$~10$^3$ \lsun; the median $L/M$ for our sample is 24.7 \lsun/\msun. Model results rarely converge for \rstar\/ and \tstar, but do for \lstar, which we take to be an indication of the multiplicity and inherently clustered nature of these sources even though, typically, only a single source dominates in the mid-infrared. The median $L/M$ value for the sample suggests that these objects may be in a transitional stage between the commonly described ``IR-quiet'' and ``IR-bright'' stages of MYSO evolution. The median $T_{dust}$ for the sample is less conclusive, but suggests that these objects are either in this transitional stage or occupy the cooler (and presumably younger) part of the IR-bright stage., Comment: 43 pages, 18 Figures, 10 Tables, 2 Appendices

Variability of pre-main-sequence stars observed at optical wavelengths has been attributed tofluctuations in the mass accretion rate from the circumstellar disk onto the forming star.Detailed models of accretion disks suggest that young deeply embedded protostars should also exhibit variations in their accretion rates, and that these changes can be tracked indirectly by monitoring the response of the dust envelope at mid-IR to millimeter wavelengths. Interferometers such as ALMA offer the resolution and sensitivity to observe small fluctuations in brightness at the scale of the disk where episodic accretion may be driven. In this work, we present novel methods for comparing interferometric observations and apply them to CARMA and ALMA 1.3mm observations of deeply embedded protostars in Serpens taken 9 years apart. We find no brightness variation above the limits of our analysis of a factor of $\gtrsim 50\%$, due to the limited sensitivity of the CARMA observations and small number of sources common to both epochs. We further show that follow up ALMA observations with a similar sample size and sensitivity may be able to uncover variability at the level of a few percent, and discuss implications for future work., Comment: 21 pages, 11 figures

Simulations of massive star formation predict the formation of discs with significant substructure, such as spiral arms and clumps due to fragmentation. Here we present a semi-analytic framework for producing synthetic observations of discs with substructure, in order to determine their observability in interferometric observations. Unlike post-processing of hydrodynamical models, the speed inherent to our approach permits us to explore a large parameter space of star and disc parameters, and thus constrain properties for real observations. We compute synthetic dust continuum and molecular line observations probing different disc masses, distances, inclinations, thermal structures, dust distributions, and number and orientation of spirals and fragments. With appropriate spatial and kinematic filtering applied, our models predict that ALMA observations of massive YSOs at <5 kpc distances should detect spirals in both gas and dust in strongly self-gravitating discs (i.e. discs with up to two spiral arms and strong kinematic perturbations). Detecting spirals will be possible in discs of arbitrary inclination, either by directly spatially resolving them for more face-on discs (inclinations up to ~50 degrees), or through a kinematic signature otherwise. Clumps resulting from disc fragmentation should be detectable in the continuum, if the clump is sufficiently hotter than the surrounding disc material., Comment: Accepted for publication in MNRAS, models available online (DOI 10.5281/zenodo.1408072)

We present the first results of a pilot program to conduct an ALMA Band 10 spectral line survey of the high-mass star-forming region NGC 6334I. The observations were taken in exceptional weather conditions (0.19 mm precipitable water) with typical system temperatures $T_{\rm{sys}}$ $<$950 K at $\sim$890 GHz. A bright, bipolar north-south outflow is seen in HDO and CS emission, driven by the embedded massive protostar MM1B. This has allowed, for the first time, a direct comparison of the thermal water in this outflow to the location of water maser emission from prior 22 GHz VLA observations. The maser locations are shown to correspond to the sites along the outflow cavity walls where high velocity gas impacts the surrounding material. We also compare our new observations to prior Herschel HIFI spectral line survey data of this field, detecting an order of magnitude more spectral lines (695 vs 65) in the ALMA data. We focus on the strong detections of the complex organic molecule glycolaldehyde (HC(O)CH$_2$OH) in the ALMA data that is not detected in the heavily beam-diluted HIFI spectra. Finally, we stress the need for dedicated THz laboratory spectroscopy to support and exploit future high-frequency molecular line observations with ALMA., Comment: Accepted in ApJ Letters

Imaging the bright maser emission produced by several molecular species at centimeter wavelengths is an essential tool for understanding the process of massive star formation because it provides a way to probe the kinematics of dense molecular gas at high angular resolution. Unimpeded by the high dust optical depths that affect shorter wavelength observations, the high brightness temperature of these emission lines offers a way to resolve accretion and outflow motions down to scales as fine as $\sim$1-10 au in deeply embedded Galactic star-forming regions, and at sub-pc scales in nearby galaxies. The Next Generation Very Large Array will provide the capabilities needed to fully exploit these powerful tracers., Comment: To be published in the ASP Monograph Series, "Science with a Next-Generation VLA", ed. E. J. Murphy (ASP, San Francisco, CA), 11 pages, 1 figure, 84 references

In this lecture, we describe a number of advanced gain calibration techniques. In particular, self-calibration is an important tool in interferometric imaging at all wavelengths. It allows the observer to determine and remove residual phase and amplitude errors that remain in the data after normal calibration while simultaneously producing a more accurate and more sensitive image of the target source. We describe the basic technique of self-calibration and attempt to dispel some common misconceptions. We proceed to give a range of useful tips, and provide continuum, spectral line, and mosaic self-calibration examples using ALMA data. We also discuss fast switching and radiometric phase correction along with advanced phase transfer techniques that can become advantageous or even essential at high frequency where the density of sufficiently bright phase calibrators becomes sparse., Comment: 31 pages, 13 figures, 56 references; to appear in the Proceedings of the 2014 Synthesis Imaging Workshop held in Socorro, NM on May 13-20, 2014

The abundance of deuterated molecules in a star-forming region is sensitive to the environment in which they are formed. Deuteration fractions therefore provide a powerful tool for studying the physical and chemical evolution of a star-forming system. While local low-mass star-forming regions show very high deuteration ratios, much lower fractions are observed towards Orion and the Galactic Centre. We derive methanol deuteration fractions at a number of locations towards the high-mass star-forming region NGC 6334I, located at a mean distance of 1.3 kpc, and discuss how these can shed light on the conditions prevailing during its formation. We use high sensitivity, high spatial and spectral resolution observations obtained with ALMA to study transitions of the less abundant, optically thin, methanol-isotopologues: (13)CH3OH, CH3(18)OH, CH2DOH and CH3OD, detected towards NGC 6334I. Assuming LTE and excitation temperatures of 120-330 K, we derive column densities for each of the species and use these to infer CH2DOH/CH3OH and CH3OD/CH3OH fractions. Interestingly, the column densities of CH3OD are consistently higher than those of CH2DOH throughout the region. All regions studied in this work show CH2DOH/CH3OH as well as CH2DOH/CH3OD ratios that are considerably lower than those derived towards low-mass star-forming regions and slightly lower than those derived for the high-mass star-forming regions in Orion and the Galactic Centre. The low ratios indicate a grain surface temperature during formation ~30 K, for which the efficiency of the formation of deuterated species is significantly reduced., Comment: Accepted for publication in Astronomy & Astrophysics

We report the detection of interstellar methoxymethanol (CH$_3$OCH$_2$OH) in ALMA Bands 6 and 7 toward the MM1 core in the high-mass star-forming region NGC 6334I at ~0.1" - 1" spatial resolution. A column density of 4(2) x $10^{18}$ cm$^{-2}$ at $T_{ex}$ = 200 K is derived toward MM1, ~34 times less abundant than methanol (CH$_3$OH), and significantly higher than predicted by astrochemical models. Probable formation and destruction pathways are discussed, primarily through the reaction of the CH$_3$OH photodissociation products, the methoxy (CH$_3$O) and hydroxymethyl (CH$_2$OH) radicals. Finally, we comment on the implications of these mechanisms on gas-phase vs grain-surface routes operative in the region, and the possibility of electron-induced dissociation of CH$_3$OH rather than photodissociation., Comment: Accepted in Astrophysical Journal Letters

In a radio interferometer, the determination of geometrical antenna positions relies on accurate calibration of the dry and wet delay of the atmosphere above each antenna. For the Atacama Large Millimeter/Submillimeter Array (ALMA), which has baseline lengths up to 16 kilometers, the geography of the site forces the height above mean sea level of the more distant antenna pads to be significantly lower than the central array. Thus, both the ground level meteorological values and the total water column can be quite different between antennas in the extended configurations. During 2015, a network of six additional weather stations was installed to monitor pressure, temperature, relative humidity and wind velocity, in order to test whether inclusion of these parameters could improve the repeatability of antenna position determinations in these configurations. We present an analysis of the data obtained during the ALMA Long Baseline Campaign of Oct. through Nov. 2015. The repeatability of antenna position measurements typically degrades as a function of antenna distance. Also, the scatter is more than three times worse in the vertical direction than in the local tangent plane, suggesting that a systematic effect is limiting the measurements. So far we have explored correcting the delay model for deviations from hydrostatic equilibrium in the measured air pressure and separating the partial pressure of water from the total pressure using water vapor radiometer (WVR) data. Correcting for these combined effects still does not provide a good match to the residual position errors in the vertical direction. One hypothesis is that the current model of water vapor may be too simple to fully remove the day-to-day variations in the wet delay. We describe possible avenues of improvement, including measuring and applying more accurate values of the sky coupling efficiency of the WVRs., Comment: 19 pages, 14 figures, 1 table; presented at SPIE Astronomical Telescopes + Instrumentation 2016, held in Edinburgh, UK on 26 June - 1 July 2016

The primary antenna elements and receivers are two of the most important components in a synthesis telescope. Together they are responsible for locking onto an astronomical source in both direction and frequency, capturing its radiation, and converting it into signals suitable for digitization and correlation. The properties and performance of antennas and receivers can affect the quality of the synthesized images in a number of fundamental ways. In this lecture, their most relevant design and performance parameters are reviewed, with emphasis on the current ALMA and VLA systems. We discuss in detail the shape of the primary beam and the components of aperture efficiency, and we present the basics of holography, pointing, and servo control. On receivers, we outline the use of amplifiers and mixers both in the cryogenic front-end and in the room temperature back-end signal path. The essential properties of precision local oscillators (LOs), phase lock loops (PLLs), and LO modulation techniques are also described. We provide a demonstration of the method used during ALMA observations to measure the receiver and system sensitivity as a function of frequency. Finally, we offer a brief derivation and numerical simulation of the radiometer equation., Comment: 31 pages, 13 figures, 153 references; to appear in the Proceedings of the 2014 Synthesis Imaging Workshop held in Socorro, NM on May 13-20, 2014

We present a derivation of the radiometer equation based on the original references and fundamental statistical concepts. We then perform numerical simulations of white noise to illustrate the radiometer equation in action. Finally, we generate 1/f and 1/f^2 noise, demonstrate that it is non-stationary, and use it to simulate the effect of gain fluctuations on radiometer performance., Comment: 4 pages, 9 figures, to appear in the Proceedings of the 26th International Symposium on Space Terahertz Technology, available at https://www.cfa.harvard.edu/events/2015/isstt2015/proceedings/

We have constructed a five station 12 GHz atmospheric phase interferometer (API) for the Submillimeter Array (SMA) located near the summit of Mauna Kea, Hawaii. Operating at the base of unoccupied SMA antenna pads, each station employs a commercial low noise mixing block coupled to a 0.7 m off-axis satellite dish which receives a broadband, white noise-like signal from a geostationary satellite. The signals are processed by an analog correlator to produce the phase delays between all pairs of stations with projected baselines ranging from 33 to 261 m. Each baseline's amplitude and phase is measured continuously at a rate of 8 kHz, processed, averaged and output at 10 Hz. Further signal processing and data reduction is accomplished with a Linux computer, including the removal of the diurnal motion of the target satellite. The placement of the stations below ground level with an environmental shield combined with the use of low temperature coefficient, buried fiber optic cables provides excellent system stability. The sensitivity in terms of rms path length is 1.3 microns which corresponds to phase deviations of about 1 degree of phase at the highest operating frequency of the SMA. The two primary data products are: (1) standard deviations of observed phase over various time scales, and (2) phase structure functions. These real-time statistical data measured by the API in the direction of the satellite provide an estimate of the phase front distortion experienced by the concurrent SMA astronomical observations. The API data also play an important role, along with the local opacity measurements and weather predictions, in helping to plan the scheduling of science observations on the telescope., Comment: 23 pages, 10 figures, accepted for publication in the Journal of Astronomical Instrumentation

We describe the successful design, implementation, and operation of a 12 GHz holography system installed on the Robert C. Byrd Green Bank Telescope (GBT). We have used a geostationary satellite beacon to construct high-resolution holographic images of the telescope mirror surface irregularities. These images have allowed us to infer and apply improved position offsets for the 2209 actuators which control the active surface of the primary mirror, thereby achieving a dramatic reduction in the total surface error (from 390 microns to ~240 microns, rms). We have also performed manual adjustments of the corner offsets for a few panels. The expected improvement in the radiometric aperture efficiency has been rigorously modeled and confirmed at 43 GHz and 90 GHz. The improvement in the telescope beam pattern has also been measured at 11.7 GHz with greater than 60 dB of dynamic range. Symmetric features in the beam pattern have emerged which are consistent with a repetitive pattern in the aperture due to systematic panel distortions. By computing average images for each tier of panels from the holography images, we confirm that the magnitude and direction of the panel distortions, in response to the combination of gravity and thermal gradients, are in general agreement with finite-element model predictions. The holography system is now fully integrated into the GBT control system, and by enabling the telescope staff to monitor the health of the individual actuators, it continues to be an essential tool to support high-frequency observations., Comment: Accepted for publication in PASP. Contains 28 pages with 2 tables and 9 figures (several at reduced quality). The full resolution version is available at http://wwwlocal.gb.nrao.edu/ptcs/papers/Hunter2011/gbtholo.ps.gz (34MB gzip file unpacks to 134MB postscript)

Wind-induced pointing errors are a serious concern for large-aperture high-frequency radio telescopes. In this paper, we describe the implementation of an optical quadrant detector instrument that can detect and provide a correction signal for wind-induced pointing errors on the 100m diameter Green Bank Telescope (GBT). The instrument was calibrated using a combination of astronomical measurements and metrology. We find that the main wind-induced pointing errors on time scales of minutes are caused by the feedarm being blown along the direction of the wind vector. We also find that wind-induced structural excitation is virtually non-existent. We have implemented offline software to apply pointing corrections to the data from imaging instruments such as the MUSTANG 3.3 mm bolometer array, which can recover ~70% of sensitivity lost due to wind-induced pointing errors. We have also performed preliminary tests that show great promise for correcting these pointing errors in real-time using the telescope's subreflector servo system in combination with the quadrant detector signal., Comment: 17 pages, 11 figures; accepted for publication in PASP

The digital phase lock loop described in this paper has been in use on the Submillimeter Array (SMA) front-end receivers for over a decade and has been a key element in obtaining all of the high quality images that have been published from this telescope over the years. The technical achievements enabled by these devices include the first phase closure observations in the 690 GHz band, the first attempts at band-to-band phase transfer at submillimeter wavelengths, and the first successful demonstration of interferometry using a fully photonic millimeter-wave local oscillator. Copies of these devices are also in place at the Caltech Submillimeter Observatory and the James Clerk Maxwell Telescope in support of the eSMA project and submillimeter VLBI experiments. Additional units of this design were used by the Princeton Millimeter Interferometer and the Microwave Anisotropy Telescope. In total, over three dozen units have been constructed and used in astronomical studies. In this paper, we briefly describe the background theory, design, performance, and calibration steps, and provide useful testing and repair information., Comment: 19 pages, 6 figures. This is mostly a superset of SMA technical memo #143 but with a new introduction and additional citations

We combine multifrequency observations from the millimeter to near infrared wavelengths that demonstrate the spatial distributions of H2, CO, and NH3 emission, which are all manifestations of various shocks driven by outflows of deeply embedded sources in NGC6334I. In addition to the well-known northeast-southwest outflow we detect at least one more outflow in the region by combining observations from APEX, ATCA, SMA, Spitzer and VLT/ISAAC. Potential driving sources will be discussed. NGC6334I exhibits several signs of active star formation and will be a major target for future observatories such as Herschel and ALMA., Comment: 6 pages, 6 figures, appeared in the proceedings of the workshop 'The Universe Under The Microscope - Astrophysics At High Angular Resolution', see http://www.iop.org/EJ/toc/1742-6596/131/1

We have used the Submillimeter Array (SMA) to make the first interferometric observations (beam size ~1") of the 12CO J=6-5 line and 435 micron (690 GHz) continuum emission toward the central region of the nearby ULIRG Arp 220. These observations resolve the eastern and western nuclei from each other, in both the molecular line and dust continuum emission. At 435 micron, the peak intensity of the western nucleus is stronger than the eastern nucleus, and the difference in peak intensities is less than at longer wavelengths. Fitting a simple model to the dust emission observed between 1.3 mm and 435 micron suggests that dust emissivity power law index in the western nucleus is near unity and steeper in the eastern nucleus, about 2, and that the dust emission is optically thick at the shorter wavelength. Comparison with single dish measurements indicate that the interferometer observations are missing ~60% of the dust emission, most likely from a spatially extended component to which these observations are not sensitive. The 12CO J=6-5 line observations clearly resolve kinematically the two nuclei. The distribution and kinematics of the 12CO J=6-5 line appear to be very similar to lower J CO lies observed at similar resolution. Analysis of multiple 12CO line intensities indicates that the molecular gas in both nuclei have similar excitation conditions, although the western nucleus is warmer and denser. The excitation conditions are similar to those found in other extreme environments, including M82, Mrk 231, and BR 1202-0725. Simultaneous lower resolution observations of the 12CO, 13CO, and C18O J=2-1 lines show that the 13CO and C18O lines have similar intensities, which suggests that both of these lines are optically thick, or possibly that extreme high mass star formation has produced in an overabundance of C18O., Comment: 13 pages (emulateapj), 10 figures, Accepted for publication in ApJ

We have designed a system to stabilize the gain of a submillimeter heterodyne receiver against thermal fluctuations of the mixing element. In the most sensitive heterodyne receivers, the mixer is usually cooled to 4 K using a closed-cycle cryocooler, which can introduce ~1% fluctuations in the physical temperature of the receiver components. We compensate for the resulting mixer conversion gain fluctuations by monitoring the physical temperature of the mixer and adjusting the gain of the intermediate frequency (IF) amplifier that immediately follows the mixer. This IF power stabilization scheme, developed for use at the Submillimeter Array (SMA), a submillimeter interferometer telescope on Mauna Kea in Hawaii, routinely achieves a receiver gain stability of 1 part in 6,000 (rms to mean). This is an order of magnitude improvement over the typical uncorrected stability of 1 part in a few hundred. Our gain stabilization scheme is a useful addition to SIS heterodyne receivers that are cooled using closed-cycle cryocoolers in which the 4 K temperature fluctuations tend to be the leading cause of IF power fluctuations., Comment: 7 pages, 6 figures accepted to IEEE Transactions on Microwave Theory and Techniques

We present multi-wavelength, high spatial resolution imaging of the IRS 7 region in the R Corona Australis molecular cloud. Our observations include 1.1 mm continuum and HCO^+ J = $3 \to 2$ images from the SMA, ^{12}CO J = $3 \to 2$ outflow maps from the DesertStar heterodyne array receiver on the HHT, 450 $\mu$m and 850 $\mu$m continuum images from SCUBA, and archival Spitzer IRAC and MIPS 24 \micron images. The accurate astrometry of the IRAC images allow us to identify IRS 7 with the cm source VLA 10W (IRS 7A) and the X-ray source X_W. The SMA 1.1 mm image reveals two compact continuum sources which are also distinguishable at 450 $\mu$m. SMA 1 coincides with X-ray source CXOU J190156.4-365728 and VLA cm source 10E (IRS 7B) and is seen in the IRAC and MIPS images. SMA 2 has no infrared counterpart but coincides with cm source VLA 9. Spectral energy distributions constructed from SMA, SCUBA and Spitzer data yield bolometric temperatures of 83 K for SMA 1 and $\leq$70 K for SMA 2. These temperatures along with the submillimeter to total luminosity ratios indicate that SMA 2 is a Class 0 protostar, while SMA 1 is a Class 0/Class I transitional object (L=$17\pm6$ \Lsun). The ^{12}CO J = $3 \to 2$ outflow map shows one major and possibly several smaller outflows centered on the IRS 7 region, with masses and energetics consistent with previous work. We identify the Class 0 source SMA 2/VLA 9 as the main driver of this outflow. The complex and clumpy spatial and velocity distribution of the HCO^+ J = $3 \to 2$ emission is not consistent with either bulk rotation, or any known molecular outflow activity., Comment: 31 pages, 8 figures, Accepted to ApJ

We performed interferometric observations of a high-mass protostellar candidate IRAS 18566+0408 in the \nh3 (J,K)=(1,1), (2,2) and (3,3) inversion transitions, the SiO J=2-1 and HCN J=1-0 lines, and the 43 and 87 GHz continuum emission using the VLA and OVRO. The 87 GHz continuum emission reveals two continuum peaks MM-1 and MM-2 along a molecular ridge. The dominant peak MM-1 coincides with a compact emission feature at 43 GHz, and arises mostly from the dust emission. For dust emissivity index $\beta$ of 1.3, the masses in the dust peaks amount to 70 \msun for MM-1, and 27 \msun for MM-2. Assuming internal heating, the central luminosities of MM-1 and MM-2 are $6 \times 10^4$ and $8 \times 10^3$ \lsun, respectively. The SiO emission reveals a well collimated outflow emanating from MM-1. The jet-like outflow is also detected in \nh3 at velocities similar to the SiO emission. The outflow, with a mass of 27 \msun, causes significant heating in the gas to temperatures of 70 K, much higher than the temperature of $\lsim 15$ K in the extended core. Compact ($< 3''$) and narrow line ($<1.5$ \kms-1) \nh3 (3,3) emission features are found associated with the outflow. They likely arise from weak population inversion in \nh3 similar to the maser emission. Toward MM-1, there is a compact \nh3 structure with a linewidth that increases from 5.5 \kms-1 FHWM measured at 3$''$ resolution to 8.7 \kms-1\ measured at 1$''$ resolution. This linewidth is much larger than the FWHM of $<$ 2 \kms-1 in the entire core, and does not appear to originate from the outflow. This large linewidth may arise from rotation/infall, or relative motions of unresolved protostellar cores., Comment: 19 pages, 9 figures. Accepted for publication in Astronomy and Astrophysics

We present studies of a massive protocluster AFGL5142 in the J=2-1 transition of the CO isotopologues, SO, CH_3OH and CH_3CN lines, as well as continuum at 225 GHz and 8.4 GHz. The 225 GHz continuum emission reveals three prominent peaks MM-1, MM-2 and MM-3. MM-1 and MM-2 are associated with strong CH_3CN emission with temperatures of 90 \pm 20 and 250 \pm 40 K, respectively, while both MM-1 and MM-3 are associated with faint continuum emission at 8.4 GHz. Additional dust continuum peaks MM-4 and MM-5 appear to be associated with \h2O masers. With many continuum sources at cm and mm wavelengths, and those already identified in the infrared, this region is forming a cluster of stars. The CO and SO emission reveals at least three molecular outflows originating from the center of the dust core. The outflows are well collimated, with terminal velocities up to 50 \kms-1 from the cloud velocity. Outflow A coincides with the SiO jet identified previously by Hunter et al. (1999). Since jet-like outflows and disk-mediated accretion process are physically connected, the well collimated outflows indicate that even in this cluster environment, accretion is responsible for the formation of individual stars in the cluster., Comment: Accepted for publication in ApJ

We report a dual-band observation at 223 and 654 GHz (460 micron) toward an ultracompact (UC) HII region, G240.31+0.07, with the Submillimeter Array. With a beam size of 1"5 X 0"8, the dust continuum emission is resolved into two clumps, with clump A coincident well with an H2O maser and the UC HII region. The newly discovered clump, B, about 1"3 (~8.3 X 10^3 AU) to the southwest of clump A, is also associated with H2O masers and may be a more recent star-forming site. The continuum flux densities imply an opacity spectral index of \beta = 1.5 +- 0.3, suggestive of a value lower than the canonical 2.0 found in the interstellar medium and in cold, massive cores. The presence of hot (~100 K) molecular gas is derived by the brightness ratio of two H2CO lines in the 223 GHz band. A radial velocity difference of 2.5 +- 0.4 km/s is found between the two clumps in C18O (6-5) emission. The total (nebular and stellar) mass of roughly 58 Msun in the central region is close to, but not by far larger than, the minimum mass required for the two clumps to be gravitationally bound for binary rotation. Our continuum data do not suggest a large amount of matter associated with the H2 knots that were previously proposed to arise from a massive disk or envelope., Comment: 12 pages, 2 tables, 3 figures, accepted by the Astrophysical Journal Letters

We report the measurement results and compensation of the antenna elevation angle dependences of the Sub-millimeter Array (SMA) antenna characteristics. Without optimizing the subreflector (focus) positions as a function of the antenna elevation angle, antenna beam patterns show lopsided sidelobes, and antenna efficiencies show degradations. The sidelobe level increases and the antenna efficiencies decrease about 1% and a few %, respectively, for every 10 degrees change in the elevation angle at the measured frequency of 237 GHz. We therefore obtained the optimized subreflector positions for X (azimuth), Y (elevation), and Z (radio optics) focus axes at various elevation angles for all the eight SMA antennas. The X axis position does not depend on the elevation angle. The Y and Z axes positions depend on the elevation angles, and are well fitted with a simple function for each axis with including a gravity term (cosine and sine of elevation, respectively). In the optimized subreflector positions, the antenna beam patterns show low level symmetric sidelobe of at most a few %, and the antenna efficiencies stay constant at any antenna elevation angles. Using one set of fitted functions for all antennas, the SMA is now operating with real-time focusing, and showing constant antenna characteristics at any given elevation angle., Comment: 10 pages, 6 figures; to appear in the Proceedings of SPIE Vol. 6275 "Millimeter and Submillimeter Detectors and Instrumentation for Astronomy III"

We present CO(1-0) observations of the high-redshift quasi-stellar objects (QSOs) BR 1202-0725 (z=4.69), PSS J2322+1944 (z=4.12), and APM 08279+5255 (z=3.91) using the NRAO Green Bank Telescope (GBT) and the MPIfR Effelsberg 100m telescope. We detect, for the first time, the CO ground-level transition in BR 1202-0725. For PSS J2322+1944 and APM 08279+5255, our observations result in line fluxes that are consistent with previous NRAO Very Large Array (VLA) observations, but they reveal the full line profiles. We report a typical lensing-corrected velocity-integrated intrinsic CO(1-0) line luminosity of L'(CO) = 5 x 10^10 K km/s pc^2 and a typical total H_2 mass of M(H2) = 4 x 10^10 M_sun for the sources in our sample. The CO/FIR luminosity ratios of these high-z sources follow the same trend as seen for low-z galaxies, leading to a combined solution of log(L_FIR) = (1.39 +/- 0.05) x log(L(CO))-1.76. It has previously been suggested that the molecular gas reservoirs in some quasar host galaxies may exhibit luminous, extended CO(1-0) components that are not observed in the higher-J CO transitions. Utilizing the line profiles and the total intensities of our observations and large velocity gradient (LVG) models based on previous results for higher-J CO transitions, we derive that emission from all CO transitions is described well by a single gas component where all molecular gas is concentrated in a compact nuclear region. Thus, our observations and models show no indication of a luminous extended, low surface brightness molecular gas component in any of the high-redshift QSOs in our sample. If such extended components exist, their contribution to the overall luminosity is limited to at most 30%., Comment: 12 pages, 5 figures, 3 tables, to appear in ApJ (accepted June 16, 2006)

A photonic millimeter wave local oscillator capable of producing two microwatts of radiated power at 224 GHz has been developed. The device was tested in one antenna of Smithsonian Institution's Submillimeter Array (SMA) and was found to produce stable phase on multiple baselines. Graphical data is presented of correlator output phase and amplitude stability. A description of the system is given in both open and closed loop modes. A model is given which is used to predict the operational behavior. A novel method is presented to determine the safe operating point of the automated system., Comment: 4 pages, 7 figures, to appear in the Proceedings of the 17th International Symposium on Space Terahertz Technology, held 10-12 May 2006 in Paris

The processes leading to the birth of low-mass stars such as our Sun have been well studied, but the formation of high-mass (> 8 x Sun's mass) stars has heretofore remained poorly understood. Recent observational studies suggest that high-mass stars may form in essentially the same way as low-mass stars, namely via an accretion process, instead of via merging of several low-mass (< 8 Msun) stars. However, there is as yet no conclusive evidence. Here, we report the discovery of a flattened disk-like structure observed at submillimeter wavelengths, centered on a massive 15 Msun protostar in the Cepheus-A region. The disk, with a radius of about 330 astronomical units (AU) and a mass of 1 to 8 Msun, is detected in dust continuum as well as in molecular line emission. Its perpendicular orientation to, and spatial coincidence with the central embedded powerful bipolar radio jet, provides the best evidence yet that massive stars form via disk accretion in direct analogy to the formation of low-mass stars.

The Submillimeter Array (SMA), a collaboration between the Smithsonian Astrophysical Observatory and the Academica Sinica Institute for Astronomy and Astrophysics of Taiwan, is an eight-element radio-interferometer designed to operate throughout the major atmospheric windows from about 180 to 900 GHz. In an effort to mitigate the effects of atmospheric instabilities which limit the phase coherence of the array especially in the higher frequency bands, the array was designed to allow simultaneous operation of a low frequency receiver (<350 GHz) with a high frequency receiver (>330 GHz). The overlap region of 330-350 GHz was included to facilitate dual polarization measurements in the frequency range considered to offer the highest sensitivity for continuum observations with the array. So far, the array is equipped with working SIS receivers covering the frequency ranges 176-256 GHz, 260-350 GHz, and 600-700 GHz, and single frequency operation has been routine in the lower two frequency bands for the past year. More recently, with the completion of IF hardware required to make full use of the SMA cross-correlator, dual receiver operation became possible. We have since made a number of Galactic and extra-galactic astronomical observations in dual-band mode with the hopes of using the 230 GHz receiver as a phase reference to enable improved interferometry in the 650 GHz band. We will present the current antenna and receiver performance, some of the first interferometric images in the 650 GHz receiver band, and our initial attempts at phase transfer., Comment: 6 pages containing text and 11 embedded figures, written in the IEEE Transactions style Latex template, based on an Oral Presentation at the 16th International Symposium on Space Terahertz Technology held at Chalmers University of Technology in Goteborg, Sweden on May 2-4, 2005

We present molecular line, H2O maser, radio continuum and near infrared maps of the bipolar outflow source AFGL5142. The high resolution of our molecular CO observations enables us to define the morphology of the large-scale bipolar outflow into a two lobe structure extending for 2' on each side of the center. In the perpendicular direction, we find consistent evidence for a second, more compact (<0.5') outflow in the form of a spatial velocity offset in the CO map and of an H2 jet-like structure derived from a near infrared narrow band image. On a smaller scale size, the radio and infrared continuum observations reveal the engines of the molecular outflows. The maser emission occurs near the position of the most embedded source of the cluster, IRS1. This is located at the center of the compact outflow and jets of shocked H2 and coincides with an ultra compact radio continuum source (most probably an ionized stellar wind). The H2O cluster is composed of five spatial components: two are within 0.3'' from the YSO (a few hundred AU) and three are at larger distance (1.5--2'', a few thousand AU). A marginal detection of proper motion of the two more distant masers may suggest a high expansion velocity at a distance 4000 AU from the YSO, similar to what is found in Orion KL and W49N. The brightest NIR source of the cluster (IRS2) is associated with an IRAS point source and lies along the axis of the large-scale bipolar outflow. We propose that the masers and the compact molecular outflow are powered concurrently by the wind from the YSO associated with IRS1, while the large-scale outflow could be the remnant from the formation of IRS2., Comment: 17 pages (A&A style), uuencoded, compressed postscript file (440kB) with 4 tables and 10 figures embedded in latex psfig style