[en] A diverse array of science goals requires 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 seven 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 allows us to achieve an unprecedented relative ALMA calibration accuracy of ∼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.

[en] We combine multi-frequency 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 source(s) in NGC 6334I. 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. NGC 6334I exhibits several signs of active star formation and will be a major target for future observatories such as Herschel and ALMA.

[en] We have used the Submillimeter Array (SMA) to make the first interferometric observations (beam size ∼1'' or ∼400 pc) of the ¹²CO J = 6 - 5 line and 435 μm (690 GHz) continuum emission toward the central region (half power field of view 17'') of the nearby ultra-luminous infrared galaxy (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 μm, 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 μm 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 the nearby starburst galaxy M82, the active galactic nucleus (AGN) hosting ULIRG Mrk 231, and the high-z QSO 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.

[en] We report the results of a pilot program for a Green Bank Telescope MUSTANG-2 Galactic Plane survey at 3 mm (90 GHz), MGPS90. The survey achieves a typical 1σ 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 < ℓ < 50 deg (total area ≈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 H ii (HCH ii) 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 HCH ii regions; of these, 10 are morphologically compact and are strong candidates for new HCH ii 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 HCH ii regions exist within dense protoclusters. Comparing the counts of HCH ii to ultracompact H ii (UCH ii) regions, we infer the HCH ii region lifetime is 16%–46% that of the UCH ii region lifetime. We additionally separated the 3 mm emission into dust and free–free emission by comparing with archival 870 $\mathrm{\mu <!--\; ??\; -->}\mathrm{m}$ and 20 cm data. In the selected pilot fields, most (≳80%) of the 3 mm emission comes from plasma, either through free–free or synchrotron emission.

[en] We report the first detection of isotopic methanol (¹³CH₃OH) maser emission in interstellar space. The emission was detected toward the high-mass young stellar object G358.93-0.03 during monitoring of a flare in the 6.7 GHz methanol (CH₃OH) maser emission in this source. We find that the spectral and spatial distribution of the ¹³CH₃OH masers differs from the CH₃OH masers imaged at the same epoch, contrary to expectations from similarity of their pumping. This conclusively demonstrates that isotopic methanol masers are bright under different physical conditions and suggests that they can provide additional, complementary information to the CH₃OH masers from the same source. We detect a rapid decay of the ¹³CH₃OH maser lines suggesting that they are transient phenomena (masing for only a few months), likely associated with rapid changes in radiation field due to an accretion burst induced by massive disk fragmentation. Changes in the line flux density are faster than required to achieve equilibrium in the energy level populations, indicating that the pumping of these masers is likely variable.