Philip R. Jewell

Discovery of the interstellar chiral molecule propylene oxide (CH3CHCH2O)

Chiral molecule discovered in space A chiral molecule is one that has two forms that are mirror images of each other: enantiomers. Biological systems overwhelmingly use one enantiomer over another, and some meteorites show an excess of one type. The two forms are almost identical chemically, so how this excess first arose is unknown. McGuire et al. used radio astronomy to detect the first known chiral molecule in space: propylene oxide. The work raises the prospect of measuring the enantiomer excess in various astronomical objects, including regions where planets are being formed, to discover how and why the excess first appeared. Science, this issue p. 1449 The first chiral molecule detected in space may offer clues to the origin of enantiomer excess. Life on Earth relies on chiral molecules—that is, species not superimposable on their mirror images. This manifests itself in the selection of a single molecular handedness, or homochirality, across the biosphere. We present the astronomical detection of a chiral molecule, propylene oxide (CH3CHCH2O), in absorption toward the Galactic center. Propylene oxide is detected in the gas phase in a cold, extended molecular shell around the embedded, massive protostellar clusters in the Sagittarius B2 star-forming region. This material is representative of the earliest stage of solar system evolution in which a chiral molecule has been found.

DETECTION OF E-CYANOMETHANIMINE TOWARD SAGITTARIUS B2(N) IN THE GREEN BANK TELESCOPE PRIMOS SURVEY

The detection of E-cyanomethanimine (E-HNCHCN) toward Sagittarius B2(N) is made by comparing the publicly available Green Bank Telescope (GBT) PRIMOS survey spectra to laboratory rotational spectra from a reaction product screening experiment. The experiment uses broadband molecular rotational spectroscopy to monitor the reaction products produced in an electric discharge source using a gas mixture of NH3 and CH3CN. Several transition frequency coincidences between the reaction product screening spectra and previously unassigned interstellar rotational transitions in the PRIMOS survey have been assigned to E-cyanomethanimine. A total of eight molecular rotational transitions of this molecule between 9 and 50?GHz are observed with the GBT. E-cyanomethanimine, often called the HCN dimer, is an important molecule in prebiotic chemistry because it is a chemical intermediate in proposed synthetic routes of adenine, one of the two purine nucleobases found in DNA and RNA. New analyses of the rotational spectra of both E-cyanomethanimine and Z-cyanomethanimine that incorporate previous millimeter-wave measurements are also reported.

A Search for l-C_3H^+ and l-C_3H in Sgr B2(N), Sgr B2(OH), and the Dark Cloud TMC-1

Pety et al. (2012) recently reported the detection of several transitions of an unknown carrier in the Horsehead PDR and attribute them to l-C_3H^+. Here, we have tested the predictive power of their fit by searching for, and identifying, the previously unobserved J = 1−0 and J = 2−1 transitions of the unknown carrier (B11244) towards Sgr B2(N) in data from the publicly available PRIMOS project. Also presented here are observations of the J = 6 − 5 and J = 7 − 6 transitions towards Sgr B2(N) and Sgr B2(OH) using the Barry E. Turner Legacy Survey and results from the Kaifu et al. (2004) survey of TMC-1. We calculate an excitation temperature and column density of B11244 of ∼10 K and ∼10^(13) cm^(−2) in Sgr B2(N) and ∼79 K with an upper limit of ≤ 1.5 × 10^(13) cm^(−2) in Sgr B2(OH) and find trace evidence for the cation’s presence in TMC-1. Finally, we present spectra of the neutral species in both Sgr B2(N) and TMC-1, and comment on the robustness of the assignment of the detected signals to l-C_3H^+.

A Polarization Survey of SiO Maser Variability in Evolved Stars

We have monitored the SiO (v = 1, J = 2 → 1) maser polarization in 17 variable stars (Miras, OH-IR stars, and supergiants) to investigate the long-term persistence of masers. The eight epochs of observations span 2.5 yr, thereby sampling multiple cycles for these stars with typical periods of ~1 yr. The average polarization was 23% with a typical dispersion of 7%, although the variability differed substantially from star to star. In the Stokes q and u spectra of individual stars, a few strong maser features tended to dominate the polarization, with the maser features persisting for less than one stellar cycle for some stars, and for multiple cycles for a few stars. Because individual masers are not resolved in our beam-averaged total intensity spectra, we correlated the polarization spectra between epochs to measure the characteristic lifetimes of the features, rather than attempting to trace the evolution of separate line components. We found that individual maser feature lifetimes ranged from a few months or less to more than 2 yr. These data indicate that for the sample of stars as a whole, the masers are not reset at a particular stellar phase.

The Green Bank Telescope

The National Radio Astronomy Observatory Green Bank Telescope (GBT) is the worlds largest fully steerable telescope. The GBT has now been in routine operation for over two years, observing at frequencies up to 50 GHz. In order to deliver the tracking accuracies required at 50 GHz, we solve simultaneously for gravitational and thermal effects in the development of the static pointing and focus tracking models. A precision temperature sensor system then generates additional real-time corrections to compensate for varying thermal gradients in the antenna. Collimation and surface accuracy requirements are met by an active surface control system which combines initial corrections derived from a finite element model of the antenna with additional terms derived from astronomical phase-retrieval holography measurements. The GBT has a rich suite of instrumentation including receivers which cover almost the complete frequency range from ~ 290 MHz to 50 GHz, and backends for spectroscopy, pulsar observing, broadband continuum, very long baseline interferometry and planetary radar reception. A 64-pixel bolometer camera is under development by a consortium including UPenn, NASA-GSFC, NIST, UCardiff and NRAO. Recent software developments include an extremely flexible application which combines traditional interactive observing, scheduling-block based observing and real-time monitoring and data display in a single, convenient interface. In this paper I will summarize the current performance of the GBT, and review some recent science results. I will also describe how plans changed with time, and review some of the lessons learned in the development of the telescope.The NRAO Robert C. Byrd Green Bank Telescope is a new 100 meter radio telescope that is now in routine astronomical operation. This paper gives a status report on astronomical performance, operational capabilities, development plans, and early scientific highlights. The telescope is now in regular operation to frequencies of 50 GHz, which achieves the base design goal of the facility. Both the surface and pointing accuracies allow good performance at that frequency. The active surface of 2004 movable panels is in regular use at all frequencies above ~8 GHz and significantly improves both gain and diffraction beam shape at high and low elevation angles. Receivers exist at most primary observing frequencies ranging from ~290 MHz to 50 GHz. A 256k-channel correlation spectrometer with up to 16 independent inputs is in routine use for spectroscopy. Backends for pulsar observing, broadband continuum, Very Long Baseline Interferometry, and planetary radar reception also exist. An advanced development program for enhancing the performance of the GBT is also underway. This program includes the Precision Telescope Control Project that will extend operation to 115 GHz or 2.6 mm wavelength (see the paper by Prestage and Constantikes in this volume), a 26-40 GHz receiver, a 68-92 GHz receiver, a fast sampling continuum backend built by a Caltech/NRAO collaboration, and a 64-pixel bolometer camera developed by a consortium of UPenn, NASA-GSFC, NIST, UCardiff, and NRAO. Software development projects for enhanced user interfaces and data handling are underway, and plans to implement queue-based dynamic scheduling and remote observing are being developed. The status of these projects and their anticipated scientific impact will be discussed.

HCO emission from H II-molecular cloud interface regions

A survey of well-known molecular clouds in the four strongest HCO NK-,K+ = 1(01)-0(00) hyperfine transitions has been carried out to determine the prevalence of HCO and to study its chemistry. HCO emission was observed in seven molecular clouds. Three of these, NGC 2264, W49, and NGC 7538, were not previously known sources of HCO. In addition, NGC 2024 and Sgr B2 were mapped and shown to have extensive HCO emission. The survey results show the HCO abundance to be enhanced in H II-molecular cloud interface regions and support a correlation between C+ and HCO emission. The strength of the HCO emission in NGC 2024 is interpreted in terms of this enhancement and the source structure and proximity to Earth.

Interstellar Carbodiimide (HNCNH): A New Astronomical Detection from the GBT PRIMOS Survey via Maser Emission Features

In this work, we identify carbodiimide (HNCNH), which is an isomer of the well-known interstellar species cyanamide (NH2CN), in weak maser emission, using data from the Green Bank Telescope PRIMOS survey toward Sgr B2(N). All spectral lines observed are in emission and have energy levels in excess of 170 K, indicating that the molecule likely resides in relatively hot gas that characterizes the denser regions of this star-forming region. The anticipated abundance of this molecule from ice mantle experiments is ∼10% of the abundance of NH2CN, which in Sgr B2(N) corresponds to ∼2 × 10 13 cm −2 . Such an abundance results in transition intensities well below the detection limit of any current astronomical facility and, as such, HNCNH could only be detected by those transitions which are amplified by masing.

The detection of rotationally excited OH emission toward the probable young planetary nebula Vy 2-2

A search for 4.7 and 6.0 GHz rotationally excited OH emission in OH/Miras, OH/IR stars, Herbig-Haro objects, Herbig Ae and Be stars, and compact planetary nebulae has been performed. Many of the objects were surveyed in all seven of the lambda doublet hyperfine transitions in the /sup 2/product/sub 3/2/, J = 5/2 and /sup 2/product/sub 1//sub ///sub 2/, J = 1/2 rotational states. In general, this is the most comprehensive and sensitive search for circumstellar rotationally excited OH yet performed. Only one new detection was made, but that was of 6035 MHz emission from the interesting object Vy 2-2. This object is a probable compact planetary nebula surrounded by a fossil molecular envelope from the progenitor red giant. The object was previously known to have a 1612 MHz ground-state OH maser. A complete summary of all observations is given. We discuss the excitation conditions and a number of pumping schemes that could explain the Vy 2-2 masers and the negative results for excited-state OH masers in the other sources. The morphology of the Vy 2-2 envelope as indicated by the velocity structure of the 6035 MHz emissions is also discussed.

Detection of Voigt Spectral Line Profiles of Hydrogen Radio Recombination Lines toward Sagittarius B2(N)

We report the detection of Voigt spectral line profiles of radio recombination lines (RRLs) toward Sagittarius B2(N) with the 100 m Green Bank Telescope (GBT). At radio wavelengths, astronomical spectra are highly populated with RRLs, which serve as ideal probes of the physical conditions in molecular cloud complexes. An analysis of the Hnα lines presented herein shows that RRLs of higher principal quantum number ( n> 90) are generally divergent from their expected Gaussian profiles and, moreover, arewell described by their respective Voigt profiles. This is in agreement with the theory that spectral lines experience pressure broadening as a result of electron collisions at lower radio frequencies. Given the inherent technical difficulties regarding the detection and profiling of true RRL wing spans and shapes, it is crucial that the observing instrumentation produce flat baselines as well as high-sensitivity, high-resolution data. The GBT has demonstrated its capabilities regarding all of these aspects, and we believe that future observations of RRL emissionvia the GBTwill be crucial toward advancing our knowledge of the larger-scale extended structures of ionized gas in the interstellar medium (ISM).

Maps of dust clouds at 1. 3 millimeters associated with bright H II regions

Maps of the dust clouds associated with W3 Main, NGC 1976, NGC 2024, Sgr B2, M17, W49A, and W51A made at 1.3 mm show brightness distributions often very different from those seen in the FIR. These differences imply that the regions of maximum dust temperature generally do not correspond with regions of maximum column density. Flux densities corrected for free-free emission are given. 29 references.