Ronald J. Maddalena

The large system of molecular clouds in Orion and Monoceros

Emission is noted over about one-eighth of an 850-sq deg region centered on Orion and Monoceros that has been surveyed in the J = 1 to 0 line of CO; most of the emission arises from giant molecular clouds associated with Orion A and B, and Mon R2. A much smaller area was surveyed for C-13O emission. A comparison of cloud masses obtained by three independent methods indicates that CO luminosity is as accurate a measure of cloud mass as other indicators. The possible relationships among clouds in the survey are discussed, including the conjecture that the overall Orion complex of clouds is a much larger system than previously considered, incorporating most of the clouds in the present survey.

Asymmetry in High-Precision Global H I Profiles of Isolated Spiral Galaxies

New high signal-to-noise ratio 21 cm H I line profiles have been obtained for 104 galaxies with the Green Bank 43 m telescope. The primary sample is composed of isolated spirals with no known optical companions within a 1° radius and a median ratio of optical diameter to beamwidth of 0.17. An effort was made to ensure linearity of baseline fitting and precise flux density calibration to better than 5%. Two quantitative measures of asymmetry are applied to assess the occurrence of lopsidedness in the global H I profiles. In agreement with previous estimates, half the galaxies show significant H I profile asymmetries. The lopsidedness cannot be explained by pointing offsets but, rather, must result from noncircular motions, confusion with unidentified companions within the telescope beam, or true distortions in the H I distribution.

A large, cold, and unusual molecular cloud in Monoceros

Observations of the J = 1 tends to 0 rotational transition of CO near the galactic plane in Monoceros (1 is approximately equal to 216 deg) reveal a molecular cloud with unusually low peak CO temperatures (less than 2 K) but wide lines (about 7 km/s) typical of much warmer clouds. At the assumed distance of 3 kpc, the cloud is large (250 x 100 pc), has a mass of 7-11 x 10 to the 5th solar masses, and is well removed from the galactic midplane (130 pc). Except for a possible H II region, all the signs of star formation usually shown by clouds of comparable mass are missing. The cloud, unlike cloud complexes of similar size, is a single, continuous object that apparently has not been torn apart by star formation. Clouds with such properties are rare in the Galaxy; only one or two similar objects have been found. The possibility that the cloud is young and not yet forming stars but will evolve into a typical cloud complex once star formation begins is discussed.

Jupiter's radio spectrum from 74 MHz up to 8 GHz

Abstract We carried out a brief campaign in September 1998 to determine Jupiter’s radio spectrum at frequencies spanning a range from 74 MHz up to 8 GHz. Eleven different telescopes were used in this effort, each uniquely suited to observe at a particular frequency. We find that Jupiter’s spectrum is basically flat shortwards of 1–2 GHz, and drops off steeply at frequencies greater than 2 GHz. We compared the 1998 spectrum with a spectrum (330 MHz–8 GHz) obtained in June 1994, and report a large difference in spectral shape, being most pronounced at the lowest frequencies. The difference seems to be linear with log(ν), with the largest deviations at the lowest frequencies (ν). We have compared our spectra with calculations of Jupiter’s synchrotron radiation using several published models. The spectral shape is determined by the energy-dependent spatial distribution of the electrons in Jupiter’s magnetic field, which in turn is determined by the detailed diffusion process across L -shells and in pitch angle, as well as energy-dependent particle losses. The spectral shape observed in September 1998 can be matched well if the electron energy spectrum at L = 6 is modeled by a double power law E − a (1+( E / E 0 )) − b , with a = 0.4, b = 3, E 0 = 100 MeV, and a lifetime against local losses τ 0 = 6 × 10 7 s. In June 1994 the observations can be matched equally well with two different sets of parameters: (1) a = 0.6, b = 3, E 0 = 100 MeV, τ 0 = 6 × 10 7 s, or (2) a = 0.4, b = 3, E 0 = 100 MeV, τ 0 = 8.6 × 10 6 s. We attribute the large variation in spectral shape between 1994 and 1998 to pitch angle scattering, coulomb scattering and/or energy degradation by dust in Jupiter’s inner radiation belts.

High-Redshift HCN Emission: Dense Star-forming Molecular Gas in IRAS F10214+4724

Hydrogen cyanide emission in the J = 1–0 transition has been detected at redshift z = 2.2858 in IRAS F10214+4724 using the Green Bank Telescope. This is the second detection of HCN emission at high redshift. The large HCN line luminosity in F10214 is similar to that in the Cloverleaf (z = 2.6) and the ultraluminous infrared galaxies Mrk 231 and Arp 220. This is also true of the ratio of HCN to CO luminosities. The ratio of far-infrared luminosity to HCN luminosity, an indicator of the star formation rate per solar mass of dense gas, follows the correlation found for normal spirals and infrared luminous starburst galaxies. F10214 clearly contains a starburst that contributes, together with its embedded quasar, to its overall infrared luminosity. A new technique for removing spectral baselines in the search for weak, broad emission lines is presented.

Radio recombination lines toward the Galactic center lobe

The Galactic center (GC) lobe is a degree-tall shell seen in radio continuum images of the GC region. If it is actually located in the GC region, formation models would require massive energy input (e. g., starburst or jet) to create it. At present, observations have not strongly constrained the location or physical conditions of the GC lobe. This paper describes the analysis of new and archival single-dish observations of radio recombination lines toward this enigmatic object. The observations find that the ionized gas has a morphology similar to the radio continuum emission, suggesting that they are associated. We study averages of several transitions from H106 alpha to H191 epsilon and find that the line ratios are most consistent with gas in local thermodynamic equilibrium. The radio recombination line widths are remarkably narrow, constraining the typical electron temperature to be less than about 4000 K. These observations also find evidence of pressure broadening in the higher electronic states, implying a gas density of n(e) = 910(-450)(+310) cm(-3). The electron temperature, gas pressure, and morphology are all consistent with the idea that the GC lobe is located in the GC region. If so, the ionized gas appears to form a shell surrounding the central 100 parsecs of the galaxy with a mass of roughly 10(5) M-circle dot, similar to ionized outflows seen in dwarf starbursts.

Holographic Measurement and Improvement of the Green Bank Telescope Surface

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 μm to ~240 μm, 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.

1.4 GHz HIGH-RESOLUTION FLUX-ACCURATE IMAGES OF SN 1006

We present interferometric+single-dish images, constructed from the Green Bank Telescope (GBT) and VLA observations, of the supernova remnant SN 1006. The image was created using a Multiscale CLEAN algorithm in conjunction with a novel approach for correcting the effect of the non-coplanar baselines. We demonstrate that integrating with a single dish to the confusion limit of the highest resolution interferometric element is not required. Instead, the noise in the map is limited by the signal-to-noise ratio in each observation. The noise in the combined map at the full resolution is dominated by uncertainties in the VLA data, not by uncertainties in our short GBT observation. The resulting image is a significant improvement over images missing short spacing information, and paves the way for future joint GBT+VLA proposals that need to accurately image objects ranging from galaxies to H II regions.

The RadioAstron Green Bank Earth Station

We present the design, commissioning, and initial results of the Green Bank Earth Station (GBES), a RadioAstron data downlink station located at the National Radio Astronomy Observatory (NRAO) in Green Bank, West Virginia. The GBES uses the modernized and refurbished NRAO 140ft telescope. Antenna optics were refurbished with new motors and drives fitted to the secondary mirror positioning system, and the deformable subreflector was refurbished with a new digital controller and new actuators. A new monitor and control system was developed for the 140ft and is based on that of the Green Bank Telescope (GBT), allowing satellite tracking via a simple scheduling block. Tools were developed to automate antenna pointing during tracking. Data from the antenna control systems and logs are retained and delivered with the science and telemetry data for processing at the Astro Space Center (ASC) of the Lebedev Physical Institute (LPI) of the Russian Academy of Sciences and the mission control centre, Lavochkin Association.

The Green Bank Telescope: current status and early results

The National Radio Astronomy Observatory Green Bank Telescope (GBT), the worlds largest fully steerable telescope, is now undergoing commissioning and early scientific operation. The GBT has many innovative design features that advance imaging quality, sensitivity, and versatility. These include an unblocked aperture, an active surface, and a six-degree of freedom Gregorian subreflector. The GBT has an advanced laser rangefinder metrology system, which will measure the position of the active surface panels, and also guide the precision pointing of the telescope. Early commissioning results have confirmed the performance of the telescope, and exciting scientific discoveries are already being made. This paper describes the various features of the telescope in more detail, and presents the latest results.