Margaret A. Frerking

The relationship between carbon monoxide abundance and visual extinction in interstellar clouds

Carbon monoxide column densities are compared to visual extinctions toward field stars in the rho Oph and Taurus molecular cloud complexes. The relationship of C/sup 18/O column density to A/sub v/ is established, and new determinations for /sup 13/CO column densities are given for a range of visual extinctions extended to beyond 20 mag. A prescription for determining hydrogen column densities and masses of molecular clouds from observations of CO isotopes is presented and discussed critically. These measurements agree well with the predictions of gas phase chemistry models which include chemical fractionation and selective isotopic photodestruction. The functional dependence of the C/sup 18/O column density on A/sub v/ is characterized by two different regimes separated by a distinct transition observed to occur at 4 mag in both molecular cloud complexes, whereas the functional dependence of /sup 13/CO is quite different in the two regions. We find that some saturation occurs for C/sup 18/O emission at high visual extinction and use the rarer isotopic species C/sup 17/O and /sup 13/C/sup 18/O to correct for it.

Subsurface properties and early activity of comet 67P/Churyumov-Gerasimenko

Heat transport and ice sublimation in comets are interrelated processes reflecting properties acquired at the time of formation and during subsequent evolution. The Microwave Instrument on the Rosetta Orbiter (MIRO) acquired maps of the subsurface temperature of comet 67P/Churyumov-Gerasimenko, at 1.6 mm and 0.5 mm wavelengths, and spectra of water vapor. The total H2O production rate varied from 0.3 kg s–1 in early June 2014 to 1.2 kg s–1 in late August and showed periodic variations related to nucleus rotation and shape. Water outgassing was localized to the “neck” region of the comet. Subsurface temperatures showed seasonal and diurnal variations, which indicated that the submillimeter radiation originated at depths comparable to the diurnal thermal skin depth. A low thermal inertia (~10 to 50 J K–1 m–2 s–0.5), consistent with a thermally insulating powdered surface, is inferred.

Novel heterojunction varactors

Several varactor structures that have been proposed for millimeter- and submillimeter-wave receiver systems are described, and their performance in a harmonic multiplier is compared. The performance is calculated using a nonlinear large-signal analysis program. >

The structure and dynamics of Bok globule B335

Results are reported from mm-wave observations CO isotopes and other molecules in the Bok globule B335 (Lynd L663). The data cover the C-12O, C-13O, C-18O, and H-12CO(+) J = 1 yields of 0 rotational transition. Spectral data were also obtained for the preceding molecules and C-17O, H-13CO(+) and DCO(+). The data were used to calculate quantitative density and mass relationships and distributions, determine the rotational parameters, and characterize the force balance within the cloud for inclusion in a spherically averaged cloud model. A nonspherical cloud model which considers magnetic fields is presented. B335 has a dense core within a moderately dense cylindrically shaped envelope, which in turn is surrounded by a more diffuse elliptical envelope. The object is 250 pc from the earth.

A 200 GHz tripler using a single barrier varactor

The authors present results for a tripler to 200 GHz using a single barrier varactor (SBV). The performance of the tripler, over an output frequency range from 186 to 207 GHz, has been measured in a crossed waveguide mount. The theoretical performance of the device and the tripler mount have been calculated using large signal analysis. An overall efficiency of 2% was achieved with efficiency at the device of above 5%. A comparison of theoretical and measured results and a discussion of various losses in the mount and the varactor are also presented. >

Spatial and diurnal variation of water outgassing on comet 67P/Churyumov-Gerasimenko observed from Rosetta/MIRO in August 2014

Aims. We present the spatial and diurnal variation of water outgassing on comet 67P/Churyumov-Gerasimenko using the (H2O)-O-16 rotational transition line at 556.936 GHz observed from Rosetta/MIRO in August 2014. Methods. The water line was analyzed with a non-LTE radiative transfer model and an optimal estimation method to retrieve the (H2O)-O-16 outgassing intensity, expansion velocity, and gas kinetic temperature. On August 7-9, 2014 and August 18-19, 2014, MIRO performed long steady nadir-pointing observations of the nucleus while it was rotating around its spin axis. The ground track of the MIRO beam during the observation was mostly on the northern hemisphere of comet 67P, covering its three distinct parts: the so-called head, body, and neck areas. Results. The MIRO spectral observation data show that the water-outgassing intensity varies by a factor of 30, from 0.1 x 1025 molecules s(-1) sr l to 3.0 x 10(25) molecules s(-1) sr, the terminal gas expansion velocity varies by 0.17 km s(-1) from 0.61 km s(-1) to 0.78 km s(-1), and the terminal gas temperature varies by 27 K from 47 K to 74 K. The retrieved coma parameters are co-registered with local environment variables such as the subsurface temperatures, measured in the MIRO continuum bands, the local solar time, illumination condition, and beam location on nucleus. The spatial variation of the outgassing activity is very noticeable, and the largest outgassing activity in August 2014 occurs near the neck region of the nucleus. The outgassing activity in the neck region is also found to be correlated with the local solar hour, which is related to the local illumination condition.

A single barrier varactor quintupler at 170 GHz

InGaAs-InAlAs single-barrier varactor (SBV) diodes are tested as frequency quintuplers. The diodes were tested in a crossed-waveguide structure and provided output frequencies between 148 and 187 GHz. The highest observed flange-to-flange efficiency was 0.78% at an output frequency of 172 GHz. This is nearly four times greater than the best quintupler efficiency obtained for previous SBV varactors made from the GaAs-AlGaAs materials system. >

Planar diode solid-state receiver for 557 GHz with state-of-the-art performance

The design and performance of a subharmonically pumped (SHP) 557-GHz mixer driven by a solid-state local oscillator (LO) are reported. Whisker contacts are not required as both the mixer and LO utilize planar Schottky devices. A measured mixer noise temperature of 2100-K double sideband (DSB) with a conversion loss of 8.9 dB has been achieved at room temperature. The mixer exhibits broad intermediate frequency (IF) bandwidth with measured DSB noise temperatures below 3400 K in the band from 1.5 to 17 GHz. An external 6-18-GHz amplifier has been added to the output of the mixer, and measured receiver noise temperatures below 7300 K have been measured across the IF band. The results are believed to represent state-of-the-art performance for a room-temperature broad-band solid-state receiver at this frequency.

Modeling of planar varactor frequency multiplier devices with blocking barriers

Models for optimization of planar frequency triplers with symmetrical C-V curves are presented. The roles and and limitations of various blocking barriers (oxide, Mott, heterojunction) are discussed. Devices with undoped drift regions (BIN) gave moderate efficiency but a broad range of power generation, whereas the devices with doped drift regions (BNN) have high efficiency in a narrower power window. In particular, an upper power limit of the BNN is caused by electron velocity saturation. Implementations in SiO/sub 2/-Si and AlAs-GaAs and means for increasing the power of BNN structures are considered. >

Determination of the hyperfine structure of HN/sup 1/u3C and HNC

The hyperfine structure of HN/sup 13/C and new frequencies for both HN/sup 13/C and HNC are determined from resolved spectra in two dark clouds. A quadrupole coupling constant of 0.28 +- 0.03 MHz has been derived from the data. The new value differs from an earlier published value. Previous confusion is due to alteration of line shapes for these two dark clouds by absorption in low excitation foreground material.