We present results of an extensive mapping survey of N2H+ (1-0) in about 60 low-mass cloud cores already mapped in the NH3 (1, 1) inversion transition line. The survey has been carried out at the FCRAO antenna with an angular resolution of 54'', about 1.5 times finer than the previous ammonia observations made at the Haystack telescope. The comparison between N2H+ and NH3 maps shows strong similarities in the size and morphology of the two molecular species, indicating that they are tracing the same material, especially in starless cores. Cores with stars typically have map sizes about a factor of 2 smaller for N2H+ than for NH3, indicating the presence of denser and more centrally concentrated gas compared to starless cores. The mean aspect ratio is ~2. Significant correlations are found between NH3 and N2H+ column densities and excitation temperatures in starless cores, but not in cores with stars, suggesting a different chemical evolution of the two species. Starless cores are less massive ( 3 M☉) than cores with stars ( 9 M☉). Velocity gradients range between 0.5 and 6 km s-1 pc-1, similar to what has been found with NH3 data, and the ratio β of rotational kinetic energy to gravitational energy has magnitudes between ~10-4 and 0.07, indicating that rotation is not energetically dominant in the support of the cores. Local velocity gradients show significant variation in both magnitude and direction, suggesting the presence of complex motions not interpretable as simple solid-body rotation. Integrated intensity profiles of starless cores present a central flattening and are consistent with a spherically symmetric density law n r-α, where α = 1.2 for r rbreak, with rbreak ~ 0.03 pc. Cores with stars are better modeled with single density power laws with α ≥ 2, in agreement with observations of submillimeter continuum emission. Line widths change across the core, but we did not find a general trend: there are cores with significant positive as well as negative linear correlations between Δv and the impact parameter b. The deviation in line width correlates with the mean line width, suggesting that the line of sight contains ~10 coherence lengths. The corresponding value of the coherence length, ~0.01 pc, is similar to the expected cutoff wavelength for MHD waves. This similarity may account for the increased coherence of line widths on small scales. Despite finer angular resolution, the majority of N2H+ and NH3 maps show a similar simple structure, with single peaks and no elongation.

http://iopscience.iop.org/article/10.1086/340195/pdf

Dense Cores in Dark Clouds. XIV. N2H+ (1-0) Maps of Dense Cloud Cores

We present results of an extensive mapping survey of N2H+(1-0) in about 60 low mass cloud cores already mapped in the NH3(1,1) inversion transition line. The survey has been carried out at the FCRAO antenna with an angular resolution about 1.5 times finer than the previous ammonia observations. Cores with stars typically have map sizes about a factor of two smaller for N2H+ than for NH3, indicating the presence of denser and more centrally concentrated gas compared to starless cores. Significant correlations are found between NH3 and N2H+ column densities and excitation temperatures in starless cores, but not in cores with stars, suggesting a different chemical evolution of the two species. Velocity gradients range between 0.5 and 6 km/s/pc, similar to what has been found with NH3 data. ``Local'' velocity gradients show significant variation in both magnitude and direction, suggesting the presence of complexmotions not interpretable as simple solid body rotation. Integrated intensity profiles of starless cores present a ``central flattening'' and are consistent with a spherically symmetric density law n ~ r^{-1.2} for r < ~0.03 pc and n ~ r^{-2} at larger r. Cores with stars are better modelled with single density power laws with n ~ r^{-2}. Line widths change across the core but we did not find a general trend. The deviation in line width correlates with the mean line width, suggesting that the line of sight contains ~ 10 coherence lengths. The corresponding value of the coherence length, ~ 0.01 pc, is similar to the expected cutoff wavelength for MHD waves. This similarity may account for the increased ``coherence'' of line widths on small scales. Despite of the finer angular resolution, the majority of N2H+ and NH3 maps show a similar ``simple'' structure, with single peaks and no elongation.

Dense Cores in Dark Clouds. XIV. N2H+(1-0) maps of dense cloud cores

Images from the FCRAO CO Survey of the Outer Galaxy and a description of the calibration and processing of the data are presented. The survey is comprised of 1,696,80012CO J = 1-0 spectra sampled every 50'' between Galactic longitudes 10249 and 14154 and latitudes -303 and 541. The VLSR ranges from -153 to +40 km s-1 sampled every 0.81 km s-1 with a velocity resolution of 0.98 km s-1. The survey provides the highest spatial dynamic range imaging of the molecular interstellar medium ever obtained. We describe the observing procedure and characterize the random and systematic noise of the survey. The images reveal a wide diversity of spatial structure within the molecular interstellar medium. We identify several voids with angular extents of 2°-5° in diameter that may result from the cumulative interactions of ultraviolet radiation fields and stellar winds from newborn massive stars with ambient interstellar gas. It is demonstrated that a significant fraction of the integrated intensity summed over the survey field originates from regions with low peak antenna temperature and low column density assuming a conversion between integrated CO emission and molecular hydrogen column density. Assuming kinematic distances to the observed emission, we derive the radial profile of mass surface density and the variations of the z midplane and scale height of the molecular gas distribution with galactocentric radius.

The Five College Radio Astronomy Observatory CO Survey of the Outer Galaxy

The authors discuss critical aspects of imaging system design and describe several different imaging systems employing focal plane array receivers operating in the 3-mm-2-mm wavelength range. Recent progress in millimeter-wavelength optics, antennas, receivers and other components permits greatly enhanced system performance in a wide range of applications. A radiometric camera for all-weather autonomous aircraft landing capability and a high sensitivity cryogenically cooled array for use in radio astronomical spectroscopy are presented. A near-focus system for identification of plastic materials concealed underneath clothing employs a two element lens, and has been demonstrated in active (transmitting) and passive (radiometric) modes. A dual-mode imaging system for plasma diagnostics utilizes both active and passive modes at its approximately=140-GHz operating frequency to study small-scale structure. The radiometric imaging systems employ between 15 and 256 Schottky barrier diode mixers, while the imaging receivers for the active systems include 64-element video detector arrays. >

Focal plane imaging systems for millimeter wavelengths

We present a survey of the distribution of 20 chemical and isotopic molecular species along the central ridge of the Orion molecular cloud from 6' north to 6' south of BN-KL observed with the QUARRY focal plane array on the FCRAO 14 m telescope, which provides an angular resolution of approximately 50" in the 3 mm wavelength region. We use standard tools of multivariate analysis for a systematic investigation of the similarities and differences among the maps of integrated intensities of the 32 lines observed. The maps fall in three broad classes: first, those strongly peaked toward BN-KL; second, those having rather flat distributions along the ridge; and third, those with a clear north-south gradient or contrast. We identify six positions or regions where we calculate relative abundances. Line velocities and line widths indicate that the optically thin lines generally trace the same volume of dense gas, except in the molecular bar, where C18O, C34S, H13CO+, CN, C2H, SO, and C3H2 have velocities characteristic of the bar itself, whereas the emission from other detected species is dominated by the background cloud. The strongest abundance variations in our data are the well-known enhancements seen in HCN, CH3OH, HC3N, and SO toward BN-KL and, less strongly, toward the Orion-South outflow 1'.3S. The principal result of this study is that along the extended quiescent ridge the chemical abundances, within factors of 3-4, exhibit an impressive degree of uniformity. The northern part of the ridge has a chemistry closest to that found in quiescent dense clouds. While temperature and density are similar around the northern radical-ion peak near 3'.5N and in the southern core near 4'.2S, some abundances, in particular, those of the ions HCO+ and N2H+, are significantly lower toward 4'.2S. The areas near 4.'2S and the molecular bar itself around (1'.7E, 2'.4S) stand out with peculiar and similar properties probably caused by stronger UV fields penetrating deeper into the clumpy molecular gas. This leads to higher electron abundances and thereby reduced abundances of the ions, as well as a lack of complex molecules.

Chemical and Physical Gradients along the OMC-1 Ridge

A focal plane imaging array receiver is described which covers the 86-115 GHz frequency range for radio astronomical observations. The 3*5 element array uses cryogenic Schottky diode mixers with integrated HEMT IF amplifiers. A cold quasi-optical filter selects the desired sideband, and terminates the image at 20 K. Polarization interleaving is used to minimize the array size on the sky. LO power is provided by a frequency tripled YIG tuned oscillator. The average receiver noise temperature of the array pixels varies from 250-350 K SSB depending on the frequency. Only three mechanical tuners are used in the system and all functions are under computer control. >

A 15 element focal plane array for 100 GHz

A cryogenic 3-mm receiver has been developed which fully utilizes the low-noise potential of Schottky diodes by approaching the shot-noise limit within 10 percent. With a broad-band mixer design which properly terminates the input sidebands and reactively terminates the second harmonic of the local oscillator and its sidebands, the double sideband (DSB) mixer noise temperature is 35 K in the best case. This design has given an average DSB receiver noise temperature of 75 K over the 80 to 115-GHz band with a best noise temperature of 62 K.

A Broad-Band, Ultra-Low-Noise Schottky Diode Receiver from 80 to 115 GHz Diode

A continuous comparison radiometer has been implemented at 97 GHz using quasi-optical techniques for local oscillator (LO) injection and realization of a 90° hybrid. Cryogenically cooled Schottky-diode mixers and FET amplifiers give a double-sideband (DSB) system temperature of 250 K. The system is self-calibrating and optimized under computer control. The root-mean-square (rms) fluctuations due to the receiver are less than 0.025 K with a 1-s integration time.

A Continuous Comparison at 97 GHz Radiometer

In the above paper, the reference to Faris should not imply that the correlation radiometer was invented by Faris. This class of radiometers was devised by Dr. Emil Blum and described in his paper "Sensibility des radiotelescopes et recepteurs a correlation," which appeared in A nnales D 'Astrophysique, vol. 22, no. 2, pp. 139-163, Mar-Apr. 1959. Dr. Blum's contribution was also cited in the paper "Radio telescopes," published in Methods of Experimental Physics, vol. 12, pt. B, pp. 218-219.

C.R. Predmore

Papers

A 15 element focal plane array for 100 GHz

A Broad-Band, Ultra-Low-Noise Schottky Diode Receiver from 80 to 115 GHz Diode

A Continuous Comparison at 97 GHz Radiometer

A Continuous Comparison Radiometer at 97 GHz (Correction)

Correction to A continuous comparison radiometer at 97 GHz