MILLIMETER-WAVELENGTH RELATIVISTIC MAGNETRON: PROBLEMS OF MICROWAVE POWER EXTRACTION

The relativistic magnetrons operating at millimeter wavelengths demonstrate problems with microwave power extraction, both in the radial and in axial direction. The preferred axial extraction concept can be implemented ei-ther as ‘diffractional output’ or via introduction of additional resonant elements into the output waveguide. In this paper several solutions for axial-directed extraction are discussed, including circular ring ‘antennas’ at the end of the anode-cathode space, and resonance-length rods at the faces of the anode-block cavities. These have allowed increasing the power extraction efficiency by a factor of 101 to 102.

High-Resolution Infrared Spectroscopy of DC3N in the Stretching Region

The perspectives opened by modern ground-based infrared facilities and the forthcoming James Webb Telescope mission have brought a great attention to the ro-vibrational spectra of simple interstellar molecules. In this view, and because of the lack of accurate spectroscopic data, we have investigated the infrared spectrum of deuterated cyanoacetylene (DC3N), a relevant astrochemical species. The ν1, ν2, and ν3 fundamentals as well as their hot-bands were observed in the stretching region (1,500–3,500 cm−1) by means of a Fourier transform infrared spectrometer. Supplementary measurements were performed at millimeter-wavelengths (243–295 GHz) with a frequency-modulation spectrometer equipped with a furnace, that allowed to probe pure rotational transitions in the investigated stretching states. Furthermore, since HC3N is observed as by-product in our spectra and suffers from the same deficiency of accurate infrared data, its ro-vibrational features have been analyzed as well. The combined analysis of both rotational and ro-vibrational data allowed us to determine precise spectroscopic constants that can be used to model the infrared spectra of DC3N and HC3N. The importance of accurate molecular data for the correct modeling of proto-planetary disks and exoplanetary atmospheres is then discussed.

Exploring HNC and HCN line emission as probes of the protoplanetary disk temperature

Context. The distributions and abundances of molecules in protoplanetary disks are powerful tracers of the physical and chemical disk structures. The abundance ratios of HCN and its isomer HNC are known to be sensitive to gas temperature. Their line ratios might therefore offer a unique opportunity to probe the properties of the emitting gas. Aims. We investigate the HNC and HCN line emission in disks at (sub-)millimeter wavelengths and explore their potential utility for probing disk temperature and other disk properties. Methods. Using the 2D thermochemical code DALI, we ran a set of disk models accounting for different stellar properties and radial and vertical disk structures, with an updated chemical network for the nitrogen chemistry. These modeling results were then compared with observations, including new observations obtained with the Atacama Large Millimeter/submillimeter Array (ALMA) of HNC J = 3−2 for the TW Hya disk and HNC J = 1−0 for 29 disks in Lupus. Results. Similar to CN, HCN and HNC have brighter line emission in models with larger disk flaring angles and higher UV fluxes. HNC and HCN are predicted to be abundant in the warm surface layer and outer midplane region, which results in ring-shaped emission patterns. However, the precise emitting regions and emission morphology depend on the probed transition, as well as on other parameters such as C and O abundances. The modeled HNC-to-HCN line intensity ratio increases from <0.1 in the inner disk to up to 0.8 in the outer disk regions, which can be explained by efficient HNC destruction at high temperatures. Disk-integrated HNC line fluxes from current scarce observations and its radial distribution in the TW Hya disk are broadly consistent with our model predictions. Conclusions. The HNC-to-HCN flux ratio robustly increases with radius (decreasing temperature), but its use as a chemical thermometer in disks is affected by other factors, including UV flux and C and O abundances. High-spatial resolution ALMA disk observations of HNC and HCN that can locate the emitting layers would have the great potential to constrain both the disk thermal and UV radiation structures, and also to verify our understanding of the nitrogen chemistry.

Characterizing galaxy clusters by their gravitational potential: Systematics of cluster potential reconstruction

Context. Biases in mass measurements of galaxy clusters are one of the major limiting systematics in constraining cosmology with clusters. Aims. We aim to demonstrate that the systematics associated with cluster gravitational potentials are smaller than the hydrostatic mass bias and that cluster potentials could therefore be a good alternative to cluster masses in cosmological studies. Methods. Using cosmological simulations of galaxy clusters, we compute the biases in the hydrostatic mass (HE mass) and those in the gravitational potential, reconstructed from measurements at X-ray and millimeter wavelengths. In particular, we investigate the effects of the presence of substructures and of nonthermal pressure support on both the HE mass and the reconstructed potential. Results. We find that the bias in the reconstructed potential (6%) is less than that of the HE mass (13%) and that the scatter in the reconstructed potential decreases by ∼35% with respect to that in the HE mass. Conclusions. This study shows that characterizing galaxy clusters by their gravitational potential is a promising alternative to using cluster masses in cluster cosmology.

The Sun at millimeter wavelengths

Context. Solar observations with the Atacama Large Millimeter/sub-millimeter Array (ALMA) facilitate studies of the atmosphere of the Sun at chromospheric heights at high spatial and temporal resolution at millimeter wavelengths. Aims. ALMA intensity data at millimeter(mm)-wavelengths are used for a first detailed systematic assessment of the occurrence and properties of small-scale dynamical features in the quiet Sun. Methods. We analyzed ALMA Band 3 data (∼3 mm/100 GHz) with a spatial resolution of ∼1.4–2.1 arcsec and a duration of ∼40 min together with SDO/HMI magnetograms. The temporal evolution of the mm maps is studied to detect pronounced dynamical features, which then are connected to dynamical events via a k-means clustering algorithm. We studied the physical properties of the resulting events and explored whether or not they show properties consistent with propagating shock waves. For this purpose, we calculated observable shock wave signatures at mm wavelengths from one- and three-dimensional model atmospheres. Results. We detect 552 dynamical events with an excess in brightness temperature (ΔTb) of at least ≥400 K. The events show a large variety in size up to ∼9″, amplitude ΔTb up to ∼1200 K with typical values in the range ∼450–750 K, and lifetime at full width at half maximum of ΔTb of between ∼43 and 360 s, with typical values between ∼55 and 125 s. Furthermore, many of the events show signature properties suggesting that they are likely produced by propagating shock waves. Conclusions. There are a lot of small-scale dynamic structures detected in the Band 3 data, even though the spatial resolution sets limitations on the size of events that can be detected. The number of dynamic signatures in the ALMA mm data is very low in areas with photospheric footpoints with stronger magnetic fields, which is consistent with the expectation for propagating shock waves.

A stringent upper limit of the PH3 abundance at the cloud top of Venus

Aims. Following the announcement of the detection of phosphine (PH3) in the cloud deck of Venus at millimeter wavelengths, we searched for other possible signatures of this molecule in the infrared range. Methods. Since 2012, we have been observing Venus in the thermal infrared at various wavelengths to monitor the behavior of SO2 and H2O at the cloud top. We identified a spectral interval recorded in March 2015 around 950 cm−1 where a PH3 transition is present. Results. From the absence of any feature at this frequency, we derive, on the disk-integrated spectrum, a 3-σ upper limit of 5 ppbv for the PH3 mixing ratio, which is assumed to be constant throughout the atmosphere. This limit is four times lower than the disk-integrated mixing ratio derived at millimeter wavelengths. Conclusions. Our result places a strong constraint on the maximum PH3 abundance at the cloud top and in the lower mesosphere of Venus.