Millimeter-wavelength Range Research Articles

Probing the Extended Atmospheres of AGB Stars. I. Synthetic Imaging of 1D Hydrodynamical Models at Radio and (Sub-)millimeter Wavelengths

We investigate the observable characteristics of the extended atmospheres of asymptotic giant branch (AGB) stars across a wide range of radio and (sub-)millimeter wavelengths using state-of-the-art 1D dynamical atmosphere and wind models over one pulsation period. We also study the relationships between the observable features and model properties. We further study practical distance ranges for observable sources assuming the capabilities of current and upcoming observatories. We present time-variable, frequency-dependent profiles of pulsating AGB stars’ atmospheres, illustrating observable features in resolved and unresolved observations, including disk brightness temperature, photosphere radius, and resolved and unresolved spectral indices. Notably, temporal variations in disk brightness temperature closely mirror the temperature variability of the stellar atmosphere. We find that while the photospheric radius decreases due to gas dilution in the layers between consecutive shocks, the increase in the observed stellar radius reflects shock propagation through the atmosphere during the expansion phase, providing a direct measurement method for the shock velocity. Furthermore, our models indicate that enhanced gas temperatures after the passage of a strong shock might be observable in the high-frequency Atacama Large Millimeter/submillimeter Array (ALMA) bands as a decrease in the brightness temperature with increasing frequency. We demonstrate that synthetic observations based on state-of-the-art dynamical atmosphere and wind models are necessary for proper interpretations of current (ALMA and Very Large Array (VLA)) and future (Square Kilometre Array and next-generation VLA) observations and that multiwavelength observations of AGB stars are crucial for empirical studies of their extended atmospheres.

X-ray radiative transfer in full 3D with SKIRT

Context. Models of active galactic nuclei (AGN) suggest that their circumnuclear media are complex with clumps and filaments, while recent observations hint towards polar extended structures of gas and dust, as opposed to the classical torus paradigm. The X-ray band could form an interesting observational window to study these circumnuclear media in great detail. Aims. We want to extend the radiative transfer code SKIRT with the X-ray processes that govern the broad-band X-ray spectra of obscured AGN, to study the structure of AGN circumnuclear media in full 3D, based on their reflected X-ray emission. Methods. We extended the SKIRT code with Compton scattering on free electrons, photo-absorption and fluorescence by cold atomic gas, scattering on bound electrons, and extinction by dust. This includes a novel treatment of extreme-forward scattering by dust, and a detailed description of anomalous Rayleigh scattering. To verify our X-ray implementation, we performed the first dedicated benchmark of X-ray torus models, comparing five X-ray radiative transfer codes. Results. The resulting radiative transfer code covers the X-ray to millimetre wavelength range self-consistently, has all the features of the established SKIRT framework, is publicly available, and is fully optimised to operate in arbitrary 3D geometries. In the X-ray regime, we find an excellent agreement with the simulation results of the MYTORUS and REFLEX codes, which validates our X-ray implementation. We find some discrepancies with other codes, which illustrates the complexity of X-ray radiative transfer and motivates the need for a robust framework that can handle non-linear 3D radiative transfer effects. We illustrate the 3D nature of the code by producing synthetic X-ray images and spectra of clumpy torus models. Conclusions. SKIRT forms a powerful new tool to model circumnuclear media in full 3D, and make predictions for the X-ray band in addition to the dust-dominated infrared-to-UV wavelength range. The new X-ray functionalities of the SKIRT code allow for uncomplicated access to a broad suite of 3D X-ray models for AGN that can easily be tested and modified. This will be particularly useful with the advent of X-ray microcalorimeter observations in the near future.

A systematic survey of millimetre-wavelength flaring variability of young stellar objects in the Orion Nebula Cluster

ABSTRACT High-energy processes are ubiquitous even in the earliest stages of protostellar evolution. Motivated by the results of our systematic search for intense centimetre radio flares in Young Stellar Objects (YSOs) and by rare findings of strong millimetre-wavelength variability, we have conducted a systematic search for such variability in the Orion Nebula Cluster (ONC) using Atacama Large Millimetre/submillimetre Array (ALMA). Rapid variability on time-scales of minutes to hours in the (centimetre) millimetre-wavelength range indicates (gyro)synchrotron radiation. Additionally, mass accretion will also affect the millimetre-wavelength luminosity but typically on longer time-scales. Beyond studies of individual YSOs, our characterization of strong millimetre-wavelength variability with ALMA in the ONC sets first systematic constraints on the occurrence of such variability in a large number of YSOs (∼130). We report the discovery of an order of magnitude millimetre-flare within just a few minutes from a known YSO previously reported as a radio flaring source at cm-wavelengths (the ‘ORBS’ source). We also present an assessment of the systematic variability effects caused by the use of time-sliced imaging of a complex region. These are mostly due to the impact of a changing synthesized beam throughout the observations. We use simulated ALMA observations to reproduce and quantify these effects, and set a lower limit for the variability that can be studied using our method in a complex region such as the ONC. Our results demonstrate that the utility of time domain analysis of YSOs extends into the millimetre-wavelength range, potentially interfering with the conversion of observed fluxes into dust masses.

Development and Experimental Study of a Pulsed Megawatt Gyroklystron Operating in the Long-Wavelength Part of the Millimeter-Wavelength Range at IAP RAS

Development and Experimental Study of a Pulsed Megawatt Gyroklystron Operating in the Long-Wavelength Part of the Millimeter-Wavelength Range at IAP RAS

CO-Ar collisions: ab initio model matches experimental spectra at a sub percent level over a wide pressure range

We use three independent spectroscopic techniques, operating in the millimeter-wavelength range, to study molecule-atom collisions, and validate our quantum-scattering calculations on two recent potential energy surfaces. We study the first pure rotational transition in a CO molecule perturbed by Ar. This molecular system is a good prototype of atmospherically relevant cases. It is, on the one hand, affordable for calculation of the line shape parameters by modern ab initio methods, and on the other hand, is very convenient for experimental studies because of its regular, well spaced rotational spectrum having a moderate intensity. We show that the simulated collision-perturbed spectra, which are based on our fully ab initio calculations, agree with the experimental line profiles at sub-percent level over a wide range (more than four orders of magnitude) of pressures. We demonstrate that the agreement between theory and experiment can be further improved if the model accounts for the collisional transfer of an optical coherence between different rotational transitions (the line-mixing effect). We show that the two surfaces tested in this work lead to a very similar agreement with the experiment. Capability of calculating line shape parameters in a broad range of temperatures is demonstrated.

Electrically tunable reflector based on ferroelectric material for millimetre wavelength range

The design of a millimeter-wave reflector based on the ferroelectric ceramic was elaborated and considered. Among the advantages of the proposed design are simplicity and cost-effectiveness compared to devices based on active repeaters and reflectarrays. The use of ferroelectric ceramic in the reflector design allows operating in a wide frequency range up to 100 GHz. Parameters of ferroelectric ceramic were experimentally measured and used in the electromagnetic simulation of the proposed reflector. Simulation of the reflector’s radiation pattern demonstrated the effective beam scan possibility in a range of 35 deg.

Design and simulation of RF MEMS switch for use in the millimetre wavelength range and SPDT RF MEMS switch

This article describes in detail the design of a shunt capacitive RF MEMS switch for Ka-frequency band, as well as the design of a single-pole-double-directional RF MEMS switch made on its basis. An electrostatic and electromagnetic analysis was performed using the MATLAB and ANSYS HFSS software package. The switch uses a 1.52 V actuation voltage and 1.3 μs of switching time to move the membrane from the up state to the down state. The operating frequency range of this switch is between 24-34 GHz with isolation of -53 dB at 29 GHz and reflection loss of -0.25 dB for this frequency range. The design of the switch is made on a GaAs substrate with a thickness of 525 μm with a double-sided insulating layer of silicon oxide with a thickness of 2 μm.

Self-scattering in protoplanetary disks with dust settling

Scattering of re-emitted flux is considered to be at least partially responsible for the observed polarisation in the (sub-)millimetre wavelength range of several protoplanetary disks. Although the degree of polarisation produced by scattering is highly dependent on the dust model, early studies investigating this mechanism relied on the assumption of single grain sizes and simple density distribution of the dust. However, in the dense inner regions where this mechanism is usually most efficient, the existence of dust grains with sizes ranging from nanometres to millimetres has been confirmed. Additionally, the presence of gas forces larger grains to migrate vertically towards the disk midplane, introducing a dust segregation in the vertical direction. Using polarisation radiative transfer simulations, we analyse the dependence of the resulting scattered light polarisation at 350 μm, 850 μm, 1.3 mm, and 2 mm on various parameters describing protoplanetary disks, including the effect of dust grain settling. We find that the different disk parameters change the degree of polarisation mostly by affecting the anisotropy of the radiation field, the optical depth, or both. It is therefore very challenging to deduce certain disk parameter values directly from polarisation measurements alone. However, assuming a high dust albedo, it is possible to trace the transition from optically thick to optically thin disk regions. The degree of polarisation in most of the considered disk configurations is lower than what is found observationally, implying the necessity to revisit models that describe the dust properties and disk structure.

High-Efficiency Relativistic Generators of Nanosecond Pulses in the Millimeter-Wavelength Range

We propose and study a scheme of a microwave pulse generator operating in the frequency ranges near 37 and 73 GHz, in which the electron interaction with both the (−1)st harmonic of the counterpropagating TM02 mode and the synchronous slowed-down TM01 mode is combined in a sectionalized slow-wave system with an average diameter of 2.5λ. An efficient current modulation at the input of the slow-wave system is ensured in the region of the cathode–anode gap and the matching section, which reduces the diffraction loss of the energy flow directed towards the cathode. Numerical modeling shows that the efficiency of converting the electron-beam power to microwave radiation is up to 50%. Experiments demonstrate a stable regime of generation of subgigawatt pulses at an efficiency of more than 40% in the upper part of the millimeterwavelength range, which have a reproducible spatial wave structure corresponding predominantly to the TM01 mode at the input of the emitting horn.

IRC + 10°216 mass loss properties through the study ofλ3 mm emission

Low-mass evolved stars are major contributors to interstellar medium enrichment as a consequence of the intense mass-loss process these stars experience at the end of their lives. The study of the gas in the envelopes surrounding asymptotic giant branch (AGB) stars through observations in the millimetre wavelength range provides information about the history and nature of these molecular factories. Here we present ALMA observations at subarsecond resolution, complemented with IRAM-30 m data, of several lines of SiO, SiS, and CS towards the best-studied AGB circumstellar envelope, IRC + 10°216. We aim to characterise their spatial distribution and determine their fractional abundances mainly through radiative transfer and chemical modelling. The three species display extended emission with several enhanced emission shells. CS displays the most extended distribution reaching distances up to approximately 20′′. SiS and SiO emission have similar sizes of approximately 11′′, but SiS emission is slightly more compact. We have estimated fractional abundances relative to H2, which on average are equal tof(SiO) ~10−7,f(SiS) ~10−6, andf(CS) ~10−6up to the photo-dissociation region. The observations and analysis presented here show evidence that the circumstellar material displays clear deviations from an homogeneous spherical wind, with clumps and low density shells that may allow UV photons from the interstellar medium (ISM) to penetrate deep into the envelope, shifting the photo-dissociation radius inwards. Our chemical model predicts photo-dissociation radii compatible with those derived from the observations, although it is unable to predict abundance variations from the starting radius of the calculations (~10R*), which may reflect the simplicity of the model. We conclude that the spatial distribution of the gas proves the episodic and variable nature of the mass loss mechanism of IRC + 10°216, on timescales of hundreds of years.

Intrinsic polarization of elongated porous dust grains

ABSTRACT Observations of the Atacama Large Millimeter Array (ALMA) revealed recently polarized radiation of several protoplanetary discs in the (sub)millimetre wavelength range. Besides self-scattering of large particles, thermal emission by elongated grains is a potential source for the detected polarization signal. We calculate the wavelength dependent absorption and intrinsic polarization of spheroidally shaped, micrometre, and submillimetre sized dust grains using the discrete dipole approximation. In particular, we analyse the impact of dust grain porosity that appears to be present in discs when small grains coagulate to form larger aggregates. For the first time, our results show that (a) the intrinsic polarization decreases for increasing grain porosity and (b) the polarization orientation flips by 90 deg for certain ratios of wavelength to grain size. We present a new method to constrain grain porosity and the grain size in protoplanetary discs using multiwavelength polarization observations in the far-infrared to millimetre wavelengths. Finally, we find that moderate grain porosities ($\mathcal {P}\lesssim 0.7$) potentially explain the observed polarization fraction in the system HD 142527 while highly porous grains ($\mathcal {P}\gt 0.7$) fail unless the grain’s axis ratio is extraordinarily large.

High-Power Relativistic Milimeter-Wave Gyrotron Operating at the Second Cyclotron Harmonic

We present the results of the theoretical and experimental studies, which are aimed at developing pulsed gyrotrons with relativistic electron beams operating in the millimeter-wavelength range and retaining the multimegawatt power level. The use of the interaction at the second gyrofrequency harmonic, which is characterized by a sufficiently high efficiency and availability of the solution for the mode selection problem, allows one to double the generation frequency without increasing the magnetic field. In the corresponding experiment, the output radiation power exceeded 2 MW, and the efficiency was about 30% at a wavelength of 5.5 mm.

Вплив опадів на роздільну здатність суднової радіолокаційної станції за кутовими координатами

An analysis of influence of rain drop down on the resolution power of marine radar by angular coordinates, working in the millimeter and centimeter wavelength ranges has been performed. It is shown that one of the directions of increase of safety of navigation using marine radar has the improvement of their resolution in angular coordinates, since the atmospheric phenomenon on the path of the ship make a significant error in the measurement of the angular coordinates by the radar surveillance navigation facilities. The study of precipitation on the resolution power of marine radar by angular coordinates, antenna which radiates a linear-polarized electromagnetic wave of millimeter and centimeter ranges has been provided. The interaction of the wave with particles of precipitation presented in the form of a power wave from the effective surface of the scattering reflective volume of rain is taken into account. The effective surface of scattering and attenuation on all particle sizes in the dropdown of rain on the way is found. It was found that for the centimeter wavelength range (λ = 3 cm...10 cm) attenuation is much higher than the scattering. The resulting conditions to improve the resolution of the antenna of marine radar by angular coordinates are in the centimeter wavelength range. In the millimeter range of wavelengths occurs as the improvement of the resolution capability of the antenna of marine radar and its deterioration depending on the intensity of precipitation. The deterioration of the resolution can reach 10 %.

Dispersion-Induced Distortions of a Rectangular Radio Pulse with Linear Chirp in a Resonantly Absorbing Gas Medium

Existing analytical results on the propagation of electromagnetic pulse in resonantly absorbing gas medium are used to analyze possible character and features of the dispersion-induced distortions of an ultrashort frequency-modulated radio pulse with the initial rectangular shape and the carrier frequency that is close to the resonance frequency of a single strong absorption band of water vapor in the millimeter-wavelength range (183.36 GHz).

Remote Sensing of the Snow cover in the millimeter-Wavelength Range

We present the results of development of a multifrequency method for passive remote sensing of the snow cover using spectral features of the downward atmospheric radiation in the millimeter-wavelength range. A description of the automated tri-band spectroradiometer realizing this method is provided. Preliminary results of test measurements of the snow cover parameters by means of the tri-band spectroradiometer are discussed.

Modeling of giant radio galaxy distribution over the sphere in the millimeter-wavelength range

We suggest a method and build a model of radio galaxy distribution over the sphere using data from the WENSS catalogs and standard radio count models by Condon. We calculate the angular power spectrum of the contribution of extended radio galaxies into the microwave background and show that it can be a distorting factor for correct estimation of the angular spectrum from the signal determined by the Sunyaev–Zeldovich effect.

Studying the features of hexaferrite film fabrication using a slurry casting for the substrates of subminiature microstrip ferrite decoupling devices in the short-wave range of millimeter wavelengths

Using polyelectrolytes, ecologically safe film elements from hexaferrites have been designed for the first time with water slurry casting instead of existing processes with the use of organic solvent binders. The use of polyelectrolyte substances as binders has made it possible to reduce the energy consumption of drying by 20–30% during the process of film casting.

ALMA spectral survey of Supernova 1987A – molecular inventory, chemistry, dynamics and explosive nucleosynthesis

We report the first molecular line survey of Supernova 1987A in the millimetre wavelength range. In the ALMA 210--300 and 340--360 GHz spectra, we detected cold (20--170 K) CO, 28SiO, HCO+ and SO, with weaker lines of 29SiO from ejecta. This is the first identification of HCO+ and SO in a young supernova remnant. We find a dip in the J=6--5 and 5--4 SiO line profiles, suggesting that the ejecta morphology is likely elongated. The difference of the CO and SiO line profiles is consistent with hydrodynamic simulations, which show that Rayleigh-Taylor instabilities cause mixing of gas, with heavier elements much more disturbed, making more elongated structure. We obtained isotopologue ratios of 28SiO/29SiO>13, 28SiO/30SiO>14, and 12CO/13CO>21, with the most likely limits of 28SiO/29SiO>128, 28SiO/30SiO>189. Low 29Si and 30Si abundances in SN 1987A are consistent with nucleosynthesis models that show inefficient formation of neutron-rich isotopes in a low metallicity environment, such as the Large Magellanic Cloud. The deduced large mass of HCO+ (~5x10^-6 Msun) and small SiS mass (<6x10^-5 Msun) might be explained by some mixing of elements immediately after the explosion. The mixing might have caused some hydrogen from the envelope to sink into carbon and oxygen-rich zones after the explosion, enabling the formation of a substantial mass of HCO+. Oxygen atoms may have penetrated into silicon and sulphur zones, suppressing formation of SiS. Our ALMA observations open up a new window to investigate chemistry, dynamics and explosive-nucleosynthesis in supernovae.

Hydration effects accompanying the formation of DNA complexes with some ligands

In the range of millimeter wavelengths the dielectric properties of aqueous solutions of some biologically active ligands (potential anticarcinogen chlorophyllin; pharmacological drug caffeine; polyamine putrescine; mutagens proflavine and ethidium bromide; actinocin derivative, an analogue of antitumor antibiotic actinomycin D) and DNA complexes with these substances were studied. It was shown that complex formation is accompanied by the change in dielectric properties of the solution. These changes during interaction of DNA with the first three compounds correspond to a decrease in hydration (compared with the total hydration of free components), and in other cases they cause an increase in hydration. The number of water molecules bound with both the ligand and DNA nucleotide in the complex was estimated. The results were compared with existing models of DNA interaction with the studied substances.

Technique of Time-Frequency Analysis of a Series of Measurements of the Radiation Spectra of Night Mesospheric Ozone in the Millimetric Wavelength Range

A technique of time-frequency analysis of unevenly sampled data sets is developed to study variations in the night mesospheric ozone emission spectra obtained from ground-based millimeter-wave observations. Basic computational formulas as well as results of numerical simulations with monochromatic and noise signals are presented. The results of the simulation are in good agreement with theoretical estimates.