Thermal Radio Emission Research Articles

Millimeter-wave radiometry of the temperature film on a sea surface

Using in-situ measurements of thermal radio emission at frequencies 53–55 GHz from a rough sea surface in the coastal zone of the eastern Crimea shore, we obtain data on the water temperature at a depth of 0.2 mm and on the reflection characteristics of the sea surface. Measuring the intensity of radio emission from a surface irradiated by a signal with known variable power allows one to obtain simultaneous data on the temperature and reflection coefficient. In our experiment, the atmospheric radio emission in the wing of the molecular oxygen absorption band, received by frequency-separated channels of a radiometer–spectrometer, plays the role of such an irradiation. The measured water temperature TSL=(5.4±1.0) °C turns out to be less than the air temperature and the in-depth water temperature. The reflection coefficient is equal to R=0.349±0.05.

Molecular gas in NGC 6946

We present imaging of molecular gas emission in the star-forming spiral galaxy NGC 6946. Our 12 CO(1{ 0) and 12 CO(3{2) images, made at 22 00 resolution with the IRAM 30-m and the Heinrich Hertz 10-m radio telescopes, are the most extensive observations of molecular gas in this galaxy and are among the most extensive observations of molecular gas in any spiral galaxy. The 12 CO(1{0) emission shows a central concentration aligned in a north-south ellipse, an extended diuse component plus concentrations in the outer spiral arms. 12 CO(1{0) is detected across most of the optical disk out to a projected radius of nearly 300 00 . The molecular component in NGC 6946 is unusually massive, with MH2=MHI =0 :57. The scale length of the disk in 12 CO(1{0) is the same, to within 5%, as the VRI, H ,1 5m, and FIR disks. The 12 CO(3{2) data shows broadly similar structure to the 12 CO(1{0) image out to the observed limit of 200 00 , although the arm-interarm contrast of 12 CO(3{2) is signicantly larger than that of 12 CO(1{0) which suggests that molecules in the spiral arms are warmer or reside longer there. The rotation curve derived from the 12 CO(1{0) velocity eld agrees, within the uncertainties, with the H i rotation curve. The velocity dispersion is 40 10 km s 1 in the inner 2 kpc and 8 3k m s 1 in the disk, without tendency to increase in the spiral arms. Spectra of the 13 CO(1{0), 13 CO(2{1), 12 C 18 O(1{0) isotopomers for several positions are used for line ratio studies of the state of the molecular ISM in NGC 6946. In the centre of NGC 6946 our LVG analysis suggests that the beam-averaged gas kinetic temperature is 40 5 K, the molecular gas density is (3:3 0:3) 10 3 cm 3 and that line opacities are high, with 12CO(3 2) 10. A star formation eciency image for NGC 6946, made from the H image divided by the molecular gas image, ranges by over two orders of magnitude with highest values found in the northeastern spiral arm. The 6 cm polarized emission image, which traces the regular part of the magnetic eld, appears anti-correlated with the star formation eciency. We present an analysis of the ISM in NGC 6946's disk by making 1-D and 2-D comparisons of images made in several wavebands. Using a point-by-point correlation technique, we investigate the distribution and kinematics of the molecular gas and its relation to the neutral and ionized gas, the mid-infrared-emitting dust, the radio continuum and the magnetic eld, and nd that the molecular gas is closely associated with the 7m-emitting dust. The highest correlation between any pair of tracers is found between the mid-infrared emission and the total radio continuum emission at 6 cm. This cannot be due to dust heating and gas ionization in star-forming regions because the thermal radio emission is less correlated with the mid-infrared emission than the nonthermal emission. A coupling of magnetic elds to gas clouds is proposed as a possible scenario.

Multi wavelength investigation of the eruption of a sigmoidal quiescent filament

We report the first observation of a filament eruption in the metric and decimetric range, where the behavior of the filament can be followed during the event via thermal radio emission diagnostics. The event, occurring on February 28th 2001, involved a quiescent filament in a sigmoid magnetic configuration, whose eruption is triggered by the birth of a small parasitic polarity. Faint radio bursts mark the beginning of the event, which shows the appearance of a brightness temperature depression associated with the filament seen in EIT, and its propagation on the disk up to the limb. The event is associated with a halo CME observed with the coronagraphs LASCO C2 and C3, which shows a significant spatial and temporal continuity with the radio observations. Finally, static dimmings, similar to what is currently observed in EUV or SXR domains, are also detected in the radio band, around the site of the eruption. Movies of the event are attached to this article.

Mercury: heat transport and thermal radio emission propagation in structurally inhomogeneous regolith

We present the results of the theoretical study of the dependence of thermal radio emission from Mercury regolith on its structure and composition. By employing different models for a regolith structure we have shown that the radiative component of the heat flux influences strongly on the effective temperature and its spectral characteristics. It is shown also that electromagnetic inhomogeneities of the regolith structure play decisive role for the interpretation of a data from the high-surface-resolution measurements. Results may be useful for comparison of the experimental data with a theory to determine physical characteristics of the upper layer of Mercury.

New effect in near-field thermal emission.

A near-field effect has been discovered experimentally in thermal radio emission of an absorbing dielectric medium. It is related to a specific character of the distribution of a quasistationary field component near a radiating surface. The effect consists of the fact that the effective depth of the received emission formation appears to be less than the skin-layer depth and depends on the size of the receiver antenna and its height above the surface. It can be considered as a new source of information about depth temperature distribution. A theory has been developed that allows for determining the relative contribution of wave and quasistationary components to the Plank emission received near the surface.

Structure of Thermal Radio Emission from the Atmosphere in the Middle Part of the Submillimeter Wavelength Range

We present the results of theoretical studies of the structure of thermal radio emission from the Earth's atmosphere in the middle part of the submillimeter wavelength range (wave numbers 1/λ≈ 20–50 cm-1). Absorption spectra of the atmosphere, heights at which the radiation is formed, and other physical factors, which determine the features of the inverse problems of remote sensing in the band considered, are analyzed.

Scaling and correlation analysis of galactic images

Different scaling and autocorrelation characteristics and their application to astronomical images are discussed: the structure function, the autocorrelation function, Fourier spectra and wavelet spectra. We recommend as the optimal mathematical tool the wavelet spectrum with a suitable choice of the analysing wavelet. We introduce the wavelet cross-correlation function which enables to study the correlation between images as a function of scale. The wavelet cross-correlation coefficient strongly depends on the scale. The classical cross-correlation coefficient can be misleading if a bright, extended central region or an extended disk exists in the galactic images. An analysis of the scaling and cross-correlation characteristics of 9 optical and radio maps of the nearby spiral galaxy NGC 6946 is presented. The wavelet spectra allow to separate structures on different scales like spiral arms and diffuse extended emission. Only the images of thermal radio emission and Halpha emission give indications of 3-dimensional Kolmogorov-type turbulence on the smallest resolved scales (160-800 pc). The cross-correlations between the images of NGC 6946 show strong similarities between the images of total radio emission, red light and mid-infrared dust emission on all scales. The best correlation is found between total radio emission and dust emission. Thermal radio continuum and Halpha emission are best correlated on a scale of about 1' \simeq 1.6 kpc, the typical width of a spiral arm. On a similar scale, the images of polarised radio and Halpha emission are anticorrelated, which remains undetected with classical ross-correlation analysis.

A Radio Study of the Evolution of Spatial Structure of an Active Region and Flare Productivity

We present the results of a radio study of the evolution of an active region through its flare productivity. The radio study was carried out with data obtained by the Nobeyama Radio Heliograph at 17 GHz. We chose the active region AR 7515, which appeared at the east limb on 1993 May 23 and then evolved during its passage across the disk. We followed its evolution until June 2. This region produced many small flares. We consider this region to be a typical active region in the sense that it did not produce any large flares, but a large number of weak flares. We investigate the optical and magnetic development of the region and show how this affects the locations of the flaring activity. We discuss a number of events in detail in order to investigate the roles of nonthermal and thermal radio emission in the flares. The nonthermal gyrosynchrotron emission generally occurs in regions of strong magnetic fields, is generally circularly polarized, and often varies rapidly in time. On the other hand, gradual radio components tend to be thermal and only weakly polarized, if at all. An interesting aspect of evolution of the flares in this region is that many of the flares in the early phase of the evolution show strong but brief nonthermal radio emission in the impulsive phase followed by gradual thermal emission, whereas in the last 3 days more gradual events without a strong spike of radio emission in the impulsive phase tend to be seen. Correspondingly, the flare images suggest that the radio sources are more compact during the early phases and more extended in the last half of the period covered. The most dominant component of the preflare region is often not the component that undergoes immediate flaring. Sometimes a number of components in the preflare region participate in the flare process together. We speculate that these component sources are unresolved compact bipolar loops that flare in sequence. Loop-loop interactions occurring at many different sites at the same time seems to be a less plausible explanation of these events.

Mercury: The Influence of the Heat Flow from the Interior on the Thermal Regime and Radio Emission of the Upper Cover

The effect of the heat flow from Mercury's interior on the thermal regime and radio emission of Mercury's upper cover was studied for various models of the upper cover structure. It was demonstrated that, in some models, the influence of the heat flow from the planet's interior is important, whereas, in other models, this effect is insignificant. For an inhomogeneous two-layer model of the upper cover structure in which a layer of a loose, unbound material rests on a sufficiently thick monolithic substrate, the heat flow from Mercury's interior is not the sole cause for the increase in temperature with depth. This temperature increase can also be explained by specific features of the upper layer structure.

Thermal Radio Emission and Scattering by a Rough Surface in the Small Slope Approximation

Thermal Radio Emission and Scattering by a Rough Surface in the Small Slope Approximation

Prospects for High Angular Resolution at Metre to Centimetre Wavelengths

The past ten years has seen very significant advances in high resolution imaging arising from the construction and routine operation of Very Long Baseline Interferometer arrays in the US, Europe and Australia. Coupled with the launch of the first VLBI antennas into earth orbit, angular resolution below a milliarcsecond has been achieved on a large number of compact objects. Nevertheless, while the application of phase referencing to VLBI observations has increased the number of sources that can be imaged by VLBI, the astrophysical impact of interferometry on baselines greater than a few tens of km, has been severely limited by sensitivity. The next decade promises to bring about very significant increases in continuum sensitivity at high angular resolution through new developments in VLBI recording devices to achieve recording bandwidths of 4 to 8 Gbits per second, and by direct optical fibre-linked long baseline arrays. In the following decade, the Square Kilometre Array will herald a new era of high resolution radio astronomy with a factor of 100 sensitivity increase. In addition to vastly increasing the sample of non-thermal sources accessible to VLBI, the SKA will open up high angular resolution imaging to a new regime of astrophysics, by enabling milliarcsecond resolution imaging of thermal radio emission. At wavelengths of a few centimetres the SKA, in combination with next generation space VLBI missions, will allow direct imaging of the x-ray emitting accretion disks in galactic nuclei.

VLA Observations of the Nearby Merger NGC 4038/4039: H [CSC]ii[/CSC] Regions and Supernova Remnants in the “Antennae”

We present 6 and 4 cm radio images of the inner disks of the nearby merger pair NGC 4038 and NGC 4039 at resolutions between ~ 1'' and 26, or ~ 100 and 260 pc. We detect numerous compact radio sources embedded in more diffuse radio emission; the compact radio sources produce ~ 12% and ~ 25% of the 6 and 4 cm radio emission in the system, respectively. The strongest radio continuum emission occurs between the galaxies, at an optically unremarkable location near but not coincident with an extremely red cluster detected by Whitmore & Schweizer. The radio peak is at a location of intense star formation (as detected in the mid-IR) and of dense molecular gas. We identify 115 individual compact 6 cm radio sources and 63 individual 4 cm sources in the system, to a limiting luminosity of ~ 2 × 1018 W Hz-1, or ~ 4 times the luminosity of Cas A; 90% of the 6 cm sources and 67% of the 4 cm sources are unresolved in these observations. Of the strongest sources, for which the flux densities are large enough to measure spectral indices with reasonable errors, one-third have nominally flat radio spectra, indicating that they are dominated by thermal radio emission from H II regions, and two-thirds have nominally steep spectra dominated by nonthermal emission from supernova remnants. For the compact sources dominated by thermal emission, we derive typical masses of 104 to 105 M⊙ in massive young stars. Using reasonable assumptions about source sizes, we estimate associated ionized gas masses of 103 to 104 M⊙. A comparison with the ~ (2–5) × 103 M⊙ typical optical clusters detected recently by Whitmore et al. suggests that the thermal radio sources detected here may be composed of tens of these typical young star clusters or else that they are more like the individual massive 105–106 M⊙ Whitmore et al. clusters. The strongest thermal radio source is associated with none of the Whitmore et al. clusters but is coincident with strong CO and 15 μm emission, implying that massive star clusters are still enshrouded by dust in this region. The second strongest thermal source is near several optical clusters, suggesting that these clusters are just now emerging from their birth cloud and are clearing away the surrounding material. Estimates of stellar ages and sound speeds indicate that we do not see star formation propagating in this system; rather, the star formation sequence is probably related to more global properties of the interaction. From the steep-spectrum sources, we derive a total system supernova rate of 0.2–0.3 yr-1, about an order of magnitude larger than that expected from the currently observed O star population. This suggests that a large number of O stars may have formed in the system in a short-lived burst, less than 106 yr in duration and about 3–4 Myr ago.

350 Micron Images of Massive Star Formation Regions

We present 350 μm continuum images of 24 massive star formation regions obtained with the Caltech Submillimeter Observatory equipped with the SHARC focal-plane array. At this wavelength the spatial resolution is 11''. The 350 μm emission is compared with thermal radio continuum emission and OH, H2O, and CH3OH masers. Emission at 350 μm is believed to be thermal emission from dust heated by embedded or nearby stars. Compact radio continuum sources are usually present in the mapped 350 μm fields, and more than 60% of the 350 μm peaks coincide with radio continuum peaks. This association lends strong support to the notion that the dust is heated primarily by hot stars. Masers are also a common property of massive star formation regions. Usually OH, H2O, and/or CH3OH masers are found near, but generally not coincident with, the 350 μm peaks. Less than 25% of the 350 μm peaks have no reported masers located close by. In most of the observed regions, the 350 μm dust maps show one or more components surrounded by fainter extended emission. In total, we identify 28 separate 350 μm components. Ten of the 28 components do not have radio continuum counterparts. These are luminous sources and should produce detectable H II regions. It is postulated that these sources may be undergoing such rapid accretion that the infalling matter quenches the H II region very close to the protostar, thereby making the H II region undetectable in free-free emission. Objects in this evolutionary state may well represent the long-sought precursors to ultracompact H II regions and should have the properties of accreting massive protostars (e.g., accretion disks, bipolar molecular outflows, hot shocked gas, infall with spin-up toward the protostar). We suggest that the 10 sources in this category in our sample merit further observational study for these properties. Two-temperature graybody models constrained by the observed infrared spectral energy distributions were calculated to estimate the total mass, luminosity, average dust temperature, and hydrogen column and number densities for each source. The graybody models do not account for possible low-level emission from the coolest dust (<25 K) and therefore may underestimate the total mass. There is, however, no evidence for emission in excess of the models at 1.3 mm, and we conclude that the mass contribution of dust colder than ~25 K cannot be large. Thus, the graybody models shown provide useful average global properties required to understand massive star formation environments.

Study of atmospheric-turbulence effects on the formation of a thermal film in the near-surface water layer and the dynamics of air-water heat exchange using measurements of thermal radio emission

On the basis of radiometric measurements of thermal radio emission from water at wavelengths 2.3 and 5 mm, we study the dynamics of heat and mass transfer through a water-air boundary, which is related to atmospheric turbulence, and reconstruct the behavior of the water-temperature profile and the heat flux through this boundary. The heat-flux components due to evaporation and thermal conductivity and the evaporation rate from unit surface are determined. Turbulence-induced variations in the depth of the atmospheric viscous sublayer are studied. Statistical parameters of the water-surface temperature variations are calculated. We find that these variations were much greater than the turbulent fluctuations of the air temperature under the measurement conditions.

The Absolute Abundance of Iron in the Solar Corona.

We present a measurement of the abundance of Fe relative to H in the solar corona using a technique that differs from previous spectroscopic and solar wind measurements. Our method combines EUV line data from the Coronal Diagnostic Spectrometer (CDS) on the Solar and Heliospheric Observatory with thermal bremsstrahlung radio data from the VLA. The coronal Fe abundance is derived by equating the thermal bremsstrahlung radio emission calculated from the EUV Fe line data to that observed with the VLA, treating the Fe/H abundance as the sole unknown. We apply this technique to a compact cool active region and find Fe&solm0;H=1.56x10-4, or about 4 times its value in the solar photosphere. Uncertainties in the CDS radiometric calibration, the VLA intensity measurements, the atomic parameters, and the assumptions made in the spectral analysis yield net uncertainties of approximately 20%. This result implies that low first ionization potential elements such as Fe are enhanced in the solar corona relative to photospheric values.

Rotation and X‐Ray Emission from Protostars

The ASCA satellite has recently detected variable hard X-ray emission from two class I protostars in the o Oph cloud, YLW 15 (IRS 43) and WL 6, with a characteristic timescale of D20 hr. In YLW 15, the X-ray emission is in the form of quasi-periodic energetic Nares, which we explain in terms of strong magnetic shearing and reconnection between the central star and the accretion disk. The Nare modeling, based on the solar analogy, gives us access to the size of the magnetic structures, which in turn allows us to calculate the rotation parameters of the star and the disk. In WL 6, X-ray Naring is rotationally modulated and appears to be more like the solar-type magnetic activity ubiquitous on T Tauri stars. On the basis of these observations, we Ðnd that YLW 15 is a fast rotator (near break-up), while WL 6 rotates with a signiÐcantly longer period. We thus use X-ray Naring as a clock II to measure the rota- tion of protostars. With the help of the mass-radius relation on the stellar birthline,II we derive masses of and for the central stars of YLW 15 and WL 6, respectively. YLW 15 thus M * D 2 M _ (0.4 M _ appears to be a future A star. In the long term, the magnetic interactions between the star and the disk result in magnetic braking and angular momentum loss of the star. A comparison of the rotation behav- ior of YLW 15 and WL 6 conÐrms that for solar-mass stars magnetic braking takes place on timescales few ) 105 yr, i.e., of the same order as the estimated duration of the class I protostar stage. The t br D a main parameter determining turns out to be the stellar mass, so that close to the birthline there must t br be a mass-rotation relation, such that stars with are fast rotators, while t br D P M * , M * Z 1E2 M _ their lower mass counterparts have had the time to spin down and reach synchronous rotation with the inner surrounding accretion disk. The rapid rotation and strong star-disk magnetic interactions of YLW 15 also naturally explain the observation of superNares II of X-ray luminosities as high as L X Z ergs s~1 during a few hours, while at the WL 6 stage the lower X-ray luminosities are likely 1033E1034 to be of purely stellar origin. The mass-rotation relation through magnetic braking may also explain why so few class I protostars have been detected in X-rays to date, and why they all lie in clusters. In the case of YLW 15, and perhaps also in other protostars, a hot coronal wind (T D 106 K) may be responsible for the VLA thermal radio emission. This paper thus proposes the Ðrst clues to the magnetic properties of protostars, which govern their rotation status and evolution. Subject headings : stars : Nare E stars : individual (YLW 15, WL 6) E stars : magnetic Ðelds E stars : preEmain-sequence E stars : rotation E X-rays : stars

Reconstruction of two-dimensional distribution of radio brightness using measurements by an antenna with the known angular pattern

We develop a method for reconstruction of two-dimensional distribution of brightness temperature using the measured antenna temperature. The method is based on the Tikhonov theory of illposed problems. The efficiency of the method is analyzed by numerical simulations of single-and multiple-beam synthetic-aperture radiometric systems. The proposed method is applied to helicopter measurements of thermal radio emission from the soil.

An Excited H2Disk and the Core Structure of the Extraordinary Bipolar Planetary Nebula KjPn 8

A disk of excited H2v=1 → 0 S(1) (2.122 μm) emission has been found in the core of the extraordinary bipolar planetary nebula KjPn 8. This surrounds the central, extended, and clumpy thermal radio emission (VLA) and near-infrared (J, H, and K' bands) source. The H2 disk is found to be contained within a previously found expanding CO disk, and they share the same orientation. The plane of the H2 disk is perpendicular to the high-velocity bipolar outflows. The 4'' diameter core of KjPn 8 has also been observed for 37 on-source hours at a wavelength of 6 cm with the MERLIN radio interferometer with an angular resolution of 005. The data have been combined with 4 hr of VLA-A observations at the same wavelength but where the resolution is 05. The most remarkable feature of these radio observations is the possible presence of a weak radio source embedded in the center of the H2 disk. It is thought plausible that this point source may be direct emission from the powerful, ionized wind of the central star that now drives this nebulosity.

VLA Observations of the Interacting‐Wind Binary System WR 147

Wolf-Rayet stars are losing mass via their strong winds. Some of them show thermal and nonthermal radio emission, and from this group, some have companions that are forming binary systems. It has been suggested that the synchrotron emission arises from the interaction region where the winds of the two stars collide. WR 147 seems to be one of these systems, having a companion and emitting thermal as well as nonthermal radiation. Besides, it has been suggested that it has an anisotropic wind. In this work, we present VLA observations that provide new information on this system. The thermal main component (WR 147S) shows a flux density increase of ~25% from 1995 to 1996. This could be due to a change in the wind parameters. The nonthermal wind-interaction zone (WR 147N) remained constant in flux. To first approximation, our maps are consistent with an isotropic wind from WR 147S. However, our results show possible faint condensations in the wind that have moved away by ~02 from WR 147S in a time interval of ~1.4 yr. This fact seems to indicate that we are dealing with an inhomogeneous wind. Finally, in a qualitative manner, the morphology of WR 147N resembles a shock like those modeled theoretically for interacting winds. In order to have a more quantitative description of the shock, we have fitted a curve that follows adequately the observed shape. This curve was obtained by fixing the ratio of momenta between the interacting stellar winds. In this way, we have derived a range of possible inclination angles of the system (45° ± 15°) by projecting the theoretical curve onto the plane of the sky. Thus, our results give further support to the interpretation of WR 147 as an interacting-wind binary system.

Ground-based microwave thermal sounding of the atmosphere

We apply the tensor transfer equation for downward radio emission in a weakly anisotropic medium and a generalized analysis of the generation conditions for such radiation to develope a unified approach to the solution of the problem on reconstruction of the atmospheric temperature prifile over a wide altitude range (from theoustasche layer to the mesosphere) using ground-based measurements of thermal radio emission. Radio emission in the oxygen spin—rotational band centered at the wavelength 5 mm was measured with radiometer equipment of various spectral resolutions. The capabilities of the remote sounding are illustrated by the results of the temperature profile reconstruction in theoustasche layer and troposphere using the radiometric measurements of the atmospheric radio emission, as well as by the results of numerical simulations of the radiometric experiment for the upper layers of the atmosphere. Trends in development of ground-based remote sounding of the temperature are discussed.