T‐matrix studies of aerosol particle shape effects on IR resonance spectral line profiles and comparison with an experiment

Context. Physical and chemical properties, such as kinetic temperature, volume density, and molecular composition of interstellar clouds are inherent in their line spectra at submillimeter wavelengths. Therefore, the spectral line profiles could be used to estimate the physical conditions of a given source. Aims. We present a new bottom-up approach, based on machine learning (ML) algorithms, to extract the physical conditions in a straightforward way from the line profiles without using radiative transfer equations. Methods. We simulated, for the typical physical conditions of dense molecular clouds and star-forming regions, the emission in spectral lines of the two isomers HCN and HNC, from J = 1–0 to J = 5–4 between 30 and 500 GHz, which are commonly observed in dense molecular clouds and star forming regions. The generated data cloud distribution has been parametrised using the line intensities and widths to enable a new way to analyse the spectral line profiles and to infer the physical conditions of the region. The line profile parameters have been charted to the HNC/HCN ratio and the excitation temperature of the molecule(s). Three ML algorithms have been trained, tested, and compared aiming to unravel the excitation conditions of HCN and HNC and their abundance ratio. Results. Machine learning results obtained with two spectral lines, one for each isomer HCN and HNC, have been compared with the local thermodynamic equilibrium (LTE) analysis for the cold source R CrA IRS 7B. The estimate of the excitation temperature and of the abundance ratio, in this case considering the two spectral lines, is in agreement with our LTE analysis. The complete optimisation procedure of the algorithms (training, testing, and prediction of the target quantities) have the potential to predict interstellar cloud properties from line profile inputs at lower computational cost than before. Conclusions. It is the first time that the spectral line profiles are mapped according to the physical conditions charting the ratio of two isomers and the excitation temperature of the molecules. In addition, a bottom-up approach starting from a set of simulated spectral data at different physical conditions is proposed to interpret line observations of interstellar regions and to estimate their physical conditions. This new approach presents the potential relevance to unravel hidden interstellar conditions with the use of ML methods.

When low-mass stars form, the collapsing cloud of gas and dust goes through several stages which are usually characterized by the shape of their spectral energy distributions. Such classification is based on the cloud morphology only and does not address the dynamical state of the object. In this paper we investigate the initial cloud collapse and subsequent disk formation through the dynamical behavior as reflected in the sub-millimeter spectral emission line profiles. If a young stellar object is to be characterized by its dynamical structure it is important to know how accurately information about the velocity field can be extracted and which observables provide the best description of the kinematics. Of particular interest is the transition from infalling envelope to rotating disk, because this provides the initial conditions for the protoplanetary disk, such as mass and size. We use a hydrodynamical model, describing the collapse of a core and formation of a disk, to produce synthetic observables which we compare to calculated line profiles of a simple parameterized model. Because we know the velocity field from the hydrodynamical simulation we can determine in a quantitative way how well our best-fit parameterized velocity field reproduces the original. We use a molecular line excitation and radiation transfer code to produce spectra of both our hydro dynamical simulation as well as our parameterized model. We find that information about the velocity field can reasonably well be derived by fitting a simple model to either single-dish lines or interferometric data, but preferentially by using a combination of the two. Our result shows that it is possible to establish relative ages of a sample of young stellar objects using this method, independently of the details of the hydrodynamical model.

Some supernova (SN) explosions show evidence for an interaction with a pre-existing nonspherically symmetric circumstellar medium (CSM) in their light curves, spectral line profiles, and polarization signatures. The origin of this aspherical CSM is unknown, but binary interactions have often been implicated. To better understand the connection with binary stars and to aid in the interpretation of observations, we performed two-dimensional axisymmetric hydrodynamic simulations where an expanding spherical SN ejecta initialized with realistic density and velocity profiles collide with various aspherical CSM distributions. We consider CSM in the form of a circumstellar disk, colliding wind shells in binary stars with different orientations and distances from the SN progenitor, and bipolar lobes representing a scaled down version of the Homunculus nebula ofηCar. We study how our simulations map onto observables, including approximate light curves, indicative spectral line profiles at late times, and estimates of a polarization signature. We find that the SN–CSM collision layer is composed of normal and oblique shocks, reflected waves, and other hydrodynamical phenomena that lead to acceleration and shear instabilities. As a result, the total shock heating power fluctuates in time, although the emerging light curve might be smooth if the shock interaction region is deeply embedded in the SN envelope. SNe with circumstellar disks or bipolar lobes exhibit late-time spectral line profiles that are symmetric with respect to the rest velocity and relatively high polarization. In contrast, SNe with colliding wind shells naturally lead to line profiles with asymmetric and time-evolving blue and red wings and low polarization. Given the high frequency of binaries among massive stars, the interaction of SN ejecta with a pre-existing colliding wind shell must occur and the observed signatures could be used to characterize the binary companion.

The orientation dynamics of small prolate and oblate spheroids in linear shear flows

It is well known that, under inertialess conditions, the orientation vector of a torque-free neutrally buoyant spheroid in an ambient simple shear flow rotates along so-called Jeffery orbits, a one-parameter family of closed orbits on the unit sphere centred around the direction of the ambient vorticity (Jeffery, Proc. R. Soc. Lond. A, vol. 102, 1922, pp. 161–179). We characterize analytically the irreversible drift in the orientation of such torque-free spheroidal particles of an arbitrary aspect ratio, across Jeffery orbits, that arises due to weak inertial effects. The analysis is valid in the limit $Re,St\ll 1$, where $Re=(\dot{{\it\gamma}}L^{2}{\it\rho}_{f})/{\it\mu}$ and $St=(\dot{{\it\gamma}}L^{2}{\it\rho}_{p})/{\it\mu}$ are the Reynolds and Stokes numbers, which, respectively, measure the importance of fluid inertial forces and particle inertia in relation to viscous forces at the particle scale. Here, $L$ is the semimajor axis of the spheroid, ${\it\rho}_{p}$ and ${\it\rho}_{f}$ are the particle and fluid densities, $\dot{{\it\gamma}}$ is the ambient shear rate, and ${\it\mu}$ is the suspending fluid viscosity. A reciprocal theorem formulation is used to obtain the contributions to the drift due to particle and fluid inertia, the latter in terms of a volume integral over the entire fluid domain. The resulting drifts in orientation at $O(Re)$ and $O(St)$ are evaluated, as a function of the particle aspect ratio, for both prolate and oblate spheroids using a vector spheroidal harmonics formalism. It is found that particle inertia, at $O(St)$, causes a prolate spheroid to drift towards an eventual tumbling motion in the flow–gradient plane. Oblate spheroids, on account of the $O(St)$ drift, move in the opposite direction, approaching a steady spinning motion about the ambient vorticity axis. The period of rotation in the spinning mode must remain unaltered to all orders in $St$. For the tumbling mode, the period remains unaltered at $O(St)$. At $O(St^{2})$, however, particle inertia speeds up the rotation of prolate spheroids. The $O(Re)$ drift due to fluid inertia drives a prolate spheroid towards a tumbling motion in the flow–gradient plane for all initial orientations and for all aspect ratios. Interestingly, for oblate spheroids, there is a bifurcation in the orientation dynamics at a critical aspect ratio of approximately 0.14. Oblate spheroids with aspect ratios greater than this critical value drift in a direction opposite to that for prolate spheroids, and eventually approach a spinning motion about the ambient vorticity axis starting from any initial orientation. For smaller aspect ratios, a pair of non-trivial repelling orbits emerge from the flow–gradient plane, and divide the unit sphere into distinct basins of orientations that asymptote to the tumbling and spinning modes. With further decrease in the aspect ratio, these repellers move away from the flow–gradient plane, eventually coalescing onto an arc of the great circle in which the gradient–vorticity plane intersects the unit sphere, in the limit of a vanishing aspect ratio. Thus, sufficiently thin oblate spheroids, similar to prolate spheroids, drift towards an eventual tumbling motion irrespective of their initial orientation. The drifts at $O(St)$ and at $O(Re)$ are combined to obtain the drift for a neutrally buoyant spheroid. The particle inertia contribution remains much smaller than the fluid inertia contribution for most aspect ratios and density ratios of order unity. As a result, the critical aspect ratio for the bifurcation in the orientation dynamics of neutrally buoyant oblate spheroids changes only slightly from its value based only on fluid inertia. The existence of Jeffery orbits implies a rheological indeterminacy, and the dependence of the suspension shear viscosity on initial conditions. For prolate spheroids and oblate spheroids of aspect ratio greater than 0.14, inclusion of inertia resolves the indeterminacy. Remarkably, the existence of the above bifurcation implies that, for a dilute suspension of oblate spheroids with aspect ratios smaller than 0.14, weak stochastic fluctuations (residual Brownian motion being analysed here as an example) play a crucial role in obtaining a shear viscosity independent of the initial orientation distribution. The inclusion of Brownian motion leads to a new smaller critical aspect ratio of approximately 0.013. For sufficiently large $Re\,Pe_{r}$, the peak in the steady-state orientation distribution shifts rapidly from the spinning- to the tumbling-mode location as the spheroid aspect ratio decreases below this critical value; here, $Pe_{r}=\dot{{\it\gamma}}/D_{r}$, with $D_{r}$ being the Brownian rotary diffusivity, so that $Re\,Pe_{r}$ measures the relative importance of inertial drift and Brownian rotary diffusion. The shear viscosity, plotted as a function of $Re\,Pe_{r}$, exhibits a sharp transition from a shear-thickening to a shear-thinning behaviour, as the oblate spheroid aspect ratio decreases below 0.013. Our results are compared in detail to earlier analytical work for limiting cases involving either nearly spherical particles or slender fibres with weak inertia, and to the results of recent numerical simulations at larger values of $Re$ and $St$.

We have observed the millimeter-wave rotational spectral lines of CH3CHO, H2CCO, cyclopropenone, and H2CO toward the cyanoployyne peak of Taurus Molecular Cloud-1 (TMC-1 CP). The spectral line profile of CH3CHO is found to reveal a well-separated double peak. It is similar to the line profile of CH3OH, but is much different from those of carbon-chain molecules and C34S. The different line profiles mean different distributions along the line of sight. The similarity of the spectral line profiles between CH3CHO and CH3OH suggests that CH3CHO is mainly formed on dust grains as CH3OH or through gas-phase reactions starting from CH3OH. On the other hand, the spectral line profiles of H2CCO and cyclopropenone are rather similar to those of carbon-chain molecules and C34S, implying their gas-phase productions. H2CO shows a composite spectral line profile reflecting the contributions of both gas-phase and grain-surface productions. In addition, we have detected the spectral lines of CH3CHO and HCOOCH3 toward the methanol peak near TMC-1 CP. We have also tentatively detected one line of (CH3)2O. Considering the chemical youth of TMC-1, the present results indicate that fairly complex organic species have already been formed in the early evolutionary phase of starless cores. TMC-1 is thus recognized as a novel source where formation processes of complex organic molecules can be studied on the basis of the line profiles.

Optical Bessel tractor beams, designed to produce a negative pulling force on a particle, are gaining increased attention for applications in noncontact remote sampling, particle manipulation, and handling, to name some examples. In the long-wavelength (Rayleigh) limit, known also as the electric dipole approximation, earlier investigations demonstrated that a zeroth-order Bessel beam incident upon a passive dielectric sphere (i.e., no radiating sources in its core) always acts as a repulsor beam, which causes the particle to be pushed away from the source in the forward direction of the linear momentum. In contrast to what has already been established, this work shows that the incident wave field can act as a tractor beam (where a small spheroid is pulled backwards towards the source due to a negative attractive force) in the dipole approximation (Rayleigh) limit, provided that the particle is made of an active material, i.e., a dielectric spheroid acting as an oscillating source for which the extinction energy efficiency is negative. Numerical computations for the Cartesian components of the optical radiation force on active prolate and oblate spheroids with arbitrary orientation are performed. Emphasis is placed on the emergence of the tractor beam behavior and its dependence upon the half-cone angle, the polarization type of the incident beam, the spheroid aspect ratio, as well as its orientation in space. The analysis is extended to calculate the Cartesian components of the spin radiation torque, which causes a rotation of the spheroid around its center of mass in either the counterclockwise or the clockwise (negative) direction of spinning. Unlike the case of a sphere, the optical spin torque arises for a nonabsorptive oblate or prolate spheroid with arbitrary orientation in the field of a zeroth-order Bessel beam. Potential applications in optically engineered metamaterials, optical tractor beams, tweezers, particle manipulation, rotation, and handling would benefit from the results of this study.

Acoustic scattering of a Bessel vortex beam by a rigid fixed spheroid

Radar parameters simulation for populations of spherical and non-spherical hydrometeors: dependence on size distributions, shapes and composition

A CCD-line module is built to upgrade conventional spectrograph to the CCD ability of advanced data acquisition as well as a cost-effective solution to the plasma diagnostic spectroscopy measurement needs. The CCD module is adapted to the ISP-51 (Lomo-Russia) spectrograph, used for light dispersion and as a wavelength pre-selector when a Fabry-Perot interferometer is positioned into the parallel optical path of the spectrograph. A high-resolution spectrometer system is developed both computerized and easy to maintain. The main advantage over conventional method of spectral line profile registration is achievement of single-shot capability. Thus the line profile distortions caused by intensity fluctuations, which occur during the scanning process are eliminated. The capability of the designed spectrometer are presented with the measurements of the resolved superfine structure of the Cd I 480 nm spectral line profile as well as analyzing Ar I 738.4 nm spectral line profile broadening resulting with the gas temperature determination of argon inductively coupled plasma at low pressure and applied power.

In this study we extract the deep features and investigate the compression of the Mg II k spectral line profiles observed in quiet Sun regions by NASA's IRIS satellite. The data set of line profiles used for the analysis was obtained on April 20th, 2020, at the center of the solar disc, and contains almost 300,000 individual Mg II k line profiles after data cleaning. The data are separated into train and test subsets. The train subset was used to train the autoencoder of the varying embedding layer size. The early stopping criterion was implemented on the test subset to prevent the model from overfitting. Our results indicate that it is possible to compress the spectral line profiles more than 27 times (which corresponds to the reduction of the data dimensionality from 110 to 4) while having a 4 DN (Data Number) average reconstruction error, which is comparable to the variations in the line continuum. The mean squared error and the reconstruction error of even statistical moments sharply decrease when the dimensionality of the embedding layer increases from 1 to 4 and almost stop decreasing for higher numbers. The observed occasional improvements in training for values higher than 4 indicate that a better compact embedding may potentially be obtained if other training strategies and longer training times are used. The features learned for the critical four-dimensional case can be interpreted. In particular, three of these four features mainly control the line width, line asymmetry, and line dip formation respectively. The presented results are the first attempt to obtain a compact embedding for spectroscopic line profiles and confirm the value of this approach, in particular for feature extraction, data compression, and denoising.

view Abstract Citations (56) References (25) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Spectral Line Profiles and Luminosities of Astrophysical Water Masers Nedoluha, Gerald E. ; Watson, William D. Abstract The spectral line narrowing and rebroadening that occurs for astrophysical masers as a function of the emergent radiative flux is calculated for the prominent, 22 GHz masing transition of water. The increased line breadths due to hyperfine structure lead to reliable, essentially model-independent upper limits to the emergent flux that tend to be lower than other estimates for these masers. For many 22 GHz masers, including the outbursts in W49 and Orion, the observed line breadths are less than 0.9 km/s. For these, the upper limit to the emergent maser flux is 10 to the 10th K-sr when expressed in terms of the brightness temperature and the solid angle for beaming. It is concluded that the extreme brightness of the interstellar water masers is due to a high degree of beaming and not to more effective pumping. Publication: The Astrophysical Journal Pub Date: February 1991 DOI: 10.1086/185932 Bibcode: 1991ApJ...367L..63N Keywords: Interstellar Masers; Luminosity; Spectral Line Width; Water Masers; Brightness Temperature; Circular Polarization; Computational Astrophysics; Interstellar Magnetic Fields; Astrophysics; LINE PROFILES; MASERS full text sources ADS |

This paper applies the singularity method to obtain analytic solutions for oblate spheroids in Stokes flow, and to obtain numerical results for prolate and oblate spheroids undergoing oscillatory translation, and oscillatory rotation. To apply the method to oblate spheroids, singularities are placed along an imaginary focal length. A novel method is used to determine the hydrodynamic torque by deriving Green’s functions for torque for the unsteady rotlet, stresslet, and potential quadrupole. The results agree with analytic solutions for high and low frequencies, the results of previous studies, and results calculated using the boundary element method.

Spectral line profiles are used to process experimental spectra when solving the inverse problem of computing the collisional parameters of the profiles [1]. The difference in their shapes is due to different physical conditions (hard/soft collisions, high/low pressures, etc.). Numerous different profiles are used in the study of the spectral line parameters of carbon dioxide, methane, methyl halides, and other molecules. The diversity of the line profiles used in the systematization of spectral line parameters adds complexity to the structures of data available in information systems and to the structures of individuals involved in ontological descriptions of the spectral line properties, which characterize the line profiles. A brief classification of spectral line profiles and their parameters is given, and the results of the systematization of spectral data relating to different line profiles used in processing carbon dioxide spectra are presented. The line profiles available in the library are described, and a system is built for importing spectral line parameters derived from the solution of the direct and inverse problems. Computer software for an automatic description of the properties of the solutions imported has been developed. The basic properties of the spectral data compiled in the W@DIS information system provide a description of the outcome of the imported data quality assessment.

The spectral line profiles of ionized emitters in plasmas play an important role in the calculation of opacity, for short-wavelength laser studies, and for the diagnostics of inertial confinement fusion plasmas. Sophisticated theoretical methods and modeling have been advanced and applied in recent years to calculate spectral line profiles in the limits where broadening by electron collisions or by ion microfield dominates. Here, the authors describe recent measurements of spectral line profiles of a z-pinch experiment employing precision plasma diagnostic techniques. In particular, the electron-collisional-broadened 2s--2p transitions in B{sub III} have been investigated because their line profiles provide an excellent test for electron-impact line shape theories and electron collision strength calculations. Although they find good agreement with semiclassical calculations, a factor of two discrepancy with the most elaborate quantum-mechanical five-state close coupling calculations is observed. They discuss the experimental error estimates of the various measured quantities and show that the observed discrepancy can not be explained by experimental shortcomings. They further discuss measurements of non-isolated spectral lines of some {Delta}n = 1 transitions in C{sub IV}--O{sub VI}. For these transitions ion broadening dominates. Excellent agreement for the whole line profile with line broadening calculations is obtained for all cases only when including ion dynamic effects. The latter are calculated using the frequency-fluctuation model and account for about 10--25% of the line width of the considered ions.