Wings Of Spectral Lines Research Articles

Yellow hypergiant V509 Cas: Stable in the ‘yellow void’

Context. The yellow hypergiant star V509 Cas is currently undergoing an extreme phase of evolution. Having experienced eruptive mass-loss outbursts in the 20th century, the star’s effective temperature reached record high values in the early 2000s. However, since then, the star’s behaviour has displayed an unprecedented level of stability. In spite of that, the star could be traversing through the ‘yellow void’ instability region. Aims. To describe the current evolutionary state of V509 Cas, we analysed its variability using photometric and spectroscopic data collected over recent years. By comparing our findings with historical records, we aim to determine whether the star’s surface shows signs of stabilisation. Additionally, we investigate the variability of emission components in the wings of certain spectral lines to highlight the contribution of the circumstellar gaseous disc to this phenomenon. Methods. Our spectroscopic monitoring observations were carried out at Tartu Observatory over the course of seven years, supplemented by echelle spectra obtained at the Nordic Optical Telescope, as well as publicly available photometric data from Gaia, AAVSO, and AAVSO’s Bright Star Monitor programme. We estimated the variability of effective temperature and radial velocity from the spectral time series and correlated it with the brightness variability of V509 Cas. Results. The results indicate that the star’s average brightness level has remained stable throughout the observed period, with an amplitude of variability ∼0.1 mag. While the amplitude of short-term temperature fluctuations has decreased compared to the early 2000s, the variability of the radial velocity remains similar to historical values from the early 20th century. Moreover, we show how the variable radial velocity affects the emission components in some absorption lines (e.g. Sc II) and how that follows the hypothesis of a disc surrounding the star.

The impact of spectral line wing cut-off: recommended standard method with application to MAESTRO opacity data base

Abstract When computing cross-sections from a line list, the result depends not only on the line strength, but also the line shape, pressure-broadening parameters, and line wing cut-off (i.e. the maximum distance calculated from each line centre). Pressure-broadening can be described using the Lorentz line shape, but it is known to not represent the true absorption in the far wings. Both theory and experiment have shown that far from the line centre, non-Lorentzian behaviour controls the shape of the wings and the Lorentz line shape fails to accurately characterize the absorption, leading to an underestimation or overestimation of the opacity continuum depending on the molecular species involved. The line wing cut-off is an often overlooked parameter when calculating absorption cross-sections, but can have a significant effect on the appearance of the spectrum since it dictates the extent of the line wing that contributes to the calculation either side of every line centre. Therefore, when used to analyse exoplanet and brown dwarf spectra, an inaccurate choice for the line wing cut-off can result in errors in the opacity continuum, which propagate into the modelled transit spectra, and ultimately impact/bias the interpretation of observational spectra, and the derived composition and thermal structure. Here, we examine the different methods commonly utilized to calculate the wing cut-off and propose a standard practice procedure (i.e. absolute value of 25 cm−1 for P ≤ 200 bar and 100 cm−1 for P > 200 bar) to generate molecular opacities which will be used by the open-access MAESTRO (Molecules and Atoms in Exoplanet Science: Tools and Resources for Opacities) data base. The pressing need for new measurements and theoretical studies of the far-wings is highlighted.

Prospects of Detecting Nonthermal Protons in Solar Flares via Lyman Line Spectroscopy: Revisiting the Orrall–Zirker Effect

Solar flares are efficient particle accelerators, with a substantial fraction of the energy released manifesting as nonthermal particles. While the role that nonthermal electrons play in transporting flare energy is well studied, the properties and importance of nonthermal protons are rather less well understood. This is in large part due to the paucity of diagnostics, particularly at the lower-energy (deka-keV) range of nonthermal proton distributions in flares. One means to identify the presence of deka-keV protons is by an effect originally described by Orrall & Zirker. In the Orrall–Zirker effect, nonthermal protons interact with ambient neutral hydrogen, and via charge exchange produce a population of energetic neutral atoms (ENAs) in the chromosphere. These ENAs subsequently produce an extremely redshifted photon in the red wings of hydrogen spectral lines. We revisit predictions of the strength of this effect using modern interaction cross sections, and numerical models capable of self-consistently simulating the flaring nonequilibrium ionization stratification, and the nonthermal proton distribution (and, crucially, their feedback on each other). We synthesize both the thermal and nonthermal emission from Ly α and Ly β, the most promising lines that may exhibit a detectable signal. These new predictions are weaker and more transient than prior estimates, but the effects should be detectable in fortuitous circumstances. We degrade the Ly β emission to the resolution of the Spectral Imaging of the Coronal Environment (SPICE) instrument on board Solar Orbiter, demonstrating that though likely difficult, it should be possible to detect the presence of nonthermal protons in flares observed by SPICE.

Explosive Events in the Quiet Sun Near and Beyond the Solar Limb Observed with the Interface Region Imaging Spectrograph (IRIS)

We study point-like explosive events (EE), characterized by emission in the far wings of spectral lines, in a quiet region near the South Pole, using Interface Region Imaging Spectrograph (IRIS) spectra at two slit positions, slit-jaw (SJ) observations, and Atmospheric Imaging Assembly (AIA) images. The events were best visible in Si iv spectra; they were weak in SJs, occasionally visible in 1600 Å and 304 Å AIA images, and invisible in higher temperature AIA images. We identified EEs from position–time images in the far wings of the Si iv lines and measured their distance from the limb. A Gaussian model of the height distribution showed that EEs occur in a narrow (0.9′′) height range, centered at 3.2′′ above the continuum limb at 2832.0 Å. On the disk, we found that they occur in network boundaries. Further, we studied the line profiles of two bright EEs above the limb and one on the disk. We found that what appears as broad-band emission is actually a superposition of 2 – 3 narrow-band Gaussian components with well-separated line profiles, indicating that material is expelled towards and/or away from the observer in discrete episodes in time and in space. The expelled plasma accelerates quickly, reaching line-of-sight (LOS) velocities up to 90 km s−1. Overall, the motion was practically along the LOS, as the velocity on the plane of the sky was small. In some cases, tilted spectra were observed that could be interpreted in terms of rotating motions of up to 30 km s−1. We did not find any strong absorption features in the wing of the Si iv lines, although in one case, a very weak absorption feature was detected. No motions indicative of jets were detected in SJ or AIA images. Reconnection in an asymmetric magnetic-field geometry, in the middle or near the top of small loops, is a plausible explanation of their observational characteristics.

New explanation of opposite position angles in red and blue wings of spectral lines

ABSTRACTWe consider the emission of resonance line radiation from rotating circular accretion disc with the progressive increasing height (the inclined ring). Our theory can also be applied to the rotating spot-like sources of resonance radiation. We assume that the atmosphere of inclined ring is homogeneous. In this case, the every part of ring emits the radiation with the wave electric field oscillations perpendicular to plane $(\boldsymbol{nN^{\prime }})$, where ${\boldsymbol{n}}$ is the direction to a telescope and $\boldsymbol{N^{\prime }}$ is the normal to considered local surface of a ring. Geometrical consideration shows that the radiation polarization angle χ (frequently denoted as position angle or PA) depends on the inclination angle α of the ring relative to the central plane of accretion disc, the inclination angle θ of the central plane of accretion disc with the normal $\boldsymbol{N}$, and on azimuthal angle φ of radiating part on the ring. The right and left parts of a ring according to the symmetry plane $(\boldsymbol{nN})$ give rise to opposite polarization angles. For rotating accretion disc, due to Doppler’s effect, this means that polarization angles have opposite signs in the red and blue wings of emerging spectral line radiation. Such behaviour is observed in many objects.

Spectral interferences and their correction in atomic emission spectral analysis

The main sources of spectral interferences in atomic emission spectral analysis (AESA) are considered, including both wide-range (bremsstrahlung and recombination continuum, radiation of hot condensed particles and electrode ends, scattered light in the spectrometer, overlapping of the analytical line by the wings of the neighbor strong spectral lines of interfering elements, imposition of the components of molecular bands with the very close lines) and narrow-band (partial or complete overlapping of the analytical line with atomic or ionic lines of the sample elements, electrodes and discharge atmosphere; superposition of spectra from higher orders of reflection in conventional diffraction spectrometers and from neighboring orders in two-dimensional echelle spectrometers). The features of their manifestation in various sources of spectrum excitation (flames, DC arc, spark discharges, arc plasma discharges, inductively coupled plasma, microwave plasma, low-pressure electric discharges, laser spark) are considered. The possibilities of reducing the level of spectral interferences or elimination of the spectral noise at the stage of design and manufacturing of AESA devices, as well as upon selecting and adjusting of operation conditions of the analysis are shown. Much attention is paid to the most easily implemented in practice off-peak correction of wide-range spectral interferences. The modern methods of background correction under the spectral peak (under-peak) using a software for atomic emission spectrometers and providing creation of various mathematical models of the background signal in the vicinity of the analytical line at the stage of developing a specific AESA technique are considered. The issues of the choice of spectral lines for analytical measurements, tables and atlases of spectral lines, electronic databases used for this purpose are considered in detail. Specific features of application of the method of inter-element correction with direct spectral overlapping of the lines are given. The operating sequence for taking into account spectral interferences when developing the analysis techniques is proposed.

Theory and simulation of spectral line broadening by exoplanetary atmospheric haze

Atmospheric haze is the leading candidate for the flattening of expolanetary spectra, as it's also an important source of opacity in the atmospheres of solar system planets, satellites, and comets. Exoplanetary transmission spectra, which carry information about how the planetary atmospheres become opaque to stellar light in transit, show broad featureless absorption in the region of wavelengths corresponding to spectral lines of sodium, potassium and water. We develop a detailed atomistic model, describing interactions of atomic or molecular radiators with dust and atmospheric haze particulates. This model incorporates a realistic structure of haze particulates from small nano-size seed particles up to sub-micron irregularly shaped aggregates, accounting for both pairwise collisions between the radiator and haze perturbers, and quasi-static mean field shift of levels in haze environments. This formalism can explain large flattening of absorption and emission spectra in haze atmospheres and shows how the radiator - haze particle interaction affects the absorption spectral shape in the wings of spectral lines and near their centers. The theory can account for nearly all realistic structure, size and chemical composition of haze particulates and predict their influence on absorption and emission spectra in hazy environments. We illustrate the utility of the method by computing shift and broadening of the emission spectra of the sodium D line in an argon haze. The simplicity, elegance and generality of the proposed model should make it amenable to a broad community of users in astrophysics and chemistry.

Concentration measurements by intracavity laser absorption spectroscopy for the case of strongly overlapped spectra

The feasibility of fiber laser intracavity absorption spectroscopy (FLICAS) for accurate measurement of pressure and concentration of CO and $$\hbox {CO}_2$$ at elevated pressures (up to 6.5 atm) was studied. FLICAS is a very sensitive absorption spectroscopy method which allows for simultaneous measurement of several species even in harsh environment, since it is not sensitive to broadband optical losses caused by particle scattering and absorption, optical window contamination, etc. However, the wings of spectral lines can overlap at elevated pressures, resulting in broadband absorption which is not monitored by FLICAS. It makes the direct evaluation of baseline challenging. Nevertheless, fitting the lineshapes of individual lines using the HITRAN database allows for recovering of the baseline position and shape in some cases. The CO and $$\hbox {CO}_2$$ spectra were studied under controlled conditions in the spectral range of 6350–6385 cm $$^{-1}$$ and processed by custom LabVIEW code to take into account the overlapping line effect. This procedure allows for accurate determination of the pressure and the CO concentration at least up to 6.5 atm. More dense and line-rich $$\hbox {CO}_2$$ spectrum allows correct pressure evaluation up to 6.5 atm, albeit with higher uncertainty. Accurate measurements of the $$\hbox {CO}_2$$ concentration are possible only up to about 4 atm.

Convective blueshifts in the solar atmosphere

Context. The solar convection manifests as granulation and intergranulation at the solar surface. In the photosphere, convective motions induce differential Doppler shifts to spectral lines. The observed convective blueshift varies across the solar disk. Aim. We focus on the impact of solar convection on the atmosphere and aim to resolve its velocity stratification in the photosphere. Methods. We performed high-resolution spectroscopic observations of the solar spectrum in the 6302 Å range with the Laser Absolute Reference Spectrograph at the Vacuum Tower Telescope. A laser frequency comb enabled the calibration of the spectra to an absolute wavelength scale with an accuracy of 1 m s−1. We systematically scanned the quiet Sun from the disk center to the limb at ten selected heliocentric positions. The analysis included 99 time sequences of up to 20 min in length. By means of ephemeris and reference corrections, we translated wavelength shifts into absolute line-of-sight velocities. A bisector analysis on the line profiles yielded the shapes and convective shifts of seven photospheric lines. Results. At the disk center, the bisector profiles of the iron lines feature a pronounced C-shape with maximum convective blueshifts of up to −450 m s−1 in the spectral line wings. Toward the solar limb, the bisectors change into a “\”-shape with a saturation in the line core at a redshift of +100 m s−1. The center-to-limb variation of the line core velocities shows a slight increase in blueshift when departing the disk center for larger heliocentric angles. This increase in blueshift is more pronounced for the magnetically less active meridian than for the equator. Toward the solar limb, the blueshift decreases and can turn into a redshift. In general, weaker lines exhibit stronger blueshifts. Conclusions. Best spectroscopic measurements enabled the accurate determination of absolute convective shifts in the solar photosphere. We convolved the results to lower spectral resolution to permit a comparison with observations from other instruments.

Space Weather Parameters: Modeling and Prediction from the Data of Groundbased Observations of Solar Activity

We consider the prospects for developing a forecast system for space weather (SPW) parameters with the use of home facilities for groundbased observations of solar activity. The space weather forecast can be conventionally divided into three components: (i) the prediction of recurrent, slowly changing events connected with the topology of the large-scale magnetic field, (ii) the estimation of fluxes of UV and high-energy radiation, and (iii) the observation of high-speed phenomena, such as solar flares and eruption processes, and the prediction of their consequences at the the Earth’s orbit. At present, to predict recurrent events, data from regular observations of the large-scale field of the Sun by the solar telescope–magnetographs for operative (realtime) prediction (STOP) are effectively used. To estimate high-energy fluxes, to register eruption events, and to estimate their geoefficiency, data from the patrol optical telescope–spectrographs may be used. Patrol telescopes operate in automatic mode and register the processes with an interval of approximately one minute. To detect eruption processes, we propose a method based on the difference between the intensity values in the wings of chromospheric spectral lines. The results of the use of the observational complex of the Kislovodsk Mountain Astronomical Station for the SW forecast are considered in the paper.

Observations of long-period oscillations of the solar active regions in the visible and UV spectral intervals

The variation of intensity in spectral line wings, which was obtained from observations of the patrol telescope at the Kislovodsk Mountain Astronomical Station of the Pulkovo Observatory, Russian Academy of Science (KMAS) and the Interface Region Imaging Spectrograph (IRIS) space observatory, are considered. A series of observations lasting a few hours near the solar active regions, in which both short- and longperiod oscillations were observed simultaneously during 2014–2015, are analyzed. It is found out that oscillations with a period of 3–5 min can exist at one time and in one place with oscillations with a period of about 100 min. The amplitude of long-period oscillations can be comparable with that for short-period oscillations. The conditions for excitation of the wave processes are considered. Oscillations with a period of 100 min have a weak dependence on the area of the active region.

Berry phases and profiles of line wings and rainbow satellites induced by optical collisions

The concept of Berry phase is included in an analysis of the intensity distribution in far wings of pressure-broadened spectral lines emitted or absorbed by atoms placed in an external cone-rotating electric field. Particular attention is focused on frequency regions where rainbow satellite bands appear. A classical-path treatment that employs the time-dependent Schr\"odinger equation is used to derive an expression for the line shape, and it uses a dipole transition moment calculated with quasimolecular wave functions given by the Berry version of the adiabatic approximation. It is found that in the presence of an external rotating electric field, the intensity distribution in far wings can be expressed in terms of the universal line shape function of the unified Franck-Condon theory once energy shifts due to Stark and Berry effects are taken into account. We show that the influence of Berry phase in the profiles of the far wings can be manifested either in the form of deviations of observed profiles from the quasistatic distribution or the appearance of additional features in the vicinity of the maximum of the rainbow satellite band. As an example, the modification of the rainbow satellite at 162.3 nm in the red wing of the self-broadened Lyman-$\ensuremath{\alpha}$ line of hydrogen, caused by an external rotating electric field, is considered.

THE DYNAMICS OF RAPID REDSHIFTED AND BLUESHIFTED EXCURSIONS IN THE SOLAR HαLINE

We analyse high temporal and spatial resolution time-series of spectral scans of the Halpha line obtained with the CRisp Imaging SpectroPolarimeter (CRISP) instrument mounted on the Swedish Solar Telescope. The data reveal highly dynamic, dark, short-lived structures known as Rapid Redshifted and Blueshifted Excursions (RREs, RBEs) that are on-disk absorption features observed in the red and blue wings of spectral lines formed in the chromosphere. We study the dynamics of RREs and RBEs by tracking their evolution in space and time, measuring the speed of the apparent motion, line-of-sight Doppler velocity, and transverse velocity of individual structures. A statistical study of their measured properties shows that RREs and RBEs have similar occurrence rates, lifetimes, lengths, and widths. They also display non-periodic, non-linear transverse motions perpendicular to their axes at speeds of 4 - 31 km/s. Furthermore, both types of structures either appear as high speed jets and blobs that are directed outwardly from a magnetic bright point with speeds of 50 - 150 km/s, or emerge within a few seconds. A study of the different velocity components suggests that the transverse motions along the line-of-sight of the chromospheric flux tubes are responsible for the formation and appearance of these redshifted/blueshifted structures. The short lifetime and fast disappearance of the RREs/RBEs suggests that, similar to type II spicules, they are rapidly heated to transition region or even coronal temperatures. We speculate that the Kelvin-Helmholtz instability triggered by observed transverse motions of these structures may be a viable mechanism for their heating.

UNRESOLVED FINE-SCALE STRUCTURE IN SOLAR CORONAL LOOP-TOPS

New and advanced space-based observing facilities continue to lower the resolution limit and detect solar coronal loops in greater detail. We continue to discover even finer sub-structures within coronal loop cross sections, in order to understand the nature of the solar corona. Here, we push this lower limit further to search for the finest coronal loop sub-structures, through taking advantage of the resolving power of the Swedish 1- m Solar Telescope (SST) / CRisp Imaging Spectro-Polarimeter (CRISP), together with co-observations from the Solar Dynamics Observatory (SDO) / Atmospheric Image Assembly (AIA). High resolution imaging of the chromospheric H-alpha 656.28 nm spectral line core and wings can, under certain circumstances, allow one to deduce the topology of the local magnetic environment of the solar atmosphere where its observed. Here, we study post-flare coronal loops, which become filled with evaporated chromosphere that rapidly condenses into chromospheric clumps of plasma (detectable in H-alpha) known as a coronal rain, to investigate their fine-scale structure. We identify, through analysis of three datasets, large-scale catastrophic cooling in coronal loop-tops and the existence of multi-thermal, multi-stranded sub-structures. Many cool strands even extend fully-intact from loop-top to foot-point. We discover that coronal loop fine-scale strands can appear bunched with as many as 8 parallel strands, within an AIA coronal loop cross-section. The strand number density vs cross-sectional width distribution, as detected by CRISP within AIA-defined coronal loops, most-likely peaks at well below 100 km and currently 69% of the sub-structure strands are statistically unresolved in AIA coronal loops.

Water in the gas phase

Water in the gas phase

Collision frequencies in density-matrix kinetic equations describing nonlinear effects in the wings of spectral lines

Using the eikonal approximation, we have calculated effective collision frequencies in density-matrix kinetic equations describing nonlinear effects in the wings of spectral lines. We have established the relation between the probabilities of absorption and stimulated emission and the characteristics of the radiation and elementary scattering event. The example of the power interaction potential shows that quantum mechanical calculation of the collision frequencies in the eikonal approximation and previously known spectral line wing theory give similar results for the probability of radiation absorption.

Kinetic equations for a density matrix describing nonlinear effects in spectral line wings

Kinetic quantum equations are derived for a density matrix with collision integrals describing nonlinear effects in spectra line wings. These equations take into account the earlier established inequality of the spectral densities of Einstein coefficients for absorption and stimulated radiation emission by a two-level quantum system in the far wing of a spectral line in the case of frequent collisions. The relationship of the absorption and stimulated emission probabilities with the characteristics of radiation and an elementary scattering event is found.

Population inversion on vibrational transitions of molecules under nonresonant optical excitation

The possibility of obtaining a population inversion on vibrational transitions of molecules through nonlinear effects when the pump radiation is absorbed in the wings of spectral lines is investigated theoretically. We show that a population inversion can be produced in molecules on vibrational transitions when intense pump radiation is absorbed in the blue wing of the R branch of the vibrational-rotational molecular spectrum. This effect is related to inequality of the probabilities of the absorption and stimulated emission of radiation and is attributable to collisional transitions between rotational levels. We have ascertained that the larger the rotational constant of the molecule and the higher the pump radiation intensity, the higher the effective frequency of the collisions that give rise to a population inversion. Using the carbon monoxide (CO) molecule as an example, we show that when intense (∼1010 W cm−2) pump radiation is absorbed in the blue wing of the R branch, a noticeable population inversion can be produced and the gain at the center of the R and P branches of the molecular spectrum can reach 0.011 and 0.250 cm−1 at temperatures T = 300 and 100 K, respectively.

A Four-Stokes-Parameter Spectral Line Polarimeter at the Caltech Submillimeter Observatory

ABSTRACTWe designed and built a new Four-Stokes-Parameter spectral line Polarimeter (FSPPol) for the Caltech Submillimeter Observatory (CSO). The simple design of FSPPol does not include any mirrors or optical components to redirect or reimage the radiation beam and simply transmits the beam to the receiver through its retarder plates. FSPPol is currently optimized for observation in the 200–260 GHz range and measures all four Stokes parameters, I, Q, U, and V. The very low level of instrument polarization makes it possible to obtain reliable measurements of the Goldreich-Kylafis effect in molecular spectral lines. Accordingly, we measured a polarization fraction of a few percent in the spectral line wings of 12CO (J = 2 → 1) in Orion KL/IRc2, which is consistent with previous observations. We also used FSPPol to study the Zeeman effect in the N = 2 → 1 transition of CN in DR21(OH) for the first time. At this point we cannot report a Zeeman detection, but more observations are ongoing.

Estimates of molecular absorption cross-sections in mercury plasmas at very high pressures using self-reversed line diagnostics

Plasmas containing mercury at pressures >100 bar are used as sources of extremely high radiance. High-resolution measurements have been made of the absolute spectral radiance of a plasma in a line of sight through its centre. Detailed comparisons of the Hg 546 nm calculated line shape with measurements provide information about the pressure, centre temperature and the temperature profile. Results are given for an arc at a pressure of 160 ± 10 bar and maximum temperature of 8350 ± 100 K. Radiation transport calculations show that absorption by mercury molecules increases progressively with decreasing wavelength towards the UV. By comparing calculations of self-reversed line maxima, a temperature-independent cross-section for Hg2 absorption has been derived. Use of this, with other published data for line and continuum radiation in radiation transport calculations, yields calculated spectra in excellent agreement with the experiment except in the extreme wings of spectral lines. The calculations can be used to predict behaviour at other pressures; at all the pressures and most wavelengths considered the plasma has appreciable optical depth. Uncertainties in pressure measurement remain a problem with this type of plasma. The 313, 365 and 435 nm line radiances are absorbed to various degrees by mercury molecules. Their radiance ratios are very sensitive to pressure and could be used, if calibrated, to provide a more satisfactory measure of pressure.