Formation Of Emission Lines Research Articles

Biosignature line ratios of [P ii] in exoplanetary and nebular environments

ABSTRACT Being the backbone element of deoxyribonucleic acid, phosphorus is a key component in the search for life in the Universe. To aid in its detection, we present line emissivity ratios for the five lowest-lying forbidden [P ii] transitions, namely those among the levels 3s23p2(3P0, 3P1, 3P2, 1D2, and 1S0). The wavelengths range between 0.44 and 70 µm, and several lie within the spectroscopic domain observable with the JWST. These line ratios have been calculated using a new collisional-radiative-cascade (CRR) model combining calculated collision strengths and level-specific recombination rate coefficients, with both data sets computed using the accurate Breit–Pauli R-matrix method. The CRR model includes a new scheme for (e + ion) recombination to emission-line formation. We compare its effect to models incorporating only electron-impact excitation and spontaneous radiative decay. We find that (e + ion) recombination has a significant impact on all line ratios, and represents a major improvement in physical accuracy of emission-line models.

Continuum Scattering and Formation of Emission Lines

The effect of radiation scattering in continuum on the spectral lines formation is treated. Compared to the early works in this direction, the conditions of emission lines appearance in the atmosphere with the temperature gradient with depth is discussed in detail. For simplicity's sake, the coherent scattering in the spectral line frequencies is adopted. By the example of atmospheres of supergiant stars of type A and adjacent classes, it is shown which lines and in what parts of the spectrum can be observed in the emission.

Formation of Hydrogen Emission Lines in the Magnetospheres of Young Stars

The formation of hydrogen emission lines in the magnetospheres of young stars is considered. The magnetosphere is assumed to be formed by a dipolar magnetic field whose axis is aligned with the stellar rotation axis. The radiative transfer in spectral lines is considered in the Sobolev approximation by taking into account the nonlocal radiative coupling. The gas density and temperature distributions in the magnetosphere are taken to be the same as those in Hartmann et al. (1994). The results of our calculations of the Hα and Hβ line intensities and profiles are presented for the case of a slowly rotating star. We separately consider the magnetospheric models in which the infalling gas is heated by radiation from an accretion spot on the stellar surface and the case where the axis of the magnetosphere is tilted with respect to the stellar rotation axis. Rotational modulation of the spectral line profiles with the stellar rotation period is observed in the latter case.

Quantifying the Influence of Key Physical Processes on the Formation of Emission Lines Observed by IRIS. I. Non-equilibrium Ionization and Density-dependent Rates

Abstract In the work described here, we investigate atomic processes leading to the formation of emission lines within the Interface Region Imaging Spectrograph wavelength range at temperatures near 105 K. We focus on (1) non-equilibrium and (2) density-dependent effects influencing the formation and radiative properties of S iv and O iv. These two effects have significant impacts on spectroscopic diagnostic measurements of quantities associated with the plasma that emission lines from S iv and O iv provide. We demonstrate this by examining nanoflare-based coronal heating to determine what the detectable signatures are in transition region emission. A detailed comparison between predictions from numerical experiments and several sets of observational data is presented to show how one can ascertain when non-equilibrium ionization and/or density-dependent atomic processes are important for diagnosing nanoflare properties, the magnitude of their contribution, and what information can be reliably extracted from the spectral data. Our key findings are the following. (1) The S/O intensity ratio is a powerful diagnostic of non-equilibrium ionization. (2) Non-equilibrium ionization has a strong effect on the observed line intensities even in the case of relatively weak nanoflare heating. (3) The density dependence of atomic rate coefficients is only important when the ion population is out of equilibrium. (4) In the sample of active regions we examined, weak nanoflares coupled with non-equilibrium ionization and density-dependent atomic rates were required to explain the observed properties (e.g., the S/O intensity ratios). (5) Enhanced S/O intensity ratios cannot be due solely to the heating strength and must depend on other processes (e.g., heating frequency, non-Maxwellian distributions).

PN G068.1+11.0: A young pre-cataclysmic variable with an extremely hot primary

An analysis of spectroscopic and photometric data for the young pre-cataclysmic variable (PCV) PN G068.1+11.0, which passed through its common-envelope stage relatively recently, is presented. The spectroscopic and photometric data were obtained with the 6-m telescope and Zeiss-1000 telescope of the Special Astrophysical Observatory. The light curves show sinusoidal brightness variations with the orbital-period time scale and brightness-variation amplitudes of Δm = 1.m41, 1.m62, and 1.m57 in the B, V, and R bands, respectively. The system’s spectrum exhibits weak HI (Hβ–Hδ) andHeII λλ4541, 4686, 5411 A absorption lines during the phases of minimum brightness, as well as HI, HeII, CIII, CIV, NIII, and OII emission lines whose intensity variations are synchronized with variations of the integrated brightness of the system. The emission-line formation in the spectra can be fully explained by the effects of fluorescence of the ultraviolet light from the primary at the surface of the cool star. All the characteristics of the optical light of PN G068.1+11.0 confirm that it is a young PCV containing sdO subdwarf. The radial velocities were measured from a blend of lines of moderately light elements, CIII+NIII λ4640 A, which is formed at the surface of the secondary due to reflection effects. The ephemeris of the system has been improved through a joint analysis of the radial-velocity curves and light curves of pre-cataclysmic variable, using modelling of the reflection effects. The fundamental parameters of PN G068.1+11.0 have been determined using two evolutionary tracks for planetary-nebula nuclei of different masses (0.7 M⊙and 0.78M⊙). The model spectra for the system and a comparison with the observations demonstrate the possibility of refining the components’ effective temperatures if the quality of the spectra used is improved.

On the origin of the correlations between the accretion luminosity and emission line luminosities in pre-main-sequence stars

Correlations between the accretion luminosity and emission line luminosities (Lacc and Lline) of pre-main-sequence (PMS) stars have been published for many different spectral lines, which are used to estimate accretion rates. Despite the origin of those correlations is unknown, this could be attributed to direct or indirect physical relations between the emission line formation and the accretion mechanism. This work shows that all (near-UV/optical/near-IR) Lacc–Lline correlations are the result of the fact that the accretion luminosity and the stellar luminosity (L*) are correlated, and are not necessarily related with the physical origin of the line. Synthetic and observational data are used to illustrate how the Lacc–Lline correlations depend on the Lacc–L* relationship. We conclude that because PMS stars show the Lacc–L* correlation immediately implies that Lacc also correlates with the luminosity of all emission lines, for which the Lacc–Lline correlations alone do not prove any physical connection with accretion but can only be used with practical purposes to roughly estimate accretion rates. When looking for correlations with possible physical meaning, we suggest that Lacc/L* and Lline/L* should be used instead of Lacc and Lline. Finally, the finding that Lacc has a steeper dependence on L* for T Tauri stars than for intermediate-mass Herbig Ae/Be stars is also discussed. That is explained from the magnetospheric accretion scenario and the different photospheric properties in the near-UV.

Spectroscopic variability of two Oe stars

The Oe stars HD45314 and HD60848 have recently been found to exhibit very different X-ray properties: whilst HD60848 has an X-ray spectrum and emission level typical of most OB stars, HD45314 features a much harder and brighter X-ray emission, making it a so-called gamma Cas analogue. Monitoring the optical spectra could provide hints towards the origin of these very different behaviours. We analyse a large set of spectroscopic observations of HD45314 and HD60848, extending over 20 years. We further attempt to fit the H-alpha line profiles of both stars with a simple model of emission line formation in a Keplerian disk. Strong variations in the strengths of the H-alpha, H-beta, and He I 5876 emission lines are observed for both stars. In the case of HD60848, we find a time lag between the variations in the equivalent widths of these lines. The emission lines are double peaked with nearly identical strengths of the violet and red peaks. The H-alpha profile of this star can be successfully reproduced by our model of a disk seen under an inclination of 30 degrees. In the case of HD45314, the emission lines are highly asymmetric and display strong line profile variations. We find a major change in behaviour between the 2002 outburst and the one observed in 2013. This concerns both the relationship between the equivalent widths of the various lines and their morphologies at maximum strength (double-peaked in 2002 versus single-peaked in 2013). Our simple disk model fails to reproduce the observed H-alpha line profiles of HD45314. Our results further support the interpretation that Oe stars do have decretion disks similar to those of Be stars. Whilst the emission lines of HD60848 are explained by a disk with a Keplerian velocity field, the disk of HD45314 seems to have a significantly more complex velocity field that could be related to the phenomenon that produces its peculiar X-ray emission.

Imaging and Spectroscopy of The Massive Binaries MWC 314 and HD 168625

VLT-NACO imaging and spectroscopic monitoring of MWC 314 shows it is a hierarchical triple system consisting of a close massive binary that contains a Luminous Blue Variable (LBV), and a third companion bound in a wide orbit. We observe Discrete Absorption Components (DACs) in extended violet absorption wings of P Cyg He i lines at orbital phases when the primary is in front of the secondary. The DACs signal fast expanding wind regions of enhanced density and variable outflow velocity. We present a model of the circumbinary disc showing that the formation region of Fe ii emission lines in the disc extends to ∼26 au from the center of gravity for i=70∘. Direct imaging of the candidate LBV HD 168625 also reveals a companion bound in a wide orbit. A kinematic analysis of VLT-UVES slit spectra signals significantly different radial velocities of the [N ii] λ6584 emission line formation regions N-S across the inner loop-like nebular structures.

Low Ionization Emission Lines in Quasars: Clues from OI 8446 and the CaII Triplet

The formation of low emission lines in quasars and active galactic nuclei is still an open issue. Aided by the organizing power of the 4D eigenvector 1 scheme, we review basic developments since th...

X-ray spectral and photoelectron study of the electronic structure of copper phthalocyanine and its fluoro-substituted analog

A complex experimental study of Cu(2p 3/2), Cu(2p 1/2) photoelectron and Kα1,2 and Lα1,2 X-ray emission spectra of copper in copper phthalocyanine CuPcH16 and its fluoro-substituted analog CuPcF16 is carried out. A charge transfer model is used to interpret the spectra. It is shown that Kα1 and Kα2 lines of the spindoublet of copper have a complex structure due to the processes of metal-to-ligand charge transfer. The role of a satellite in the formation of emission lines is revealed.

The influence of rotation on optical emission profiles of O stars

We study the formation of photospheric emission lines in O stars and show that the rectangular profiles, sometimes double peaked, that are observed for some stars are a direct consequence of rotation, and it is unnecessary to invoke an enhanced density structure in the equatorial regions. Emission lines, such as N IV 4058 and the N III 4634-4640-4642 multiplet, exhibit non-standard "limb darkening" laws. The lines can be in absorption for rays striking the center of the star and in emission for rays near the limb. Weak features in the flux spectrum do not necessarily indicate an intrinsically weak feature -- instead the feature can be weak because of cancellation between absorption in "core" rays and emission from rays near the limb. Rotation also modifies line profiles of wind diagnostics such as He II 4686 and Halpha and should not be neglected when inferring the actual stratification, level and nature of wind structures.

Nitrogen line spectroscopy of O-stars

This is the second paper in a series aiming at the analysis of nitrogen abundances in O-type stars, to enable further constraints on the early evolution of massive stars. We investigate the NIV lambda4058 emission line formation, provide nitrogen abundances for a substantial O-star sample in the LMC, and compare our findings with recent predictions from stellar evolutionary models. Stellar and wind parameters are determined by line profile fitting of hydrogen, helium and nitrogen lines, based on synthetic spectra calculated by FASTWIND. We derive nitrogen abundances for 20 O- and 5 B-stars, by analyzing all nitrogen lines present in the available optical spectra. The dominating process responsible for emission at NIV lambda4058 in O-stars is the strong depopulation of the lower level of the transition, which increases as a function of Mdot. Unlike the NIII triplet emission, resonance lines do not play a role for typical mass-loss rates and below. The bulk of our sample O-stars seems to be strongly nitrogen-enriched, and a clear correlation of nitrogen and helium enrichment is found. By comparing the nitrogen abundances as a function of vsini ('Hunter-plot') with tailored evolutionary calculations, we identify a considerable number of highly enriched objects at low rotation. Due to the low initial abundance, the detection of strong Nitrogen enrichment in the bulk of O-stars indicates that efficient mixing takes place already during the very early phases of stellar evolution of LMC O-stars.

Shock-induced polarized hydrogen emission lines in the Mira staroCeti

In the spectra of pulsating stars, especially Mira stars, the detection of intense hydrogen emission lines has been explained by a radiative shock wave, periodically propagating throughout the atmosphere. Previous observation of the Mira star omicron Ceti around a bright maximum of light led to the detection of a strong linear polarization associated to Balmer emissions, although the origin of this phenomenon is not fully explained yet. With the help of spectropolarimetry, we propose to investigate the nature of shock waves propagating throughout the stellar atmosphere and present, for omicron Ceti (the prototype of Mira stars), a full observational study of hydrogen emission lines formed in the radiative region of such a shock. Using the instrument NARVAL, we performed a spectropolarimetric monitoring of omicron Ceti during three consecutive pulsation cycles. All Stokes parameters were systematically collected, with a particular emphasis on the maxima of luminosity, i.e. when a radiative shock wave is supposed to emerge from the photosphere and propagate outward. On Balmer lines, over a large part of the luminosity cycle, we report detections in Stokes spectra which are evolving with time. These signatures appear to be strongly correlated to the presence of an intense shock wave responsible for the hydrogen emission lines. We establish that those lines are polarized by a process inherent to the mechanism responsible for the emission line formation: the shock wave itself. Two mechanisms are considered: a global one that implies a polarization induced by convective cells located around the photosphere and a local one that implies a charge separation due to the passage of the shock wave, inducing an electrical current. Combined with the existing turbulence, this may generate a magnetic field, hence polarization.

Optical Investigation of Plasma Formation Process by Interaction of Intense Electron Beams with Metallic Targets

Summary form only given. Intense pulsed electron beams are investigated for surface modifications since several years. Different applications like surface hardening or improvement of corrosion resistance are investigated. However, physical processes of beam target interaction and plasma formation are not yet fully understood. These effects might have influence on the functional parameters of such an electron accelerator. The target plasma as an ion source might change the electron beam parameters. Especially for longer pulses in the 10th of microsecond range an intensification of this feedback is expected. To investigate the influence of the plasma formation process on the most important parameters of the accelerator like course of impedance, beam profile and beam stability an optical diagnostic system with high time and spatial resolution was developed. The system consists of a spectrometer combined with a framing and streaming camera. The target plasma has been analyzed with a 0.5m polychromator with fiber-optically couples array of 6 photo- multiple. This allows the simultaneous detection of different spectral lines emitted from the target plasma. The moment of plasma formation is under normal target conditions, without excessive adsorbents at the surface, about 20mus after the onset of the electron beam. The timely evolution of the carbon lines indicates a non thermal plasma formation mechanism. The start of plasma formation or evolution of characteristic emission lines depends on the target material. The somewhat earlier visibility in case of Ta target is related to the release of hydrogen stored in the bulk. Until the formation of the plasma ions are mainly produced by direct ionization of gas molecules. The ignition of the plasma above the target in a later stage is a result of ionization by secondary electrons.

Quasi‐equilibrium emission of excited semiconductors

AbstractAs a first detailed study on the basis of a recently found basic law for the non‐equilibrium energy flow in bounded media, we examine the absorption spectra of an excited semiconductor slab and present detailed calculations for its emission spectra in quasi‐equilibrium. The density of excited carriers is varied from the excitonic regime up to densities where gain appears.Different slab lengths are regarded, and the dependence on the angle of incidence is studied. The absorption and emission spectra follow the Fabry‐Perot resonances in the slab. Depending on its length, excitation, and the position of the resonances, the formation of sharp and strong emission lines is observed, and degeneracy effects become apparent. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Molecular Emission Line Formation in Prestellar Cores

We investigate general aspects of molecular line formation under conditions typical of prestellar cores. Focusing on simple linear molecules, we study the formation of their rotational lines with radiative transfer simulations. We present a thermalization diagram to show the effects of collisions and radiation on the level excitation. We construct a detailed scheme (contribution chart) to illustrate the formation of emission-line profiles. This chart can be used as an efficient tool to identify which parts of the cloud contribute to a specific line profile. We show how molecular line characteristics for uniform model clouds depend on hydrogen density, molecular column density, and kinetic temperature. The results are presented in a two-dimensional plane to illustrate mutual effects of the physical factors. We also use a core model with a nonuniform density distribution and chemical stratification to study the effects of cloud contraction and rotation on spectral line maps. We discuss the main issues that should be taken into account when dealing with interpretation and simulation of observed molecular lines.

New results of magnetic field measurements for BP Tau

We present measurements of the longitudinal magnetic field component B ‖ of the young star BP Tau in the He I 5876 emission line formation region, i.e., in the accretion flow near the stellar surface. The values obtained (≃1.7 kG and ≃1.0 kG in 2000 and 2001, respectively) agree with the results of similar measurements by other authors. At the same time, we show that the previously obtained field strength at the magnetic pole, B p, and the inclination of the magnetic axis to the rotation axis, β, are untrustworthy. In our opinion, based on the B ‖ measurements available to date, it is not possible to conclude whether the star’s magnetic field is a dipole one or has a more complex configuration and to solve the question of whether this field is stationary. However, we argue that at least in the He I 5876 line formation region, the star’s magnetic field is not stationary and can be restructured in a time of the order of several hours. Nonstationary small-scale magnetic fields of active regions on the stellar surface and/or magnetospheric field line reconnection due to the twisting of these field lines as the star rotates could be responsible for the short-term magnetic field variability. It seems highly likely that there are no strictly periodic variations in brightness and emission line profiles in BP Tau due to the irregular restructuring of the star’s magnetic field.

Line formation regions of the UV spectrum of CI Cygni

Aims. They are the interpretation of the emission line formation regions in CI Cygni Methods. They involve the examination of radial velocities and fluxes of ultraviolet emission lines at different epochs, deduced from archival IUE and GRHS/HST spectra. Results. The line fluxes give electron densities and were in addition used to calculate emission measures, suggesting line formation in regions rather smaller than the binary separation. Examination of the radial velocities led to us to find a systematic redshift of the high ionization resonance lines with respect to the intercombination, and He ii lines. Possible explanations of the redshift and the high resolution GHRS C iv profile are discussed. We favour that involving resonance line absorption by a circum-binary region most probably in an asymmetric wind interaction shell or in a wind from the accretion disk.

Edge-on T Tauri stars

Using the U echelle spectrograph at the ESO VLT we obtained two-dimensional high-resolution (R = 50 000) spectra of the edge-on disk objects HH30 ∗ , HK Tau B, and HV Tau C. For comparison purposes we also observed with the same equipment both the classical T Tauri star HL Tau and the active late-type star LDN 1551-9. The spectra of all three observed edge-on disks consist of a T Tauri emission and absorption line spectrum with superimposed jet emission lines. Analysis of the spectra confirmed that the disks are completely opaque at visible wavelengths and that light from the central objects reaches us only via scattering layers above and below the disk planes. The central objects of our targets were found to be normal T Tauri stars showing moderate but different amounts of veiling of their photospheric spectra, indicating different accretion rates or evolutionary stages. We suggest that all classical T Tauri stars (CTTSs) show this observed morphology when viewed edge-on. Part of the jet emission from edge-on systems is directly visible to us in the forbidden lines as well as in Hα and He , a finding which contradicts the present paradigm of a pure magnetospheric accretion origin for the formation of hydrogen and helium emission lines in moderately active CTTSs. From a comparison with those Taurus-Auriga CTTSs for which the inclination is reliably known, we conclude that the view angle of CTTS systems is one of the key parameters governing apparent Hα emission strength in the T Tauri class. We discuss the various possible formation regions for the Na  D lines and show that profiles similar to observed ones can be formed at the base of the disk wind.

Diagnostics of the collisionally pumped plasma and search of the optimal plasma parameters of x-ray lasing: calculation of electron-collision strengths and rate coefficients for Ne-like plasma

The problem of diagnostics for the collisionally pumped plasma and search of the optimal plasma parameters of x-ray lasing are studied. We present the uniform energy approach, formally based on the quantum electrodynamics with using gauge invariant scheme of generation of the optimal one-electron representation, for the calculation of electron collision strengths and rate coefficients for electron-collisional excitation. The aim is to study, in a uniform manner, elementary processes responsible for emission-line formation in plasmas. The electron collision excitation cross-sections and rate coefficients for some Ne-like ions (Fe, Ba) are calculated. To test the results of calculations we compare them with other authors calculations and with available experimental data.