Angstrom Line Research Articles

Local atomic structure of Fe-Ni/V multilayer nanostructures: EXAFS spectroscopy and synchrotron radiation data

Fe, Ni, and V K EXAFS spectroscopic experiments were carried out at the European Synchrotron Radiation Center (ESRF) in the fluorescent mode on the 26 angstrom line with the use of the [(Fe82Ni18)( ...

Tracing jet–ISM interaction in young AGN: correlations between [O III] λ 5007 Å and 5-GHz emission

Aims. I study the interaction between young AGN and their host galaxies based on their ionized gas and radio emission, and analyze possible implications for radio galaxy evolution. Methods. The [O III] lambda 5007 angstrom line and 5-GHz radio properties are compared and studied for a large, representative sample of GPS and CSS (i.e., young) quasars and radio galaxies, and large-scale sources using [O III] lambda 5007 angstrom line and 5-GHz radio data from the literature and our own observations. Results. Several correlations between the [O III] lambda 5007 angstrom line and 5-GHz radio emission were found. The main result is that the [O III] lambda 5007 angstrom emission is strongly correlated with the GPS/CSS source size, which indicates that the [O III] lambda 5007 angstrom emission is enhanced significantly by the jet expansion through the host galaxy ISM. Shocks are the most likely enhancement mechanism, although jet-induced star formation could be also, partly, responsible for the [O III] lambda 5007 angstrom emission. The data also suggests that the jet possibly decelerates as it grows, although the correlation expansion speed versus radio size is weak.

On the Binary Nature of Dust-encircled BD +20 307

Three epochs of high resolution spectra of the star BD+20 307 show that it is a short period (~3.5 day) spectroscopic binary of two nearly identical stars. Surprisingly, the two stars, though differing in effective temperature by only ~250 K and having a mass ratio of 0.91, show very different Li line equivalent widths. A Li 6707 Angstrom line is only detected from the primary star, and it is weak. This star is therefore likely to be older than 1 Gyr. If so, the large amount of hot circumbinary dust must be from a very large and recent, but very late evolutionarily, collision of planetesimals.

Interstellar Iron and Silicon Depletions in Translucent Sight Lines

We report interstellar FeII and SiII column densities toward six translucent sight lines (A_V >~ 1) observed with the Space Telescope Imaging Spectrograph (STIS). The abundances were determined from the absorption of SiII] at 2335 Angstroms, and several weak Fe transitions including the first reported detections of the 2234 Angstrom line. We derive an empirical f-value for the FeII 2234 Angstrom transition of log(f lambda) = -1.54 +/- 0.05. The observed sight lines sample a variety of extinction characteristics as indicated by their R_V values, which range from 2.6 - 5.8. The dust-phase abundances of both Si and Fe are positively correlated with the small-grain population (effective radii smaller than a few hundred micron) toward the targets. The physical conditions along the sight lines suggest that this relationship may be due to differences in the survival of small particles in some interstellar environments. The chemical composition of the small grains could either resemble dust mantles or be silicate rich.

An Atlas of [N ii ] and [O iii ] Images and Spectra of Planetary Nebulae

Abstract : We present an atlas of Hubble Space Telescope images and ground-based, long-slit, narrowband spectra centered on the 6584 Angstrom line of [N ii] and the 5007 Angstrom line of [O iii]. The spectra were obtained for a variety of slit positions across each target (as shown on the images) in an effort to account for nonspherical nebular geometries in a robust manner. We have extended the prolate ellipsoidal shell model originally devised by Aaquist, Zhang, and Kwok to generate synthetic images, as well as long-slit spectra. Using this model, we have derived basic parameters for the subsample of PNe that present ellipsoidal appearances and regular kinematic patterns. We find differences between our parameters for the target PNe as compared to those of previous studies, which we attribute to increased spatial resolution for our image data and the inclusion of kinematic data in the model fits. The data and analysis presented in this paper can be combined with detections of nebular angular expansion rates to determine precise distances to the PN targets.

Spectral Analysis and Atmospheric Models of Microflares

By use of the high-resolution spectral data obtained with THEMIS on 2002 September 5, the spectra and characteristics of five well-observed microflares have been analyzed. Our results indicate that some of them are located near the longitudinal magnetic polarity inversion lines. All the microflares are accompanied by mass motions. The most obvious characteristic of the H alpha microflare spectra is the emission at the center of both H alpha and Call 8542 angstrom lines. For the first time both thermal and non-thermal semi-empirical atmospheric models for the conspicuous and faint microflares are computed. In computing the non-thermal models, we assume that the electron beam resulting from magnetic reconnection is produced in the chromosphere, because it requires lower energies for the injected particles. It is found there is obvious heating in the low chromosphere. The temperature enhancement is about 1000-2200 K in the thermal models. If the non-thermal effects are included, then the required temperature increase can be reduced by 100-150 K. These imply that the H alpha microflares can probably be produced by magnetic reconnection in the solar lower atmosphere. The radiative and kinetic energies of the Ha microflares are estimated and the total energy is found to be 10(27) - 4 x 10(28) erg.

Multi-Site Spectrographic and Heliographic Observations of Radio Fine Structure on April 10, 2001

We present results of the first simultaneous observations of zebra patterns (ZPs) with super-fine spiky structure in the microwave range made at two observatories similar to 1000 km apart (Beijing and Nanjing, China). The fine structure was recorded by a spectra polarimeter in the 5.2 - 7.6 and 2.8 - 3.6 GHz ranges at the Huairou station and by the spectrometer in the 4.5 - 7.5 GHz range at the Purple Mountain Observatory. Simultaneously, the locations of radio sources were observed by the Siberian Solar Radio Telescope (SSRT) at 5.7 GHz. For a general analysis of the April 10, 2001 event, the Solar and Heliospheric Observatory/Michelson Doppler Imager (SOHO/MDI) data and Transition Region and Coronal Explorer (TRACE) images in EUV 171 angstrom line were used. The circular polarization degree was very weak for the burst background radio emission and moderate to strong for the fine structure. The polarization sign in all the cases probably corresponds to the extraordinary wave mode. Estimations of the magnetic field values in the whistler model for fine structure agree well with the extrapolated values from magnetic maps. Given the possibility of wave transformation in the perpendicular magnetic field and the spiky structure of the ZP, the whistler wave model appears to be the most appropriate explanation for the zebra stripe phenomenon.

Chromospheric Internetwork Oscillations at Various Locations of the Quiet Sun

We analyze oscillation behaviours in chromospheric internetwork regions using spectral observations of the CII 1334 angstrom line obtained with the Solar Ultraviolet Measurements of Emitted Radiation spectrograph (SUMER) aboard Solar and Heliospheric Observatory (SOHO). Three areas, 26 x 120 arcsec(2) each, at the various latitudes from the disk center to the north polar coronal hole, were rastered with a cadence of about 40-60 s in the solar minimum year. We obtained the time evolution of two-dimensional (2D) line intensity, continuum and line core shift. The continuum and the line shift show similar to 3 min chromospheric oscillations in the internetwork regions underlying the coronal hole as well as at the disk center: We find that the CII 1334 angstrom line shift oscillates with an average speed of similar to 1.7 km s(-1), independent of the latitude, while its coherent scale decreases with latitude. On the other hand, the oscillation amplitude of the continuum around the 1334 angstrom and the phase delay between the Doppler shift and continuum slightly increase with latitude.

On the formation of the He I 10 830 Å line in a flaring atmosphere

We explore the formation of the He. 10 830 Angstrom line in a flaring atmosphere, with special attention to the nonthermal effects of an electron beam. Using non-LTE calculations we obtain the line profiles from different model atmospheres. Without the nonthermal effects, the line changes from weak absorption in a cool atmosphere to emission in a hot and condensed atmosphere, as expected. However, the presence of an electron beam can significantly change the line strength, producing much stronger absorption and emission in these two cases. We find that in the nonthermal case, the collisional ionization of He. followed by recombinations becomes an important process in populating the triplet levels corresponding to the He I 10 830 Angstrom line. These results suggest that the He I 10 830 Angstrom line is also a potential diagnostic tool for nonthermal effects in solar or stellar flares.

On the Formation of Extreme‐Ultraviolet Helium Lines in the Sun: Analysis ofSOHOData

The resonance lines of in the Sun are several times stronger than expected, relative to lines of other ions. To explore the origins of this helium enhancement,'' we have studied data from the SUMER, CDS, MDI, and EIT instruments on the Solar and Heliospheric Observatory (SOHO). Time series data obtained in a quiet region and a coronal hole indicate that the spatio-temporal properties of the He I 584 Angstrom, and He II 304 and 1084 Angstrom lines are qualitatively unlike other chromospheric and transition region lines. Helium line intensities vary slowly compared to chromospheric oscillations and to transient transition region brightenings seen in other lines, yet they vary rapidly (both spatially and temporally) compared to the coronal radiation. This suggests that photoionization/recombination plays a minor excitation role in these lines. The Doppler shift of the 584 Angstrom line shows a remarkably clear 4-8 mHz oscillation, with no accompanying line intensity oscillation. Transient brightenings are used to show that nonradial photon scattering might explain the previously found diffuse appearance of the network but cannot account for the reduced network/internetwork intensity contrast in the 584 Angstrom line. We propose a new enhancement mechanism, based on arguments in a companion paper, in which predominantly neutral species such as diffuse across magnetic field lines into regions of hot coronal plasma, but charged ions do not. The enhanced internetwork emission may arise from the diffusion of atoms across the chromospheric canopy; the enhanced network emission may have contributions from diffusion from spicules. This mechanism may naturally explain why the 584 Angstrom line shows Doppler shift, but not intensity, oscillations. Finally, if a mechanism such as velocity redistribution'' dominates emission, spectroheliograms of lines will be bright in regions of large temperature gradients parallel to the magnetic field. Differences between and other spectroheliograms may then reveal the relative contributions of classical'' and cool loop'' models to the observed emission. (Less)

On the Formation of the Resonance Lines of Helium in the Sun

To investigate the cause of anomalously bright resonance lines of helium in the Sun, we have studied the magnitude of the enhancements and some proposals made to explain them. Calculations in new semiempirical solar models indicate that the resonance lines of helium are enhanced by factors of 2-5 for He I and between 2 and 9 for He II, depending on the elemental abundances assumed. These enhancements are substantially less than earlier work has suggested, with the differences arising from radiative transfer not only in helium lines but also in lines of less abundant elements. Photon scattering, even with small line center optical depths (of order unity or less) throughout the transition region, is shown to significantly modify line intensities and their center-to-limb variation. This effect has important consequences both for our analysis and also for the analysis of solar EUV lines in general, including emission measure analyses. We have re-examined some proposals to explain the enhancements based on the plasma'' picture. The proposals include explicit heating (the burst'' picture) and heating via advection (diffusive and/or flowing models and the velocity redistribution'' [VR] proposal). We argue that the original VR mechanism must be modified to include kinetic effects for helium atoms, which reduce the effects of VR. The VR mechanism also naturally predicts helium lines that are blue-shifted relative to lines less sensitive to VR, which contradicts observations for the He I 584 Angstrom line. Ionizing plasma models also potentially lead to enhancement of Deltangreater than or equal to1 transitions (n is the principal quantum number) in other atoms and ions. Existing observational evidence for enhanced Deltangreater than or equal to1 transitions in other ions is weak. We conclude that the ionizing plasma scenarios are not the sole cause of the helium enhancement. Instead, in a companion paper, we propose that the thermodynamic properties of noble gases in the upper chromosphere will lead to enhancement of their spectral lines if cross-field diffusion into coronal plasma is important. Finally, we show that, surprisingly, multifluid flows of the kind computed by Fontenla and colleagues can reproduce observed intensities of several lines of C and Si and may contain a resolution to the helium enhancement problem. (Less)

The lithium isotope ratio in metal-poor stars

Methods of deriving the Li-6/Li-7 isotope ratio from high resolution observations of the Li 16708 Angstrom line by means of spectral synthesis using both 1D hydrostatic and 3D hydrodynamical model atmospheres are discussed. Recent results are compared to

Theoretical values for the [O iii] 5007/4959 line-intensity ratio and homologous cases

The magnetic-dipole probabilities for the D-1(2)-P-3(2) and D-1(2)-P-3(1) transitions in carbon-like and oxygen-like ions are calculated up to atomic number Z = 12, including relativistic corrections to the magnetic dipole operator. The ratio of the probabilities for these two transitions is found to change by up to 5 per cent compared with previous theoretical work, none of which included these relativistic corrections, with the effect being largest for the near neutral ions. The transition probability ratio for the [O III] 5007 and 4959 Angstrom lines is found to be 3.01, implying an intensity ratio of 2.98, in significantly better agreement with the observed ratio than the earlier theoretical work.

On Photospheric Fluorescence and the Nature of the 17.62 A Feature in Solar X‐Ray Spectra

The identification of the emission line feature at 17.62 Angstroms in solar x-ray spectra is re-examined. Using a Monte Carlo technique, we compute a realistic theoretical upper limit to the observed Fe L-alpha photospheric fluorescent line strength caused by irradiation from an overlying corona. These calculations demonstrate that the photospheric Fe L-alpha characteristic line is much too weak to account for the observed 17.62 Angstrom line flux. Instead, we identify this line with the configuration interaction 2s2p3p2P-2s2p6 2S transition in Fe XVIII seen in Electron Beam Ion Trap spectra and predicted in earlier theoretical work on the Fe XVIII x-ray spectrum.

What caused an unusually broad He I 10830 A emission line in a solar limb flare?

This Letter discusses an observation of an unusually broad Helium I (He I) 10830 angstrom line from a flare on the solar limb. The line profile is close to a dispersion profile with full half-width of 4.2 angstrom. One interpretation of this observation could be a peculiar distribution of mass motions in the flare that is just right to mimic a dispersion-like line profile. Pressure broadening is usually indicated by a dispersion profile but is also an unlikely cause of the broadening because it would imply an electron density in the flare near 4 x 10(17) cm3. We found no other credible explanations for the observed broadening. We therefore propose a critical experiment to clearly distinguish between the two interpretations or to point the way toward another.

C IV line ratios in the sun

Recent R-matrix calculations of electron impact excitation rates for transitions in C IV are used to derive the theoretical electron temperature-sensitive emission-line ratios R1 = I(312.43 angstrom)/I(419.74 angstrom), R2 = I(3 84.14 angstrom)/I(419.74 angstrom), R3 = I(312.43 angstrom)/I(419.49 angstrom), and R4 = I(384.14 angstrom)/I(419.49 angstrom). A comparison of these with solar observational data obtained with the Naval Research Laboratory's S082A slitless spectrograph on board Skylab reveals good agreement between theory and observation for R2 and R4, which provides experimental support for the accuracy of the atomic data adopted in the analysis. However, most of the observed values of R1 and R3 lie above the theoretical high-temperature limit, which is probably due to blending of the 312.43 angstrom line.

Measurement of the shift-to-width ratio of Ar II 4806 Angstrom

Measurements of the shift-to-width ratio of the Ar II 4806 Angstrom line are reported in this paper. The profile-splitting interferometer developed by Burgess and Cooper (1965 a) was used for these measurements on the line radiated by a Z pinch filled with argon. Observations cover the temperature range 25 000-45 000 °k.

Excitation induced by 0$middot$15-1$middot$0 MeV protons in argon

The spectrum emitted as a result of proton impact on an argon target has been investigated, and cross sections determined for the collisionally induced emission of the 4200 Angstrom Ar I line and the 4431 Angstrom Ar II line. The cross section for the 4431 Angstrom line is 80 × 10-20 cm2 at 150 keV and decreases linearly with increasing energy. The 4200 Angstrom emission has a cross section of similar magnitude, but decreases less rapidly with energy.

Broadening of the hyperfine components of Cs I 4555 Angstrom line by argon

Broadening by argon of the caesium line 4555 Angstrom was investigated at low densities. The hyperfine components were found to be broadened equally with a broadening constant of 5?6 ? 0?7 ? 10-20 cm-1/(atoms cm-3). This is appreciably less than that which has previously been observed at higher argon densities when multiple interactions cannot be neglected.

The excitation of the 6 1P1 and 6 3P0,1,2 states of mercury by electron impact

The mercury atom is treated as a two-electron system with the orbitals determined using the Coulomb approximation, and intermediate-coupling wave functions are employed which allow for the spin-orbit interaction of the 6p electron of the 6 1P1 and 6 3P1 states of mercury. The oscillator strengths of the 6 1P1 -> 6 1S0 1850 Angstrom line and the 6 3P1 -> 6 1S0 2537 Angstrom line are calculated to be 1.17 and 0.037, respectively, which are in very good agreement with the experimentally determined values.