Some comments on the limb shift of solar lines

It has been suggested that pressure shifts are the sole cause for the systematic blue shift of solar fraunhofer lines (Hart, 1974). To check this we evaluate the significance of pressure shifts for Fe i lines. The observed wavelength shifts of a large number of lines are compared with the shifts calculated for the Lennard-Jones potential following Hindmarsh et al. (1967). The Lennard-Jones potential for the interatomic forces yield pressure shifts caused by neutral hydrogen, which explain only a small fraction of the observed blue shift. It is also shown that the quadratic Stark effect contributes insignificantly to the position of Fe i lines. Table I summarizes the average line shifts for iron lines at the center of the solar disk after correction for pressure shifts.Operated by the Association of Universities for Research in Astronomy, Inc. under contract AST 74-04129 with the National Science Foundation.     

An electron temperature distribution derived from optical and radio measurements of HII regions

Radio measurements of the electron temperature ofHii regions are obtained from the ratio of the brightness temperature of a hydrogen recombination line to that of the adjacent continuum, while optical measurements are obtained from the ratio of [Oiii] forbidden-line intensities. The radio and optical measurements made under the assumption of an isothermalHii region,T _R andT _opt respectively, are combined to derive a temperature distribution for an entire nebula. A sphericalHii region in local thermodynamic equilibrium with constant density which is optically thin in both the line and the continuum is used as a model. Assuming linear temperature gradients withT _R=6000K andT _opt=10000K, it is found thatT=12000K (1–0.74r/R), wherer is the distance from the center andR is the radius of the nebula.     

How bright is the [Si X] 1431 nm coronal emission line?

Airborne eclipse observations of the [Six] 1430.5 nm coronal emission line are reviewed, and new ground-based out-of-eclipse coronagraph observations obtained at NSO/Sacramento Peak are reported. We find that the [Six] 1430.5 nm coronal emission line brightness is less than 8 × 10⁻⁶ B⊙ in small active region corona which showed [Fexiii] 1074.7 nm emission (corrected for sky background) of about 20 × 10⁻⁶ B⊙. Operated by the Association of Universities for Research in Astronomy, Inc. (AURA) under cooperative agreement with the National Science Foundation.     

Steady flows in active regions observed with the HeI 10830 Å line

We show that the He i 10830 A line gives reliable Doppler shift measurements in the upper chromosphere above active regions. Persistent flow patterns in active regions observed near the solar limb show features previously noted in Dopplergrams using the Civ transition region ultraviolet emission line. Unlike the Civ measurements, however, the He i absorption shows a strong correlation with the line-of-sight velocity images in certain regions of some active regions.Operated by the Association of Universities for Research in Astronomy, Inc., under contract AST 78-17292 with the United States National Science Foundation.The National Center for Atmospheric Research is operated by the University Corporation for Atmospheric Research under sponsorship of the National Science Foundation.Visiting Astronomers, Sacramento Peak Observatory.     

Comparison of observed Ca xix and Ca xviii relative line intensities with current theory

A comparison is made between Ca xix and Ca xviii line ratios observed in solar flares with the Bent Crystal Spectrometer (BCS) on the Solar Maximum Mission (SMM) satellite and currently available atomic data. Close agreement is found with the excitation rates recently published by Pradhan et al. (1981). The observations show little dependence of line ratios on electron temperature, supporting a further conclusion that cascade contributions to the 2³ P and 2³ S levels are not significant.     

A comparison of theoreticalCIV emission line strengths with active region observations obtained with the Solar EUV Rocket Telescope and Spectrograph (SERTS)

Theoretical line ratios involving 2s ² S - 3p ² P, 2p ² P - 3s ² S, and 2p ² S - 3d ² D transitions inCiv between 312 and 420 Å are presented. A comparison of these with solar active region observational data obtained during a rocket flight by the Solar EUV Rocket Telescope and Spectrograph (SERTS) reveals good agreement between theory and experiment, with discrepancies that average only 22%. This provides experimental support for the accuracy of the atomic data adopted in the line ratio calculations, and also resolves discrepancies found previously when the theoretical results were compared with solar data from the S082A instrument on boardSkylab. The potential usefulness of theCIV line ratios as electron temperature diagnostics for the solar transition region is briefly discussed.     

Theoretical models of planetary nebulae

Theoretical models are calculated for 15 planetary nebulae of medium-to-high excitation, following procedures previously described. Initial stellar energy distributions are adopted from Cassinelli (1971), but are subsequently modified to obtain the best representation of optical spectra for the selected objects. Other adjustable parameters include the stellar radius,R ^(*), the nebular density,N _H, the truncation radius,r _c, for the nebular shell, and the chemical composition. Excitationsensitive ratios are usually well-represented as are the actual observed intensities of spectral lines. Forbidden lines arising from 3p ³ configurations, e.g., those of [SII], [ArIV], and [ClIII] offer difficulties. For this sample of nebulae, the mean abundances seem to agree well with those found in an earlier study where the models were used as interpolation devices (Aller, 1978). Our objective is not to use the models to derive abundances explicitly, but rather to use them to find ionization correction factors. Some cautions and limitations of this procedure are described.     

Removing instrumental polarization from infrared solar polarimetric observations

Full Stokes polarimetry is obtained using the National Solar Observatory Vacuum Tower Telescope at Sacramento Peak while observing the magnetically sensitive infrared Fei line at wavelength of 1.56. A technique is described which makes use of the high magnetic resolution in this spectral range to remove instrumental polarization from observed StokesQ, U, andV line profiles.Supported under a USAF/AFOSR research initiative.Operated by the Association of Universities for Research in Astronomy, Inc. (AURA) under cooperative agreement with the National Science Foundation.     

X-ray bright points and He i λ 10830 dark points

Using near-simultaneous full disk solar X-ray images and Hei 10830 spectroheliograms from three rocket flights, we compare dark points identified on the Hei maps with X-ray bright points identified on the X-ray images. We find that for the largest and most obvious features there is a strong correlation: most Hei dark points correspond to X-ray bright points. However, about two-thirds of the X-ray bright points were not identified on the basis of the helium data alone. Once an X-ray feature is identified it is almost always possible to find an underlying dark patch of enhanced Hei absorption which, however, would not a priori have been selected as a dark point. Therefore, the Hei dark points, using current selection criteria, cannot be used as a one-to-one proxy for the X-ray data. Hei dark points do, however, identify the locations of the stronger X-ray bright points.Visitor, National Solar Observatory. National Optical Astronomy Observatories operated by the Association of Universities for Research in Astronomy, Inc. under contract with the National Science Foundation.     

The effects of acoustic-gravity waves on the K i 7699 line

We examine the effects of acoustic-gravity waves with long and short periods on the solar profile of the K i7699 line using a dynamic model of line formation.First we studied the kinetic equilibrium of the K i atoms in a static atmosphere confirming, with up-to-date atomic data and atmospheric model, that a good fit of the resonance line 7699 is possible only when non-LTE effects are accounted for.Then the static non-LTE line source function and lower-level population are used as input data for calculating the line formation in the presence of waves.The time behaviour of the synthetic profiles corresponding to 300 s and 30 s waves is extensively discussed. The characteristic redshift induced by the 30 s wave is explained within the framework of the S-S line formation model. Long-period waves yield an anticorrelation between the asymmetry at different residual intensities and the line core shift, as observed. The short-period waves with velocity amplitude of about 100 m s^–1 (at the base of the photosphere) produce a mean bisector whose lower part has a slope in agreement with the observed one. The efficiency of waves to produce macro and microturbulence is also discussed.Currently NAS/NRC Senior Research Associate at Sacramento Peak Obs., New Mexico, U.S.A.     

Civ plasma flow near active region filaments

Selected Civ V ₀ lines (lines separating adjacent regions of strong blue shift and strong red shift) located near the solar limb and oriented parallel to the radius vector from disk center are shown to be closely aligned with H dark filaments in active regions. The filaments, in turn, are known to lie in the vicinity of magnetic neutral lines. The radial orientation of the V ₀ lines minimizes uncertainties in image registration and their location near the limb ensures that the observed fluid motion has major components paralleling V ₀. It follows that the filaments are located at sites of velocity shear, and, by inference, of magnetic shear. For a case in which a given V ₀ line is observed near both east and west limbs, the gradient of Doppler velocity across the V ₀ line reverses sign from one limb to the other as is expected for horizontal steady flow. Thus, the velocity vectors remain fixed with respect to the filament.The National Center for Atmospheric Research is sponsored by the National Science Foundation.     

On the temperature structure of sunspot umbrae

From a high-resolution spectrum of a sunspot umbra (1.1 < < 2.3 m) we derive models of the temperature stratification in the deep layers of the umbra. The observed spectrum is corrected for straylight using the Hi Paschen line at gl = 1.282 m. A method is described for the iterative fitting of empirical temperature models to spectral information, and the method is applied to the present data. We find that the observed profiles of 3 high-excitation lines of Sii and the observed continuum contrast between umbra and photosphere cannot be reproduced with a single one-component model of the umbral atmosphere: the Si i lines require a model that is 460 K hotter at gt _0.5 = 3 than the continuum model. This indicates that hot and cool components coexist within the umbra. A temperature model derived from the relative intensity in the wings of 3 low-excitation lines of Mgi, Ali, and Sii is not significantly different from the continuum model.Based on observations obtained at Kitt Peak National Observatory, which is operated by the Association of Universities for Research in Astronomy, Inc. (AURA), under contract with the National Science Foundation.     

Density perturbations in cosmological models

Differential equations are derived, following the methods ofLifshitz (1946) andLifshitz andKhalatnikov (1963), for density perturbations in isotropic, spatially homogeneous cosmological models of arbitrary space curvature. The unperturbed models contain both matter and radiation. An explicit third-order equation is obtained for the time development of the perturbations, and it is shown that one of the solutions is not covariantly defined. The two remaining solutions are compared with existing solutions for the limiting cases of radiation-filled and dust-filled models. The results ofBonnor's (1957) Newtonian analysis are shown to be a valid limiting case of our equation when the pressurep is finite, but small compared with the densityp timesc ².A detailed analysis is given of a model containing coupled radiation (p=pc ²/3) and dust (p=0). It is shown that density perturbations with long wavelengths are unstable (with slow growth rate) for all time. The instability exists because for a long-wavelength disturbance, the time scale governing the propagation of pressure effects (which stabilize perturbations) is longer than the time scale for which pressure falls to the point of ineffectiveness. The present value of the critical wavelength is 60 Mpc in models based on flat space sections in which the present background radiation temperature is 3 °K.The research reported herein was supported in part by the Atomic Energy Commission under contract number AT(11-1)-34, Project Agreement No. 125, and by the National Science Foundation, under Grant GP-4975.     

Physical properties of solar chromospheric plages

We compute a new grid of plage models to determine the difference in temperature versus mass column density structure T(m) between plage regions and the quiet solar chromosphere, and to test whether the solar chromosphere is geometrically thinner in plages. We compare partial redistribution calculations of Mg ii h and k and Ca ii K to NRL Skylab observations of Mg ii h and k in six active regions and Ca ii K intensities obtained from spectroheliograms taken at approximately the same time as the Mg ii observations. We find that the plage observations are better matched by models with linear (in log m) temperature distributions and larger values of m ₀ (the mass column density at the 8000 K layer in the chromosphere), than by models with larger low chromosphere temperature gradients but values of m ₀ similar to the quiet Sun. Our derived temperature structures are in agreement with the grid originally proposed by Shine and Linsky, but our analysis is in contrast to the study by Kelch which implies that stellar chromospheric geometrical thickness is not affected by chromospheric activity. We conclude that either the stellar Mg ii observations upon which the Kelch study was based are of poorer quality than had been assumed, or that the spatial averaging of inhomogeneous structures, which is inherent in the stellar data, does not lead to a best fit one-component model similar in detail to that of a stellar or a solar plage.Visiting Astronomer at Kitt Peak National Observatory, which is operated by the Association of Universities for Research in Astronomy, Inc., under contract with the National Science Foundation.Staff member, Quantum Physics Division, National Bureau of Standards.     

On the relative residual intensities of the calcium H and K lines

We have observed the solar Caii H and K lines to obtain well-calibrated ratios of their core residual intensities. From three independent calibrations, one using a standard lamp, we conclude that the residual intensity ratio r(K₃)/r(H₃) is 1.048 ± 0.03 in the quiet chromosphere and 1.20 ± 0.03 in a plage region. These ratios correspond closely to those observed in stars with quiet and active chromospheres, respectively. For a chromospheric model suggested by the calcium lines and a four-level Caii ion, we compute H and K line profiles varying the direct collisional coupling and indirect radiative and collisional coupling via the 3 ² D level. We conclude that enhanced chromospheric activity in the sun and late-type stars results more from a steepening of the chromospheric thermal gradient than from a change in density.Kitt Peak National Observatory Contribution No. 530.Of the University of Colorado and the National Bureau of Standards.Operated by the Association of Universities for Research in Astronomy, Inc., under contract with the National Science Foundation.     

Physical parameters of reflection nebulae in the galaxy

We show how, given observed equivalent widths of Mgii and Mgi absorptions due to an interstellar cloud in which a late-B star is embedded, the basic physical parameters: kinetic temperature, mean density, electron density, and radius can be constrained. Hydrogen ionization by means of cosmic rays and the effect of the stellar radiation field on the magnesium ionization equilibrium are taken into account.The method is applied to the reflection nebula surrounding the star HD 26676. The resulting solutions for the radius and temperature of the nebulosity are comparable to the typical values derived for diffuse interstellar clouds from optical and 21-cm measurements, if a cosmic-ray ionization rate 10^–16s^–1 — in agreement with recent determinations — is assumed. The results are not strongly dependent on the gas pressureP forP varying in a range of values typical of interstellar clouds.     

Emission line ratios for Si XI applicable to CDS/SUMER observations from Soho

Calculations of electron temperature (T _e) and density (N _e) sensitive line ratios in Sixi involving transitions in the 358–604 wavelength range are presented. These are shown in the form of ratio-ratio diagrams, which should in principle allow bothN _e andT _e to be deduced for the Sixi line-emitting region of a plasma. However a comparison of these with observational data for two solar flares, obtained with the Naval Research Laboratory's S082A spectrograph on boardSkylab, reveals that the experimental ratios are much larger than expected from theory, which is probably due to the Sixi lines in the S082A spectra being blended with transitions from species including Nev, Fexi, and Fexii. Possible future applications of the Sixi results to spectral data from the Coronal Diagnostic Spectrometer on the Solar and Heliospheric Observatory are briefly discussed.     

A proposed correction to the solar abundances of carbon and oxygen utilizing new and accurate theoretical forbidden transition probabilities

Previously published solar abundances of oxygen and carbon can be corrected to be logN(O) = 8.93 and logN(C) = 8.60 on the hydrogen log-scale when new accurate forbidden electric quadrupole transition probabilities A _Q(s^–1) are used. Such A _Q's, based on the new atomic structure and electron correlation theory, developed recently by Sinanolu and coworkers, are reported for the (¹ S ₀-¹ D ₂) lines of [C i], [N ii, [O i] and [O iii] and the (² P-² D) lines of [N i] and [O ii]. The available experimental values are also given for comparison.Work supported by Grant No. GP-29471 from the U.S. National Science Foundation.     

Gravitational collapse of magnetic neutron stars

Pulsars are presently believed to be rotating neutron stars with frozen-in magnetic fields. Because of the high density of neutron stars, general relativistic effects are important since they effect both the structure and stability of such stars. Besides this, the magnetic field outside the star is also affected. Instead of falling of asr ^(2+l) as in flat space, it is shown that each magnetic multipole varies as a hypergeometric function of radius. A closed form of these hypergeometric functions is given in terms of Legendre functions of the second kind. If the mass of a neutron star exceeds about 2.4m , the star becomes unstable and coliapses. For a quasistatically collapsing body, it is shown that the magnetic field seen by a distant observer vanishes as the radius approaches the gravitational radius.This work was supported in part by the Air Force Office of Scientific Research, Office of Aerospace Research under AFOSR Grant 70-1866.     

Log(gf) for singly-ionized elements of the iron group

Moderately accurate oscillator strengths are obtained by application of systematic corrections to Warner's-log(gf) for the once ionized elements of the iron group. Corrections are derived by comparison of Warner's values with those of Kurucz-Peytremann (Scii, Tiii,Vii, Crii, Mnii, Coii, and Niii) and Kurucz (Feii).K–P andK derived log(gf) by semi-empirical methods for many transitions using scaled Thomas-Fermi-Dirac potentials for the atoms of the elements from B to Ni. Although their individual values may be seriously in error, it has been shown that their mean scales are acceptable, being affected at most by rather small errors. It is known, for example, that when interacting terms are not properly accounted for in the semi-empirical method, very small values are derived for log(gf); these were dropped away in calculating mean corrections as they exhibited gross deviations.The relations loggf(KP)–loggf(W) vs , loggf(W), andE _u (energy of the upper level) have been investigated, and constant corrections, or weak dependences are found.Using Warner's corrected log(gf) for these elements, we have shown in a rather qualitative way that abundances consistent with the values accepted at present are found in the solar photosphere.Research supported in part by the SECYT.