Oscillator strengths for optically allowed transitions in carbon-like ions

Using configuration interaction wavefunctions, the excitation thresholds for the twelve lowest terms arising from the configurations 1s ²2s ²2p ², 1s ²2s2p ³, and 1s ²2p ⁴ of Mgvii have been calculated. The same wavefunctions have been used to calculate the oscillator strengths for the optically allowed transitions in Mgvii. Combining these results with earlier published values for Oiii, Nev, Siix, Caxv, and Fexxi, oscillator strengths for other ions in the carbon isoelectronic sequence (F through Ni) have been predicted. The predicted values are found to be slightly lower when compared with the available published results.     

Excitation equilibrium for low lying levels in Cii,Niii, Oiv,Nevi, Mgviii,Six, and Siii

The populations of the excited state ² P _3/2 relative to the ground state ² P _1/2 have been investigated in C ii, N iii, O iv, Ne vi, Mg viii, Si x, and Si ii by considering all the radiative and collisional processes including the collisional transitions to the higher states which cascade to the upper level. The relative populations are used for the calculation of the line emissivities. The intensities of 76 320, 30 258 and 14 302 lines of Nevi, Mgviii, and Six ions respectively in the chromosphere-corona transition region are also calculated.     

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.     

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.     

The forbidden transitions within 3s 2 3p 5 3d of Fe ix and Ni xi and 3s 2 3p 4 3d of Fe x and Ni xii

Improved level values for 3s ² 3p ⁵ 3d and 3s ² 3p ⁴ 3d of Fe ix and Fe x are derived from the observed forbidden lines with special regard to recent accurate measurements in the vacuum-ultraviolet part of the coronal spectrum. A procedure for estimating expected relative intensities is proposed and used for an additional check on the consistency of the identifications, which now comprise 12 lines of Fe IX and 11 lines of Fe x. Finally, by use of a suitable extrapolation technique the wavelengths of corresponding lines in nickel are predicted, which leads to some new identifications and a total of 6 identified lines of Ni xi and 4 of Ni xii.     

Intensity of lines from low-lying levels in C ii,N iii,O iv,Ne vi,Mg viii,Si x,and Si ii

The intensities of the lines for the transition ² P ₃ ^2/0 -² P ₁ ^2/0 in C ii, N iii, O iv, Ne vi, Mg viii, Si x, and Si ii in the chromosphere-corona transition region are investigated. The populations of the excited state ² P _f3/2 ^p0 relative to the ground state ² P _f1/2 ⁰ are obtained by considering all the radiative and collisional transition processes including the collisional transitions to the higher states which cascade to the upper level. It is found that in the transition region the intensity (which may be expressed as a function of temperature only) increases with the charge on the ion for a sequence.On leave from D.N. College, Gulaothi 245 408, India.     

Theoretical Neix line ratios compared to solar observations

R-matrix calculations of the 1¹S - 2³S and 1¹S - 2³P electron excitation rates in He - like Cv, Ovii, and Mgxi by Kingston and Tayal are used to interpolate results for Neix. Adoption of these in emission line strength calculations leads to values for the density-sensitiveR ratio very similar to those of Pradhanet al. and Wolfsonet al., although the temperature-sensitiveG ratios are approximately 10 to 20 % lower than those deduced by these authors. However the present theoretical value ofG at the temperature of maximum Neix emission,G(T _m) = 0.82, is in excellent agreement with those observed by the SMM and P78-1 satellites for the 1980, November5 flare (G = 0.83 ± 0.01) and nonflaring active regions (G = 0.80 ± 0.05), respectively.     

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.     

Some physical considerations on NGC 1313

The ionized gas in NGC 1313 was studied by spectrophotometric means. The radial behaviour of theI(H)/I(6584),I(6717)/I(6731),N(Nii)/N(Hii), andN(Nii)/N(Sii) ratios and the deduced electron densities are discussed. The abundance ratiosN(N)/N(H) andN(N)/N(S) for the nucleus and two emission regions were also derived and compared with previous data.     

The stellar mass spectrum of the open cluster NGC 3293

We have obtained and analyzed UBVRI CCD frames of the young, 4–10 Myr, open cluster NGC 3293 and the surrounding field in order to study its stellar content and determine the cluster’s IMF. We found significantly fewer lower mass stars, M≤2.5M _⊙, than expected. This is particularly so if a single age for the cluster of 4.6 Myr is adopted as derived from fitting evolutionary models to the upper main sequence. Some intermediate-mass stars near the main sequence in the HR diagram imply an age for the cluster of about 10 Myr. When compared with the Scalo (The stellar initial mass function. ASP conference series, vol. 24, p. 201, 1998) IMF scaled to the cluster IMF in the intermediate mass range, 2.5≤M/M _⊙≤8.0 where there is good agreement, the high mass stars have a distinctly flatter IMF, indicating an over abundance of these stars, and there is a sharp turnover in the distribution at lower masses. The radial density distribution of cluster stars in the massive and intermediate mass regimes indicate that these stars are more concentrated to the cluster core whereas the lower-mass stars show little concentration. We suggest that this is evidence supporting the formation of massive stars through accretion and/or coagulation processes in denser cluster cores at the expense of the lower mass proto-stars. R.W. Slawson and E.P. Horch are guest investigators at the University of Toronto Southern Observatory, Las Campanas, Chile.     

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.     

Planetary nebula with a neutral envelope?

IUE ultraviolet spectral recording for a low excitating planetary nebula NGC 6369 is obtained. The very strong doublet 2800 Mgii in emission as well as not less strong absorption line 2852 Mgi are discovered in the spectrum of this nebula. It is shown that the resonance line 2852 Mgi may originate only in a neutral envelope, around the nebula, consisting of neutral hydrogen, neutral magnesium, and dust particles (Hi+Mgi). The importance of this absorption line as a powerful indicator of the discovery of neutral envelopes around the planetary nebulae is outlined.The possibility of the existence of one more envelope—transition zone—immediately contacting with the bright that is ionized part of nebula (Hii+Mgii) is also shown. The transition zone consists of neutral hydrogen, ionized magnesium, and dust particles (Hi+Mgii), main parameters of this zone are also obtained (Table IV).The temperature of the central star of this nebula is obtained for the first time:T _*=48000 K. Continuous background in the interval 2600–3000 Å is identified with Balmer continuum with electron temperatureT _e =12500 K.     

Coronal survey in X-rays of O vii and Ne ix

We report some results of a rocket experiment flown on 29 April, 1971. A survey of the solar corona was carried out with a pair of collimated Bragg spectrometers to study the resonance, intersystem and forbidden line emission from the helium-like ions O vii (22 Å) and Ne ix (13 Å). In the direction of dispersion the collimator provided a field of view of 1.7. Also, the continuum radiation near 3 Å was monitored by a collimated proportional counter within a view angle of 4.2. The observed X-ray emission came from the general corona, seven plage regions, and one dynamic feature- the late stage of a small flare. From the intensity of the O vii and Ne ix resonance lines the electron temperature and emission measure of the individual emitting regions are derived on the basis of two models, one (a) in which the region is assumed to be isothermal and another (b) in which the emission measure decreases exponentially with increasing temperature. The latter model, which is the most adequate of the two, yields for the electron temperature of the time-varying feature 2–3 × 10⁶ K, for the other active regions 1.5–2.5 × 10⁶ K, and for the general corona 1.3–1.7 × 10⁶ K. The Ne ix emitting regions are about 1.5 times as hot as the O vii regions. The emission measure ranges from 0.4–2.3 × 10⁴⁸ cm^–3 for all active regions and is about 2 × 10⁴⁹ cm^–3 for one hemisphere of the general corona above 10⁶ K. From an analysis of the ratio, R, of the forbidden and intersystem lines of O vii we conclude that none of the regions producing these lines at the time of the rocket flight had electron densities exceeding about 3 × 10⁹ cm^–3. Our data demonstrate a dependence of R upon temperature in agreement with the theory of Blumenthal et al. (1971). The wavelengths for the intersystem, the 1s ²2s ² S ^e–1s2p2s ² P ⁰ satellite, and the forbidden transition show in the case of Ne ix improved agreement with predictions. The observed strength of the satellite lines for both O vii and Ne ix agrees with the predictions of Gabriel's (1972) theory, which attributes their formation to dielectronic recombination.We are saddened to report the death of A. J. Meyerott on 13 November, 1971.     

Satellite lines in the solar X-ray spectrum

Observations of solar X-ray line emission using crystal spectrometers during a large chromospheric flare have provided a list of wavelengths with a precision of 0.003 Å in first order of diffraction and correspondingly better in higher orders. In addition to the resonance, intersystem (1 ¹ S ₀-2 ³ P ₁) and forbidden (1 ¹ S ₀-2 ³ S ₁) transitions of ions of the Hei isoelectronic sequence, we have recorded satellite lines arising from ions in the Lii, Bei and Bi isoelectronic sequences. These satellite features are most prominent in the iron spectrum. Apparent decreases in the ratio of forbidden and intersystem line intensities of Mgxi and Sixiii during the flare are used to derive electron densities possibly as high as 1 × 10¹³ cm^–3 in the Mgxi emitting region and 1 × 10¹⁴ cm^–3 in the Sixiii region during the event. A search for satellite lines on the long-wavelength side of the Lyman-alpha line of Hi-like ions has yielded no positive identifications.     

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.     

Coronagraphic observations of an enhanced coronal region: I,Fexii and Nixv emission line data

Nine coronal emission lines representing five stages of Fe ionization and one stage of Ni were observed in an enhanced coronal region. The data from these observations are presented along with a density model of the enhanced region obtained from the FeXIII and NiXV emission line ratios as a function of position angle. The electron densities obtained from FeXIII lines range from N _e = 10⁸ to 10⁹ cm^–3, and are slightly lower for NiXV line data. Estimates of the variation of temperature over the enhanced region are inferred from the observed line intensities.     

NIV line ratios in the Sun

Theoretical Niv emission line ratios, which incorporate several improvements over previous estimates, are presented for R ₁ = I(923.2 Å)/I(765.1 Å) and R ₂ = I(1718.6 Å)/I(1486.5 Å), which are electron density and temperature sensitive, respectively. A comparison of R ₁ with observational data for several solar features obtained with the Harvard S-055 spectrometer on board Skylab reveals generally good agreement between theory and observation, except for the quiet Sun, which is probably due to the 923.2 Å line being blended with an Feiii transition in this instance. The observed value of R ₂, determined from a quiet-Sun spectrum obtained by the S082-B spectrograph on board Skylab, implies an electron temperature in excellent agreement with that of maximum Niv fractional abundance in ionisation equilibrium, which provides observational support for the accuracy of the diagnostic calculations.     

The empirical determination of damping constants in the solar photosphere

The evaluation of damping constants is discussed and the results of their empirical determination by various authors are listed in the Appendix.The results of the determination of damping constants by analysing the wings of 38 strong Fe i lines are discussed and compared with previous results obtained by the same method from 27 moderate Fe i lines.The van der Waals damping constant ₆ multiplied by an enhancement factor E = 2.5 is suggested for the interpretation of Fe i Fraunhofer lines.     

The photospheric barium spectrum: Solar abundance and collision broadening of Baii lines by hydrogen

A study of the solar Ba ii spectrum leads to a solar abundance of barium of log ba = = 2.11±0.12, on the scale log h = 12. The observed asymmetry of the resonance line 4554 is consistent with an isotopic abundance ratio equal to the terrestrial one. The meteoritic Ba/Si abundance ratio found in carbonaceous chondrites appears to exceed the solar ratio by 0.1 to 0.2 dex (Section 5).The broadening by collisions with hydrogen atoms is determined from the solar spectrum (Section 4). Damping half-widths, h, of the three stronger Ba ii lines turn out to be larger by a factor of about 3.0 than predicted from pure van der Waals interaction of dipoles. Departures from LTE appear to be present in the cores of the resonance lines and of the lines arising from the metastable 5D levels (Section 6). The equivalent widths, however, remain practically unaffected.Equivalent widths of neutral barium lines are predicted and some new identifications of photospheric Ba i lines are suggested (Section 7).     

Density dependence of solar emission lines of boron-like ions

Steady state level density of 20 levels of boron-like ions, Mg viii and Si x, have been computed as a function of electron density and temperature. We have accounted for collisional and spontaneous radiative processes. Photo-excitation between the two levels of the ground term has also been considered. Using the computed level density, line intensities have been obtained as a function of electron density and temperature. In case of Mg viii, line intensity ratio I(430.47)/I(436.62) is found to be electron density sensitive. Likewise other pairs of lines namely, I(75.03)/I(74.86), I(315.02)/I(430.47), and I(315.02)/I(335.23) are also found to be density sensitive. Similar density sensitive line intensity ratios have been found for Si x. Absolute line fluxes from these ions at earth distance have been computed and are found to be comparable with values obtained, using various satellite and rocket measurements.Paper presented at the 5th Astronomical Society of India Meeting, held at Uttar Pradesh State Observatory, Naini Tal, India, 5–9 November, 1979.