Extreme ultraviolet transitions of Fe xxi in solar, stellar and laboratory spectra

Recent R -matrix calculations of electron impact excitation rates for transitions among the 2s²2p², 2s2p³ and 2p⁴ levels of Fe  xxi are used to derive theoretical electron density ( N _e) sensitive emission-line ratios involving 2s²2p²–2s2p³ transitions in the ∼98–146Å wavelength range. A comparison of these with observations from the PLT tokamak plasma, for which the electron density has been independently determined, reveals generally very good agreement between theory and experiment, and in some instances removes discrepancies found previously. The observed Fe  xxi ratios for a solar flare, obtained with the OSO–5 satellite, imply electron densities which are consistent, with discrepancies that do not exceed 0.2 dex. In addition, the derived values of N _e are similar to those estimated for the high-temperature regions of other solar flares. The good agreement between theory and observation, in particular for the tokamak spectra, provides experimental support for the accuracy of the present line-ratio calculations, and hence for the atomic data on which they are based.     

Si x emission lines in spectra obtained with the Solar EUV Rocket Telescope and Spectrograph (SERTS)

New R -matrix calculations of electron impact excitation rates for transitions among the 2s²2p, 2s2p² and 2p³ levels of Si  x are presented. These data are subsequently used, in conjunction with recent estimates for proton excitation rates, to derive theoretical electron density sensitive emission-line ratios involving transitions in the ∼253–356 Å wavelength range. A comparision of these with observations of a solar active region and subflare, obtained during the 1989 flight of the Solar EUV Rocket Telescope and Spectrograph ( SERTS ), reveals that the electron densities determined from most of the Si  x line ratios are consistent with one another for both solar features. In addition, the derived densities are also in good agreement with the values of N _e estimated from diagnostic lines in other species formed at similar electron temperatures to Si  x , such as Fe  xii and Fe  xiii . These results provide observational support for the general accuracy of the adopted atomic data, and hence line ratio calculations, employed in the present analysis. However, we find that the Si  x 256.32-Å line is blended with the He  ii transition at the same wavelength, while the feature at 292.25 Å is not due to Si  x , but currently remains unidentified. The intensity of the 253.81-Å line in the SERTS active region spectrum is about a factor of 3 larger than expected from theory, but the reason for this is unclear, and requires additional observations to explain the discrepancy.     

Emission lines of Fe xv in spectra obtained with the Solar Extreme-Ultraviolet Research Telescope and Spectrograph

Recent R-matrix calculations of electron impact excitation rates in Mg-like Fe  xv are used to derive theoretical emission-line ratios involving transitions in the 243–418 Å  wavelength range. A comparison of these with a data set of solar active region, subflare and off-limb spectra, obtained during rocket flights by the Solar Extreme-Ultraviolet Research Telescope and Spectrograph (SERTS), reveals generally very good agreement between theory and observation, indicating that most of the Fe  xv emission lines may be employed with confidence as electron density diagnostics. In particular, the 312.55-Å  line of Fe  xv is not significantly blended with a Co  xvii transition in active region spectra, as suggested previously, although the latter does make a major contribution in the subflare observations. Most of the Fe  xv transitions which are blended have had the species responsible clearly identified, although there remain a few instances where this has not been possible. We briefly address the long-standing discrepancy between theory and experiment for the intensity ratio of the  3s^2 1S–3s3p ³P₁  intercombination line at 417.25 Å  to the  3s^2 1S–3s3p ¹P  resonance transition at 284.16 Å.     

Fe xiixii emission lines in solar active regions observed by the RES-C spectroheliograph on the CORONAS-I mission

Theoretical line intensity ratios involving Fe  xii transitions in the 186–201 Å wavelength range are compared with observational data for five solar active regions, obtained by the RES-C spectroheliograph on the CORONAS-I mission. Generally good agreement is found between theory and observation, hence resolving discrepancies previously found in the comparison of calculations with active region and subflare spectra from the Solar EUV Rocket Telescope and Spectrograph ( SERTS ). However, the Fe  xii 190.06- and 201.12-Å lines are blended with Fe  x 190.04 Å and Fe  xiii 201.13 Å, respectively. In addition, a weak feature at ∼197 Å, tentatively identified as Fe  xii 196.87 Å, does not appear to be due to this ion.     

Effective collision strengths for fine-structure forbidden transitions among the 2s22p3 levels of Ne iv

Effective collision strengths for electron-impact excitation of the N-like ion Ne  iv are calculated in the close-coupling approximation using the multichannel R-matrix method. Specific attention is given to the 10 astrophysically important fine-structure forbidden transitions among the ⁴S^o, ²D^o and ²P^o levels in the 2s²2p³ ground-state configuration. The expansion of the total wavefunction incorporates the lowest 11 LS eigenstates of Ne  iv , consisting of eight n  = 2 terms with configurations 2s²2p³, 2s2p⁴ and 2p⁵, together with three n  = 3 states of configuration 2s²2p²3s. We present in graphical form the effective collision strengths obtained by thermally averaging the collision strengths over a Maxwellian distribution of velocities, for all 10 fine-structure transitions, over the range of electron temperatures log T (K) = 3.6 to log T (K) = 6.1 (the range appropriate for astrophysical applications). Comparisons are made with the earlier, less sophisticated close-coupling calculation of Giles, and excellent agreement is found in the limited temperature region where a comparison is possible [log T (K) = 3.7 to log T (K) = 4.3]. At higher temperatures the present data are the only reliable results currently available.     

Effective collision strengths for forbidden transitions among the 3s23p3 fine-structure levels of Cl IIIIII

Effective collision strengths for the 10 astrophysically important fine-structure forbidden transitions among the ⁴S^o, ²D^o and ²P^o levels in the 3s²3p³ configuration of Cl  iii are presented. The calculation employs the multichannel R-matrix method to compute the electron-impact excitation collision strengths in a close-coupling expansion, which incorporates the lowest 23 LS target eigenstates of Cl  iii . These states are formed from the 3s²3p³, 3s3p⁴, 3s²3p²3d and 3s²3p²4s configurations. The Maxwellian-averaged effective collision strengths are presented graphically for all 10 fine-structure transitions over a wide range of electron temperatures appropriate for astrophysical applications [log  T (K)=3.3−log  T (K)=5.9]. Comparisons are made with the earlier seven-state close-coupling calculation of Butler & Zeippen, and in general excellent agreement is found in the low-temperature region where a comparison is possible [log  T (K)=3.3−log  T (K)=4.7]. However, discrepancies of up to 30 per cent are found to occur for the forbidden transitions which involve the ⁴S^o ground state level, particularly for the lowest temperatures considered. At the higher temperatures, the present data are the only reliable results currently available.     

Molecular hydrogen emission in Cygnus A

We present J , H and K -band spectroscopy of Cygnus A, spanning 1.0–2.4 μm in the rest-frame and hence several rovibrational H₂, H recombination and [Fe  ii ] emission lines. The lines are spatially extended by up to 6 kpc from the nucleus, but their distinct kinematics indicate that the three groups (H, H₂ and [Fe  ii ]) are not wholly produced in the same gas. The broadest line, [Fe  ii ] λ 1.644, exhibits a non-Gaussian profile with a broad base (FWHM≃1040 km s⁻¹), perhaps because of the interaction with the radio source. Extinctions to the line-emitting regions substantially exceed earlier measurements based on optical H recombination lines.
Hard X-rays from the quasar nucleus are likely to dominate the excitation of the H₂ emission. The results of Maloney, Hollenbach & Tielens are thus used to infer the total mass of gas in H₂ v=1–0 S(1)-emitting clouds as a function of radius, for gas densities of 10³ and 10⁵ cm⁻³, and stopping column densities N _H=10²²–10²⁴ cm⁻². Assuming azimuthal symmetry, at least 2.3×10⁸ M_⊙ of such material is present within 5 kpc of the nucleus, if the line-emitting clouds see an unobscured quasar spectrum. Alternatively, if the bulk of the X-ray absorption to the nucleus inferred by Ueno et al. actually arises in a circumnuclear torus, the implied gas mass rises to ∼10¹⁰ M_⊙. The latter plausibly accounts for 10⁹ yr of mass deposition from the cluster cooling flow, for which within this radius.     

X-ray and extreme-ultraviolet emission from the coronae of Capella

The primary objective of this work is the analysis and interpretation of coronal observations of Capella obtained in 1999 September with the High Energy Transmission Grating Spectrometer on the Chandra X-ray Observatory and the Extreme Ultraviolet Explorer ( EUVE ). He-like lines of O (O  vii ) are used to derive a density of 1.7×10¹⁰ cm⁻³ for the coronae of the binary, consistent with the upper limits derived from Fe  xxi , Ne  ix and Mg  xi line ratios. Previous estimates of the electron density based on Fe  xxi should be considered as upper limits. We construct emission measure distributions and compare the theoretical and observed spectra to conclude that the coronal material has a temperature distribution that peaks around 4–6 MK , implying that the coronae of Capella were significantly cooler than in the previous years. In addition, we present an extended line list with over 100 features in the 5–24 Å wavelength range, and find that the X-ray spectrum is very similar to that of a solar flare observed with SMM . The observed to theoretical Fe  xvii 15.012-Å line intensity reveals that opacity has no significant effect on the line flux. We derive an upper limit to the optical depth, which we combine with the electron density to derive an upper limit of 3000 km for the size of the Fe  xvii emitting region. In the same context, we use the Si  iv transition region lines of Capella from HST /Goddard High-Resolution Spectrometer observations to show that opacity can be significant at T =10⁵ K , and derive a path-length of ≈75 km for the transition region. Both the coronal and transition region observations are consistent with very small emitting regions, which could be explained by small loops over the stellar surfaces.     

Effective collision strengths for fine-structure forbidden transitions among the 2s22p3 levels of S x

Effective collision strengths for electron-impact excitation of the N-like ion S  x are calculated in the close-coupling approximation using the multichannel R -matrix method. Specific attention is given to the 10 astrophysically important fine-structure forbidden transitions among the ⁴S^o, ²D^o and ²P^o levels in the 2s²2p³ ground configuration. The total (e⁻+ion) wavefunction is expanded in terms of the 11 lowest LS eigenstates of S  x , and each eigenstate is represented by extensive configuration-interaction wavefunctions. The collision strengths obtained are thermally averaged over a Maxwellian distribution of velocities, for all 10 fine-structure transitions, over the range of electron temperatures log  T (K)=4.6–6.7 (the range appropriate for astrophysical applications). The present effective collision strengths are the only results currently available for these fine-structure transition rates.     

[Al ii] in the ultraviolet spectrum of the symbiotic star RR Telescopii

An inspection of a GHRS/ HST spectrum of the symbiotic star RR Telescopii reveals the presence of the [Al  ii ] 3s²¹S – 3s3p ³P₂ line at a vacuum wavelength of 2661.06±0.08 Å, 8.89±0.08 Å away from the Al  ii ] 3s²¹S – 3s3p ³P₁ intercombination transition at 2669.95 Å, in good agreement with the theoretical prediction of Δ λ =8.80 Å. We also find that the Al  ii ] line profile is asymmetric, showing a strong low-density component with a weak high-density wing, redshifted by 30 km s⁻¹, in agreement with the findings of Schild & Schmid, which were based on optical observations. Our measurement of the emission-line ratio R I (2661.06 Å)/ I (2669.95 Å)=0.027±0.003 implies log  N _e=5.8±0.2, in good agreement with the densities found from other ions, such as Si  iii . These results provide strong evidence that we have detected the [Al  ii ] line, the first time (to our knowledge) that this feature has been reliably identified in an astrophysical or laboratory spectrum.     

An investigation of Fe xvi emission lines in solar and stellar extreme-ultraviolet and soft X-ray spectra

New fully relativistic calculations of radiative rates and electron impact excitation cross-sections for Fe  xvi are used to determine theoretical emission-line ratios applicable to the 251–361 and 32–77 Å portions of the extreme-ultraviolet (EUV) and soft X-ray spectral regions, respectively. A comparison of the EUV results with observations from the Solar Extreme-Ultraviolet Research Telescope and Spectrograph (SERTS) reveals excellent agreement between theory and experiment. However, for emission lines in the 32–49 Å portion of the soft X-ray spectral region, there are large discrepancies between theory and measurement for both a solar flare spectrum obtained with the X-Ray Spectrometer/Spectrograph Telescope (XSST) and for observations of Capella from the Low-Energy Transmission Grating Spectrometer (LETGS) on the Chandra X-ray Observatory . These are probably due to blending in the solar flare and Capella data from both first-order lines and from shorter wavelength transitions detected in second and third order. By contrast, there is very good agreement between our theoretical results and the XSST and LETGS observations in the 50–77 Å wavelength range, contrary to previous results. In particular, there is no evidence that the Fe  xvi emission from the XSST flare arises from plasma at a much higher temperature than that expected for Fe  xvi in ionization equilibrium, as suggested by earlier work.     

Do binaries in clusters form in the same way as in the field?

We examine the dynamical destruction of binary systems in star clusters of different densities. We find that at high densities  (10⁴– 10⁵ M_⊙ pc⁻³)  almost all binaries with separations  >10³  au are destroyed after a few crossing times. At low densities [     ], many binaries with separations  >10³  au are destroyed, and no binaries with separations  >10⁴  au survive after a few crossing times. Therefore, the binary separations in clusters can be used as a tracer of the dynamical age and past density of a cluster.
We argue that the central region of the Orion nebula cluster was ∼100 times denser in the past with a half-mass radius of only 0.1–0.2 pc as (i) it is expanding, (ii) it has very few binaries with separations  >10³  au and (iii) it is well mixed and therefore dynamically old.
We also examine the origin of the field binary population. Binaries with separations  <10²  au are not significantly modified in any cluster, therefore at these separations the field reflects the sum of all star formation. Binaries with separations in the range  10²– 10⁴  au are progressively more and more heavily affected by dynamical disruption in increasingly dense clusters. If most star formation is clustered, these binaries must be overproduced relative to the field. Finally, no binary with a separation  >10⁴  au can survive in any cluster and so must be produced by isolated star formation, but only if all isolated star formation produces extremely wide binaries.     

HST/STIS observations of the high-velocity interstellar cloud HVC 291.2−41.2+80: a warm, mainly ionized high-velocity cloud

We present intermediate-resolution HST /STIS spectra of a high-velocity interstellar cloud ( v _LSR=+80 km s⁻¹) towards DI 1388, a young star in the Magellanic Bridge located between the Small and Large Magellanic Clouds. The STIS data have a signal-to-noise ratio (S/N) of 20–45 and a spectral resolution of about 6.5 km s⁻¹ (FWHM). The high-velocity cloud absorption is observed in the lines of C  ii , O  i , Si  ii , Si  iii , Si  iv and S  iii . Limits can be placed on the amount of S  ii and Fe  ii absorption that is present. An analysis of the relative abundances derived from the observed species, particularly C  ii and O  i , suggests that this high-velocity gas is warm ( T _k∼10³–10⁴ K) and predominantly ionized. This hypothesis is supported by the presence of absorption produced by highly ionized species, such as Si  iv . This sightline also intercepts two other high-velocity clouds that produce weak absorption features at v _LSR=+113 and +130 km s⁻¹ in the STIS spectra.     

Radiative decay data for highly excited Zr i levels

Radiative lifetimes of 17 excited levels in Zr  i , in the energy interval 29 000–40 974 cm⁻¹, have been investigated using the time-resolved laser-induced fluorescence method. The levels belong to the 4d²5s5p, 4d³5p and 4d5s²5p electronic configurations and were excited in a single-step process from either the ground term, 4d²5s² a ³F, or from the low-lying 4d²5s² a ³P and a ⁵F terms. For three levels, we confirm previous measurements while for 14 of the levels the lifetimes have been measured for the first time. The experimental results are compared to theoretical calculations performed with a multiconfiguration relativistic Hartree–Fock method including core-polarization effects. Theoretical transition probabilities of astrophysical interest, scaled by the experimental lifetimes, for the depopulating channels of the investigated levels are also presented.     

Neutral sodium atoms release from the surface of the Moon induced by meteoroid impacts

In this work, we calculate the vapour and neutral Na production rates on the Moon, as due to the impacts of meteoroids in the radius range of 10⁻⁸–0.15 m. We limit our calculations to this size range, since meteoroids with radius larger than 0.15 m have not been found to be important for the production of the exosphere in a time interval comparable with that of the observations.
We have considered a new dynamical model of the flux of meteoroids at the heliocentric distance of the Moon, regarding objects in the radius range of 10⁻²–0.15 m. Instead, the flux of smaller meteoroids (radius range 10⁻⁸–10⁻² m) has been calculated using the two distributions adopted by Cintala and Love & Brownlee.
The results of our model are that (i) the neutral Na production rate is  ∼3–4.9 × 10⁴ atoms cm⁻² s⁻¹  , slightly larger than the previous estimates  (∼2–3 × 10⁴ atoms cm⁻² s⁻¹)  , and (ii) only about 6 per cent of neutral Na is produced by the impacts of meteoroids in the size range 10⁻³–0.15 m, whereas about 94 per cent of the Na comes from the  10⁻⁵–10⁻³ m  size range.     

3s23p–3s3p2 transitions in S iv

We use both the civ3 and mchf codes to calculate oscillator strengths of allowed and intercombination lines in the  3s²3p–3s3p²  multiplets. Valence, core–valence and some core–core correlation effects are included. The two approaches give results in excellent agreement. Core effects are particularly important for the intercombination lines, though relatively minor for allowed transitions.
We obtain a branching ratio of 1.42 with an estimated accuracy of 0.02 for the     transitions, compared with an experimental value of  1.12±0.1  . The A -values of the     intercombination lines are substantially different from those of previous calculations.     

Inner-shell photoionization of O iii

A 25-state R -matrix calculation is performed to obtain photoionization cross-sections for transitions from the 1s²2s²2p²³P ground state of the O  iii ion. Results are obtained for a range of photon energies, including those at which K-shell photoionization processes take place. We compare our results with those from previous calculations. Excellent agreement is obtained. We also consider resonances owing to transitions of a 1s electron excited into the 2p orbital and compare with a recent calculation.     

RR Pic (1925): a Chandra X-ray view

We present the Chandra ACIS-S3 data of the old classical nova RR Pic (1925). The source has a count rate of 0.067 ± 0.002 count s⁻¹ in the 0.3–5.0 keV energy range. We detect the orbital period of the underlying binary system in the X-ray wavelengths. We also find that the neutral hydrogen column density differs for orbital minimum and orbital maximum spectra with values  0.25^+0.23_−0.18× 10²²  and  0.64^+0.13_−0.14× 10²² cm⁻²  at 3σ confidence level. The X-ray spectrum of RR Pic can be represented by a composite model of bremsstrahlung with a photoelectric absorption, two absorption lines centered around 1.1–1.4 keV and five Gaussian lines centered at emission lines around 0.3–1.1 keV corresponding to various transitions of S, N, O, C, Ne and Fe. The bremsstrahlung temperature derived from the fits ranges from 0.99 to 1.60 keV and the unabsorbed X-ray flux is found to be  2.5^+0.4_−1.2× 10⁻¹³ erg  cm⁻² s⁻¹  in the 0.3–5.0 keV range with a luminosity of 1.1 ± 0.2  10³¹ erg s⁻¹  at 600 pc. We also detect excess emission in the spectrum possibly originating from the reverse shock in the ejecta. A fit with a cooling flow plasma emission model shows enhanced abundances of He, C, N, O and Ne in the X-ray emitting region indicating existence of diffusive mixing.     

Calculated spectra for HeH+ and its effect on the opacity of cool metal-poor stars

The wavelength and Einstein A coefficient are calculated for all rotation–vibration transitions of  ⁴He¹H⁺, ³He¹H⁺, ⁴He²H⁺  and  ³ He²H⁺  , giving a complete line list and the partition function for  ⁴HeH⁺  and its isotopologues. This opacity is included in the calculation of the total opacity of low-metallicity stars and its effect is analysed for different conditions of temperature, density and hydrogen number fraction. For a low helium number fraction (as in the Sun), it is found that HeH⁺ has a visible but small effect for very low densities  (ρ≤ 10⁻¹⁰ g cm⁻³)  , at temperatures around 3500 K. However, for high helium number fraction, the effect of HeH⁺ becomes important for higher densities  (ρ≤ 10⁻⁶ g cm⁻³)  , its effect being most important for a temperature around 3500 K. Synthetic spectra for a variety of different conditions are presented.     

Evolution of massive binary black holes

Since many or most galaxies have central massive black holes (BHs), mergers of galaxies can form massive binary black holes (BBHs). In this paper we study the evolution of massive BBHs in realistic galaxy models, using a generalization of techniques used to study tidal disruption rates around massive BHs. The evolution of BBHs depends on BH mass ratio and host galaxy type. BBHs with very low mass ratios (say, ≲0.001) are hardly ever formed by mergers of galaxies, because the dynamical friction time-scale is too long for the smaller BH to sink into the galactic centre within a Hubble time. BBHs with moderate mass ratios are most likely to form and survive in spherical or nearly spherical galaxies and in high-luminosity or high-dispersion galaxies; they are most likely to have merged in low-dispersion galaxies (line-of-sight velocity dispersion ≲90 km s⁻¹) or in highly flattened or triaxial galaxies.
The semimajor axes and orbital periods of surviving BBHs are generally in the range  10^-3–10 pc  and  10–10⁵ yr;  they are also larger in high-dispersion galaxies than in low-dispersion galaxies, larger in nearly spherical galaxies than in highly flattened or triaxial galaxies, and larger for BBHs with equal masses than for BBHs with unequal masses. The orbital velocities of surviving BBHs are generally in the range  10²–10⁴ km s^-1  . The methods of detecting surviving BBHs are also discussed.
If no evidence of BBHs is found in AGNs, this may be either because gas plays a major role in BBH orbital decay or because nuclear activity switches on soon after a galaxy merger, and ends before the smaller BH has had time to spiral to the centre of the galaxy.