Sextet transitions in Fe ii

We have calculated oscillator strengths for the J – J ' levels of the a  ⁶D– z  ⁶P^o, the a  ⁶D– z  ⁶D^o and the a  ⁶D– z  ⁶F^o multiplets in Fe  ii , using configuration interaction wave functions. The importance of the mixing of the ⁶P^o, ⁶D^o and ⁶F^o states is shown, as is the effect of core polarization of both the 3 p and 3 s orbitals. Good agreement with recent experimental results is obtained.     

Statistics of structure and rotation parameters of disk galaxies

Through an analysis of the optical and radio rotation curves of 57 galaxies, the following conclusions are drawn. 1) As we go from early-type to late-type along the Hubble sequence, dynamical quantities characterizing the structure and rotation show systematic variations. The bulge-to-disk mass-ratio decreases, the peak velocity V_m decreases, the period at the peak-velocity-point P_m increases, the peak-velocity-radius r_m shifts outward relative to the optical Holmberg radius r_H, the spiral arms become more loosely-wound and the galaxy flattens. 2) Within a given morphological type, the dispersion in each of the dynamical quantities is related to the mass, the more massive galaxies have larger V_m and r_m and small k (= V_m²/r_m) and β (= r_H/r_m.3) another important factor in influencing the dynamical state of a galaxy and, within a morphological type, the tidal galaxies have greater k and β and smaller r_m.     

Auroral N2(c′4Σu → aΠg emission

Inspection of recent spectra presented by Sivjee (1983) show evidence of the 0–4 and 0–5 bands of the N₂(c′₄¹Σ_u⁺ → a¹Π_g) Gaydon-Herman system. In conjunction with earlier spectra, it is now possible that this band system is a significant auroral component, with an intensity approx. 7% that of the N2 2P system. The absence in aurorae of the potentially far stronger N₂(c′₄¹Σ_u⁺ → X¹Π_g) system is discussed. It is that the O₂(A³Σ_u⁺ → X³Σ_g⁻) band system is indiscernible in Sivjee's auroral spectra, under conditio the foreground nightglow is expected to be clearly visible. On the other hand, at least one relatively strong O₂(A′³Δ_u → a¹Δ_g) band appears to be present in these spectra.     

Effects of E-region electric fields on F-region parameters at the magnetic equator

The effects of E-region electric fields on F-region parameters like height of constant electron density contours (h_N) and semi-thickness, at an equatorial station, Trivandrum, are investigated. The E-region east-west electric field (E_y) has been deduced using the horizontal magnetic field values from the ground magnetograms. It is found that the semi-thickness parameter and h_N follow closely the E-region electric field (E_y) variations in the forenoon and noon periods. In the afternoon, it is shown that there is close association between h_N and E_y, when the electric field variations are large. On a day-to-day basis also, h_N and the E-region electric field show very good correlation in the forenoon and noon hours. It is also shown that higher levels are affected more than the lower levels in the F-region by E_y.     

The impact of cooling flows on the TX–LBol relation for the most luminous clusters

We examine the effects of cooling flows on the T _X– L _Bol relation for a sample of the most X-ray luminous ( L _Bol > 10⁴⁵ erg s⁻¹) clusters of galaxies known. Using high-quality ASCA X-ray spectra and ROSAT images we explicitly account for the effects of cooling flows on the X-ray properties of the clusters and show that this reduces the previously noted dispersion in the T _X– L _Bol relationship. More importantly, the slope of the relationship is flattened from L _Bol ∝  T ³_X to approximately L _Bol ∝  T ²_X, in agreement with recent theoretical models which include the effects of shocks and pre-heating on the X-ray gas. We find no evidence for evolution in the T _X– L _Bol relation within z  ∼ 0.3. Our results demonstrate that the effects of cooling flows must be accounted for before cosmological parameters can be determined from X-ray observations of clusters. The results presented here should provide a reliable basis for modelling the T _X– L _Bol relation at high X-ray luminosities.     

On the role of the motion of the photospheric footpoints of the magnetic field lines

By means of a simple relation between the velocity v of the fluid particle and the velocity v_f of the photospheric footpoint of the magnetic field line v_z and B_z being respectively the components of v and the magnetic field B normal to the photospheric surface, it is shown formally that through the phtospheric surface the transport of all the quantities attributed to the magnetic field, such as the magnetic flux, the magnetic energy and the helicity, is independent of v_z, and v_f is the only kinematical quantity on which the transport depends. In addition, in the neighborhood of the neutral line the velocity v_l of the moving curve of constant B_z is found to be equal approximately to the component of v or v_f in the direction of v_l. Since v_l can be measured or extimated, so can the components of v and v_f near the neutral line.     

Radiative lifetimes and transition probabilities in Hf I and Hf III

Radiative lifetimes of nine odd levels in Hf III (5d6p z³P_0,1, z¹D₂, z³D_1,2,3, z³F_2,3,4) and of two odd levels in Hf I (5d6s²6p z³D_2,3) have been determined using Time-Resolved Laser-Induced Fluorescence. The investigated levels in Hf I were excited in a single-step process from the ground term (5d²6s² a³F), whereas in Hf III we started from either the ground term 5d² a³F or the low-lying 5d6s a³D term. For all of the investigated levels, the lifetimes have been measured for the first time. A multiconfiguration relativistic Hartree–Fock method, including core-polarization effects, has been used to compute radiative lifetimes of 15 Hf III levels, including those measured in this work. Transition probabilities for 55 transitions in Hf III are also given.     

Single star scidar: atmospheric parameters profiling using the simulated annealing algorithm

In this paper, we present a new method to estimate, for each turbulent layer labelled i , the horizontal wind speed   v ( h _i )  , the standard deviation of the horizontal wind speed fluctuations  σ _v ( h_i )  and the integrated value of   C ² _n   over the thickness  Δ h_i   of the turbulent layer   C ² _n ( h_i )Δ h_i   , where   h_i   is the altitude of the turbulent layer. These parameters are extracted from single star scintillation spatiotemporal cross-correlation functions of atmospheric speckles obtained within the generalized mode. This method is based on the simulated annealing algorithm to find the optimal solution required to solve the problem. Astrophysics parameters for adaptive optics are also calculated using   C ² _n ( h_i )  and   v ( h_i )  values. The results of other techniques support this new method.     

Charged boson stars and vacuum instabilities

We consider charged boson stars and study their effect on the structure of the vacuum. For very compact particle like “stars”, with constituent mass m_* close to the Planck mass m_Pl, i.e. m²_* = O(m²_Pl), we argue that there is electric charge Z_c, which, primarily, is due to the formation of a pion condensate (Z_c 0.5⁻¹e, where is the fine structure constant and e is the electric charge of the positron). If the charge of the “star” is larger than Z_c we find numerical evidence for a complete screening indicating a limiting charge for a very compact object. There is also a less efficient competing charge screening mechanism due to spontaneous electron-positron pair creation in which case Z_c ⁻¹e. Astrophysical and cosmological abundances of charged compact boson stars are briefly discussed in terms of dark matter.     

Deactivation of N2A Σumolecules in the aurora

Recent rocket observations of the N₂ V-K (Vegard-Kaplan) system in the aurora have been reinterpreted using an atmospheric model based on mass spectrometer measurements in an aurora of similar intensity at the same time of year. In contrast to the original interpretation, we find that population by cascade from the C³Π_u and B³Π_g states in the A³Σ_u⁺v=0,1 levels, as calculated using recently measured electron excitation cross sections, accurately accounts for the observed relative emission rates (IV-K/12PG_0.0). In addition there is no need to change the production rate of A ³ Σ _u⁺ molecules relative to that of C³Π_uv=0 as a function of altitude in order to fit the profile of the deactivation probability to the atmospheric model. Quenching of A ³ Σ _u⁺ molecules at high altitudes is dominated by atomic oxygen. The rate constants for the v=0 and v=1 levels are 8 × 10⁻¹¹ cm³ sec⁻¹ and 1.7 × 10⁻¹⁰ cm³ sec⁻¹ respectively, as determined using the model atmosphere mentioned above. Recent observations with a helium cooled mass spectrometer suggest that conventional mass spectrometer measurements tend to underestimate the atomic oxygen relative concentration. The rate coefficients may therefore be too large by as much as a factor of 3. Below 130 Km we find that it is possible to account for the deactivation in bright auroras by invoking large nitric oxide concentrations, similar to those recently observed mass spectrometrically and using a rate constant of 8 × 10⁻¹¹ cm³ sec⁻¹ for both the v=1 levels. This rate constant is very nearly the same as that measured in the laboratory (7 × 10⁻¹¹ cm³ sec⁻¹). Molecular oxygen appears not to play a significant role in deactivating the lower A ³ Σ _u⁺ levels.     

Seismological studies of ZZ Ceti stars – II. Application to the ZZ Ceti class

We used the detected pulsation modes and adiabatic pulsation models to do seismology of the class of ZZ Ceti stars and measure the H layer mass for 83 stars. We found the surface hydrogen layer to be within the range  10^−9.5≥ M _H/ M _*≥ 10⁻⁴  , with an average of   M _H/ M _*= 10^−6.3  , which is thinner than the predicted value of   M _H/ M _*= 10⁻⁴  , indicating that the stars lose more mass during their evolution than previously expected. These results are preliminary and do not include the possible effects of realistic C/O profiles on the fits.     

Electron density diagnostic potential of the Si x ion and its application in Procyon

Theoretical electron density sensitive line ratios   R ₁– R ₆  of Si  x soft X-ray emission lines are presented. We found that these line ratios are sensitive to electron density n _e, and the ratio R ₁ is insensitive to electron temperature T _e. For reliable determination of the electron density of laboratory and astrophysical plasmas, atomic data, such as electron impact excitation rates, are very important. Our results reveal that the discrepancy of the line ratios from different atomic data calculated with the distorted wave (DW) approximation and the R-matrix method is up to 19 per cent at   n _e= 2 × 10⁸ cm⁻³  . We applied the theoretical intensity ratio R ₁ to the Low Energy Transmission Grating Spectrometer (LETGS) spectrum of the solar-like star Procyon. By comparing the observed value (1.29) with the theoretical calculation, the derived electron density n _e is  2.6 × 10⁸ cm⁻³  , which is consistent with that derived from  (C  v < 8.3 × 10⁸ cm⁻³)  . When the temperature structure of the Procyon corona is taken into account, the derived electron density increases from   n _e= 2.6 × 10⁸  to  2.8 × 10⁸ cm⁻³  .     

Anomalous mixing of the w2Po3/2 and x6Po3/2 levels in Fe ii

The neighbouring lines a ⁶D_5/2– x ⁶P^o_3/2 (1272.617 Å) and a ⁶D_5/2– w ²P^o_3/2 (1272.657 Å) have been observed in the UV spectrum of χ Lupi to be of comparable intensity. The latter, Δ S  = 2, transition would be expected to be very weak. The two upper states should display negligible mixing. We give a detailed, quantitative discussion of how the two upper states are in fact strongly mixed through their mixing with 3d⁶(³D)4p ⁴P^o_3/2, and hence we explain the relative strengths of the two UV lines.     

On the maximum amplitude and coherence of the kilohertz quasi-periodic oscillations in low-mass X-ray binaries

I study the behaviour of the maximum rms fractional amplitude, r _max, and the maximum coherence, Q _max, of the kilohertz quasi-periodic oscillations (kHz QPOs) in a dozen low-mass X-ray binaries. I find that (i) the maximum rms amplitudes of the lower- and upper-kHz QPOs,   r ^ℓ_max  and   r ^u_max  , respectively, decrease more or less exponentially with increasing luminosity of the source; (ii) the maximum coherence of the lower-kHz QPO,   Q ^ℓ_max  , first increases and then decreases exponentially with luminosity, at a faster rate than both   r ^ℓ_max  and   r ^u_max  ; (iii) the maximum coherence of the upper-kHz QPO,   Q ^u_max  , is more or less independent of luminosity; and (iv) r _max and Q _max show the opposite behaviour with hardness of the source, consistent with the fact that there is a general anticorrelation between luminosity and spectral hardness in these sources. Both r _max and Q _max in the sample of sources, and the rms amplitude and coherence of the kHz QPOs in individual sources show a similar behaviour with hardness. This similarity argues against the interpretation that the drop of coherence and rms amplitude of the lower-kHz QPO at high QPO frequencies in individual sources is a signature of the innermost stable circular orbit around a neutron star. I discuss possible interpretations of these results in terms of the modulation mechanisms that may be responsible for the observed variability.     

Interpreting vertical plasma drift in the mid-lattitude ionosphere using ionosonde measurements

Measurements of the density at the F₂ peak (N_mF₂) were obtained by the Boulder, Colorado, ionosonde as part of the SUNDIAL-86 campaign. The measurements were made during a period of low to moderate geomagnetic activity following a “disturbed” day. These measurements were then used to estimate the height of the F₂ peak (h_mF₂). A three-dimensional time-dependent model of Earth's ionosphere was used to calculate N_mF₂ and h_mF₂ using the vertical plasma drift as a free parameter. Since the plasmasphere-ionosphere exchange flux can remain upward during the night for these conditions, different feasible flux scenarios were inputed to the ionospheric model. These different flux scenarios had a large effect on the “induced” vertical plasma drifts required to match the measurements (i.e. at times greater than a factor of 2 in speed or a difference in direction). Futhermore, uncertainty in the O⁺---O collision frequency changes the required vertical plasma drift at night. Despite knowledge of h_mF₂, interpretation of the vertical plasma drifts as meridional neutral winds is compromised by a lack of knowledge of the plasmasphere-ionosphere exchange flux following disturbed days.     

On the transfer of radiation at asteroidal surfaces in relation to their orbit deflection – I

One of the methods discussed in deflecting the orbit of an Earth-colliding asteroid is the use of nuclear explosives. In assessing its feasibility, apart from political considerations, it is important to quantify how effective it is in orbit deflection. The transfer of radiation incident at the surface is governed by a non-linear diffusion equation. For low-yield explosions with a slab geometry ( S ₀≃10⁸ kJ μs⁻¹), the temperature at depth x and time t is well approximated by a similarity solution of the form T ( x , t )= T ₀ f (ξ), ξ= x/ ( T ⁿ ₀ t )^1/2, with T ₀ given by ( S ₀/σ)^1/4, where σ is the Stefan–Boltzmann constant, n is an index that specifies the radiation transfer and f (ξ) is the solution of a non-linear differential equation subject to the condition f (0)=1 and lim_ξ→∞ f (ξ)=0. For high-yield explosions ( S ₀≃10¹⁰ kJ μs⁻¹), numerical solutions to the non-linear diffusion equation can be obtained. These solutions have properties similar to the case of low-yield explosions. If the duration of the explosion is d ×10⁻⁸ s, where d is close to 3, the fraction of energy absorbed by the surface is found to be 7, 12 and 23 per cent for S ₀=10⁸, 10⁹ and 10¹⁰ kJ μs⁻¹ respectively.     

Electron excitation and auroral emission parameters

Auroral luminosities of the main emission lines in the aurora have been calculated for excitation by an isotopic primary electron flux with spectra of the form J(E) = AE exp (−E/E₁) + B(E₂)E exp (−E/E₁). The variation of emissions from O and N₂⁺ with height are shown, as are the variations of column integrated intensities and pertinent intensity ratios with the characteristic energy E₂, this leading to a method of estimating the electron spectrum from ground observation.     

Generalized limits to the number of light particle degrees of freedom from big bang nucleosynthesis

We compute the big bang nucleosynthesis limit on the number of light neutrino degrees of freedom in a model-independent likelihood analysis based on the abundances of ⁴He and ⁷Li. We use the two-dimensional likelihood functions to simultaneously constrain the baryon-to-photon ratio and the number of light neutrinos for a range of ⁴He abundances Y_p = 0.225–0.250, as well as a range in primordial ⁷Li abundances from (1.6 to 4.1) ×10⁻¹⁰. For (⁷Li/H)_p = 1.6 × 10⁻¹⁰, as can be inferred from the ⁷Li data from Population II halo stars, the upper limit to N_ν based on the current best estimate of the primordial ⁴He abundance of Y_p = 0.238 is N_ν < 4.3 and varies from N_ν < 3.3 (at 95% C.L.) when Y_p = 0.225 to N_ν < 5.3 when Y_p = 0.250. If ⁷Li is depleted in these stars the upper limit to N_ν is relaxed. Taking (⁷Li/H)_p = 4.1 × 10⁻¹⁰, the limit varies from N_ν < 3.9 when Y_p = 0.225 to N_ν 6 when Y_p = 0.250. We also consider the consequences on the upper limit to N_ν if recent observations of deuterium in high-redshift quasar absorption-line systems are confirmed.     

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.     

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.