Electronimpact excitation of Hlike Cr, Mn, Fe, Co, and Ni for applications in modeling Xray astrophysical sources^{⋆}
^{1}
NASA Goddard Space Flight Center,
Greenbelt,
MD
20771,
USA
email: Charles.A.Malespin@nasa.gov
^{2}
Auburn University, Auburn, AL
36849,
USA
Received: 11 November 2010
Accepted: 29 November 2010
Context. Accurate atomic data for the less abundance Fepeak elements are required for use in Xray astrophysical studies.
Aims. We calculate high quality electronimpact excitation collision strengths and effective collision strengths for hydrogenic Cr, Mn, Fe, Co, and Ni.
Methods. We use the Dirac Rmatrix method, the intermediate coupling frame transformation Rmatrix method, the semirelativistic distortedwave method and the fullyrelativistic distortedwave method to calculate collision strengths for each of the ions. The ADAS collisionalradiative codes are used to produce photon emissivity coefficients for each ion.
Results. Results are presented for atomic energy levels, spontaneous emission coefficients, electronimpact excitation collision strengths and associated effective collision strengths for each of the five species under consideration. We find relativistic effects can contribute an approximate 10% increase to the background cross section in relation to semirelativistic collision calculations. We also confirm that radiation damping plays a prominent role for certain near threshold resonances. In order check the integration of our results within collisionalradiative modeling codes, we have used the ADAS package for some preliminary modeling of photon emissivities. The atomic data shall be made available online through the OPENADAS site and the CFADC database
Key words: atomic data / atomic processes / supernovae: general
Final datasets for each ion are only available in electronic form at CDS via anonymous ftp to cdsarc.ustrasbg.fr (130.79.128.5) or via http://cdsarc.ustrasbg.fr/vizbin/qcat?J/A+A/526/A115
© ESO, 2011
1. Introduction
Recent Xray observations of Fepeak elements from supernova remnants have highlighted the need for accurate atomic data for highly charged, less abundant Fepeak elements such as Mn, Cr, Co, and Ni. The first Xray observation of spectral emission from Co and Mn was reported for the Helike ion stages in the Galactic supernova remnant W49B using observations by the ASCA Xray satellite (Hwang et al. 2000). These detections were later confirmed by XMMNewton observations (Miceli et al. 2006). Since no photon emissivity data existed for the Helike stages of Co or Mn, Hwang et al. (2000) estimated emissivities by interpolating the emissivity data generated for Ca, Fe, Ni by the Raymond and Smith thermal models Raymond & Smith 1977; these were then used to infer the abundances of these elements. Subsequently, Cr line emission has been reported for W49B, Tycho’s SNR, Kepler’s SNR, and Cas A with Chandra observations (Yang et al. 2009). Generally the Cr ion states (as estimated by the centroid energy of the Cr emission in these moderateresolution CCD spectra) track the ion stages of Fe in the same objects and range from an estimated Nelike to Helike ions. Tamagawa et al. (2009) also used the Suzaku Xray observatory to tentatively identify the detected Mn and Cr emissions in Tycho’s SNR with the Nelike stage. Mn and Cr emission have now also been reported in the Xray emission from clusters of galaxies (e.g., Tamura et al. 2010). Aside from their presence in Xray spectra, the line emission of the Fepeak elements in supernova remnants are important diagnostics of supernova explosions. For example, the MntoCr line strengths are sensitive to the neutron excess in Type Ia (thermonuclear) explosions, and are diagnostics of the progenitor metallicity (Badenes et al. 2008). These are important constraints given that we do not yet know the details of the progenitor systems that produce Ia supernovae.
The interpretation of these diagnostics depend on having accurate atomic data for the Fepeak elements. In this paper we calculate a set of atomic data for Hlike Fe peak elements that can be used in analyzing Xray spectra from these Hlike ions. Work on further isoelectronic sequences is currently in progress. In our calculations, we compare with NIST (Ralchenko et al. 2010) level energies and spontaneous emission coefficients for Hlike Co, Mn, Fe, Cr and Ni. We also present calculations for electronimpact excitation for each of these ions. In the case of Co, Mn, Cr and Ni these are the first complete Rmatrix calculations. However, for Hlike Fe there have been several previous calculations. Aggarwal & Kingston (1993) performed an LS coupling Rmatrix calculation for all transitions within the first 5 nshells. Radiation damping was shown to be important by Gorczyca and Badnell (1996) in a small n = 2 Rmatrix calculation. Kisielius et al. (1996) performed a fullyrelativistic Dirac Rmatrix calculation for the first 5 nshells, but without radiation damping. Radiation damping was included in the n = 5 BreitPauli Rmatrix calculation of Ballance et al. (2002). Recently, a Dirac Rmatrix calculation by Aggarwal et al. (2008) was performed but without the effects of radiation damping. Aggarwal et al. (2008) reported that for most of the transitions there was reasonable agreement with the results of Ballance et al. (2002), however they saw differences up to 20% on some of the low temperature rate coefficients and even larger differences for a small selection of transitions. Chen et al. (2010) calculated relativistic distortedwave data using the flexible atomic code (Gu 2003) for the first 6 nshells, including radiation damping. Effective collision strengths for allowed transitions between nearly degenerate levels for all Hlike ions from Z = 6 to 30 were calculating using the CoulombBorn method by Hamada et al. (2010). Although discrepancies exist between these Fe calculations, which we discuss later, it is clear that both radiation damping and a fullyrelativistic treatment of the scattering problem are important. Until this work, these two aspects have not been included in the same Rmatrix calculation. The main purpose of this paper is to report on new atomic data for Hlike Mn, Co, Cr, and Ni using the new Fe data to verify the accuracy of our results. We note that while current astrophysical observations of Mn and Cr are for ion stages less charged than Hlike (i.e. Helike and Nelike), the calculations presented here are with a view to assisting the search and identification of possible Hlike lines, and as an initial study with calculations of the ion stages beyond Hlike to follow.
To the best of our knowledge experimental measurements for excitation cross sections of Hlike Fepeak elements only exist for Hlike Fe. Measurements of the Lyman α_{1} excitation cross sections for Fe^{25+} and Ni^{27+} were carried out at the Lawrence Livermore National Laboratory SuperEBIT experiment (Thorn et al. 2009). These measurements were taken at very high electron impact energies beyond our area of interest.
In the next section we describe the theoretical methods used in this study. Section 3 will describe the atomic structure and cross section results. In Sect. 4 we will do some preliminary modeling with the new data and in Sect. 5 we will summarize the results of the work.
2. Calculation methods and details
We use a number of methods to calculate the electronimpact excitation cross sections of Hlike Mn, Cr, Fe, Co and Ni. As all of these methods are well described elsewhere, we give here just a brief outline of the various approximations. Our recommended data will come from the fullyrelativistic Dirac Rmatrix calculations, which we compare with semirelativistic Rmatrix calculations. With both of these Rmatrix methods, we calculate results with and without radiation damping. We also compare with semi and fullyrelativistic distortedwave calculations for the 1s2s transition in Hlike Fe. By comparing these different methods we seek to quantify the importance of using a fullyrelativistic treatment and to look at the roles of radiation damping and resonance resolution. The details for each calculation method are presented below.
2.1. Distortedwave
The configurationaverage distortedwave (CADW) method (Pindzola et al. 1986) has been used with much success to calculate electronimpact excitation cross sections for light species. We generate semirelativistic energies and bound radial wave functions using Cowan’s atomic structure code (Cowan 1981) where the massvelocity and Darwin corrections have been included. The method can be used to calculate direct excitation cross sections or resonant excitation cross sections. Radiation damping of the resonances can also be included. This method has been used for studies on Fe^{23+} (Pindzola et al. 2002) and on Fe^{16+} (Pindzola et al. 2006).
We also perform fully relativistic, subconfiguration average distortedwave calculations. In this method, the fullyrelativistic energies and boundradial orbitals are generated using Grant’s atomic structure code (Grant 2007). The details of this method can be found in Pindzola et al. (1988) where it was used to calculate the electronimpact ionization of U^{89+} and in Pindzola et al. (1989) for the ionization plus excitationautoionization of U^{16+}.
2.2. Rmatrix
Two Rmatrix methods are used in this paper, namely the semirelativistic intermediate coupling frame transformation (ICFT) method (Griffin et al. 1998) and the relativistic DiracCoulomb method (Norrington et al. 1987; AitTahar et al. 1996). The main strength of the ICFT method is that the majority of the calculation is carried out in an LS coupling scheme. Therefore, the formation and diagonalization of the smaller LS resolved Hamiltonians (including massvelocity and Darwin terms) is less computationally demanding than the associated jK or jJ resolved Hamiltonians. Only in the final stages of the calculation are the termresolved K matrices transformed into leveltolevel collision strengths. For the current study, the ICFT results are used to quantify the importance of having fullyrelativistic Rmatrix data for Hlike systems. We note that it has previously been shown (Griffin et al. 1998 and Munoz Burgos et al. 2009) that the ICFT method gives results that are very close to the BreitPauli Rmatrix method.
For the ICFT calculations, the hydrogenic 1s5g orbitals were generated from the atomic structure code AUTOSTRUCTURE (Badnell 1986) resulting in 15 LS terms which were used in our close coupling expansion. The scattering calculation was performed with our set of parallel Rmatrix programs (Mitnik et al. 2003; Ballance et al. 2004), which are extensively modified versions of the serial RMATRIX I programs (Berrington et al. 1995). A basis of 70 continuum functions was used to span the energy range from the ground state to four times the ionization threshold for each of the hydrogenic systems under consideration. In the scattering calculation, 50 partial waves ranging over angular momenta L = 0–12 were employed in the exchange scattering calculation. A further 162 higher partial waves, L = 13–50, were also calculated where exchange effects were neglected. The results were then topped up at higher L using the method described by Burgess (1970) for dipole transitions and a geometric series for nondipole transitions. AUTOSTRUCTURE was used to calculate infiniteenergy Bethe/Born limit points using our semirelativistic structure to allow us to interpolate our collision strengths to higher energies.
For the Dirac Rmatrix calculations, we obtained our target orbitals, energy levels and radiative rates using the multiconfiguration DiracFock (MCDF) atomic structure program GRASP0 (Dyal et al. 1989; Parpia et al. 1996). The resulting 25 relativistic orbitals produced 25 Jπ levels, all of which were used in our closecoupling expansion. For the scattering calculation we used a recently parallelized version of the Dirac Atomic Rmatrix Codes (DARC) (Ballance et al. 2006) which are based on portions of DARC from Norrington et al. (1987) and AitTahar et al. (1996). In the DARC calculations, full exchange effects were included for J = 0–11 and neglected for J = 12–50. Higher partial wave results were then approximated using the same topup procedure described above (Burgess 1970). As these results shall be our recommended data sets for use in plasma modeling, the energy levels were shifted slightly to match NIST spectroscopic values. Given the highly charged nature of the five hydrogenic species under consideration, radiation damping will affect the resonance contribution to the cross sections and the subsequent Maxwellianaveraged rates based upon them. Both Rmatrix calculations implement radiation damping of resonances as described in Robicheaux et al. (1995). It is important that the resonance structure be carefully resolved over the entire energy range. Therefore, in order to make meaningful comparisons between the two Rmatrix methods, a very fine energy mesh of 400 000 points from the first to the last threshold was used for both the ICFT and DARC calculations.
The Rmatrix calculations provide dimensionless collision strengths for all transitions between the levels of the target ion. For modeling purposes, Maxwellian excitation rate coefficients are required which can be expressed in terms of effective collision strengths. The effective collision strengths (Υ) are obtained by convolving the collision strengths with a Maxwellian electron distribution. BurgessTully plots (Burgess Tully 1992) are used to show effective collision strengths versus a scaled energy from threshold to the infiniteenergy limit on a scale from zero to one. For the type 2 (nondipole, nonspinchanging) transitions that we will consider (1s–2s), this involves the following transformations: where E_{ij} is the energy of the transition i to j, and C is an arbitrary constant, for which we will use a value of 0.5.
2.3. Collisionalradiative modeling
As well as archiving our fundamental atomic collision data, we also archive a set of photon emissivity coefficients (PECs) produced from a collisionalradiative model. These PECs can be used in direct comparison with an observed spectrum. They also serve as an illustration of how the new data can be used in collisionalradiative modeling of Xray sources.
To produce photon emissivity coefficients for each element in our study, we use collisionalradiative theory (Bates et al. 1962) as implemented in the ADAS suite of codes (http://www.adas.ac.uk), see Summers et al. (2006) for more details on the method. The photon emissivity coefficients for a given transition j → k, which include collisional and radiative redistribution between the excited states (i and j) are given by where A_{j → k} is the spontaneous emission coefficient for the transition j → k and the Cmatrix elements contain the collisional and radiative rates connecting the excited levels of the atom. C_{i1} is the excitation rate coefficient from the ground of the Hlike ion to level i and R_{i + } is the total recombition rate coefficient from the ground of the fully stripped ion into level i of the Hlike ion. For most plasma applications the PEC^{exc} dominates the line emission. The temperature and densitydependent photon emissivity coefficients for transition j → k are in units of photons cm^{3} s^{1}. When each of these coefficients is multiplied by the electron density and the density of the relevant ground state ion density, the result is the number of photons emitted per unit volume per unit time. For PEC^{exc} the relevant ground population is the ground of the Hlike ion stage and for PEC^{rec} the relevant driving population is that of the bare ion. We archive PEC coefficients for the 50 strongest transitions for each ion on a temperaturedensity grid. These can then be used for direct comparison with spectral observations.
3. Results and discussion
We used both AUTOSTRUCTURE (Badnell 1986) and the GRASP0 multiconfiguration DiracFock code (Dyal et al. 1989; Parpia et al. 1996) to provide our atomic structure. As expected for hydrogenic systems, the calculated level energies were very close to NIST values with an average difference of 0.05% for each of the ions. However, since the relativistic calculations (GRASP0, DARC) are to form the basis of our recommended data set, before the diagonalization of Hamiltonians representing our partial wave expansion we shifted our GRASP0 calculated energies to experimental spectroscopic NIST values. The relativistic spontaneous emission coefficients calculated from our DARC calculations for the 5 species under consideration here were within 2% of the available NIST values.
Fig. 1 Electron impact excitation cross section for the 1s–2s transition in Hlike Fe. The cross sections have been convolved with a 25 eV FWHM Gaussian distribution. The solid line shows the DARC results with radiation damping included, the dashed line show the ICFT results with radiation damping included. The dotdashed line shows the semirelativistic CADW results with radiation damping included. The solid squares show the direct excitation calculated using a fullyrelativistic distortedwave method. The solid circles show the DARC calculation of Aggarwal et al. (2008). 

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For our collision calculations, a number of systematic checks on Fe^{25+} were performed to investigate differences between existing datasets in the literature. Our first conclusion is that the fully relativistic Rmatrix results show an increase in the background excitation cross section of about 10% compared with the semirelativistic Rmatrix results. This is not entirely surprising given the relativistic nature of the atomic structure of these Hlike ions. A 1s orbital for atomic systems ranging from 23 to 27 times ionized should exhibit relativistic effects. Indeed, the Einstein A coefficient from the AUTOSTRUCTURE calculation was approximately 10% less than the DiracFock value from GRASP0 for the 1s_{1/2} → 2s_{1/2} transition in Fe^{25+}. Figure 1 shows the difference in the semi and fullyrelativistic excitation cross sections for the 1s2s tranistion in Fe^{25+}, with the background of the ICFT being about 10% below the DARC results. This result is confirmed with semi and fullyrelativistic distortedwave calculations for the 1s–2s excitation, also shown in Fig. 1. The semirelativistic distortedwave results include both direct and resonant excitation and are very close to the ICFT results. The fully relativistic subconfiguration average distortedwave direct excitation cross section results are in good agreement with the background of the DARC results. We note that this 10% difference is seen in all excitations from the 1s subshell, and in the 1s transitions for the others members of the Hlike Fepeak. This difference presumably decreases as one moves to less ionized ion stages. The close agreement of the distortedwave and Rmatrix results shown in Fig. 1 indicates that a fullyrelativistic distortedwave calculation which includes resonant excitation would provide accurate excitation data for these Fepeak elements. This is investigated in detail by Chen et al. (2010), who compare fully relativistic distortedwave results using the FAC code (Gu 2003) with the DARC calculations of Aggarwal et al. (2008) and the ICFT calculations of Ballance et al. (2002). They report both damped and undamped FAC effective collision strengths, with the undamped FAC results being very close to the undamped DARC calculation of Aggarwal et al.(2008), showing the similarity of perturbative and nonperturbative calculations for such a highly charged system.
Fig. 2 Electron impact excitation cross section for the 1s_{1/2}–2p_{1/2} transition in Hlike Fe. In a) the solid line shows the DARC results with damping included, the dashed line shows the DARC results without radiation damping. The cross sections have been convolved with a 25 eV FWHM Gaussian distribution. In b) the solid line shows the DARC effective collision strengths with radiation damping included, the dashed lines show the DARC results without radiation damping. The solid circles are the DARC results of Aggarwal et al. (2008) and the solid squares are the ICFT results (with radiation damping) calculated as part of this work. The solid diamonds show the damped FAC calculations of Chen et al. (2010). 

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Our study also confirms the earlier conclusions of Gorczyca and Badnell (1996), that the low energy resonances are significantly reduced by radiation damping. It is well known that low temperature Maxwellianaveraged collision strengths are very sensitive to the position and magnitudes of near threshold resonances, and therefore to radiation damping that effects these magnitudes. Figure 2 shows a comparison for the 1s_{1/2} → 2p_{1/2} transition for Fe^{25+}. Figure 2a shows the DARC cross section results with and without radiation damping. Note that the resonances just above threshold are almost completely damped out. These high n resonances are attached to the 2s_{1/2} and 2p_{3/2} thresholds. The strong radiative decay of the 2p electron effectively damps out these resonances. Furthermore, there is also noticeable damping of the 3l3l′ , 3l4l′ resonances between the n = 2 and n = 3 thresholds, however this does not have as strong an effect on the Maxwellian averaged rate coefficients as the damping of the near threshold resonances. We note that the sensitivity of effective collision strengths to near threshold resonances was also noted by Chen et al. (2010) in their FAC calculations of Fe^{25+}. Indeed, their damped FAC results shown on Fig. 2 are in good agreement with our damped DARC calculation over the entire energy and temperature range of the plots. Comparing the Maxwellian effective collision strengths (Fig. 2b) one can see that the radiationally damped rate coefficients for the 1s_{1/2} → 2p_{1/2} differ from nondamped results by 10% at 116 eV (1.35 × 10^{6} K), with this difference reducing as the temperature increases. By 5748 eV (6.67 × 10^{7} K) the difference is about 1%. Similar results were seen for the other excitations from the 1s subshell in Fe^{25+}, and for the same transitions in the other ions. Thus, our final DARC calculations for each ion include radiation damping of the resonances.
Fig. 3 Electron impact excitation cross sections for the 1s–2s transition in Hlike Fe. The results of the damped DARC calculation are shown on a BurgessTully plot with C = 0.5. The solid circles show the last finite energy point and the infinite energy point. 

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Our last calculated incident electron energy of approximately four times the ionization threshold was determined from a series of calculations to progressively higher energies, each of which was then extrapolated to the infinite limit Bethe/Born limits. The limit points for the dipole excitations were calculated as part of the DARC calculations, while the nondipole limit points were taken from AUTOSTRUCTURE calculations. Care must be taken to ensure that the cross section is approaching its asymptotic limit sufficiently before extrapolating to the infinite limit point, or there is a danger of grossly underestimating/overestimating the Maxwellian averaged rate coefficient. We carried out a series of calculations, extrapolating from progressively higher energies, checking the results on BurgessTully plots (Burgess Tully 1992) until the Maxwellian averaged rates converged to a consistent set of rates. Figure 3 shows the 1s–2s reduced effective collision strength on a BurgessTully plot, with the last calculated energy (at four times the ionization potential) and the infinite energy points shown. We note that the effective collision strengths shown in Fig. 3, even at the last explicitly calculated energy has still to turn over towards the limit point, but that the difference in the effective collision strength was relatively small for the highest calculated temperature. As a consequence of the large energy span of our DARC calculation, we had to include a large basis set size (70) and a large number of partial waves (up to J = 50) in our DARC collision calculations. We note that Chen et al. (2010) also investigated the high energy behaviour of the collision strengths, and included relativistic limit points in both their dipole and nondipole transitions.
Finally, we looked at the convergence of the excitation rate coefficients with the energy mesh used to resolve the resonance region of the calculation. We progressively doubled the mesh used in this region, and generated effective collision strengths each time. We found that the resonances were extremely narrow. With 400 000 energy mesh points in the resonance region we achieved rate coefficients that were converged to better than 1% for all transitions from the ground. Thus, our final set of calculations for each of the Hlike ions consisted of a DARC calculation including radiation damping, using 400,000 energy mesh points in the resonance region. Note that we archive both the levelresolved data and a bundledn version of the data, where the data has been reduced to nshell resolution, see Summers et al. (2006) for more details.
Fig. 4 Effective collision strengths for a) Cr, b) Mn, c) Co and d) Ni. The solid lines show the 1s → 2p_{1/2}, the dashed lines show the 1s → 2p_{3/2}, the dotdashed lines show the 1s → 3p_{1/2}, the doubledotted dashed lines show the 1s → 3p_{3/2}, the doubledashed dotted lines show the 1s → 4p_{1/2}, the dotted lines show the 1s → 4p_{3/2} transitions. 

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With these convergence checks complete for Fe^{25+}, we generated DARC excitation data based on GRASP0 atomic structure calculations for Mn, Cr, Co and Ni. In the DARC calculations we used the same basis set size, partial wave expansion and energy mesh resolution as in the Fe^{25+} calculation. We also generated undamped DARC and damped/undamped ICFT collision strengths for each ion. Comparing the ICFT with DARC results, there were similar 10% differences in the background cross sections, as seen in Fig. 1. The effect of radiation damping was also similar to that found in Fe^{25+}. Thus, our final recommended data for Hlike Mn, Cr, Co and Ni consisted of radiatively damped DARC calculations. The main purpose of this paper is to report on this new data, with this being the first calculations for these ions. The availability of a large amount of data in the literature for Hlike Fe allows us to investigate all of the issues mentioned above, with the likelihood that the other Fepeak elements will behave similarly. Fig. 4 shows a sample of the levelresolved effective collision strength results for Cr, Mn, Co and Ni. Our final datasets for each ion are stored in ADAS adf04 file format (http://www.adas.ac.uk), and are available for download via the OPENADAS web site (http://open.adas.ac.uk) or via the CFADC web archive (http://wwwcfadc.phy.ornl.gov/data_and_codes/home.html). The data is also been imported into the ATOMDB database used by the XSPEC modeling codes.
4. Modeling results
For use in spectral modeling, and as an illustration of how the atomic data can be used, we generated a set of photon emissivity coefficients for each of the Hlike Fepeak element ions considered here. An absolute line intensity for the transition is generated by multiplying each PEC by the electron density and by the ground population (for PEC^{exc} this is the ground population of the Hlike ion stage and for for PEC^{rec} this is the bare ion density). These ground populations can be evaluated from an ionization balance calculation. For conditions where the excited states are driven primarily from excitation and not from recombination, the ratio of PEC^{exc} for the two transitions will give the predicted line intensity ratio.
Fig. 5 Photon emissivity coefficients for the 1s → 2p_{3/2} (solid line) and the 1s → 2p_{1/2} (dashed line) as a function of electron temperature. a) for Cr; b) for Mn; c) for Co; and d) is for Ni. 

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We show a sample of the results in Figs. 5 and 6. We first show the photon emissivity coefficients for both 1s2p transitions for Cr, Mn, Co and Ni. For cases with sufficient electron density, one can get collisional mixing of the 2p and 2s populations. However, our initial collisionalradiative modeling indicates that this does not start to occur until about N_{e} = 1 × 10^{10} cm^{3}. Thus for most supernova remnant studies (where the electron density is typically 1 cm^{3}), the Lyman α emission will be coming from the sum of the two 2p transitions from the ground. In the limit of higher densities, collisions will drive the 2s and 2p populations to be statistical, relative to each other. This does not happen until about N_{e} = 1 × 10^{24} cm^{3} if electron collisions are the only collisional mixing process, though we note that proton collisions can mix the subshells much more efficiently than electron collisions. For cases where the subshells are strongly collisionally mixed, then the bundledn data is more appropriate. For intermediate densities one needs to do a densitydependent collisionalradiative calculation to calculate the expected line intensities.
As a further illustration of the use of the new data, we show in Fig. 6 the expected Lymanα to Lymanβ line intensity ratio for Cr, Mn, Co and Ni, for the low density case (Ne < 1 × 10^{10} cm^{3}). Thus, if one sees Lymanα emission from one of these ions, then the plotted results would allow one to determine whether the Lymanβ could be observed, or if it would be buried in the background noise of the spectrum. For example, at T = 1 × 10^{7} K, the Lymanβ intensity would be down by an order of magnitude from the Lymanα intensity for all of the ions studied. Also, as one might expect, the ratio of the Lymanα to Lymanβ is clearly strongly temperature dependent at the lowest temperatures.
Fig. 6 Lyman α to Lyman β line intensity ratio as a function of electron temperature. Fig. a) is for Cr, Fig.b) is for Mn, Fig. c) is for Co and Fig. d) is for Ni. 

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5. Conclusions

1.
Results of electronimpact collision calculations for Hlike Cr, Mn, Fe, Co and Ni are presented.

2.
Fully relativistic effects results in an approximately 10% rise in the background cross section for the excitations from the ground.

3.
Radiation damping must be included for accurate low temperature rate coefficients, and a fine energy mesh is required to resolve all of the resonant structure.

4.
Photon emisivity coefficients have been generated on a T_{e}/N_{e} grid for Hlike Cr, Mn, Fe, Co and Ni.

5.
The new atomic data is archived online at the OPENADAS web site (http://open.adas.ac.uk) and the CFADC web archive (http://wwwcfadc.phy.ornl.gov/data_and_codes/home.html).

6.
Future work will include similar calculations for the Helike stages of these same elements.
Acknowledgments
This work was supported by NASA ROSES ADAP grant NNX10AD46G. The computational work was carried out on the Alabama supercomputer and on the NERSC facilities in California. We would like to thank Una Hwang for the many useful discussions and help that she provided throughout the project.
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All Figures
Fig. 1 Electron impact excitation cross section for the 1s–2s transition in Hlike Fe. The cross sections have been convolved with a 25 eV FWHM Gaussian distribution. The solid line shows the DARC results with radiation damping included, the dashed line show the ICFT results with radiation damping included. The dotdashed line shows the semirelativistic CADW results with radiation damping included. The solid squares show the direct excitation calculated using a fullyrelativistic distortedwave method. The solid circles show the DARC calculation of Aggarwal et al. (2008). 

Open with DEXTER  
In the text 
Fig. 2 Electron impact excitation cross section for the 1s_{1/2}–2p_{1/2} transition in Hlike Fe. In a) the solid line shows the DARC results with damping included, the dashed line shows the DARC results without radiation damping. The cross sections have been convolved with a 25 eV FWHM Gaussian distribution. In b) the solid line shows the DARC effective collision strengths with radiation damping included, the dashed lines show the DARC results without radiation damping. The solid circles are the DARC results of Aggarwal et al. (2008) and the solid squares are the ICFT results (with radiation damping) calculated as part of this work. The solid diamonds show the damped FAC calculations of Chen et al. (2010). 

Open with DEXTER  
In the text 
Fig. 3 Electron impact excitation cross sections for the 1s–2s transition in Hlike Fe. The results of the damped DARC calculation are shown on a BurgessTully plot with C = 0.5. The solid circles show the last finite energy point and the infinite energy point. 

Open with DEXTER  
In the text 
Fig. 4 Effective collision strengths for a) Cr, b) Mn, c) Co and d) Ni. The solid lines show the 1s → 2p_{1/2}, the dashed lines show the 1s → 2p_{3/2}, the dotdashed lines show the 1s → 3p_{1/2}, the doubledotted dashed lines show the 1s → 3p_{3/2}, the doubledashed dotted lines show the 1s → 4p_{1/2}, the dotted lines show the 1s → 4p_{3/2} transitions. 

Open with DEXTER  
In the text 
Fig. 5 Photon emissivity coefficients for the 1s → 2p_{3/2} (solid line) and the 1s → 2p_{1/2} (dashed line) as a function of electron temperature. a) for Cr; b) for Mn; c) for Co; and d) is for Ni. 

Open with DEXTER  
In the text 
Fig. 6 Lyman α to Lyman β line intensity ratio as a function of electron temperature. Fig. a) is for Cr, Fig.b) is for Mn, Fig. c) is for Co and Fig. d) is for Ni. 

Open with DEXTER  
In the text 