Daniel Wolf Savin

A NEW APPROACH TO ANALYZING SOLAR CORONAL SPECTRA AND UPDATED COLLISIONAL IONIZATION EQUILIBRIUM CALCULATIONS. II. UPDATED IONIZATION RATE COEFFICIENTS

We have re-analyzed Solar Ultraviolet Measurement of Emitted Radiation (SUMER) observations of a parcel of coronal gas using new collisional ionization equilibrium (CIE) calculations. These improved CIE fractional abundances were calculated using state-of-the-art electron–ion recombination data for K-shell, L-shell, Na-like, and Mg-like ions of all elements from H through Zn and, additionally, Al- through Ar-like ions of Fe. They also incorporate the latest recommended electron impact ionization data for all ions of H through Zn. Improved CIE calculations based on these recombination and ionization data are presented here. We have also developed a new systematic method for determining the average emission measure (EM) and electron temperature (Te )o f an isothermal plasma. With our new CIE data and a new approach for determining average EM and Te ,w e have re-analyzed SUMER observations of the solar corona. We have compared our results with those of previous studies and found some significant differences for the derived EM and Te. We have also calculated the enhancement of coronal elemental abundances compared to their photospheric abundances, using the SUMER observations themselves to determine the abundance enhancement factor for each of the emitting elements. Our observationally derived first ionization potential factors are in reasonable agreement with the theoretical model of Laming.

Spectroscopic Challenges of Photoionized Plasmas

The conference “Photoionized Plasmas 2000: The Challenge of High Resolution X-Ray through IR Spectroscopy of Photoionized Plasmas” was held on 2000 November 13–17 at the Lexington campus of the University of Kentucky. Sponsorship for the conference was provided by the NASA Applied Information Systems Research Program and by the University of Kentucky. Conference participants came from Australia, Brazil, Canada, Denmark, France, Germany, India, Ireland, Israel, Kazakhstan, Mexico, the Netherlands, Northern Ireland, the United Kingdom, and the United States. There were a total of 83 participants. This conference was the third in a series which begin in 1985 with a meeting on modeling ionized nebulae in Meudon, France. The second meeting was held in 1994 at the Lexington campus of the University of Kentucky and was in conjunction with the 70th birthdays of Professors Donald Osterbrock and Michael Seaton. The timing of this third conference was motivated by recent advances in observational astrophysics. With the recent launches of Chandra and XMM-Newton, high-resolution spectroscopy of photoionized plasmas at X-ray energies has become routine. HST and FUSE make the vacuum ultraviolet readily accessible. Ground-based optical telescopes can now obtain spectra of faint galaxies at the very edge of the visible universe. And within a few years SOFIA and SIRTF will routinely provide high-resolution spectra in the midand far-infrared regions. Understanding the astrophysical messages contained in these spectra makes unprecedented demands on our understanding of atomic processes and our ability to simulate conditions in these nonequilibrium plasmas. This meeting brought together developers of plasma emission codes, experts in atomic physics and radiation transport, and the observers who are working to unravel the message in the spectrum. A broad range of physical processes determine the observed spectrum, and a complete simulation of the plasma is an intricate computational problem. The

COLLISIONAL IONIZATION EQUILIBRIUM FOR OPTICALLY THIN PLASMAS. I. UPDATED RECOMBINATION RATE COEFFICIENTS FOR BARE THROUGH SODIUM-LIKE IONS

Reliably interpreting spectra from electron-ionized cosmic plasmas requires accurate ionization balance calculations for the plasma in question. However, much of the atomic data needed for these calculations have not been generated using modern theoretical methods and are often highly suspect. This translates directly into the reliability of the collisional ionization equilibrium (CIE) calculations. We make use of state-of-the-art calculations of dielectronic recombination (DR) rate coefficients for the hydrogenic through Na-like ions of all elements from He up to and including Zn. Where measurements exist, these published theoretical DR data agree with recent laboratory work to within typically 35% or better at the temperatures relevant for CIE. We also make use of state-of-the-art radiative recombination (RR) rate coefficient calculations for the bare through Na-like ions of all elements from H through to Zn. Here we present improved CIE calculations for temperatures from 104 to 109 K using our data and the recommended electron impact ionization data of Mazzotta et al. for elements up to and including Ni and Mazzotta for Cu and Zn. DR and RR data for ionization stages that have not been updated are also taken from these two additional sources. We compare our calculated fractional ionic abundances using these data with those presented by Mazzotta et al. for all elements from H to Ni. The differences in peak fractional abundance are up to 60%. We also compare with the fractional ionic abundances for Mg, Si, S, Ar, Ca, Fe, and Ni derived from the modern DR calculations of Gu for the H-like through Na-like ions, and the RR calculations of Gu for the bare through F-like ions. These results are in better agreement with our work, with differences in peak fractional abundance of less than 10%.

Interstellar X-ray absorption spectroscopy of oxygen, neon, and iron with the CHANDRA LETGS Spectrum of X0614+091

We —nd resolved interstellar O K, Ne K, and Fe L absorption spectra in the Chandra X-Ray Observatory Low-Energy Transmission Grating Spectrometer (LETGS) spectrum of the low-mass X-ray binary X0614]091. We measure the column densities in O and Ne and —nd direct spectroscopic constraints on the chemical state of the interstellar O. These measurements probably probe a low-density line of sight through the Galaxy, and we discuss the results in the context of our knowledge of the properties of interstellar matter in regions between the spiral arms.

Experimental Results for H2 Formation from H- and H and Implications for First Star Formation

Early Rising Hydrogen Formation of molecular hydrogen through electron-expelling collisions of H atoms and H− anions is regarded as a key step in the cooling process that led to assembly of the first stars in the early universe. Kreckel et al. (p. 69; see the Perspective by Bromm) performed highly precise laboratory measurements of the rate of this reaction at a range of different energies. The study required construction of a dedicated apparatus to carefully tune the relative velocity of merged atom and ion beams. The data validated prior theoretically calculated reaction cross sections, which were then extended for use in cosmological models. Precise measurements of molecular hydrogen formation rates help improve models of star assembly in the early universe. During the epoch of first star formation, molecular hydrogen (H2) generated via associative detachment (AD) of H− and H is believed to have been the main coolant of primordial gas for temperatures below 104 kelvin. The uncertainty in the cross section for this reaction has limited our understanding of protogalaxy formation during this epoch and of the characteristic masses and cooling times for the first stars. We report precise energy-resolved measurements of the AD reaction, made with the use of a specially constructed merged-beams apparatus. Our results agreed well with the most recent theoretically calculated cross section, which we then used in cosmological simulations to demonstrate how the reduced AD uncertainty improves constraints of the predicted masses for Population III stars.

Cosmological implications of the uncertainty in H- destruction rate coefficients

In primordial gas, molecular hydrogen forms primarily through associative detachment of Hand H, thereby destroying the H � .T he Hanion can also be destroyed by a number of other reactions, most notably by mutual neutralization with protons. However, neither the associative detachment nor the mutual neutralization rate co- efficients are well determined: both may be uncertain by as much as an order of magnitude. This introduces a correspondinguncertaintyintotheH2formationrate,whichmayhavecosmologicalimplications.Hereweexamine the effect that these uncertainties have on the formation of H2 and the cooling of protogalactic gas in a variety of situations. We show that the effect is particularly large for protogalaxies forming in previously ionized regions, affecting our predictions of whether or not a given protogalaxy can cool and condense within a Hubble time, and altering the strength of the ultraviolet background that is required to prevent collapse. Subject headingg atomic data — atomic processes — galaxies: formation — molecular data — molecular processes — stars: formation

Is H+3 cooling ever important in primordial gas?

Studies of the formation of metal-free Population III stars usually focus primarily on the role played by H2 cooling, on account of its large chemical abundance relative to other possible molecular or ionic coolants. However, while H2 is generally the most important coolant at low gas densities, it is not an effective coolant at high gas densities, owing to the low critical density at which it reaches local thermodynamic equilibrium (LTE) and to the large opacities that develop in its emission lines. It is therefore possible that emission from other chemical species may play an important role in cooling high-density primordial gas. A particularly interesting candidate is the H + molecular ion. This ion has an LTE cooling rate that is roughly a billion times larger than that of H2, and unlike other primordial molecular ions such as H + or HeH + , it is not easily removed from the gas by collisions with H or H2 .I t is already known to be an important coolant in at least one astrophysical context – the upper atmospheres of gas giants – but its role in the cooling of primordial gas has received little previous study. In this paper, we investigate the potential importance of H + cooling in primordial gas using a newly developed H + cooling function and the most detailed model of primordial chemistry published to date. We show that although H + is, in most circumstances, the third most important coolant in dense primordial gas (after H2 and HD), it is nevertheless unimportant, as it contributes no more than a few per cent of the total cooling. We also show that in gas irradiated by a sufficiently strong flux of cosmic rays or X-rays, H + can become the dominant coolant in the gas, although the size of the flux required renders this scenario unlikely to occur.

Experimental M1 Transition Rates of Coronal Lines from Ar X, Ar XIV, and Ar XV

Transition probabilities of three magnetic dipole (M1) transitions in multiply charged ions of Ar have been measured using the Livermore electron-beam ion trap. Two of the transitions are in the ground con—gurations of Ar XIV (B-like) and Ar IX (F-like), and are associated with the coronal lines at 4412.4 and 5533.4 respectively. The third is in the excited 2s2p con—guration of Be-like Ar XV and produces Ae , the coronal line at 5943.73 Our results for the three atomic level lifetimes are 9.32 ^ 0.12 ms for the Ae . Ar X 2s22p5 level, 9.70 ^ 0.15 ms for the Ar XIV 2s22p level, and 15.0 ^ 0.8 ms for the Ar XV 2P 1@2 2P 3@2 2s2p level. These results diUer signi—cantly from earlier measurements and are the most accurate 3P 2 ones to date. Subject headings: atomic datamethods: laboratory

High resolution mass spectrometry using a linear electrostatic ion beam trap

Abstract We describe a new mass spectrometric technique that is based on the use of a linear electrostatic ion trap and a newly discovered self-bunching phenomenon. Ions are stored in the trap and their oscillation frequencies are determined by Fourier transform of their oscillation times. Using this system, we demonstrate that it is possible to simultaneously trap several masses and obtain their mass spectra with high resolution. The instrument is compared to time-of-flight mass, as well as to ion cyclotron resonance mass spectrometers.

Dielectronic recombination in photoionized gas. II. Laboratory measurements for Fe xviii and Fe xix

In photoionized gases with cosmic abundances, dielectronic recombination (DR) proceeds primarily via nlj ) nl@j@ core excitations (*n \ 0 DR). We have measured the resonance strengths and energies for Fe XVIII to Fe XVII and Fe XIX to Fe XVIII *n \ 0 DR. Using our measurements, we have calculated the Fe XVIII and Fe XIX *n \ 0 DR rate coefficients. Signi—cant discrepancies exist between our inferred rates and those of published calculations. These calculations overestimate the DR rates by factors of D 2o r underestimate it by factors of D2 to orders of magnitude, but none are in good agreement with our results. Almost all published DR rates for modeling cosmic plasmas are computed using the same theo- retical techniques as the above-mentioned calculations. Hence, our measurements call into question all theoretical *n \ 0 DR rates used for ionization balance calculations of cosmic plasmas. At temperatures where the Fe XVIII and Fe XIX fractional abundances are predicted to peak in photoionized gases of cosmic abundances, the theoretical rates underestimate the Fe XVIII DR rate by a factor of D2 and over- estimate the Fe XIX DR rate by a factor of D1.6. We have carried out new multicon—guration Dirac- Fock and multicon—guration Breit-Pauli calculations which agree with our measured resonance strengths and rate coefficients to within typically better than We provide a —t to our inferred rate coeffi- (30%. cients for use in plasma modeling. Using our DR measurements, we infer a factor of D2 error in the Fe XX through Fe XXIV *n \ 0 DR rates. We investigate the eUects of this estimated error for the well- known thermal instability of photoionized gas. We —nd that errors in these rates cannot remove the instability, but they do dramatically aUect the range in parameter space over which it forms. Subject headings: atomic dataatomic processesgalaxies: activeinstabilitiesX-rays: general