An experimental concept to measure opacities under solar-relevant conditions

Paul A. Keiter (1), Katie Mussack (2) and Salle R. Klein (1) ((1) University of Michigan, Ann Arbor, MI 48105, USA; (2) XTD-2, MS T-086, Los Alamos National Laboratory, Los Alamos, NM 87545, USA)

Recent solar abundance models (Asplund 2009) use a significantly lower abundance for C, N, O compared to models used roughly a decade ago. Although the models used now are much more sophisticated than before, a discrepancy still exists between the abundances in the models and the abundances determined by helioseismic measurements. Agreement can be obtained by ad hoc adjustments to the opacity of high-Z (Z > 2) elements ranging from a few percent in the solar interior to as much as 30 just below the convection zone (CZ). Although many of the opacity models are thought to agree within a few percent, a recent element-by-element study (Blancard 2012) indicates a larger disagreement between models for certain elements. Experimental opacity measurements for these elements in the regimes of interest will provide valuable information to help resolve these discrepancies. We will present an experimental platform designed to measure the opacity of C, N, and O and discuss the achievable parameter regime. We will also briefly discuss how this platform can be extended to include other high-Z elements.

See complete preprint –> http://www.sciencedirect.com/science/article/pii/S1574181813000177

Expanded Iron UTA spectra — probing the thermal stability limits in AGN clouds

Ferland, G. J.; Kisielius, R.; Keenan, F. P.; van Hoof, P. A. M.; Jonauskas, V.; Lykins, M. L.; Porter, R. L.; Williams, R. J. R 

ApJ in press; eprint arXiv:1303.0316

The Fe unresolved transition array (UTAs) produce prominent features in the 15-17?A wavelength range in the spectra of Active Galactic Nuclei (AGN). Here we present new calculations of the energies and oscillator strengths of inner- shell lines from Fe XIV, Fe XV, and Fe XVI. These are crucial ions since they are dominant at inflection points in the gas thermal stability curve, and UTA excitation followed by autoionization is an important ionization mechanism for these species. We incorporate these, and data reported in previous papers, into the plasma simulation code Cloudy. This updated physics is subsequently employed to reconsider the thermally stable phases in absorbing media in Active Galactic Nuclei. We show how the absorption profile of the Fe XIV UTA depends on density, due to the changing populations of levels within the ground configuration.

See complete preprint -> http://adsabs.harvard.edu/abs/2013arXiv1303.0316F

 

Suppression of Dielectronic Recombination Due to Finite Density Effects

D. Nikolić, T. W. Gorczyca, K. T. Korista, G. J. Ferland, N. R. Badnell (Submitted on 10 Mar 2013)

We have developed a general model for determining density-dependent effective dielectronic recombination (DR) rate coefficients in order to explore finite-density effects on the ionization balance of plasmas. Our model consists of multiplying by a suppression factor those highly-accurate total zero-density DR rate coefficients which have been produced from state-of-the-art theoretical calculations and which have been benchmarked by experiment. The suppression factor is based-upon earlier detailed collision-radiative calculations which were made for a wide range of ions at various densities and temperatures, but used a simplified treatment of DR. A general suppression formula is then developed as a function of isoelectronic sequence, charge, density, and temperature. These density-dependent effective DR rate coefficients are then used in the plasma simulation code Cloudy to compute ionization balance curves for both collisionally ionized and photoionized plasmas at very low (ne = 1 cm^-3) and finite (ne=10^10 cm^-3) densities. We find that the denser case is significantly more ionized due to suppression of DR, warranting further studies of density effects on DR by detailed collisional-radiative calculations which utilize state-of-the-art partial DR rate coefficients. This is expected to impact the predictions of the ionization balance in denser cosmic gases such as those found in nova and supernova shells, accretion disks, and the broad emission line regions in active galactic nuclei.

See complete preprint –> http://arxiv.org/abs/1303.2338

Photoionization modeling of oxygen K absorption in the interstellar medium: the Chandra grating spectra of XTE J1817-330

E. Gatuzz (1), J. García (2), C. Mendoza (1,4), T. R. Kallman (3), M. Witthoeft (3), A. Lohfink (2), M. A. Bautista (5), P. Palmeri (6) and P. Quinet (7) ((1) Centro de Física, Instituto Venezolano de Investigaciones Científicas, PO Box 20632, Caracas 1020A,  Venezuela; (2) Department of Astronomy and Maryland Astronomy Center for Theory and Computation, University of Maryland, College Park, MD 20742, USA; (3) NASA Goddard Space Flight Center, Greenbelt, MD 20771, USA; (4) Centro Nacional de Cálculo Científico, Universidad de Los Andes, Corporación Parque Tecnológico de Mérida 5101, Venezuela; (5) Department of Physics, Western Michigan University, Kalamazoo, MI 49008, USA; (6) Astrophysique et Spectroscopie, Université de Mons – UMONS, B-7000 Mons, Belgium; (7) IPNAS, Sart Tilman B15, Université de Liége, B-4000 Liége,
Belgium)

We present detailed analyses of oxygen K absorption in the interstellar medium (ISM) using four high-resolution Chandra spectra towards the X-ray low-mass binary XTE J1817-330. The 11-25 A broadband is described with a simple absorption model that takes into account the pileup effect and results in an estimate of the hydrogen column density. The oxygen K-edge region (21-25 A) is fitted with the physical warmabs model, which is based on a photoionization model grid generated with the xstar code with the most up-to-date atomic database. This approach allows a benchmark of the atomic data which involves wavelength shifts of both the K lines and photoionization cross sections in order to fit the observed spectra accurately. As a result we obtain: a column density of N(H)=1.38+/-0.01\times 10^21 cm^-2; ionization parameter of log(xi)=-2.70+/-0.023; oxygen abundance of A(O)= 0.689^{+0.015}_{-0.010}; and ionization fractions of OI/O = 0.911, OII/O = 0.077, and OIII/O = 0.012 that are in good agreement with previous studies. Since the oxygen abundance in warmabs is given relative to the solar standard of Grevesse et al. (1998), a rescaling with the revision by Asplund et al. (2009) yields A(O)=0.952^{+0.020}_{-0.013}, a value close to solar that reinforces the new standard. We identify several atomic absorption lines Kalpha, Kbeta, and Kgamma in OI and OII; and Kalpha in OIII, OVI, and OVII – last two probably residing in the neighborhood of the source rather than in the ISM. This is the first firm detection of oxygen K resonances with principal quantum numbers n>2 associated to ISM cold absorption.

See complete preprint –> http://arxiv.org/abs/1303.2396

The 2013 Release of Cloudy

G. J. Ferland (1), R. L. Porter (2), P. A. M. van Hoof (3), R. J. R. Williams (4), N. P. Abel (5), M. L. Lykins (1), Gargi Shaw (6), W. J. Henney (7), P. C. Stancil (2) ((1) University of Kentucky, (2) University of Georgia, (3) Royal Observatory of Belgium, (4) AWE plc, UK, (5) University of Cincinnati, (6) CEBS, University of Mumbai, India, (7) CRyA-UNAM, Mexico)

This is a summary of the 2013 release of the plasma simulation code Cloudy. Cloudy models the ionization, chemical, and thermal state of material that may be exposed to an external radiation field or other source of heating, and predicts observables such as emission and absorption spectra. It works in terms of elementary processes, so is not limited to any particular temperature or density regime. This paper summarizes advances made since the last major review in 1998. Much of the recent development has emphasized dusty molecular environments, improvements to the ionization / chemistry solvers, and how atomic and molecular data are used. We present two types of simulations to demonstrate the capability of the code. We consider a molecular cloud irradiated by an X-ray source such as an Active Nucleus and show how treating EUV recombination lines and the full SED affects the observed spectrum. A second example illustrates the very wide range of particle and radiation density that can be considered.

See complete preprint –> http://arxiv.org/abs/1302.4485

Influence of Departures from LTE on Oxygen Abundance Determination

Sitnova, T. M. (1,2), Mashonkina, L. I. (1) and Ryabchikova, T. A. (1) ((1) Institut of Astronomy of RAS, Moscow, Russia; (2) Moscow M.V. Lomonosov State University, Moscow, 119991 Russia)

We performed non-LTE calculations for O I with the plane-parallel model atmospheres for a set of stellar parameters corresponding to A-K type stars. The model atom of Przybilla et al. (2000) was updated using the best theoretical and experimental atomic data available so far. Non-LTE leads to strengthening the O I lines, and the difference between the non-LTE and LTE abundances (non-LTE correction) is negative. The departures from LTE grow toward higher effective temperature and lower surface gravity. In the entire temperature range and log g = 4, the non-LTE correction does not exceed 0.05 dex in absolute value for O I lines in the visible spectral range. The non-LTE corrections are significantly larger for the infrared O I 7771-5 A lines and reach -1.9 dex in the model atmosphere with Teff = 10000 K and log g = 2. To differentiate the effects of inelastic collisions with electrons and neutral hydrogen atoms on the statistical equilibrium (SE) of O I, we derived the oxygen abundance for the three well studied A-type stars Vega, Sirius, and HD 32115. For each star, non-LTE leads to smaller difference between the infrared and visible lines. For example, for Vega, this difference reduces from 1.17 dex in LTE down to 0.14 dex when ignoring LTE. To remove the difference between the infrared and visible lines in A-type stars completely, one needs to reduce the used electron-impact excitation rates by Barklem (2007) by a factor of 4. In the case of Procyon and the Sun, inelastic collisions with H I affect the SE of O I, and agreement between the abundances from different lines is achieved when using the Drawin’s formalism to compute collisional rates. The solar mean oxygen abundance from the O I 6300, 6158, 7771-5, and 8446 A lines is log epsilon = 8.74+-0.05, when using the MAFAGS-OS solar model atmosphere and log epsilon = 8.78+-0.03, when applying the 3D corrections from the literature.

See complete preprint –> http://arxiv.org/abs/1302.1048

Effect of a High Opacity on the Light Curves of Radioactively Powered Transients from Compact Object Mergers

Barnes, Jennifer (1) & Kasen D. (1) ((1) University of California, Berkeley; Lawrence Berkeley National Laboratory)

The coalescence of compact objects is a promising source of detectable gravitational wave (GW) signals. The science returns from a GW detection can be greatly enhanced by identifying an electromagnetic (EM) counterpart. The radioactive debris ejected in tidal tails during the inspiral of a binary neutron star system is thought to power a dim supernova-like event lasting for days after the merger. A major challenge to accurately modeling these transients is determining the opacities of the r-process material synthesized in the ejecta, as very little experimental line is data available for these heavy elements. Earlier radiative transfer studies calculated light curves assuming the opacities were similar to that of iron group elements. We test this assumption using the available experimental line data, and find that the presence of heavy elements, particularly the lanthanides, results in opacities orders of magnitude greater than that of iron alone. We refine our estimates using synthetic line lists generated through ab-initio atomic structure modeling. We compute light curves for a one-dimensional model of r-process ejecta using reasonable parameters (ejecta masses = 1e-2Msun and velocities = 0.1c). We find that the higher opacities lead to longer, flatter, and dimmer light curves than previously predicted.

See complete preprint –> http://adsabs.harvard.edu/abs/2013AAS…22134604B