Knowing accurate Pb abundances of metal-poor stars provides constraints on the Pb production mechanisms in the early Galaxy. Accurately deriving Th abundances permits a nucleo-chronometric age determination of the star. We improve the calculation of the Pb I and Th II lines in stellar atmospheres based on non-LTE line formation and evaluate the influence of departures from LTE on Pb and Th abundance determinations through a range of stellar parameters. Comprehensive model atoms for Pb I and Th II are presented. The departures from LTE lead to systematically depleted total absorption in the Pb I lines and positive abundance corrections. Non-LTE removes the discrepancy between the solar and the meteoritic Pb abundance. With the Holweger & Mueller (1974) solar model atmosphere, log eps(Pb, non-LTE) = 2.09. We revise the Pb and Eu abundances of the strongly r-process enhanced (r-II) stars CS 31082-001 and HE 1523-0901 and the Roederer et al. (2010) stellar sample. Our results provide strong evidence for universal Pb/Eu relative r-process yields during course of the Galaxy evolution. The stars with -2.3<[Fe/H]< -1.4 have, on average, 0.51 dex higher Pb/Eu ratios compared with that of the r-II stars suggesting that the s-process synthesis of Pb started as early as the time when Galactic metallicity had grown to [Fe/H] = -2.3. The average Pb/Eu ratio of the -1.4<[Fe/H]< -0.59 stars is close to the solar value, in line with the predictions of Travaglio et al. (2001) that AGB stars with [Fe/H] ~ -1 provided the largest contribution to the solar s-nuclei of Pb. Non-LTE leads to weakened Th II lines. Overall, the abundance correction does not exceed +0.2 dex when collisions with H I atoms are taken into account in non-LTE calculations.

Complete preprint ==> http://arxiv.org/abs/1202.2630

Using near-ultraviolet spectra obtained with the Space Telescope Imaging Spectrograph onboard the Hubble Space Telescope, we detect neutral tellurium in three metal-poor stars enriched by products of r-process nucleosynthesis, BD+17 3248, HD 108317, and HD 128279. Tellurium (Te, Z=52) is found at the second r-process peak (A=130) associated with the N=82 neutron shell closure, and it has not been detected previously in Galactic halo stars. The derived tellurium abundances match the scaled solar system r-process distribution within the uncertainties, confirming the predicted second peak r-process residuals. These results suggest that tellurium is predominantly produced in the main component of the r-process, along with the rare earth elements.

Complete preprint ==> http://arxiv.org/abs/1202.2378

Sodium abundances have been determined in a large number of giants of open clusters but conflicting results, ranging from solar values to overabundances of up to five orders of magnitude, have been found. The reasons for this disagreement are not well-understood. As these Na overabundances can be the result of deep mixing, their proper understanding has consequences for models of stellar evolution. As discussed in the literature, part of this disagreement comes from the adoption of different corrections for non-LTE effects and from the use of different atomic data for the same set of lines. However, a clear picture of the Na behaviour in giants is still missing. To contribute in this direction, this work presents a careful redetermination of the Na abundances of the Hyades giants, motivated by the recent measurement of their angular diameters. An average of [Na/Fe] = +0.30, in NLTE, has been found. This overabundance can be explained by hydrodynamical models with high initial rotation velocities. This result, and a trend of increasing Na with increasing stellar mass found in a previous work, suggests that there is no strong evidence of Na overabundances in red giants beyond those values expected by evolutionary models of stars with more than ~ 2 Msun.

Complete preprint ==> http://arxiv.org/abs/1202.2200

 
Sulfur abundances derived from optical emission line measurements and ionization correction factors in planetary nebulae are systematically lower than expected for the objects’ metallicities. We have carefully considered a large range of explanations for this “sulfur anomaly”, including: (1) correlations between the size of the sulfur deficit and numerous nebular and central star properties; (2) ionization correction factors which under-correct for unobserved ions; (3) effects of dielectronic recombination on the sulfur ionization balance; (4) sequestering of S into dust and/or molecules; and (5) excessive destruction of S or production of O by AGB stars. It appears that all but the second scenario can be ruled out. However, we find evidence that the sulfur deficit is generally reduced but not eliminated when S^{+3} abundances determined directly from IR measurements are used in place of the customary sulfur ionization correction factor. We tentatively conclude that the sulfur anomaly is caused by the inability of commonly used ICFs to properly correct for populations of ionization stages higher than S^{+2}.

Complete preprint ==> http://arxiv.org/abs/1202.1563

We report the discovery of a “super-damped” Lyman-alpha absorber at z_{abs}=2.2068 toward QSO Q1135-0010 in the Sloan Digital Sky Survey, and follow-up VLT UVES spectroscopy. Voigt profile fit to the DLA line indicates log N_{H I} = 22.05 ± 0.1. This is the second QSO DLA discovered to date with such high N_{H I}. We derive element abundances [Si/H] = –1.10 ± 0.10, [Zn/H] = –1.06 ± 0.10, [Cr/H] = –1.55 ± 0.10, [Ni/H] =  –1.60 ± 0.10, [Fe/H] = –1.76 ± 0.10, [Ti/H] = –1.69 ± 0.11, [P/H] = –0.93 ± 0.23, and [Cu/H] = –0.75 ± 0.14. Our data indicate detection of Lyα emission in the DLA trough, implying a star formation rate of ~10 M(sun) yr^{-1} in the absence of dust attenuation. C II* λ1336 absorption is also detected, suggesting SFR surface density –2 < log (dψ_{*}/dt) < 0 M(sun) yr^{-1} kpc^{-2}. We estimate electron density in the range 3.5 × 10^{-4} to 24.7 cm^{-3} from C II*/C II, and ~0.5–0.9 cm^{-3} from Si II*/Si II. Overall, this is a robustly star-forming, moderately enriched absorber, but with relatively low dust depletion. Fitting of the SDSS spectrum yields low reddening for Milky Way, LMC, or SMC extinction curves. No CO absorption is detected, and C I absorption is weak. The low dust and molecular content, reminiscent of some SMC sight-lines, may result from the lower metallicity, and a stronger radiation field (due to higher SFR). Finally, we compare this absorber with other QSO and GRB DLAs.

Complete preprint ==> http://arxiv.org/abs/1202.0882

Grazyna Stasinska (LUTH, Observatoire de Paris, Meudon, France)

The discrepancy between abundances derived from collisionally excited lines (CEL) and recombination lines (RL) in  H II regions and planetary nebulae is still not fully understood.

As I understand, the effective recombination rates for the lines have been computed for electron temperatures “typical” of photoionized nebulae, i.e. Te < 20 000 K. What if inside the nebulae there are zones of much higher temperatures? Such zones can be produced inside dusty filamentary nebulae, as shown by Stasinska & Szczerba (2001), or in zones of shocked gas inside the nebulae. Could there not be a process occurring only at temperatures higher that 20 000 K, that would enhance the production of recombination lines (something similar to dielectronic recombination at high temperatures) ?  Of course, CELs would also be affected by such high temperature zones, but differently and this could perhaps resolve the RL/CEL discrepancy.

I know that, presently, temperature diagnostics from recombination lines always indicate low temperatures. But those could be flawed as well, if the high temperature zones give a noticeable contribution.

You are invited to attend the inaugural gathering of the Laboratory Astrophysics Division (LAD) of the American Astronomical Society (AAS) during the 220th AAS meeting in Anchorage, Alaska, 10-14 June 2012. There LAD will convene several Meeting-in-a-Meeting sessions devoted to the interplay between laboratory astrophysics and other fields in astronomy and related sciences.

The first new AAS division in more than 30 years, LAD is the successor to the AAS Working Group on Laboratory Astrophysics, established in 2007. The transition to a full-fledged division was approved by the AAS Council at the 219th AAS meeting in Austin, Texas, in January 2012. LAD’s mission is to advance our understanding of the universe through the promotion of fundamental theoretical and experimental research into the underlying processes that drive cosmic evolution.

Among the special events planned in Anchorage is the Kavli Lecture opening the conference, which will be given by Prof. Ewine van Dishoeck (University of Leiden) on a topic relevant to the new division. Over the next four days a Meeting-in-a-Meeting entitled “Bridging Laboratory and Astrophysics” will feature 21 talks covering atomic, molecular, solid state, plasma, planetary, nuclear, and particle laboratory astrophysics, as well as an associated poster session.

To attend this event and support the new division, please visit http://aas.org/meetings/aas220 for registration and abstract-submission deadlines and guidelines. Please note that abstracts are due Thursday, 1 March 2012. Student participation is particularly encouraged. More information will soon follow on how to join the LAD and on other division activities.

(a) CEA/IRFU/SAp, F-91191 Gif-sur-Yvette France; (b) CEA/IRAMIS/SPAM, F-91191 Gif-sur-Yvette France; (c) CEA/DAM/DIF, F-91297 Arpajon, France; (d) Theoretical Division, LANL, Los Alamos NM 87545, USA; (e) AWE Reading, Berkshire, RG7 4PR, UK; (f) ARTEP, Ellicott City MD 21042; (g) LERMA, Observatoire de Paris, France; (h) LULI, Ecole Polytechnique, CNRS, CEA, UPMC, F-91128 Palaiseau Cedex, France.

Opacity is an important ingredient of the evolution of stars. The calculation of opacity coefficients is complicated by the fact that the plasma contains partially ionized heavy ions that contribute to opacity dominated by H and He. Up to now, the astrophysical community has greatly benefited from the work of the contributions of Los Alamos [1], Livermore [2] and the Opacity Project (OP) [3]. However unexplained differences of up to 50% in the radiative forces and Rosseland mean values for Fe have been noticed for conditions corresponding to stellar envelopes. Such uncertainty has a real impact on the understanding of pulsating stellar envelopes, on the excitation of modes, and on the identification of the mode frequencies. Temperature and density conditions equivalent to those found in stars can now be produced in laboratory experiments for various atomic species. Recently the photo-absorption spectra of nickel and iron plasmas have been measured during the LULI 2010 campaign, for temperatures between 15 and 40 eV and densities of ~3 mg/cm3. A large theoretical collaboration, the “OPAC”, has been formed to prepare these experiments. We present here the set of opacity calculations performed by eight different groups for conditions relevant to the LULI 2010 experiment and to astrophysical stellar envelope conditions.

Complete preprint ==> http://arxiv.org/abs/1201.6245

]]> http://astroatom.wordpress.com/2012/01/31/comparison-of-fe-and-ni-opacity-calculations-for-a-better-understanding-of-pulsating-stellar-envelopes/feed/ 0 astroatom http://astroatom.wordpress.com/2012/01/30/iron-and-nickel-spectral-opacity-calculations-in-conditions/ http://astroatom.wordpress.com/2012/01/30/iron-and-nickel-spectral-opacity-calculations-in-conditions/#comments Mon, 30 Jan 2012 17:33:26 +0000 AstroAtom http://astroatom.wordpress.com/?p=692 Continue reading →]]> D. Gilles (1), S. Turck-Chièze (1), M. Busquet (2), F. Thais (3), G. Loisel (4), L. Piau (1), J.E. Ducret (1), T. Blenski (3), C. Blancard (5), P. Cossé (5), G. Faussurier, F. Gilleron (5), J.C. Pain (5), Q. Porcherot (5), J.A. Guzik (6), D.P. Kilcrease (6), N.H. Magee (6), J. Harris (7), S. Bastiani-Ceccotti (8), F. Delahaye (9), C.J Zeippen (9).

(1) CEA/IRFU/Sap, F-91191 Gif-sur-Yvette, Cedex, France; (2) ARTEP Ellicott City, MD 21042, USA;  (3) CEA/IRAMIS/SPAM, F-91191 Gif-sur-Yvette, France; (4) Sandia National Laboratories, Albuquerque, NM 87185-1196, USA; (5) CEA/DIF, F-91297 Arpajon, France; (6) Theoretical Division, LANL, Los Alamos NM 87545, USA;  (7) AWE Readings Berkshire, RG7 4PR, UK; (8) LULI, Ecole Polytechnique, 91128 Palaiseau, France; (9) LERMA Observatoire de Paris-Meudon, France.

Seismology of stars is strongly developing. To address this question we have formed an international collaboration OPAC to perform specific experimental measurements, compare opacity calculations and improve the opacity calculations in the stellar codes [1]. We consider the following opacity codes: SCO, CASSANDRA, STA, OPAS, LEDCOP, OP, SCO-RCG. Their comparison has shown large differences for Fe and Ni in equivalent conditions of envelopes of type II supernova precursors, temperatures between 15 and 40 eV and densities of a few mg/cm3 [2, 3, 4]. LEDCOP, OPAS, SCO-RCG structure codes and STA give similar results and differ from OP ones for the lower temperatures and for spectral interval values [3]. In this work we discuss the role of Configuration Interaction (CI) and the influence of the number of used configurations. We present and include in the opacity code comparisons new HULLAC-v9 calculations [5, 6] that include full CI. To illustrate the importance of this effect we compare different CI approximations (modes) available in HULLAC-v9 [7]. These results are compared to previous predictions and to experimental data. Differences with OP results are discussed.

Complete preprint ==> http://arxiv.org/abs/1201.4692

In the recent paper  ”Solar mixture opacity calculations using detailed configuration and level accounting treatments” by Christophe Blancard (1, 2),  Philippe Cossé (1)  & Gérald Faussurier (1) ((1) CEA, DAM, DIF, F-91297 Arpajon, France;  (2) Observatoire de Paris, LUTH, CNRS UMR 8102, F-92195 Meudon, France), new opacities have been computed for a solar mixture using a numerical method referred to as OPAS. Rosseland mean opacities are compared with both the OPAL and OP data sets, finding very good agreement for a range of densities and temperatures typical of the solar radiative zone. However large discrepancies  between OPAS and OP are  reported for the relative elemental contributions, particularly for the heavier species where the former are higher near the radiation-convection interface. Such differences are believed to be due to the large number of excited configurations considered by OPAS. The OP Heα line profile is also found to be broader than that predicted by OPAS.