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At least 739 records · Page 41
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DOE OSTI2014

The signature of single-degenerate accretion-induced collapse

The accretion-induced collapse (AIC) of a white dwarf to a neutron star has long been suggested as a natural theoretical outcome in stellar evolution, but there has never been a direct detection of such an event. This is not surprising since the small amount of radioactive nickel synthesized (∼10{sup –3} M {sub ☉}) implies a relatively dim optical transient. Here we argue that a particularly strong signature of an AIC would occur for an oxygen-neon-magnesium (ONeMg) white dwarf accreting from a star that is experiencing Roche-lobe overflow as it becomes a red giant. In such cases, the ∼10{sup 50} erg explosion from the AIC collides with and shock-heats the surface of the extended companion, creating an X-ray flash lasting ∼1 hr followed by an optical signature that peaks at an absolute magnitude of ∼ – 16 to –18 and lasts for a few days to a week. These events would be especially striking in old stellar environments where hydrogen-rich supernova-like transients would not normally be expected. Although the rate of such events is not currently known, we describe observing strategies that could be utilized with high cadence surveys that should either detect these events or place strong constraints on their rates.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2015

PHYSICAL CONDITIONS IN THE X-RAY EMISSION-LINE GAS IN NGC 1068

We present a detailed, photoionization modeling analysis of XMM-Newton/Reflection Grating Spectrometer observations of the Seyfert 2 galaxy NGC 1068. The spectrum, previously analyzed by Kinkhabwala et al., reveals a myriad of soft X-ray emission lines, including those from H- and He-like carbon, nitrogen, oxygen, and neon, and M- and L-shell iron. As noted in the earlier analysis, based on the narrowness of the radiative recombination continua, the electron temperatures in the emission-line gas are consistent with photoionization, rather than collisional ionization. The strengths of the carbon and nitrogen emission lines, relative to those of oxygen, suggest unusual elemental abundances, which we attribute to the star formation history of the host galaxy. Overall, the emission lines are blueshifted with respect to systemic, with radial velocities ∼160 km s{sup –1}, similar to that of [O III] λ5007, and thus consistent with the kinematics and orientation of the optical emission-line gas and, hence, likely part of an active galactic nucleus driven outflow. We were able to achieve an acceptable fit to most of the strong emission lines with a two-component photoionization model, generated with CLOUDY. The two components have ionization parameters and column densities of logU = –0.05 and 1.22 and logN {sub H} = 20.85 and 21.2 and covering factors of 0.35 and 0.84, respectively. The total mass of the X-ray gas is roughly an order of magnitude greater than the mass of ionized gas determined from optical and near-IR spectroscopy, which indicates that it may be the dominant component of the narrow-line region. Furthermore, we suggest that the medium that produces the scattered/polarized optical emission in NGC 1068 possesses similar physical characteristics to those of the more highly ionized of the X-ray model components.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2015

ISMabs: A COMPREHENSIVE X-RAY ABSORPTION MODEL FOR THE INTERSTELLAR MEDIUM

We present an X-ray absorption model for the interstellar medium, to be referred to as ISMabs, that takes into account both neutral and ionized species of cosmically abundant elements, and includes the most accurate atomic data available. Using high-resolution spectra from eight X-ray binaries obtained with the Chandra High Energy Transmission Grating Spectrometer, we proceed to benchmark the atomic data in the model particularly in the neon K-edge region. Compared with previous photoabsorption models, which solely rely on neutral species, the inclusion of ions leads to improved spectral fits. Fit parameters comprise the column densities of abundant contributors that allow direct estimates of the ionization states. ISMabs is provided in the appropriate format to be implemented in widely used X-ray spectral fitting packages such as XSPEC, ISIS, and SHERPA.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2015

Pre-supernova neutrino emissions from ONe cores in the progenitors of core-collapse supernovae: Are they distinguishable from those of Fe cores?

Aiming to distinguish two types of progenitors of core-collapse supernovae, i.e., one with a core composed mainly of oxygen and neon (abbreviated as ONe core) and the other with an iron core (or Fe core), we calculated the luminosities and spectra of neutrinos emitted from these cores prior to gravitational collapse, taking neutrino oscillation into account. We found that the total energies emitted as ν-bar {sub e} from the ONe core are ≲10{sup 46} erg, which is much smaller than ∼10{sup 47} erg for Fe cores. The average energy, on the other hand, is twice as large for the ONe core as those for the Fe cores. The neutrinos produced by the plasmon decays in the ONe core are more numerous than those from the electron–positron annihilation in both cores, but they have much lower average energies ≲1 MeV. Although it is difficult to detect the pre-supernova neutrinos from the ONe core even if it is located within 200 pc from Earth, we expect ∼9–43 and ∼7–61 events for Fe cores at KamLAND and Super-Kamiokande, respectively, depending on the progenitor mass and neutrino-mass hierarchy. These numbers might be increased by an order of magnitude if we envisage next-generation detectors such as JUNO. We will hence be able to distinguish the two types of progenitors by the detection or nondetection of the pre-supernova neutrinos if they are close enough (≲1 kpc).

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2015

Temperature-based metallicity measurements at z = 0.8: Direct calibration of strong-line diagnostics at intermediate redshift

We present the first direct calibration of strong-line metallicity diagnostics at significant cosmological distances using a sample at z≃0.8 drawn from the DEEP2 Galaxy Redshift Survey. Oxygen and neon abundances are derived from measurements of electron temperature and density. We directly compare various commonly used relations between gas-phase metallicity and strong line ratios of O, Ne, and H at z≃0.8 and z = 0. There is no evolution with redshift at high precision (Δlog O/H=−0.01±0.03, Δlog Ne/O=0.01±0.01). O, Ne, and H line ratios follow the same locus at z≃0.8 as at z = 0, with ≲0.02 dex evolution and low scatter (≲0.04 dex). This suggests little or no evolution in physical conditions of H ii regions at fixed oxygen abundance, in contrast to models which invoke more extreme properties at high redshifts. We speculate that offsets observed in the [N ii]/Ha versus [O iii]/Hβ diagram at high redshift are therefore due to [N ii] emission, likely as a result of relatively high N/O abundance. If this is indeed the case, then nitrogen-based metallicity diagnostics suffer from systematic errors at high redshift. Our findings indicate that locally calibrated abundance diagnostics based on α-capture elements can be reliably applied at z≃1 and possibly at much higher redshifts. This constitutes the first firm basis for the widespread use of empirical calibrations in high redshift metallicity studies.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2015

Chemical abundances of planetary nebulae in the substructures of M31

We present deep spectroscopy of planetary nebulae (PNe) that are associated with the substructures of the Andromeda Galaxy (M31). The spectra were obtained with the Optical System for Imaging and low-intermediate-Resolution Integrated Spectroscopy spectrograph on the 10.4 m Gran Telescopio Canarias. Seven targets were selected for the observations, three in the Northern Spur and four associated with the Giant Stream. The most distant target in our sample, with a rectified galactocentric distance ⩾ 100 kpc, was the first PN discovered in the outer streams of M31. The [O iii] λ4363 auroral line is well detected in the spectra of all targets, enabling electron temperature determination. Ionic abundances are derived based on the [O iii] temperatures, and elemental abundances of helium, nitrogen, oxygen, neon, sulfur, and argon are estimated. The relatively low N/O and He/H ratios, as well as abundance ratios of α-elements, indicate that our target PNe might belong to populations as old as ∼2 Gyr. Our PN sample, including the current seven and the previous three observed by Fang et al., have rather homogeneous oxygen abundances. The study of abundances and the spatial and kinematical properties of our sample leads to the tempting conclusion that their progenitors might belong to the same stellar population, which hints at a possibility that the Northern Spur and the Giant Stream have the same origin. This may be explained by the stellar orbit proposed by Merrett et al. Judging from the position and kinematics, we emphasize that M32 might be responsible for the two substructures. Deep spectroscopy of PNe in M32 will help to assess this hypothesis.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2013

THE SPECTRUM AND TERM ANALYSIS OF V II

The spectrum and extended term analysis of V II are presented. Fourier transform spectrometry was used to record high resolution spectra of singly ionized vanadium in the region 1492-5800 A (67020-17260 cm{sup -1}) with vanadium-neon and vanadium-argon hollow cathode lamps as sources. The wavenumber uncertainty for the center of gravity of the strongest lines is typically 0.002 cm{sup -1}, an improvement of an order of magnitude over previous measurements. Most of the lines exhibit partly resolved hyperfine structure. The V II energy levels in the 1985 compilation of Sugar and Corliss have been confirmed and revised, with the exception of the high-lying 4f levels and eight of the lower levels. Thirty-nine of the additional eighty-five high levels published by Iglesias et al. have also been confirmed and revised, and three of their missing levels have been found. The energy uncertainty of the revised levels has been reduced by about an order of magnitude. In total, 176 even levels and 233 odd levels are presented. Wavenumbers and classifications are given for 1242 V II lines.

74 ATOMIC AND MOLECULAR PHYSICS↗
DOE OSTI2014

THE SPECTRUM OF THORIUM FROM 250 nm TO 5500 nm: RITZ WAVELENGTHS AND OPTIMIZED ENERGY LEVELS

We have made precise observations of a thorium-argon hollow cathode lamp emission spectrum in the region between 350 nm and 1175 nm using a high-resolution Fourier transform spectrometer. Our measurements are combined with results from seven previously published thorium line lists to re-optimize the energy levels of neutral, singly, and doubly ionized thorium (Th I, Th II, and Th III). Using the optimized level values, we calculate accurate Ritz wavelengths for 19, 874 thorium lines between 250 nm and 5500 nm (40, 000 cm{sup –1} to 1800 cm{sup –1}). We have also found 102 new thorium energy levels. A systematic analysis of previous measurements in light of our new results allows us to identify and propose corrections for systematic errors in Palmer and Engleman and typographical errors and incorrect classifications in Kerber et al. We also found a large scatter with respect to the thorium line list of Lovis and Pepe. We anticipate that our Ritz wavelengths will lead to improved measurement accuracy for current and future spectrographs that make use of thorium-argon or thorium-neon lamps as calibration standards.

74 ATOMIC AND MOLECULAR PHYSICS↗
DOE OSTI2013

ELECTRON-CAPTURE SUPERNOVAE AS ORIGIN OF {sup 48}Ca

We report that electron-capture supernovae (ECSNe), arising from collapsing oxygen-neon-magnesium cores, are a possible source of {sup 48}Ca, whose origin has remained a longstanding puzzle. Our two-dimensional, self-consistent explosion model of an ECSN predicts ejection of neutron-rich matter with electron fractions Y{sub e} Almost-Equal-To 0.40-0.42 and relatively low entropies, s Almost-Equal-To 13-15 k{sub B} per nucleon (k{sub B} is the Boltzmann constant). Post-processing nucleosynthesis calculations result in appreciable production of {sup 48}Ca in such neutron-rich and low-entropy matter during the quasi-nuclear equilibrium and subsequent freezeout phases. The amount of ejected {sup 48}Ca can account for that in the solar inventory when we consider possible uncertainties in the entropies. ECSNe could thus be a site of {sup 48}Ca production in addition to a hypothetical, rare class of high-density Type Ia supernovae.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2011

Characterization of the ground X{sub 1} state of {sup 204}Pb{sup 19}F, {sup 206}Pb{sup 19}F, {sup 207}Pb{sup 19}F, and {sup 208}Pb{sup 19}F

Pure rotational spectra of the ground electronic-vibrational X{sub 1} state of {sup 204}Pb{sup 19}F, {sup 206}Pb{sup 19}F, {sup 207}Pb{sup 19}F, and {sup 208}Pb{sup 19}F are measured with a resonator pulsed supersonic jet Fourier-transform microwave spectrometer. Also reported is a new measurement of the Stark effect on the optical spectra of A(leftarrow)X{sub 1} transitions. These spectra are combined with published high-resolution infrared spectra of X{sub 2}{r_reversible}X{sub 1} transitions in order to create a complete picture of the ground state of lead monofluoride. For the microwave data, molecules are prepared by laser ablation of lead target rods and stabilized in a supersonic jet of neon mixed with sulfur hexafluoride. For the optical Stark spectra, a continuous source of molecules is created in a nozzle heated to 1000 deg. C. The microwave spectra confirm, improve, and extend previously reported constants that describe the rotational, spin-orbit, and hyperfine interactions of the ground electronic state of the PbF molecule. A discrepancy concerning the sign of the hyperfine constant describing the {sup 207}Pb nucleus is discussed. Magnetic-field-dependent microwave spectra are used to characterize the Zeeman interaction in terms of two g factors of the body-fixed electronic wave function. The optical Stark spectra are used to characterize the electric dipole moment of the X{sub 1} and A states.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
DOE OSTI2011

Single attosecond pulse from terahertz-assisted high-order harmonic generation

High-order harmonic generation by few-cycle 800 nm laser pulses in neon gas in the presence of a strong terahertz (THz) field is investigated numerically with propagation effects taken into account. Our calculations show that the combination of THz fields with up to 12 fs laser pulses can be an effective gating technique to generate single attosecond pulses. We show that in the presence of the strong THz field only a single attosecond burst can be phase matched, whereas radiation emitted during other half cycles disappears during propagation. The cutoff is extended and a wide supercontinuum appears in the near-field spectra, extending the available spectral width for isolated attosecond pulse generation from 23 to 93 eV. We demonstrate that phase-matching effects are responsible for the generation of isolated attosecond pulses, even in conditions when single-atom response yields an attosecond pulse train.

74 ATOMIC AND MOLECULAR PHYSICS↗
DOE OSTI2011

Correction method for obtaining the variationally best ground-state pair density

We present a correction method for the pair density (PD) to get close to the ground-state one. The PD is corrected to be a variationally best PD within the search region that is extended by adding the uniformly scaled PDs to its elements. The corrected PD is kept N-representable and satisfies the virial relation rigorously. The validity of the present method is confirmed by numerical calculations of neon atom. It is shown that the root-mean-square error of the electron-electron interaction and external potential energies, which is a good benchmark for the error of the PD, is reduced by 69.7% without additional heavy calculations.

74 ATOMIC AND MOLECULAR PHYSICS↗
DOE OSTI2012

In situ high-pressure synchrotron x-ray diffraction study of CeVO[subscript 4] and TbVO[subscript 4] up to 50 GPa

Room-temperature angle-dispersive x-ray diffraction measurements on zircon-type TbVO{sub 4} and CeVO{sub 4} were performed in a diamond-anvil cell up to 50 GPa using neon as a pressure-transmitting medium. In TbVO{sub 4}, we found at 6.4 GPa evidence of a nonreversible pressure-induced structural phase transition from zircon to a scheelite-type structure. A second transition to an M-fergusonite-type structure was found at 33.9 GPa, which is reversible. Zircon-type CeVO{sub 4} exhibits two pressure-induced transitions: first, an irreversible transition to a monazite-type structure at 5.6 GPa and, second, at 14.7 GPa, a reversible transition to an orthorhombic structure. No additional phase transitions or evidences of chemical decomposition are found in the experiments. The equations of state and axial compressibility for the different phases are also determined. Finally, the sequence of structural transitions and the compressibilities are discussed in comparison with other orhtovanadates and the influence of nonhydrostaticity commented.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2011

Unveiling and Driving Hidden Resonances with High-Fluence, High-Intensity X-Ray Pulses

We show that high fluence, high-intensity x-ray pulses from the world's first hard x-ray free-electron laser produce nonlinear phenomena that differ dramatically from the linear x-ray-matter interaction processes that are encountered at synchrotron x-ray sources. We use intense x-ray pulses of sub-10-fs duration to first reveal and subsequently drive the 1s{r_reversible}2p resonance in singly ionized neon. This photon-driven cycling of an inner-shell electron modifies the Auger decay process, as evidenced by line shape modification. Our work demonstrates the propensity of high-fluence, femtosecond x-ray pulses to alter the target within a single pulse, i.e., to unveil hidden resonances, by cracking open inner shells energetically inaccessible via single-photon absorption, and to consequently trigger damaging electron cascades at unexpectedly low photon energies.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2013

A prototype fan-beam optical CT scanner for 3D dosimetry

Purpose: The objective of this work is to introduce a prototype fan-beam optical computed tomography scanner for three-dimensional (3D) radiation dosimetry. Methods: Two techniques of fan-beam creation were evaluated: a helium-neon laser (HeNe, {lambda} = 543 nm) with line-generating lens, and a laser diode module (LDM, {lambda} = 635 nm) with line-creating head module. Two physical collimator designs were assessed: a single-slot collimator and a multihole collimator. Optimal collimator depth was determined by observing the signal of a single photodiode with varying collimator depths. A method of extending the dynamic range of the system is presented. Two sample types were used for evaluations: nondosimetric absorbent solutions and irradiated polymer gel dosimeters, each housed in 1 liter cylindrical plastic flasks. Imaging protocol investigations were performed to address ring artefacts and image noise. Two image artefact removal techniques were performed in sinogram space. Collimator efficacy was evaluated by imaging highly opaque samples of scatter-based and absorption-based solutions. A noise-based flask registration technique was developed. Two protocols for gel manufacture were examined. Results: The LDM proved advantageous over the HeNe laser due to its reduced noise. Also, the LDM uses a wavelength more suitable for the PRESAGE{sup TM} dosimeter. Collimator depth of 1.5 cm was found to be an optimal balance between scatter rejection, signal strength, and manufacture ease. The multihole collimator is capable of maintaining accurate scatter-rejection to high levels of opacity with scatter-based solutions (T < 0.015%). Imaging protocol investigations support the need for preirradiation and postirradiation scanning to reduce reflection-based ring artefacts and to accommodate flask imperfections and gel inhomogeneities. Artefact removal techniques in sinogram space eliminate streaking artefacts and reduce ring artefacts of up to {approx}40% in magnitude. The flask registration technique was shown to achieve submillimetre and subdegree placement accuracy. Dosimetry protocol investigations emphasize the need to allow gel dosimeters to cool gradually and to be scanned while at room temperature. Preliminary tests show that considerable noise reduction can be achieved with sinogram filtering and by binning image pixels into more clinically relevant grid sizes. Conclusions: This paper describes a new optical CT scanner for 3D radiation dosimetry. Tests demonstrate that it is capable of imaging both absorption-based and scatter-based samples of high opacities. Imaging protocol and gel dosimeter manufacture techniques have been adapted to produce optimal reconstruction results. These optimal results will require suitable filtering and binning techniques for noise reduction purposes.

62 RADIOLOGY AND NUCLEAR MEDICINE↗
DOE OSTI2013

Influence of the chemical nature of implanted ions on the structure of a silicon layer damaged by implantation

The influence of the implantation of silicon single crystals by fluorine, nitrogen, oxygen, and neon ions on the distribution of strain and the static Debye-Waller factor in the crystal lattice over the implanted-layer depth has been investigated by high-resolution X-ray diffraction. The density depth distribution in the surface layer of native oxide has been measured by X-ray reflectometry. Room-temperature implantation conditions have ensured the equality of the suggested ranges of ions of different masses and the energies transferred by them to the target. It is convincingly shown that the change in the structural parameters of the radiation-damaged silicon layer and the native oxide layer depend on the chemical activity of the implanted ions.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2012

Narrow-band N-resonance formed in thin rubidium atomic layers

The narrow-band N-resonance formed in a {Lambda} system of D{sub 1}-line rubidium atoms is studied in the presence of a buffer gas (neon) and the radiations of two continuous narrow-band diode lasers. Special-purpose cells are used to investigate the dependence of the process on vapor column thickness L in millimeter, micrometer, and nanometer ranges. A comparison of the dependences of the N-resonance and the electromagnetically induced transparency (EIT) resonance on L demonstrates that the minimum (record) thickness at which the N-resonance can be detected is L = 50 {mu}m and that a high-contrast EIT resonance can easily be formed even at L Almost-Equal-To 800 nm. The N-resonance in a magnetic field for {sup 85}Rb atoms is shown to split into five or six components depending on the magnetic field and laser radiation directions. The results obtained indicate that levels F{sub g} = 2, 3 are initial and final in the N-resonance formation. The dependence of the N-resonance on the angle between the laser beams is analyzed, and practical applications are noted.

74 ATOMIC AND MOLECULAR PHYSICS↗
DOE OSTI2017

On the atomic-number similarity of the binding energies of electrons in filled shells of elements of the periodic table

An expression for the binding energies of electrons in the ground state of an atom is derived on the basis of the Bohr–Sommerfeld quantization rule within the Thomas–Fermi model. The validity of this relation for all elements from neon to uranium is tested within a more perfect quantum-mechanical model with and without the inclusion of relativistic effects, as well as with experimental binding energies. As a result, the ordering of electronic levels in filled atomic shells is established, manifested in an approximate atomic-number similarity. It is proposed to use this scaling property to analytically estimate the binding energies of electrons in an arbitrary atom.

74 ATOMIC AND MOLECULAR PHYSICS↗