Wavelengths of the 3d6(5D)4s a6D - 3d6(5D)4p y6P Multiplet of Fe II (UV 8)

We investigate the wavenumber scale of Fe I and Fe II lines using new spectra recorded with Fourier transform spectroscopy and using a re-analysis of archival spectra. We find that standards in Ar II, Mg I, Mg II and Ge I give a consistent wavenumber calibration. We use the recalibrated spectra to derive accurate wavelengths for the a6D-y6P multiplet of Fe II (UV 8) using both directly measured lines and Ritz wavelengths. Lines from this multiplet are important for astronomical tests of the invariance of the fine structure constant on a cosmological time scale. We recommend a wavelength of 1608.45081 {\AA} with a one standard deviation uncertainty of 0.00007 {\AA} for the a6D9/2-y6P7/2 transition.Comment: 29 pages, 7 figures, 8 tables. Accepted for publication in J. Opt. Soc. Am

Atomic Data Needs in Laboratory Astrophysics: Experimental Methods for Spectroscopy and Charge Exchange with Ions

Modeling the spectra of astrophysical environments requires knowledge of the absolute cross sections for the relevant interactions occuring between ions and neutrals. Two of the common processes are a) charge transfer, which has been shown to be the source of cometary x-rays, and b) spontaneous emission, which is used as an abundance diagnostic in studies of stellar atmospheres and other exotic environments. Understanding the source of observed emission lines, e.g. charge exchange or de-excitation, will require an understanding of the electronic structure of each element in the environment being studied. This dissertation examines the charge exchange process with singly charged ions and investigates spontaneous emission from heavy atoms and ions. Charge exchange measurements are typically performed either with a gas cell or with a gas jet. For absolute cross sections, a gas cell provides a well defined pressure and path length. The design and experimental procedure for a gas cell based experiment optimized for interactions expected to have large cross sections is discussed. Charge exchange cross sections are reported for the four symmetric ion-neutral reactions He+ - He, Ne+ - Ne, Ar+ - Ar, and Kr+ - Kr between 0.2 - 5.0 keV and compared to the available experimental and theoretical results. Extrapolation of a fit to the current data using the theoretically suggested functional form provides good agreement even with the available high energy experimental results. A modified gas cell with improved gas conductance and the ability to scan the collision energy without altering the ion source parameters is also described. For interactions where it is desired to extract both the product ions and/or photons from the interaction region, a crossed beam set-up is required. A gas jet provides near-complete optical access to the interaction region but a less rigidly defined pressure and path length. Two gas jets were designed for future cossed-beam experiments involving solar wind ions and simple neutrals. The first design is intended for delivering neutral beams with central densities of order 10^6 - 10^7 cm-3 to the CUEBIT drift tubes for producing metal ions. The operation of the jet is consistent with a simple theoretical model. A second gas jet, intended for operation at higher backing pressures, is presented and expected to produce dense targets (n \u3e 10^10 cm-3) to a crossed-beam apparatus downstream from CUEBIT. In environments containing heavy elements, emission lines may be used to estimate the abundances of these heavy species. Therefore accurate knowledge of the electronic structure of both the ground and higher charge states of the elements is required for interpretation of the astrophysical spectra where these elements may be found, e.g. kilonovae or solar wind interactions. The electronic structure of Au I - II was studied by observing emission from gold targets ablated inside the Compact Toroidal Hybrid plasma apparatus at Auburn University. Gold lines were identified by their time-dependent behavior when compared to similar spectra of nickel targets. New emission lines are found by comparing the observed spectra to Ritz wavelengths calculated from level energies in the literature. Level lists and line lists of dipole-allowed transitions spanning 187 - 800nm are reported for Au I \& II with resolution d(Lambda) / Lambda ~ 10^-4. It is expected that these results will contribute to the understanding and interpretation of spectra from neutron star mergers and chemically peculiar stars

Fourier-transform VUV spectroscopy of 14,15^{14,15}N and 12,13^{12,13}C

Accurate Fourier-transform spectroscopic absorption measurements of vacuum ultraviolet transitions in atomic nitrogen and carbon were performed at the Soleil synchrotron. For $^{14}$N transitions from the 2s^22p^3\,^4S$_{3/2}$ ground state and from the 2s^22p^3\,^2P and $^2$D metastable states were determined in the $95 - 124$ nm range at an accuracy of $0.025\,\mathrm{cm}^{-1}$. Combination of these results with data from previous precision laser experiments in the vacuum ultraviolet range reveal an overall and consistent offset of -0.04 \wn\ from values reported in the NIST database. %The splitting of the 2s^22p^3\,^4S$_{3/2}$ -- %2s2p^4\,^4P$_{5/2,3/2,1/2}$ The splittings of the 2s^22p^3\,^4S$_{3/2}$ -- 2s2p^4\,^4P$_{J}$ transitions are well-resolved for $^{14}$N and $^{15}$N and isotope shifts determined. While excitation of a $2p$ valence electron yields very small isotope shifts, excitation of a $2s$ core electron results in large isotope shifts, in agreement with theoretical predictions. For carbon six transitions from the ground 2s^22p^2\,^3P$_{J}$ and 2s^22p3s\, ^3P$_{J}$ excited states at $165$ nm are measured for both $^{12}$C and $^{13}$C isotopes

Deep Modeling of Quasar Variability

Quasars have long been known as intrinsically variable sources, but the physical mechanism underlying the temporal optical/UV variability is still not well understood. We propose a novel nonparametric method for modeling and forecasting the optical variability of quasars utilizing an AE neural network to gain insight into the underlying processes. The AE is trained with ~15,000 decade-long quasar light curves obtained by the Catalina Real-time Transient Survey selected with negligible flux contamination from the host galaxy. The AE's performance in forecasting the temporal flux variation of quasars is superior to that of the damped random walk process. We find a temporal asymmetry in the optical variability and a novel relation—the amplitude of the variability asymmetry decreases as luminosity and/or black hole mass increases—is suggested with the help of autoencoded features. The characteristics of the variability asymmetry are in agreement with those from the self-organized disk instability model, which predicts that the magnitude of the variability asymmetry decreases as the ratio of the diffusion mass to inflow mass in the accretion disk increases

Deep Modeling of Quasar Variability

Quasars have long been known as intrinsically variable sources, but the physical mechanism underlying the temporal optical/UV variability is still not well understood. We propose a novel nonparametric method for modeling and forecasting the optical variability of quasars utilizing an AE neural network to gain insight into the underlying processes. The AE is trained with ~15,000 decade-long quasar light curves obtained by the Catalina Real-time Transient Survey selected with negligible flux contamination from the host galaxy. The AE's performance in forecasting the temporal flux variation of quasars is superior to that of the damped random walk process. We find a temporal asymmetry in the optical variability and a novel relation—the amplitude of the variability asymmetry decreases as luminosity and/or black hole mass increases—is suggested with the help of autoencoded features. The characteristics of the variability asymmetry are in agreement with those from the self-organized disk instability model, which predicts that the magnitude of the variability asymmetry decreases as the ratio of the diffusion mass to inflow mass in the accretion disk increases

One and two-color photoassociation spectroscopy of ultracold 40Ca

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