arXiv subjects

J. E. Lawler

Publications and source records attributed to J. E. Lawler.

At least 19 recordsLinked to original sources

arXiv2023

Atomic Transition Probabilities for Transitions of Si I and Si II and the Silicon Abundances of Several Very Metal-Poor Stars

We report new measurements of branching fractions for 20 UV and blue lines in the spectrum of neutral silicon (Si I) originating in the 3$s^{2}$3$p$4$s$ $^{3}$P$^{\rm o}_{1,2}$, $^{1}$P$^{\rm o}_{1}$ and 3$s$3$p^{3}$ $^{1}$D$^{\rm o}_{1,2}$ upper levels. Transitions studied include both strong, nearly pure LS multiplets as well as very weak spin-forbidden transitions connected to these upper levels. We also report a new branching fraction measurement of the $^{4}$P$_{1/2}$ - $^{2}$P$^{\rm o}_{1/2,3/2}$ intercombination lines in the spectrum of singly-ionized silicon (Si II). The weak spin-forbidden lines of Si I and Si II provide a stringent test on recent theoretical calculations, to which we make comparison. The branching fractions from this study are combined with previously reported radiative lifetimes to yield transition probabilities and log($gf$)s for these lines. We apply these new measurements to abundance determinations in five metal-poor stars.

astro-ph.SR↗

arXiv2021

Atomic Transition Probabilities of Neutral Calcium

The goals of this study are 1) to test the best theoretical transition probabilities for Ca I (a relatively light alkaline earth spectrum) from a modern ab initio calculation using configuration interaction plus many body perturbation theory against the best modern experimental transition probabilities, and 2) to produce as accurate and comprehensive a line list of Ca I transition probabilities as is currently possible based on this comparison. We report new Ca I radiative lifetime measurements from a laser-induced fluorescence (LIF) experiment and new emission branching fraction measurements from a 0.5 m focal length grating spectrometer with a detector array. We combine these data for upper levels that have both a new lifetime and new branching fractions to report log(gf)s for two multiplets consisting of nine transitions. Detailed comparisons are made between theory and experiment, including the measurements reported herein and a selected set of previously published experimental transition probabilities. We find that modern theory compares favorably to experimental measurements in most instances where such data exist. A final list of 202 recommended transition probabilities is presented, which covers lines of Ca I with wavelengths ranging from 2200 - 10,000 Angstroms. These are mostly selected from theory, but are augmented with high quality experimental measurements from this work and from the literature. The recommended transition probabilities are used in a redetermination of the Ca abundance in the Sun and in the metal-poor star HD 84937.

astro-ph.SR↗

arXiv2021

Improved Atomic Transition Probabilities for UV and Optical Lines of Hf II and Determination of the Hf Abundance in Two Metal-Poor Stars

We report new branching fraction measurements for 199 UV and optical transitions of Hf II. These transitions range in wavelength (wavenumber) from 2068- 6584 A (48322-15183 cm-1) and originate in 17 odd-parity upper levels ranging in energy from 38578-53227 cm-1. The branching fractions are combined with radiative lifetimes reported in an earlier study to produce a set of transition probabilities and log(gf) values with accuracy ranging from 5-25%. Comparison is made to transition probabilities from the literature where such data exist. We use these new transition probabilities to derive improved Hf abundances in two metal-poor stars. HD 196944 is enhanced in s-process elements, and we derive log epsilon (Hf) = -0.72 +/- 0.03 (sigma = 0.09) from 12 Hf II lines. HD 222925 is enhanced in r-process elements, and we derive log epsilon (Hf) = 0.32 +/- 0.03 (sigma = 0.11) from 20 Hf II lines. These measurements greatly expand the number of potentially useful Hf II lines for analysis in UV and optical spectra.

astro-ph.SR↗

arXiv2019

Atomic transition probabilities for UV and blue lines of Fe II and abundance determinations in the photospheres of the Sun and metal-poor star HD 84937

We report new branching fractions for 121 UV lines from the low-lying odd-parity levels of Fe II belonging to the z6Do, z6Fo, z6Po, z4Fo, z4Do and z4Po terms of the 3d6(5D)4p configuration. These lines range in wavelength from 2250 - 3280 Å and originate in levels ranging in energy from 38459 - 47626 cm-1. In addition, we report branching fractions for 10 weak blue lines connecting to the z4Do term which range in wavelength from 4173 - 4584 Å. The BFs are combined with radiative lifetimes from the literature to determine transition probabilities and log(gf) values. Comparison is made to selected experimental and theoretical data from the literature. Our new data are applied to iron abundance determinations in the Sun and in metal-poor star HD 84937. For the Sun, eight blue lines yield log ε(Fe) = 7.46 +/- 0.03, in agreement with standard solar abundance estimates. For HD 84937 the observable wavelength range extends to the vacuum UV (λ >= 2327 Å), and from 75 lines we derive log ε(Fe) = 5.26 +/- 0.01 (σ = 0.07), near to the metallicity estimates of past HD 84937 studies.

astro-ph.SR↗

arXiv2018

FRIB and the GW170817 Kilonova

In July 2018 an FRIB Theory Alliance program was held on the implications of GW170817 and its associated kilonova for r-process nucleosynthesis. Topics of discussion included the astrophysical and nuclear physics uncertainties in the interpretation of the GW170817 kilonova, what we can learn about the astrophysical site or sites of the r process from this event, and the advances in nuclear experiment and theory most crucial to pursue in light of the new data. Here we compile a selection of scientific contributions to the workshop, broadly representative of progress in r-process studies since the GW170817 event.

astro-ph.HE↗

arXiv2018

Transition probabilities of Co II, weak lines to the ground and low metastable levels

New branching fraction measurements based primarily on data from a cross dispersed echelle spectrometer are reported for 84 lines of Co II. The branching fractions for 82 lines are converted to absolute atomic transition probabilities using radiative lifetimes from laser induced fluorescence (LIF) measurements on 19 upper levels of the lines. A lifetime of 3.3(2) ns for the z5D0 level is used based on LIF measurements for lifetimes of the four other levels in the z5D term. Twelve of the 84 lines are weak transitions connecting to the ground and low metastable levels of Co+. Another 46 lines are strong transitions connecting to the ground and low metastable levels of Co+. For these lines log(gf) values were measured in earlier studies and, with a few exceptions, are confirmed in this study. Such lines, if unblended in stellar spectra, have the potential to yield Co abundance values unaffected by any breakdown of the Local Thermodynamic Equilibrium approximation in stellar photospheres because the ground and low metastable levels of Co+ are the primary population reservoirs of Co in photospheres of interest. Weak lines, if unblended, are useful in photospheres with high Co abundance and strong lines are useful in metal-poor photospheres. New hyperfine structure A constants for 28 levels of ionized Co from least square fits to Fourier transform spectra line profiles are reported. These laboratory data are applied to re-determine the Co abundance in metal-poor halo star HD 84937. Branching fractions and transition probabilities for 19 lines are reported for the first time.

astro-ph.SR↗

arXiv2017

Vanadium Transitions in the Spectrum of Arcturus

We derive a new abundance for vanadium in the bright, mildly metal-poor red giant Arcturus. This star has an excellent high-resolution spectral atlas and well-understood atmospheric parameters, and it displays a rich set of neutral vanadium lines that are available for abundance extraction. We employ a newly recorded set of laboratory FTS spectra to investigate any potential discrepancies in previously reported V I log(gf) values near 900 nm. These new spectra support our earlier laboratory transition data and the calibration method utilized in that study. We then perform a synthetic spectrum analysis of weak V I features in Arcturus, deriving log ε(V) = 3.54 $\pm$ 0.01 (σ = 0.04) from 55 lines. There are no significant abundance trends with wavelength, line strength, or lower excitation energy.

astro-ph.SR↗

arXiv2017

Fe I oscillator strengths for transitions from high-lying odd-parity levels

We report new experimental Fe I oscillator strengths obtained by combining measurements of branching fractions measured with a Fourier Transform spectrometer and time-resolved laser-induced fluorescence lifetimes. The study covers the spectral region ranging from 213 to 1033 nm. A total of 120 experimental log(gf)-values coming from 15 odd-parity energy levels are provided, 22 of which have not been reported previously and 63 values with lower uncertainty than the existing data. Radiative lifetimes for 60 upper energy levels are presented, 39 of which have no previous measurements.

astro-ph.SR↗

arXiv2015

New H-band Stellar Spectral Libraries for the SDSS-III/APOGEE survey

The Sloan Digital Sky Survey--III (SDSS--III) Apache Point Observatory Galactic Evolution Experiment (APOGEE) has obtained high resolution (R $\sim$ 22,500), high signal-to-noise ratio ($>$ 100) spectra in the H$-$band ($\sim$1.5$-$1.7 $μ$m) for about 146,000 stars in the Milky Way galaxy. We have computed spectral libraries with effective temperature ($T\rm{_{eff}}$) ranging from 3500 to 8000 K for the automated chemical analy\-sis of the survey data. The libraries, used to derive stellar parameters and abundances from the APOGEE spectra in the SDSS--III data release 12 (DR12), are based on ATLAS9 model atmospheres and the ASS$ε$T spectral synthesis code. We present a second set of libraries based on MARCS model atmospheres and the spectral synthesis code Turbospectrum. The ATLAS9/ASS$ε$T ($T\rm{_{eff}}$ = 3500$-$8000 K) and MARCS/Turbospectrum ($T\rm{_{eff}}$ = 3500$-$5500 K) grids cover a wide range of metallicity ($-$2.5 $\leq$ [M/H] $\leq$ $+$0.5 dex), surface gravity (0 $\leq$ log $g$ $\leq$ 5 dex), microturbulence (0.5 $\leq$ $ξ$ $\leq$ 8 km~s$^{-1}$), carbon ($-$1 $\leq$ [C/M] $\leq$ $+$1 dex), nitrogen ($-$1 $\leq$ [N/M] $\leq$ $+$1 dex), and $α$-element ($-$1 $\leq$ [$α$/M] $\leq$ $+$1 dex) variations, having thus seven dimensions. We compare the ATLAS9/ASS$ε$T and MARCS/Turbospectrum libraries and apply both of them to the analysis of the observed H$-$band spectra of the Sun and the K2 giant Arcturus, as well as to a selected sample of well-known giant stars observed at very high-resolution. The new APOGEE libraries are publicly available and can be employed for chemical studies in the H$-$band using other high-resolution spectrographs.

astro-ph.SR↗

arXiv2014

Measurement of Holmium Rydberg series through MOT depletion spectroscopy

We report measurements of the absolute excitation frequencies of $^{165}$Ho $4f^{11}6sns$ and $4f^{11}6snd$ odd-parity Rydberg series. The states are detected through depletion of a magneto-optical trap via a two-photon excitation scheme. Measurements of 162 Rydberg levels in the range $n=40-101$ yield quantum defects well described by the Rydberg-Ritz formula. We observe a strong perturbation in the $ns$ series around $n=51$ due to an unidentified interloper at 48515.47(4) cm$^{-1}$. From the series convergence, we determine the first ionization potential $E_\mathrm{IP}=48565.939(4)$ cm$^{-1}$, which is three orders of magnitude more accurate than previous work. This work represents the first time such spectroscopy has been done in Holmium and is an important step towards using Ho atoms for collective encoding of a quantum register.

physics.atom-ph↗

arXiv2014

Fe I Oscillator Strengths for Transitions from High-lying Even-Parity Levels

New radiative lifetimes, measured to $\pm$ 5 % accuracy, are reported for 31 even-parity levels of Fe I ranging from 45061 cm$^{-1}$ to 56842 cm$^{-1}$. These lifetimes have been measured using single-step and two-step time-resolved laser-induced fluorescence on a slow atomic beam of iron atoms. Branching fractions have been attempted for all of these levels, and completed for 20 levels. This set of levels represents an extension of the collaborative work reported in Ruffoni et al. (2014). The radiative lifetimes combined with the branching fractions yields new oscillator strengths for 203 lines of Fe I. Utilizing a 1D-LTE model of the solar photosphere, spectral syntheses for a subset of these lines which are unblended in the solar spectrum yields a mean iron abundance of = 7.45 $\pm$ 0.06.

astro-ph.SR↗

arXiv2014

Improved V II log($gf$) Values, Hyperfine Structure Constants, and Abundance Determinations in the Photospheres of the Sun and Metal-poor Star HD 84937

New experimental absolute atomic transition probabilities are reported for 203 lines of V II. Branching fractions are measured from spectra recorded using a Fourier transform spectrometer and an echelle spectrometer. The branching fractions are normalized with radiative lifetime measurements to determine the new transition probabilities. Generally good agreement is found between this work and previously reported V II transition probabilities. Use of two spectrometers, independent radiometric calibration methods, and independent data analysis routines enables a reduction in systematic uncertainties, in particular those due to optical depth errors. In addition, new hyperfine structure constants are measured for selected levels by least squares fitting line profiles in the FTS spectra. The new V II data are applied to high resolution visible and UV spectra of the Sun and metal-poor star HD 84937 to determine new, more accurate V abundances. Lines covering a range of wavelength and excitation potential are used to search for non-LTE effects. Very good agreement is found between our new solar photospheric V abundance, log ε(V) = 3.95 from 15 V II lines, and the solar-system meteoritic value. In HD 84937, we derive [V/H] = -2.08 from 68 lines, leading to a value of [V/Fe] = 0.24.

astro-ph.SR↗

arXiv2014

Fe I Oscillator Strengths for the Gaia-ESO Survey

The Gaia-ESO Public Spectroscopic Survey (GES) is conducting a large-scale study of multi-element chemical abundances of some 100 000 stars in the Milky Way with the ultimate aim of quantifying the formation history and evolution of young, mature and ancient Galactic populations. However, in preparing for the analysis of GES spectra, it has been noted that atomic oscillator strengths of important Fe I lines required to correctly model stellar line intensities are missing from the atomic database. Here, we present new experimental oscillator strengths derived from branching fractions and level lifetimes, for 142 transitions of Fe I between 3526 Å and 10864 Å, of which at least 38 are urgently needed by GES. We also assess the impact of these new data on solar spectral synthesis and demonstrate that for 36 lines that appear unblended in the Sun, Fe abundance measurements yield a small line-by-line scatter (0.08 dex) with a mean abundance of 7.44 dex in good agreement with recent publications.

astro-ph.SR↗

arXiv2014

Improved Ni I log(gf) Values and Abundance Determinations in the Photospheres of the Sun and Metal-poor Star HD 84937

Atomic transition probability measurements for 371 Ni I lines in the UV through near IR are reported. Branching fractions from data recorded using a Fourier transform spectrometer and a new echelle spectrograph are combined with published radiative lifetimes to determine these transition probabilities. Generally good agreement is found in comparisons to previously reported Ni I transition probability measurements. Use of the new echelle spectrograph, independent radiometric calibration methods, and independent data analysis routines enable a reduction of systematic errors and overall improvement in transition probability uncertainty over previous measurements. The new Ni I data are applied to high resolution visible and UV spectra of the Sun and metal-poor star HD 84937 to derive new, more accurate Ni abundances. Lines covering a wide range of wavelength and excitation potential are used to search for non-LTE effects.

astro-ph.SR↗

arXiv2013

Improved Ti II log(gf) Values and Abundance Determinations in the Photospheres of the Metal-Poor Star HD 84937

Atomic transition probability measurements for 364 lines of Ti II in the UV through near IR are reported. Branching fractions from data recorded using a Fourier transform spectrometer and a new echelle spectrometer are combined with published radiative lifetimes to determine these transition probabilities. The new results are in generally good agreement with previously reported FTS measurements. Use of the new echelle spectrometer, independent radiometric calibration methods, and independent data analysis routines enables a reduction of systematic errors and overall improvement in transition probability accuracy over previous measurements. The new Ti II data are applied to high resolution visible and UV spectra of the Sun and metal-poor star HD 84937 to derive new, more accurate Ti abundances. Lines covering a range of wavelength and excitation potential are used to search for non-LTE effects. The Ti abundances derived using Ti II for these two stars match those derived using Ti I and support the relative Ti/Fe abundance ratio versus metallicity seen in previous studies.

astro-ph.SR↗

arXiv2009

Improved Laboratory Transition Probabilities for Ce II, Application to the Cerium Abundances of the Sun and Five r-process Rich, Metal-Poor Stars, and Rare Earth Lab Data

Recent radiative lifetime measurements accurate to +/- 5% using laser-induced fluorescence (LIF) on 43 even-parity and 15 odd-parity levels of Ce II have been combined with new branching fractions measured using a Fourier transform spectrometer (FTS) to determine transition probabilities for 921 lines of Ce II. This improved laboratory data set has been used to determine a new solar photospheric Ce abundance, log epsilon = 1.61 +/- 0.01 (sigma = 0.06 from 45 lines), a value in excellent agreement with the recommended meteoritic abundance, log epsilon = 1.61 +/- 0.02. Revised Ce abundances have also been derived for the r-process-rich metal-poor giant stars BD+17 3248, CS 22892-052, CS 31082-001, HD 115444 and HD 221170. Between 26 and 40 lines were used for determining the Ce abundance in these five stars, yielding a small statistical uncertainty of 0.01 dex similar to the Solar result. The relative abundances in the metal-poor stars of Ce and Eu, a nearly pure r-process element in the Sun, matches r-process only model predictions for Solar System material. This consistent match with small scatter over a wide range of stellar metallicities lends support to these predictions of elemental fractions. A companion paper includes an interpretation of these new precision abundance results for Ce as well as new abundance results and interpretations for Pr, Dy and Tm.

astro-ph.SR↗

arXiv2008

Improved Laboratory Transition Probabilities for Er II and Applications to the Erbium Abundances of the Sun and Five r-Process Rich, Metal-Poor Stars

Recent radiative lifetime measurements accurate to +/- 5% (Stockett et al. 2007, J. Phys. B 40, 4529) using laser-induced fluorescence (LIF) on 8 even-parity and 62 odd-parity levels of Er II have been combined with new branching fractions measured using a Fourier transform spectrometer (FTS) to determine transition probabilities for 418 lines of Er II. This work moves Er II onto the growing list of rare earth spectra with extensive and accurate modern transition probability measurements using LIF plus FTS data. This improved laboratory data set has been used to determine a new solar photospheric Er abundance, log epsilon = 0.96 +/- 0.03 (sigma = 0.06 from 8 lines), a value in excellent agreement with the recommended meteoric abundance, log epsilon = 0.95 +/- 0.03. Revised Er abundances have also been derived for the r-process-rich metal-poor giant stars CS 22892-052, BD+17 3248, HD 221170, HD 115444, and CS 31082-001. For these five stars the average Er/Eu abundance ratio, = 0.42, is in very good agreement with the solar-system r-process ratio. This study has further strengthened the finding that r-process nucleosynthesis in the early Galaxy which enriched these metal-poor stars yielded a very similar pattern to the r-process which enriched later stars including the Sun.

astro-ph↗

arXiv2006

Hf Transition Probabilities and Abundances

Radiative lifetimes from laser-induced fluorescence measurements, accurate to about +/- 5 percent, are reported for 41 odd-parity levels of Hf II. The lifetimes are combined with branching fractions measured using Fourier transform spectrometry to determine transition probabilities for 150 lines of Hf II. Approximately half of these new transition probabilities overlap with recent independent measurements using a similar approach. The two sets of measurements are found to be in good agreement for measurements in common. Our new laboratory data are applied to refine the hafnium photospheric solar abundance and to determine hafnium abundances in 10 metal-poor giant stars with enhanced r-process abundances. For the Sun we derive log epsilon (Hf) = 0.88 +/- 0.08 from four lines; the uncertainty is dominated by the weakness of the lines and their blending by other spectral features. Within the uncertainties of our analysis, the r-process-rich stars possess constant Hf/La and Hf/Eu abundance ratios, log epsilon (Hf/La) = -0.13 +/- 0.02 (sigma = 0.06) and log epsilon (Hf/Eu) = +0.04 +/- 0.02 (sigma = 0.06). The observed average stellar abundance ratio of Hf/Eu and La/Eu is larger than previous estimates of the solar system r-process-only value, suggesting a somewhat larger contribution from the r-process to the production of Hf and La. The newly determined Hf values could be employed as part of the chronometer pair, Th/Hf, to determine radioactive stellar ages.

astro-ph↗