arXiv subjects

David L. Lambert

Publications and source records attributed to David L. Lambert.

At least 19 recordsLinked to original sources

arXiv2024

Chemical compositions of semi-regular variable red giants

A sample of warm low level semi-regular variables chosen from the General Catalogue of Variable Stars is studied for their chemical compositions by analysing high resolution optical spectra. The abundance ratios from Na/Fe to Eu/Fe displayed by these and previously analysed semi-regular variables are quite similar to ratios displayed by normal red giants across the Galactic thin and thick disks and halo populations in the solar neighbourhood suggesting from this perspective that the variables may be among the more photometrically active red giants.

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arXiv2024

A refined search for high-velocity gas in the Cygnus Loop supernova remnant

We present the results of a sensitive search for high-velocity gas in interstellar absorption lines associated with the Cygnus Loop supernova remnant (SNR). We examine high-resolution, high signal-to-noise ratio optical spectra of six stars in the Cygnus Loop region with distances greater than ~700 pc. All stars show low-velocity Na I and Ca II absorption. However, only one star, HD 198301, exhibits high-velocity Ca II absorption components, at velocities of +62, +82, and +96 km/s. The distance to this star of ~870 pc helps to constrain the distance to the receding edge of the Cygnus Loop's expanding shock front. One of our targets, HD 335334, was previously thought to exhibit high positive and high negative velocity interstellar Na I and Ca II absorption. This was one factor leading Fesen et al. to derive a distance to the Cygnus Loop of 725 pc. However, we find that HD 335334 is in fact a double-line spectroscopic binary and shows no evidence of high-velocity interstellar absorption. As such, the distance to HD 335334 cannot be used to constrain the distance to the Cygnus Loop. Our detection of Ca II absorption approaching 100 km/s toward HD 198301 is the first conclusive detection of high-velocity absorption from a low ionization species associated with the Cygnus Loop SNR. A large jump in the Na I column density toward BD+31 4218, a star located beyond the northwestern boundary of the Cygnus Loop, helps to constrain the distance to a large molecular cloud complex with which the Cygnus Loop is evidently interacting.

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arXiv2023

The Distribution of Metallicities in the Local Galactic Interstellar Medium

In this investigation, we present an analysis of the metallicity distribution that pertains to neutral gas in the local Galactic interstellar medium (ISM). We derive relative ISM metallicities for a sample of 84 sight lines probing diffuse atomic and molecular gas within 4 kpc of the Sun. Our analysis is based, in large part, on column density measurements reported in the literature for 22 different elements that are commonly studied in interstellar clouds. We supplement the literature data with new column density determinations for certain key elements and for several individual sight lines important to our analysis. Our methodology involves comparing the relative gas-phase abundances of many different elements for a given sight line to simultaneously determine the strength of dust depletion in that direction and the overall metallicity offset. We find that many sight lines probe multiple distinct gas regions with different depletion properties, which complicates the metallicity analysis. Nevertheless, our results provide clear evidence that the dispersion in the metallicities of neutral interstellar clouds in the solar neighborhood is small ($\sim$0.10 dex) and only slightly larger than the typical measurement uncertainties. We find no evidence for the existence of very low metallicity gas (as has recently been reported by De Cia et al.) along any of the 84 sight lines in our sample. Our results are consistent with a local Galactic ISM that is well mixed and chemically homogeneous.

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arXiv2021

Lithium in red giants: the roles of the He-core flash and the luminosity bump

Lithium abundances for red giants in the GALAH DR3 survey are studied. The rare examples of Li-enriched stars with abundances A(Li) $\geq 1.5$ are confirmed to be He-core burning stars belonging to or evolved from the red clump with similar masses and metallicity: $M\simeq 1.1\pm0.2{\rm M_\odot}$ and [Fe/H] $\simeq-0.3\pm0.3$. Li enrichment over the Li abundance present in a star's predecessor at the tip of the red giant branch likely occurs in all these red clump stars. Examination of the elemental abundances (C to Eu) in the GALAH catalogue shows no anomalous abundances in red clump giants and, in particular, no dependence on the Li abundance, which ranges over at least five dex. Lithium synthesis is attributed to the He-core flash occurring in stars at the tip of the red giant branch. Models from the Modules for Experiments in Stellar Astrophysics (MESA) match the observed evolution of these stars along the red giant branch and to the red clump but only at the low effective temperature end of the observed spread of red clump giants. Run of Li abundance on the red giant branch is fairly well reproduced by MESA models. A speculation is presented that the series of He-core flashes not only leads to $^7$Li synthesis from a star's internal reservoir of $^3$He but also may lead to internal restructuring leading to the observed effective temperature spread of red clump stars at about a constant luminosity. Giants exhibiting marked Li enrichments are not found at other evolutionary phases and, in particular, not directly associated with the luminosity bump on the red giant branch for which the Li abundance increase does not exceed 0.3 dex.

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arXiv2021

The Transition from Diffuse Molecular Gas to Molecular Cloud Material in Taurus

We study four lines of sight that probe the transition from diffuse molecular gas to molecular cloud material in Taurus. Measurements of atomic and molecular absorption are used to infer the distribution of species and the physical conditions toward stars behind the Taurus Molecular Cloud (TMC). New high-resolution spectra at visible and near infrared wavelengths of interstellar Ca II, Ca I, K I, CH, CH^+, C2, CN, and CO toward HD28975 and HD29647 are combined with data at visible wavelengths and published CO results from ultraviolet measurements for HD27778 and HD30122. Gas densities and temperatures are inferred from C2, CN, and CO excitation and CN chemistry. Our results for HD29647 are noteworthy because the CO column density is 10^{18} cm^{-2} while C2 and CO excitation reveals a temperature of 10 K and density about 1000 cm^{-3}, more like conditions found in dark molecular clouds. Similar results arise from our chemical analysis for CN through reactions involving observations of CH, C2, and NH. Enhanced potassium depletion and a reduced CH/H2 column density ratio also suggest the presence of a dark cloud. The directions toward HD27778 and HD30122 probe molecule-rich diffuse clouds, which can be considered CO-dark gas, while the sight line toward HD28975 represents an intermediate case. Maps of dust temperature help refine the description of the material along the four sight lines and provide an estimate of the distance between HD29647 and a clump in the TMC. An Appendix provides results for the direction toward HD26571; this star also probes diffuse molecular gas.

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arXiv2021

Lithium abundances and asteroseismology of red giants: understanding the evolution of lithium in giants based on asteroseismic parameters

In this study, we explore the evolution of lithium in giant stars based on data assembled from the literature on asteroseismology and Li abundances for giants. Our final sample of 187 giants consists of 44 red giant branch (RGB), 140 core He-burning (CHeB) and three giants with an unclassified evolutionary phase. For all 187 stars, the seismic parameters $ν\rm_{max}$ (frequency of maximum oscillation power) and $Δν$ (large frequency spacing) are available, while $ΔΠ\rm_{1}$ (the asymptotic gravity-mode period spacing) is available for a subset of 64. For some of the CHeB giants, mass estimates from the asteroseismic scaling relations are found to be underestimated when compared with mass estimates from isochrones based on seismic data. Whilst most of the Li-rich giants in the sample have masses less than 1.5 $M_\odot$, they are also present up to and beyond the maximum mass expected to have suffered a core He-flash, i.e. $M$ $\leq$ 2.25 $M_\odot$: this suggests contributions from other processes towards Li enrichment. To understand the evolution of giants in the $ΔΠ\rm_{1}$ $-$ $Δν$ plane, we use the {\it Modules for Experiments in Stellar Astrophysics} models which show the presence of mini He-flashes following the initial strong core He-flash. From the distribution of A(Li) as a function of $Δν$, which is similar to the distribution of A(Li) as a function of luminosity, we find no indication of Li enrichment near the luminosity bump. Also, A(Li) trends to $\sim$ -1.5 dex near the RGB tip. The data also suggest a decrease in A(Li) with an increase in $ΔΠ\rm_{1}$ for CHeB giants.

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arXiv2020

Detection of CH$^{+}$, CH and H$_2$ molecules in the Young Planetary Nebula IC 4997

We have detected CH$^{+}$ and CH molecular absorption lines from the young compact planetary nebula IC 4997 from high resolution optical spectra. A high-resolution infra-red (H and K bands) spectrum provides detection of H$_2$ emission lines amongst many other lines. The H$_2$ lines provide an excitation temperature of 2100 K which may result from UV fluorescence in the envelope or from shocks formed at the interface between an expanding outflow of ionized gas and the neutral envelope ejected when the star was on the AGB. It is suggested that the CH$^+$ may result from the endothermic reaction C + H$_2$ $\rightarrow$ CH$^+$ + H. Intriguingly, CH$^{+}$ and also CH show a higher expansion velocity than H$_{\rm 2}$ emission suggesting they may be part of the post-shocked gas.

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arXiv2020

Abundance analyses of Li-enriched and normal giants in the GALAH survey

Compositions of lithium-enriched and normal giants among the GALAH survey are compared. Except for Li, the only detectable abundance difference between lithium-enriched and normal giants among the investigated elements from carbon to europium occurs for carbon. Among Li-rich giants with A(Li) = 1.8 to 3.1, the C deficiency is very similar to that reported for the normal giants (with A(Li) $<$ 1.8) where the slight C deficiency arises from the first dredge-up. Carbon is slightly under abundant relative to normal giants among the super Li-rich giants where the Li abundance exceeds A(Li) = 3.2. The C abundance as well as the $^{12}$C/$^{13}$C ratios from the literature suggest that addition of Li to create a Li-rich giant may occur independently of the abundance changes wrought by the first dredge-up. Creation of a super Li-rich giant, however, appears to occur with additional CN-cycle conversion of C to N. The probability of becoming a Li-rich giant is approximately independent of a star's mass, although the majority of the Li-rich giants are found to be low mass ($M \leq$ 2 M$_\odot$). The frequency of occurrence of Li-enriched giants among normal giants is about one percent and slightly dependent on metallicity ([Fe/H]). Li-enriched and normal giants are found to have similar projected rotational velocity which suggest that Li-enrichment in giants is not linked to scenarios such as mergers and tidal interaction between binary stars.

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arXiv2020

SALT revisits DY Cen: a rapidly-evolving strontium-rich single helium star

The hydrogen-deficient star DY Cen has been reported as an R CrB-type variable, an extreme helium star (with some hydrogen), and as a single-lined spectroscopic binary. It has been associated with a dramatic change in visual brightness and colour corresponding to a change in effective temperature ($T_{\mathrm eff}$) of some 20,000 K in the last century. To characterize the binary orbit and $T_{\mathrm eff}$ changes more precisely, new high-resolution spectroscopy has been obtained with SALT. The previous orbital period is not confirmed; previous measurements may have been confused by the presence of pulsations. Including data from earlier epochs (1987, 2002, and 2010), self-consistent spectral analyses from all four epochs demonstrate an increase in $T_{\mathrm eff}$ from 18,800 to 24,400 K between 1987 and 2015. Line profiles demonstrate that the surface rotation has increased by a factor two over the same interval. This is commensurate with the change in $T_{\mathrm eff}$ and an overall contraction. Rotation will exceed critical if contraction continues. The 1987 spectrum shows evidence of a very high abundance of the s-process element strontium. The very rapid evolution, non-negligible surface hydrogen and high surface strontium point to a history involving a very late thermal pulse. Observations over the next thirty years should look for a decreasing pulsation period, reactivation of R CrB-type activity as the star seeks to shed angular momentum and increasing illumination by emission lines from nebular material ejected in the past.

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arXiv2020

Detection of fluorine in hot Extreme Helium Stars

The main objective of this paper is to explore abundances of fluorine in hot Extreme Helium Stars (EHes). Overabundance of fluorine is a characteristic feature for cool EHes and R Coronae Borealis (RCB) stars and further enforces their close connection. For hot EHes this relationship with the cooler EHes, based on their fluorine abundance is unexplored. We present in this paper the first abundance estimates of fluorine determined from singly ionised fluorine lines (F\,{\sc ii}) for 10 hot EHe stars from optical spectra. Fluorine abundances were determined using the F\,{\sc ii} lines in two windows centered at 3505 Å\ and 3850 Å. Six of the 10 stars show significant enhancement of fluorine similar to the cool EHes. Two carbon-poor hot EHes show no signature of fluorine and have a significant low upper limit for the F abundance. These fluorine abundances are compared with the other elemental abundances observed in these stars which provide an idea about the formation and evolution of these stars. The trends of fluorine with C, O, and Ne show that significant helium burning after a CO-He white dwarf merger can account for a majority of the observed abundances. Predictions from simulations of white dwarf mergers are discussed in light of the observed abundances.

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arXiv2020

Unveiling Vela -- Variability of Interstellar Lines in the Direction of the Vela Supernova Remnant III. Na D and Ca II K

High-resolution optical spectra were obtained in 2017-2019 with The Southern African Large Telescope of fifteen stars in the direction of the Vela supernova remnant. Interstellar Ca ii H and K and Na i D lines are discussed in this paper. In particular, the line profiles are compared with profiles at a comparable spectral resolution obtained in 1993-1996 by Cha & Sembach. Ten of the lines of sight show changes to one or more of the components in that line of sight. Changes include small changes (1-2 km/s) in radial velocity and/or increases/decreases in equivalent width over the two decades between the periods of observation. Changes are more obvious in the Ca K line than in the Na D lines. These changes are attributed to gas disturbed by interactions between the supernova ejecta and the surrounding interstellar medium. A representative timescale may be 20-50 years. Small-scale variations in line profiles across the face of the remnant suggest, as previously remarked, that a linear scale for interactions is a small fraction of the 40 pc size of the present remnant.

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arXiv2019

Carbon and Oxygen Isotopic Ratios. II. Semiregular Variable M Giants

Carbon and oxygen isotopic ratios are reported for a sample of 51 SRb- and Lb-type variable asymptotic giant branch stars. Vibration-rotation first- and second-overtone CO lines in 1.5-2.5 mum spectra were measured to derive isotopic ratios for 12C/13C, 16O/17O, and 16O/18O. Comparisons with previous measurements for individual stars and with various samples of evolved stars, as available in the extant literature, are discussed. Using the oxygen isotopic ratios, the masses of the SRb stars can be derived. Combining the masses with Gaia luminosities, the SRb stars are shown to be antecedents of the Mira variables. The limiting parameters where plane-parallel, hydrostatic equilibrium model atmospheres can be used for abundance analysis of M giants are explored.

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arXiv2019

Fluorine Abundances in the Galactic Disk

The chemical evolution of fluorine is investigated in a sample of Milky Way red giantstars that span a significant range in metallicity from [Fe/H] $\sim$ -1.3 to 0.0 dex. Fluorine abundances are derived from vibration-rotation lines of HF in high-resolution infraredspectra near $λ$ 2.335 $μ$m. The red giants are members of the thin and thick disk / halo,with two stars being likely members of the outer disk Monoceros overdensity. At lowermetallicities, with [Fe/H]<-0.4 to -0.5, the abundance of F varies as a primary element with respect to the Fe abundance, with a constant subsolar value of [F/Fe] $\sim$ -0.3 to -0.4 dex. At larger metallicities, however, [F/Fe] increases rapidly with [Fe/H] anddisplays a near-secondary behavior with respect to Fe. Comparisons with various models of chemical evolution suggest that in the low-metallicity regime (dominated hereby thick disk stars), a primary evolution of $^{19}$F with Fe, with a subsolar [F/Fe] valuethat roughly matches the observed plateau can be reproduced by a model incorporatingneutrino nucleosynthesis in the aftermath of the core collapse in supernovae of type II (SN II). A primary behavior for [F/Fe] at low metallicity is also observed for a model including rapid rotating low-metallicity massive stars but this overproduces [F/Fe] atlow metallicity. The thick disk red giants in our sample span a large range of galactocentric distance (Rg $\sim$ 6--13.7 kpc), yet display a $\sim$constant value of [F/Fe], indicating a very flat gradient (with a slope of 0.02 $\pm$ 0.03 dex/kpc) of this elemental ratio over asignificant portion of the Galaxy having|Z|>300 pc away from the Galaxy mid-plane.

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arXiv2019

Fluorine Abundances in the Globular Cluster M 4

We present chemical abundances for the elements carbon, sodium, and fluorine in 15 red giants of the globular cluster M 4, as well as six red giants of the globular cluster $ω$ Centauri. The chemical abundances were calculated in LTE via spectral synthesis. The spectra analyzed are high-resolution spectra obtained in the near-infrared region around $λ$2.3$μ$m with the Phoenix spectrograph on the 8.1m Gemini South Telescope, the IGRINS spectrograph on the McDonald Observatory 2.7m Telescope, and the CRIRES spectrograph on the ESO 8.2m Very Large Telescope. The results indicate a significant reduction in the fluorine abundances when compared to previous values from the literature for M 4 and $ω$ Centauri, due to a downward revision in the excitation potentials of the HF(1-0) R9 line used in the analysis. The fluorine abundances obtained for the M 4 red giants are found to be anti-correlated with those of Na, following the typical pattern of abundance variations seen in globular clusters between distinct stellar populations. In M 4, as the Na abundance increases by $\sim$+0.4 dex, the F abundance decreases by $\sim$-0.2 dex. A comparison with abundance predictions from two sets of stellar evolution models finds that the models predict somewhat less F depletion ($\sim$-0.1 dex) for the same increase of +0.4 dex in Na.

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arXiv2019

Comprehensive abundance analysis of red giants in the open clusters Stock 2, NGC 2168, 6475, 6991 and 7762

We have analysed high-dispersion echelle spectra ($R = 60000$) of red giant members of five open clusters to derive abundances for many elements from Na to Eu. The [Fe/H] values are $-0.06\pm0.03$ for Stock 2, $-0.11\pm0.03$ for NGC 2168, $-0.01\pm0.03$ for NGC 6475, $0.00\pm0.03$ for NGC 6991 and $-0.07\pm0.03$ for NGC 7662. Sodium is enriched in the giants relative to the abundance expected of main sequence stars of the same metallicity. This enrichment of [Na/Fe] by about $+0.25$ attributed to the first dredge-up is discussed in the light of theoretical predictions and recently published abundance determinations. Abundance ratios [El/Fe] for other elements are with very few exceptions equal to those of field giants and dwarfs, i.e., [El/Fe] $\simeq 0.00$ for [Fe/H] $\sim 0.0$. An exception is the overabundance of La, Ce, Nd and Sm in NGC 6991 but this is consistent with our previous demonstration that the abundances of these $s$-process products vary by about $\pm0.2$ among clusters of the same [Fe/H], a variation found also among field giants and dwarfs.

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arXiv2019

Chemical compositions of giants in the Hyades And Sirius superclusters

An abundance analysis for 20 elements from Na to Eu is reported for 34 K giants from the Hyades supercluster and for 22 K giants from the Sirius supercluster. Observed giants were identified as highly probable members of their respective superclusters by Famaey et al. (2005, A&A, 430, 165). Three giants each from the Hyades and Praesepe open clusters were similarly observed and analysed. Each supercluster shows a range in metallicity: $-0.20 \leq$ [Fe/H] $\leq +0.25$ for the Hyades supercluster and $-0.22 \leq $ [Fe/H] $\leq +0.15$ for the Sirius supercluster with the metal-rich tail of the metallicity distribution of the Hyades supercluster extending beyond that of the Sirius supercluster and spanning the metallicity of the Hyades and Praesepe cluster giants. Relative elemental abundances [El/Fe] across the supercluster giants are representative of the Galactic thin disc as determined from giants in open clusters analysed in a similar way to our approach. Judged by metallicity and age, very few and likely none of the giants in these superclusters originated in an open cluster: the pairings include the Hyades supercluster with the Hyades - Praesepe open clusters and the Sirius supercluster with the U Ma open cluster. Literature on main sequence stars attributed to the two superclusters and the possible relation to the associated open cluster is reviewed. It is suggested that the Hyades supercluster's main sequence population contains few stars from the two associated open clusters. As suggested by some previous investigations, the Sirius supercluster, when tightly defined kinematically, appears to be well populated by stars shed by the U Ma open cluster.

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arXiv2018

Abundances and Depletions of Neutron-Capture Elements in the Interstellar Medium

We present an extensive analysis of the gas-phase abundances and depletion behaviors of neutron-capture elements in the interstellar medium (ISM). Column densities (or upper limits to the column densities) of Ga II, Ge II, As II, Kr I, Cd II, Sn II, and Pb II are determined for a sample of 69 sight lines with high- and/or medium-resolution archival spectra obtained with the Space Telescope Imaging Spectrograph onboard the Hubble Space Telescope. An additional 59 sight lines with column density measurements reported in the literature are included in our analysis. Parameters that characterize the depletion trends of the elements are derived according to the methodology developed by Jenkins (2009; arXiv:0905.3173). (In an appendix, we present similar depletion results for the light element B.) The depletion patterns exhibited by Ga and Ge comport with expectations based on the depletion results obtained for many other elements. Arsenic exhibits much less depletion than expected, and its abundance in low-depletion sight lines may even be supersolar. We confirm a previous finding by Jenkins (2009; arXiv:0905.3173) that the depletion of Kr increases as the overall depletion level increases from one sight line to another. Cadmium shows no such evidence of increasing depletion. We find a significant amount of scatter in the gas-phase abundances of Sn and Pb. For Sn, at least, the scatter may be evidence of real intrinsic abundance variations due to s-process enrichment combined with inefficient mixing in the ISM.

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arXiv2017

Non-Local Thermodynamic Equilibrium abundance analyses of the extreme helium stars: V652 Her and HD 144941

Optical high-resolution spectra of V652 Her and HD 144941, the two extreme helium stars with exceptionally low C/He ratios, have been subjected to a non-LTE abundance analysis using the tools TLUSTY and SYNSPEC. Defining atmospheric parameters were obtained from a grid of non-LTE atmospheres and a variety of spectroscopic indicators including He I and He II line profiles, ionization equilibrium of ion pairs such as C II/C III and N II/N III. The various indicators provide a consistent set of atmospheric parameters: $T_{\rm eff}$=25000$\pm$300K, $\log g$ = 3.10$\pm$0.12(cgs), and $ξ=13\pm2 {\rm km\,s^{-1}}$ are provided for V652 Her, and $T_{\rm eff}$=22000$\pm$600K, $\log g$ = 3.45$\pm$0.15 (cgs), and $ξ=10 {\rm km\,s^{-1}}$ are provided for HD 144941. In contrast to the non-LTE analyses, the LTE analyses - LTE atmospheres and a LTE line analysis - with the available indicators do not provide a consistent set of atmospheric parameters. The principal non-LTE effect on the elemental abundances is on the neon abundance. It is generally considered that these extreme helium stars with their very low C/He ratio result from the merger of two helium white dwarfs. Indeed, the derived composition of V652 Her is in excellent agreement with predictions by Zhang & Jeffery (2012) who model the slow merger of helium white dwarfs; a slow merger results in the merged star having the composition of the accreted white dwarf. In the case of HD 144941 which appears to have evolved from metal-poor stars a slow merger is incompatible with the observed composition but variations of the merger rate may account for the observed composition. More detailed theoretical studies of the merger of a pair of helium white dwarfs are to be encouraged.

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