arXiv subjects

Ann Merchant Boesgaard

Publications and source records attributed to Ann Merchant Boesgaard.

At least 19 recordsLinked to original sources

arXiv2024

Primordial Lithium from Globular Cluster Turn-Off Stars: M13 AND M71

During Big Bang nucleosynthesis (BBN) in the first 15 minutes of the Universe some $^7$Li was created along with isotopes of H and He. The determination of that primordial value of Li can help constrain the conditions at that time. The oldest stars with known ages can be found in globular clusters which have well-determined ages through stellar evolution models. High-resolution spectra have been obtained with the Keck I telescope and HIRES of Li in several unevolved stars in the clusters M13 and M71 with V magnitudes of 17.6 -- 17.9. Abundances of Li have been determined with spectrum synthesis techniques and show a range of a factor of 4. We attribute that spread to differences in initial angular momentum resulting in different amounts of spin-down, related mixing, and destruction of Li. Our results are compared with similar results for main-sequences and turn-off stars in other globular clusters. The range in age of these clusters is 11.2 to 14.2 Gyr for an age span of 3 Gyr. These clusters range in [Fe/H] from $-$0.75 to $-$2.24 corresponding to a factor of 30 in metallicity. The maximum in the Li abundance for these unevolved stars in all eight clusters is the same corresponding to Li/H = 3.16 x 10$^{-10}$ while the predicted Li abundance, based on the deuterium abundance and the BBN predictions, is 5.24 x 10$^{-10}$.

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arXiv2023

Lithium in Turn-off Stars in the Globular Cluster M5: A Quest for Primordial Lithium

The light element lithium is formed by nucleosynthesis during the Big Bang. Its abundance can help to define the parameters of the early universe. To find this primordial value, it is necessary to determine Li abundances in the oldest stars because it is readily destroyed by nuclear reactions in stellar interiors. We have made high-resolution (~45,000) spectroscopic observations of five identical unevolved turn-off stars in the 13 Gyr old globular cluster M5. In our analysis we find a range in Li abundance of a factor of two; the spread is five times the individual error. The comparison of these results with those for turn-off stars from five other globular clusters reveals similarly large range in Li. Lithium in M5 and the other clusters all have stars above the field star Li plateau, but none are as high as the predictions for primordial Li. The maximum values for Li are the same in all six clusters. Multiple generations of stars are found in many globular clusters; those later generations are expected to have formed from Li-depleted gas. Such second- and later-generation stars would have no Li. However, only one of the six clusters has a few unevolved stars with upper limits on the Li abundance.

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arXiv2023

Iron-Peak Element Abundances in Warm Very Metal-Poor Stars

We have derived new detailed abundances of Mg, Ca, and the Fe-group elements Sc through Zn (Z = 21-30) for 37 main sequence turnoff very metal-poor stars ([Fe/H] <= -2.1). We analyzed Keck HIRES optical and near-UV high signal-to-noise spectra originally gathered for a beryllium abundance survey. Using typically about 400 Fe-group lines with accurate laboratory transition probabilities for each star, we have determined accurate LTE metallicities and abundance ratios for neutral and ionized species of the 10 Fe-group elements as well as alpha elements Mg and Ca. We find good neutral/ion abundance agreement for the 6 elements that have detectable transitions of both species in our stars in the 3100-5800 Angstrom range. Earlier reports of correlated Sc-Ti-V relative overabundances are confirmed, and appear to slowly increase with decreasing metallicity. To this element trio we add Zn; it also appears to be increasingly overabundant in the lowest metallicity regimes. Co appears to mimic the behavior of Zn, but issues surrounding its abundance reliability cloud its interpretation.

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arXiv2022

Intriguing Revelations from Lithium, Beryllium, and Boron

This is a report on some highlights of some research on the rare light elements, lithium (Li), beryllium (Be), and boron (B), that I presented in my Henry Norris Russell Lecture in January, 2020. It is not a comprehensive review of work on these light elements, but contains sections on Big Bang nucleosynthesis of Li and the rarity of these light elements. It includes information on how they are observed, both historically and currently, and the difficulties entailed in determining their abundances. %It traces the evolution of these elements through the history of the Galaxy. The production of Li, Be, and B is ongoing so the youngest stars contain the most Li, and they have had less time to destroy it. All three elements are readily destroyed in stellar interiors, but have differing degrees of susceptibility to the particular nuclear fusion reactions which deplete their surface content. This feature makes them remarkably good probes into the otherwise unobservable interiors of stars. It also enhances the use of two or more of the three in sorting out the various processes at work in the insides of stars.

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arXiv2022

Lithium and Beryllium in One Solar Mass Stars

The surface content of lithium (Li) and beryllium (Be) in stars can reveal important information about the temperature structure and physical processes in their interior regions. This study focuses on solar-type stars with a sample that is more precisely defined than done previously. Our selection of stars studied for Be is constrained by five parameters: mass, temperature, surface gravity, metallicity, and age to be similar to the Sun and is focussed on stars within +-0.02 of 1 M_sun. We have used the Keck I telescope with HIRES to obtain spectra of the Be II spectral region of 52 such stars at high spectral resolution ($\sim$45,000) and high signal-to-noise ratios. While the spread in Li in these stars is greater than a factor of 400, the spread in Be is only 2.7 times. Two stars were without any Be, perhaps due to a merger or a mass transfer with a companion. We find a steep trend of Li with temperature but little for Be. While there is a downward trend in Li with [Fe/H] from -0.4 to +0.4 due to stellar depletion, there is a small increase in Be with Fe from Galactic Be enrichment. While there is a broad decline in Li with age, there may be a small increase in Be with age, though age is less well-determined. In the subset of stars closest to the Sun in temperature and other parameters we find that the ratio of the abundances of Be to Li is much lower than predicted by models; there may be other mixing mechanisms causing additional Li depletion.

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arXiv2022

Lithium and Beryllium in NGC 752 -- An Open Cluster Twice the Age of the Hyades

The surface abundances of the light elements lithium (Li) and beryllium (Be) reveal information about the physical processes taking place in stellar interiors. The investigation of the amount of these two elements in stars in open clusters shows the effect of age on those mechanisms. We have obtained spectra of both Li and Be in main-sequence stars in NGC 752 at high spectral resolution and high signal-to-noise ratios with HIRES on the Keck I telescope. In order to make meaningful comparisons with other clusters, we have determined the stellar parameters on a common scale. We have found abundances of Li and Be by spectral synthesis techniques. NGC 752 is twice the age of the well-studied Hyades cluster. We find that 1) The Li dip centered near 6500 K is wider in NGC 752, having expanded toward cooler temperatures; 2) The Be dip is deeper in the older NGC 752; 3) The Li "peak" near 6200 K is lower by about 0.3 dex; 4) Although there is little Be depletion in the cooler stars, it is possible that Be may be lower in NGC 752 than in the Hyades; 5) The Li content in both clusters declines with decreasing temperature, but there is less Li in NGC 752 at a given temperature by $\sim$0.4 dex. These differences are consistent with the transport of the light-element nuclei below the surface convection zone as predicted by theory. That connection to rotational spin-down is indicated by the pattern of rotation with temperature in the two clusters.

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arXiv2020

Detailed Iron-Peak Element Abundances in Three Very Metal-Poor Stars

We have obtained new detailed abundances of the Fe-group elements Sc through Zn (Z=21-30) in three very metal-poor ([Fe/H] $\approx -3$) stars: BD 03 740, BD -13 3442 and CD -33 1173. High-resolution ultraviolet HST/STIS spectra in the wavelength range 2300-3050Å were gathered, and complemented by an assortment of optical echelle spectra. The analysis featured recent laboratory atomic data for number of neutral and ionized species for all Fe-group elements except Cu and Zn. A detailed examination of scandium, titanium, and vanadium abundances in large-sample spectroscopic surveys indicates that they are positively correlated in stars with [Fe/H]<-$2. The abundances of these elements in BD 03 740, BD -13 3442 and CD -33 1173 and HD 84937. (studied in a previous paper of this series) are in accord with these trends and lie at the high end of the correlations. Six elements have detectable neutral and ionized features, and generally their abundances are in reasonable agreement. For Cr we find only minimal abundance disagreement between the neutral (mean of [Cri/Fe]=+0.01) and ionized species (mean of [Crii/Fe]=+0.08), unlike most studies in the past. The prominent exception is Co, for which the neutral species indicates a significant overabundance (mean of [Co/H]=-2.53), while no such enhancement is seen for the ionized species (mean of [Coii/H]=-2.93). These new stellar abundances, especially the correlations among Sc, Ti, and V, suggest that models of element production in early high-mass metal-poor stars should be revisited.

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arXiv2019

Correlated Depletion and Dilution of Lithium and Beryllium Revealed by Subgiants in M 67

The surface content of lithium (Li) and beryllium (Be) provide insights into the mixing and circulation mechanisms in stellar interiors. The old open cluster, M 67, has been well-studied for Li abundances in both main-sequence and evolved stars. The Be abundances give us a probe to a deeper level in stars. We have taken high-resolution spectra with Keck I with HIRES to determine Be abundances along the subgiant branch of M 67, where there are dramatic depletions of Li. These subgiants range in mass from 1.26 to 1.32 M$_{\odot}$ and have evolved from main-sequence stars that would have occupied the region of the Li-Be dip found in younger clusters. Lithium abundances have been adjusted to the same scale for 103 stars in M 67 by Pace et al. The more massive stars $-$ now the coolest and furthest-evolved from the main-sequence $-$ show a drop in Li by a factor of 400 across the subgiant branch. Our new Be abundances also show a decline, but by a factor of $\sim$50. The two elements decline together with Li showing a steeper decline in these subgiants than it does in the Li-Be dip stars. The relative decline in Be abundance compared to Li is remarkably well fit by the models of Sills & Deliyannis, made specifically for the subgiants in M 67. Those models include the effects of mixing induced by stellar rotation. These M 67 subgiants show the effects of both main-sequence depletion and post-main-sequence dilution of both Li and Be.

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arXiv2019

A Comparison of the Chemical Composition of Main Sequence and Giant Stars in the Open Cluster NCC 752

The chemical composition of stars in open clusters provides the best information on the chemical evolution of stars via comparison of the main sequence stars with the evolved giants. This is a case study of the abundances in the dwarfs and giants in the old, open cluster, NGC 752. It is also a pilot program for automated abundance determinations, including equivalent width measurements, stellar parameter determinations, and abundance analysis. We have found abundances of 31 element-ion combinations in 23 dwarfs and 6 giants. The mean cluster abundance of Fe is solar with [Fe/H]$=-0.01\pm0.06$ with no significant difference between the dwarfs and giants. We find that the cluster abundances of other elements, including alpha-elements, to be at, or slightly above solar levels. We find some evidence for CNO processing in the spectra of the giants. The enhancement of Na in giants indicates that the NeNa cycle has occurred. The abundances of Mg and Al are similar in the dwarfs and giants indicating that the hotter MgAl cycle has not occurred. We find no evidence of s-process enhancements in the abundances of heavy elements in the giants.

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arXiv2018

Consistent Iron Abundances Derived from Neutral and Singly-Ionized Iron Lines in Ultraviolet and Optical Spectra of Six Warm Metal-Poor Stars

Neutral Fe lines in metal-poor stars yield conflicting abundances depending on whether and how deviations from local thermodynamic equilibrium (LTE) are considered. We have collected new high resolution and high signal-to-noise ultraviolet (UV) spectra of three warm dwarf stars with [Fe/H] = -2.9 with the Space Telescope Imaging Spectrograph on the Hubble Space Telescope. We locate archival UV spectra for three other warm dwarfs with [Fe/H] = -3.3, -2.2, and -1.6, supplemented with optical spectra for all six stars. We calculate stellar parameters using methods that are largely independent of the spectra, adopting broadband photometry, color-temperature relations, Gaia parallaxes, and assumed masses. We use the LTE line analysis code MOOG to derive Fe abundances from hundreds of Fe I and Fe II lines with wavelengths from 2290 to 6430 Angstroms. The [Fe/H] ratios derived separately from Fe I and Fe II lines agree in all six stars, with [Fe II/H] - [Fe I/H] ranging from +0.00 +/- 0.07 to -0.12 +/- 0.09 dex, when strong lines and Fe I lines with lower excitation potential < 1.2 eV are excluded. This constrains the extent of any deviations from LTE that may occur within this parameter range. While our result confirms non-LTE calculations for some warm, metal-poor dwarfs, it may not be generalizable to more metal-poor dwarfs, where deviations from LTE are predicted to be larger. We also investigate trends of systematically lower abundances derived from Fe I lines in the Balmer continuum region (3100-3700 Angstroms), and we conclude that no proposed explanation for this effect can fully account for the observations presently available.

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arXiv2018

Identifying Young $Kepler$ Planet Host Stars from Keck-HIRES Spectra of Lithium

The lithium doublet at 6708 Å provides an age diagnostic for main sequence FGK dwarfs. We measured the abundance of lithium in 1305 stars with detected transiting planets from the Kepler Mission using high-resolution spectroscopy. Our catalog of lithium measurements from this sample have a range of abundance from A(Li) = 3.11 $\pm$ 0.07 to an upper limit of $-$0.84 dex. For a magnitude-limited sample that comprises 960 of the 1305 stars, our Keck-HIRES spectra have a median S/N = 45 per pixel at $\sim$6700 Å with spectral resolution $\fracλ{Δλ}$ = $R$ = 55,000. We identify 80 young stars that have A(Li) values greater than the Hyades at their respective effective temperatures; these stars are younger than $\sim$650 Myr old, the approximate age of the Hyades. We then compare the distribution of A(Li) with planet size, multiplicity, orbital period, and insolation flux. We find larger planets preferentially in younger systems, with an A-D two-sided test p-value = 0.002, a $>3σ$ confidence that the older and younger planet samples do not come from the same parent distribution. This is consistent with planet inflation/photoevaporation at early ages. The other planet parameters ($Kepler$ planet multiplicity, orbital period, and insolation flux) are uncorrelated with age.

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arXiv2016

Boron Abundances Across the "Li-Be Dip" in the Hyades

Dramatic deficiencies of Li in the mid-F dwarf stars of the Hyades cluster were discovered by Boesgaard & Tripicco. Boesgaard & King discovered corresponding, but smaller, deficiencies in Be in the same narrow temperature region in the Hyades. With the Space Telescope Imaging Spectrograph on the Hubble Space Telescope we investigate B abundances in the Hyades F stars to look for a potential B dip using the B I resonance line at 2496.8 A. The light elements, Li, Be, and B, are destroyed inside stars at increasingly hotter temperatures: 2.5, 3.5, and 5x10^6 K respectively. Consequently, these elements survive to increasingly greater depths in a star and their surface abundances indicate the depth and thoroughness of mixing in the star. We have (re)determined Li abundances/upper limits for 79 Hyades dwarfs, Be for 43 stars, and B in five stars. We find evidence for a small drop in the B abundance across the Li-Be dip. The B abundances for the four stars in the temperature range 6100-6730 K fit the B-Be correlation found previously by Boesgaard et al. Models of rotational mixing produce good agreement with the relative depletions of Be and B in the dip region. We have compared our nLTE B abundances for the three high B stars on either side of the Li-Be dip with those found by Duncan et al. for the two Hyades giants. This confirms the factor of ~10 decline in the B abundance in the Hyades giants as predicted by dilution due to the deepening of the surface convection zone.

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arXiv2015

The Old, Super-Metal-Rich Open Cluster, NGC 6791 - Elemental Abundances in Turn-off Stars from Keck/HIRES Spectra

The study of star clusters has advanced our understanding of stellar evolution, Galactic chemical evolution and nucleosynthesis. Here we investigate the composition of turn-off stars in the intriguing open cluster, NGC 6791, which is old, but super-metal-rich with high-resolution (46,000) Keck/HIRES spectra. We find [Fe/H] = +0.30 +/-0.02 from measurements of some 40 unblended, unsaturated lines of both Fe I and Fe II in eight turn-off stars. Our O abundances come from the O I triplet near 7774 A and we do a differential analysis relative to the Sun from our Lunar spectrum also obtained with Keck/HIRES. The O results are corrected for small nLTE effects. We find consistent ratios of [O/Fe]n with a mean of $-$0.06 +/-0.02. This continues the trend of decreasing [O/Fe] with increasing [Fe/H] found in field stars that are also both old and metal-rich. The small range in our oxygen abundances is consistent with a single population of stars. Our results for the alpha elements [Mg/Fe], [Si/Fe], [Ca/Fe], and [Ti/Fe] are near solar and compare well with those of the old, metal-rich field stars. The two Fe-peak elements, Cr and Ni, are consistent with Fe. These turn-off-star abundances provide benchmark abundances to investigate changes in the giants that might arise from nuclear-burning and dredge-up processes. Determinations of upper limits were found for Li by spectrum synthesis and these results support the prediction from standard theory that higher-metallicity stars deplete more Li. Probably no stars in NGC 6791 have retained their initial Li.

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arXiv2013

The Chemical Composition of Praesepe (M44)

Star clusters have long been used to illuminate both stellar evolution and Galactic evolution. They also hold clues to the chemical and nucleosynthetic processes throughout the history of the Galaxy. We have taken high signal-to-noise, high-resolution spectra of 11 solar-type stars in the Praesepe open cluster to determine the chemical abundances of 16 elements: Li, C, O, Na, Mg, Al, Si, Ca, Sc, Ti, V, Cr, Fe, Ni, Y, and Ba. We have determined Fe from Fe I and Fe II lines and find [Fe/H] = +0.12 $\pm$0.04. We find that Li decreases with temperature due to increasing Li depletion in cooler stars; it matches the Li-temperature pattern found in the Hyades. The [C/Fe] and [O/Fe] abundances are below solar and lower than the field star samples due to the younger age of Praesepe (0.7 Gyr) than the field stars. The alpha-elements, Mg, Si, Ca, and Ti, have solar ratios with respect to Fe, and are also lower than the field star samples. The Fe-peak elements, Cr and Ni, track Fe and have solar values. The neutron capture element [Y/Fe] is found to be solar, but [Ba/Fe] is enhanced relative to solar and to the field stars. Three Praesepe giants were studied by Carrera and Pancino; they are apparently enhanced in Na, Mg, and Ba relative to the Praesepe dwarfs. The Na enhancement may indicate proton-capture nucleosynthesis in the Ne -> Na cycling with dredge-up into the atmospheres of the red giants.

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arXiv2013

A Spectroscopic Analysis of the Eclipsing Short-Period Binary v505 Per and the Origin of the Lithium Dip

As a test of rotationally-induced mixing causing the well-known Li dip in older mid-F dwarfs in the local Galactic disk, we utilize high-resolution and -S/N Keck/HIRESspectroscopy to measure the Li abundance in the components of the1 Gyr, [Fe/H]=-0.15 eclipsing short-period binary V505 Per. We find A(Li)=2.7+/-0.1 and 2.4+/-0.2 in the Teff=6500 and 6450 K primary and secondary components, respectively. Previous Teff determinations and uncertainties suggest that each component is located in the midst of the Li dip. If so, their A(Li) are >=2-5 times larger than A(Li) detections and upper limits observed in the similar metallicity and intermediate-age open clusters NGC 752 and 3680, as well as the more metal-rich and younger Hyades and Praesepe. These differences are even larger if the consistent estimates of the scaling ofinitial Li with metallicity inferred from nearby disk stars, open clusters, and recent Galactic chemical evolution models are correct. Our results suggest, independently of complementary evidence based on Li/Be ratios, Be/B ratios, and Li in subgiants evolving out of the Li dip, that main-sequence angular momentum evolution is the origin of the Li dip. Specifically, our stars' A(Li) indicates tidal synchronization can be sufficiently efficient and occur early enough in short-period binary mid-F stars to reduce the effects of rotationally-induced mixing and destruction of Li occuring during the main-sequence in otherwise similar stars that are not short-period tidally-locked binaries.

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arXiv2011

Beryllium and Alpha-Element Abundances in a Large Sample of Metal-Poor Stars

The light elements, Li, Be, and B, provide tracers for many aspects of astronomy including stellar structure, Galactic evolution, and cosmology. We have taken spectra of Be in 117 metal-poor stars ranging in metallicity from [Fe/H] = -0.5 to -3.5 with Keck I + HIRES at a resolution of 42,000 and signal-to-noise ratios of near 100. We have determined the stellar parameters spectroscopically from lines of Fe I, Fe II, Ti I and Ti II. The abundances of Be and O were derived by spectrum synthesis techniques, while abundances of Fe, Ti, and Mg were found from many spectral line measurements. There is a linear relationship between [Fe/H] and A(Be) with a slope of +0.88 +-0.03 over three orders of magnitude in [Fe/H]. We fit the relationship between A(Be) and [O/H] with both a single slope and with two slopes. The relationship between [Fe/H] and [O/H] seems robustly linear and we conclude that the slope change in Be vs. O is due to the Be abundance. Although Be is a by-product of CNO, we have used Ti and Mg abundances as alpha-element surrogates for O in part because O abundances are rather sensitive to both stellar temperature and surface gravity. We find that A(Be) tracks [Ti/H] very well with a slope of 1.00 +-0.04. It also tracks [Mg/H] very well with a slope of 0.88 +-0.03. We find that there are distinct differences in the relationships of A(Be) and [Fe/H] and of A(Be) and [O/H] for our dissipative stars and our accretive stars. We suggest that the Be in the dissipative stars was primarily formed by GCR spallation and Be in the accretive stars was formed in the vicinity of SN II.

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arXiv2009

Beryllium, Oxygen and Iron Abundances in Extremely Metal-Deficient Stars

The abundance of beryllium in the oldest, most metal-poor stars acts as a probe of early star formation and Galactic chemical evolution. We have analyzed high-resolution, high signal-to-noise Keck/HIRES spectra of 24 stars with [Fe/H] from -2.3 to -3.5 in order to determine the history of Be abundances and explore the possibility of a Be plateau. We have determined stellar parameters spectroscopically. Oxygen abundances have been derived from three OH features which occur in the same spectral region. We have supplemented this sample with reanalyzed spectra of 25 stars from previous observations; our total sample ranges in [Fe/H] from -0.5 to -3.5. We find that the relationship between Be and [Fe/H] continues to lower metallicities with a slope of 0.92 +-0.04. Although there is no indication of a plateau with constant Be abundance, the four lowest metallicity stars do show a Be enhancement relative to Fe. A single relationship between Be and [O/H] has a slope of 1.21 +-0.08, but there is also a good fit with two slopes: 1.59 for [O/H] > -1.6 and 0.74 for stars with [O/H] < -1.6. This change in slope could result from a change in the dominant production mechanism for Be. In the era of the formation of the more metal poor stars Be would be formed by acceleration of CNO atoms in the vicinity of SNII and in later times by high-energy cosmic rays bombarding CNO in the ambient interstellar gas. We find an excellent correlation between [Fe/H] and [O/H].

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arXiv2009

Abundances in Turn-off Stars in the Old, Metal-Rich Open Cluster, NGC 6791

Open clusters have long been used to illuminate both stellar evolution and Galactic evolution. The oldest clusters, though rather rare, can reveal the chemical and nucleosynthetic processes early in the history of the Galaxy. We have studied two turn-off stars in the old, metal-rich open cluster, NGC 6791. The Keck + HIRES spectra have a resolution of 45,000 and signal-to-noise ratios of 40 per pixel. We confirm the high value for [Fe/H] finding +0.30 $\pm$0.08, in agreement with earlier results from evolved stars in other parts of the HR diagram. We have also determined abundances for Na, Si, Ca, Ti, Cr, Ni, Y and Ba. These are compared to a sample of old, metal-rich field stars. With the probable exception of enhanced Ni in the cluster stars, the field and cluster stars show similar abundances of the elements. Model predictions show that the Ni enhancement could result from enrichment of the pre-cluster gas by SN Ia. Orbital evidence indicates that NGC 6791 could have originated near the inner regions of the Galaxy where the metallicity is generally higher than it is in the disk or halo. Subsequent perturbations and migrations may have resulted in its current heliocentric distance of 4 kpc and 1 kpc above the Galactic plane.

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