arXiv subjects

D. L. Lambert

Publications and source records attributed to D. L. Lambert.

67 records · Page 4Linked to original sources

arXiv2002

Chemical Abundances in Twelve Red Giants of the Large Magellanic Cloud from High-Resolution Infrared Spectroscopy

High-resolution infrared spectra (R=50,000) have been obtained for twelve red-giant members of the LMC with the Gemini South 8.3-meter telescope plus Phoenix spectrometer. Quantitative chemical abundances of carbon-12, carbon-13, nitrogen-14, and oxygen-16 were derived from molecular lines of CO, CN, and OH, while sodium, scandium, titanium, and iron abundances were derived from neutral atomic lines. The LMC giants have masses from about 1 to 4 solar masses and span a metallicity range from [Fe/H]= -1.1 to -0.3. The program red giants all show evidence of first dredge-up mixing, with low 12C/13C ratios, and low 12C correlated with high 14N abundances. Comparisons of the oxygen-to-iron ratios in the LMC and the Galaxy indicate that the trend of [O/Fe] versus [Fe/H] in the LMC falls about 0.2 dex below the Galactic trend. Such an offset can be modeled as due to an overall lower rate of supernovae per unit mass in the LMC relative to the Galaxy, as well as a slightly lower ratio of supernovae of type II to supernovae of type Ia.

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arXiv2002

Detection of Water Vapor in the Photosphere of Arcturus

We report detections of pure rotation lines of OH and H2O in the K1.5 III red-giant star Arcturus (alpha Bootis) using high-resolution, infrared spectra covering the regions 806-822 cm-1 (12.2-12.4 um) and 884-923 cm-1 (10.8-11.3 um). Arcturus is the hottest star yet to show water-vapor features in its disk-averaged spectrum. We argue that the water vapor lines originate from the photosphere, albeit in the outer layers. We are able to predict the observed strengths of OH and H2O lines satisfactorily after lowering the temperature structure of the very outer parts of the photosphere (log tau_500=-3.8 and beyond) compared to a flux-constant, hydrostatic, standard MARCS model photosphere. Our new model is consistently calculated including chemical equilibrium and radiative transfer from the given temperature structure. Possible reasons for a temperature decrease in the outer-most parts of the photosphere and the assumed break-down of the assumptions made in classical model-atmosphere codes are discussed.

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arXiv2002

O/Fe in metal-poor main sequence and subgiant stars

A study of the O/Fe ratio in metal-poor main sequence and subgiant stars is presented using the [OI] 6300A line, the OI 7774A triplet, and a selection of weak FeII lines observed on high-resolution spectra acquired with the VLT UVES spectrograph. The [OI] line is detected in the spectra of 18 stars with -0.5 < [Fe/H] < -2.4, and the triplet is observed for 15 stars with [Fe/H] ranging from -1.0 to -2.7. The abundance analysis was made first using standard model atmospheres taking into account non-LTE effects on the triplet: the [OI] line and the triplet give consistent results with [O/Fe] increasing quasi-linearly with decreasing [Fe/H] reaching [O/Fe] ~ +0.7 at [Fe/H] = -2.5. When hydrodynamical model atmospheres representing stellar granulation in dwarf and subgiant stars replace standard models, the [O/Fe] from the [OI] and FeII lines is decreased by an amount which increases with decreasing [Fe/H]. The [O/Fe] vs [Fe/H] relation remains quasi-linear extending to [O/Fe] ~ +0.5 at [Fe/H] = -2.5, but with a tendency of a plateau with [O/Fe] ~ +0.3 for -2.0 < [Fe/H] < -1.0, and a hint of cosmic scatter in [O/Fe] at [Fe/H] ~ -1.0. Use of the hydrodynamical models disturbs the broad agreement between the oxygen abundances from the [OI], OI, and OH lines, but 3D non-LTE effects may serve to erase these differences.

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arXiv2002

Boron in the Small Magellanic Cloud: A Novel Test of Light Element Production

Hubble Space Telescope STIS observations of the B III resonance line at 2066A have been obtained and analyzed for two Small Magellanic Cloud (SMC) B-type stars. While boron is not detected in either star, upper limits to the boron abundance are set, with 12+log(B/H) le 1.6 for both AV 304 and NGC 346-637. The upper limits are consistent with the relationship between boron and oxygen previously reported for Galactic disk stars. The SMC upper limits are discussed in light of that galaxy's star formation history, present oxygen abundance, and its present cosmic ray flux. The UV spectrum has also been used to determine the iron-group abundances in the SMC stars. For AV 304, [Fe/H]=-0.6 pm 0.2, from both an absolute and a differential analysis (with respect to the Galactic B-type star HD 36591). This is consistent with results from A-F supergiants in the SMC. A lower iron abundance is found for NGC 346-637, [Fe/H]=-1.0 pm 0.3, but this is in good agreement with the supergiant iron abundances in NGC 330,another young SMC cluster. We propose NGC 346-637 may be an unrecognized binary though, which complicates its spectral analysis.

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arXiv2001

Boron Abundances in Main Sequence B-type Stars: A Test of Rotational Depletion during Main Sequence Evolution

Boron abundances have been derived for seven main sequence B-type stars from HST STIS spectra around the B III 2066 A line. In two stars, boron appears to be undepleted with respect to the presumed initial abundance. In one star, boron is detectable but it is clearly depleted. In the other four stars, boron is undetectable implying depletions of 1 to 2 dex. Three of these four stars are nitrogen enriched, but the fourth shows no enrichment of nitrogen. Only rotationally induced mixing predicts that boron depletions are unaccompanied by nitrogen enrichments. The inferred rate of boron depletion from our observations is in good agreement with these predictions. Other boron-depleted nitrogen-normal stars are identified from the literature. Also, several boron-depleted nitrogen-rich stars are identified, and while all fall on the boron-nitrogen trend predicted by rotationally-induced mixing, a majority have nitrogen enrichments that are not uniquely explained by rotation. The spectra have also been used to determine iron-group (Cr, Mn, Fe, and Ni) abundances. The seven B-type stars have near solar iron-group abundances, as expected for young stars in the solar neighborhood. We have also analysed the halo B-type star, PG0832+676. We find [Fe/H] = -0.88 +/- 0.10, and the absence of the B III line gives the upper limit [B/H]<2.5. These and other published abundances are used to infer the star's evolutionary status as a post-AGB star.

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arXiv2001

Nucleosynthesis and mixing on the Asymptotic Giant Branch. III. Predicted and observed s-process abundances

We present the results of s-process nucleosynthesis calculations for AGB stars of different metallicities and initial masses. The computations were based on previously published stellar evolutionary models that account for the III dredge up phenomenon occurring late on the AGB. Neutron production is driven by the 13C(alpha,n)16O reaction during the interpulse periods in a tiny layer in radiative equilibrium at the top of the He- and C-rich shell. The s-enriched material is subsequently mixed with the envelope by the III dredge up, and the envelope composition is computed after each thermal pulse. We follow the changes in the photospheric abundance of the Ba-peak elements (heavy s, or `hs') and that of the Zr-peak ones (light s, or `ls'), whose logarithmic ratio [hs/ls] has often been adopted as an indicator of the s-process efficiency. The theoretical predictions are compared with published abundances of s elements for Galactic AGB giants of classes MS, S, SC, post-AGB supergiants, and for various classes of binary stars. The observations in general confirm the complex dependence of n captures on metallicity. They suggest that a moderate spread exists in the abundance of 13C that is burnt in different stars. Although additional observations are needed, a good understanding has been achieved of s-process operation in AGB. The detailed abundance distribution including the light elements (CNO) of a few s-enriched stars at different metallicity are examined.

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arXiv2000

The Chemical Compositions of the SRd Variable Stars. III. KK Aquilae, AG Aurigae, Z Aurigae, W Leo Minoris, and WW Tauri

Chemical compositions are derived from high-resolution spectra for five field SRd variables. These supergiants not previously analysed are shown to be metal-poor: KK Aql with [Fe/H] = -1.2, AG Aur with [Fe/H] = -1.8, Z Aur with [Fe/H] = -1.4, W LMi with [Fe/H] = -1.1, and WW Tau with [Fe/H] = -1.1. Their compositions are, except for two anomalies, identical to within the measurement errors with the compositions of subdwarfs, subgiants, and less evolved giants of the same [Fe/H]. One anomaly is an s-process enrichment for KK Aql, the first such enrichment reported for a SRd variable. The second and more remarkable anomaly is a strong lithium enrichment for W LMi, also a first for field SRds. The Li I 6707 A profile is not simply that of a photospheric line but includes strong absorption from red-shifted gas, suggesting, perhaps, that lithium enrichment results from accretion of Li-rich gas. This potential clue to lithium enrichment is discussed in light of various proposals for lithium synthesis in evolved stars.

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arXiv1999

Isotopic lithium abundances in five metal-poor disk stars

High resolution (R=110000), very high S/N spectra, centered on the 6707.8A LiI line have been obtained with the ESO Coude Echelle Spectrometer for five, metal-poor (-0.8 < [Fe/H] < -0.6) disk stars in the turnoff region of the HR-diagram. The instrumental and stellar atmospheric line broadening have been determined from two unblended iron lines and used in a model atmosphere synthesis of the profile of the LiI line as a function of the lithium isotope ratio. This has led to a detection of 6Li in HD68284 and HD130551 with 6Li/7Li = 0.05, whereas the other stars, HR2883, HR3578 and HR8181, have 6Li/7Li close to zero. By comparing Teff-values and absolute magnitudes based on Hipparcos parallaxes with recent stellar evolutionary tracks, the masses of the stars have been derived. It is shown that the two stars with 6Li present have a significantly higher mass, M = 1.05 M(sun), than the other three stars for which values between 0.85 and 1.0 M(sun) are obtained. The results are discussed in terms of models for the galactic evolution of the light elements and depletion of the lithium isotopes in stellar envelopes. It is shown that the measured 6Li abundances are in agreement with standard cosmic ray production of 6Li in the galactic disk and a moderate depletion (0.5 dex) in the stars. Recent models for the evolution of 6Li including alpha + alpha fusion reactions and predicting a high lithium isotopic ratio, 6Li/7Li = 0.3 at [Fe/H] = -0.6, require a high degree of 6Li depletion (1.0 dex) to fit the observations. Furthermore, these models imply a 7Li abundance about 0.2 dex higher than observed for metal-poor disk stars.

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arXiv1998

The rapid evolution of the born-again giant Sakurai's object

The extraordinarily rapid evolution of the born-again giant Sakurai's object can be traced both in a continued cooling of the stellar surface and dramatic changes in chemical composition on a timescale of a mere few months. The abundance alterations are the results of the mixing and nuclear reactions which have ensued due to the final He-shell flash which occurred during the descent along the white dwarf cooling track. The observed changes in the H and Li abundances can be explained by ingestion and burning of the H-rich envelope and Li-production through the Cameron-Fowler mechanism. The rapidly increasing abundances of the light s-elements (including Sc) is consistent with current s-processing by neutrons released from the concomitantly produced 13C. However, the possibility that the s-elements have previously been synthesized during the AGB-phase and only mixed to the surface in connection with the final He-shell flash cannot be convincingly ruled out either. Since Sakurai's object shows substantial abundance similarities with the R CrB stars and has recently undergone RCrB-like visual fading events, the ``birth'' of an RCrB star may have been witnessed for the first time ever.

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arXiv1998

Boron in Very Metal-Poor Stars

We have observed the B I 2497 A line to derive the boron abundances of two very metal-poor stars selected to help in tracing the origin and evolution of this element in the early Galaxy: BD +23 3130 and HD 84937. The observations were conducted using the Goddard High Resolution Spectrograph on board the Hubble Space Telescope. A very detailed abundance analysis via spectral synthesis has been carried out for these two stars, as well as for two other metal-poor objects with published spectra, using both Kurucz and OSMARCS model photospheres, and taking into account consistently the NLTE effects on the line formation. We have also re-assessed all published boron abundances of old disk and halo unevolved stars. Our analysis shows that the combination of high effective temperature (Teff > 6000 K, for which boron is mainly ionized) and low metallicity ([Fe/H]<-1) makes it difficult to obtain accurate estimates of boron abundances from the B I 2497 A line. This is the case of HD 84937 and three other published objects (including two stars with [Fe/H] ~ -3), for which only upper limits can be established. BD +23 3130, with [Fe/H] ~ -2.9 and logN(B)_NLTE=0.05+/-0.30, appears then as the most metal-poor star for which a firm measurement of the boron abundance presently exists. The evolution of the boron abundance with metallicity that emerges from the seven remaining stars with Teff < 6000 K and [Fe/H]<-1, for which beryllium abundances were derived using the same stellar parameters, shows a linear increase with a slope ~ 1. Furthermore, the B/Be ratio found is constant at a value ~ 20 for stars in the range -3<[Fe/H]<-1. These results point to spallation reactions of ambient protons and alpha particles with energetic particles enriched in CNO as the origin of boron and beryllium in halo stars.

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arXiv1998

Abundance similarities between the R CrB star V854 Cen and the born-again Sakurai's object

The elemental abundances of the mildly hydrogen-deficient R Coronae Borealis (R CrB) star V854 Cen have been estimated. The R CrB stars have been divided into majority and minority classes judging by their abundance patterns. Class assignment has previously been unambiguous but V854 Cen has traits of both the minority and majority class. Neither V854 Cen nor the three obvious minority members show any clear abundance signatures of having been affected by e.g. dust-gas separation as often observed in post-AGB stars. By chemical composition, V854 Cen closely resembles Sakurai's object, which has probably recently experienced a final He-shell flash. Therefore V854 Cen and Sakurai's object may share the same evolutionary background, which would add support for the final-flash scenario as a viable origin of the R CrB stars. Most of the few differences in abundance ratios between the stars could if so be attributed to milder H-ingestion in connection with the final He-shell flash of V854 Cen. The identification of either the majority or the minority group, if any, as final flash objects, remain uncertain, however, due to the unclear membership status of V854 Cen.

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arXiv1997

A stellar endgame -- the born-again Sakurai's object

The surface chemical composition of this remarkable star shows that it is hydrogen-deficient, carbon-rich and enriched in the light s-process elements. Spectra taken in May and October 1996 indicate a decrease in the surface hydrogen abundance by 0.7 dex in five months along with an increase in the abundances of Li, Sr, Y and Zr. The abundance changes are in agreement with the hypothesis of the star being a rapidly evolving ``born-again'' AGB star experiencing a final He-shell flash, similar to FG Sge. The C^12/C^13 ratio in October is very low, also suggesting hydrogen ingestion. By chemical composition, Sakurai's object resembles the R Coronae Borealis (R CrB) stars.

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arXiv1994

Boron in the Extreme Pop II Star HD 140283 and the Production of Light Elements in the Early Galaxy

Using observations of the 2496.7 A B I line with the HST GHRS at a nominal resolution of 90,000, we have found the abundance of boron of HD 140283 to be log epsilon_B (= 12 + log (N_B/N_H)) = 0.34 \pm 0.20. This result is found when a significant non-LTE effect in the formation of the B I line is taken into account. The resulting N_B/N_Be ratio is about 17 (in the range 9 -- 34), which is in very good agreement with what is expected from spallation by cosmic rays. We conclude that this origin of Be and B in the Early Galaxy is the most probable of recently suggested formation mechanisms. (Submitted to A&A. Postscript figures available via annymous ftp to "chiron.astro.uu.se" in directory pub/Preprints/P87)

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