K. Butler

A cosmic abundance standard: chemical homogeneity of the solar neighborhood and the ISM dust-phase composition

A representative sample of unevolved early B-type stars in nearby OB associations and the field is analyzed to unprecedented precision using NLTE techniques. The resulting chemical composition is found to be more metal-rich and much more homogeneous than indicated by previous work. A rms scatter of ~10% in abundances is found for the six stars (and confirmed by six evolved stars), the same as reported for ISM gas-phase abundances. A cosmic abundance standard for the present-day solar neighborhood is proposed, implying mass fractions for hydrogen, helium, and metals of -->X = 0.715, -->Y = 0.271, and -->Z = 0.014. Good agreement with solar photospheric abundances as reported from recent 3D radiative-hydrodynamical simulations of the solar atmosphere is obtained. As a first application we use the cosmic abundance standard as a proxy for the determination of the local ISM dust-phase composition, putting tight observational constraints on dust models.

Quantitative spectroscopy of BA-type supergiants

Luminous BA-type supergiants have enormous potential for modern astrophysics. They allow topics ranging from non-LTE physics and the evolution of massive stars to the chemical evolution of galaxies and cosmology to be addressed. A hybrid non-LTE technique for the quantitative spectroscopy of these stars is discussed. Thorough tests and first applications of the spectrum synthesis method are presented for the bright Galactic objects η Leo (A0 Ib), HD 111613 (A2 Iabe), HD 92207 (A0 Iae) and β Ori (B8 Iae), based on high-resolution and high-S/N Echelle spectra. Stellar parameters are derived from spectroscopic indicators, consistently from multiple non-LTE ionization equilibria and Starkbroadened hydrogen line profiles, and they are verified by spectrophotometry. The internal accuracy of the method allows the 1σ-uncertainties to be reduced to <1−2% in Teff and to 0.05−0.10 dex in log g. Elemental abundances are determined for over 20 chemical species, with many of the astrophysically most interesting in non-LTE (H, He, C, N, O, Mg, S, Ti, Fe). The non-LTE computations reduce random errors and remove systematic trends in the analysis. Inappropriate LTE analyses tend to systematically underestimate iron group abundances and overestimate the light and α-process element abundances by up to factors of two to three on the mean. This is because of the different responses of these species to radiative and collisional processes in the microscopic picture, which is explained by fundamental differences of their detailed atomic structure, and not taken into account in LTE. Contrary to common assumptions, significant non-LTE abundance corrections of ∼0.3 dex can be found even for the weakest lines (Wλ < 10 mA). Non-LTE abundance uncertainties amount to typically 0.05−0.10 dex (random) and ∼0.10 dex (systematic 1σ-errors). Near-solar abundances are derived for the heavier elements in the sample stars, and patterns indicative of mixing with nuclear-processed matter for the light elements. These imply a blue-loop scenario for η Leo because of first dredge-up abundance ratios, while the other three objects appear to have evolved directly from the main sequence. In the most ambitious computations several ten-thousand spectral lines are accounted for in the spectrum synthesis, permitting the accurate reproduction of the entire observed spectra from the visual to near-IR. This prerequisite for the quantitative interpretation of intermediate-resolution spectra opens up BA-type supergiants as versatile tools for extragalactic stellar astronomy beyond the Local Group. The technique presented here is also well suited to improve quantitative analyses of less extreme stars of similar spectral types.

Abundance analysis of prime B-type targets for asteroseismology. I. Nitrogen excess in slowly-rotating beta Cephei stars

Seismic modelling of the beta Cephei stars promises major advances in our understanding of the physics of early B-type stars on (or close to) the main sequence. However, a precise knowledge of their physical parameters and metallicity is a prerequisite for correct mode identification and inferences regarding their internal structure. Here we present the results of a detailed NLTE abundance study of nine prime targets for theoretical modelling: gamma Peg , delta Cet , nu Eri , beta CMa , xi1 CMa , V836 Cen , V2052 Oph , beta Cep and DD (12) Lac (hereafter 12 Lac ). The following chemical elements are considered: He, C, N, O, Mg, Al, Si, S and Fe. Our curve-of-growth abundance analysis is based on a large number of time-resolved, high-resolution optical spectra covering in most cases the entire oscillation cycle of the stars. Nitrogen is found to be enhanced by up to 0.6 dex in four stars, three of which have severe constraints on their equatorial rotational velocity, Omega R, from seismic or line-profile variation studies: beta Cep (Omega R ~ 26 km s -1 ), V2052 Oph (Omega R ~ 56 km s -1 ), delta Cet (Omega R -1 ) and xi1 CMa (Omega R sin i  10 km s -1 ). The existence of core-processed material at the surface of such largely unevolved, slowly-rotating objects is not predicted by current evolutionary models including rotation. We draw attention to the fact that three stars in this subsample have a detected magnetic field and briefly discuss recent theoretical work pointing to the occurrence of diffusion effects in beta Cephei stars possibly capable of altering the nitrogen surface abundance. On the other hand, the abundances of all the other chemical elements considered are, within the errors, indistinguishable from the values found for OB dwarfs in the solar neighbourhood. Despite the mild nitrogen excess observed in some objects, we thus find no evidence for a significantly higher photospheric metal content in the studied beta Cephei stars compared to non-pulsating B-type stars of similar characteristics.

Non-LTE Line Formation for Hydrogen Revisited

We discuss aspects of non-LTE line formation for hydrogen in early-type stars. We evaluate the effect of variations in the electron-impact excitation cross sections in model atoms of differing complexity by comparison with observation. While the Balmer lines are basically unaffected by the choice of atomic data, the Paschen, Brackett, and Pfund series members allow us to discriminate between the different models. Non-LTE calculations based on the widely used approximation formulae of Mihalas, Heasley, & Auer and of Johnson fail to simultaneously reproduce the optical and IR spectra over the entire parameter range. The use of data from ab initio calculations up to principal quantum number n ≤ 7 largely solves the problem. We recommend a reference model using the available data. This model is of general interest because of the ubiquity of the hydrogen spectrum.

Statistical equilibrium of silicon in the solar atmosphere

Aims. The statistical equilibrium of neutral and ionised silicon in the solar photosphere is investigated. Line formation is discussed and the solar silicon abundance determined. Methods. High-resolution solar spectra were used to determine solar log g f eSi values by comparison with Si line synthesis based on LTE and NLTE level populations. The results will be used in a forthcoming paper for differential abundance analyses of metalpoor stars. A detailed analysis of silicon line spectra leads to setting up realistic model atoms, which are exposed to interactions in plane-parallel solar atmospheric models. The resulting departure coefficients are entered into a line-by-line analysis of the visible and near-infrared solar silicon spectrum. Results. The statistical equilibrium of Si i turns out to depend marginally on bound-free interaction processes, both radiative and collisional. Bound-bound interaction processes do not play a significant role either, except for hydrogen collisions, which have to be chosen adequately for fitting the cores of the near-infrared lines. Except for some near-infrared lines, the NLTE influence on the abundances is weak. Conclusions. Taking the deviations from LTE in silicon into account, it is possible to calculate the ionisation equilibrium from neutral and ionised lines. The solar abundance based on the experimental f -values of Garz corrected for the Becker et al.’s measurement is 7.52 ± 0.05. Combined with an extended line sample with selected NIST f -values, the solar abundance is 7.52 ± 0.06, with a nearly perfect ionisation equilibrium of Δ log e� (Siii/Sii) = −0.01.

Chemical Abundances of OB Stars with High Projected Rotational Velocities

Elemental abundances of carbon, nitrogen, oxygen, magnesium, aluminum, and silicon are presented for a sample of 12 rapidly rotating OB star (v sin i > 60 km s-1) members of the Cep OB2, Cyg OB3, and Cyg OB7 associations. The abundances are derived from spectrum synthesis, using both LTE and non-LTE calculations. As found in almost all previous studies of OB stars, the average abundances are slightly below solar, by about 0.1 to 0.3 dex. In the case of oxygen, even with the recently derived low solar abundances, the OB stars are closer to, but still below, the solar value. Results for the nine Cep OB2 members in this sample can be combined with results published previously for eight Cep OB2 stars with low projected rotational velocities to yield the most complete set of abundances, to date, for this particular association. These abundances provide a clear picture of both the general chemical and individual stellar evolution that has occurred within this association. By placing the Cep OB2 stars studied in an HR-diagram we identify the presence of two distinct age subgroups, with both subgroups having quite uniform chemical abundances. Two stars are found in the older subgroup that show significant N/O overabundances, with both stars being two of the most massive, the most evolved, and the most rapidly rotating of the members studied in Cep OB2. These characteristics of increased N abundances being tied to high mass, rapid rotation, and an evolved phase are those predicted from models of rotating stars that undergo rotationally driven mixing.

The neon content of nearby B-type stars and its implications for the solar model problem

The recent downward revision of the solar photospheric abundances now leads to severe inconsistencies between the theoretical predictions for the internal structure of the Sun and the results of helioseismology. There have been claims that the solar neon abundance may be underestimated and that an increase in this poorly-known quantity could alleviate (or even completely solve) this problem. Early-type stars in the solar neighbourhood are well-suited to testing this hypothesis because they are the only stellar objects whose absolute neon abundance can be derived from the direct analysis of photospheric lines. Here we present a fully homogeneous NLTE abundance study of the optical Ne i and Ne ii lines in a sample of 18 nearby, early B-type stars, which suggests log � (Ne) = 7.97 ± 0.07 dex (on the scale in which log � [H] = 12) for the present-day neon abundance of the local interstellar medium (ISM). Chemical evolution models of the Galaxy only predict a very small enrichment of the nearby interstellar gas in neon over the past 4.6 Gyr, implying that our estimate should be representative of the Sun at birth. Although higher by about 35% than the new recommended solar abundance, such a value appears insufficient by itself to restore the past agreement between the solar models and the helioseismological constraints.

LMC origin of the hyper-velocity star HE 0437 5439 ⋆ Beyond the supermassive black hole paradigm

Context. Hyper-velocity stars move so fast that only a supermassive black hole (SMBH) seems to be capable to accelerate them. Hence the Galactic centre (GC) is their only suggested place of ori gin. Edelmann et al. (2005) found the early B-type star HE 0437−5439 to be too short-lived to have reached its current position in th e Galactic halo if ejected from the GC, except if being a blue straggler star. Its proximity to the Large Magellanic Cloud (LMC) suggested an origin from this galaxy. Aims. The chemical signatures of stars at the GC are significantly d ifferent from those in the LMC. Hence, an accurate measurement of the abundance pattern of HE 0437−5439 will yield a new tight constraint on the place of birth of this hyper-velocity star. Methods. High-resolution spectra obtained with UVES on the VLT are analysed using state-of-the-art non-LTE modelling techniques. Results. We measured abundances of individual elements to very high accuracy in HE 0437−5439 as well as in two reference stars, from the LMC and the solar neighbourhood, respectively. The abundance pattern is not consistent at all with that observe d in stars near the GC, ruling our an origin from the GC. However, there is a high degree of consistency with the LMC abundance pattern. Our abundance results cannot rule out an origin in the outskirts of the Galactic disk. However, we find the life time of HE 0437−5439 to be more than three times shorter than the time of flight to the e dge of the disk, rendering a Galactic origin unlikely. Conclusions. Only one SMBH is known to be present in Galaxy and none in the LMC. Hence the exclusion of an GC origin challenges the SMBH paradigm. We conclude that there must be other mechanism(s) to accelerate stars to hyper-velocity speed than th e SMBH. We draw attention to dynamical ejection from dense massive clusters, that has recently been proposed by Gvaramadze et al. (2008).

Testing common classical LTE and NLTE model atmosphere and line-formation codes for quantitative spectroscopy of early-type stars

It is generally accepted that the atmospheres of cool/lukewarm stars of spectral types A and later are described well by LTE model atmospheres, while the O-type stars require a detailed treatment of NLTE effects. Here model atmosphere structures, spectral energy distributions and synthetic spectra computed with ATLAS9/SYNTHE and TLUSTY/SYNSPEC, and results from a hybrid method combining LTE atmospheres and NLTE line-formation with DETAIL/SURFACE are compared. Their ability to reproduce observations for effective temperatures between 15000 and 35000 K are verified. Strengths and weaknesses of the different approaches are identified. Recommendations are made as to how to improve the models in order to derive unbiased stellar parameters and chemical abundances in future applications, with special emphasis on Gaia science.

Non-LTE line-formation for neutral and singly-ionized carbon - Model atom and first results on BA-type stars

A comprehensive model atom for non-LTE line-formation calculations for neutral and singly-ionized carbon is presented. Highly accurate radiative and collisional atomic data are incorporated, recently determined for astrophysical and fusion research using the R-matrix method in the close-coupling approximation. As a test and rst application of the model, carbon abundances are determined on the basis of line-blanketed LTE model atmospheres for ve stars, the main sequence object Vega (A0 V) and the supergiants Leo (A0 Ib), HD 111613 (A2 Iabe), HD 92207 (A0 Iae) and Ori (B8 Iae), using high S=N and high-resolution spectra at visual and near-IR wavelengths. The computed non-LTE line proles t the observations well for a single carbon abundance in each object. For two supergiants, Leo and HD 111613, lines of both species are simultaneously present in the spectra, giving consistent C i and C ii abundances (within the error bars). However, the uncertainties of the abundances are large, on the order of 0.3 dex (statistical + systematical error), thus the ionization equilibrium of C i/ii is of restricted use for the determination of stellar parameters. All supergiants within our sample show a depletion of carbon on the order of 0.2{0.5 dex, indicating the mixing of CN-cycled material into the atmospheric layers, with the sum of the CNO abundances remaining close to solar. This nding is in accordance with recent stellar evolution models accounting for mass-loss and rotation. For Vega, an underabundance of carbon by 0.3 dex is found, in excellent agreement with the similar underabundance of other light elements. The dependence of the non-LTE eects on the atmospheric parameters is discussed and the influence of systematic errors is estimated. Special emphasis is given to the supergiants where a strong radiation eld at low particle densities favours deviations from LTE. Non-LTE eects systematically strengthen the C i/ii lines. For the C i lines in the infrared, a strong sensitivity to modications in the photoionization and collisional excitation data is found. An increasing discrepancy between our model predictions and the observations for the C ii doublet 6578-82 is perceived with rising luminosity, while the other C ii doublet and quartet lines remain consistent. Furthermore, the influence of microturbulence on the statistical-equilibrium calculations is investigated.