N. Przybilla

Present-day cosmic abundances A comprehensive study of nearby early B-type stars and implications for stellar and Galactic evolution, and interstellar dust models ⋆,⋆⋆,⋆⋆⋆

Aims. A carefully selected sample of early B-type stars in OB associations and the field within the solar neighbourhood is studi ed comprehensively. Present-day abundances for the astrophysically most interesting chemical elements are derived in order to investigate whether a present-day cosmic abundance standard can be established. Methods. High-resolution and high-S/N spectra of well-studied sharp-lined early B-type stars ar e analysed in non-LTE. Atmospheric parameters are derived from the simultaneous establishment of independent indicators, from multiple ionization equi libria and the hydrogen Balmer lines, and they are confirmed by reproductio n of the stars’ global spectral energy distributions. Results. Effective temperatures are constrained to 1-2% and surface gravities to less than 15% uncertainty, along with accurate rotational, micro- and macroturbulence velocities. Good agreement of the resulting spectroscopic parallaxes with those from the new reduction of the Hipparcos catalogue is obtained. Absolute values for abundances of He, C, N, O, Ne, Mg, Si and Fe are determined to better than 25% uncertainty. The synthetic spectra match the observations reliably over almost the entire visual spe ctral range. Conclusions. A present-day cosmic abundance standard is established from a sample of 29 early B-type stars, indicating abundance fluctuations of less than 10% around the mean. Our results (i) resolve the long-standing discrepancy between a chemical homogeneous gas-phase ISM and a chemically inhomogeneous young stellar component out to several hundred parsec from the Sun, (ii) facilitate the amount of heavy elements locked up in the interstellar dust to be constrained precisely ‐ the results imply that carbo naceous dust is largely destroyed inside the Orion Hii region, unlike the silicates, and that graphite is only a min ority species in interstellar dust ‐, (iii) show that the mixing of CNO-burning products in the course of massive star evolution follows tightly the predicted nucl ear path, (iv) provide reliable present-day reference points for anchor ing Galactic chemical evolution models to observation, and (v) imply that the Sun has migrated outwards from the inner Galactic disk over its lifetime from a birthplace at a distance around 5-6 kpc from the Galactic Centre; a cancellation of the effects of Galactic chemical evolution and abundance gradients leads to the similarity of solar and present-day cosmic abundances in the solar neighbourhood, with a telltaling signature of the Sun’s origin left in th e C/O ratio. (ABRIDGED)

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.

Mixing of CNO-cycled matter in massive stars

Aims. We test predictions of evolution models on mixing of CNO-cycled products in massive stars from a fundamental perspective. Relative changes within the theoretical C:N:O abundance ratios and the buildup of helium are compared with observational results. Methods. A sample of well-studied Galactic massive stars is presented. High-quality optical spectra are carefully analysed using improved NLTE line-formation and comprehensive analysis strategies. The results are put in the context of the existing literature data. Results. A tight trend in the observed N/ Cv s. N/O ratios and the buildup of helium is found from the self-consistent analysis of main-sequence to supergiant stars for the first time. The catalytic nature of the CNO-cycles is confirmed quantitatively, though further investigations are required to derive a fully consistent picture. Our observational results support the case of strong mixing, as predicted e.g. by evolution models that consider magnetic fields or by models that have gone through the first dredge-up in the case of many supergiants.

The hot subdwarf B + white dwarf binary KPD 1930+2752 A supernova type Ia progenitor candidate ,,, †

Context. The nature of the progenitors of type Ia supernovae is still under debate. KPD 1930+2752 is one of the best SN Ia progenitor candidates known today. The object is a double degenerate system consisting of a subluminous B star (sdB) and a massive white dwarf (WD). Maxted et al. (2000) conclude that the system mass exceeds the Chandrasekhar mass. This conclusion, however, rests on the assumption that the sdB mass is 0.5 M� . However, recent binary population synthesis calculations suggest that the mass of an sdB star may range from 0.3 Mto more than 0.7 M� . Aims. It is therefore important to measure the mass of the sdB star simultaneously with that of the white dwarf. Since the rotation of the sdB star is tidally locked to the orbit, the inclination of the system can be constrained if the sdB radius and the projected rotational velocity can be measured with high precision. An analysis of the ellipsoidal variations in the light curve allows the constraints derived from spectroscopy to be tightened. Methods. We derived the mass-radius relation for the sdB star from a quantitative spectral analysis of 150 low-resolution spectra obtained with the Calar Alto 2.2 m telescope using metal-rich, line-blanketed LTE model atmospheres with and without NLTE line formation. The projected rotational velocity was determined for the first time from 200 high-resolution spectra obtained with the Keck I 10 m and with the ESO-VLT 8.2 m telescopes. In addition a reanalysis of the published light curve was performed. Results. The atmospheric and orbital parameters were measured with unprecedented accuracy. In particular the projected rotational velocity vrot sini = 92.3 ± 1. 5k m s −1 was determined. Assuming the companion to be a white dwarf, the mass of the sdB is limited between 0.45 Mand 0.64 Mand the corresponding total mass of the system ranges from 1.33 Mto 2.04 M� . This constrains the inclination to i > 68 ◦ . The photometric analysis allows the parameters to be constrained even more. A neutron star companion can be ruled out and the mass of the sdB is limited to the range between 0.45 Mand 0.52 M� . The total mass of the system ranges from 1.36 Mto 1.48 Mand hence is likely to exceed the Chandrasekhar mass. The inclination angle is 80 ◦ and the light curve shows weak and shallow signs of eclipses. A high-precision light curve is needed in order to accurately measure these eclipses. So KPD 1930+2752 qualifies as an excellent double degenerate supernova Ia progenitor candidate. Conclusions.

Exploring the origin of magnetic fields in massive stars: a survey of O-type stars in clusters and in the field

Context. Although the effects of magnetic fields in massive stars have been found to be substantial by recent models and observations, the magnetic fields of only a small number of massive O-type stars have so far been investigated. Additional observations are of the utmost importance to constraining the conditions that are conducive to magnetic fields and to determine the first trends about their occurrence rate and field strength distribution. Aims. To investigate statistically whether magnetic fields in massive stars are ubiquitous or appear only in stars with a specific spectral classification, certain ages, or in a special environment, we acquired 41 new spectropolarimetric observations for 36 stars. Among the observed sample, roughly half of the stars are probable members of clusters at different ages, whereas the remaining stars are field stars not known to belong to any cluster or association. Methods. Spectropolarimetric observations were obtained during three different nights using the low-resolution spectropolarimetric mode of FORS 2 (FOcal Reducer low dispersion Spectrograph) mounted on the 8-m Antu telescope of the VLT. To assess the membership in open clusters and associations, we used astrometric catalogues with the highest quality kinematic and photometric data currently available. Results. A field at a significance level of 3σ was detected in ten O-type stars. The strongest longitudinal magnetic fields were

Carbon abundances of early B-type stars in the solar vicinity - Non-LTE line-formation for C II/III/IV and self-consistent atmospheric parameters

Context. Precise determinations of the chemical composition in early B-type stars constitute fundamental observational constraints on stellar and galactochemical evolution. Carbon, in particular, is one of the most abundant metals in the Universe but analyses in early-type stars are known to show inconclusive results. Large discrepancies between analyses of different lines in C ii, a failure to establish the C ii/iii ionization balance, and the derivation of systematically lower abundances than from other indicators like H ii regions and young FG-type stars all pose long-standing problems. Aims. We discuss improvements to the non-LTE modelling of the visual line spectrum and to the spectral analysis of early B-type stars, as well as their consequences for stellar parameter and abundance derivations. The most relevant sources of systematic uncertainies and their effects on the analysis are investigated. Consequences for the present-day carbon abundance in the solar vicinity are discussed. Methods. We present a comprehensive and robust C ii/iii/iv model for non-LTE line-formation calculations based on carefully selected atomic data. The model is calibrated with high-S/N spectra of six apparently slow-rotating early B-type dwarfs and giants, which cover a wide parameter range and are randomly distributed in the solar neighbourhood. A self-consistent quantitative spectrum analysis is performed using an extensive iteration scheme to determine stellar atmospheric parameters and to select the appropriate atomic data used for deriving chemical abundances. Results. We establish the carbon ionization balance for all sample stars based on a unique set of input atomic data. Consistency is achieved for all modelled carbon lines of the sample stars. Highly accurate atmospheric parameters and a homogeneous carbon abundance of log (C/H) + 12 = 8.32 ± 0.04 are derived with reduced systematic errors. Present evolution models for massive stars indicate that this value may require only a small adjustment because of the effects of rotational mixing, by <+0.05 dex per sample star. This results in a present-day stellar carbon abundance in the solar neighbourhood, which is in good agreement with recent determinations of the solar value and with the gas-phase abundance of the Orion H ii region. Our finding of a homogeneous present-day carbon abundance also conforms to predictions of chemical-evolution models for the Galaxy. Moreover, the present approach allows us to constrain the effects of systematic errors on fundamental parameters and abundances. This suggests that most of the difficulties found in previous work may be related to large systematic effects in the atmospheric parameter determination and/or inaccuracies in the atomic data.

An asteroseismic study of the O9V star HD 46202 from CoRoT space-based photometry

The O9V star HD46202, which is a member of the young open cluster NGC 2244, was observed by the CoRoT satellite in October/November 2008 during a short run of 34 days. From the very high-precision light curve, we clearly detect β Cep-like pulsation frequencies with amplitudes of ∼0.1 mmag and below. A comparison with stellar models was performed using a χ 2 as a measure for the goodness-of-fit between the observed and theoretically computed frequencies. The physical parameters of our best-fitting models are compatible with the ones deduced spectroscopically. A core overshooting parameter αov = 0.10 ± 0.05 pressure scale height is required. None of the observed frequencies are theoretically excited with the input physics used in our study. More theoretical work is thus needed to overcome this shortcoming in how we understand the excitation mechanism of pulsation modes in such a massive star. A similar excitation problem has also been encountered for certain pulsation modes in β Cep stars recently modelled asteroseismically.

C II Abundances in Early-Type Stars: Solution to a Notorious Non-LTE Problem

We address a long-standing discrepancy between non-LTE analyses of the prominent C II 4267 and 6578/82 A multiplets in early-type stars. A comprehensive non-LTE model atom of C II is constructed based on critically selected atomic data. This model atom is used for an abundance study of six apparently slow-rotating main-sequence and giant early B-type stars. High-resolution and high S/N spectra allow us to derive highly consistent abundances not only from the classical features but also from up to 18 additional C II lines in the visual—including two so far unreported emission features equally well reproduced in non-LTE. These results require the stellar atmospheric parameters to be determined with care. A homogeneous (slightly) subsolar present-day carbon abundance from young stars in the solar vicinity (in associations and in the field) of log(C/H) + 12 = 8.29 ± 0.03 is indicated.

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).

Non-LTE modelling of the He I 10830 Å line in early-type main sequence stars

The near-IR He  10 830 A transition is a highly sensitive diagnostic for non-LTE effects in astrophysical plasmas. So far, non-LTE lineformation computations have failed to quantitatively reproduce observations of this line in the entire range of early-A to late-O main sequence stars. It is shown that the non-LTE modelling was insufficient, for the most part, either because of inaccurate photoionization cross-sections for the 2s 3 S state or because of neglecting line blocking. New calculations based on state-of-the-art atomic data give excellent agreement with observation for the He  10 830 A feature, while profiles of the He  lines in the visual are retained.