Norbert Przybilla

The VLT-FLAMES Survey of Massive Stars: Observations in the Galactic Clusters NGC 3293, NGC 4755 and NGC 6611 ⋆

We introduce a new survey of massive stars in the Galaxy and the Magellanic Clouds using the Fibre Large Array Multi-Element Spectrograph (FLAMES) instrument at the Very Large Telescope (VLT). Here we present observations of 269 Galactic stars with the FLAMES-Giraffe Spectrograph (R � 25 000), in fields centered on the open clusters NGC 3293, NGC 4755 and NGC 6611. These data are supplemented by a further 50 targets observed with the Fibre-Fed Extended Range Optical Spectrograph (FEROS, R = 48 000). Following a description of our scientific motivations and target selection criteria, the data reduction methods are described; of critical importance the FLAMES reduction pipeline is found to yield spectra that are in excellent agreement with less automated methods. Spectral classifications and radial velocity measurements are presented for each star, with particular attention paid to morphological peculiarities and evidence of binarity. These observations represent a significant increase in the known spectral content of NGC 3293 and NGC 4755, and will serve as standards against which our subsequent FLAMES observations in the Magellanic Clouds will be compared.

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 24 A Supergiants in the Sculptor Galaxy NGC 300: Flux-weighted Gravity-Luminosity Relationship, Metallicity, and Metallicity Gradient

A quantitative spectral analysis of 24 A supergiants in the Sculptor Group spiral galaxy NGC 300 at a distance of 1.9 Mpc is presented. A new method is introduced to analyze low-resolution (~5 A) spectra, which yields metallicities accurate to 0.2 dex including the uncertainties arising from the errors in Teff (5%) and log g (0.2 dex). For the first time the stellar metallicity gradient based on elements such as titanium and iron in a galaxy beyond the Local Group is investigated. Solar metallicity is measured in the center and 0.3 solar in the outskirts and a logarithmic gradient of –0.08 dex kpc−1. An average reddening of E(B − V) ~ 0.12 mag is obtained, however, with a large variation from 0.07 to 0.24 mag. We also determine stellar radii, luminosities, and masses and discuss the evolutionary status. Finally, the observed relationship between absolute bolometric magnitudes Mbol and flux-weighted gravities gF = g/T4eff is investigated. At high temperatures the strength of the Balmer lines depends solely on the flux-weighted gravity, which allows a precise direct determination of log gF with an accuracy of 0.05-0.1 dex. We find a tight relationship between Mbol and log gF in agreement with stellar evolution theory. Combining these new results with previous work on Local Group galaxies, we obtain a new flux-weighted gravity-luminosity relationship (FGLR), which is very well defined and appears to be an excellent alternative tool to determine distances to galaxies.

Helium enhanced stars and multiple populations along the horizontal branch of NGC 2808: Direct spectroscopic measurements

We present an abundance analysis of 96 horizontal branch (HB) stars in NGC 2808, a globular cluster exhibiting a complex multiple stellar population p attern. These stars are distributed in different portions of the HB and cover a wide range of temperature. By studying the chemical abundances of this sample, we explore the connection between HB morphology and the chemical enrichment history of multiple stellar populatio ns. For stars lying on the red HB, we use GIRAFFE and UVES spectra to determine Na, Mg, Si, Ca, Sc, Ti, Cr, Mn, Fe, Ni, Zn, Y, Ba, and Nd abundances. For colder, blue HB stars, we derive abundances for Na, primarily from GIRAFFE spectra. We were also able to measure direct NLTE He abundances for a subset of these blue HB stars with temperature higher than∼9000 K. Our results show that: (i) HB stars in NGC 2808 show different content in Na depending on their position in the color-magnitude diagram, with blue HB stars having higher Na than red HB stars; (ii) the red HB is not consistent with an uniform chemical abundance, with slightly warmer stars exhibiting a statistically significant higher Na content; and (iii) our subsample of blue HB stars with He abundances shows evidence of enhancement with respect to the predicted primordial He ‐ ‐ ‐ ‐

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.

QUANTITATIVE SPECTROSCOPY OF BLUE SUPERGIANT STARS IN THE DISK OF M81: METALLICITY, METALLICITY GRADIENT, AND DISTANCE

The quantitative spectral analysis of low-resolution (~5 ?) Keck LRIS spectra of blue supergiants in the disk of the giant spiral galaxy M81 is used to determine stellar effective temperatures, gravities, metallicities, luminosities, interstellar reddening, and a new distance using the flux-weighted gravity-luminosity relationship. Substantial reddening and extinction are found with E(B ? V) ranging between 0.13 and 0.38?mag and an average value of 0.26?mag. The distance modulus obtained after individual reddening corrections is 27.7 ? 0.1?mag. The result is discussed with regard to recently measured tip of the red giant branch and Cepheid distances. The metallicities (based on elements such as iron, titanium, magnesium) are supersolar (0.2?dex) in the inner disk (R? 5?kpc) and slightly subsolar ( ? 0.05?dex) in the outer disk (R? 10?kpc) with a shallow metallicity gradient of 0.034?dex?kpc?1. The comparison with published oxygen abundances of planetary nebulae and metallicities determined through fits of Hubble Space Telescope color-magnitude diagrams indicates a late metal enrichment and a flattening of the abundance gradient over the last 5?Gyr. This might be the result of gas infall from metal-rich satellite galaxies. Combining these M81 metallicities with published blue supergiant abundance studies in the Local Group and the Sculptor Group, a galaxy mass-metallicity relationship based solely on stellar spectroscopic studies is presented and compared with recent studies of Sloan Digital Sky Survey star-forming galaxies.

The Araucaria Project: The Local Group Galaxy WLM—Distance and Metallicity from Quantitative Spectroscopy of Blue Supergiants

The quantitative analysis of low-resolution spectra of A and B supergiants is used to determine a distance modulus of 24.99 ± 0.10 mag (995 ± 46 kpc) to the Local Group galaxy WLM. The analysis yields stellar effective temperatures and gravities, which provide a distance through the flux-weighted gravity-luminosity relationship (FGLR). Our distance is 0.07 mag larger than the most recent results based on Cepheids and the tip of the red giant branch. This difference is within the 1 σ overlap of the typical uncertainties quoted in these photometric investigations. In addition, non-LTE spectral synthesis of the rich metal-line spectra (mostly iron, chromium, and titanium) of the A supergiants is carried out, which allows the determination of stellar metallicities. An average metallicity of –0.87 ± 0.06 dex with respect to solar metallicity is found.

Spectroscopy of Blue Supergiants in the Spiral Galaxy NGC 300

We have obtained VLT low-resolution (~5 ?) multiobject spectroscopy in the 4000-5000 ? spectral range of about 70 blue supergiant candidates in the Sculptor Group spiral galaxy NGC 300, selected from previous wide-field photometry. Of the 62 spectroscopically confirmed blue supergiants, with spectral types ranging from late O to F, 57 have types between early B and mid-A. We present a detailed spectral catalog containing identification, magnitudes, colors, and spectral types. We employ synthetic spectra calculated from blue supergiant model atmospheres for different metallicities to determine metal abundances for two A0 supergiants of the sample. In agreement with the expectations, the star closer to the galactic center is found to be more metal-rich than the object at a larger galactocentric distance. We will employ this technique on the whole supergiant sample to determine the stellar abundance gradient in the disk of NGC 300, together with the internal reddening from a comparison of the observed versus synthetic colors. This will allow, among other things, an accurate calibration of the effect of metallicity on the Cepheid period-luminosity relation. Using the Balmer H? line profile, we have estimated the mass-loss rate for one of the brightest A2 supergiants in the sample. Under additional reasonable assumptions, we determined the wind momentum of the star and compared it with the value expected from the empirical wind momentum-luminosity relationship (WLR) for A-type supergiants of Kudritzki et al. Good agreement is obtained. We will derive mass-loss rates and wind momenta for all stars in our sample from the H? line profiles in forthcoming work, and we will then thoroughly test the usefulness of the WLR for distance measurement out to about 15 Mpc.

B fields in OB stars (BOB): Low-resolution FORS2 spectropolarimetry of the first sample of 50 massive stars

Within the context of the collaboration “B fields in OB stars” (BOB), we used the FORS2 low-resolution spectropolarimeter to search for a magnetic field in 50 massive stars, including two reference magnetic massive stars. Because of the many controversies of magnetic field detections obtained with the FORS instruments, we derived the magnetic field values with two completely independent reduction and analysis pipelines. We compare and discuss the results obtained from the two pipelines. We obtained a general good agreement, indicating that most of the discrepancies on magnetic field detections reported in the literature are caused by the interpretation of the significance of the results (i.e., 3–4σ detections considered as genuine, or not), instead of by significant differences in the derived magnetic field values. By combining our results with past FORS1 measurements of HD 46328, we improve the estimate of the stellar rotation period, obtaining P = 2.17950 ± 0.00009 days. For HD 125823, our FORS2 measurements do not fit the available magnetic field model, based on magnetic field values obtained 30 years ago. We repeatedly detect a magnetic field for the O9.7V star HD 54879, the HD 164492C massive binary, and the He-rich star CPD −57 3509. We obtain a magnetic field detection rate of 6 ± 4%, while by considering only the apparently slow rotators we derive a detection rate of 8 ± 5%, both comparable with what was previously reported by other similar surveys. We are left with the intriguing result that, although the large majority of magnetic massive stars is rotating slowly, our detection rate is not a strong function of the stellar rotational velocity.

Evolution of surface CNO abundances in massive stars

The nitrogen to carbon (N/C) and nitrogen to oxygen (N/O) ratios are the most sensitive quantities to mixing in stellar interiors of intermediate and massive stars. We further investigate the theoretical properties of these ratios as well as put in context recent observational results obtained by the VLT-FLAMES Survey of massive stars in the Galaxy and the Magellanic Clouds. We consider analytical relations and numerical models of stellar evolution as well as our own stellar atmosphere models, and we critically re-investigate observed spectra. On the theoretical side, the N/C vs N/O plot shows little dependence on the initial stellar masses, rotation velocities, and nature of the mixing processes up to relative enrichment of N/O by a factor of about four, thus this plot constitutes an ideal quality test for observational results. The comparison between the FLAMES Survey and theoretical values shows overall agreement, despite the observational scatter of the published results. The existence of some mixing of CNO products is clearly confirmed, however the accuracy of the data is not sufficient for allowing a test of the significant differences between different models of rotating stars and the Geneva models. We discuss reasons (for the most part due to observational bias) why part of the observational data points should not be considered for this comparison. When these observational data points are not considered, the scatter is reduced. Finally, the N/C vs N/O plot potentially offers a powerful way for discriminating blue supergiants before the red supergiant stage from those after it. Also, red supergiants of similar low velocities may exhibit different N enrichments, depending on their initial rotation during the main-sequence phase.