The chemical composition of the Orion star forming region: stars, gas and dust

We present a summary of main results from the studies performed in the series of papers "The chemical composition of the Orion star forming region". We reinvestigate the chemical composition of B-type stars in the Orion OB1 association by means of state-of-the-art stellar atmosphere codes, atomic models and techniques, and compare the resulting abundances with those obtained from the emission line spectra of the Orion nebula (M42), and recent determinations of the Solar chemical composition.Comment: 5 pages, 4 figures, 2 tables. Poster contribution to the proceedings of the LIAC2010 conference "The multi-wavelength view of hot, massive stars

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.Comment: 12 pages, 8 figures; accepted for publication in Journal of Physics: Conference Series, GREAT-ESF Workshop: Stellar Atmospheres in the Gaia Er

NLTE spectroscopic analysis of the 3^3He anomaly in subluminous B-type stars

Several B-type main-sequence stars show chemical peculiarities. A particularly striking class are the $^3$He stars, which exhibit a remarkable enrichment of $^3$He with respect to $^4$He. This isotopic anomaly has also been found in blue horizontal branch (BHB) and subdwarf B (sdB) stars, which are helium-core burning stars of the extreme horizontal branch. Using a hybrid local/non-local thermodynamic equilibrium (LTE/NLTE) approach for B-type stars, we analyzed high-quality spectra of two known $^3$He BHBs and nine known $^3$He sdBs to determine their isotopic helium abundances and $^4$He/$^3$He abundance ratios. We redetermined their atmospheric parameters and analyzed selected neutral helium lines, including $\lambda$4922 $\mathring{A}$ and $\lambda$6678 $\mathring{A}$, which are very sensitive to $^4$He/$^3$He. Most of the $^3$He sdBs cluster in a narrow temperature strip between 26000 K and 30000 K and are helium deficient in accordance with previous LTE analyses. BD+48$^\circ$ 2721 is reclassified as a BHB star because of its low temperature ($T_{\mathrm{eff}}=$ 20700 K). Whereas $^4$He is almost absent ($^4$He/$^3$He$<$ 0.25) in most of the known $^3$He stars, other sample stars show abundance ratios up to $^4$He/$^3$He$=$2.51. A search for $^3$He stars in the ESO SPY survey led to the discovery of two new $^3$He sdB stars (HE 0929-0424 and HE 1047-0436). The observed helium line profiles of all BHBs and of three sdBs are not matched by chemically homogeneous atmospheres, but hint at vertical helium stratification. This phenomenon has been seen in other peculiar B-type stars, but is found for the first time for sdBs. We estimate helium to increase from the outer to the inner atmosphere by factors ranging from 1.4 (SB 290) up to 8.0 (BD+48$^\circ$ 2721).Comment: 19 pages, 79 figures submitted to Astronomy&Astrophysic

HVS7: a chemically peculiar hyper-velocity star

Context: Hyper-velocity stars are suggested to originate from the dynamical interaction of binary stars with the supermassive black hole in the Galactic centre (GC), which accelerates one component of the binary to beyond the Galactic escape velocity. Aims: The evolutionary status and GC origin of the HVS SDSS J113312.12+010824.9 (HVS7) is constrained from a detailed study of its stellar parameters and chemical composition. Methods: High-resolution spectra of HVS7 obtained with UVES on the ESO VLT were analysed using state-of-the-art NLTE/LTE modelling techniques that can account for a chemically-peculiar composition via opacity sampling. Results: Instead of the expected slight enrichments of alpha-elements and near-solar Fe, huge chemical peculiarities of all elements are apparent. The He abundance is very low (<1/100 solar), C, N and O are below the detection limit, i.e they are underabundant (<1/100, <1/3 and <1/10 solar). Heavier elements, however, are overabundant: the iron group by a factor of ~10, P, Co and Cl by factors ~40, 80 and 440 and rare-earth elements and Hg even by ~10000. An additional finding, relevant also for other chemically peculiar stars are the large NLTE effects on abundances of TiII and FeII (~0.6-0.7dex). The derived abundance pattern of HVS7 is characteristic for the class of chemical peculiar magnetic B stars on the main sequence. The chemical composition and high vsini=55+-2km/s render a low mass nature of HVS7 as a blue horizontal branch star unlikely. Conclusions: Such a surface abundance pattern is caused by atomic diffusion in a possibly magnetically stabilised, non-convective atmosphere. Hence all chemical information on the star's place of birth and its evolution has been washed out. High precision astrometry is the only means to validate a GC origin for HVS7.Comment: 9 pages, 3 figure

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/C vs. 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.Comment: 6 pages, 6 figures. A&A, in pres

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 further 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) sub-solar 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.Comment: 8 pages, 5 figure

B fields in OB stars (BOB): FORS2 spectropolarimetric follow-up of the two rare rigidly rotating magnetosphere stars HD23478 and HD345439

Massive B-type stars with strong magnetic fields and fast rotation are very rare and provide a mystery for theories of both star formation and magnetic field evolution. Only two such stars, called sigma Ori E analogs, were previously known. Recently, a team involved in APOGEE, one of the Sloan Digital Sky Survey III programs, announced the discovery of two additional rigidly rotating magnetosphere stars, HD23478 and HD345439. The presence of magnetic fields in these newly discovered sigma Ori E analogs was not investigated in the past. In the framework of our ESO Large Programme, and one normal ESO programme, we carried out low-resolution FORS2 spectropolarimetric observations of HD23478 and HD345439. From the measurements using hydrogen lines, we discover a rather strong longitudinal magnetic field of the order of up to 1.5kG in HD23478, and up to 1.3kG using the entire spectrum. The analysis of HD345439 using four subsequent spectropolarimetric subexposures does not reveal the presence of a magnetic field at a significance level of 3sigma. On the other hand, the inspection of individual subexposures indicates that HD345439 may host a strong magnetic field, rapidly varying over 88 minutes. A hint at the fast rotation of HD345439 is also given by the behaviour of several metallic and He I lines in the low-resolution FORS2 spectra, showing profile variations already on such a short time scale.Comment: 5 pages, 4 figures, 1 table, accepted for publication as a letter to A&