Present-day cosmic abundances

Abstract

Context. Early B-type stars are ideal indicators for present-day cosmic abundances since they preserve their pristine abundances and typically do not migrate far beyond their birth environments over their short lifetimes, in contrast to older stars like the Sun. They are also unaffected by depletion onto dust grains, unlike the cold/warm interstellar medium (ISM) or H ii regions. Aims. A carefully selected sample of early B-type stars in OB associations and the field within the solar neighbourhood is studied comprehensively. Quantitative spectroscopy is used to characterise their atmospheric properties in a self-consistent way. 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 FOCES, FEROS and ELODIE spectra of well-studied sharp-lined early B-type stars are analysed in non-LTE. Line-profile fits based on extensive model grids and an iterative analysis methodology are used to constrain stellar parameters and elemental abundances at high accuracy and precision. Atmospheric parameters are derived from the simultaneous establishment of independent indicators, from multiple ionization equilibria and the Stark-broadened hydrogen Balmer lines, and they are confirmed by reproduction 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 spectral range. Three sample stars, γ Ori, o Per and θ 1 Ori D, are identified as double-lined, indicating the presence of an early/mid B-type companion. 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 carbonaceous dust is largely destroyed inside the Orion H ii region, unlike the silicates, and that graphite is only a minority species in interstellar dust -, (iii) show that the mixing of CNO-burning products in the course of massive star evolution follows tightly the predicted nuclear path, (iv) provide reliable present-day reference points for anchoring Galactic chemical evolution models to observation, and (v) imply that the Sun has migrated outwards from the inne...

Possible Overionization of C II, N II, and O II Ions in the Atmospheres of Early B- and Late O-Type Stars

“…In addition the accuracy of the determination of the C, N and O abundances was increased, also for the B-type MS stars. The most recent work on these stars [6,7] shows that the average abundances of nitrogen and oxygen in B-type MS stars are now in good agreement with modern estimates for the sun. However, there is still a small deficiency for C. For example, an average carbon abundance of ( ) 04 0 33 8 C log .…”

“…. ± = ε has been obtained for a sample of 29 B-type MS stars [6], and a close value of ( ) 13 0 31 8 C log . .…”

“…However, some recent calculations provide an alternative view, according to which the sun was substantially closer to the center of the galaxy when it was formed than now. In that case its chemical composition may differ from that of the young stars in its current environment [6]. It should be noted that, on one hand, calculations of the migration of stars (including the sun) in the galactic disk are quite difficult and are still quite uncertain.…”

Forbidden Iron Lines and Dust Destruction in Supernova Remnant Shocks: The Case of N49 in the Large Magellanic Cloud

“…In these models, we have adopted a chemical abundance set of 0.5 times the local Galactic concordance (LGC) abundances (see Nicholls 2016, in press). These are based upon the Nieva & Przybilla (2012) observations of early B-star data, and have the advantage that they sample the abundances in the local region of the Galaxy (out to 500 pc), providing the current abundances in this region. The Nieva & Przybilla (2012) data also provide the abundances of the main coolants: H, He, C, N, O, Ne, Mg, Si, and Fe.…”

“…These are based upon the Nieva & Przybilla (2012) observations of early B-star data, and have the advantage that they sample the abundances in the local region of the Galaxy (out to 500 pc), providing the current abundances in this region. The Nieva & Przybilla (2012) data also provide the abundances of the main coolants: H, He, C, N, O, Ne, Mg, Si, and Fe. For the light elements, we use the Lodders et al (2009) abundances, while for all other elements the abundances are based upon Scott et al (2015aScott et al ( , 2015b and Grevesse et al (2015).…”