arXiv subjects

P. E. Nissen

Publications and source records attributed to P. E. Nissen.

40 records · Page 3Linked to original sources

arXiv1999

Chemical composition of 90 F and G disk dwarfs

High resolution, high S/N spectra have been obtained for a sample of 90 F and G main-sequence disk stars covering the metallicity range -1.0 < [Fe/H] < +0.1, and have been analysed in a parallel way to the work of Edvardsson et al. (1993). Effective temperatures are based on the Alonso et al. (1996) calibration of color indices and surface gravities are calculated from Hipparcos parallaxes, which also allow more accurate ages to be calculated. In addition, more reliable kinematical parameters are derived from Hipparcos distances and proper motions. Finally, a larger spectral coverage, 5600 - 8800 A, makes it possible to improve the abundance accuracy by studying more lines and to discuss several elements not included in the work of Edvardsson et al. The present paper provides the data and discusses some general results of the abundance survey. A group of stars in the metallicity range of -1.0 < [Fe/H] < -0.6 having a small mean Galactocentric distance in the stellar orbits, Rm < 7 kpc, are shown to be older than the other disk stars and probably belong to the thick disk. Excluding these stars, a slight decreasing trend of [Fe/H] with increasing Rm and age is found, but a large scatter in [Fe/H] (up to 0.5 dex) is present at a given age and Rm. The derived trends of O, Mg, Si, Ca, Ti, Ni and Ba as a function of [Fe/H] agree rather well with those of Edvardsson et al., but the overabundance of Na and Al for metal-poor stars found in their work is not confirmed. Furthermore, the Galactic evolution of elements not included in Edvardsson et al., K, V and Cr, is studied.

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arXiv1999

Sc and Mn abundances in disk and metal-rich halo stars

Sc and Mn abundances are determined for 119 F and G main-sequence stars with -1.4 < [Fe/H] < +0.1, representing stars from the thin disk, the thick disk and the halo. The results indicate that Sc behaves like an alpha-element, showing a decreasing [Sc/Fe] with increasing metallicity in disk stars and a dual pattern in the kinematically selected halo stars. In contrast, Mn shows an increase from [Mn/Fe] = -0.5 at [Fe/H] = -1.4 to zero at solar metallicity. There appears to be a discontinuity or sharp increase of [Mn/Fe] at [Fe/H] = -0.7 corresponding to the transition between the thick and the thin disk. It is discussed if supernovae of Type Ia are a major source of Mn in the Galactic disk or if the trend of [Mn/Fe] vs. [Fe/H] can be explained by nucleosynthesis in Type II supernovae with a strong metallicity dependence of the yield.

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arXiv1999

Isotopic lithium abundances in five metal-poor disk stars

High resolution (R=110000), very high S/N spectra, centered on the 6707.8A LiI line have been obtained with the ESO Coude Echelle Spectrometer for five, metal-poor (-0.8 < [Fe/H] < -0.6) disk stars in the turnoff region of the HR-diagram. The instrumental and stellar atmospheric line broadening have been determined from two unblended iron lines and used in a model atmosphere synthesis of the profile of the LiI line as a function of the lithium isotope ratio. This has led to a detection of 6Li in HD68284 and HD130551 with 6Li/7Li = 0.05, whereas the other stars, HR2883, HR3578 and HR8181, have 6Li/7Li close to zero. By comparing Teff-values and absolute magnitudes based on Hipparcos parallaxes with recent stellar evolutionary tracks, the masses of the stars have been derived. It is shown that the two stars with 6Li present have a significantly higher mass, M = 1.05 M(sun), than the other three stars for which values between 0.85 and 1.0 M(sun) are obtained. The results are discussed in terms of models for the galactic evolution of the light elements and depletion of the lithium isotopes in stellar envelopes. It is shown that the measured 6Li abundances are in agreement with standard cosmic ray production of 6Li in the galactic disk and a moderate depletion (0.5 dex) in the stars. Recent models for the evolution of 6Li including alpha + alpha fusion reactions and predicting a high lithium isotopic ratio, 6Li/7Li = 0.3 at [Fe/H] = -0.6, require a high degree of 6Li depletion (1.0 dex) to fit the observations. Furthermore, these models imply a 7Li abundance about 0.2 dex higher than observed for metal-poor disk stars.

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arXiv1994

Boron in the Extreme Pop II Star HD 140283 and the Production of Light Elements in the Early Galaxy

Using observations of the 2496.7 A B I line with the HST GHRS at a nominal resolution of 90,000, we have found the abundance of boron of HD 140283 to be log epsilon_B (= 12 + log (N_B/N_H)) = 0.34 \pm 0.20. This result is found when a significant non-LTE effect in the formation of the B I line is taken into account. The resulting N_B/N_Be ratio is about 17 (in the range 9 -- 34), which is in very good agreement with what is expected from spallation by cosmic rays. We conclude that this origin of Be and B in the Early Galaxy is the most probable of recently suggested formation mechanisms. (Submitted to A&A. Postscript figures available via annymous ftp to "chiron.astro.uu.se" in directory pub/Preprints/P87)

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