Subscribe to the gold package and get unlimited access to Shamra Academy
Register a new userBimodal production of Be and B in the early Galaxy
44
0
0.0
(
0
)
Ask ChatGPT about the research
No Arabic abstract
Recently models based on the acceleration of metal-rich material inside superbubbles have been proposed to account for the observed abundances of Be and B in metal-poor halo stars. We analyse some of the implications of these models for the distribution of the Be/O and B/O abundance ratios. In particular, we discuss the possible scatter in the data and argue that LiBeB production in the very early Galaxy was probably bimodal, with isolated supernovae giving rise to a low-efficiency mechanism, and collective supernovae exploding in an OB association inducing a high-efficiency mechanism. This should produce two populations of halo stars, one with high L/M ratios (light elements/metals), and the other with L/M ratios about ten times lower. The relative weight of these two populations depends on the fraction of supernovae exploding inside superbubbles. In this context, we discuss the recent observation of the B-depleted, Li-normal star HD 160617 (Primas, et al., 1998), as well as the reported spread in the Be data at [Fe/H] ~ -2.2 (Boesgaard, et al., 1999). Finally, we predict that Be will be found to be even more deficient than B in HD 160617.
rate research
Read More
We reassess the problem of the production and evolution of the light elements Li, Be and B and of their isotopes in the Milky Way, in the light of new observational and theoretical developments. The main novelty is the introduction of a new scheme for the origin of Galactic cosmic rays (GCR), which for the first time enables a self-consistent calculation of their composition during galactic evolution. The scheme accounts for key features of the present-day GCR source composition, it is based on the wind yields of the Geneva models of rotating, mass losing stars and it is fully coupled to a detailed galactic chemical evolution code. We find that the adopted GCR source composition accounts naturally for the observations of primary Be and helps understanding why Be follows closer Fe than O. We find that GCR produce ~70% of the solar B11/B10 isotopic ratio; the remaining 30% of B11 presumably result from neutrino-nucleosynthesis in massive star explosions. We find that GCR and primordial nucleosynthesis can make at most 30% of solar Li. At least half of solar Li has to originate in low-mass stellar sources (red giants, asymptotic giant branch stars or novae), but the required average yields of those sources are found to be much larger than obtained in current models of stellar nucleosynthesis. We also present radial profiles of LiBeB elemental and isotopic abundances in the Milky Way disc. We argue that the shape of those profiles - and the late evolution of LiBeB in general - reveals important features of the production of those light elements through primary and secondary processes.
We investigate the spallative production of the light elements, Li, Be and B (LiBeB), associated with the evolution of a superbubble (SB) blown by repeated SNe in an OB association. It is shown that if about ten percent of the SN energy can power the acceleration of particles from the material inside the SB, the observed abundances of LiBeB in halo stars, as a function of O, can be explained in a fully consistent way over several decades of metallicity. In this model, the energetic particles (EPs) reflect the SB material, which is a mixing of the ejecta of previous SNe and of the swept-up ISM gas evaporated off the shell. We investigated two different energy spectra for the EPs: the standard cosmic ray source spectrum, or `SNR spectrum, and a specific `SB spectrum, as results from the SB acceleration mechanism of Bykov & Fleishman (1992). While the latter spectrum is more efficient in producing LiBeB, the SNR spectrum can be reconciled with the observational data if an imperfect mixing of the SN ejecta with the rest of the SB material and/or a selective acceleration is invoked (enhancing the C and O abundance amongst the EPs by a factor of ~ 6). One of the main consequences of our model is that the observed linear growth of Be and B abundances as a function of Fe/H expresses a dilution line rather than a continuous, monotonic increase of the metallicity. We propose an observational test of this feature. We also show that the recent 6Li observations in halo stars fit equally well in the framework of the SB model. Finally, we conjecture the existence of two sets of low-metallicity stars, differing in their Be/Fe or B/O abundance ratios, resulting from a `bimodal LiBeB production in the Galaxy, namely from correlated (in SBs) or isolated SN explosions.
We examine the Pb and Th abundances in 27 metal-poor stars (-3.1 < [Fe/H] < -1.4) whose very heavy metal (Z > 56) enrichment was produced only by the rapid (r-) nucleosynthesis process. New abundances are derived from HST/STIS, Keck/HIRES, and VLT/UVES spectra and combined with other measurements from the literature to form a more complete picture of nucleosynthesis of the heaviest elements produced in the r-process. In all cases, the abundance ratios among the rare earth elements and the 3rd r-process peak elements considered (La, Eu, Er, Hf, and Ir) are constant and equivalent to the scaled solar system r-process abundance distribution. We compare the stellar observations with r-process calculations within the classical waiting-point approximation. In these computations a superposition of 15 weighted neutron-density components in the range 23 < log(n_n) < 30 is fit to the r-process abundance peaks to successfully reproduce both the stable solar system isotopic distribution and the stable heavy element abundance pattern between Ba and U in low-metallicity stars. Under these astrophysical conditions, which are typical of the main r-process, we find very good agreement between the stellar Pb r-process abundances and those predicted by our model. For stars with anomalously high Th/Eu ratios (the so-called actinide boost), our observations demonstrate that any nucleosynthetic deviations from the main r-process affect--at most--only the elements beyond the 3rd r-process peak, namely Pb, Th, and U. Our theoretical calculations also indicate that possible r-process abundance losses by nuclear fission are negligible for isotopes along the r-process path between Pb and the long-lived radioactive isotopes of Th and U.
We investigate the possibility that inhomogeneous nucleosynthesis may eventually be used to explain the abundances of li6, be9 and B in population II stars. The present work differs from previous studies in that we have used a more extensive reaction network. It is demonstrated that in the simplest scenario the abundances of the light elements with $Ale7$ constrain the separation of inhomogeneities to sufficiently small scales that the model is indistinguishable from homogeneous nucleosynthesis and that the abundances of li6, be9 and B are then below observations by several orders of magnitude. This conclusion does not depend on the li7 constraint. We also examine alternative scenarios which involve a post-nucleosynthesis reprocessing of the light elements to reproduce the observed abundances of Li and B, while allowing for a somewhat higher baryon density (still well below the cosmological critical density). Future B/H measurements may be able to exclude even this exotic scenario and further restrict primordial nucleosynthesis to approach the homogeneous model conclusions.
In early-type stars a fossil magnetic field may be generated during the star formation process or be the result of a stellar merger event. Surface magnetic fields are thought to be erased by (sub)surface convection layers, which typically leave behind weak disordered fields. However, if the fossil field is strong enough it can prevent the onset of (sub)surface convection and so be preserved onto the main sequence. We calculate the critical field strength at which this occurs, and find that it corresponds well with the lower limit amplitude of observed fields in strongly magnetised Ap/Bp stars ($approx$ 300 G). The critical field strength is predicted to increase slightly during the main sequence evolution, which could also explain the observed decline in the fraction of magnetic stars. This supports the conclusion that the bimodal distribution of observed magnetic fields in early-type stars reflects two different field origin stories: strongly magnetic fields are fossils fields inherited from star formation or a merger event, and weak fields are the product of on-going dynamo action.
Log in to be able to interact and post comments
comments
Fetching comments
Sign in to be able to follow your search criteria
