Accretion of Planetary Material onto Host Stars

Accretion of planetary material onto host stars may occur throughout a star's life. Especially prone to accretion, extrasolar planets in short-period orbits, while relatively rare, constitute a significant fraction of the known population, and these planets are subject to dynamical and atmospheric influences that can drive significant mass loss. Theoretical models frame expectations regarding the rates and extent of this planetary accretion. For instance, tidal interactions between planets and stars may drive complete orbital decay during the main sequence. Many planets that survive their stars' main sequence lifetime will still be engulfed when the host stars become red giant stars. There is some observational evidence supporting these predictions, such as a dearth of close-in planets around fast stellar rotators, which is consistent with tidal spin-up and planet accretion. There remains no clear chemical evidence for pollution of the atmospheres of main sequence or red giant stars by planetary materials, but a wealth of evidence points to active accretion by white dwarfs. In this article, we review the current understanding of accretion of planetary material, from the pre- to the post-main sequence and beyond. The review begins with the astrophysical framework for that process and then considers accretion during various phases of a host star's life, during which the details of accretion vary, and the observational evidence for accretion during these phases.Comment: 18 pages, 5 figures (with some redacted), invited revie

The Fe-only nitrogenase from Rhodobacter capsulatus: identification of the cofactor, an unusual, high-nuclearity iron-sulfur cluster, by FeK-edge EXAFS and Fe-57 Mossbauer spectroscopy

Krahn E, Weiss BJR, Krockel M, et al. The Fe-only nitrogenase from Rhodobacter capsulatus: identification of the cofactor, an unusual, high-nuclearity iron-sulfur cluster, by FeK-edge EXAFS and Fe-57 Mossbauer spectroscopy. JOURNAL OF BIOLOGICAL INORGANIC CHEMISTRY. 2002;7(1-2):37-45.Samples of the dithionite-reduced FeFe protein (the dinitrogenase component of the Fe-only nitrogenase) from Rhodobacter capsulatus have been investigated by Fe-57 Mossbauer spectroscopy and by Fe and Zn EXAFS as well as XANES spectroscopy. The analyses were performed on the basis of data known for the FeMo cofactor and the P cluster of Mo nitrogenases. The prominent Fourier transform peaks of the Fe K-edge spectrum are assigned to Fe-S and Fe-Fe interactions at distances of 2.29 Angstrom and 2.63 Angstrom, respectively. A significant contribution to the Fe EXAFS must be assigned to an Fe backscatterer shell at 3.68 Angstrom, which is an unprecedented feature of the trigonal prismatic arrangement of iron atoms found in the FeMo cofactor of nitrogenase MoFe protein crystal structures. Additional (FeFe)-Fe-... interactions at 2.92 Angstrom and 4.05 Angstrom clearly indicate that the principal geometry, of the P cluster is also conserved. Mossbauer spectra of Fe-57-enriched FeFe protein preparations were recorded at 77 K (20 mT) and 4.2 K (20 mT, 6.2 T), whereby the 4.2 K high-field spectrum clearly demonstrates that the cofactor of the Fe-only nitrogenase (FeFe cofactor) is diamagnetic in the dithionite-reduced ("as isolated") state. The evaluation of the 77 K spectrum is in agreement with the assumption that this cofactor contains eight Fe atoms. In the literature, several genetic and biochemical lines of evidence are presented pointing to a significant structural similarity of the FeFe, the FeMo and and the FeV cofactors. The data reported here provide the first spectroscopic evidence for a structural homology of the FeFe cofactor to the heterometal-containing cofactors, thus substantiating that the FeFe cofactor is the largest iron-sulfur cluster so far found in nature