C_2 in Peculiar DQ White Dwarfs

White dwarfs (WDs) with carbon absorption features in their optical spectra are known as DQ WDs. The subclass of peculiar DQ WDs are cool objects (T_eff<6000 K) which show molecular absorption bands that have centroid wavelengths ~100-300 Angstroms shortward of the bandheads of the C_2 Swan bands. These "peculiar DQ bands" have been attributed to a hydrocarbon such as C_2H. We point out that C_2H does not show strong absorption bands with wavelengths matching those of the peculiar DQ bands and neither does any other simple molecule or ion likely to be present in a cool WD atmosphere. The most straightforward explanation for the peculiar DQ bands is that they are pressure-shifted Swan bands of C_2. While current models of WD atmospheres suggest that, in general, peculiar DQ WDs do not have higher photospheric pressures than normal DQ WDs do, that finding requires confirmation by improved models of WD atmospheres and of the behavior of C_2 at high pressures and temperatures. If it is eventually shown that the peculiar DQ bands cannot be explained as pressure-shifted Swan bands, the only explanation remaining would seem to be that they arise from highly rotationally excited C_2 (J_peak>45). In either case, the absorption band profiles can in principle be used to constrain the pressure and the rotational temperature of C_2 in the line-forming regions of normal and peculiar DQ WD atmospheres, which will be useful for comparison with models. Finally, we note that progress in understanding magnetic DQ WDs may require models which simultaneously consider magnetic fields, high pressures and rotational excitation of C_2.Comment: ApJ in press. 8 pages emulateapj style, 1 figur

Fermat, Leibniz, Euler, and the gang: The true history of the concepts of limit and shadow

Fermat, Leibniz, Euler, and Cauchy all used one or another form of approximate equality, or the idea of discarding "negligible" terms, so as to obtain a correct analytic answer. Their inferential moves find suitable proxies in the context of modern theories of infinitesimals, and specifically the concept of shadow. We give an application to decreasing rearrangements of real functions.Comment: 35 pages, 2 figures, to appear in Notices of the American Mathematical Society 61 (2014), no.

Entanglement and chaos in the kicked top

The standard kicked top involves a periodically kicked angular momentum. By considering this angular momentum as a collection of entangled spins, we compute the bipartite entanglement dynamics as a function of the dynamics of the classical counterpart. Our numerical results indicate that the entanglement of the quantum top depends on the specific details of the dynamics of the classical top rather than depending universally on the global properties of the classical regime. These results are grounded on linking the entanglement rate to averages involving the classical angular momentum, thereby explaining why regular dynamics can entangle as efficiently as the classically chaotic regime. The findings are in line with previous results obtained with a 2-particle top model, and we show here that the standard kicked top can be obtained as a limiting case of the 2-particle top

Strong fragmentation of low-energy electromagnetic excitation strength in 117^{117}Sn

Results of nuclear resonance fluorescence experiments on $^{117}$Sn are reported. More than 50 $\gamma$ transitions with $E_{\gamma} < 4$ MeV were detected indicating a strong fragmentation of the electromagnetic excitation strength. For the first time microscopic calculations making use of a complete configuration space for low-lying states are performed in heavy odd-mass spherical nuclei. The theoretical predictions are in good agreement with the data. It is concluded that although the E1 transitions are the strongest ones also M1 and E2 decays contribute substantially to the observed spectra. In contrast to the neighboring even $^{116-124}$Sn, in $^{117}$Sn the $1^-$ component of the two-phonon $[2^+_1 \otimes 3^-_1]$ quintuplet built on top of the 1/2$^+$ ground state is proved to be strongly fragmented.Comment: 4 pages, 3 figure

[NiFe]-hydrogenase maturation in vitro: analysis of the roles of the HybG and HypD accessory proteins

[NiFe]-hydrogenases (Hyd) bind a nickel-iron-based cofactor. The Fe ion of the cofactor is bound by two cyanide ligands and a single carbon monoxide ligand. Minimally six accessory proteins (HypA–HypF) are necessary for NiFe(CN)2CO cofactor biosynthesis in Escherichia coli. It has been shown that the anaerobically purified HypC–HypD–HypE scaffold complex carries the Fe(CN)2CO moiety of this cofactor. In the present study, we have purified the HybG–HypDE complex and used it to successfully reconstitute in vitro active Hyd from E. coli. HybG is a homologue of HypC that is specifically required for the maturation of Hyd-2 and also functions in the maturation of Hyd-1 of E. coli. Maturation of active Hyd-1 and Hyd-2 could be demonstrated in extracts derived from HybG- and HypD-deficient E. coli strains by adding anaerobically purified HybG–HypDE complex. In vitro maturation was dependent on ATP, carbamoylphosphate, nickel and reducing conditions. Hydrogenase maturation was prevented when the purified HybG–HypDE complex used in the maturation assay lacked a bound Fe(CN)2CO moiety. These findings demonstrate that it is possible to isolate incompletely processed intermediates on the maturation pathway and to use these to activate apo-forms of [NiFe]-hydrogenase large subunits

Search for the electric dipole excitations to the 3s1/2⊗[21+⊗31−]3s_{1/2} \otimes [2^{+}_{1} \otimes 3^{-}_{1}] multiplet in 117^{117}Sn

The odd-mass $^{117}$Sn nucleus was investigated in nuclear resonance fluorescence experiments up to an endpoint energy of the incident photon spectrum of 4.1 MeV at the bremsstrahlung facility of the Stuttgart University. More than 50 mainly hitherto unknown levels were found. From the measurement of the scattering cross sections model independent absolute electric dipole excitation strengths were extracted. The measured angular distributions suggested the spins of 11 excited levels. Quasi-particle phonon model calculations including a complete configuration space were performed for the first time for a heavy odd-mass spherical nucleus. These calculations give a clear insight in the fragmentation and distribution of the $E1$, $M1$, and $E2$ excitation strength in the low energy region. It is proven that the $1^{-}$ component of the two-phonon $[2^{+}_{1} \otimes 3^{-}_{1}]$ quintuplet built on top of the $1/2^{+}$ ground state is strongly fragmented. The theoretical calculations are consistent with the experimental data.Comment: 10 pages, 5 figure

Optical excitations in organic molecules, clusters and defects studied by first-principles Green's function methods

Spectroscopic and optical properties of nanosystems and point defects are discussed within the framework of Green's function methods. We use an approach based on evaluating the self-energy in the so-called GW approximation and solving the Bethe-Salpeter equation in the space of single-particle transitions. Plasmon-pole models or numerical energy integration, which have been used in most of the previous GW calculations, are not used. Fourier transforms of the dielectric function are also avoided. This approach is applied to benzene, naphthalene, passivated silicon clusters (containing more than one hundred atoms), and the F center in LiCl. In the latter, excitonic effects and the $1s \to 2p$ defect line are identified in the energy-resolved dielectric function. We also compare optical spectra obtained by solving the Bethe-Salpeter equation and by using time-dependent density functional theory in the local, adiabatic approximation. From this comparison, we conclude that both methods give similar predictions for optical excitations in benzene and naphthalene, but they differ in the spectra of small silicon clusters. As cluster size increases, both methods predict very low cross section for photoabsorption in the optical and near ultra-violet ranges. For the larger clusters, the computed cross section shows a slow increase as function of photon frequency. Ionization potentials and electron affinities of molecules and clusters are also calculated.Comment: 9 figures, 5 tables, to appear in Phys. Rev. B, 200

Rotational and vibrational spectra of quantum rings

One can confine the two-dimensional electron gas in semiconductor heterostructures electrostatically or by etching techniques such that a small electron island is formed. These man-made ``artificial atoms'' provide the experimental realization of a text-book example of many-particle physics: a finite number of quantum particles in a trap. Much effort was spent on making such "quantum dots" smaller and going from the mesoscopic to the quantum regime. Far-reaching analogies to the physics of atoms, nuclei or metal clusters were obvious from the very beginning: The concepts of shell structure and Hund's rules were found to apply -- just as in real atoms! In this Letter, we report the discovery that electrons confined in ring-shaped quantum dots form rather rigid molecules with antiferromagnetic order in the ground state. This can be seen best from an analysis of the rotational and vibrational excitations