Very deep spectroscopy of the bright Saturn Nebula NGC 7009 -- I. Observations and plasma diagnostics

We present very deep CCD spectrum of the bright, medium-excitation planetary nebula NGC 7009, with a wavelength coverage from 3040 to 11000 A. Traditional emission line identification is carried out to identify all the emission features in the spectra, based on the available laboratory atomic transition data. Since the spectra are of medium resolution, we use multi-Gaussian line profile fitting to deblend faint blended lines, most of which are optical recombination lines (ORLs) emitted by singly ionized ions of abundant second-row elements such as C, N, O and Ne. Computer-aided emission-line identification, using the code EMILI developed by Sharpee et al., is then employed to further identify all the emission lines thus obtained. In total about 1200 emission features are identified, with the faintest ones down to fluxes 10^{-4} of H_beta. The flux errors for all emission lines, estimated from multi-Gaussian fitting, are presented. Plots of the whole optical spectrum, identified emission lines labeled, are presented along with the results of multi-Gaussian fits. Plasma diagnostics using optical forbidden line ratios are carried out. Also derived are electron temperatures and densities from the H I, He I and He II recombination spectrum.Comment: 66 pages, 16 figures, 7 tables, paper accepted by MNRAS in Marc

Spectroscopic Observations of Planetary Nebulae in the Northern Spur of M31

We present spectroscopy of three planetary nebulae (PNe) in the Northern Spur of the Andromeda Galaxy (M31) obtained with the Double Spectrograph on the 5.1 m Hale Telescope at the Palomar Observatory. The samples are selected from the observations of Merrett et al. Our purpose is to investigate formation of the substructures of M31 using PNe as a tracer of chemical abundances. The [O III] 4363 auroral line is detected in the spectra of two objects, enabling temperature determinations. Ionic abundances are derived from the observed collisionally excited lines, and elemental abundances of nitrogen, oxygen, and neon as well as sulphur and argon are estimated. Correlations between oxygen and the alpha-element abundance ratios are studied, using our sample and the M31 disk and bulge PNe from the literature. In one of the three PNe, we observed relatively higher oxygen abundance compared to the disk sample in M31 at similar galactocentric distances. The results of at least one of the three Northern Spur PNe might be in line with the proposed possible origin of the Northern Spur substructure of M31, i.e. the Northern Spur is connected to the Southern Stream and both substructures comprise the tidal debris of the satellite galaxies of M31.Comment: 5 tables, 17 figures; accepted for publication in Ap

Tailoring microstructure and texture of annealed Al-Mn alloy through the variation of homogenization and prior cold deformation strain

The grain structures and crystallographic textures desired for various applications of aluminium alloys are usually modified by recrystallization during annealing. In this study, the interactions between recrystallization and precipitation were investigated using the same Al-Mn alloy but with different homogenization procedures (which gives different microchemistries in terms of solute level and second phase particle state) and prior cold deformation strains, both of which are easy to realize in industrial practice. The results show that recrystallization and precipitation kinetics are both promoted at large deformations but their mutual interactions also exis

Controlling soliton interactions in Bose-Einstein condensates by synchronizing the Feshbach resonance and harmonic trap

We present how to control interactions between solitons, either bright or dark, in Bose-Einstein condensates by synchronizing Feshbach resonance and harmonic trap. Our results show that as long as the scattering length is to be modulated in time via a changing magnetic field near the Feshbach resonance, and the harmonic trapping frequencies are also modulated in time, exact solutions of the one-dimensional nonlinear Schr\"{o}dinger equation can be found in a general closed form, and interactions between two solitons are modulated in detail in currently experimental conditions. We also propose experimental protocols to observe the phenomena such as fusion, fission, warp, oscillation, elastic collision in future experiments.Comment: 7 pages, 7 figure

Mechanism for the Nonadiabatic Photooxidation of Benzene to Phenol: Orientation-Dependent Proton-Coupled Electron Transfer

An efficient catalytic one-step conversion of benzene to phenol was achieved recently by selective photooxidation under mild conditions with 2,3-dichloro-5,6-dicyano-p-benzoquinone (DDQ) as the photocatalyst. Herein, high-level electronic structure calculations in the gas phase and in acetonitrile solution are reported to explore the underlying mechanism. The initially populated 1ππ* state of DDQ can relax efficiently through a nearby dark 1nπ* doorway state to the 3ππ* state of DDQ, which is found to be the precursor state involved in the initial intermolecular electron transfer from benzene to DDQ. The subsequent triplet-state reaction between DDQ radical anions, benzene radical cations, and water is computed to be facile. The formed DDQH and benzene-OH radicals can undergo T1→S0 intersystem crossing and concomitant proton-coupled electron transfer (PCET) to generate the products DDQH2 and phenol. Two of the four considered nonadiabatic pathways involve an orientation-dependent triplet PCET process, followed by intersystem crossing to the ground state (S0). The other two first undergo a nonadiabatic T1→S0 transition to produce a zwitterionic S0 complex, followed by a barrierless proton transfer. The present theoretical study identifies novel types of nonadiabatic PCET processes and provides detailed mechanistic insight into DDQ-catalyzed photooxidation

Labor protection and stock price crash risk: Evidence from international equity markets

We utilize the exogenous intertemporal variation in employment protection across countries and study the impact of employment protection on international equity markets. We find robust evidence that firms located in countries with weak labor protection regulation exhibit a low level of one-year-ahead stock price crash risks relative to those in countries with strong labor protection regulation. It is consistent with the view that when employees face material unemployment risk, they are more incentivized to engage in information search and analysis, thereby curtailing the effectiveness of managerial bad news hoarding activities (i.e., the protection heightening risk hypothesis). Our findings are robust to a battery of sensitivity tests. Further evidence shows that the impact of employment protection on crash risk is more pronounced for firms with a higher proclivity to suppress bad news and for firms in countries with stronger legal enforcement. Our findings have important implications for policymakers concerning equity extreme risk and labor protection around the globe