Efficient electrocatalytic water oxidation at neutral and high pH by adventitious nickel at nanomolar concentrations

Electrolytic water oxidation using earth-abundant elements is a key challenge in the quest to develop cheap, large surface area arrays for solar-to-hydrogen conversion. There have been numerous studies in this area in recent years, but there remains an imperative to demonstrate that the current densities reported are indeed due to the species under consideration and not due to the presence of adventitious (yet possibly highly active) contaminants at low levels. Herein, we show that adventitious nickel at concentrations as low as 17 nM can act as a water oxidation catalyst in mildly basic aqueous solutions, achieving stable (tens of hours) current densities of 1 mA cm–2 at overpotentials as low as 540 mV at pH 9.2 and 400 mV at pH 13. This nickel was not added to the electrolysis baths deliberately, but it was found to be present in the electrolytes as an impurity by ICP-MS. The presence of nickel on anodes from extended-time bulk electrolysis experiments was confirmed by XPS. In showing that such low levels of nickel can perform water oxidation at overpotentials comparable to many recently reported water oxidation catalysts, this work serves to raise the burden of proof required of new materials in this field: contamination by adventitious metal ions at trace loadings must be excluded as a possible cause of any observed water oxidation activity

Improved preparation of 9-octadecenes

Organic synthesis of cis-9 and trans-9 octadecenes from oleyl alcohol and elaidyl alcohol, respectively, by conversion to tosylates followed by lithium aluminum hydride reductio

Comparison of Global and Local Adaptive Coordinates for Density Functional Calculations

A globally-adaptive curvilinear coordinate formalism is shown to be easily convertible to a class of curvilinear transformations locally optimized around atom sites by a few parameters. Parameter transferability is established for a demanding test case, and the results of the two methods are shown to be comparable. Computational efficiencies realized in the local method are discussed.Comment: 21 pages, 4 figure

Quasar outflow energetics from broad absorption line variability

Quasar outflows have long been recognized as potential contributors to the co-evolution between supermassive black holes (SMBHs) and their host galaxies. The role of outflows in AGN feedback processes can be better understood by placing observational constraints on wind locations and kinetic energies. We utilize broad absorption line (BAL) variability to investigate the properties of a sample of 71 BAL quasars with P$\thinspace$V broad absorption. The presence of P$\thinspace$V BALs indicates that other BALs like C$\thinspace$IV are saturated, such that variability in those lines favours clouds crossing the line of sight. We use these constraints with measurements of BAL variability to estimate outflow locations and energetics. Our data set consists of multiple-epoch spectra from the Sloan Digital Sky Survey and MDM Observatory. We detect significant (4$\sigma$) BAL variations from 10 quasars in our sample over rest frame time-scales between < 0.2-3.8 yr. Our derived distances for the 10 variable outflows are nominally < 1-10 pc from the SMBH using the transverse-motion scenario, and < 100-1000 pc from the central source using ionization-change considerations. These distances, in combination with the estimated high outflow column densities (i.e. $N_{\textrm{H}}$ > 10$^{22}$ cm$^{-2}$), yield outflow kinetic luminosities between ~ 0.001-1 times the bolometric luminosity of the quasar, indicating that many absorber energies within our sample are viable for AGN feedback.Comment: 19 pages, 3 figures, 4 tables, 1 supplementary figure, accepted to MNRA

Weak magnetic fields in central stars of planetary nebulae?

It is not yet clear whether magnetic fields play an essential role in shaping planetary nebulae (PNe), or whether stellar rotation alone and/or a close binary companion can account for the variety of the observed nebular morphologies. In a quest for empirical evidence verifying or disproving the role of magnetic fields in shaping PNe, we follow up on previous attempts to measure the magnetic field in a representative sample of PN central stars. We obtained low-resolution polarimetric spectra with FORS 2 at VLT for a sample of twelve bright central stars of PNe with different morphology, including two round nebulae, seven elliptical nebulae, and three bipolar nebulae. Two targets are Wolf-Rayet type central stars. For the majority of the observed central stars, we do not find any significant evidence for the existence of surface magnetic fields. However, our measurements may indicate the presence of weak mean longitudinal magnetic fields of the order of 100 Gauss in the central star of the young elliptical planetary nebula IC 418, as well as in the Wolf-Rayet type central star of the bipolar nebula Hen2-113 and the weak emission line central star of the elliptical nebula Hen2-131. A clear detection of a 250 G mean longitudinal field is achieved for the A-type companion of the central star of NGC 1514. Some of the central stars show a moderate night-to-night spectrum variability, which may be the signature of a variable stellar wind and/or rotational modulation due to magnetic features. We conclude that strong magnetic fields of the order of kG are not widespread among PNe central stars. Nevertheless, simple estimates based on a theoretical model of magnetized wind bubbles suggest that even weak magnetic fields below the current detection limit of the order of 100 G may well be sufficient to contribute to the shaping of PNe throughout their evolution.Comment: 16 pages, 11 figures, 3 tables, accepted for publication in A&A; References updated, minor correction

Constraining FeLoBAL outflows from absorption line variability

FeLoBALs are a rare class of quasar outflows with low-ionization broad absorption lines (BALs), large column densities, and potentially large kinetic energies that might be important for `feedback' to galaxy evolution. In order to probe the physical properties of these outflows, we conducted a multiple-epoch, absorption line variability study of 12 FeLoBAL quasars spanning a redshift range between 0.7 and 1.9 over rest frame time-scales of approximately 10 d to 7.6 yr. We detect absorption line variability with greater than 8 sigma confidence in 3 out of the 12 sources in our sample over time-scales of 0.6 to 7.6 yr. Variable wavelength intervals are associated with ground and excited state Fe II multiplets, the Mg II 2796, 2803 doublet, Mg I 2852, and excited state Ni II multiplets. The observed variability along with evidence of saturation in the absorption lines favors transverse motions of gas across the line of sight (LOS) as the preferred scenario, and allows us to constrain the outflow distance from the supermassive black hole (SMBH) to be less than 69, 7, and 60 pc for our three variable sources. In combination with other studies, these results suggest that the outflowing gas in FeLoBAL quasars resides on a range of scales and includes matter within tens of parsecs of the central source.Comment: 21 pages, 6 figures, 2 supplementary figures (attached at the end of the manuscript), accepted to Monthly Notices of the Royal Astronomical Societ

Highly Ionized Collimated Outflow from HE 0238 - 1904

We present a detailed analysis of a highly ionized, multiphased and collimated outflowing gas detected through O V, O VI, Ne VIII and Mg X absorption associated with the QSO HE 0238 - 1904 (z_em ~ 0.629). Based on the similarities in the absorption line profiles and estimated covering fractions, we find that the O VI and Ne VIII absorption trace the same phase of the absorbing gas. Simple photoionization models can reproduce the observed N(Ne VIII), N(O VI) and N(Mg X) from a single phase whereas the low ionization species (e.g. N III, N IV, O IV) originate from a different phase. The measured N(Ne VIII)/N(O VI) ratio is found to be remarkably similar (within a factor of ~ 2) in several individual absorption components kinematically spread over ~ 1800 km/s. Under photoionization this requires a fine tuning between hydrogen density (nH) and the distance of the absorbing gas from the QSO. Alternatively this can also be explained by collisional ionization in hot gas with T > 10^{5.7} K. Long-term stability favors the absorbing gas being located outside the broad line region (BLR). We speculate that the collimated flow of such a hot gas could possibly be triggered by the radio jet interaction.Comment: Minor revision (accepted for publication in MNRAS letter

Low-metallicity massive single stars with rotation. II. Predicting spectra and spectral classes of chemically-homogeneously evolving stars

Context. Metal-poor massive stars are supposed to be progenitors of certain supernovae, gamma-ray bursts and compact object mergers, potentially contributing to the early epochs of the Universe with their strong ionizing radiation. However, they remain mainly theoretical as individual spectroscopic observations of such objects have rarely been carried out below the metallicity of the SMC. Aims. This work aims at exploring what our state-of-the-art theories of stellar evolution combined with those of stellar atmospheres predict about a certain type of metal-poor (0.02 Z$_{\odot}$) hot massive stars, the chemically homogeneously evolving ones, called TWUIN stars. Methods. Synthetic spectra corresponding to a broad range in masses (20-130 M$_{\odot}$) and covering several evolutionary phases from the zero-age main-sequence up to the core helium-burning stage were computed. Results. We find that TWUIN stars show almost no emission lines during most of their {core hydrogen-burning} lifetimes. Most metal lines are completely absent, including nitrogen. During their core helium-burning stage, lines switch to emission and even some metal lines (oxygen and carbon, but still almost no nitrogen) show up. Mass loss and clumping play a significant role in line-formation in later evolutionary phases, particularly during core helium-burning. Most of our spectra are classified as an early O type giant or supergiant, and we find Wolf-Rayet stars of type WO in the core helium-burning phase. Conclusions. An extremely hot, early O type star observed in a low-metallicity galaxy could be the outcome of chemically homogeneous evolution $-$ and therefore the progenitor of a long-duration gamma-ray burst or a type Ic supernova. TWUIN stars may play an important role in reionizing the Universe due to their being hot without showing prominent emission lines during the majority of their lifetimes.Comment: Accepted by Astronomy and Astrophysics. In Pres

Assumptions of the primordial spectrum and cosmological parameter estimation

The observables of the perturbed universe, CMB anisotropy and large structures, depend on a set of cosmological parameters, as well as, the assumed nature of primordial perturbations. In particular, the shape of the primordial power spectrum (PPS) is, at best, a well motivated assumption. It is known that the assumed functional form of the PPS in cosmological parameter estimation can affect the best fit parameters and their relative confidence limits. In this paper, we demonstrate that a specific assumed form actually drives the best fit parameters into distinct basins of likelihood in the space of cosmological parameters where the likelihood resists improvement via modifications to the PPS. The regions where considerably better likelihoods are obtained allowing free form PPS lie outside these basins. In the absence of a preferred model of inflation, this raises a concern that current cosmological parameters estimates are strongly prejudiced by the assumed form of PPS. Our results strongly motivate approaches toward simultaneous estimation of the cosmological parameters and the shape of the primordial spectrum from upcoming cosmological data. It is equally important for theorists to keep an open mind towards early universe scenarios that produce features in the PPS.Comment: 11 pages, 2 figures, discussions extended, main results unchanged, matches published versio