Structural and vibrational properties of two-dimensional MnxOy\rm Mn_xO_y nanolayers on Pd(100)

Using different experimental techniques combined with density functional based theoretical methods we have explored the formation of interface-stabilized manganese oxide structures grown on Pd(100) at (sub)monolayer coverage. Amongst the multitude of phases experimentally observed we focus our attention on four structures which can be classified into two distinct regimes, characterized by different building blocks. Two oxygen-rich phases are described in terms of MnO(111)-like O-Mn-O trilayers, whereas the other two have a lower oxygen content and are based on a MnO(100)-like monolayer structure. The excellent agreement between calculated and experimental scanning tunneling microscopy images and vibrational electron energy loss spectra allows for a detailed atomic description of the explored models.Comment: 14 pages, 11 figure

Interplay between magnetic, electronic and vibrational effects in monolayer Mn_3O_4 grown on Pd(100)

The surface stabilized MnO(100)-like monolayer, characterised by a regular c(4x2) distribution of Mn vacancies, is studied by hybrid functionals and discussed in the light of available scanning tunneling microscopy and high-resolution electron energy loss spectroscopy data. We show that the use of hybrid functionals is crucial to account for the intermingled nature of magnetic ineractions, electron localization, structural distortions and surface phonons. The proposed Pd(100) supported Mn3O4 structure is excellently compatible with the experiments previously reported in literature.Comment: 16 pages, 5 figure

The Very Highly Ionized Broad Absorption Line System of the QSO SBS1542+541

We have analyzed the broad absorption line system of the bright (V=16.5) high-redshift (z=2.361) QSO SBS1542+541 using UV spectra from the HST FOS along with optical data from the MMT and the Steward Observatory 2.3m telescope. These spectra give continuous wavelength coverage from 1200 to 8000 Angstroms, corresponding to 340 to 2480 Angstroms in the QSO rest frame. This object therefore offers a rare opportunity to study broad absorption lines in the rest-frame extreme UV. We find that the absorption system is dominated by very high-ionization species, including O VI, NeVIII, and SiXII. We also identify apparently saturated broad Lyman-series lines of order Ly-gamma and higher. There is strong evidence for partial occultation of the QSO emission source, particularly from the higher-order Lyman lines which indicate a covered fraction less than 0.2. Overall, the data suggest a correlation between a larger covered fraction and a higher state of ionization. We suggest that the different covered fractions can be explained by either a special line of sight through a disk-like geometry or by the existence of density fluctuations of a factor >2 in the BAL gas. Our photoionization models of the system indicate a large column density and high ionization state similar to that found in X-ray ``warm absorbers''.Comment: 31 pages, 13 figures, to be published in Ap

X-ray imaging of the Seyfert 2 galaxy Circinus with Chandra

We present results from the zeroth-order imaging of a Chandra HETGS observation of the nearby Seyfert 2 galaxy Circinus. Twelve X-ray sources were detected in the ACIS-S image of the galaxy, embedded in diffuse X-ray emission. The latter shows a prominent (~18arcsec) soft ``plume'' in the N-W direction, coincident with the [OIII] ionization cone. The radial profiles of the brightest X-ray source at various energies are consistent with an unresolved (FWHM ~0.8arcsec) component, which we identify as the active nucleus, plus two extended components with FWHMs ~ 2.3arcsec and 18arcsec, respectively. In a radius of 3arcsec, the nucleus contributes roughly the same flux as the extended components at the softest energies (< 2 keV). However, at harder energies (> 2 keV), the contribution of the nucleus is dominant. The zeroth-order ACIS spectrum of the nucleus exhibits emission lines at both soft and hard X-rays, including a prominent Fe Kalpha line at 6.4 keV, showing that most of the X-ray lines previously detected with ASCA originate in a compact region (<15 pc). Based on its X-ray spectrum, we argue that the 2.3arcsec extended component is scattered nuclear radiation from nearby ionized gas. The large-scale extended component includes the emission from the N-W plume and possibly from the outer starburst ring.Comment: Figure 1 in color. ApJ Letters, in pres

X-ray Properties of the Weak Seyfert 1 Nucleus in NGC 4639

We obtained observations of NGC 4639 with ASCA in order to investigate its mildly active Seyfert 1 nucleus at hard X-ray energies. Koratkar et al. (1995) have previously shown that the nucleus is a pointlike source in the ROSAT soft X-ray band. We detected in the 2-10 keV band a compact central source with a luminosity of 8.3E+40 erg/s. Comparison of the ASCA data with archival data taken with the Einstein and ROSAT satellites shows that the nucleus varies on timescales of months to years. The variability could be intrinsic, or it could be caused by variable absorption. More rapid variability, on a timescale of \~10^4 s, may be present in the ASCA data. The spectrum from 0.5 to 10 keV is well described by a model consisting of a lightly absorbed (N_H = 7.3E+20 cm^-2) power law with a photon index of 1.68. We find no evidence for significant emission from a thermal plasma; if present, it can account for no more than 25% of the flux in the 0.5-2.0 keV band. The limited photon statistics of our data do not allow us to place significant limits on the presence of iron K emission. (abridged)Comment: To appear in The Astrophysical Journal. LaTex, 18 pages including embedded figures and table

A search for broad infrared recombination lines in NGC 1068

We report infrared spectroscopy of the prototypical Seyfert 2 galaxy NGC 1068, aiming at detection of broad components of hydrogen recombination lines that originate in the obscured broad-line region. Using the Short Wavelength Spectrometer on board the Infrared Space Observatory, we have observed for the first time the regions of Brackett beta 2.626um and Pfund alpha 7.460um, and present improved data for Brackett alpha 4.052um. No significant broad components are detected, implying an equivalent visual extinction to the broad-line region of at least 50 magnitudes and an obscuring column density of at least 10^23 cm^-2. While consistent with a highly obscured broad-line region, as required by the classical unified scenario, these limits are not yet significant enough to discriminate strongly between different torus models or to constrain properties of the gas causing the very large X-ray obscuration. We discuss the systematic limitations of infrared broad-line region searches and suggest that Brackett alpha may often be the most favorable transition for future searches.Comment: aastex (V4), 4 eps figures. Accepted by Ap

Optically thick clumps: not the solution to the Wolf-Rayet wind momentum problem?

The hot star wind momentum problem &#951; = M&#957;&#8734;/(L/c)&#187; much greater than 1 is revisited, and it is shown that the conventional belief, that it can be solved by a combination of clumping of the wind and multiple scattering of photons, is not self-consistent for optically thick clumps. Clumping does reduce the mass loss rate M, and hence the momentum supply, required to generate a specified radio emission measure epsilon, while multiple scattering increases the delivery of momentum from a specified stellar luminosity L. However, in the case of thick clumps, when combined the two effects act in opposition rather than in unison since clumping reduces multiple scattering. From basic geometric considerations, it is shown that this reduction in momentum delivery by clumping more than offsets the reduction in momentum required, for a specified &#949;. Thus the ratio of momentum deliverable to momentum required is maximal for a smooth wind and the momentum problem remains for the thick clump case. In the case of thin clumps, all of the benefit of clumping in reducing &#951; lies in reducing M for a given &#949; so that extremely small filling factors f &#8776; 10-4 are needed. It is also shown that clumping affects the inference of M from radio &#949; not only by changing the emission measure per unit mass but also by changing the radio optical depth unity radius Rrad, and hence the observed wind volume, at radio wavelengths. In fact, for free-free opacity proportional to &#945;n2, contrary to intuition, Rrad increases with increasing clumpiness

Gd disilicide nanowires attached to Si(111) steps

Self-assembled electronic devices, such as quantum dots or switchable molecules, need self-assembled nanowires as connections. We explore the growth of conducting Gd disilicide nanowires at step arrays on Si(111). Atomically smooth wires with large aspect ratios are formed at low coverage and high growth rate (length >1 micron, width 10nm, height 0.6nm). They grow parallel to the steps in the [-1 1 0 ] direction, which is consistent with a lattice match of 0.8% with the a-axis of the hexagonal silicide, together with a large mismatch in all other directions. This mechanism is similar to that observed previously on Si(100). In contrast to Si(100), the wires are always attached to step edges on Si(111) and can thus be grown selectively on regular step arrays.Comment: 3 pages including 4 figure

Simultaneous Ultraviolet and X-ray Observations of the Seyfert Galaxy NGC 4151. II. Physical Conditions in the UV Absorbers

We present a detailed analysis of the intrinsic absorption in the Seyfert 1 galaxy NGC 4151 using UV spectra from the HST/STIS and FUSE, obtained 2002 May as part of a set of contemporaneous observations that included Chandra/HETGS spectra. In our analysis of the Chandra spectra, we determined that the soft X-ray absorber was the source of the saturated UV lines of O VI, C IV, and N V associated with the absorption feature at a radial velocity of ~ -500 km/sec, which we referred to as component D+E. In the present work, we have derived tighter constrains on the the line-of-sight covering factors, densities, and radial distances of the absorbers. We find that the Equivalent Widths (EWs) of the low-ionization lines associated with D+E varied over the period from 1999 July to 2002 May. The drop in the EWs of these lines between 2001 April and 2002 May are suggestive of bulk motion of gas out of our line-of-sight. If these lines from these two epochs arose in the same sub-component, the transverse velocity of the gas is ~ 2100 km/sec. Transverse velocities of this order are consistent with an origin in a rotating disk, at the roughly radial distance we derived for D+E.Comment: 51 pages, including 12 figures. Accepted for publication in ApJ Supplement