Excitons in insulating cuprates

We study the electronic excitations near the charge-transfer gap in insulating CuO$_2$ planes, starting from a six-band model which includes $p_\pi$ and $d_{xy}$ orbitals and Cu-O nearest-neighbor repulsion $U_{pd}$. While the low lying electronic excitations in the doped system are well described by a modified $t-J$ model, the excitonic states of the insulator include hybrid $d_{xy}-$ $p_\pi$ states of $A_{2g}$ symmetry. We also obtain excitons of symmetries $B_{1g}$ and $E_u$, and eventually $A_{1g}$, which can be explained within a one-band model. The results agree with observed optical absorption and Raman excitations.Comment: 10 pages and 3 figures in postscript format, compressed with uufile

Sher 25: pulsating but apparently alone

The blue supergiant Sher25 is surrounded by an asymmetric, hourglass-shaped circumstellar nebula, which shows similarities to the triple-ring structure seen around SN1987A. From optical spectroscopy over six consecutive nights, we detect periodic radial velocity variations in the stellar spectrum of Sher25 with a peak-to-peak amplitude of ~12 km/s on a timescale of about 6 days, confirming the tentative detec-tion of similar variations by Hendry et al. From consideration of the amplitude and timescale of the signal, coupled with observed line profile variations, we propose that the physical origin of these variations is related to pulsations in the stellar atmosphere, rejecting the previous hypothesis of a massive, short-period binary companion. The radial velocities of two other blue supergiants with similar bipolar nebulae, SBW1 and HD 168625, were also monitored over the course of six nights, but these did not display any significant radial velocity variations.Comment: 9 pages, 7 figures. Accepted for publication in MNRA

B-type supergiants in the SMC: Rotational velocities and implications for evolutionary models

High-resolution spectra for 24 SMC and Galactic B-type supergiants have been analysed to estimate the contributions of both macroturbulence and rotation to the broadening of their metal lines. Two different methodologies are considered, viz. goodness-of-fit comparisons between observed and theoretical line profiles and identifying zeros in the Fourier transforms of the observed profiles. The advantages and limitations of the two methods are briefly discussed with the latter techniques being adopted for estimated projected rotational velocities (\vsini) but the former being used to estimate macroturbulent velocities. Only one SMC supergiant, SK 191, shows a significant degree of rotational broadening (\vsini $\simeq$ 90 \kms). For the remaining targets, the distribution of projected rotational velocities are similar in both our Galactic and SMC samples with larger values being found at earlier spectral types. There is marginal evidence for the projected rotational velocities in the SMC being higher than those in the Galactic targets but any differences are only of the order of 5-10 \kms, whilst evolutionary models predict differences in this effective temperature range of typically 20 to 70 \kms. The combined sample is consistent with a linear variation of projected rotational velocity with effective temperature, which would imply rotational velocities for supergiants of 70 \kms at an effective temperature of 28 000 K (approximately B0 spectral type) decreasing to 32 \kms at 12 000 K (B8 spectral type). For all targets, the macroturbulent broadening would appear to be consistent with a Gaussian distribution (although other distributions cannot be discounted) with an $\frac{1}{e}$ half-width varying from approximately 20 \kms at B8 to 60 \kms at B0 spectral types.Comment: 4 figures, 8 pages, submitted to Astronomy and Astrophysic

PN fast winds: Temporal structure and stellar rotation

To diagnose the time-variable structure in the fast winds of central stars of planetary nebulae (CSPN), we present an analysis of P Cygni line profiles in FUSE satellite far-UV spectroscopic data. Archival spectra are retrieved to form time-series datasets for the H-rich CSPN NGC 6826, IC 418, IC 2149, IC 4593 and NGC 6543. Despite limitations due to the fragmented sampling of the time-series, we demonstrate that in all 5 CSPN the UV resonance lines are variable primarily due to the occurrence of blueward migrating discrete absorption components (DACs). Empirical (SEI) line-synthesis modelling is used to determine the range of fluctuations in radial optical depth, which are assigned to the temporal changes in large-scale wind structures. We argue that DACs are common in CSPN winds, and their empirical properties are akin to those of similar structures seen in the absorption troughs of massive OB stars. Constraints on PN central star rotation velocities are derived from Fast-Fourier Transform analysis of photospheric lines for our target stars. Favouring the causal role of co-rotating interaction regions, we explore connections between normalised DAC accelerations and rotation rates of PN central stars and O stars. The comparative properties suggest that the same physical mechanism is acting to generate large-scale structure in the line-driven winds in the two different settings.Comment: Accepted for publication in MNRAS; 10 pages, 5 figure

The Library of Babel

We show that heavy pure states of gravity can appear to be mixed states to almost all probes. Our arguments are made for $\rm{AdS}_5$ Schwarzschild black holes using the field theory dual to string theory in such spacetimes. Our results follow from applying information theoretic notions to field theory operators capable of describing very heavy states in gravity. For certain supersymmetric states of the theory, our account is exact: the microstates are described in gravity by a spacetime ``foam'', the precise details of which are invisible to almost all probes.Comment: 7 pages, 1 figure, Essay receiving honorable mention in the 2005 Gravity Research Foundation essay competitio

Stachorutes cabagnerensis sp. n., Collembola (Neanuridae) de la región central de España, y una aproximación preliminar a la filogenia del género

A new species of the genus Stachorutes, Stachorutes cabagnerensis n. sp., from central Spain is described. It is characterized by the presence of 6+6 eyes in the head, retinaculum 2+2 teeth, dentes with 5 hairs, and the absence of mucron. A phylogenetic analysis of this genus was attempted. Potential synapomorphies supporting the monophyly of Stachorutes are presented. One member of the genus (the Nearctic S. navajellus) appears as a basal form, phylogenetically distant from the remaining (Old World) species. There is evidence for a monophyletic infrageneric clade with the species S. dematteisi, S. jizuensis and S. sphagnophilus. However, more information is required for further phylogenetic resolution.Se describe una nueva especie del genero Stachorutes de la region central de España. Stachorutes cabagnerensis nov. sp. se caracteriza por la presencia de 6+6 ojos en la cabeza, retinaculum con 2+2 dientes y 5 sedas en cada rama del dentes; la furca carece de mucrón. Se ha efectuado un análisis filogenético. Las sinapomorfias potenciales establecen la monofilia del género. Una especie del mismo, S. navajellus, aparece como forma basal, filogenéticamente distante del resto de especies (Viejo Mundo). Se podría establecer un clado infragenérico con las especies S. dematteisi, S. jizuensis y S. sphagnophilus. Sin embargo, se precisa de mayor información para poder confirmarlo

The IACOB project. VI. On the elusive detection of massive O-type stars close to the ZAMS

The apparent lack of massive O-type stars near the zero-age main sequence (at ages < 2 Myr) is a topic widely discussed. Different explanations for this elusive detection have been proposed, but no firm conclusions have been reached yet. We reassess this empirical result benefiting from the high-quality spectroscopic observations of >400 Galactic O-type stars gathered by the IACOB and OWN surveys. We used temperatures and gravities from a iacob-gbat/fastwind spectroscopic analysis to locate our sample in the Kiel and spectroscopic HR diagrams. We evaluated the completeness of our sample of stars, observational biases using information from the Galactic O star catalog (GOSC), systematics of our methodology, and compare with other recent studies using smaller samples of Galactic O-type stars. We base our discussion on the spectroscopic HR diagram to avoid the use of uncertain distances. We performed a detailed study of the young cluster Trumpler-14 as an example of how Gaia cluster distances can help to construct the associated classical HR diagram. The apparent lack of massive O-type stars near the ZAMS with masses between 30 and 70 Msol persist even when spectroscopic results from a large, non-biased sample of stars are used. We do not find correlation between the dearth of stars and observational biases, limitations of our methodology, or the use of spectroscopic HR diagram instead of the classical one. Investigating the efficiency of mass accretion during the formation process we conclude that an adjustment of the accretion rate towards lower values could reconcile the hotter boundary of detected O-type stars and the theoretical birthline. Last, we discuss that the presence of a small sample of O2-O3.5 stars found closer to the ZAMS might be explained taking into account non-standard star evolution (e.g. binary interaction, mergers, or homogeneous evolution).Comment: 20 pages, 15 figures, accepted for publication in Astronomy & Astrophysic

Phase separation and valence instabilities in cuprate superconductors. Effective one-band model approach

We study the Cu-O valence instability (VI) and the related phase separation (PS) driven by Cu-O nearest-neighbor repulsion $U_{pd}$, using an effective extended one-band Hubbard model ($H_{eff}$) obtained from the extended three-bandHubbard model, through an appropriate low-energy reduction. $H_{eff}$ is solved by exact diagonalization of a square cluster with 10 unit cells and also within a slave-boson mean-field theory. Its parameters depend on doping for $U_{pd}\neq 0$ or on-site O repulsion $U_p\neq 0$. The results using both techniques coincide in that there is neither VI nor PS for doping levels $x<0.5$ if $U_{pd}\lesssim 2$ eV. The PS region begins for $U_{pd}\gtrsim 2$ eV at large doping $x>0.6$ and increases with increasing $U_{pd}$. The PS also increases with increasing on-site Cu repulsion $U_d$.Comment: 16 pages and 10 figures in postscript format, compressed with uufile

La Pileta (Benaoján, Málaga) cien años después. Aportaciones al conocimiento de su secuencia arqueológica

La cueva de La Pileta es un yacimiento muy conocido por la riqueza de las manifestaciones artísticas parietales prehistóricas que atesora. Sin embargo, el conocimiento de la secuencia cronocultural documentada en las excavaciones arqueológicas llevadas a cabo en 1912 y 1942 es muy pobre. En este trabajo hacemos un repaso del registro arqueológico de La Pileta a partir de la revisión historiográfica y del estudio de los materiales depositados en el Museo de Málaga y de algunos documentos del archivo de esta institución. Los resultados, aunque limitados, permiten deducir la existencia de una secuencia más amplia de la conocida por lo general, compuesta por niveles del Paleolítico medio, Paleolítico superior; Neolítico, Calcolítico, Edad del Bronce y Edad Media

The VLT-FLAMES Tarantula Survey XXIII. Two massive double-lined binaries in 30 Doradus

Aims. We investigate the characteristics of two newly discovered short-period, double-lined, massive binary systems in the Large Magellanic Cloud, VFTS 450 (O9.7 II–Ib + O7::) and VFTS 652 (B1 Ib + O9: III:). Methods. We perform model-atmosphere analyses to characterise the photospheric properties of both members of each binary (denoting the “primary” as the spectroscopically more conspicuous component). Radial velocities and optical photometry are used to estimate the binary-system parameters. Results. We estimate Teff = 27 kK, log g = 2.9 (cgs) for the VFTS 450 primary spectrum (34 kK, 3.6: for the secondary spectrum); and Teff = 22 kK, log g = 2.8 for the VFTS 652 primary spectrum (35 kK, 3.7: for the secondary spectrum). Both primaries show surface nitrogen enrichments (of more than 1 dex for VFTS 652), and probable moderate oxygen depletions relative to reference LMC abundances. We determine orbital periods of 6.89 d and 8.59 d for VFTS 450 and VFTS 652, respectively, and argue that the primaries must be close to filling their Roche lobes. Supposing this to be the case, we estimate component masses in the range ∼20–50 M⊙. Conclusions. The secondary spectra are associated with the more massive components, suggesting that both systems are high-mass analogues of classical Algol systems, undergoing case-A mass transfer. Difficulties in reconciling the spectroscopic analyses with the light-curves and with evolutionary considerations suggest that the secondary spectra are contaminated by (or arise in) accretion disks