The nearby Galaxy structure toward the Vela Gum nebula

We report on $UBVI$ photometry and spectroscopy for MK classification purposes carried out in the fields of five open clusters projected against the Vela Gum in the Third Galactic Quadrant of the Galaxy. They are Ruprecht 20, Ruprecht 47, Ruprecht 60, NGC 2660 and NGC 2910. We could improve/confirm the parameters of these objects derived before. The spectroscopic parallax method has been applied to several stars located in the fields of four out of the five clusters to get their distances and reddenings. With this method we found two blue stars in the field of NGC 2910 at distances that make them likely members of Vela OB1 too. Also, projected against the fields of Ruprecht 20 and Ruprecht 47 we have detected other young stars favoring not only the existence of Puppis OB1 and OB2 but conforming a young stellar group at $\sim1$ kpc from the Sun and extending for more than 6 kpc outward the Galaxy. If this is the case, there is a thickening of the thin Galactic disk of more than 300 pc at just 2-3 kpc from the Sun. Ruprecht 60 and NGC 2660 are too old objects that have no physical relation with the associations under discussion. An astonishing result has been the detection in the background of Ruprecht 47 of a young star at the impressive distance of 9.5 kpc from the Sun that could be a member of the innermost part of the Outer Arm. Another far young star in the field of NGC 2660, at near 6.0 kpc, may become a probable member of the Perseus Arm or of the inner part of the Local Arm. The distribution of young clusters and stars onto the Third Galactic Quadrant agrees with recent findings concerning the extension of the Local Arm as revealed by parallaxes of regions of star formation. We show evidences too that added to previous ones found by our group explain the thickening of the thin disk as a combination of flare and warp.Comment: Accepted for publication in New Astronom

Pressure-induced structural, electronic, and magnetic effects in BiFeO3

We present a first-principles study of multiferroic BiFeO3 at high pressures. Our work reveals the main structural (change in Bi's coordination and loss of ferroelectricity), electronic (spin crossover and metallization), and magnetic (loss of order) effects favored by compression and how they are connected. Our results are consistent with the striking manifold transition observed experimentally by Gavriliuk et al. [Phys. Rev. B 77, 155112 (2008)] and provide an explanation for it.Comment: 4 pages with 4 figures embedded. More information at http://www.icmab.es/dmmis/leem/jorg

Numerical analysis of the stability of the Electrohydrodynamic (EHD) electroconvection between two plates

The time evolution of the problem of Electrohydrodynamic (EHD) convection in a liquid between two plates is analysed numerically. The equations are nondimensionalized using the ion drift velocity and the viscous time scales. Following the non-dimensionalisation of the respective model, two different techniques have been used to describe the charge evolution, namely the Finite-Element Flux-Corrected Transport Method and the Particle-In-Cell technique. The results obtained with the two schemes, apart from showing good agreement, have revealed the appearance of a two-roll structure not described in previous works. This is investigated in detail for both strong and weak injection.Ministerio de ciencia y tecnología FQM-42

Examples of signature (2,2) manifolds with commuting curvature operators

We exhibit Walker manifolds of signature (2,2) with various commutativity properties for the Ricci operator, the skew-symmetric curvature operator, and the Jacobi operator. If the Walker metric is a Riemannian extension of an underlying affine structure A, these properties are related to the Ricci tensor of A

Photometric study of the young open cluster NGC 3293

Deep and extensive CCD photometric observations $UBV(RI)_{C}H_{\alpha}$ were carried out in the area of the open cluster NGC 3293. The new data set allows to see the entire cluster sequence down to $M_{V} \approx +4.5$, revealing that stars with $M_{V} < -2$ are evolving off the main sequence; stars with $-2 < M_{V} +2$ are placed above it. According to our analysis, the cluster distance is $d = 2750 \pm 250 pc$ ($V_{0}-M_{V} = 12.2 \pm 0.2$) and its nuclear age is $8 \pm 1 Myr$. NGC 3293 contains an important fraction of pre--main sequence (PMS) stars distributed along a parallel band to the ZAMS with masses from 1 to 2.5 \cal M_{\sun} and a mean contraction age of $10 Myr$. This last value does not differ too much from the nuclear age estimate. Actually, if we take into account the many factors that may affect the PMS star positions onto the colour--magnitude diagram, both ages can be perfectly reconciled. The star formation rate, on the other hand, suggests that NGC 3293 stars formed surely in one single event, therefore favouring a coeval process of star formation. Besides, using the $H_{\alpha}$ data, we detected nineteen stars with signs of having $H_{\alpha}$ emission in the region of NGC 3293, giving another indication that the star formation process is still active in the region. The computed initial mass function for the cluster has a slope value $x = 1.2 \pm 0.2$, a bit flatter than the typical slope for field stars and similar to the values found for other young open clusters.Comment: 17 pages, 13 eps figures, in press in Astronomy and Astrophysic

The inhomogeneous evolution of subgraphs and cycles in complex networks

Subgraphs and cycles are often used to characterize the local properties of complex networks. Here we show that the subgraph structure of real networks is highly time dependent: as the network grows, the density of some subgraphs remains unchanged, while the density of others increase at a rate that is determined by the network's degree distribution and clustering properties. This inhomogeneous evolution process, supported by direct measurements on several real networks, leads to systematic shifts in the overall subgraph spectrum and to an inevitable overrepresentation of some subgraphs and cycles.Comment: 4 pages, 4 figures, submitted to Phys. Rev.

Searching for spiral features in the outer Galactic disk. The field towards WR38 and WR38a

The detailed spiral structure in the outer Galactic disk is still poorly known, and for several Galactic directions we rely on model extrapolations. One of these regions is the fourth Galactic quadrant, in the sector comprised between Vela and Carina (270 <l< 300) where no spiral arms have been detected so far in the optical beyond 270. By means of deep UBVI photometry, we search for spiral features in known low absorption windows.U photometry, although demanding, constitutes a powerful tool to detect and characterize distant aggregates, and allows to derive firmer distance estimates. We studied a direction close to the tangent (l=290) to the Carina arm, in an attempt to detect optical spiral tracers beyond the Carina branch, where radio observations and models predictions indicate the presence of the extension of the Perseus and Norma-Cygnus spiral arms in the fourth quadrant.Along this line of sight, we detect three distinct groups of young stars. Two of them, at 2.5 and 6.0 kpc, belong to the Carina spiral arm (which is crossed twice in this particular direction).The latter is here detected for the first time. The third group, at a distance of 12.7 kpc, is part of the Perseus arm which lies beyond the Carina arm, and constitutes the first optical detection of this arm in the fourth Galactic quadrant. The position of this feature is compatible with HI observations and model predictions. We furthermore present evidence that this extremely distant group, formerly thought to be a star cluster (Shorlin 1), is in fact a diffuse young population. In addition, our data-set does not support the possible presence of the Monoceros Ring toward this direction. This study highlights how multicolor optical studies can be effective to probe the spiral structure in the outer Galactic disk.Comment: 9 pages, 13 eps figure, in press in A&A, abstract rephrased and a few figures degraded in resolution to fit i