The VISTA Orion mini-survey: star formation in the Lynds 1630 North cloud

The Orion cloud complex presents a variety of star formation mechanisms and properties and it is still one of the most intriguing targets for star formation studies. We present VISTA/VIRCAM near-infrared observations of the L1630N star forming region, including the stellar clusters NGC 2068 and NGC 2071, in the Orion molecular cloud B and discuss them in combination with Spitzer data. We select 186 young stellar object (YSO) candidates in the region on the basis of multi-colour criteria, confirm the YSO nature of the majority of them using published spectroscopy from the literature, and use this sample to investigate the overall star formation properties in L1630N. The K-band luminosity function of L1630N is remarkably similar to that of the Trapezium cluster, i.e., it presents a broad peak in the range 0.3-0.7 M$_\odot$ and a fraction of sub-stellar objects of $\sim$20%. The fraction of YSOs still surrounded by disk/envelopes is very high ($\sim$85%) compared to other star forming regions of similar age (1-2 Myr), but includes some uncertain corrections for diskless YSOs. Yet, a possibly high disk fraction together with the fact that 1/3 of the cloud mass has a gas surface density above the threshold for star formation ($\sim$129 M$_\odot$ pc$^{-2}$), points towards a still on-going star formation activity in L1630N. The star formation efficiency (SFE), star formation rate (SFR) and density of star formation of L1630N are within the ranges estimated for galactic star forming regions by the Spitzer "core to disk" and "Gould's Belt" surveys. However, the SFE and SFR are lower than the average value measured in the Orion A cloud and, in particular, lower than that in the southern regions of L1630. This might suggest different star formation mechanisms within the L1630 cloud complex.Comment: 22 pages, 9 figure

Buckling Analysis and Stability of Compressed Low-Carbon Steel Rods in the Elastoplastic Region of Materials

[EN] This paper presents new approaches for solving a problem of the stability of compressed rods in the elastoplastic working region of materials. It is known that the columns of buildings, supports of engineering devices, drill rods of oil, and gas extraction industry may be subjected to significant risk of stability loss. Nowadays, there are design methods based on test results defining the relations (e.g., critical stresses-slenderness) to avoid this risk due to stability loss, but the precision and limits of definition are not always known. The main objectives of the study were to develop new approaches that would allow specifying the values of critical stresses of compressed elements beyond the proportional limit. The problem of stability of the compressed elements in the elastoplastic region was studied according to the stability theory. The authors suggested an original approach to the issue; in particular, the determination of values of the critical stresses and the finding of the points of the bifurcation were carried out by the tangent established by experimental results and by the approximation of the so-called double modulus. Comparative analysis showed the advantage of the proposed approach, particularly that the new critical curves were located below the curves of Engesser-Karman and Shanley and above the critical curves established by building codes. A new approach for the determination of critical stresses in the elastoplastic region was developed through which the structural reliability and economic efficiency was increased by almost 12% compared to the existing approaches.This research was financially supported by the Erasmus Mundus Action 2 Project Electra: Enhancing Learning in ENPI Countries through Clean Technologies and Research related Activities (project: ELEC1400294) and the Spanish Ministry of Economy and Competitiveness, along with FEDER funding (project: BIA2017-85098-R).Partskhaladze, G.; Mshvenieradze, I.; Medzmariashvili, E.; Chavleshvili, G.; Yepes, V.; Alcalá-González, J. (2019). Buckling Analysis and Stability of Compressed Low-Carbon Steel Rods in the Elastoplastic Region of Materials. Advances in Civil Engineering. 2019:1-9. https://doi.org/10.1155/2019/7601260S192019Braun, D. J. (2008). On the optimal shape of compressed rotating rod with shear and extensibility. International Journal of Non-Linear Mechanics, 43(2), 131-139. doi:10.1016/j.ijnonlinmec.2007.11.001Rossi, B., & Rasmussen, K. J. R. (2013). Carrying Capacity of Stainless Steel Columns in the Low Slenderness Range. Journal of Structural Engineering, 139(6), 1088-1092. doi:10.1061/(asce)st.1943-541x.0000666Cheng, X., Chen, Y., Niu, L., & Nethercot, D. A. (2018). Experimental study on H-section steel beam-columns under cyclic biaxial bending considering the effect of local buckling. Engineering Structures, 174, 826-839. doi:10.1016/j.engstruct.2018.08.001Goto, Y., Muraki, M., & Obata, M. (2009). Ultimate State of Thin-Walled Circular Steel Columns under Bidirectional Seismic Accelerations. Journal of Structural Engineering, 135(12), 1481-1490. doi:10.1061/(asce)st.1943-541x.0000076Lu, J., Wu, B., & Mei, Y. (2018). Buckling mechanism of steel core and global stability design method for fixed-end buckling-restrained braces. Engineering Structures, 174, 447-461. doi:10.1016/j.engstruct.2018.07.024Razdolsky, A. G. (2014). Revision of Engesser’s Approach to the Problem of Euler Stability for Built-Up Columns with Batten Plates. Journal of Engineering Mechanics, 140(3), 566-574. doi:10.1061/(asce)em.1943-7889.0000677Zapata-Medina, D. G., Arboleda-Monsalve, L. G., & Aristizabal-Ochoa, J. D. (2010). Static Stability Formulas of a Weakened Timoshenko Column: Effects of Shear Deformations. Journal of Engineering Mechanics, 136(12), 1528-1536. doi:10.1061/(asce)em.1943-7889.0000193Ziółkowski, A., & Imiełowski, S. (2010). Buckling and Post-buckling Behaviour of Prismatic Aluminium Columns Submitted to a Series of Compressive Loads. Experimental Mechanics, 51(8), 1335-1345. doi:10.1007/s11340-010-9455-yLi, P., Liu, X., & Zhang, C. (2018). Interactive buckling of cable-stiffened steel columns with pin-connected crossarms. Journal of Constructional Steel Research, 146, 97-108. doi:10.1016/j.jcsr.2018.03.037Yang, L., Shi, G., Zhao, M., & Zhou, W. (2017). Research on interactive buckling behavior of welded steel box-section columns. Thin-Walled Structures, 115, 34-47. doi:10.1016/j.tws.2017.01.030Papp, F. (2016). Buckling assessment of steel members through overall imperfection method. Engineering Structures, 106, 124-136. doi:10.1016/j.engstruct.2015.10.021Simão, P. D. (2017). Influence of shear deformations on the buckling of columns using the Generalized Beam Theory and energy principles. European Journal of Mechanics - A/Solids, 61, 216-234. doi:10.1016/j.euromechsol.2016.09.015Li, X.-F., & Lee, K. Y. (2018). Effects of Engesser’s and Haringx’s Hypotheses on Buckling of Timoshenko and Higher-Order Shear-Deformable Columns. Journal of Engineering Mechanics, 144(1), 04017150. doi:10.1061/(asce)em.1943-7889.0001363Becque, J. (2010). Inelastic Plate Buckling. Journal of Engineering Mechanics, 136(9), 1123-1130. doi:10.1061/(asce)em.1943-7889.0000075Ahmed, M., Liang, Q. Q., Patel, V. I., & Hadi, M. N. S. (2018). Nonlinear analysis of rectangular concrete-filled double steel tubular short columns incorporating local buckling. Engineering Structures, 175, 13-26. doi:10.1016/j.engstruct.2018.08.032Long, Y.-L., & Zeng, L. (2018). A refined model for local buckling of rectangular CFST columns with binding bars. Thin-Walled Structures, 132, 431-441. doi:10.1016/j.tws.2018.09.019Moen, C. D., Schudlich, A., & von der Heyden, A. (2013). Experiments on Cold-Formed Steel C-Section Joists with Unstiffened Web Holes. Journal of Structural Engineering, 139(5), 695-704. doi:10.1061/(asce)st.1943-541x.0000652Szalai, J. (2017). Complete generalization of the Ayrton-Perry formula for beam-column buckling problems. Engineering Structures, 153, 205-223. doi:10.1016/j.engstruct.2017.10.031Zhang, C., Li, F., & Wang, B. (2013). Estimation of the elasto-plastic properties of metallic materials from micro-hardness measurements. Journal of Materials Science, 48(12), 4446-4451. doi:10.1007/s10853-013-7263-3Ban, H., & Shi, G. (2018). Overall buckling behaviour and design of high-strength steel welded section columns. Journal of Constructional Steel Research, 143, 180-195. doi:10.1016/j.jcsr.2017.12.026Ma, T.-Y., Hu, Y.-F., Liu, X., Li, G.-Q., & Chung, K.-F. (2017). Experimental investigation into high strength Q690 steel welded H-sections under combined compression and bending. Journal of Constructional Steel Research, 138, 449-462. doi:10.1016/j.jcsr.2017.06.008Kervalishvili, A., & Talvik, I. (2016). Modified procedure for buckling of steel columns at elevated temperatures. Journal of Constructional Steel Research, 127, 108-119. doi:10.1016/j.jcsr.2016.07.008Tankova, T., Martins, J. P., Simões da Silva, L., Marques, L., Craveiro, H. D., & Santiago, A. (2018). Experimental lateral-torsional buckling behaviour of web tapered I-section steel beams. Engineering Structures, 168, 355-370. doi:10.1016/j.engstruct.2018.04.084Tullini, N., Tralli, A., & Baraldi, D. (2013). Buckling of Timoshenko Beams in Frictionless Contact with an Elastic Half-Plane. Journal of Engineering Mechanics, 139(7), 824-831. doi:10.1061/(asce)em.1943-7889.0000529Xie, B., Hou, J., Xu, Z., & Dan, M. (2018). Component-based model of fin plate connections exposed to fire-part I: Plate in bearing component. Journal of Constructional Steel Research, 149, 1-13. doi:10.1016/j.jcsr.2018.07.011Aristizabal-Ochoa, J. D. (2011). Stability of columns with semi-rigid connections including shear effects using Engesser, Haringx and Euler approaches. Engineering Structures, 33(3), 868-880. doi:10.1016/j.engstruct.2010.12.008Mitenkov, F. M., Bazhenov, V. G., Lomunov, V. K., & Osetrov, S. L. (2011). Effects of elasticity, plasticity, and geometrical nonlinearity in problems of static and dynamic bending of plates. Doklady Physics, 56(12), 622-625. doi:10.1134/s102833581112010xBielski, J., & Bochenek, B. (2008). On a compressed elastic–plastic column optimized for post-buckling behaviour. Engineering Optimization, 40(12), 1101-1114. doi:10.1080/03052150802313365Fergani, O., Lazoglu, I., Mkaddem, A., El Mansori, M., & Liang, S. Y. (2014). Analytical modeling of residual stress and the induced deflection of a milled thin plate. The International Journal of Advanced Manufacturing Technology, 75(1-4), 455-463. doi:10.1007/s00170-014-6146-

Statistical tools for the improvement and optimization of electrochemical sensors

The response of electrochemical sensors for substance detection critically depends on the sensing potential, the value of which is often selected by the visual inspection of the sensor's response, as given by, for example, electrochemical methods like cyclic voltammetry (CV). Using experimental data from CV, we show how the selection of the sensing potential can affect the sensitivity and linear range of the measurements. Whenever the magnitude of the sensor's response is crucial, it can be better to optimize the sensor for its sensitivity; however, if the testing conditions involve a variable range of concentrations, with putative very small or high concentrations, a reliable response can be obtained if the sensor is optimized for the linear range.ITESO, A.C

X-Shooter spectroscopy of young stellar objects: II. Impact of chromospheric emission on accretion rate estimates

Context. The lack of knowledge of photospheric parameters and the level of chromospheric activity in young low-mass pre-main sequence stars introduces uncertainties when measuring mass accretion rates in accreting (Class II) Young Stellar Objects. A detailed investigation of the effect of chromospheric emission on the estimates of mass accretion rate in young low-mass stars is still missing. This can be undertaken using samples of young diskless (Class III) K and M-type stars. Aims. Our goal is to measure the chromospheric activity of Class III pre main sequence stars to determine its effect on the estimates of accretion luminosity (Lacc) and mass accretion rate (Macc) in young stellar objects with disks. Methods. Using VLT/X-Shooter spectra we have analyzed a sample of 24 non-accreting young stellar objects of spectral type between K5 and M9.5. We identify the main emission lines normally used as tracers of accretion in Class II objects, and we determine their fluxes in order to estimate the contribution of the chromospheric activity to the line luminosity. Results. We have used the relationships between line luminosity and accretion luminosity derived in the literature for Class II objects to evaluate the impact of chromospheric activity on the accretion rate measurements. We find that the typical chromospheric activity would bias the derived accretion luminosity by Lacc,noise< 10-3Lsun, with a strong dependence with the Teff of the objects. The noise on Macc depends on stellar mass and age, and the typical values of log(Macc,noise) range between -9.2 to -11.6Msun/yr. Conclusions. Values of Lacc< 10-3Lsun obtained in accreting low-mass pre main sequence stars through line luminosity should be treated with caution as the line emission may be dominated by the contribution of chromospheric activity.Comment: accepted for publication in Astronomy & Astrophysic

X-Shooter spectroscopy of young stellar objects: IV -- Accretion in low-mass stars and sub-stellar objects in Lupus

We present X-Shooter/VLT observations of a sample of 36 accreting low-mass stellar and sub-stellar objects (YSOs) in the Lupus star forming region, spanning a range in mass from ~0.03 to ~1.2Msun, but mostly with 0.1Msun < Mstar < 0.5Msun. Our aim is twofold: firstly, analyse the relationship between excess-continuum and line emission accretion diagnostics, and, secondly, to investigate the accretion properties in terms of the physical properties of the central object. The accretion luminosity (Lacc), and from it the accretion rate (Macc), is derived by modelling the excess emission, from the UV to the near-IR, as the continuum emission of a slab of hydrogen. The flux and luminosity (Ll) of a large number of emission lines of H, He, CaII, etc., observed simultaneously in the range from ~330nm to 2500nm, were computed. The luminosity of all the lines is well correlated with Lacc. We provide empirical relationships between Lacc and the luminosity of 39 emission lines, which have a lower dispersion as compared to previous relationships in the literature. Our measurements extend the Pab and Brg relationships to Lacc values about two orders of magnitude lower than those reported in previous studies. We confirm that different methodologies to measure Lacc and Macc yield significantly different results: Ha line profile modelling may underestimate Macc by 0.6 to 0.8dex with respect to Macc derived from continuum-excess measures. Such differences may explain the likely spurious bi-modal relationships between Macc and other YSOs properties reported in the literature. We derive Macc in the range 2e-12 -- 4e-8 Msun/yr and conclude that Macc is proportional to Mstar^1.8(+/-0.2), with a dispersion lower by a factor of about 2 than in previous studies. A number of properties indicate that the physical conditions of the accreting gas are similar over more than 5 orders of magnitude in Macc

X-Shooter spectroscopy of young stellar objects III. Photospheric and chromospheric properties of Class III objects

We analyzed X-Shooter/VLT spectra of 24 ClassIII sources from three nearby star-forming regions (sigmaOrionis, LupusIII, and TWHya). We determined the effective temperature, surface gravity, rotational velocity, and radial velocity by comparing the observed spectra with synthetic BT-Settl model spectra. We investigated in detail the emission lines emerging from the stellar chromospheres and combined these data with archival X-ray data to allow for a comparison between chromospheric and coronal emissions. Both X-ray and Halpha luminosity as measured in terms of the bolometric luminosity are independent of the effective temperature for early-M stars but decline toward the end of the spectral M sequence. For the saturated early-M stars the average emission level is almost one dex higher for X-rays than for Halpha: log(L_x/L_bol) = -2.85 +- 0.36 vs. log(L_Halpha/L_bol) = -3.72 +- 0.21. When all chromospheric emission lines (including the Balmer series up to H11, CaII HK, the CaII infrared triplet, and several HeI lines) are summed up the coronal flux still dominates that of the chromosphere, typically by a factor 2-5. Flux-flux relations between activity diagnostics that probe different atmospheric layers (from the lower chromosphere to the corona) separate our sample of active pre-main sequence stars from the bulk of field M dwarfs studied in the literature. Flux ratios between individual optical emission lines show a smooth dependence on the effective temperature. The Balmer decrements can roughly be reproduced by an NLTE radiative transfer model devised for another young star of similar age. Future, more complete chromospheric model grids can be tested against this data set.Comment: accepted for publication in Astronomy & Astrophysic

Proposal of novel single-phase power quality indicators considering subsynchronous frequency perturbations in voltage and current under non-sinusoidal conditions

This work pretends to reconsider power quality (PQ), in AC single-phase low voltage systems, considering perturbation sources with frequency components below the fundamental frequency in voltage and current signals. These perturbations induced by sources like Geomagnetic Induced Currents (GMC), Arc Furnaces, switching VSC, etc. [1]-[2],[30]-[32], can occur in a frequency range comprised between DC and the fundamental frequency of the system. Standard PQ indexes do not characterize properly these subsynchronous frequency perturbations (SFFP), [2]-[3] and this work pretends to analyze the spectra from 0 to 50Hz for voltage and current, proposing new formulation for some PQ as a function of SSFP, with the intention of explaining the observed degradation of the power quality in single-phase low voltage electric systems.Peer ReviewedPostprint (published version

Scientific Programming Tools for Water Management

This special issue delivers a platform in which researchers expose intersections between algorithm design, software platforms, and hardware architectures to deal with emerging challenges in the scientific field of management of water and water-dependent resources. Since the call for papers was announced in June 2019, this special issue has received 10 manuscripts. After a rigorous review process, 6 papers have been finally accepted for publication. Published papers deal with groundwater quality monitoring, coastal groundwater-dependent irrigation agriculture, desertification risk, water recovery from tailings, future scenarios of water resources, and vulnerability of coastal aquifers

Nanomechanics of flexoelectric switching

We examine the phenomenon of flexoelectric switching of polarization in ultrathin films of barium titanate induced by a tip of an atomic force microscope (AFM). The spatial distribution of the tip-induced flexoelectricity is computationally modeled both for perpendicular mechanical load (point measurements) and for sliding load (scanning measurements), and compared with experiments. We find that (i) perpendicular load does not lead to stable ferroelectric switching in contrast to the load applied in the sliding contact load regime, due to nontrivial differences between the strain distributions in both regimes: ferroelectric switching for the perpendicular load mode is impaired by a strain gradient inversion layer immediately underneath the AFM tip; while for the sliding load regime, domain inversion is unimpaired within a greater material volume subjected to larger values of the mechanically induced electric field that includes the region behind the sliding tip; (ii) beyond a relatively small value of an applied force, increasing mechanical pressure does not increase the flexoelectric field inside the film, but results instead in a growing volume of the region subjected to such field that aids domain nucleation processes; and (iii) the flexoelectric coefficients of the films are of the order of few nC/m, which is much smaller than for bulk BaTiO3 ceramics, indicating that there is a “flexoelectric size effect” that mirrors the ferroelectric one

Nanomechanics of flexoelectric switching

We examine the phenomenon of flexoelectric switching of polarization in ultrathin films of barium titanate induced by a tip of an atomic force microscope (AFM). The spatial distribution of the tip-induced flexoelectricity is computationally modeled both for perpendicular mechanical load (point measurements) and for sliding load (scanning measurements), and compared with experiments. We find that (i) perpendicular load does not lead to stable ferroelectric switching in contrast to the load applied in the sliding contact load regime, due to nontrivial differences between the strain distributions in both regimes: ferroelectric switching for the perpendicular load mode is impaired by a strain gradient inversion layer immediately underneath the AFM tip; while for the sliding load regime, domain inversion is unimpaired within a greater material volume subjected to larger values of the mechanically induced electric field that includes the region behind the sliding tip; (ii) beyond a relatively small value of an applied force, increasing mechanical pressure does not increase the flexoelectric field inside the film, but results instead in a growing volume of the region subjected to such field that aids domain nucleation processes; and (iii) the flexoelectric coefficients of the films are of the order of few nC/m, which is much smaller than for bulk BaTiO3 ceramics, indicating that there is a “flexoelectric size effect” that mirrors the ferroelectric one