Gas physical conditions and kinematics of the giant outflow Ou4

Ou4 is a recently discovered bipolar outflow with a projected size of more than one degree in the plane of the sky. It is apparently centred on the young stellar cluster -whose most massive representative is the triple system HR8119- inside the HII region Sh 2-129. The driving source, the nature, and the distance of Ou4 are not known. Deep narrow-band imagery of the whole nebula at arcsec resolution was obtained to study its morphology. Long-slit spectroscopy of the tips of the bipolar lobes was secured to determine the gas ionization mechanism, physical conditions, and line-of-sight velocities. An estimate of the proper motions at the tip of the south lobe using archival images is attempted. The existing multi-wavelength data for Sh 2-129 and HR 8119 are also comprehensively reviewed. The morphology of Ou4, its emission-line spatial distribution, line flux ratios, and the kinematic modelling adopting a bow-shock parabolic geometry, illustrate the expansion of a shock-excited fast collimated outflow. The radial velocities and reddening are consistent with those of Sh 2-129 and HR 8119. The improved determination of the distance to HR8119 (composed of two B0 V and one B0.5 V stars) and Sh 2-129 is 712 pc. We identify in WISE images a 5 arcmin-radius (1 pc at the distance above) bubble of emission at 22 micron emitted by hot (107 K) dust, located inside the central part of Ou4 and corresponding to several [O III] features of Ou4. The apparent position and the properties studied in this work are consistent with the hypothesis that Ou4 is located inside the Sh 2-129 HII region, suggesting that it was launched some 90 000 yrs ago by HR8119. The outflow total kinetic energy is estimated to be ~4e47~ergs. However, the alternate possibility that Ou4 is a bipolar planetary nebula, or the result of an eruptive event on a massive AGB or post-AGB star not yet identified, cannot be ruled out.Comment: Accepted for publication in Astronomy and Astrophysics. Also available at http://hal.archives-ouvertes.fr/hal-0102228

Designing Dirac points in two-dimensional lattices

We present a framework to elucidate the existence of accidental contacts of energy bands, particularly those called Dirac points which are the point contacts with linear energy dispersions in their vicinity. A generalized von-Neumann-Wigner theorem we propose here gives the number of constraints on the lattice necessary to have contacts without fine tuning of lattice parameters. By counting this number, one could quest for the candidate of Dirac systems without solving the secular equation. The constraints can be provided by any kinds of symmetry present in the system. The theory also enables the analytical determination of k-point having accidental contact by selectively picking up only the degenerate solution of the secular equation. By using these frameworks, we demonstrate that the Dirac points are feasible in various two-dimensional lattices, e.g. the anisotropic Kagome lattice under inversion symmetry is found to have contacts over the whole lattice parameter space. Spin-dependent cases, such as the spin-density-wave state in LaOFeAs with reflection symmetry, are also dealt with in the present scheme.Comment: 15pages, 9figures (accepted to Phys. Rev. B

Fermion Soliton Stars with Asymmetric Vacua

Fermion soliton stars are a motivated model of exotic compact objects in which a nonlinear self-interacting real scalar field couples to a fermion via a Yukawa term, giving rise to an effective fermion mass that depends on the fluid properties. Here we continue our investigation of this model within General Relativity by considering a scalar potential with generic asymmetric vacua. This case provides fermion soliton stars with a parametrically different scaling of the maximum mass relative to the model parameters, showing that the special case of symmetric vacua, in which we recover our previous results, requires fine tuning. In the more generic case studied here the mass and radius of a fermion soliton star are comparable to those of a neutron star for natural model parameters at the GeV scale. Finally, the asymmetric scalar potential inside the star can provide either a positive or a negative effective cosmological constant in the interior, being thus reminiscent of gravastars or anti-de Sitter bubbles, respectively. In the latter case we find the existence of multiple, disconnected, branches of solutions.Comment: 9 pages, 7 figures. v2: matches version accepted in PR

Defending OC-SVM based IDS from poisoning attacks

Machine learning techniques are widely used to detect intrusions in the cyber security field. However, most machine learning models are vulnerable to poisoning attacks, in which malicious samples are injected into the training dataset to manipulate the classifier's performance. In this paper, we first evaluate the accuracy degradation of OC-SVM classifiers with 3 different poisoning strategies with the ADLA-FD public dataset and a real world dataset. Secondly, we propose a saniti-zation mechanism based on the DBSCAN clustering algorithm. In addition, we investigate the influences of different distance metrics and different dimensionality reduction techniques and evaluate the sensitivity of the DBSCAN parameters. The ex-perimental results show that the poisoning attacks can degrade the performance of the OC-SVM classifier to a large degree, with an accuracy equal to 0.5 in most settings. The proposed sanitization method can filter out poisoned samples effectively for both datasets. The accuracy after sanitization is very close or even higher to the original value.</p

Life cycle assessment of a floating offshore wind farm in Italy

Mitigation of climate change requires consistent actions toward the reduction of emissions from the energy sector: in the last years, renewable energy technologies, such as wind power, have become a cost-effective option to pursue the transition to low emission systems for power generation. Offshore wind energy can provide access to additional wind resources, also overcoming some issues related to onshore wind deployments such as land-use competition and social acceptability. The Life Cycle Assessment (LCA) methodology can be used to gain insight into the environmental performances of different technologies, e.g. renewable energy generation technologies, along the lifecycle stages and across a number of impact categories. This paper reports the cradle-to-grave LCA of a floating offshore wind farm, consisting of 190 wind turbines with 14.7 MW rated power, intended to be deployed in the Mediterranean Sea. The employed technology is represented by the IEA 15 MW reference wind turbine supported by the reference semi-submersible platform. The selected functional unit is the delivery of 1 GWh of electricity to the onshore grid and the impact assessment method is the EPD (version 2018), which is usually used for the creation of Environmental Product Declarations (EPDs) and considers 8 impact categories. The results of the analysis show that the supply of raw materials, especially steel, for aerogenerators and floaters is the most significant contributor to the overall potential impacts in all the impact categories, except for abiotic depletion of elements, where power cables are the hotspot. In the perspective of decarbonisation, the estimated carbon intensity is 31 g CO2eq/kWh and so it results competitive with other low emissions electricity generation technologies. To compare the estimated global warming impacts to other studies, some harmonisations efforts on capacity factor and lifetime of turbines are made. Moreover, the wind farm performance has been evaluated in terms of carbon and energy payback time, estimated in 2 and 3 years respectively, showing a substantial benefit when compared to the expected 30-year lifetime. As a conclusion, despite the number of approximations and conservative assumptions, floating offshore wind power, represented by the modelled case study, can be considered a promising technology and has been found to be already competitive with other renewable electricity generation technologies. Future research should address the uncertainty rooted to the data: repeating the analysis relying on the executive project, and therefore on a more detailed modelling, would help to get more accurate results

Sensitive survey for 13CO, CN, H2CO, and SO in the disks of T Tauri and Herbig Ae stars II: Stars in ρ\rho Oph and upper Scorpius

We attempt to determine the molecular composition of disks around young low-mass stars in the $\rho$ Oph region and to compare our results with a similar study performed in the Taurus-Auriga region. We used the IRAM 30 m telescope to perform a sensitive search for CN N=2-1 in 29 T Tauri stars located in the $\rho$ Oph and upper Scorpius regions. $^{13}$CO J=2-1 is observed simultaneously to provide an indication of the level of confusion with the surrounding molecular cloud. The bandpass also contains two transitions of ortho-H$_2$CO, one of SO, and the C$^{17}$O J=2-1 line, which provides complementary information on the nature of the emission. Contamination by molecular cloud in $^{13}$CO and even C$^{17}$O is ubiquitous. The CN detection rate appears to be lower than for the Taurus region, with only four sources being detected (three are attributable to disks). H$_2$CO emission is found more frequently, but appears in general to be due to the surrounding cloud. The weaker emission than in Taurus may suggest that the average disk size in the $\rho$ Oph region is smaller than in the Taurus cloud. Chemical modeling shows that the somewhat higher expected disk temperatures in $\rho$ Oph play a direct role in decreasing the CN abundance. Warmer dust temperatures contribute to convert CN into less volatile forms. In such a young region, CN is no longer a simple, sensitive tracer of disks, and observations with other tracers and at high enough resolution with ALMA are required to probe the gas disk population.Comment: 18 pages, 5 figures, accepted for publication in A&