Chemical analysis of giant stars in the young open cluster NGC 3114

Context: Open clusters are very useful targets for examining possible trends in galactocentric distance and age, especially when young and old open clusters are compared. Aims: We carried out a detailed spectroscopic analysis to derive the chemical composition of seven red giants in the young open cluster NGC 3114. Abundances of C, N, O, Li, Na, Mg, Al, Ca, Si, Ti, Ni, Cr, Y, Zr, La, Ce, and Nd were obtained, as well as the carbon isotopic ratio. Methods: The atmospheric parameters of the studied stars and their chemical abundances were determined using high-resolution optical spectroscopy. We employed the local-thermodynamic-equilibrium model atmospheres of Kurucz and the spectral analysis code MOOG. The abundances of the light elements were derived using the spectral synthesis technique. Results: We found that NGC 3114 has a mean metallicity of [Fe/H] = -0.01+/-0.03. The isochrone fit yielded a turn-off mass of 4.2 Msun. The [N/C] ratio is in good agreement with the models predicted by first dredge-up. We found that two stars, HD 87479 and HD 304864, have high rotational velocities of 15.0 km/s and 11.0 km/s; HD 87526 is a halo star and is not a member of NGC 3114. Conclusions: The carbon and nitrogen abundance in NGC 3114 agree with the field and cluster giants. The oxygen abundance in NGC 3114 is lower compared to the field giants. The [O/Fe] ratio is similar to the giants in young clusters. We detected sodium enrichment in the analyzed cluster giants. As far as the other elements are concerned, their [X/Fe] ratios follow the same trend seen in giants with the same metallicity.Comment: 17 pages, 9 figures, 10 tables; accepted for publication in A&

Diamond degradation in hadron fields

The energy dependence of the concentration of primary displacements induced by protons and pions in diamond has been calculated in the energy range 50 MeV - 50 GeV, in the frame of the Lindhard theory. The concentrations of primary displacements induced by protons and pions have completely different energy dependencies: the proton degradation is very important at low energies, and is higher than the pion one in the whole energy range investigated, with the exception of the delta33 resonance region. Diamond has been found, theoretically, to be one order of magnitude more resistant to proton and pion irradiation in respect to silicon.Comment: 7 pages, 3 figure

Detection of new eruptions in the Magellanic Clouds LBVs R 40 and R 110

We performed a spectroscopic and photometric analysis to study new eruptions in two luminous blue variables (LBVs) in the Magellanic Clouds. We detected a strong new eruption in the LBV R40 that reached $V \sim 9.2$ in 2016, which is around $1.3$ mag brighter than the minimum registered in 1985. During this new eruption, the star changed from an A-type to a late F-type spectrum. Based on photometric and spectroscopic empirical calibrations and synthetic spectral modeling, we determine that R\,40 reached $T_{\mathrm{eff}} = 5800-6300$~K during this new eruption. This object is thereby probably one of the coolest identified LBVs. We could also identify an enrichment of nitrogen and r- and s-process elements. We detected a weak eruption in the LBV R 110 with a maximum of $V \sim 9.9$ mag in 2011, that is, around $1.0$ mag brighter than in the quiescent phase. On the other hand, this new eruption is about $0.2$ mag fainter than the first eruption detected in 1990, but the temperature did not decrease below 8500 K. Spitzer spectra show indications of cool dust in the circumstellar environment of both stars, but no hot or warm dust was present, except by the probable presence of PAHs in R\,110. We also discuss a possible post-red supergiant nature for both stars

The different origins of magnetic fields and activity in the Hertzsprung gap stars, OU Andromedae and 31 Comae

Context: When crossing the Hertzsprung gap, intermediate-mass stars develop a convective envelope. Fast rotators on the main sequence, or Ap star descendants, are expected to become magnetic active subgiants during this evolutionary phase. Aims: We compare the surface magnetic fields and activity indicators of two active, fast rotating red giants with similar masses and spectral class but diferent rotation rates - OU And (Prot=24.2 d) and 31 Com (Prot=6.8 d) - to address the question of the origin of their magnetism and high activity. Methods: Observations were carried out with the Narval spectropolarimeter in 2008 and 2013.We used the least squares deconvolution technique to extract Stokes V and I profiles to detect Zeeman signatures of the magnetic field of the stars. We provide Zeeman-Doppler imaging, activity indicator monitoring, and a precise estimation of stellar parameters. We use stellar evolutionary models to infer the evolutionary status and the initial rotation velocity on the main sequence. Results: The detected magnetic field of OU And is a strong one. Its longitudinal component Bl reaches 40 G and presents an about sinusoidal variation with reversal of the polarity. The magnetic topology of OU And is dominated by large scale elements and is mainly poloidal with an important dipole component, and a significant toroidal component. The detected magnetic field of 31 Com is weaker, with a magnetic map showing a more complex field geometry, and poloidal and toroidal components of equal contributions. The evolutionary models show that the progenitors of OU And and 31 Com must have been rotat Conclusions: OU And appears to be the probable descendant of a magnetic Ap star, and 31 Com the descendant of a relatively fast rotator on the main sequence.Comment: 16 pages, 12 figure

Two-scale structure of the electron dissipation region during collisionless magnetic reconnection

Particle in cell (PIC) simulations of collisionless magnetic reconnection are presented that demonstrate that the electron dissipation region develops a distinct two-scale structure along the outflow direction. The length of the electron current layer is found to decrease with decreasing electron mass, approaching the ion inertial length for a proton-electron plasma. A surprise, however, is that the electrons form a high-velocity outflow jet that remains decoupled from the magnetic field and extends large distances downstream from the x-line. The rate of reconnection remains fast in very large systems, independent of boundary conditions and the mass of electrons.Comment: Submitted to Physical Review Letters, 4 pages, 4 figure

Mixed Phase in Compact Starts : M-R relations and radial oscillations

It is believed that quark stars or neutron stars with mixed phase in the core have smaller radii compared to ordinary compact stars. With the recent observation of several low radius objects, typically a radius of $<10 Km.$ for star of mass $< 1M_0$ in low mass X-ray binaries (LMXB), it has become very important to understand the nature of these objects. An accurate determination of mass-radius relationship of these objects provide us with a physical laboratory to study the composition of high density matter and the nature of phase transition. We study the effect of quark and nuclear matter mixed phase on mass radius relationship and radial oscillations of neutron stars. We find that the effect of the mixed phase is to decrease the maximum mass of a stable neutron star and to decrease the radial frequencies .Comment: guest contribution at Int. Workshop on Astronomy & Relativistic Astrophysics (IWARA 03)held at Olinda-PE (Brazil) from Oct. 12-17,200