Investigating the Rotational Phase of Stellar Flares on M dwarfs Using K2 Short Cadence Data

We present an analysis of K2 short cadence data of 34 M dwarfs which have spectral types in the range M0 - L1. Of these stars, 31 showed flares with a duration between $\sim$10-90 min. Using distances obtained from Gaia DR2 parallaxes, we determined the energy of the flares to be in the range $\sim1.2\times10^{29}-6\times10^{34}$ erg. In agreement with previous studies we find rapidly rotating stars tend to show more flares, with evidence for a decline in activity in stars with rotation periods longer than $\sim$10 days. The rotational modulation seen in M dwarf stars is widely considered to result from a starspot which rotates in and out of view. Flux minimum is therefore the rotation phase where we view the main starspot close to the stellar disk center. Surprisingly, having determined the rotational phase of each flare in our study we find none show any preference for rotational phase. We outline three scenarios which could account for this unexpected finding. The relationship between rotation phase and flare rate will be explored further using data from wide surveys such as NGTS and TESS.Comment: Accepted main Journal MNRA

Residual stress redistribution during elastic shake down in welded plates

Residual stresses are a consequence of welding in various structures such as ships and offshore structures. Residual stresses can be relaxed or redistributed according to the load levels during operation. The elastic shakedown phenomenon can be considered as one of the reasons for this change. This paper studies the relaxation/redistribution of weld residual stress during different levels of shakedown in a butt-welded plate chosen according to ship design and welding procedures. Welding was performed on DH36, a ship structural steel. Neutron diffraction was used to measure residual stresses in these plates in the as-welded state and after different levels of shakedown. A mixed hardening model in line with the Chaboche model is determined for both weld and base material. A numerical model is developed to estimate the shakedown limit on butt-welded plate. Further, the redistribution of residual stress in a numerical weld model according to the different levels of shakedown limit is studied. Based on the shakedown limit of the butt-welded plate, a shakedown region is determined, where the structure will undergo elastic shakedown in the presence of an existing residual stress field if the maximum stress on the load section after a few initial cycles is in the shakedown region

The Luminosity - E_p Relation within Gamma--Ray Bursts and Implications for Fireball Models

Using a sample of 2408 time-resolved spectra for 91 BATSE gamma-ray bursts (GRBs) presented by Preece et al., we show that the relation between the isotropic-equivalent luminosity (L_iso) and the spectral peak energy (E_p) in the cosmological rest frame, L_iso \propto E_p^2, not only holds within these bursts, but also holds among these GRBs, assuming that the burst rate as a function of redshift is proportional to the star formation rate. The possible implications of this relation for the emission models of GRBs are discussed. We suggest that both the kinetic-energy-dominated internal shock model and the magnetic-dissipation-dominated external shock model can well interpret this relation. We constrain the parameters for these two models, and find that they are in a good agreement with the parameters from the fittings to the afterglow data (abridged).Comment: 3 pages plus 5 figures, emulateapj style, accepted for publication in ApJ Letter

Non-Relativistic Limit of Dirac Equations in Gravitational Field and Quantum Effects of Gravity

Based on unified theory of electromagnetic interactions and gravitational interactions, the non-relativistic limit of the equation of motion of a charged Dirac particle in gravitational field is studied. From the Schrodinger equation obtained from this non-relativistic limit, we could see that the classical Newtonian gravitational potential appears as a part of the potential in the Schrodinger equation, which can explain the gravitational phase effects found in COW experiments. And because of this Newtonian gravitational potential, a quantum particle in earth's gravitational field may form a gravitationally bound quantized state, which had already been detected in experiments. Three different kinds of phase effects related to gravitational interactions are discussed in this paper, and these phase effects should be observable in some astrophysical processes. Besides, there exists direct coupling between gravitomagnetic field and quantum spin, radiation caused by this coupling can be used to directly determine the gravitomagnetic field on the surface of a star.Comment: 12 pages, no figur

Surface Contribution to Raman Scattering from Layered Superconductors

Generalizing recent work, the Raman scattering intensity from a semi-infinite superconducting superlattice is calculated taking into account the surface contribution to the density response functions. Our work makes use of the formalism of Jain and Allen developed for normal superlattices. The surface contributions are shown to strongly modify the bulk contribution to the Raman-spectrum line shape below $2\Delta$, and also may give rise to additional surface plasmon modes above $2\Delta$. The interplay between the bulk and surface contribution is strongly dependent on the momentum transfer $q_\parallel$ parallel to layers. However, we argue that the scattering cross-section for the out-of-phase phase modes (which arise from interlayer Cooper pair tunneling) will not be affected and thus should be the only structure exhibited in the Raman spectrum below $2\Delta$ for relatively large $q_\parallel\sim 0.1\Delta/v_F$. The intensity is small but perhaps observable.Comment: 14 pages, RevTex, 6 figure

A New Spin Gapless Semiconductors Family: Quaternary Heusler Compounds

Using first-principles calculations, we investigate the band structures of a series of quaternary LiMgPdSn-type Heusler compounds. Our calculation results show that five compounds CoFeMnSi, CoFeCrAl, CoMnCrSi, CoFeVSi and FeMnCrSb possess unique electronic structures characterized by a half-metallic gap in one spin direction while a zero-width gap in the other spin direction showing spin gapless semiconducting behavior. We further analysis the electronic and magnetic properties of all quaternary Heusler alloys involved, and reveal a semi-empirical general rule (total valence electrons number being 26 or 28) for indentifying spin gapless semiconductors in Heusler compounds. The influences of lattice distortion and main-group element change have also been discussed.Comment: 20 pages, 5 figures, 1 supplementary file, submitted for publicatio

Radiant energy absorption studies for laser propulsion

A study of the energy absorption mechanisms and fluid dynamic considerations for efficient conversion of high power laser radiation into a high velocity flow is presented. The objectives of the study are: (1) to determine the most effective absorption mechanisms for converting laser radiation into translational energy, and (2) to examine the requirements for transfer of the absorbed energy into a steady flow which is stable to disturbances in the absorption zone. A review of inverse Bremsstrahlung, molecular and particulate absorption mechanisms is considered and the steady flow and stability considerations for conversion of the laser power to a high velocity flow in a nozzle configuration is calculated. A quasi-one-dimensional flow through a nozzle was formulated under the assumptions of perfect gas

Triplet-singlet relaxation in semiconductor single and double quantum dots

We study the triplet-singlet relaxation in two-electron semiconductor quantum dots. Both single dots and vertically coupled double dots are discussed. In our work, the electron-electron Coulomb interaction, which plays an important role in the electronic structure, is included. The spin mixing is caused by spin-orbit coupling which is the key to the triplet-singlet relaxation. We show that the selection rule widely used in the literature is incorrect unless near the crossing/anticrossing point in single quantum dots. The triplet/singlet relaxation in double quantum dots can be markedly changed by varying barrier height, inter-dot distance, external magnetic field and dot size.Comment: 7 pages, 4 figures, PRB in pres