The Experts below are selected from a list of 252 Experts worldwide ranked by ideXlab platform
Dong Lai - One of the best experts on this subject based on the ideXlab platform.
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evolution of Spin Direction of accreting magnetic protostars and Spin orbit misalignment in exoplanetary systems ii warped discs
Monthly Notices of the Royal Astronomical Society, 2011Co-Authors: Francois Foucart, Dong LaiAbstract:Magnetic interactions between a protostar and its accretion disc can induce warping in the disc and produce secular changes in the stellar Spin Direction, so that the Spin axis may not always be perpendicular to the disc. This may help to explain the 7° misalignment between the ecliptic plane of the Solar system and the Sun’s equatorial plane as well as play a role in producing the recently observed Spin–orbit misalignment in a number of exoplanetary systems. We study the dynamics of warped protoplanetary discs under the combined effects of magnetic warping/precession torques and internal stresses in the disc, including viscous damping of warps and propagation of bending waves. We show that when the outer disc axis is misaligned with the stellar Spin axis, the disc evolves towards a warped steady state on a time-scale that depends on the disc viscosity or the bending wave propagation speed, but in all cases is much shorter than the time-scale for the Spin evolution (of the order of a million years). Moreover, for the most likely physical parameters characterizing magnetic protostars, circumstellar discs and their interactions, the steady-state disc, averaged over the stellar rotation period, has a rather small warp such that the whole disc lies approximately in a single plane determined by the outer disc boundary conditions, although more extreme parameters may give rise to larger disc warps. In agreement with our recent analysis based on flat discs, we find that the back-reaction magnetic torques of the slightly warped disc on the star can either align the stellar Spin axis with the disc axis or push it towards misalignment, depending on the parameters of the star–disc system. This implies that newly formed planetary systems may have a range of inclination angles between the stellar Spin axis and the orbital angular momentum axis of the planetary orbits.
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evolution of Spin Direction of accreting magnetic protostars and Spin orbit misalignment in exoplanetary systems ii warped discs
arXiv: Earth and Planetary Astrophysics, 2010Co-Authors: Francois Foucart, Dong LaiAbstract:Magnetic interactions between a protostar and its accretion disc tend to induce warping in the disc and produce secular changes in the stellar Spin Direction, so that the Spin axis may not always be perpendicular to the disc. This may help explain the recently observed Spin-orbit misalignment in a number of exoplanetary systems. We study the dynamics of warped protoplanetary discs under the combined effects of magnetic warping/precession torques and internal stresses in the disc, including viscous damping of warps and propagation of bending waves. We show that when the outer disc axis is misaligned with the stellar Spin axis, the disc evolves towards a warped steady-state on a timescale that depends on the disc viscosity or the bending wave propagation speed, but in all cases is much shorter than the timescale for the Spin evolution (of order of a million years). Moreover, for the most likely physical parameters characterizing magnetic protostars, circumstellar discs and their interactions, the steady-state disc has a rather small warp, such that the whole disc lies approximately in a single plane determined by the outer disc boundary conditions, although more extreme parameters may give rise to larger disc warps. In agreement with our recent analysis (Lai et al. 2010) based on flat discs, we find that the back-reaction magnetic torques of the slightly warped disc on the star can either align the stellar Spin axis with the disc axis or push it towards misalignment, depending on the parameters of the star-disc system. This implies that newly formed planetary systems may have a range of inclination angles between the stellar Spin axis and the symmetry axis of the planetary orbits.
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evolution of Spin Direction of accreting magnetic protostars and Spin orbit misalignment in exoplanetary systems
arXiv: Earth and Planetary Astrophysics, 2010Co-Authors: Francois Foucart, Dong Lai, Douglas N C LinAbstract:Recent observations have shown that in many exoplanetary systems the Spin axis of the parent star is misaligned with the planet's orbital axis. These have been used to argue against the scenario that short-period planets migrated to their present-day locations due to tidal interactions with their natal discs. However, this interpretation is based on the assumption that the Spins of young stars are parallel to the rotation axes of protostellar discs around them. We show that the interaction between a magnetic star and its circumstellar disc can (but not always) have the effect of pushing the stellar Spin axis away from the disc angular momentum axis toward the perpendicular state and even the retrograde state. Planets formed in the disc may therefore have their orbital axes misaligned with the stellar Spin axis, even before any additional planet-planet scatterings or Kozai interactions take place. In general, magnetosphere--disc interactions lead to a broad distribution of the Spin--orbit angles, with some systems aligned and other systems misaligned.
Wenhong Wang - One of the best experts on this subject based on the ideXlab platform.
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A New Spin Gapless Semiconductors Family: Quaternary Heusler Compounds
EPL (Europhysics Letters), 2013Co-Authors: Enke Liu, Guodong Liu, Wenhong WangAbstract: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.
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A new Spin gapless semiconductors family: Quaternary Heusler compounds
EPL (Europhysics Letters), 2013Co-Authors: Enke Liu, Guodong Liu, Wenhong WangAbstract:By 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 they have a zero-width gap in the other Spin Direction showing a Spin gapless semiconducting behavior. We further analyse the electronic and magnetic properties of all quaternary Heusler alloys involved, and reveal a semi-empirical general rule (the total valence electrons number should be 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.
Francois Foucart - One of the best experts on this subject based on the ideXlab platform.
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evolution of Spin Direction of accreting magnetic protostars and Spin orbit misalignment in exoplanetary systems ii warped discs
Monthly Notices of the Royal Astronomical Society, 2011Co-Authors: Francois Foucart, Dong LaiAbstract:Magnetic interactions between a protostar and its accretion disc can induce warping in the disc and produce secular changes in the stellar Spin Direction, so that the Spin axis may not always be perpendicular to the disc. This may help to explain the 7° misalignment between the ecliptic plane of the Solar system and the Sun’s equatorial plane as well as play a role in producing the recently observed Spin–orbit misalignment in a number of exoplanetary systems. We study the dynamics of warped protoplanetary discs under the combined effects of magnetic warping/precession torques and internal stresses in the disc, including viscous damping of warps and propagation of bending waves. We show that when the outer disc axis is misaligned with the stellar Spin axis, the disc evolves towards a warped steady state on a time-scale that depends on the disc viscosity or the bending wave propagation speed, but in all cases is much shorter than the time-scale for the Spin evolution (of the order of a million years). Moreover, for the most likely physical parameters characterizing magnetic protostars, circumstellar discs and their interactions, the steady-state disc, averaged over the stellar rotation period, has a rather small warp such that the whole disc lies approximately in a single plane determined by the outer disc boundary conditions, although more extreme parameters may give rise to larger disc warps. In agreement with our recent analysis based on flat discs, we find that the back-reaction magnetic torques of the slightly warped disc on the star can either align the stellar Spin axis with the disc axis or push it towards misalignment, depending on the parameters of the star–disc system. This implies that newly formed planetary systems may have a range of inclination angles between the stellar Spin axis and the orbital angular momentum axis of the planetary orbits.
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evolution of Spin Direction of accreting magnetic protostars and Spin orbit misalignment in exoplanetary systems ii warped discs
arXiv: Earth and Planetary Astrophysics, 2010Co-Authors: Francois Foucart, Dong LaiAbstract:Magnetic interactions between a protostar and its accretion disc tend to induce warping in the disc and produce secular changes in the stellar Spin Direction, so that the Spin axis may not always be perpendicular to the disc. This may help explain the recently observed Spin-orbit misalignment in a number of exoplanetary systems. We study the dynamics of warped protoplanetary discs under the combined effects of magnetic warping/precession torques and internal stresses in the disc, including viscous damping of warps and propagation of bending waves. We show that when the outer disc axis is misaligned with the stellar Spin axis, the disc evolves towards a warped steady-state on a timescale that depends on the disc viscosity or the bending wave propagation speed, but in all cases is much shorter than the timescale for the Spin evolution (of order of a million years). Moreover, for the most likely physical parameters characterizing magnetic protostars, circumstellar discs and their interactions, the steady-state disc has a rather small warp, such that the whole disc lies approximately in a single plane determined by the outer disc boundary conditions, although more extreme parameters may give rise to larger disc warps. In agreement with our recent analysis (Lai et al. 2010) based on flat discs, we find that the back-reaction magnetic torques of the slightly warped disc on the star can either align the stellar Spin axis with the disc axis or push it towards misalignment, depending on the parameters of the star-disc system. This implies that newly formed planetary systems may have a range of inclination angles between the stellar Spin axis and the symmetry axis of the planetary orbits.
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evolution of Spin Direction of accreting magnetic protostars and Spin orbit misalignment in exoplanetary systems
arXiv: Earth and Planetary Astrophysics, 2010Co-Authors: Francois Foucart, Dong Lai, Douglas N C LinAbstract:Recent observations have shown that in many exoplanetary systems the Spin axis of the parent star is misaligned with the planet's orbital axis. These have been used to argue against the scenario that short-period planets migrated to their present-day locations due to tidal interactions with their natal discs. However, this interpretation is based on the assumption that the Spins of young stars are parallel to the rotation axes of protostellar discs around them. We show that the interaction between a magnetic star and its circumstellar disc can (but not always) have the effect of pushing the stellar Spin axis away from the disc angular momentum axis toward the perpendicular state and even the retrograde state. Planets formed in the disc may therefore have their orbital axes misaligned with the stellar Spin axis, even before any additional planet-planet scatterings or Kozai interactions take place. In general, magnetosphere--disc interactions lead to a broad distribution of the Spin--orbit angles, with some systems aligned and other systems misaligned.
Enke Liu - One of the best experts on this subject based on the ideXlab platform.
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A New Spin Gapless Semiconductors Family: Quaternary Heusler Compounds
EPL (Europhysics Letters), 2013Co-Authors: Enke Liu, Guodong Liu, Wenhong WangAbstract: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.
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A new Spin gapless semiconductors family: Quaternary Heusler compounds
EPL (Europhysics Letters), 2013Co-Authors: Enke Liu, Guodong Liu, Wenhong WangAbstract:By 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 they have a zero-width gap in the other Spin Direction showing a Spin gapless semiconducting behavior. We further analyse the electronic and magnetic properties of all quaternary Heusler alloys involved, and reveal a semi-empirical general rule (the total valence electrons number should be 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.
Arye Nehorai - One of the best experts on this subject based on the ideXlab platform.
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Linear independence of steering vectors of an electromagnetic vector sensor
IEEE Transactions on Signal Processing, 1996Co-Authors: Kah-chye Tan, Arye NehoraiAbstract:We investigate linear independence of steering vectors of one electromagnetic vector sensor. We show that every three steering vectors with distinct Directions of arrival (DOAs) are linearly independent. We also show that four steering vectors with distinct DOAs are linearly independent if the ellipticity angles of the signals associated with any two of the four steering vectors are distinct. Moreover, every four steering vectors corresponding to circularly polarized signals with the same Spin Direction are linearly dependent. We then establish that five steering vectors are linearly independent if exactly two or three of them correspond to circularly polarized signals with the same Spin Direction. Finally, we demonstrate that given any five steering vectors, then for any DOA there exists a steering vector that is linearly dependent on the five steering vectors.