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The impact of the metallicity and star formation rate on the time-dependent galaxy-wide stellar initial mass function
The stellar initial mass function (IMF) is commonly assumed to be an
invariant probability density distribution function of initial stellar masses
being represented by the canonical IMF. As a consequence the galaxy-wide IMF
(gwIMF), defined as the sum of the IMFs of all star forming regions, should
also be invariant. Recent observational and theoretical results challenge the
hypothesis that the gwIMF is invariant. In order to study the possible reasons
for this variation we use the IMF determined in resolved star clusters and
apply the IGIMF-theory to calculate a grid of gwIMF models for metallicities,
-3<[Fe/H]<1, and galaxy-wide star formation rates,
<SFR<. For a galaxy with metallicy
[Fe/H]/yr, which is a common condition in the early
Universe, we find that the gwIMF is top-heavy (more massive stars), when
compared to the canonical IMF. For a SFR the gwIMF
becomes top-light regardless of the metallicity. For metallicities
the gwIMF can become bottom-heavy regardless of the SFR.
The IGIMF models predict that massive elliptical galaxies should have formed
with a gwIMF that is top-heavy within the first few hundred Myr of the galaxy's
life and that it evolves into a bottom-heavy gwIMF in the metal-enriched
galactic center. We study the SFRH relation, its dependency on
metallicity and the SFR, the correction factors to the Kennicutt SFRH relation, and provide new fitting functions Late-type dwarf
galaxies show significantly higher SFRs with respect to Kennicutt SFRs, while
star forming massive galaxies have significantly lower SFRs than hitherto
thought. This has implications for the gas-consumption time scales and for the
main sequence of galaxies. The Leo P and ultra-faint dwarf galaxies are
discussed explicitly. [abridged]Comment: Astronomy and Astrophysics (A&A) in press. 15 pages, 8 figure
Low thermal conductivity in A-site high entropy perovskite relaxor ferroelectric
An A-site disordered high entropy perovskite (Pb1/6Ba1/6Sr1/6Ca1/6Na1/6Bi1/6)TiO3 (PBSCNBi) ceramic was prepared by a solid-state reaction method. XRD and scanning electron microscopy-energy dispersive x ray confirmed a single-phase tetragonal solid solution. Dielectric and hysteresis loop measurements showed relaxor ferroelectricity at room temperature; Curie Weiss fitting gives a Burns temperature (Tb) of 123 °C, and Vogel-Fulcher fitting gives a freezing temperature (Tf) of -67.24 °C, which confirms the room-temperature relaxor ferroelectricity of PBSCNBi. This is attributed to local chemical inhomogeneities in the high entropy ceramics. PBSCNBi also has a low thermal conductivity (1.15 W m-1 K-1 at room temperature) compared to all of its constituent simple perovskites (e.g., BaTiO3, PbTiO3, SrTiO3 CaTiO3, and Na1/2Bi1/2TiO3 in the range of 25-100 °C), which is attributed to the enhanced phonon scattering by both polar nanoregions and the mass contrast effect in the multi-element perovskite. This work demonstrates the great potential of making A-site high entropy ceramics with relaxor ferroelectric properties
Long-range interactions for He()--He and He()--He
The energetically lowest five states of a helium atom are: He(),
He(), He(), He(), and He(). Long-range interaction
coefficients , , , , and for all and
pairs of these states are calculated precisely using correlated wave functions
in Hylleraas coordinates. Finite nuclear isotope mass effects are included
Long-range interactions between a He() atom and a He() atom for like isotopes
For the interactions between a He() atom and a He() atom for
like isotopes, we report perturbation theoretic calculations using accurate
variational wave functions in Hylleraas coordinates of the coefficients
determining the potential energies at large internuclear separations. We
evaluate the coefficient of the first order resonant dipole-dipole
energy and the van der Waals coefficients , , and for
the second order energies arising from the mutual perturbations of
instantaneous electric dipole, quadrupole, and octupole interactions. We also
evaluate the coefficient of the leading contribution to the third order
energy. We establish definitive values including treatment of the finite
nuclear mass for the He()--He() and He()--He() interactions.Comment: This article has been accepted by Physical Review
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