22 research outputs found
Quantum fluctuation driven first order phase transition in weak ferromagnetic metals
In a local Fermi liquid (LFL), we show that there is a line of weak first
order phase transitions between the ferromagnetic and paramagnetic phases due
to purely quantum fluctuations. We predict that an instability towards
superconductivity is only possible in the ferromagnetic state. At T=0 we find a
point on the phase diagram where all three phases meet and we call this a
quantum triple point (QTP). A simple application of the Gibbs phase rule shows
that only these three phases can meet at the QTP. This provides a natural
explanation of the absence of superconductivity at this point coming from the
paramagnetic side of the phase diagram, as observed in the recently discovered
ferromagnetic superconductor, .Comment: 5 pages, 5 figure
Physical properties of ferromagnetic-superconducting coexistent system
We studied the nuclear relaxation rate 1/T1 of a
ferromagnetic-superconducting system from the mean field model proposed in
Ref.14. This model predicts the existence of a set of gapless excitations in
the energy spectrum which will affect the properties studied here, such as the
density of states and, hence, 1/T1. The study of the temperature variation of
1/T1(for T<Tc) shows that the usual Hebel-Slichter peak exists, but will be
reduced because of the dominant role of the gapless fermions and the background
magnetic behavior. We have also presented the temperature dependence of
ultrasonic attenuation and the frequency dependence of electromagnetic
absorption within this model. We are successful in explaining certain
experimental results.Comment: 10 Pages, 9 figute
Challenges in Cepheid Evolution and Pulsation Modeling
Cepheids have long been used as standard candles to determine distances
around the Milky Way and to nearby galaxies. A discrepancy still remains for
Hubble Constant determinations using Cepheids vs. the cosmic microwave
background or calibrations to the tip of the red-giant branch. Therefore,
refinement of Cepheid period-luminosity relations continues to be an active
topic of research.
Cepheids are also important laboratories for testing stellar physics. This
paper explores outstanding questions in Cepheid evolution and pulsation
modeling. We examine the discrepancy between Cepheid masses determined from
pulsation properties and binary orbital dynamics and those determined using
stellar evolution models. We review attempts to resolve the discrepancy by
including rotation, convective overshooting, and mass loss. We review the
impact of uncertainties in nuclear reaction rates on Cepheid evolution and the
extent of blue loops in the Hertzsprung-Russell diagram. We consider
implications for Cepheids of stellar opacity revisions suggested in light of
findings for the Sun and other types of variable stars.
We apply the 1-D open-source MESA stellar evolution code and the MESA radial
stellar pulsation (RSP) nonlinear hydrodynamics code to investigate changes in
input physics for Cepheid models. We touch on progress in 2-D and 3-D stellar
modeling applied to Cepheids. Additional areas in which Cepheid models are
being tested against observations include: predicting the edges of the Cepheid
pulsation instability strip; predicting period-change rates and implications
for instability strip crossings; explaining period and amplitude modulations
and periodicities that may be non-radial pulsation modes; discovering what can
be learned from Cepheid observations in X-ray, ultraviolet, and radio
wavelengths. We also show a few examples of Cepheid light curves from NASA TESS
photometry.Comment: 10 pages, 17 figures. Revised and updated version of conference
proceedings submitted for 2023 Society for Astronomical Sciences Symposium on
Telescope Science, June 22-24, 2023, eds. J.C. Martin, R.K. Buchheim, R.M.
Gill, W. Green, and J. Menk
Variable Blue Straggler Stars in Open Cluster NGC 6819 Observed in the Kepler 'Superstamp' Field
NGC 6819 is an open cluster of age 2.4 Gyr that was in the NASA Kepler
spacecraft field of view from 2009 to 2013. The central part of the cluster was
observed in a 200 x 200 pixel `superstamp' during these four years in 30-minute
cadence photometry, providing a unique long time-series high-precision data
set. The cluster contains 'blue straggler' stars, i.e., stars on the main
sequence above the cluster turnoff that should have left the main sequence to
become red giants. We present light curves and pulsation frequency analyses
derived from custom photometric reductions for five confirmed cluster
members--four blue stragglers and one star near the main-sequence turnoff. Two
of these stars show a rich spectrum of Scuti pulsation modes, with 236
and 124 significant frequencies identified, respectively, while two stars show
mainly low-frequency modes, characteristic of Doradus variable stars.
The fifth star, a known active x-ray binary, shows only several harmonics of
two main frequencies. For the two Scuti stars, we use a frequency
separation--mean-density relation to estimate mean density, and then use this
value along with effective temperature to derive stellar mass and radius. For
the two stars showing low frequencies, we searched for period-spacing sequences
that may be representative of gravity-mode or Rossby-mode sequences, but found
no clear sequences. The common age for the cluster members, considered along
with the frequencies, will provide valuable constraints for asteroseismic
analyses, and may shed light on the origin of the blue stragglers.Comment: Accepted for publication in The Astronomical Journal February 23,
2023. 36 pages, 14 figures, 4 tables in text, 5 in Appendi