101 research outputs found
An experimental setup for high resolution 10.5 eV laser-based angle-resolved photoelectron spectroscopy using a time-of-flight electron analyzer
We present an experimental setup for laser-based angle-resolved
time-of-flight (LARTOF) photoemission. Using a picosecond pulsed laser, photons
of energy 10.5 eV are generated through higher harmonic generation in xenon.
The high repetition rate of the light source, variable between 0.2-8 MHz,
enables high photoelectron count rates and short acquisition times. By using a
Time-of-Flight (ToF) analyzer with angle-resolving capabilities electrons
emitted from the sample within a circular cone of up to \pm15 degrees can be
collected. Hence, simultaneous acquisition of photoemission data for a complete
area of the Brillouin zone is possible. The current photon energy enables bulk
sensitive measurements, high angular resolution and the resulting covered
momentum space is large enough to enclose the entire Brillouin zone in cuprate
high-Tc superconductors. Fermi edge measurements on polycrystalline Au shows an
energy resolution better than 5 meV. Data from a test measurement of the
Au(111) surface state is presented along with measurements of the Fermi surface
of the high-Tc superconductor Bi2212.Comment: 9 pages, 7 figure
Direct observation of decoupled Dirac states at the interface between topological and normal insulators
Several proposed applications and exotic effects in topological insulators
rely on the presence of helical Dirac states at the interface between a
topological and a normal insulator. In the present work, we have used
low-energy angle-resolved photoelectron spectroscopy to uncover and
characterize the interface states of BiSe thin films and
BiTe/BiSe heterostuctures grown on Si(111). The results
establish that Dirac fermions are indeed present at the
topological-normal-insulator boundary and absent at the
topological-topological-insulator interface. Moreover, it is demonstrated that
band bending present within the topological-insulator films leads to a
substantial separation of the interface and surface states in energy. These
results pave the way for further studies and the realization of
interface-related phenomena in topological-insulator thin-film
heterostructures.Comment: 9 pages, 5 figure
Sensitivity of the lower edge of the pair-instability black hole mass gap to the treatment of time-dependent convection
Binary-stripped Stars as Core-collapse Supernovae Progenitors
Most massive stars experience binary interactions in their lifetimes that can alter both the surface and core structure of the stripped star with significant effects on their ultimate fate as core-collapse supernovae. However, core-collapse supernovae simulations to date have focused almost exclusively on the evolution of single stars. We present a systematic simulation study of single and binary-stripped stars with the same initial mass as candidates for core-collapse supernovae (11–21 M⊙). Generally, we find that binary-stripped stars core tend to have a smaller compactness parameter, with a more prominent, deeper silicon/oxygen interface, and explode preferentially to the corresponding single stars of the same initial mass. Such a dichotomy of behavior between these two modes of evolution would have important implications for supernovae statistics, including the final neutron star masses, explosion energies, and nucleosynthetic yields. Binary-stripped remnants are also well poised to populate the possible mass gap between the heaviest neutron stars and the lightest black holes. Our work presents an improvement along two fronts, as we self-consistently account for the pre-collapse stellar evolution and the subsequent explosion outcome. Even so, our results emphasize the need for more detailed stellar evolutionary models to capture the sensitive nature of explosion outcome
Observational predictions for Thorne-\.Zytkow objects
Thorne-ytkow objects (TO) are potential end products of the merger
of a neutron star with a non-degenerate star. In this work, we have computed
the first grid of evolutionary models of TOs with the MESA stellar
evolution code. With these models, we predict several observational properties
of TOs, including their surface temperatures and luminosities, pulsation
periods, and nucleosynthetic products. We expand the range of possible TO
solutions to cover and
. Due to the much
higher densities our TOs reach compared to previous models, if TOs
form we expect them to be stable over a larger mass range than previously
predicted, without exhibiting a gap in their mass distribution. Using the GYRE
stellar pulsation code we show that TOs should have fundamental pulsation
periods of 1000--2000 days, and period ratios of 0.2--0.3. Models
computed with a large 399 isotope fully-coupled nuclear network show a
nucleosynthetic signal that is different to previously predicted. We propose a
new nucleosynthetic signal to determine a star's status as a TO: the
isotopologues and , which will
have a shift in their spectral features as compared to stable
titanium-containing molecules. We find that in the local Universe (~SMC
metallicities and above) TOs show little heavy metal enrichment,
potentially explaining the difficulty in finding TOs to-date.Comment: 17 pages, 16 figures, 3 Tables, Sumbitedd to MNRAS, Zenodo data
available https://doi.org/10.5281/zenodo.453442
Sensitivity of the lower-edge of the pair instability black hole mass gap to the treatment of time dependent convection
Gravitational-wave detections are now probing the black hole (BH) mass
distribution, including the predicted pair-instability mass gap. These data
require robust quantitative predictions, which are challenging to obtain. The
most massive BH progenitors experience episodic mass ejections on timescales
shorter than the convective turn-over timescale. This invalidates the
steady-state assumption on which the classic mixing-length theory relies. We
compare the final BH masses computed with two different versions of the stellar
evolutionary code \texttt{MESA}: (i) using the default implementation of
\cite{paxton:18} and (ii) solving an additional equation accounting for the
timescale for convective deceleration. In the second grid, where stronger
convection develops during the pulses and carries part of the energy, we find
weaker pulses. This leads to lower amounts of mass being ejected and thus
higher final BH masses of up to \,. The differences are much
smaller for the progenitors which determine the maximum mass of BHs below the
gap. This prediction is robust at , at least
within the idealized context of this study. This is an encouraging indication
that current models are robust enough for comparison with the present-day
gravitational-wave detections. However, the large differences between
individual models emphasize the importance of improving the treatment of
convection in stellar models, especially in the light of the data anticipated
from the third generation of gravitational wave detectors.Comment: 7 pages + 1 appendix, accepted in MNRAS, online results at
https://zenodo.org/record/340632
Predictions for the hydrogen-free ejecta of pulsational pair-instability supernovae
Present time-domain astronomy efforts will unveil a variety of rare
transients. We focus here on pulsational pair-instability evolution, which can
result in signatures observable with electromagnetic and gravitational waves.
We simulate grids of bare helium stars to characterize the resulting black hole
(BH) masses and ejecta composition, velocity, and thermal state. The stars do
not react "elastically" to the thermonuclear explosion: there is not a
one-to-one correspondence between pair-instability driven ignition and mass
ejections, causing ambiguity in what is an observable pulse. In agreement with
previous studies, we find that for carbon-oxygen core masses 28Msun<
M_CO<30.5Msun the explosions are not strong enough to affect the surface. With
increasing mass, they first cause large radial expansion
(30.5Msun<M_CO<31.4Msun), and finally, also mass ejection episodes
(M_CO>31.4Msun). The lowest mass to be fully disrupted in a pair-instability
supernova is M_CO=57Msun. Models with M_CO>121Msun reach the
photodisintegration regime, resulting in BHs with M_BH>125Msun. If the
pulsating models produce BHs via (weak) explosions, the previously-ejected
material might be hit by the blast wave. We characterize the H-free
circumstellar material from the pulsational pair-instability of helium cores
assuming simply that the ejecta maintain a constant velocity after ejection.
Our models produce He-rich ejecta with mass 10^{-3}Msun<M_CSM<40Msun. These
ejecta are typically launched at a few thousand \kms and reach distances of
~10^{12}-10^{15} cm before core-collapse. The delays between mass ejection
events and the final collapse span a wide and mass-dependent range (from
sub-hour to 10^4 years), and the shells ejected can also collide with each
other. The range of properties we find suggests a possible connection with
(some) type Ibn supernovae.Comment: accepted versio
Massive runaway and walkaway stars:A study of the kinematical imprints of the physical processes governing the evolution and explosion of their binary progenitors
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