150 research outputs found
Simulation of stellar instabilities with vastly different timescales using domain decomposition
Strange mode instabilities in the envelopes of massive stars lead to shock
waves, which can oscillate on a much shorter timescale than that associated
with the primary instability. The phenomenon is studied by direct numerical
simulation using a, with respect to time, implicit Lagrangian scheme, which
allows for the variation by several orders of magnitude of the dependent
variables. The timestep for the simulation of the system is reduced appreciably
by the shock oscillations and prevents its long term study. A procedure based
on domain decomposition is proposed to surmount the difficulty of vastly
different timescales in various regions of the stellar envelope and thus to
enable the desired long term simulations. Criteria for domain decomposition are
derived and the proper treatment of the resulting inner boundaries is
discussed. Tests of the approach are presented and its viability is
demonstrated by application to a model for the star P Cygni. In this
investigation primarily the feasibility of domain decomposition for the problem
considered is studied. We intend to use the results as the basis of an
extension to two dimensional simulations.Comment: 15 pages, 10 figures, published in MNRA
Zinc oxide as an ozone sensor
Journal of Applied Physics, Vol. 96, nº3This work presents a study of intrinsic zinc oxide thin film as ozone sensor based on the ultraviolet sUVd photoreduction and subsequent ozone re oxidation of zinc oxide as a fully reversible process performed at room temperature. The films analyzed were produced by spray pyrolysis, dc and rf
magnetron sputtering. The dc resistivity of the films produced by rf magnetron sputtering and constituted by nanocrystallites changes more than eight orders of magnitude when exposed to an UV dose of 4 mW/cm2. On the other hand, porous and textured zinc oxide films produced by spray pyrolysis at low substrate temperature exhibit an excellent ac impedance response where the reactance changes by more than seven orders of magnitude when exposed to the same UV dose,
with a response frequency above 15 kHz, thus showing improved ozone ac sensing
discrimination
Self-powered, flexible and room temperature operated solution processed hybrid metal halide p-type sensing element for efficient hydrogen detection
Hydrogen (H2) is a well-known reduction gas and for safety reasons is very
important to be detected. The most common systems employed along its detection
are metal oxide-based elements. However, the latter demand complex and
expensive manufacturing techniques, while they also need high temperatures or
UV light to operate effectively. In this work, we first report a solution
processed hybrid mixed halide spin coated perovskite films that have been
successfully applied as portable, flexible, self-powered, fast and sensitive
hydrogen sensing elements, operating at room temperature. The minimum
concentrations of H2 gas that could be detected was down to 10 ppm. This work
provides a new pathway on gases interaction with perovskite materials, launches
new questions that must be addressed regarding the sensing mechanisms involved
due to the utilization of halide perovskite sensing elements while also
demonstrates the potential that these materials have on beyond solar cell
applications
On the stability of very massive primordial stars
The stability of metal-free very massive stars ( = 0; M = 120 - 500
\msol) is analyzed and compared with metal-enriched stars. Such zero-metal
stars are unstable to nuclear-powered radial pulsations on the main sequence,
but the growth time scale for these instabilities is much longer than for their
metal-rich counterparts. Since they stabilize quickly after evolving off the
ZAMS, the pulsation may not have sufficient time to drive appreciable mass loss
in Z = 0 stars. For reasonable assumptions regarding the efficiency of
converting pulsational energy into mass loss, we find that, even for the larger
masses considered, the star may die without losing a large fraction of its
mass. We find a transition between the - and -mechanisms for
pulsational instability at Z\sim 2\E{-4} - 2\E{-3}. For the most metal-rich
stars, the -mechanism yields much shorter -folding times, indicating
the presence of a strong instability. We thus stress the fundamental difference
of the stability and late stages of evolution between very massive stars born
in the early universe and those that might be born today.Comment: 7 pages, 5 figures. Minor changes, more results given in Table 1,
accepted for publication in Ap
Instability of LBV-stars against radial oscillations
In this study we consider the nonlinear radial oscillations exciting in
LBV--stars with effective temperatures 1.5e4 K <= Teff <= 3e4 K, bolometric
luminosities 1.2e6 L_odot <= L <= 1.9e6 L_odot and masses 35.7 M_odot <= M <=
49.1 M_odot. Hydrodynamic computations were carried out with initial conditions
obtained from evolutionary sequences of population I stars (X=0.7, Z=0.02) with
initial masses from 70M_odot to 90 M_odot. All hydrodynamical models show
instability against radial oscillations with amplitude growth time comparable
with dynamical time scale of the star. Radial oscillations exist in the form of
nonlinear running waves propagating from the boundary of the compact core to
the upper boundary of the hydrodynamical model. The velocity amplitude of outer
layers is of several hundreds of km/s while the bolometric light amplitude does
not exceed 0.2 mag. Stellar oscillations are not driven by the kappa-mechanism
and are due to the instability of the gas with adiabatic exponent close to the
critical value Gamma_1 = 4/3 due to the large contribution of radiation in the
total pressure. The range of the light variation periods (6 day <= P <= 31 day)
of hydrodynamical models agrees with periods of microvariability observed in
LBV--stars.Comment: 14 pages, 5 figures, submitted to Astronomy Letter
Most Detects G- and P-Modes in the B Supergiant HD 163899 (B2Ib/II)
The {\it Microvariability and Oscillations of Stars (MOST)} satellite
observed the B supergiant HD 163899 (B2 Ib/II) for 37 days as a guide star and
detected 48 frequencies \la 2.8 c d with amplitudes of a few
milli-magnitudes (mmag) and less. The frequency range embraces g- and p-mode
pulsations. It was generally thought that no g-modes are excited in less
luminous B supergiants because strong radiative damping is expected in the
core. Our theoretical models, however, show that such g-modes are excited in
massive post-main-sequence stars, in accordance with these observations. The
nonradial pulsations excited in models between at and at are roughly
consistent with the observed frequency range. Excitation by the Fe-bump in
opacity is possible because g-modes can be partially reflected at a convective
zone associated with the hydrogen-burning shell, which significantly reduces
radiative damping in the core. The {\it MOST} light curve of HD 163899 shows
that such a reflection of g-modes actually occurs, and reveals the existence of
a previously unrecognized type of variable, slowly pulsating B supergiants
(SPBsg) distinct from Cyg variables. Such g-modes have great potential
for asteroseismology.Comment: 24 pages, 9 figures, Astrophysical Journal in pres
Local Radiative Hydrodynamic and Magnetohydrodynamic Instabilities in Optically Thick Media
We examine the local conditions for radiative damping and driving of short
wavelength, propagating hydrodynamic and magnetohydrodynamic (MHD) waves in
static, optically thick, stratified equilibria. We show that so-called strange
modes in stellar oscillation theory and magnetic photon bubbles are intimately
related and are both fundamentally driven by the background radiation flux
acting on compressible waves. We identify the necessary criteria for unstable
driving of these waves, and show that this driving can exist in both gas and
radiation pressure dominated media, as well as pure Thomson scattering media in
the MHD case. The equilibrium flux acting on opacity fluctuations can drive
both hydrodynamic acoustic waves and magnetosonic waves unstable. In addition,
magnetosonic waves can be driven unstable by a combination of the equilibrium
flux acting on density fluctuations and changes in the background radiation
pressure along fluid displacements. We briefly describe the conditions under
which these instabilities might be manifested in both main sequence stellar
envelopes and accretion disks.Comment: 55 pages, revised version accepted for publication by ApJ. New
appendix added justifying WKB analysi
Revisiting the Modified Eddington Limit for Massive Stars
We have determined the location of the line-opacity modified Eddington limit
for stars in the LMC using the most recent atmosphere models combined with a
precise mapping to the HR Diagram through up-to-date stellar evolution
calculations. While we find, in agreement with previous studies, that the shape
of the modified Eddington limit qualitatively corresponds to the
Humphreys-Davidson (HD) limit defined by the most luminous supergiants, the
modified limit is actually a full magnitude higher than the upper luminosity
limit observed for LMC stars. The observed limit is consistent with atmosphere
models in which the maximum value of the ratio of the radiation force outwards
to the gravitational force inwards, Y_max, is 0.9, i.e., the photospheres of
stars at the observed luminosity limit are bound. As massive stars evolve, they
move to higher, and therefore less stable values of Y_max, so mass loss, either
sporadic or continuous, may halt their natural redward evolution as they
approach the Y_max = 0.9 limit. We assess the metallicity dependence of this
limit. If the limit does determine the most luminous stars, and the value of
Y_max corresponding to the luminosity limit in the LMC is universal, then the
brightest supergiants the SMC should be only marginally brighter (0.3 mag) than
those of the LMC, in agreement with observations. Moreover, the brightest
supergiants in M31 should be 0.75~mag fainter than those in the LMC.Comment: 13 pages with 4 figures, AAS Latex, ApJ Submitted (August
MOST detects corotating bright spots on the mid-O type giant {\xi} Persei
We have used the MOST (Microvariability and Oscillations of STars)
microsatellite to obtain four weeks of contiguous high-precision broadband
visual photometry of the O7.5III(n)((f)) star {\xi} Persei in November 2011.
This star is well known from previous work to show prominent DACs (Discrete
Absorption Components) on time-scales of about 2 d from UV spectroscopy and NRP
(Non Radial Pulsation) with one (l = 3) p-mode oscillation with a period of 3.5
h from optical spectroscopy. Our MOST-orbit (101.4 min) binned photometry fails
to reveal any periodic light variations above the 0.1 mmag 3-sigma noise level
for periods of hours, while several prominent Fourier peaks emerge at the 1
mmag level in the two-day period range. These longer-period variations are
unlikely due to pulsations, including gravity modes. From our simulations based
upon a simple spot model, we deduce that we are seeing the photometric
modulation of several co-rotating bright spots on the stellar surface. In our
model, the starting times (random) and lifetimes (up to several rotations) vary
from one spot to another yet all spots rotate at the same period of 4.18 d, the
best-estimated rotation period of the star. This is the first convincing
reported case of co-rotating bright spots on an O star, with important
implications for drivers of the DACs (resulting from CIRs - Corotating
Interaction Regions) with possible bright-spot generation via a breakout at the
surface of a global magnetic field generated by a subsurface convection zone.Comment: 9 pages, 4 figures, 2 tables, MNRAS in pres
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