536 research outputs found
Implications of WMAP Observations On the Population III Star Formation Processes
In an earlier paper (Cen 2003) we pointed out the strong likelihood for the
universal reionization to occur twice, giving rise to a larger Thomson optical
depth. Here we perform a more focused analysis of the Thomson optical depth in
light of the WMAP observations. While the current uncertainties on the observed
Thomson optical depth are still relatively large, with tau_e=0.17+-0.04 (68%)
(Kogut et al 2003), important implications on Pop III star formation processes
at high redshift can already be inferred. We are able to draw four conclusions:
(1) in the absence of a top-heavy initial stellar mass function (IMF) for Pop
III metal-free stars and without a dramatic upturn in the star formation
efficiency and ionizing photon escape fraction at high redshift (z>6), we find
tau_e =< 0.09; (2) with a top-heavy IMF for the Pop III metal-free stars and
plausible star formation efficiency and ionizing photon escape fraction, it is
expected that tau_e =< 0.12; (3) it is possible to reach tau_e = 0.15, if the
metal enrichment efficiency of the intergalactic medium by Pop III stars is
very low thus Pop III era is prolonged; (4) to reach tau_e >= 0.17 requires
either of the following two conditions: the cosmological model power index n is
positively tilted to n >= 1.03, Pop III star formation in minihalos with
molecular hydrogen cooling has an efficiency c_*(H_2,III)>0.01 (with ionizing
photon escape fraction greater than 30%). If the current observed value of
Thomson optical depth withstands future data, we will have strong observational
evidence that Pop III stars are massive and their formation efficiency may be
much higher than current theoretical works suggest. Alternatively, there may be
unknown, non-stellar ionizing sources at very high redshift.Comment: a numerical error corrected, conclusions strengthened, submitted to
ApJ Letters, 13 page
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
Instabilities of captured shocks in the envelopes of massive stars
The evolution of strange mode instabilities into the non linear regime has
been followed by numerical simulation for an envelope model of a massive star
having solar chemical composition, M=50M_sun, T_eff=10^4K and L=1.17*10^6
L_sun. Contrary to previously studied models, for these parameters shocks are
captured in the H-ionisation zone and perform rapid oscillations within the
latter. A linear stability analysis is performed to verify that this behaviour
is physical. The origin of an instability discovered in this way is identified
by construction of an analytical model. As a result, the stratification turns
out to be essential for instability. The difference to common stratification
instabilities, e.g., convective instabilities, is discussed.Comment: 16 pages, 6 figures, accepted for publication in MNRA
Nonradial oscillations in classical Cepheids: the problem revisited
We analyse the presence of nonradial oscillations in Cepheids, a problem
which has not been theoretically revised since the work of Dziembowsky (1977)
and Osaki (1977). Our analysis is motivated by a work of Moskalik et al. (2004)
which reports the detec tion of low amplitude periodicities in a few Cepheids
of the large Magellanic cloud. These newly discovered periodicities were
interpreted as nonradial modes.} {Based on linear nonadiabatic stability
analysis, our goal is to reanalyse the presence and stability of nonradial
modes, taking into account improvement in the main input phys ics required for
the modelling of Cepheids.} {We compare the results obtained from two different
numerical methods used to solve the set of differential equations: a matrix
method and the Ricatti method.} {We show the limitation of the matrix method to
find low order p-modes (), because of their dual character in evolved
stars such as Cepheids. For higher order p-modes, we find an excellent
agreement between the two methods.} {No nonradial instability is found below
, whereas many unstable nonradial modes exist for higher orders. We also
find that nonradial modes remain unstable, even at hotter effective
temperatures than the blue edge of the Cepheid instability strip, where no
radial pulsations are expected.Comment: Accepted for publication in A&A; 7 pages, 8 figure
Galactic Twins of the Ring Nebula Around SN1987A and a Possible LBV-like Phase for Sk-69 202
Some core-collapse supernovae show clear signs of interaction with dense
circumstellar material that often appears to be non-spherical. Circumstellar
nebulae around supernova progenitors provide clues to the origin of that
asymmetry in immediate pre-supernova evolution. Here I discuss outstanding
questions about the formation of the ring nebula around SN1987A and some
implications of similar ring nebulae around Galactic B supergiants. Several
clues hint that SN1987A's nebula may have been ejected in an LBV-like event,
rather than through interacting winds in a transition from a red supergiant to
a blue supergiant.Comment: 2 pages, to appear in procedings of "Massive stars: fundamental
parameters and circumstellar interactions", conference in honor of Virpi
Niemela's 70th birthda
A Multi-scale Approach for Simulations of Kelvin Probe Force Microscopy with Atomic Resolution
The distance dependence and atomic-scale contrast observed in nominal contact
potential difference (CPD) signals recorded by KPFM on surfaces of insulating
and semiconducting samples, have stimulated theoretical attempts to explain
such effects. We attack this problem in two steps. First, the electrostatics of
the macroscopic tip-cantilever-sample system is treated by a finite-difference
method on an adjustable nonuniform mesh. Then the resulting electric field
under the tip apex is inserted into a series of atomistic wavelet-based density
functional theory (DFT) calculations. Results are shown for a realistic neutral
but reactive silicon nano-scale tip interacting with a NaCl(001) sample.
Bias-dependent forces and resulting atomic displacements are computed to within
an unprecedented accuracy. Theoretical expressions for amplitude modulation
(AM) and frequency modulation (FM) KPFM signals and for the corresponding local
contact potential differences (LCPD) are obtained by combining the macroscopic
and atomistic contributions to the electrostatic force component generated at
the voltage modulation frequency, and evaluated for several tip oscillation
amplitudes A up to 10 nm. Being essentially constant over a few Volts, the
slope of atomistic force versus bias is the basic quantity which determines
variations of the atomic-scale LCPD contrast. Already above A = 0.1 nm, the
LCPD contrasts in both modes exhibit almost the same spatial dependence as the
slope. In the AM mode, this contrast is approximately proportional to
, but remains much weaker than the contrast in the FM mode, which
drops somewhat faster as A is increased. These trends are a consequence of the
macroscopic contributions to the KPFM signal, which are stronger in the AM-mode
and especially important if the sample is an insulator even at sub-nanometer
separations where atomic-scale contrast appears.Comment: 19 pages, 13 figure
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
The Structure of the Homunculus. III. Forming a Disk and Bipolar Lobes in a Rotating Surface Explosion
We present a semi-analytic model for shaping the nebula around eta Carinae
that accounts for the simultaneous production of bipolar lobes and an
equatorial disk through a rotating surface explosion. Material is launched
normal to the surface of an oblate rotating star with an initial kick velocity
that scales approximately with the local escape speed. Thereafter, ejecta
follow ballistic orbital trajectories, feeling only a central force
corresponding to a radiatively reduced gravity. Our model is conceptually
similar to the wind-compressed disk model of Bjorkman & Cassinelli, but we
modify it to an explosion instead of a steady line-driven wind, we include a
rotationally-distorted star, and we treat the dynamics somewhat differently.
Continuum-driving avoids the disk inhibition that normally operates in
line-driven winds. Our model provides a simple method by which rotating hot
stars can simultaneously produce intrinsically bipolar and equatorial mass
ejections, without an aspherical environment or magnetic fields. Although
motivated by eta Carinae, the model may have generic application to other LBVs,
B[e] stars, or SN1987A's nebula. When near-Eddington radiative driving is less
influential, our model generalizes to produce bipolar morphologies without
disks, as seen in many PNe.Comment: ApJ accepted, 9 page
The Nature of the Radiative Hydrodynamic Instabilities in Radiatively Supported Thomson Atmospheres
Atmospheres having a significant radiative support are shown to be
intrinsically unstable at luminosities above a critical fraction Gamma_crit ~
0.5-0.85 of the Eddington limit, with the exact value depending on the boundary
conditions. Two different types of absolute radiation-hydrodynamic
instabilities of acoustic waves are found to take place even in the electron
scattering dominated limit. Both instabilities grow over dynamical time scales
and both operate on non radial modes. One is stationary and arises only after
the effects of the boundary conditions are taken into account, while the second
is a propagating wave and is insensitive to the boundary conditions. Although a
significant wind can be generated by these instabilities even below the
classical Eddington luminosity limit, quasi-stable configurations can exist
beyond the Eddington limit due to the generally reduced effective opacity.
The study is done using a rigorous numerical linear analysis of a gray plane
parallel atmosphere under the Eddington approximation. We also present more
simplified analytical explanations.Comment: 18 Pages, 7 figures, uses emulateapj5.sty, accepted to Ap
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