181 research outputs found
Relative Importance of Convective Uncertainties in Massive Stars
In this work, we investigate the impact of uncertainties due to convective boundary mixing (CBM), commonly called âovershootâ, namely the boundary location and the amount of mixing at the convective boundary, on stellar structure and evolution. For this we calculated two grids of stellar evolution models with the MESA code, each with the Ledoux and the Schwarzschild boundary criterion, and vary the amount of CBM. We calculate each grid with the initial masses 15, 20 and . We present the stellar structure of the models during the hydrogen and helium burning phases. In the latter, we examine the impact on the nucleosynthesis. We find a broadening of the main-sequence with more CBM, which is more in agreement with observations. Furthermore during the core hydrogen burning phase there is a convergence of the convective boundary location due to CBM. The uncertainties of the intermediate convective zone remove this convergence. The behaviour of this convective zone strongly affects the surface evolution of the model, i.e. how fast it evolves red-wards. The amount of CBM impacts the size of the convective cores and the nucleosynthesis, e.g. the 12C to 16O ratio and the weak s-process. Lastly, we determine the uncertainty that the range of parameter values investigated introduce and we find differences of up to for the core masses and the total mass of the star
Neutrino-driven Explosions
The question why and how core-collapse supernovae (SNe) explode is one of the
central and most long-standing riddles of stellar astrophysics. A solution is
crucial for deciphering the SN phenomenon, for predicting observable signals
such as light curves and spectra, nucleosynthesis, neutrinos, and gravitational
waves, for defining the role of SNe in the evolution of galaxies, and for
explaining the birth conditions and properties of neutron stars (NSs) and
stellar-mass black holes. Since the formation of such compact remnants releases
over hundred times more energy in neutrinos than the SN in the explosion,
neutrinos can be the decisive agents for powering the SN outburst. According to
the standard paradigm of the neutrino-driven mechanism, the energy transfer by
the intense neutrino flux to the medium behind the stagnating core-bounce
shock, assisted by violent hydrodynamic mass motions (sometimes subsumed by the
term "turbulence"), revives the outward shock motion and thus initiates the SN
blast. Because of the weak coupling of neutrinos in the region of this energy
deposition, detailed, multidimensional hydrodynamic models including neutrino
transport and a wide variety of physics are needed to assess the viability of
the mechanism. Owing to advanced numerical codes and increasing supercomputer
power, considerable progress has been achieved in our understanding of the
physical processes that have to act in concert for the success of
neutrino-driven explosions. First studies begin to reveal observational
implications and avenues to test the theoretical picture by data from
individual SNe and SN remnants but also from population-integrated observables.
While models will be further refined, a real breakthrough is expected through
the next Galactic core-collapse SN, when neutrinos and gravitational waves can
be used to probe the conditions deep inside the dying star. (abridged)Comment: Author version of chapter for 'Handbook of Supernovae,' edited by A.
Alsabti and P. Murdin, Springer. 54 pages, 13 figure
Explosive Nucleosynthesis: What we learned and what we still do not understand
This review touches on historical aspects, going back to the early days of
nuclear astrophysics, initiated by BFH and Cameron, discusses (i) the
required nuclear input from reaction rates and decay properties up to the
nuclear equation of state, continues (ii) with the tools to perform
nucleosynthesis calculations and (iii) early parametrized nucleosynthesis
studies, before (iv) reliable stellar models became available for the late
stages of stellar evolution. It passes then through (v) explosive environments
from core-collapse supernovae to explosive events in binary systems (including
type Ia supernovae and compact binary mergers), and finally (vi) discusses the
role of all these nucleosynthesis production sites in the evolution of
galaxies. The focus is put on the comparison of early ideas and present, very
recent, understanding.Comment: 11 pages, to appear in Springer Proceedings in Physics (Proc. of
Intl. Conf. "Nuclei in the Cosmos XV", LNGS Assergi, Italy, June 2018
The Extremes of Thermonuclear Supernovae
The majority of thermonuclear explosions in the Universe seem to proceed in a
rather standardised way, as explosions of carbon-oxygen (CO) white dwarfs in
binary systems, leading to 'normal' Type Ia supernovae (SNe Ia). However, over
the years a number of objects have been found which deviate from normal SNe Ia
in their observational properties, and which require different and not seldom
more extreme progenitor systems. While the 'traditional' classes of peculiar
SNe Ia - luminous '91T-like' and faint '91bg-like' objects - have been known
since the early 1990s, other classes of even more unusual transients have only
been established 20 years later, fostered by the advent of new wide-field SN
surveys such as the Palomar Transient Factory. These include the faint but
slowly declining '02es-like' SNe, 'Ca-rich' transients residing in the
luminosity gap between classical novae and supernovae, extremely short-lived,
fast-declining transients, and the very luminous so-called
'super-Chandrasekhar' SNe Ia. Not all of them are necessarily thermonuclear
explosions, but there are good arguments in favour of a thermonuclear origin
for most of them. The aim of this chapter is to provide an overview of the zoo
of potentially thermonuclear transients, reviewing their observational
characteristics and discussing possible explosion scenarios.Comment: Author version of a chapter for the 'Handbook of Supernovae', edited
by A. Alsabti and P. Murdin, Springer. 50 pages, 7 figure
Convective core entrainment in 1D main-sequence stellar models
3D hydrodynamics models of deep stellar convection exhibit turbulent entrainment at the convective-radiative boundary which follows the entrainment law, varying with boundary penetrability. We implement the entrainment law in the 1D Geneva stellar evolution code. We then calculate models between 1.5 and 60âMâ at solar metallicity (Z = 0.014) and compare them to previous generations of models and observations on the main sequence. The boundary penetrability, quantified by the bulk Richardson number, RiB, varies with mass and to a smaller extent with time. The variation of RiB with mass is due to the mass dependence of typical convective velocities in the core and hence the luminosity of the star. The chemical gradient above the convective core dominates the variation of RiB with time. An entrainment law method can therefore explain the apparent mass dependence of convective boundary mixing through RiB. New models including entrainment can better reproduce the mass dependence of the main-sequence width using entrainment law parameters A ⌠2 Ă 10â4 and n = 1. We compare these empirically constrained values to the results of 3D hydrodynamics simulations and discuss implications
Hypernova Nucleosynthesis and Galactic Chemical Evolution
We study nucleosynthesis in 'hypernovae', i.e., supernovae with very large
explosion energies ( \gsim 10^{52} ergs) for both spherical and aspherical
explosions. The hypernova yields compared to those of ordinary core-collapse
supernovae show the following characteristics: 1) Complete Si-burning takes
place in more extended region, so that the mass ratio between the complete and
incomplete Si burning regions is generally larger in hypernovae than normal
supernovae. As a result, higher energy explosions tend to produce larger [(Zn,
Co)/Fe], small [(Mn, Cr)/Fe], and larger [Fe/O], which could explain the trend
observed in very metal-poor stars. 2) Si-burning takes place in lower density
regions, so that the effects of -rich freezeout is enhanced. Thus
Ca, Ti, and Zn are produced more abundantly than in normal
supernovae. The large [(Ti, Zn)/Fe] ratios observed in very metal poor stars
strongly suggest a significant contribution of hypernovae. 3) Oxygen burning
also takes place in more extended regions for the larger explosion energy. Then
a larger amount of Si, S, Ar, and Ca ("Si") are synthesized, which makes the
"Si"/O ratio larger. The abundance pattern of the starburst galaxy M82 may be
attributed to hypernova explosions. Asphericity in the explosions strengthens
the nucleosynthesis properties of hypernovae except for "Si"/O. We thus suggest
that hypernovae make important contribution to the early Galactic (and cosmic)
chemical evolution.Comment: To be published in "The Influence of Binaries on Stellar Population
Studies", ed. D. Vanbeveren (Kluwer), 200
3D Simulations and MLT. I. Renziniâs Critique
Renzini wrote an influential critique of âovershootingâ in mixing-length theory (MLT), as used in stellar evolution codes, and concluded that three-dimensional fluid dynamical simulations were needed. Such simulations are now well tested. Implicit large eddy simulations connect large-scale stellar flow to a turbulent cascade at the grid scale, and allow the simulation of turbulent boundary layers, with essentially no assumptions regarding flow except the number of computational cells. Buoyant driving balances turbulent dissipation for weak stratification, as in MLT, but with the dissipation length replacing the mixing length. The turbulent kinetic energy in our computational domain shows steady pulses after 30 turnovers, with no discernible diminution; these are caused by the necessary lag in turbulent dissipation behind acceleration. Interactions between coherent turbulent structures give multi-modal behavior, which drives intermittency and fluctuations. These cause mixing, which may justify use of the instability criterion of Schwarzschild rather than the Ledoux. Chaotic shear flow of turning material at convective boundaries causes instabilities that generate waves and sculpt the composition gradients and boundary layer structures. The flow is not anelastic; wave generation is necessary at boundaries. A self-consistent approach to boundary layers can remove the need for ad hoc procedures of âconvective overshootingâ and âsemi-convection.â In Paper II, we quantify the adequacy of our numerical resolution in a novel way, determine the length scale of dissipationâthe âmixing lengthââwithout astronomical calibration, quantify agreement with the four-fifths law of Kolmogorov for weak stratification, and deal with strong stratification
A low energy core-collapse supernova without a hydrogen envelope
The final fate of massive stars depends on many factors, including mass,
rotation rate, magnetic fields and metallicity. Theory suggests that some
massive stars (initially greater than 25-30 solar masses) end up as Wolf-Rayet
stars which are deficient in hydrogen because of mass loss through strong
stellar winds. The most massive of these stars have cores which may form a
black hole and theory predicts that the resulting explosion produces ejecta of
low kinetic energy, a faint optical display and a small mass fraction of
radioactive nickel(1,2,3). An alternative origin for low energy supernovae is
the collapse of the oxygen-neon core of a relatively lowmass star (7-9 solar
masses) through electron capture(4,5). However no weak, hydrogen deficient,
core-collapse supernovae are known. Here we report that such faint, low energy
core-collapse supernovae do exist, and show that SN2008ha is the faintest
hydrogen poor supernova ever observed. We propose that other similar events
have been observed but they have been misclassified as peculiar thermonuclear
supernovae (sometimes labelled SN2002cx-like events(6)). This discovery could
link these faint supernovae to some long duration gamma-ray bursts. Extremely
faint, hydrogen-stripped core-collapse supernovae have been proposed to produce
those long gamma-ray bursts whose afterglows do not show evidence of
association with supernovae (7,8,9).Comment: Submitted 12 January 2009 - Accepted 24 March 200
Hydrogen-poor superluminous stellar explosions
Supernovae (SNe) are stellar explosions driven by gravitational or
thermonuclear energy, observed as electromagnetic radiation emitted over weeks
or more. In all known SNe, this radiation comes from internal energy deposited
in the outflowing ejecta by either radioactive decay of freshly-synthesized
elements (typically 56Ni), stored heat deposited by the explosion shock in the
envelope of a supergiant star, or interaction between the SN debris and
slowly-moving, hydrogen-rich circumstellar material. Here we report on a new
class of luminous SNe whose observed properties cannot be explained by any of
these known processes. These include four new SNe we have discovered, and two
previously unexplained events (SN 2005ap; SCP 06F6) that we can now identify as
members. These SNe are all ~10 times brighter than SNe Ia, do not show any
trace of hydrogen, emit significant ultra-violet (UV) flux for extended periods
of time, and have late-time decay rates which are inconsistent with
radioactivity. Our data require that the observed radiation is emitted by
hydrogen-free material distributed over a large radius (~10^15 cm) and
expanding at high velocities (>10^4 km s^-1). These long-lived, UV-luminous
events can be observed out to redshifts z>4 and offer an excellent opportunity
to study star formation in, and the interstellar medium of, primitive distant
galaxies.Comment: Accepted to Nature. Press embargoed until 2011 June 8, 18:00 U
- âŠ