270 research outputs found
Convective proton and He3 ingestion into helium burning: Nucleosynthesis during a post-AGB thermal pulse
A thermal pulse during the post-AGB phase of stellar evolution may lead to a
unique mode of light element nucleosynthesis. The stage is set by the ingestion
of the unprocessed envelope material into the hot He-flash convection zone
below. If the temperature is sufficiently large and the C12 abundance high
enough (e.g. T_8 > 0.8, X(C12) ~ 0.4 and X(H) ~ 1E-3) protons react faster with
C12 and form C13 than destroying Be7 The latter forms by alpha-capture of He3
after an initial reduction of the He3 abundance to about 3E-5 X(He4) by the ppI
reaction He3(He3,2p)He4 (for T_8 ~ 1). All He3 is burned within minutes to
weeks depending on the temperature. Be7 is now present at about the previously
mentioned level of He3. Its further fate is determined by the reactions
Be7(e-,nu)Li7 and the alpha-capture reactions of Be7 and Li7. These captures
lead to the production of Be11 which in turn is finally destroyed by
Be11(alpha,n)N14$. The details of this mechanism of light element production in
real stars is expected to be fairly dependent on the description of mixing.Comment: 4 pages, 2 figures, conference NIC2000, Aarhus, Denmark, to appear in
Nucl. Phys.
Wolf-Rayet stars in the Small Magellanic Cloud as testbed for massive star evolution
The majority of Wolf-Rayet (WR) stars represent the stripped cores of evolved
massive stars who lost most of their hydrogen envelope. In low metallicity
environments, such as the Small Magellanic Cloud (SMC), stellar winds are
weaker and binary interaction is expected to dominate WR-star formation.
However, the WR binary fraction appears to be ~40% at any metallicity. We use
the recently determined physical properties of the twelve known SMC WR stars to
explore their possible formation channels through comparisons with grids of SMC
models, simulated with the detailed stellar evolution code MESA. These include
models of rapidly rotating single stars, which experience (partial) chemically
homogeneous evolution (CHE). We find that CHE is not able to account for the
majority of the SMC WR stars. However, the apparently single WN star SMC AB12
and the double WR system SMC AB5 (HD 5980) appear consistent with this channel.
We also analyze core helium burning stellar models assuming constant hydrogen
gradients in their envelopes. We find a dichotomy in the envelope hydrogen
gradients required to explain the observed temperatures of the SMC WR stars.
Shallow gradients are found for the WR stars with O star companions, consistent
with binary models where mass transfer occurs early, which is in agreement with
their binary properties. On the other hand, much steeper hydrogen gradients are
inferred for the group of hot apparently single WR stars. Since the hydrogen
profiles in post main sequence models of massive stars become steeper with
time, we conclude that these stars have likely been stripped by a companion
during a phase of common envelope evolution. The companions, either main
sequence stars or compact objects, are expected to still be present. A
corresponding search might identify the first immediate double black hole
binary progenitor with masses as high as those detected in GW150914.Comment: 17 pages, 23 figures, accepted by Astronomy and Astrophysic
Pulsations of red supergiant pair-instability supernova progenitors leading to extreme mass loss
Recent stellar evolution models show consistently that very massive
metal-free stars evolve into red supergiants shortly before they explode. We
argue that the envelopes of these stars, which will form pair-instability
supernovae, become pulsationally unstable and that this will lead to extreme
mass-loss rates despite the tiny metal content of the envelopes. We investigate
the pulsational properties of such models and derive pulsationally induced
mass-loss rates, which take the damping effects of the mass loss on the
pulsations selfconsistently into account. We find that the pulsations may
induce mass-loss rates of ~ 1e-4 - 1e-2 Msun/yr shortly before the explosions,
which may create a dense circumstellar medium. Our results show that very
massive stars with dense circumstellar media may stem from a wider initial mass
range than pulsational-pair instability supernovae. The extreme mass loss will
cease when so much of the hydrogen-rich envelope is lost that the star becomes
more compact and stops pulsating. The helium core of these stars therefore
remains unaffected, and their fate as pair-instability supernovae remains
unaltered. The existence of dense circumstellar media around metal-free
pair-instability supernovae can make them brighter and bluer, and they may be
easier to detect at high redshifts than previously expected. We argue that the
mass-loss enhancement in pair-instability supernova progenitors can naturally
explain some observational properties of superluminous supernovae: the
energetic explosions of stars within hydrogen-rich dense circumstellar media
with little 56Ni production and the lack of a hydrogen-rich envelope in
pair-instability supernova candidates with large 56Ni production.Comment: 11 pages, 10 figures, 1 table, accepted by Astronomy & Astrophysics,
proofed in v
3D Smoothed Particle Hydrodynamics Models of Betelgeuse's Bow Shock
Betelgeuse, the bright red supergiant (RSG) in Orion, is a runaway star. Its
supersonic motion through the interstellar medium has resulted in the formation
of a bow shock, a cometary structure pointing in the direction of motion. We
present the first 3D hydrodynamic simulations of the formation and evolution of
Betelgeuse's bow shock. We show that the bow shock morphology depends
substantially on the growth timescale for Rayleigh-Taylor versus
Kelvin-Helmholtz instabilities. We discuss our models in light of the recent
Herschel, GALEX and VLA observations. If the mass in the bow shock shell is low
(~few x 0.001 Msun), as seems to be implied by the AKARI and Herschel
observations, then Betelgeuse's bow shock is very young and is unlikely to have
reached a steady state. The circular, smooth bow shock shell is consistent with
this conclusion. We further discuss the implications of our results, in
particular, the possibility that Betelgeuse may have only recently entered the
RSG phase.Comment: 9 pages, 4 figures, Betelgeuse workshop, November 2012, Paris. To be
published in the European Astronomical Society Publications Series, editors:
Pierre Kervella, Thibaut Le Bertre & Guy Perri
Progenitors of ultra-stripped supernovae
The explosion of ultra-stripped stars in close binaries may explain new
discoveries of weak and fast optical transients. We have demonstrated that
helium star companions to neutron stars (NSs) may evolve into naked metal cores
as low as ~1.5 Msun, barely above the Chandrasekhar mass limit, by the time
they explode. Here we present a new systematic investigation of the progenitor
evolution leading to such ultra-stripped supernovae (SNe), in some cases
yielding pre-SN envelopes of less than 0.01 Msun. We discuss the nature of
these SNe (electron-capture vs iron core-collapse) and their observational
light-curve properties. Ultra-stripped SNe are highly relevant for binary
pulsars, as well as gravitational wave detection of merging NSs by LIGO/VIRGO,
since these events are expected to produce mainly low-kick NSs in the mass
range 1.10-1.80 Msun.Comment: 7 pages, 5 figures, NS4 talk presented at the Marcel Grossmann
Meeting (MG14), Rome, July 201
The formation of low-mass helium white dwarfs orbiting pulsars: Evolution of low-mass X-ray binaries below the bifurcation period
Millisecond pulsars (MSPs) are generally believed to be old neutron stars
(NSs) which have been spun up to high rotation rates via accretion of matter
from a companion star in a low-mass X-ray binary (LMXB). However, many details
of this recycling scenario remain to be understood. Here we investigate binary
evolution in close LMXBs to study the formation of radio MSPs with low-mass
helium white dwarf companions (He WDs) in tight binaries with orbital periods
P_orb = 2-9 hr. In particular, we examine: i) if such observed systems can be
reproduced from theoretical modelling using standard prescriptions of orbital
angular momentum losses (i.e. with respect to the nature and the strength of
magnetic braking), ii) if our computations of the Roche-lobe detachments can
match the observed orbital periods, and iii) if the correlation between WD mass
and orbital period (M_WD, P_orb) is valid for systems with P_orb < 2 days.
Numerical calculations with a detailed stellar evolution code were used to
trace the mass-transfer phase in ~ 400 close LMXB systems with different
initial values of donor star mass, NS mass, orbital period and the so-called
gamma-index of magnetic braking. Subsequently, we followed the orbital and the
interior evolution of the detached low-mass (proto) He WDs, including stages
with residual shell hydrogen burning. We find that a severe fine-tuning is
necessary to reproduce the observed MSPs in tight binaries with He WD
companions of mass < 0.20 M_sun, which suggests that something needs to be
modified or is missing in the standard input physics of LMXB modelling. We
demonstrate that the theoretically calculated (M_WD, P_orb)-relation is in
general also valid for systems with P_orb < 2 days, although with a large
scatter in He WD masses between 0.15-0.20 M_sun. The results of the thermal
evolution of the (proto) He WDs are reported in a follow-up paper (Paper II).Comment: 14 pages, 13 figures, 1 table, A&A, accepte
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