265 research outputs found
The first binary star evolution model producing a Chandrasekhar mass white dwarf
Today, Type Ia supernovae are essential tools for cosmology, and recognized
as major contributors to the chemical evolution of galaxies. The construction
of detailed supernova progenitor models, however, was so far prevented by
various physical and numerical difficulties in simulating binary systems with
an accreting white dwarf component, e.g., unstable helium shell burning which
may cause significant expansion and mass loss. Here, we present the first
binary evolution calculation which models both stellar components and the
binary interaction simultaneously, and where the white dwarf mass grows up to
the Chandrasekhar limit by mass accretion. Our model starts with a 1.6 Msun
helium star and a 1.0 Msun CO white dwarf in a 0.124 day orbit. Thermally
unstable mass transfer starts when the CO core of the helium star reaches 0.53
Msun, with mass transfer rates of 1...8 times 10^{-6} Msun/yr. The white dwarf
burns the accreted helium steadily until the white dwarf mass has reached ~ 1.3
Msun and weak thermal pulses follow until carbon ignites in the center when the
white dwarf reaches 1.37 Msun. Although the supernova production rate through
this channel is not well known, and this channel can not be the only one as its
progenitor life time is rather short (~ 10^7 - 10^8 yr), our results indicate
that helium star plus white dwarf systems form a reliable route for producing
Type Ia supernovae.Comment: 4 pages, 5 figure
Type Ib/c supernovae in binary systems I. Evolution and properties of the progenitor stars
We investigate the evolution of Type Ib/c supernova (SN Ib/c) progenitors in
close binary systems, using new evolutionary models that include the effects of
rotation, with initial masses of 12 - 25 Msun for the primary components, and
of single helium stars with initial masses of 2.8 - 20 Msun. We find that,
despite the impact of tidal interaction on the rotation of primary stars, the
amount of angular momentum retained in the core at the presupernova stage in
different binary model sequences converge to a value similar to those found in
previous single star models. This amount is large enough to produce millisecond
pulsars, but too small to produce magnetars or long gamma-ray bursts. We employ
the most up-to-date estimate for the Wolf-Rayet mass loss rate, and its
implications for SN Ib/c progenitors are discussed in detail. In terms of
stellar structure, SN Ib/c progenitors in binary systems are predicted to have
a wide range of final masses even up to 7 Msun, with helium envelopes of 0.16 -
1.5 Msun. Our results indicate that, if the lack of helium lines in the spectra
of SNe Ic were due to small amounts of helium, the distribution of both initial
and final masses of SN Ic progenitors should be bimodal. Furthermore, we find
that a thin hydrogen layer (0.001 - 0.01 Msun) is expected to be present in
many SN Ib progenitors at the presupernova stage. We show that the presence of
hydrogen, together with a rather thick helium envelope, can lead to a
significant expansion of some SN Ib/c progenitors by the time of supernova
explosion. This may have important consequences for the shock break-out and
supernova light curve. We also argue that some SN progenitors with thin
hydrogen layers produced via Case AB/B transfer might be related to Type IIb
supernova progenitors with relatively small radii of about 10 Rsun.Comment: 16 pages, 15 figures, 2 tables, ApJ, in pres
The most massive progenitors of neutron stars: CXO J164710.2-455216
The evolution leading to the formation of a neutron star in the very young
Westerlund 1 star cluster is investigated. The turnoff mass has been estimated
to be 35 Msun, indicating a cluster age ~ 3-5 Myr. The brightest X-ray source
in the cluster, CXO J164710.2-455216, is a slowly spinning (10 s) single
neutron star and potentially a magnetar. Since this source was argued to be a
member of the cluster, the neutron star progenitor must have been very massive
(M_zams > 40 Msun) as noted by Muno et al. (2006). Since such massive stars are
generally believed to form black holes (rather than neutron stars), the
existence of this object poses a challenge for understanding massive star
evolution. We point out while single star progenitors below M_zams < 20 Msun
form neutron stars, binary evolution completely changes the progenitor mass
range. In particular, we demonstrate that mass loss in Roche lobe overflow
enables stars as massive as 50-80 Msun, under favorable conditions, to form
neutron stars. If the very high observed binary fraction of massive stars in
Westerlund 1 (> 70 percent) is considered, it is natural that CXO
J164710.2-455216 was formed in a binary which was disrupted in a supernova
explosion such that it is now found as a single neutron star. Hence, the
existence of a neutron star in a given stellar population does not necessarily
place stringent constraints on progenitor mass when binary interactions are
considered. It is concluded that the existence of a neutron star in Westerlund
1 cluster is fully consistent with the generally accepted framework of stellar
evolution.Comment: 5 pages of text and 4 figures (submitted to Astrophysical Journal
Theoretical Black Hole Mass Distributions
We derive the theoretical distribution function of black hole masses by
studying the formation processes of black holes. We use the results of recent
2D simulations of core-collapse to obtain the relation between remnant and
progenitor masses and fold it with an initial mass function for the
progenitors. We examine how the calculated black-hole mass distributions are
modified by (i) strong wind mass loss at different evolutionary stages of the
progenitors, and (ii) the presence of close binary companions to the black-hole
progenitors. Thus, we are able to derive the binary black hole mass
distribution. The compact remnant distribution is dominated by neutron stars in
the mass range 1.2-1.6Msun and falls off exponentially at higher remnant
masses. Our results are most sensitive to mass loss from winds which is even
more important in close binaries. Wind mass-loss causes the black hole
distribution to become flatter and limits the maximum possible black-hole mass
(<10-15Msun). We also study the effects of the uncertainties in the explosion
and unbinding energies for different progenitors. The distributions are
continuous and extend over a broad range. We find no evidence for a gap at low
values (3-5Msun) or for a peak at higher values (~7Msun) of black hole masses,
but we argue that our black hole mass distribution for binaries is consistent
with the current sample of measured black-hole masses in X-ray transients. We
discuss possible biases against the detection or formation of X-ray transients
with low-mass black holes. We also comment on the possibility of black-hole
kicks and their effect on binaries.Comment: 22 pages, submitted to Ap
Late Emission from the Type Ib/c SN 2001em: Overtaking the Hydrogen Envelope
The Type Ib/c supernova SN 2001em was observed to have strong radio, X-ray,
and Halpha emission at an age of about 2.5 yr. Although the radio and X-ray
emission have been attributed to an off-axis gamma-ray burst, we model the
emission as the interaction of normal SN Ib/c ejecta with a dense, massive (3
Msun) circumstellar shell at a distance about 7 x 10^{16} cm. We investigate
two models, in which the circumstellar shell has or has not been overtaken by
the forward shock at the time of the X-ray observation. The circumstellar shell
was presumably formed by vigorous mass loss with a rate (2-10) x 10^{-3}
Msun/yr at 1000-2000 yr prior to the supernova explosion. The hydrogen envelope
was completely lost, and subsequently was swept up and accelerated by the fast
wind of the presupernova star up to a velocity of 30-50 km/s. Although
interaction with the shell can explain most of the late emission properties of
SN 2001em, we need to invoke clumping of the gas to explain the low absorption
at X-ray and radio wavelengths.Comment: 26 pages, 4 figures, ApJ submitte
- …