1,393 research outputs found
The Slow Merger of Massive Stars
We study the complete merger of two massive stars inside a common envelope
and the subsequent evolution of the merger product, a rapidly rotating massive
supergiant. Three qualitatively different types of mergers have been identified
and investigated in detail, and the post-merger evolution has been followed to
the immediate presupernova stage. The ``quiet merger'' case does not lead to
significant changes in composition, and the star remains a red supergiant. In
the case of a ``moderate merger'', the star may become a blue supergiant and
end its evolution as a blue supergiant, depending on the core to total mass
ratio (as may be appropriate for the progenitor of SN 1987A). In the case of
the most effective ``explosive merger'', the merger product stays a red giant.
In last two cases, the He abundance in the envelope is increased drastically,
but significant s-processing is mainly expected in the ``explosive merger''
case.Comment: 4 pages, 1 figure, proc. ESO/MPA/MPE Workshop "From Twilight to
Highlight: The Physics of Supernovae
The single degenerate channel for the progenitors of type Ia supernovae
We have carried out a detailed study of one of the most popular evolutionary
channels for the production of Type Ia supernova (SN Ia) progenitors, the
semi-degenerate channel (CO+MS), where a carbon/oxygen (CO) white dwarf (WD)
accretes matter from an unevolved or slightly evolved non-degenerate star until
it reaches the Chandrasekhar mass limit. Employing Eggleton's stellar evolution
code and adopting the prescription of Hachisu et al. (1999) for the accretion
efficiency, we have carried out full binary evolution calculations for about
2300 close WD binary systems and mapped out the initial parameters in the
orbital period -- secondary mass (--) plane (for a range of WD masses)
which lead to a successful Type Ia supernova. We obtained accurate, analytical
fitting formulae to describe this parameter range which can be used for binary
population synthesis (BPS) studies. The contours in the -- plane differ
from those obtained by Hachisu et al. (1999) for low-mass CO WDs, which are
more common than massive CO WDs. We show that white dwarfs with a mass as low
as can accrete efficiently and reach the Chandrasekhar limit. We
have implemented these results in a BPS study to obtain the birthrates for SNe
Ia and the evolution of birthrates with time of SNe Ia for both a constant star
formation rate and a single star burst. The birthrates are somewhat lower than
(but comparable to) those inferred observationally.Comment: 10 pages, 14 figures, submitted to MNRA
The birth rate of SNe Ia from hybrid CONe white dwarfs
Considering the uncertainties of the C-burning rate (CBR) and the treatment
of convective boundaries, Chen et al. (2014) found that there is a regime where
it is possible to form hybrid CONe white dwarfs (WDs), i.e. ONe WDs with
carbon-rich cores. As these hybrid WDs can be as massive as 1.30 ,
not much mass needs to be accreted for these objects to reach the Chandrasekhar
limit and to explode as Type Ia supernovae (SNe Ia). We have investigated their
contribution to the overall SN Ia birth rate and found that such SNe Ia tend to
be relatively young with typical time delays between 0.1 and 1 Gyr, where some
may be as young as 30 Myr. SNe Ia from hybrid CONe WDs may contribute several
percent to all SNe Ia, depending on the common-envelope ejection efficiency and
the CBR. We suggest that these SNe Ia may produce part of the 2002cx-like SN Ia
class.Comment: 4 figures, accepted for publication for ApJ Lette
The metallicity dependence of the long-duration GRB rate from host galaxy luminosities
We investigate the difference between the host galaxy properties of
core-collapse supernovae and long-duration gamma-ray bursts (LGRBs), and
quantify a possible metallicity dependence of the efficiency of producing
LGRBs. We use a sample of 16 CC SNe and 16 LGRBs from Fruchter et al. (2006)
which have similar redshift distributions to eliminate galaxy evolution biases.
We make a forward prediction of their host galaxy luminosity distributions from
the overall cosmic metallicity distribution of star formation. This appoach is
supported by the finding that LGRB hosts follow the L-Z relations of
star-forming galaxies. We then compare predictions for metallicity-dependent
event efficiencies with the observed host data. We find that UV-based SFR
estimates predict the hosts distribution of CC SNe perfectly well in a
metallicity-independent form. In contrast, LGRB hosts are fainter on average by
one magnitude, almost as faint as the Large Magellanic Cloud. Assuming this is
a metallicity effect, the present data are insufficient to discriminate between
a sharp metallicity cutoff and a soft decrease in efficiency towards higher
metallicity. For a sharp cut-off, however, we find a best value for the cutoff
metallicity, as reflected in the oxygen abundance, 12+log (O/H)_lim ~ 8.7+/-0.3
at 95% confidence including systematic uncertainties on the calibration of
Kobulnicky & Kewley (2004). This value is somewhat lower than the traditionally
quoted value for the Sun, but is comparable to the revised solar oxygen
abundance (Asplund, Grevesse & Sauval 2005). LGRB models that require sharp
metallicity cutoffs well below ~1/2 the revised solar metallicity appear to be
effectively ruled out (abridged).Comment: MNRAS resubmitte
A common-envelope wind model for Type Ia supernovae (I): binary evolution and birth rate
The single-degenerate (SD) model is one of the principal models for the
progenitors of type Ia supernovae (SNe Ia), but some of the predictions in the
most widely studied version of the SD model, i.e. the optically thick wind
(OTW) model, have not been confirmed by observations. Here, we propose a new
version of the SD model in which a common envelope (CE) is assumed to form when
the mass-transfer rate between a carbon-oxygen white dwarf (CO WD) and its
companion exceeds a critical accretion rate. The WD may gradually increase its
mass at the base of the CE. Due to the large nuclear luminosity for stable
hydrogen burning, the CE may expand to giant dimensions and will lose mass from
the surface of the CE by a CE wind (CEW). Because of the low CE density, the
binary system will avoid a fast spiral-in phase and finally re-emerge from the
CE phase. Our model may share the virtues of the OTW model but avoid some of
its shortcomings. We performed binary stellar evolution calculations for more
than 1100 close WD + MS binaries. Compared with the OTW model, the parameter
space for SNe Ia from our CEW model extends to more massive companions and less
massive WDs. Correspondingly, the Galactic birth rate from the CEW model is
higher than that from the OTW model by 30\%. Finally, we discuss the
uncertainties of the CEW model and the differences between our CEW model and
the OTW model.Comment: 28 pages, 24 figures, accepted for publication in MNRA
Constraining the spin-down timescale of the white-dwarf progenitors of Type Ia supernovae
Justham (2011) and DiStefano et al.\ (2011) proposed that the white-dwarf
progenitor of a Type Ia supernova (SN Ia) may have to spin down before it can
explode. As the white dwarf spin-down timescale is not well known
theoretically, we here try to constrain it empirically (within the framework of
this spin-down model) for progenitor systems that contain a giant donor and for
which circumbinary material has been detected after the explosion: we obtain an
upper limit of a few . Based on the study of Di Stefano &
Kilic (2012) this means that it is too early to rule out the existence of a
surviving companion in SNR 0509-67.5.Comment: 4 figures, accepted for publication in ApJ lette
Fast orbital shrinkage of black hole X-ray binaries driven by circumbinary disks
Recently, the black hole X-ray binary (BHXB) Nova Muscae 1991 has been
reported to be experiencing an extremely rapid orbital decay. So far, three
BHXBs have anomalously high orbital period derivatives, which can not be
interpreted by the standard stellar evolution theory. In this work, we
investigate whether the resonant interaction between the binary and a
surrounding circumbinary (CB) disk could produce the observed orbital period
derivatives. Analytical calculations indicate that the observed orbital period
derivatives of XTE J1118+480 and A0620-00 can originate from the tidal torque
between the binary and a CB disk with a mass of , which
is approximately in agreement with the dust disk mass detected in these two
sources. However, Nova Muscae 1991 was probably surrounded by a heavy CB disk
with a mass of . Based on the CB disk model and the
anomalous magnetic braking theory, we simulate the evolution of the three BHXBs
with intermediate-mass donor stars by using the MESA code. Our simulated
results are approximately consistent with the observed donor star masses,
orbital periods, and orbital-period derivatives. However, the calculated
effective temperatures of the donor stars are higher than indicated by the
observed spectral types of two sources.Comment: 6 pages, 5 figures, accepted for publication by ApJ
Common-Envelope Evolution: the Nucleosynthesis in Mergers of Massive Stars
We study the merging of massive stars inside a common envelope for binary
systems consisting of a red supergiant with a mass of 15-20 Msun and a
main-sequence companion of 1-5 Msun. We are particularly interested in the
stage when the secondary, having overfilled its Roche lobe inside the common
envelope, starts to transfer mass to the core of the primary at a very high
mass-transfer rate and the subsequent nucleo-synthesis in the core-impact
region. Using a parametrized model for the structure of the envelope at this
stage, we perform 2-dimensional hydrodynamical calculations with the Munich
Prometheus code to calculate the dynamics of the stream emanating from the
secondary and its impact on the core of the primary. We find that, for the
lower end of the estimated mass-transfer rate, low-entropy, hydrogen-rich
material can penetrate deep into the primary core where nucleosynthesis through
the hot CNO cycle can take place and that the associated neutron exposure may
be sufficiently high for significant s-processing. For mass-transfer rates at
the high end of our estimated range and higher densities in the stream, the
stream impact can lead to the dredge-up of helium, but the neutron production
is too low for significant s-processing.Comment: 5 pages, 2 figures, to appear in the proceeding of ``Binary and
Multiple Star Systems'' (Bormio (Italy), June 2000
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
Dust Formation in the Ejecta of Common Envelope Systems
The material that is ejected in a common-envelope (CE) phase in a close
binary system provides an ideal environment for dust formation. By constructing
a simple toy model to describe the evolution of the density and the temperature
of CE ejecta and using the \emph{AGBDUST} code to model dust formation, we show
that dust can form efficiently in this environment. The actual dust masses
produced in the CE ejecta depend strongly on their temperature and density
evolution. We estimate the total dust masses produced by CE evolution by means
of a population synthesis code and show that, compared to dust production in
AGB stars, the dust produced in CE ejecta may be quite significant and could
even dominate under certain circumstances.Comment: 11pages, 7 figures, accepted for publication by Ap
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