1,254 research outputs found
Constraints on explosive silicon burning in core-collapse supernovae from measured Ni/Fe ratios
Measurements of explosive nucleosynthesis yields in core-collapse supernovae
provide tests for explosion models. We investigate constraints on explosive
conditions derivable from measured amounts of nickel and iron after radioactive
decays using nucleosynthesis networks with parameterized thermodynamic
trajectories. The Ni/Fe ratio is for most regimes dominated by the production
ratio of 58Ni/(54Fe + 56Ni), which tends to grow with higher neutron excess and
with higher entropy. For SN 2012ec, a supernova that produced a Ni/Fe ratio of
times solar, we find that burning of a fuel with neutron excess
is required. Unless the progenitor metallicity
is over 5 times solar, the only layer in the progenitor with such a neutron
excess is the silicon shell. Supernovae producing large amounts of stable
nickel thus suggest that this deep-lying layer can be, at least partially,
ejected in the explosion. We find that common spherically symmetric models of
Msun stars exploding with a delay time of less than
one second ( Msun) are able to achieve such silicon-shell
ejection. Supernovae that produce solar or sub-solar Ni/Fe ratios, such as SN
1987A, must instead have burnt and ejected only oxygen-shell material, which
allows a lower limit to the mass cut to be set. Finally, we find that the
extreme Ni/Fe value of 60-75 times solar derived for the Crab cannot be
reproduced by any realistic-entropy burning outside the iron core, and
neutrino-neutronization obtained in electron-capture models remains the only
viable explanation.Comment: 13 pages, 9 figures, accepted for publication in Ap
The peculiar Type Ia supernova iPTF14atg: Chandrasekhar-mass explosion or violent merger?
iPTF14atg, a subluminous peculiar Type Ia supernova (SN Ia) similar to SN
2002es, is the first SN Ia for which a strong UV flash was observed in the
early-time light curves. This has been interpreted as evidence for a
single-degenerate (SD) progenitor system where such a signal is expected from
interactions between the SN ejecta and the non-degenerate companion star. Here,
we compare synthetic observables of multi-dimensional state-of-the-art
explosion models for different progenitor scenarios to the light curves and
spectra of iPTF14atg. From our models, we have difficulties explaining the
spectral evolution of iPTF14atg within the SD progenitor channel. In contrast,
we find that a violent merger of two carbon-oxygen white dwarfs with 0.9 and
0.76 solar masses, respectively, provides an excellent match to the spectral
evolution of iPTF14atg from 10d before to several weeks after maximum light.
Our merger model does not naturally explain the initial UV flash of iPTF14atg.
We discuss several possibilities like interactions of the SN ejecta with the
circum-stellar medium and surface radioactivity from a He ignited merger that
may be able to account for the early UV emission in violent merger models.Comment: 12 pages, 7 figures, accepted for publication in MNRA
A Failed Gamma-Ray Burst with Dirty Energetic Jets Spirited Away? New Implications for the GRB-SN Connection from Supernova 2002ap
(Abridged) SN 2002ap is an interesting event with broad spectral features
like the famous SN 1998bw / GRB 980425. Here we examine the recently proposed
jet hypothesis from SN 2002ap by a spectropolarimetric observation. We show
that jets should be moving at about 0.23c with a jet kinetic energy of ~5 x
10^{50} erg, a similar energy scale to the GRB jets. The weak radio emission
from SN 2002ap has been used to argue against the jet hypothesis, but we show
that this problem can be avoided. However, the jet cannot be kept ionized
because of adiabatic cooling without external photoionization or heating
source. We found that only the radioactivity of 56Ni is a possible source,
indicating that the jet is formed and ejected from central region of the core
collapse. Then we point out that the jet will eventually sweep up enough
interstellar medium and generate shocks in a few to 10 years, producing strong
radio emission that can be spatially resolved, giving us a clear test for the
jet hypothesis. Discussions are given on possible implications for the GRB-SN
connection in the case that the jet is real. We suggest existence of two
distinct classes of GRBs from similar core-collapse events but by completely
different mechanisms. Cosmologically distant GRBs (~10^{50} erg) are collimated
jets generated by central activity of core collapses. SN 2002ap could be a
failed GRB of this type with a large baryon load. On the other hand, much less
energetic ones like GRB 980425 are rather isotropic, which may be produced by
hydrodynamical shock acceleration at the outer envelope. We propose that the
radioactive ionization for the SN 2002ap jet may give a new explanation also
for the X-ray line features often observed in GRB afterglows.Comment: 14 pages, 5 figures. Version accepted to Ap
Constraints on the luminosity of the stellar remnant in SNR1987A
We obtain photometric constraints on the luminosity of the stellar remnant in
SNR1987A using XMM-Newton and INTEGRAL data. The upper limit in the 2--10 keV
band based on the XMM-Newton data is L<5*10^{34}erg/s. We note, however, that
the optical depth of the envelope is still high in the XMM-Newton band,
therefore, this upper limit does not constrain the true unabsorbed luminosity
of the central source. The optical depth is expected to be small in the hard
X-ray band of the IBIS telescope aboard the INTEGRAL observatory, therefore it
provides an unobscured look at the stellar remnant. We did not detect
statistically significant emission from SN1987A in the 20-60 keV band with the
upper limit of L<1.1*10^{36}erg/s. We also obtained an upper limit on the mass
of radioactive 44Ti M(44Ti)<10^{-3}Msun.Comment: 5 pages, 3 figures, accepted for publication in Astronomy Letter
Optical and near infrared observations of SN 1998bu
Infrared and optical spectra of SN 1998bu at an age of one year after
explosion are presented. The data show evidence for the radioactive decay of
56Co to 56Fe, long assumed to be the powering source for the supernova light
curve past maximum light. The spectra provide direct evidence for at least 0.4
solar masses of iron being present in the ejecta of the supernova. The fits to
the data also show that the widths of the emission lines increase with time.
Photometric measurements in the H-band show that the supernova is not fading
during the observation period. This is consistent with theoretical
expectations.Comment: accepted A&A, 7 pages, 9 figure
The tunnel magnetoresistance in chains of quantum dots weakly coupled to external leads
We analyze numerically the spin-dependent transport through coherent chains
of three coupled quantum dots weakly connected to external magnetic leads. In
particular, using the diagrammatic technique on the Keldysh contour, we
calculate the conductance, shot noise and tunnel magnetoresistance (TMR) in the
sequential and cotunneling regimes. We show that transport characteristics
greatly depend on the strength of the interdot Coulomb correlations, which
determines the spacial distribution of electron wave function in the chain.
When the correlations are relatively strong, depending on the transport regime,
we find both negative TMR as well as TMR enhanced above the Julliere value,
accompanied with negative differential conductance (NDC) and super-Poissonian
shot noise. This nontrivial behavior of tunnel magnetoresistance is associated
with selection rules that govern tunneling processes and various high-spin
states of the chain that are relevant for transport. For weak interdot
correlations, on the other hand, the TMR is always positive and not larger than
the Julliere TMR, although super-Poissonian shot noise and NDC can still be
observed
Dust Production and Particle Acceleration in Supernova 1987A Revealed with ALMA
Supernova (SN) explosions are crucial engines driving the evolution of
galaxies by shock heating gas, increasing the metallicity, creating dust, and
accelerating energetic particles. In 2012 we used the Atacama Large
Millimeter/Submillimeter Array to observe SN 1987A, one of the best-observed
supernovae since the invention of the telescope. We present spatially resolved
images at 450um, 870um, 1.4mm, and 2.8mm, an important transition wavelength
range. Longer wavelength emission is dominated by synchrotron radiation from
shock-accelerated particles, shorter wavelengths by emission from the largest
mass of dust measured in a supernova remnant (>0.2Msun). For the first time we
show unambiguously that this dust has formed in the inner ejecta (the cold
remnants of the exploded star's core). The dust emission is concentrated to the
center of the remnant, so the dust has not yet been affected by the shocks. If
a significant fraction survives, and if SN 1987A is typical, supernovae are
important cosmological dust producers.Comment: ApJL accepte
Three-dimensional distribution of ejecta in Supernova 1987A at 10 000 days
Due to its proximity, SN 1987A offers a unique opportunity to directly
observe the geometry of a stellar explosion as it unfolds. Here we present
spectral and imaging observations of SN 1987A obtained ~10,000 days after the
explosion with HST/STIS and VLT/SINFONI at optical and near-infrared
wavelengths. These observations allow us to produce the most detailed 3D map of
H-alpha to date, the first 3D maps for [Ca II] \lambda \lambda 7292, 7324, [O
I] \lambda \lambda 6300, 6364 and Mg II \lambda \lambda 9218, 9244, as well as
new maps for [Si I]+[Fe II] 1.644 \mu m and He I 2.058 \mu m. A comparison with
previous observations shows that the [Si I]+[Fe II] flux and morphology have
not changed significantly during the past ten years, providing evidence that it
is powered by 44Ti. The time-evolution of H-alpha shows that it is
predominantly powered by X-rays from the ring, in agreement with previous
findings. All lines that have sufficient signal show a similar large-scale 3D
structure, with a north-south asymmetry that resembles a broken dipole. This
structure correlates with early observations of asymmetries, showing that there
is a global asymmetry that extends from the inner core to the outer envelope.
On smaller scales, the two brightest lines, H-alpha and [Si I]+[Fe II] 1.644
\mu m, show substructures at the level of ~ 200 - 1000 km/s and clear
differences in their 3D geometries. We discuss these results in the context of
explosion models and the properties of dust in the ejecta.Comment: Accepted for publication in Ap
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