1,081 research outputs found
Explosive Nucleosynthesis from GRB and Hypernova Progenitors: Direct Collapse versus Fallback
The collapsar engine behind long-duration gamma-ray bursts extracts the
energy released from the rapid accretion of a collapsing star onto a
stellar-massed black hole. In a collapsing star, this black hole can form in
two ways: the direct collapse of the stellar core into a black hole and the
delayed collapse of a black hole caused by fallback in a weak supernova
explosion. In the case of a delayed-collapse black hole, the strong
collapsar-driven explosion overtakes the weak supernova explosion before shock
breakout, and it is very difficult to distinguish this black hole formation
scenario from the direct collapse scenario. However, the delayed-collapse
mechanism, with its double explosion, produces explosive nucleosynthetic yields
that are very different from the direct collapse scenario. We present
1-dimensional studies of the nucleosynthetic yields from both black hole
formation scenarios, deriving differences and trends in their nucleosynthetic
yields.Comment: 47 pages, submitted to Ap
Light Curve Calculations of Supernovae from Fallback Gamma-Ray Bursts
The currently-favored model for long-duration gamma-ray bursts (GRBs) invokes
explosions from the collapse of a massive star down to a black hole: either
directly or through fallback. Those GRBs forming via fallback will produce much
less radioactive nickel, and hence it has been argued (without any real
calculation) that these systems produce dim supernovae. These fallback
black-hole GRBs have been recently been argued as possible progenitors of a
newly discovered set of GRBs lacking any associated supernovae. Here we present
the first ever radiation-hydrodynamics calculations of the light-curves
produced in the hypernova explosion by a delayed-fallback gamma-ray burst. We
find that the bolometric light-curve is dominated by shock-deposited energy,
not the decay of radioactive elements. As such, observations of such bursts
actually probe the density in the progenitor wind more than it does the
production of radioactive nickel.Comment: 11 pages (including 3 figures), submitted to ApJ, comments welcom
A line-binned treatment of opacities for the spectra and light curves from neutron star mergers
The electromagnetic observations of GW170817 were able to dramatically
increase our understanding of neutron star mergers beyond what we learned from
gravitational waves alone. These observations provided insight on all aspects
of the merger from the nature of the gamma-ray burst to the characteristics of
the ejected material. The ejecta of neutron star mergers are expected to
produce such electromagnetic transients, called kilonovae or macronovae.
Characteristics of the ejecta include large velocity gradients, relative to
supernovae, and the presence of heavy -process elements, which pose
significant challenges to the accurate calculation of radiative opacities and
radiation transport. For example, these opacities include a dense forest of
bound-bound features arising from near-neutral lanthanide and actinide
elements. Here we investigate the use of fine-structure, line-binned opacities
that preserve the integral of the opacity over frequency. Advantages of this
area-preserving approach over the traditional expansion-opacity formalism
include the ability to pre-calculate opacity tables that are independent of the
type of hydrodynamic expansion and that eliminate the computational expense of
calculating opacities within radiation-transport simulations. Tabular opacities
are generated for all 14 lanthanides as well as a representative actinide
element, uranium. We demonstrate that spectral simulations produced with the
line-binned opacities agree well with results produced with the more accurate
continuous Monte Carlo Sobolev approach, as well as with the commonly used
expansion-opacity formalism. Additional investigations illustrate the
convergence of opacity with respect to the number of included lines, and
elucidate sensitivities to different atomic physics approximations, such as
fully and semi-relativistic approaches.Comment: 27 pages, 22 figures. arXiv admin note: text overlap with
arXiv:1702.0299
Probing the Density in the Galactic Center Region: Wind-Blown Bubbles and High-Energy Proton Constraints
Recent observations of the Galactic center in high-energy gamma-rays (above
0.1TeV) have opened up new ways to study this region, from understanding the
emission source of these high-energy photons to constraining the environment in
which they are formed. We present a revised theoretical density model of the
inner 5pc surrounding Sgr A* based on the fact that the underlying structure of
this region is dominated by the winds from the Wolf-Rayet stars orbiting Sgr
A*. An ideal probe and application of this density structure is this high
energy gamma-ray emission. We assume a proton-scattering model for the
production of these gamma-rays and then determine first whether such a model is
consistent with the observations and second whether we can use these
observations to further constrain the density distribution in the Galactic
center.Comment: 36 pages including 17 figures, submitted to ApJ, comments welcom
A Case Study of Small Scale Structure Formation in 3D Supernova Simulations
It is suggested in observations of supernova remnants that a number of large-
and small-scale structures form at various points in the explosion.
Multidimensional modeling of core-collapse supernovae has been undertaken since
SN1987A, and both simulations and observations suggest/show that
Rayleigh-Taylor instabilities during the explosion is a main driver for the
formation of structure in the remnants.
We present a case study of structure formation in 3D in a \msol{15} supernova
for different parameters. We investigate the effect of moderate asymmetries and
different resolutions of the formation and morphology of the RT unstable
region, and take first steps at determining typical physical quantities (size,
composition) of arising clumps. We find that in this progenitor the major RT
unstable region develops at the He/OC interface for all cases considered. The
RT instabilities result in clumps that are overdense by 1-2 orders of magnitude
with respect to the ambient gas, have size scales on the level of a few % of
the remnant diameter, and are not diffused after the first yrs of the
remnant evolution, in the absence of a surrounding medium.Comment: 59 pages, 34 figure
Spectra and Light Curves of Failed Supernovae
Astronomers have proposed a number of mechanisms to produce supernova
explosions. Although many of these mechanisms are now not considered primary
engines behind supernovae, they do produce transients that will be observed by
upcoming ground-based surveys and NASA satellites. Here we present the first
radiation-hydrodynamics calculations of the spectra and light curves from three
of these "failed" supernovae: supernovae with considerable fallback, accretion
induced collapse of white dwarfs, and energetic helium flashes (also known as
type .Ia supernovae).Comment: 33 pages, 14 figure
Structural, UV-VIS-NIR luminescence and decay associated spectral profiles of Sm3+ doped calcium phosphate glass
Rare-Earths or Lanthanide ions (Ln3+) are a group of elements from lanthanum to lutetium (Z=57 to 71), plus scandium (Z=21) and Yttrium (Z=39). Ln3+ doped glasses have been included within the Hydroxyapatite (HA) matrix, inducing promising changes in their physicochemical and biological properties. Because of the lanthanides' ability to modulate bone metabolism, inclusion of Ln3+ in the composition of calcium phosphate biomaterials for bone tissue regeneration has been considered. Earlier, the authors studied Samarium (Sm3+) doped glass-reinforced hydroxyapatite with enhanced osteoblastic performance and antibacterial properties for bone tissue regeneration. Moreover, Sm3+ substituted calcium phosphate is a non-hazardous material that luminesces under UV-visible light. In this work, we study a calcium phosphate host glass doped with samarium oxide, which has been prepared and characterized by FTIR, SEM, EDS analysis, and X-ray mapping. Pumping with two visible excitation sources at 405nm and 423nm, we observed intense, sharp Green, yellow, orange emission peaks (4G5/2â 6H5/2,7/2, 9/2) at 560nm, 596nm and 643nm respectively. A weak red emission was also observed at 704nm. Two NIR peaks at 1134nm (4G5/2â 6F11/2) and 1310 nm (4G5/2â 6F9/2) are monitored by using an excitation at 1060nm. Furthermore, by making use of time-resolved emission spectroscopy (TRES) measurements, the decay associated spectra were obtained allowing the kinetic parameters for the different emission bands to be elucidated and compared with steady state emission spectra.European Union's Seventh Framework Programme (FP7/2007-2013) under grant agreement nº REGPOT-CT2012-316331-POLARIS. Also, SHN would like to thank to Cost Action, ref. # MP 1205 under materials (soft, bio & nano) and technologies for optofluidic devices, and further acknowledge to the FCT -Fundação para a Ciência e a Tecnologia, Portugal, through the project PTDC/SAU-BEB/103034/2008
The Sgr B2 X-ray Echo of the Galactic Center Supernova Explosion that Produced Sgr A East
The possible impact Sgr A East is having on the Galactic center has fueled
speculation concerning its age and the energetics of the supernova explosion
that produced it. We have carried out the first in-depth analysis of the
remnant's evolution and its various interactions: with the stellar winds
flowing out from the inner ~2 pc, with the supermassive black hole, Sgr A*, and
with the 50 km/s molecular cloud behind and to the East of the nucleus. We have
found that a rather "standard" supernova explosion with energy ~1.5e51 ergs is
sufficient to create the remnant we see today, and that the latter is probably
only ~1,700 years old. The X-ray Ridge between ~9" and 15" to the NE of Sgr A*
appears to be the product of the current interaction between the remaining
supernova ejecta and the outflowing winds. Perhaps surprisingly, we have also
found that the passage of the remnant across the black hole would have enhanced
the accretion rate onto the central object by less than a factor 2. Such a
small increase cannot explain the current Fe fluorescence observed from the
molecular cloud Sgr B2; this fluorescence would have required an increase in
Sgr A*'s luminosity by 6 orders of magnitude several hundred years ago.
Instead, we have uncovered what appears to be a more plausible scenario for
this transient irradiation--the interaction between the expanding remnant and
the 50 km/s molecular cloud. The first impact would have occurred about 1,200
years after the explosion, producing a 2-200 keV luminosity of ~1e39 ergs/s.
During the intervening 300-400 years, the dissipation of kinetic energy
subsided considerably, leading to the much lower luminosity (~1e36 ergs/s at
2-10 keV) we see today.Comment: 27 pages (including 9 figures), submitted to ApJ, for hi-res/color
figures, see http://qso.lanl.gov/~cl
- …