26 research outputs found
R-process Nucleosynthesis from Three-Dimensional Magnetorotational Core-Collapse Supernovae
We investigate r-process nucleosynthesis in three-dimensional (3D)
general-relativistic magnetohydrodynamic simulations of rapidly rotating
strongly magnetized core collapse. The simulations include a microphysical
finite-temperature equation of state and a leakage scheme that captures the
overall energetics and lepton number exchange due to postbounce neutrino
emission and absorption. We track the composition of the ejected material using
the nuclear reaction network SkyNet. Our results show that the 3D dynamics of
magnetorotational core-collapse supernovae (CCSN) are important for their
nucleosynthetic signature. We find that production of r-process material beyond
the second peak is reduced by a factor of 100 when the magnetorotational jets
produced by the rapidly rotating core undergo a kink instability. Our results
indicate that 3D magnetorotationally powered CCSNe are a robust r-process
source only if they are obtained by the collapse of cores with unrealistically
large precollapse magnetic fields of order G. Additionally, a
comparison simulation that we restrict to axisymmetry, results in overly
optimistic r-process production for lower magnetic field strengths.Comment: 10 pages, 9 figures, 2 tables. submitted to Ap
Density Profiles of Collapsed Rotating Massive Stars Favor Long Gamma-Ray Bursts
Long-duration gamma-ray bursts (lGRBs) originate in relativistic collimated
outflows -- jets -- that drill their way out of collapsing massive stars.
Accurately modeling this process requires realistic stellar profiles for the
jets to propagate through and break out of. Most previous studies have used
simple power laws or pre-collapse models for massive stars. However, the
relevant stellar profile for lGRB models is in fact that of a star after its
core has collapsed to form a compact object. To self-consistently compute such
a stellar profile, we use the open-source code GR1D to simulate the
core-collapse process for a suite of low-metallicity, rotating, massive stellar
progenitors that have undergone chemically homogeneous evolution. Our models
span a range of zero-age main sequence (ZAMS) masses: , and . All of these models, at the onset of
core-collapse, feature steep density profiles, with
, which would result in jets that are inconsistent with lGRB
observables. We follow the collapse of four out of our seven models until they
form BHs and the other three proto-neutron stars (PNSs). We find, across all
models, that the density profile outside of the newly-formed BH or PNS is
well-represented by a flatter power law with . Such
flat density profiles are conducive to successful formation and breakout of
BH-powered jets and, in fact, required to reproduce observable properties of
lGRBs. Future models of lGRBs should be initialized with shallower
\textit{post-collapse} stellar profiles like those presented here instead of
the much steeper pre-collapse profiles that are typically used.Comment: 9 pages, 4 figures+1 table, submitted to ApJL, comments welcom
Mahakala: a Python-based Modular Ray-tracing and Radiative Transfer Algorithm for Curved Space-times
We introduce Mahakala, a Python-based, modular, radiative ray-tracing code
for curved space-times. We employ Google's JAX framework for accelerated
automatic differentiation, which can efficiently compute Christoffel symbols
directly from the metric, allowing the user to easily and quickly simulate
photon trajectories through non-Kerr metrics. JAX also enables Mahakala to run
in parallel on both CPUs and GPUs and achieve speeds comparable to C-based
codes. Mahakala natively uses the Cartesian Kerr-Schild coordinate system,
which avoids numerical issues caused by the "pole" of spherical coordinates. We
demonstrate Mahakala's capabilities by simulating the 1.3 mm wavelength images
(the wavelength of Event Horizon Telescope observations) of general
relativistic magnetohydrodynamic simulations of low-accretion rate supermassive
black holes. The modular nature of Mahakala allows us to easily quantify the
relative contribution of different regions of the flow to image features. We
show that most of the emission seen in 1.3 mm images originates close to the
black hole. We also quantify the relative contribution of the disk, forward
jet, and counter jet to 1.3 mm images.Comment: 15 pages, 11 figure
Discovery and Follow-up Observations of the Young Type Ia Supernova 2016coj
The Type~Ia supernova (SN~Ia) 2016coj in NGC 4125 (redshift ) was
discovered by the Lick Observatory Supernova Search 4.9 days after the fitted
first-light time (FFLT; 11.1 days before -band maximum). Our first detection
(pre-discovery) is merely day after the FFLT, making SN 2016coj one
of the earliest known detections of a SN Ia. A spectrum was taken only 3.7 hr
after discovery (5.0 days after the FFLT) and classified as a normal SN Ia. We
performed high-quality photometry, low- and high-resolution spectroscopy, and
spectropolarimetry, finding that SN 2016coj is a spectroscopically normal SN
Ia, but with a high velocity of \ion{Si}{2} 6355 (\,\kms\
around peak brightness). The \ion{Si}{2} 6355 velocity evolution can
be well fit by a broken-power-law function for up to a month after the FFLT. SN
2016coj has a normal peak luminosity ( mag), and it
reaches a -band maximum \about16.0~d after the FFLT. We estimate there to be
low host-galaxy extinction based on the absence of Na~I~D absorption lines in
our low- and high-resolution spectra. The spectropolarimetric data exhibit weak
polarization in the continuum, but the \ion{Si}{2} line polarization is quite
strong () at peak brightness.Comment: Submitte
SN 2016iyc : a Type IIb supernova arising from a low-mass progenitor
In this work, photometric and spectroscopic analyses of a very low-luminosity Type IIb supernova (SN) 2016iyc have been performed. SN 2016iyc lies near the faint end among the distribution of similar supernovae (SNe). Given lower ejecta mass (Mej) and low nickel mass (MNi) from the literature, combined with SN 2016iyc lying near the faint end, one-dimensional stellar evolution models of 9–14 M⊙ zero-age main-sequence (ZAMS) stars as the possible progenitors of SN 2016iyc have been performed using the publicly available code mesa. Moreover, synthetic explosions of the progenitor models have been simulated, using the hydrodynamic evolution codes stella and snec. The bolometric luminosity light curve and photospheric velocities produced through synthetic explosions of ZAMS stars of mass in the range of 12–13 M⊙ having a pre-supernova radius R0 = (204–300) R⊙, with Mej = (1.89–1.93) M⊙, explosion energy Eexp = (0.28–0.35) × 1051 erg, and MNi < 0.09 M⊙, are in good agreement with observations; thus, SN 2016iyc probably exploded from a progenitor near the lower mass limits for SNe IIb. Finally, hydrodynamic simulations of the explosions of SN 2016gkg and SN 2011fu have also been performed to compare intermediate- and high-luminosity examples among well-studied SNe IIb. The results of progenitor modelling and synthetic explosions for SN 2016iyc, SN 2016gkg, and SN 2011fu exhibit a diverse range of mass for the possible progenitors of SNe IIb
What Does the Geometry of the HβBLR Depend On?
We combine our dynamical modeling black-hole mass measurements from the Lick AGN Monitoring Project 2016 sample with measured cross-correlation time lags and line widths to recover individual scale factors, f, used in traditional reverberation-mapping analyses. We extend our sample by including prior results from Code for AGN Reverberation and Modeling of Emission Lines (CARAMEL) studies that have utilized our methods. Aiming to improve the precision of black-hole mass estimates, as well as uncover any regularities in the behavior of the broad-line region (BLR), we search for correlations between f and other AGN/BLR parameters. We find (i) evidence for a correlation between the virial coefficient log10(fmean,σ) and black-hole mass, (ii) marginal evidence for a similar correlation between log10( frms,σ) and black-hole mass, (iii) marginal evidence for an anticorrelation of BLR disk thickness with log10( fmean,FWHM) and log10( frms,FWHM), and (iv) marginal evidence for an anticorrelation of inclination angle with log10( fmean,FWHM), log10( frms,σ), and log10( fmean,σ). Last, we find marginal evidence for a correlation between line-profile shape, when using the root-mean-square spectrum, log10(FWHM/σ)rms, and the virial coefficient, log10( frms,σ), and investigate how BLR properties might be related to line-profile shape using CARAMEL models