293 research outputs found
Infrared emission from kilonovae: the case of the nearby short hard burst GRB 160821B
We present constraints on Ks-band emission from one of the nearest short hard
gamma-ray bursts, GRB 160821B, at z=0.16, at three epochs. We detect a reddened
relativistic afterglow from the jetted emission in the first epoch but do not
detect any excess kilonova emission in the second two epochs. We compare upper
limits obtained with Keck I/MOSFIRE to multi-dimensional radiative transfer
models of kilonovae, that employ composition-dependent nuclear heating and LTE
opacities of heavy elements. We discuss eight models that combine toroidal
dynamical ejecta and two types of wind and one model with dynamical ejecta
only. We also discuss simple, empirical scaling laws of predicted emission as a
function of ejecta mass and ejecta velocity. Our limits for GRB 160821B
constrain the ejecta mass to be lower than 0.03 Msun for velocities greater
than 0.1c. At the distance sensitivity range of advanced LIGO, similar
ground-based observations would be sufficiently sensitive to the full range of
predicted model emission including models with only dynamical ejecta. The color
evolution of these models shows that I-K color spans 7--16 mag, which suggests
that even relatively shallow infrared searches for kilonovae could be as
constraining as optical searches.Comment: Accepted for Publication in Astrophysical Journal Letter
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
The first direct double neutron star merger detection: implications for cosmic nucleosynthesis
The astrophysical r-process site where about half of the elements heavier
than iron are produced has been a puzzle for several decades. Here we discuss
the role of neutron star mergers (NSMs) in the light of the first direct
detection of such an event in both gravitational (GW) and electromagnetic (EM)
waves. We analyse bolometric and NIR lightcurves of the first detected double
neutron star merger and compare them to nuclear reaction network-based
macronova models. The slope of the bolometric lightcurve is consistent with the
radioactive decay of neutron star ejecta with (but not
larger), which provides strong evidence for an r-process origin of the
electromagnetic emission. This rules out in particular "nickel winds" as major
source of the emission. We find that the NIR lightcurves can be well fitted
either with or without lanthanide-rich ejecta. Our limits on the ejecta mass
together with estimated rates directly confirm earlier purely theoretical or
indirect observational conclusions that double neutron star mergers are indeed
a major site of cosmic nucleosynthesis. If the ejecta mass was {\em typical},
NSMs can easily produce {\em all} of the estimated Galactic r-process matter,
and --depending on the real rate-- potentially even more. This could be a hint
that the event ejected a particularly large amount of mass, maybe due to a
substantial difference between the component masses. This would be compatible
with the mass limits obtained from the GW-observation. The recent observations
suggests that NSMs are responsible for a broad range of r-process nuclei and
that they are at least a major, but likely the dominant r-process site in the
Universe.Comment: 11 pages, 8 figures; accepted for A \&
Measuring Streetscape Design for Livability Using Spatial Data and Methods
City streets are the most widely distributed and heavily trafficked urban public spaces. As cities strive to improve livability in the built environment, it is important for planners and designers to have a concise understanding of what contributes to quality streetscapes. The proportions and scale of buildings and trees, which define the three-dimensional extents of streetscapes, provide enduring, foundational skeletons. This thesis investigates how characteristics of such streetscape skeletons can be quantified and tested for appeal among human users.
The first of two journal-style papers identifies a concise set of skeleton variables that urban design theorists have described as influential to streetscape appeal. It offers an automated GIS-based method for identifying and cataloging these skeleton variables, which are practical to measure using widely available spatial data. Such an approach allows measurement of tens of thousands of street segments precisely and efficiently, a dramatically larger sample than can be feasibly collected using the existing auditing techniques of planners and researchers. Further, this paper examines clustering patterns among skeleton variables for street segments throughout Boston, New York, and Baltimore, identifying four streetscape skeleton types that describe a ranking of enclosure from surrounding buildings--upright, compact, porous, and open. The types are identifiable in all three cities, demonstrating regional consistency in streetscape design. Moreover, the types are poorly associated with roadway functional classifications--arterial, collector, and local--indicating that streetscapes are a distinct component of street design and must receive separate planning and design attention.
The second paper assesses relationships between skeleton variables and crowdsourced judgments of streetscape visual appeal throughout New York City. Regression modeling indicates that streetscapes with greater tree canopy coverage, lined by a greater number of buildings, and with more upright cross-sections, are more visually appealing. Building and tree canopy geometry accounts for more than 40% of variability in perceived safety, which is used as an indicator of appeal. While unmeasured design details undoubtedly influence overall streetscape appeal, basic skeletal geometry may contribute important baseline conditions for appealing streetscapes that are enduring and can meet a broad variety of needs
Monte Carlo Radiation Transport for Astrophysical Transients Powered by Circumstellar Interaction
In this paper, we introduce \texttt{SuperLite}, an open-source Monte Carlo
radiation transport code designed to produce synthetic spectra for
astrophysical transient phenomena affected by circumstellar interaction.
\texttt{SuperLite} utilizes Monte Carlo methods for semi-implicit,
semi-relativistic radiation transport in high-velocity shocked outflows,
employing multi-group structured opacity calculations. The code enables rapid
post-processing of hydrodynamic profiles to generate high-quality spectra that
can be compared with observations of transient events, including superluminous
supernovae, pulsational pair-instability supernovae, and other peculiar
transients. We present the methods employed in \texttt{SuperLite} and compare
the code's performance to that of other radiative transport codes, such as
\texttt{SuperNu} and CMFGEN. We show that \texttt{SuperLite} has successfully
passed standard Monte Carlo radiation transport tests and can reproduce spectra
of typical supernovae of Type Ia, Type IIP and Type IIn.Comment: Accepted for publication at the Astrophysics Journa
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