7,611 research outputs found
Outside the Wall: Hydrodynamics of Type I Supernovae Interacting with a Partially Swept-Up Circumstellar Medium
Explaining the observed diversity of supernovae (SNe) and the physics of
explosion requires knowledge of their progenitor stars, which can be obtained
by constraining the circumstellar medium (CSM). Models of the SN ejecta
colliding with CSM are necessary to infer the structure of the CSM and tie it
back to a progenitor model. Recent SNe I revealed CSM concentrated at a
distance cm, for which models of SN interaction are extremely
limited. In this paper, we assume the concentrated region is a "wall"
representing swept-up material, and unswept material lies outside the wall. We
simulate one-dimensional hydrodynamics of SNe Ia & Ib impacting 300 unique CSM
configurations using RT1D, which captures the Rayleigh-Taylor instability. We
find that the density ratio between the wall and ejecta -- denoted or
"wall height" -- is key, and higher walls deviate more from self-similar
evolution. Functional fits accounting for are presented for the forward
shock radius evolution. We show that higher walls have more degeneracy between
CSM properties in the deceleration parameter, slower shocks, deeper-probing
reverse shocks, slower shocked ejecta, less ejecta mass than CSM in the shock,
and more mixing of ejecta into the CSM at early times. We analyze observations
of SN 2014C (Type Ib) and suggest that it had a moderately high wall () and wind-like outer CSM. We also postulate an alternate interpretation
for the radio data of SN 2014C, that the radio rise occurs in the wind rather
than the wall. Finally, we find that hydrodynamic measurements at very late
times cannot distinguish the presence of a wall, except perhaps as an
anomalously wide shock region.Comment: 17 pages, 13 figures, accepted to Ap
Skill set profile clustering based on student capability vectors computed from online tutoring data
In educational research, a fundamental goal is identifying which skills students have mastered, which skills they have not, and which skills they are in the process of mastering. As the number of examinees, items, and skills increases, the estimation of even simple cognitive diagnosis models becomes difficult. To address this, we introduce a capability matrix showing for each skill the proportion correct on all items tried by each student involving that skill. We apply variations of common clustering methods to this matrix and discuss conditioning on sparse subspaces. We demonstrate the feasibility and scalability of our method on several simulated datasets and illustrate the difficulties inherent in real data using a subset of online mathematics tutor data. We also comment on the interpretability and application of the results for teachers
Skill set profile clustering: the empty K-means algorithm with automatic specification of starting cluster centers
While students’ skill set profiles can be estimated with formal cognitive diagnosis models [8], their computational complexity makes simpler proxy skill estimates attractive [1, 4, 6]. These estimates can be clustered to generate groups of similar students. Often hierarchical agglomerative clustering or k-means clustering is utilized, requiring, for K skills, the specification of 2^K clusters. The number of skill set profiles/clusters can quickly become computationally intractable. Moreover, not all profiles may be present in the population. We present a flexible version of k-means that allows for empty clusters. We also specify a method to determine efficient starting centers based on the Q-matrix. Combining the two substantially improves the clustering results and allows for analysis of data sets previously thought impossible
Against the Wind: Radio Light Curves of Type Ia Supernovae Interacting with Low-Density Circumstellar Shells
For decades, a wide variety of observations spanning the radio through
optical and on to the x-ray have attempted to uncover signs of type Ia
supernovae (SNe Ia) interacting with a circumstellar medium (CSM). The goal of
these studies is to constrain the nature of the hypothesized SN Ia mass-donor
companion. A continuous CSM is typically assumed when interpreting observations
of interaction. However, while such models have been successfully applied to
core-collapse SNe, the assumption of continuity may not be accurate for SNe Ia,
as shells of CSM could be formed by pre-supernova eruptions (novae). In this
work, we model the interaction of SNe with a spherical, low density,
finite-extent CSM and create a suite of synthetic radio synchrotron light
curves. We find that CSM shells produce sharply peaked light curves, and
identify a fiducial set of models that all obey a common evolution and can be
used to generate radio light curves for interaction with an arbitrary shell.
The relations obeyed by the fiducial models can be used to deduce CSM
properties from radio observations; we demonstrate this by applying them to the
non-detections of SN 2011fe and SN 2014J. Finally, we explore a multiple shell
CSM configuration and describe its more complicated dynamics and resultant
radio light curves.Comment: 15 pages, 11 figures, ApJ accepte
Intermediate Palomar Transient Factory: Realtime Image Subtraction Pipeline
A fast-turnaround pipeline for realtime data reduction plays an essential
role in discovering and permitting follow-up observations to young supernovae
and fast-evolving transients in modern time-domain surveys. In this paper, we
present the realtime image subtraction pipeline in the intermediate Palomar
Transient Factory. By using high-performance computing, efficient database, and
machine learning algorithms, this pipeline manages to reliably deliver
transient candidates within ten minutes of images being taken. Our experience
in using high performance computing resources to process big data in astronomy
serves as a trailblazer to dealing with data from large-scale time-domain
facilities in near future.Comment: 18 pages, 6 figures, accepted for publication in PAS
Low Hubble Constant from Type Ia Supernovae by van den Bergh's Method
An interesting way to calibrate the absolute magnitudes of remote Type Ia
supernovae (SNe Ia) that are well out in the Hubble flow, and thus determine
the value of the Hubble constant, H_0, has been introduced by van den Bergh.
His approach relies on calculations of the peak absolute magnitudes and
broad--band colors for SN Ia explosion models. It does not require any
corrections for extinction by interstellar dust, and no SNe Ia are excluded on
grounds of peculiarity. Within the last few years distances have been
determined to the parent galaxies of six SNe Ia by means of Cepheid variables.
Cepheid--based distances also have become available for three other SNe Ia if
one is willing to use the distance to a galaxy in the same group in lieu of the
distance to the parent galaxy itself. Here we determine the value of H_0 in a
way that is analogous to that of van den Bergh, but now using Cepheid--based
distances instead of calculated light curves. We obtain H_0 = 55 km/s/Mpc. This
value, with Lambda=0 and Omega=1, corresponds to a cosmic expansion time of 12
Gyr, which is consistent with several recent determinations of the ages of
globular clusters.Comment: Latex, 4 pages, 1 table, 1 figure, Submitted to Nature March 28,
1996. PostScript version available at http://www.nhn.ou.edu/~nugent
The Host Galaxies of Sub-Chandrasekhar Mass Type Ia Supernovae
In recent years, there has been ample evidence for the existence of multiple
progenitor pathways that can result in Type Ia supernova (SNe Ia), including
SNe Ia of sub-Chandrasekhar mass origin best distinguished by their redder
colors and higher Si II velocities near peak brightness. These SNe can
contaminate the population of normal events used for cosmological analyses,
creating unwanted biases in the final analyses. Given that many current and
future surveys using SNe Ia as cosmological probes will not have the resources
to take a spectrum of all the events, likely only getting host redshifts long
after the SNe Ia have faded, we need to turn to methods that could separate
these populations based purely on photometry or host properties. Here, we
present a study of a sample of well observed, nearby SNe Ia and their hosts to
determine if there are significant enough difference between these populations
that can be discerned only from the stellar population properties of their
hosts. Our results indicate that the global host properties, including star
formation, stellar mass, stellar population age, and dust attenuation, of
sub-Chandrasekhar mass explosions do not differ significantly from those of
normal mass origin. However, we do find evidence using Na I D equivalent widths
that the local environments of sub-Chandrasekhar mass explosions are more
dust-affected than normal SNe Ia. Future work requires strengthening
photometric probes of sub-Chandrasekhar SNe and their local environments to
distinguish these events.Comment: 16 pages, 10 figures, 2 tables, submitte
The volumetric rate of calcium-rich transients in the local universe
We present a measurement of the volumetric rate of `calcium-rich' optical
transients in the local universe, using a sample of three events from the
Palomar Transient Factory (PTF). This measurement builds on a detailed study of
the PTF transient detection efficiencies, and uses a Monte Carlo simulation of
the PTF survey. We measure the volumetric rate of calcium-rich transients to be
higher than previous estimates: events
yr Mpc. This is equivalent to 33-94% of the local volumetric type
Ia supernova rate. This calcium-rich transient rate is sufficient to reproduce
the observed calcium abundances in galaxy clusters, assuming an asymptotic
calcium yield per calcium-rich event of ~0.05. We also
study the PTF detection efficiency of these transients as a function of
position within their candidate host galaxies. We confirm as a real physical
effect previous results that suggest calcium-rich transients prefer large
physical offsets from their host galaxies.Comment: Accepted for publication in ApJ. 9 pages, 5 figure
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