6,658 research outputs found
Synthetic Spectra of Hydrodynamic Models of Type Ia Supernovae
We present detailed NLTE synthetic spectra of hydrodynamic SNe Ia models. We
make no assumptions about the form of the spectrum at the inner boundary. We
calculate both Chandrasekhar-mass deflagration models and sub-Chandrasekhar
``helium detonators.'' Gamma-ray deposition is handled in a simple, accurate
manner. We have parameterized the storage of energy that arises from the time
dependent deposition of radioactive decay energy in a reasonable manner, that
spans the expected range. We find that the Chandrasekhar-mass deflagration
model W7 of Nomoto etal shows good agreement with the observed spectra of SN
1992A and SN 1994D, particularly in the UV, where our models are expected to be
most accurate. The sub-Chandrasekhar models do not reproduce the UV deficit
observed in normal SNe Ia. They do bear some resemblance to sub-luminous SNe
Ia, but the shape of the spectra (i.e. the colors) are opposite to that of the
observed ones and the intermediate mass element lines such as Si II, and Ca II
are extremely weak, which seems to be a generic difficulty of the models.
Although the sub-Chandrasekhar models have a significant helium abundance
(unlike Chandrasekhar-mass models), helium lines are not prominent in the
spectra near maximum light and thus do not act as a spectral signature for the
progenitor.Comment: submitted to ApJ, 26 pages, 10 figures, uses LaTeX styles aasms4.sty
and natbib.sty Also available at: http://www.nhn.ou.edu/~baron
“Transfer Talk” in Talk about Writing in Progress: Two Propositions about Transfer of Learning
This article tracks the emergence of the concept of “transfer talk”—a concept distinct from transfer of learning—and teases out the implications of transfer talk for theories of transfer of learning. The concept of transfer talk was developed through a systematic examination of 30 writing center transcripts and is defined as “the talk through which individuals make visible their prior learning (in this case, about writing) or try to access the prior learning of someone else.” In addition to including a taxonomy of transfer talk and analysis of which types occur most often in this set of conferences, this article advances two propositions about the nature of transfer of learning: (1) transfer of learning may have an important social, even collaborative, component and (2) although meta-awareness about writing has long been recognized as valuable for transfer of learning, more automatized knowledge may play an important role as well
The Rising Light Curves of Type Ia Supernovae
We present an analysis of the early, rising light curves of 18 Type Ia
supernovae (SNe Ia) discovered by the Palomar Transient Factory (PTF) and the
La Silla-QUEST variability survey (LSQ). We fit these early data flux using a
simple power-law to determine the time of first
light , and hence the rise-time from first light to
peak luminosity, and the exponent of the power-law rise (). We find a mean
uncorrected rise time of days, with individual SN rise-times
ranging from to days. The exponent n shows significant
departures from the simple 'fireball model' of (or ) usually assumed in the literature. With a mean value of , our data also show significant diversity from event to event. This
deviation has implications for the distribution of 56Ni throughout the SN
ejecta, with a higher index suggesting a lesser degree of 56Ni mixing. The
range of n found also confirms that the 56Ni distribution is not standard
throughout the population of SNe Ia, in agreement with earlier work measuring
such abundances through spectral modelling. We also show that the duration of
the very early light curve, before the luminosity has reached half of its
maximal value, does not correlate with the light curve shape or stretch used to
standardise SNe Ia in cosmological applications. This has implications for the
cosmological fitting of SN Ia light curves.Comment: 19 pages, 19 figures, accepted for publication in MNRA
Evidence for a Spectroscopic Sequence Among SNe Ia
In this Letter we present evidence for a spectral sequence among Type Ia
supernovae (SNe Ia). The sequence is based on the systematic variation of
several features seen in the near-maximum light spectrum. This sequence is
analogous to the recently noted photometric sequence among SNe Ia which shows a
relationship between the peak brightness of a SN Ia and the shape of its light
curve. In addition to the observational evidence we present a partial
theoretical explanation for the sequence. This has been achieved by producing a
series of non-LTE synthetic spectra in which only the effective temperature is
varied. The synthetic sequence nicely reproduces most of the differences seen
in the observed one and presumably corresponds to the amount of 56Ni produced
in the explosion.Comment: To appear in Astrophysical Journal Letters, uuencoded, gzipped
postscript file, also available from http://www.nhn.uoknor.edu/~baron
Single-Shot Electron Diffraction using a Cold Atom Electron Source
Cold atom electron sources are a promising alternative to traditional
photocathode sources for use in ultrafast electron diffraction due to greatly
reduced electron temperature at creation, and the potential for a corresponding
increase in brightness. Here we demonstrate single-shot, nanosecond electron
diffraction from monocrystalline gold using cold electron bunches generated in
a cold atom electron source. The diffraction patterns have sufficient signal to
allow registration of multiple single-shot images, generating an averaged image
with significantly higher signal-to-noise ratio than obtained with unregistered
averaging. Reflection high-energy electron diffraction (RHEED) was also
demonstrated, showing that cold atom electron sources may be useful in
resolving nanosecond dynamics of nanometre scale near-surface structures.Comment: This is an author-created, un-copyedited version of an article
published in Journal of Physics B: Atomic, Molecular and Optical Physics. IOP
Publishing Ltd is not responsible for any errors or omissions in this version
of the manuscript or any version derived from it. The Version of Record is
available online at http://dx.doi.org/10.1088/0953-4075/48/21/21400
NEOWISE Reactivation Mission Year One: Preliminary Asteroid Diameters and Albedos
We present preliminary diameters and albedos for 7,959 asteroids detected in
the first year of the NEOWISE Reactivation mission. 201 are near-Earth
asteroids (NEAs). 7,758 are Main Belt or Mars-crossing asteroids. 17% of these
objects have not been previously characterized using WISE or NEOWISE thermal
measurements. Diameters are determined to an accuracy of ~20% or better. If
good-quality H magnitudes are available, albedos can be determined to within
~40% or better.Comment: 42 pages, 5 figure
NEOWISE Reactivation Mission Year Three: Asteroid Diameters and Albedos
The Near-Earth Object Wide-field Infrared Survey Explorer (NEOWISE)
reactivation mission has completed its third year of surveying the sky in the
thermal infrared for near-Earth asteroids and comets. NEOWISE collects
simultaneous observations at 3.4 um and 4.6 um of solar system objects passing
through its field of regard. These data allow for the determination of total
thermal emission from bodies in the inner solar system, and thus the sizes of
these objects. In this paper we present thermal model fits of asteroid
diameters for 170 NEOs and 6110 MBAs detected during the third year of the
survey, as well as the associated optical geometric albedos. We compare our
results with previous thermal model results from NEOWISE for overlapping sample
sets, as well as diameters determined through other independent methods, and
find that our diameter measurements for NEOs agree to within 26% (1-sigma) of
previously measured values. Diameters for the MBAs are within 17% (1-sigma).
This brings the total number of unique near-Earth objects characterized by the
NEOWISE survey to 541, surpassing the number observed during the fully
cryogenic mission in 2010.Comment: Accepted for publication in A
Precise time delays from strongly gravitationally lensed type Ia supernovae with chromatically microlensed images
Time delays between the multiple images of strongly gravitationally lensed Type Ia supernovae (glSNe Ia) have the potential to deliver precise cosmological constraints, but the effects of microlensing on time delay extraction have not been studied in detail. Here we quantify the effect of microlensing on the glSN Ia yield of the Large Synoptic Survey Telescope (LSST) and the effect of microlensing on the precision and accuracy of time delays that can be extracted from LSST glSNe Ia. Microlensing has a negligible effect on the LSST glSN Ia yield, but it can be increased by a factor of ∼2 over previous predictions to 930 systems using a novel photometric identification technique based on spectral template fitting. Crucially, the microlensing of glSNe Ia is achromatic until three rest-frame weeks after the explosion, making the early-time color curves microlensing-insensitive time delay indicators. By fitting simulated flux and color observations of microlensed glSNe Ia with their underlying, unlensed spectral templates, we forecast the distribution of absolute time delay error due to microlensing for LSST, which is unbiased at the sub-percent level and peaked at 1% for color curve observations in the achromatic phase, while for light-curve observations it is comparable to state-of-the-art mass modeling uncertainties (4%). About 70% of LSST glSN Ia images should be discovered during the achromatic phase, indicating that microlensing time delay uncertainties can be minimized if prompt multicolor follow-up observations are obtained. Accounting for microlensing, the 1-2 day time delay on the recently discovered glSN Ia iPTF16geu can be measured to 40% precision, limiting its cosmological utility
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