208 research outputs found
Onto what planes should Coulomb stress perturbations be resolved?
[1] Coulomb stress maps are produced by computing the tensorial stress perturbation due to an earthquake rupture and resolving this tensor onto planes of a particular orientation. It is often assumed that aftershock fault planes are ‘‘optimally oriented’’; in other words, the regional stress and coseismic stress change are used to compute the orientation of planes most likely to fail and the coseismic stress is resolved onto these orientations. This practice assumes that faults capable of sustaining aftershocks exist at all orientations, an assumption contradicted by the observation that aftershock focal mechanisms have strong preferred orientations consistent with mapped structural trends. Here we systematically investigate the best planes onto which stress should be resolved for the Landers, Hector Mine, Loma Prieta, and Northridge earthquakes by quantitatively comparing observed aftershock distributions with stress maps based on optimally oriented planes (two- and three-dimensional), main shock orientation, and regional structural trend. We find that the best model differs between different tectonic regions but that in all cases, models that incorporate the regional stress field tend to produce stress maps that best fit the observed aftershock distributions, although not all such models do so equally well. Our results suggest that when the regional stress field is poorly defined, or in structurally complex areas, the best model may be to fix the strike of the planes upon which the stress is to be resolved to that of the main shock but allow the dip and rake to vary
Photometric selection of high-redshift type Ia supernovae
We present a method for selecting high-redshift type Ia supernovae (SNe Ia)
located via rolling SN searches. The technique, using both color and magnitude
information of events from only 2-3 epochs of multi-band real-time photometry,
is able to discriminate between SNe Ia and core collapse SNe. Furthermore, for
the SNe Ia, the method accurately predicts the redshift, phase and light-curve
parameterization of these events based only on pre-maximum-light data. We
demonstrate the effectiveness of the technique on a simulated survey of SNe Ia
and core-collapse SNe, where the selection method effectively rejects most
core-collapse SNe while retaining SNe Ia. We also apply the selection code to
real-time data acquired as part of the Canada-France-Hawaii Telescope Supernova
Legacy Survey (SNLS). During the period May 2004 to January 2005 in the SNLS,
440 SN candidates were discovered of which 70 were confirmed spectroscopically
as SNe Ia and 15 as core-collapse events. For this test dataset, the selection
technique correctly identifies 100% of the identified SNe II as non-SNe Ia with
only a 1-2% false rejection rate. The predicted parameterization of the SNe Ia
has a precision of |delta_z|/(1+z_spec)<0.09 in redshift, and +/- 2-3
rest-frame days in phase, providing invaluable information for planning
spectroscopic follow-up observations. We also investigate any bias introduced
by this selection method on the ability of surveys such as SNLS to measure
cosmological parameters (e.g., w and omega matter), and find any effect to be
negligible.Comment: Accepted for publication in A
The effect of progenitor age and metallicity on luminosity and 56Ni yield in Type Ia supernovae
Timmes, Brown & Truran found that metallicity variations could theoretically
account for a 25% variation in the mass of 56Ni synthesized in Type Ia
supernovae (SNe Ia), and thus account for a large fraction of the scatter in
observed SN Ia luminosities. Higher-metallicity progenitors are more
neutron-rich, producing more stable burning products relative to radioactive
56Ni. We develop a new method for estimating bolometric luminosity and 56Ni
yield in SNe Ia and use it to test the theory with data from the Supernova
Legacy Survey. We find that the average 56Ni yield does drop in SNe Ia from
high metallicity environments, but the theory can only account for 7%--10% of
the dispersion in SN Ia 56Ni mass, and thus luminosity. This is because the
effect is dominant at metallicities significantly above solar, whereas we find
that SN hosts have predominantly subsolar or only moderately above-solar
metallicities. We also show that allowing for changes in O/Fe with the
metallicity [Fe/H] does not have a major effect on the theoretical prediction
of Timmes, Brown & Truran, so long as one is using the O/H as the independent
variable. Age may have a greater effect than metallicity -- we find that the
luminosity weighted age of the host galaxy is correlated with 56Ni yield, and
thus more massive progenitors give rise to more luminous explosions. This is
hard to understand if most SNe Ia explode when the primaries reach the
Chandrasekhar mass. Finally, we test the findings of Gallagher et al., that the
residuals of SNe Ia from the Hubble diagram are correlated with host galaxy
metallicity, and we find no such correlation.Comment: ApJ, accepted, 34 pages, 11 figures, apologies for one column format
-- necessary for the equation
Type Ia Supernovae Rates and Galaxy Clustering from the CFHT Supernova Legacy Survey
The Canada-France-Hawaii Telescope Supernova Legacy Survey (SNLS) has created
a large homogeneous database of intermediate redshift (0.2 < z < 1.0) type Ia
supernovae (SNe Ia). The SNLS team has shown that correlations exist between SN
Ia rates, properties, and host galaxy star formation rates. The SNLS SN Ia
database has now been combined with a photometric redshift galaxy catalog and
an optical galaxy cluster catalog to investigate the possible influence of
galaxy clustering on the SN Ia rate, over and above the expected effect due to
the dependence of SFR on clustering through the morphology-density relation. We
identify three cluster SNe Ia, plus three additional possible cluster SNe Ia,
and find the SN Ia rate per unit mass in clusters at intermediate redshifts is
consistent with the rate per unit mass in field early-type galaxies and the SN
Ia cluster rate from low redshift cluster targeted surveys. We also find the
number of SNe Ia in cluster environments to be within a factor of two of
expectations from the two component SNIa rate model.Comment: 21 pages, 2 figures, 6 tables, accepted for publication in A
PHotometry Assisted Spectral Extraction (PHASE) and identification of SNLS supernovae
Aim: We present new extraction and identification techniques for supernova
(SN) spectra developed within the Supernova Legacy Survey (SNLS) collaboration.
Method: The new spectral extraction method takes full advantage of
photometric information from the Canada-France-Hawai telescope (CFHT) discovery
and reference images by tracing the exact position of the supernova and the
host signals on the spectrogram. When present, the host spatial profile is
measured on deep multi-band reference images and is used to model the host
contribution to the full (supernova + host) signal. The supernova is modelled
as a Gaussian function of width equal to the seeing. A chi-square minimisation
provides the flux of each component in each pixel of the 2D spectrogram. For a
host-supernova separation greater than <~ 1 pixel, the two components are
recovered separately and we do not use a spectral template in contrast to more
standard analyses. This new procedure permits a clean extraction of the
supernova separately from the host in about 70% of the 3rd year ESO/VLT spectra
of the SNLS. A new supernova identification method is also proposed. It uses
the SALT2 spectrophotometric template to combine the photometric and spectral
data. A galaxy template is allowed for spectra for which a separate extraction
of the supernova and the host was not possible.
Result: These new techniques have been tested against more standard
extraction and identification procedures. They permit a secure type and
redshift determination in about 80% of cases. The present paper illustrates
their performances on a few sample spectra.Comment: 27 pages, 18 Figures, 1 Table. Accepted for publication in A&
Ingestive behaviour and forage intake responses of young and mature steers to the vertical differentiation of sugarcane in pen and grazing studies
SUMMARY Sugarcane is an important forage resource in sub-tropical and tropical areas as it is used during the winter or dry season when the growth rate of pastures is significantly reduced. The current research study assessed the effect of four vertical sections of sugarcane in a pen trial and the level of sugarcane utilization in a grazing trial on the ingestive behaviour and forage intake of two age groups of steers (1 and 2 years old). The pen trial was comprised of two simultaneous 4 × 4 balanced Latin square designs (one for each age group of animals) of four periods, four animals and four feeding treatments, which consisted of four equal vertical sections of sugarcane. Dry matter (DM) and digestible DM (DDM) intake per kilogram of metabolic weight declined gradually from top to bottom of the sugarcane, with no significant differences between the age groups of steers. This difference in intake was associated with a decline in intake of neutral detergent fibre (NDF) as a proportion of the liveweight of the animal and an increase of total chewing time per kilogram of DM or NDF from top to bottom of the sugarcane. It was concluded that the toughness of plant material played a significant role regulating intake, which was higher for the top sections of sugarcane. In the grazing trial, steers of both age groups grazed down sugarcane in three plots over 9 days. Steers grazed up to four distinctive grazing strata. Digestible DM intake (DDM intake) was high at low levels of horizontal utilization of the top grazing stratum but DDM intake started to decline sharply when this stratum was removed in 0·92 of paddock area (i.e. equivalent to 0·08 of the pasture area remaining un-grazed). It was concluded that the proportion of un-grazed area of the pasture can be used as a grazing management strategy to control forage intake for sugarcane
The Supernova Legacy Survey 3-year sample: Type Ia Supernovae photometric distances and cosmological constraints
We present photometric properties and distance measurements of 252 high
redshift Type Ia supernovae (0.15 < z < 1.1) discovered during the first three
years of the Supernova Legacy Survey (SNLS). These events were detected and
their multi-colour light curves measured using the MegaPrime/MegaCam instrument
at the Canada-France-Hawaii Telescope (CFHT), by repeatedly imaging four
one-square degree fields in four bands. Follow-up spectroscopy was performed at
the VLT, Gemini and Keck telescopes to confirm the nature of the supernovae and
to measure their redshifts. Systematic uncertainties arising from light curve
modeling are studied, making use of two techniques to derive the peak
magnitude, shape and colour of the supernovae, and taking advantage of a
precise calibration of the SNLS fields. A flat LambdaCDM cosmological fit to
231 SNLS high redshift Type Ia supernovae alone gives Omega_M = 0.211 +/-
0.034(stat) +/- 0.069(sys). The dominant systematic uncertainty comes from
uncertainties in the photometric calibration. Systematic uncertainties from
light curve fitters come next with a total contribution of +/- 0.026 on
Omega_M. No clear evidence is found for a possible evolution of the slope
(beta) of the colour-luminosity relation with redshift.Comment: (The SNLS Collaboration) 40 pages, 32 figures, Accepted in A&
Evolution in the Volumetric Type Ia Supernova Rate from the Supernova Legacy Survey
We present a measurement of the volumetric Type Ia supernova (SN Ia) rate
(SNR_Ia) as a function of redshift for the first four years of data from the
Canada-France-Hawaii Telescope (CFHT) Supernova Legacy Survey (SNLS). This
analysis includes 286 spectroscopically confirmed and more than 400 additional
photometrically identified SNe Ia within the redshift range 0.1<z<1.1. The
volumetric SNR_Ia evolution is consistent with a rise to z~1.0 that follows a
power-law of the form (1+z)^alpha, with alpha=2.11+/-0.28. This evolutionary
trend in the SNLS rates is slightly shallower than that of the cosmic
star-formation history over the same redshift range. We combine the SNLS rate
measurements with those from other surveys that complement the SNLS redshift
range, and fit various simple SN Ia delay-time distribution (DTD) models to the
combined data. A simple power-law model for the DTD (i.e., proportional to
t^-beta) yields values from beta=0.98+/-0.05 to beta=1.15+/-0.08 depending on
the parameterization of the cosmic star formation history. A two-component
model, where SNR_Ia is dependent on stellar mass (Mstellar) and star formation
rate (SFR) as SNR_Ia(z)=AxMstellar(z) + BxSFR(z), yields the coefficients
A=1.9+/-0.1 SNe/yr/M_solar and B=3.3+/-0.2 SNe/yr/(M_solar/yr). More general
two-component models also fit the data well, but single Gaussian or exponential
DTDs provide significantly poorer matches. Finally, we split the SNLS sample
into two populations by the light curve width (stretch), and show that the
general behavior in the rates of faster-declining SNe Ia (0.8<s<1.0) is
similar, within our measurement errors, to that of the slower objects
(1.0<s<1.3) out to z~0.8.Comment: Accepted in A
SALT2: using distant supernovae to improve the use of Type Ia supernovae as distance indicators
We present an empirical model of Type Ia supernovae spectro-photometric
evolution with time. The model is built using a large data set including
light-curves and spectra of both nearby and distant supernovae, the latter
being observed by the SNLS collaboration. We derive the average spectral
sequence of Type Ia supernovae and their main variability components including
a color variation law. The model allows us to measure distance moduli in the
spectral range 2500-8000 A with calculable uncertainties, including those
arising from variability of spectral features. Thanks to the use of
high-redshift SNe to model the rest-frame UV spectral energy distribution, we
are able to derive improved distance estimates for SNe Ia in the redshift range
0.8<z<1.1. The model can also be used to improve spectroscopic identification
algorithms, and derive photometric redshifts of distant Type Ia supernovae.Comment: Accepted for publication in A&A. Data and source code available at :
http://supernovae.in2p3.fr/~guy/salt
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