6,875 research outputs found
Exploratory wind tunnel tests of a shock-swallowing air data sensor at a Mach number of approximately 1.83
The test probe was designed to measure free-stream Mach number and could be incorporated into a conventional airspeed nose boom installation. Tests were conducted in the Langley 4-by 4-foot supersonic pressure tunnel with an approximate angle of attack test range of -5 deg to 15 deg and an approximate angle of sideslip test range of + or - 4 deg. The probe incorporated a variable exit area which permitted internal flow. The internal flow caused the bow shock to be swallowed. Mach number was determined with a small axially movable internal total pressure tube and a series of fixed internal static pressure orifices. Mach number error was at a minimum when the total pressure tube was close to the probe tip. For four of the five tips tested, the Mach number error derived by averaging two static pressures measured at horizontally opposed positions near the probe entrance were least sensitive to angle of attack changes. The same orifices were also used to derive parameters that gave indications of flow direction
MAESTRO, CASTRO, and SEDONA -- Petascale Codes for Astrophysical Applications
Performing high-resolution, high-fidelity, three-dimensional simulations of
Type Ia supernovae (SNe Ia) requires not only algorithms that accurately
represent the correct physics, but also codes that effectively harness the
resources of the most powerful supercomputers. We are developing a suite of
codes that provide the capability to perform end-to-end simulations of SNe Ia,
from the early convective phase leading up to ignition to the explosion phase
in which deflagration/detonation waves explode the star to the computation of
the light curves resulting from the explosion. In this paper we discuss these
codes with an emphasis on the techniques needed to scale them to petascale
architectures. We also demonstrate our ability to map data from a low Mach
number formulation to a compressible solver.Comment: submitted to the Proceedings of the SciDAC 2010 meetin
Diversity of Decline-Rate-Corrected Type Ia Supernova Rise Times: One Mode or Two?
B-band light-curve rise times for eight unusually well-observed nearby Type
Ia supernovae (SNe) are fitted by a newly developed template-building
algorithm, using light-curve functions that are smooth, flexible, and free of
potential bias from externally derived templates and other prior assumptions.
From the available literature, photometric BVRI data collected over many
months, including the earliest points, are reconciled, combined, and fitted to
a unique time of explosion for each SN. On average, after they are corrected
for light-curve decline rate, three SNe rise in 18.81 +- 0.36 days, while five
SNe rise in 16.64 +- 0.21 days. If all eight SNe are sampled from a single
parent population (a hypothesis not favored by statistical tests), the rms
intrinsic scatter of the decline-rate-corrected SN rise time is 0.96 +0.52
-0.25 days -- a first measurement of this dispersion. The corresponding global
mean rise time is 17.44 +- 0.39 days, where the uncertainty is dominated by
intrinsic variance. This value is ~2 days shorter than two published averages
that nominally are twice as precise, though also based on small samples. When
comparing high-z to low-z SN luminosities for determining cosmological
parameters, bias can be introduced by use of a light-curve template with an
unrealistic rise time. If the period over which light curves are sampled
depends on z in a manner typical of current search and measurement strategies,
a two-day discrepancy in template rise time can bias the luminosity comparison
by ~0.03 magnitudes.Comment: As accepted by The Astrophysical Journal; 15 pages, 6 figures, 2
tables. Explanatory material rearranged and enhanced; Fig. 4 reformatte
The Rise Times of High and Low Redshift Type Ia Supernovae are Consistent
We present a self-consistent comparison of the rise times for low- and
high-redshift Type Ia supernovae. Following previous studies, the early light
curve is modeled using a t-squared law, which is then mated with a modified
Leibundgut template light curve. The best-fit t-squared law is determined for
ensemble samples of low- and high-redshift supernovae by fitting simultaneously
for all light curve parameters for all supernovae in each sample. Our method
fully accounts for the non-negligible covariance amongst the light curve
fitting parameters, which previous analyses have neglected. Contrary to Riess
et al. (1999), we find fair to good agreement between the rise times of the
low- and high-redshift Type Ia supernovae. The uncertainty in the rise time of
the high-redshift Type Ia supernovae is presently quite large (roughly +/- 1.2
days statistical), making any search for evidence of evolution based on a
comparison of rise times premature. Furthermore, systematic effects on rise
time determinations from the high-redshift observations, due to the form of the
late-time light curve and the manner in which the light curves of these
supernovae were sampled, can bias the high-redshift rise time determinations by
up to +3.6/-1.9 days under extreme situations. The peak brightnesses - used for
cosmology - do not suffer any significant bias, nor any significant increase in
uncertainty.Comment: 18 pages, 4 figures, Accepted for publication in the Astronomical
Journal. Also available at http://www.lbl.gov/~nugent/papers.html Typos were
corrected and a few sentences were added for improved clarit
The Impact of Strong Gravitational Lensing on Observed Lyman-Break Galaxy Numbers at 4<z<8 in the GOODS and the XDF Blank Fields
Detection of Lyman-Break Galaxies (LBGs) at high-redshift can be affected by
gravitational lensing induced by foreground deflectors not only in galaxy
clusters, but also in blank fields. We quantify the impact of strong
magnification in the samples of , , , LBGs () observed in the XDF and GOODS/CANDELS fields, by investigating the
proximity of dropouts to foreground objects. We find that of bright
LBGs () by
foreground objects. This fraction decreases from at to
at . Since the observed fraction of strongly lensed
galaxies is a function of the shape of the luminosity function (LF), it can be
used to derive Schechter parameters, and , independently
from galaxy number counts. Our magnification bias analysis yields
Schechter-function parameters in close agreement with those determined from
galaxy counts albeit with larger uncertainties. Extrapolation of our analysis
to suggests that future surveys with JSWT, WFIRST and EUCLID
should find excess LBGs at the bright-end, even if there is an intrinsic
exponential cutoff of number counts. Finally, we highlight how the
magnification bias measurement near the detection limit can be used as probe of
the population of galaxies too faint to be detected. Preliminary results using
this novel idea suggest that the magnification bias at is not
as strong as expected if extends well below the current
detection limits in the XDF. At face value this implies a flattening of the LF
at . However, selection effects and completeness estimates
are difficult to quantify precisely. Thus, we do not rule out a steep LF
extending to .Comment: Submitted to ApJ on 18/12/201
Near-infrared observations of type Ia supernovae: The best known standard candle for cosmology
We present an analysis of the Hubble diagram for 12 Type Ia supernovae (SNe
Ia) observed in the near-infrared J and H bands. We select SNe exclusively from
the redshift range 0.03 < z < 0.09 to reduce uncertainties coming from peculiar
velocities while remaining in a cosmologically well-understood region. All of
the SNe in our sample exhibit no spectral or B-band light-curve peculiarities
and lie in the B-band stretch range of 0.8-1.15. Our results suggest that SNe
Ia observed in the near-infrared (NIR) are the best known standard candles. We
fit previously determined NIR light-curve templates to new high-precision data
to derive peak magnitudes and to determine the scatter about the Hubble line.
Photometry of the 12 SNe is presented in the natural system. Using a standard
cosmology of (H_0, Omega_m, Lambda) = (70,0.27,0.73) we find a median J-band
absolute magnitude of M_J = -18.39 with a scatter of 0.116 and a median H-band
absolute magnitude of M_H = -18.36 with a scatter of 0.085. The scatter in the
H band is the smallest yet measured. We search for correlations between
residuals in the J- and H-band Hubble diagrams and SN properties, such as SN
colour, B-band stretch and the projected distance from host-galaxy centre. The
only significant correlation is between the J-band Hubble residual and the J-H
pseudo-colour. We also examine how the scatter changes when fewer points in the
near-infrared are used to constrain the light curve. With a single point in the
H band taken anywhere from 10 days before to 15 days after B-band maximum light
and a prior on the date of H-band maximum set from the date of B-band maximum,
we find that we can measure distances to an accuracy of 6%. The precision of
SNe Ia in the NIR provides new opportunities for precision measurements of both
the expansion history of the universe and peculiar velocities of nearby
galaxies.Comment: 6 pages, 2 figures. Accepted for publication in MNRA
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