11,831 research outputs found
Selection Effects, Biases, and Constraints in the Calan/Tololo Supernova Survey
We use Monte Carlo simulations of the Calan/Tololo photographic supernova
survey to show that a simple model of the survey's selection effects accounts
for the observed distributions of recession velocity, apparent magnitude,
angular offset, and projected radial distance between the supernova and the
host galaxy nucleus for this sample of Type Ia supernovae (SNe Ia). The model
includes biases due to the flux-limited nature of the survey, the different
light curve morphologies displayed by different SNe Ia, and the difficulty of
finding events projected near the central regions of the host galaxies. From
these simulations we estimate the bias in the zero-point and slope of the
absolute magnitude-decline rate relation used in SNe Ia distance measurements.
For an assumed intrinsic scatter of 0.15 mag about this relation, these
selection effects decrease the zero-point by 0.04 mag. The slope of the
relation is not significantly biased. We conclude that despite selection
effects in the survey, the shape and zero-point of the relation determined from
the Calan/Tololo sample are quite reliable. We estimate the degree of
incompleteness of the survey as a function of decline rate and estimate a
corrected luminosity function for SNe Ia in which the frequency of SNe appears
to increase with decline rate (the fainter SNe are more common). Finally, we
compute the integrated detection efficiency of the survey in order to infer the
rate of SNe Ia from the 31 events found. For a value of Ho=65 km/sec/Mpc we
obtain a SN Ia rate of 0.21(+0.30)(-0.13) SNu. This is in good agreement with
the value 0.16+/-0.05 SNu recently determined by Capellaro et al. (1997).Comment: 36 pages, 19 figures as extra files, to appear in the A
The Extreme Ultraviolet Variability of Quasars
We study the extreme ultraviolet (EUV) variability (rest frame wavelengths
500 - 920 ) of high luminosity quasars using HST (low to intermediate
redshift sample) and SDSS (high redshift sample) archives. The combined HST and
SDSS data indicates a much more pronounced variability when the sampling time
between observations in the quasar rest frame is sec
compared to sec. Based on an excess variance analysis, for
time intervals sec in the quasar rest frame, of the
quasars (4/40) show evidence of EUV variability. Similarly, for time intervals
sec in the quasar rest frame, of the quasars (21/38)
show evidence of EUV variability. The propensity for variability does not show
any statistically significant change between sec and
sec (1 yr). The temporal behavior is one of a threshold
time interval for significant variability as opposed to a gradual increase on
these time scales. A threshold time scale can indicate a characteristic spatial
dimension of the EUV region. We explore this concept in the context of the slim
disk models of accretion. We find that for rapidly spinning black holes, the
radial infall time to the plunge region of the optically thin surface layer of
the slim disk that is responsible for the preponderance of the EUV flux
emission (primarily within 0 - 7 black hole radii from the inner edge of the
disk) is consistent with the empirically determined variability time scale.Comment: To appear in Ap
Identifying infestation probabilities of Emerald Ash Borer (Agrilus planipennis, Fairmaire) in the Mid-Atlantic region
Emerald Ash Borer (EAB) impacts all species of North American ash trees, and has caused several million dollars (U.S.) in damage to trees across the affected region. EAB is primarily spread through the movement of trees and wood products, such as nursery stock and firewood. This thesis assessed the potential risk of EAB introduction in the Mid-Atlantic region of the U.S., where the species has not yet been as widely reported. Using a Geographic Information Systems-based approach, a risk prioritization framework was developed to assess and rank various mapped factors for EAB introduction. Results indicated high risk areas throughout the study region with approximately 30 counties being cited for potential risk. From an analysis of risk versus ash basal area for all counties, three management strategies were derived; quarantine, plan harvest, public outreach and monitoring
Statistics for Middle and High School Teachers: A Resource for Middle and High School Teachers to Feel Better Prepared To Teach the Common Core State Standards (CCSS) Relating to Statistics
The purpose of this project is to create a two-day workshop to better prepare middle and high school teachers to teach probability and statistics as required by the Common Core State Standards (CCSS), which have broadened the mathematics curriculum to include in depth understanding of probability and statistics. Many teachers are not prepared to address probability and statistics concepts. Research has demonstrated a need for greater professional development and resources for teachers in this area. The two-day workshop will allow teachers to review their knowledge and enhance their understanding of statistics by emphasizing student-centered teaching examples. Technology and/or software will be used in connection with the problems. The goal of the workshop is to provide professional development for teachers by helping them to be better qualified and more confident in their ability to teach statistics at the middle and high school level
Quantitative chemical tagging, stellar ages and the chemo-dynamical evolution of the Galactic disc
The early science results from the new generation of high-resolution stellar
spectroscopic surveys, such as GALAH and the Gaia-ESO survey, will represent
major milestones in the quest to chemically tag the Galaxy. Yet this technique
to reconstruct dispersed coeval stellar groups has remained largely untested
until recently. We build on previous work that developed an empirical chemical
tagging probability function, which describes the likelihood that two field
stars are conatal, that is, they were formed in the same cluster environment.
In this work we perform the first ever blind chemical tagging experiment, i.e.,
tagging stars with no known or otherwise discernable associations, on a sample
of 714 disc field stars with a number of high quality high resolution
homogeneous metal abundance measurements. We present evidence that chemical
tagging of field stars does identify coeval groups of stars, yet these groups
may not represent distinct formation sites, e.g. as in dissolved open clusters,
as previously thought. Our results point to several important conclusions,
among them that group finding will be limited strictly to chemical abundance
space, e.g. stellar ages, kinematics, colors, temperature and surface gravity
do not enhance the detectability of groups. We also demonstrate that in
addition to its role in probing the chemical enrichment and kinematic history
of the Galactic disc, chemical tagging represents a powerful new stellar age
determination technique.Comment: 12 pages, 9 figures, accepted for publication in Monthly Notices of
the Royal Astronomical Society (MNRAS
Modeling the HeII Transverse Proximity Effect: Constraints on Quasar Lifetime and Obscuration
The HeII transverse proximity effect - enhanced HeII Ly{\alpha} transmission
in a background sightline caused by the ionizing radiation of a foreground
quasar - offers a unique opportunity to probe the emission properties of
quasars, in particular the emission geometry (obscuration, beaming) and the
quasar lifetime. Building on the foreground quasar survey published in
Schmidt+2017, we present a detailed model of the HeII transverse proximity
effect, specifically designed to include light travel time effects, finite
quasar ages, and quasar obscuration. We post-process outputs from a
cosmological hydrodynamical simulation with a fluctuating HeII UV background
model, plus the added effect of the radiation from a single bright foreground
quasar. We vary the age and obscured sky fractions
of the foreground quasar, and explore the resulting
effect on the HeII transverse proximity effect signal. Fluctuations in IGM
density and the UV background, as well as the unknown orientation of the
foreground quasar, result in a large variance of the HeII Ly{\alpha}
transmission along the background sightline. We develop a fully Bayesian
statistical formalism to compare far UV HeII Ly{\alpha} transmission spectra of
the background quasars to our models, and extract joint constraints on
and for the six Schmidt+2017 foreground
quasars with the highest implied HeII photoionization rates. Our analysis
suggests a bimodal distribution of quasar emission properties, whereby one
foreground quasar, associated with a strong HeII transmission spike, is
relatively old and unobscured ,
whereas three others are either younger than or highly
obscured .Comment: 19 pages, 6 figures, submitted to Ap
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